Material and methods: For the analysis, we considered a physiologic model of abdominal aorta with an iliac bifurcation set at 30°, 45° and 70° without stenosis. Subsequently, a bilateral ostial common iliac stenosis of 80% was considered for each type of bifurcation. For the stent simulation, we reconstructed Zilver vascular self-expanding (Zilver; Cook, Bloomington, MN) and Palmaz Genesis Peripheral (Cordis, Miami, FL) stents.

Results: The physiologic model, across the different angles, static pressure, Reynolds number and stream function, were lower for the 30° bifurcation angle with a gradient from 70° to 30° angles, whereas all the other parameters were inversely higher. After stenting, all the fluid parameters decreased homogenously independent of the stent type, maintaining a gradient in favour of 30° compared to 45° and 70° angles. The absolute greater deviation from physiology was observed for low kissing when self-expandable stents were used across all angles; in particular, the wall shear stress was high at at 45° angle.

Conclusion: Bifurcation angle deeply impacts the physiology of aortoiliac bifurcations, which are used to predict the fluid dynamic profile after stenting. CFD, having the potential to be derived both from computed tomography scan or invasive angiography, appears to be an ideal tool to predict fluid dynamic profile before and after stenting in aortoiliac bifurcation.}, } @article {pmid30424640, year = {2018}, author = {Sadri, M and Hejranfar, K and Ebrahimi, M}, title = {Prediction of fluid flow and acoustic field of a supersonic jet using vorticity confinement.}, journal = {The Journal of the Acoustical Society of America}, volume = {144}, number = {3}, pages = {1521}, doi = {10.1121/1.5055215}, pmid = {30424640}, issn = {1520-8524}, abstract = {In this study, the numerical simulation of the fluid flow and acoustic field of a supersonic jet is performed by using high-order discretization and the vorticity confinement (VC) method on coarse grids. The three-dimensional Navier-Stokes equations are considered in the generalized curvilinear coordinate system and the high-order compact finite-difference scheme is applied for the space discretization, and the time integration is performed by the fourth-order Runge-Kutta scheme. A low-pass high-order filter is applied to stabilize the numerical solution. The non-reflecting boundary conditions are adopted for all the free boundaries, and the Kirchhoff surface integration is utilized to obtain the far-field sound pressure levels in a number of observer locations. Comparisons of the jet mean flow and jet aeroacoustics results with the other numerical and experimental data at similar flow conditions are made and show a reasonable agreement. The study shows that the proposed solution methodology based on the high-order compact finite-difference scheme in conjunction with the VC method can reasonably predict the near-field flow and the far-field noise of high Reynolds number jets with a fairly coarser grid than that used in the large eddy simulations and, thus, the computational cost can be significantly decreased.}, } @article {pmid30424188, year = {2018}, author = {Jung, BJ and Kim, J and Kim, JA and Jang, H and Seo, S and Lee, W}, title = {PDMS-Parylene Hybrid, Flexible Microfluidics for Real-Time Modulation of 3D Helical Inertial Microfluidics.}, journal = {Micromachines}, volume = {9}, number = {6}, pages = {}, doi = {10.3390/mi9060255}, pmid = {30424188}, issn = {2072-666X}, support = {NRF-2015M2A2A4A02044826//National Research Foundation of Korea/ ; 10054488//Ministry of Trade, Industry & Energy (MOTIE, Korea)/ ; }, abstract = {Inertial microfluidics has drawn much attention for its applications for circulating tumor cell separations from blood. The fluid flows and the inertial particle focusing in inertial microfluidic systems are highly dependent on the channel geometry and structure. Flexible microfluidic systems can have adjustable 3D channel geometries by curving planar 2D channels into 3D structures, which will enable tunable inertial separation. We present a poly(dimethylsiloxane) (PDMS)-parylene hybrid thin-film microfluidic system that can provide high flexibility for 3D channel shaping while maintaining the channel cross-sectional shape. The PDMS-parylene hybrid microfluidic channels were fabricated by a molding and bonding technique using initiated chemical vapor deposition (iCVD) bonding. We constructed 3D helical inertial microfluidic channels by coiling a straight 2D channel and studied the inertial focusing while varying radius of curvature and Reynolds number. This thin film structure allows for high channel curvature and high Dean numbers which leads to faster inertial particle focusing and shorter channel lengths than 2D spiral channels. Most importantly, the focusing positions of particles and cells in the microchannel can be tuned in real time by simply modulating the channel curvature. The simple mechanical modulation of these 3D structure microfluidic systems is expected to provide unique advantages of convenient tuning of cell separation thresholds with a single device.}, } @article {pmid30424137, year = {2018}, author = {Ansari, MA and Kim, KY and Kim, SM}, title = {Numerical and Experimental Study on Mixing Performances of Simple and Vortex Micro T-Mixers.}, journal = {Micromachines}, volume = {9}, number = {5}, pages = {}, doi = {10.3390/mi9050204}, pmid = {30424137}, issn = {2072-666X}, support = {NRF- 2016R1A2B4006987//National Research Foundation of Korea/ ; //Inha University/ ; }, abstract = {Vortex flow increases the interface area of fluid streams by stretching along with providing continuous stirring action to the fluids in micromixers. In this study, experimental and numerical analyses on a design of micromixer that creates vortex flow were carried out, and the mixing performance was compared with a simple micro T-mixer. In the vortex micro T-mixer, the height of the inlet channels is half of the height of the main mixing channel. The inlet channel connects to the main mixing channel (micromixer) at the one end at an offset position in a fashion that creates vortex flow. In the simple micro T-mixer, the height of the inlet channels is equal to the height of the channel after connection (main mixing channel). Mixing of fluids and flow field have been analyzed for Reynolds numbers in a range from 1⁻80. The study has been further extended to planar serpentine microchannels, which were combined with a simple and a vortex T-junction, to evaluate and verify their mixing performances. The mixing performance of the vortex T-mixer is higher than the simple T-mixer and significantly increases with the Reynolds number. The design is promising for efficiently increasing mixing simply at the T-junction and can be applied to all micromixers.}, } @article {pmid30424044, year = {2018}, author = {Raza, W and Ma, SB and Kim, KY}, title = {Multi-Objective Optimizations of a Serpentine Micromixer with Crossing Channels at Low and High Reynolds Numbers.}, journal = {Micromachines}, volume = {9}, number = {3}, pages = {}, doi = {10.3390/mi9030110}, pmid = {30424044}, issn = {2072-666X}, abstract = {In order to maximize the mixing performance of a micromixer with an integrated three-dimensional serpentine and split-and-recombination configuration, multi-objective optimizations were performed at two different Reynolds numbers, 1 and 120, based on numerical simulation. Numerical analyses of fluid flow and mixing in the micromixer were performed using three-dimensional Navier-Stokes equations and convection-diffusion equation. Three dimensionless design variables that were related to the geometry of the micromixer were selected as design variables for optimization. Mixing index at the exit and pressure drop through the micromixer were employed as two objective functions. A parametric study was carried out to explore the effects of the design variables on the objective functions. Latin hypercube sampling method as a design-of-experiment technique has been used to select design points in the design space. Surrogate modeling of the objective functions was performed by using radial basis neural network. Concave Pareto-optimal curves comprising of Pareto-optimal solutions that represents the trade-off between the objective functions were obtained using a multi-objective genetic algorithm at Re = 1 and 120. Through the optimizations, maximum enhancements of 18.8% and 6.0% in mixing index were achieved at Re = 1 and 120, respectively.}, } @article {pmid30411937, year = {2018}, author = {Galitski, V and Kargarian, M and Syzranov, S}, title = {Dynamo Effect and Turbulence in Hydrodynamic Weyl Metals.}, journal = {Physical review letters}, volume = {121}, number = {17}, pages = {176603}, doi = {10.1103/PhysRevLett.121.176603}, pmid = {30411937}, issn = {1079-7114}, abstract = {The dynamo effect is a class of macroscopic phenomena responsible for generating and maintaining magnetic fields in astrophysical bodies. It hinges on the hydrodynamic three-dimensional motion of conducting gases and plasmas that achieve high hydrodynamic and/or magnetic Reynolds numbers due to the large length scales involved. The existing laboratory experiments modeling dynamos are challenging and involve large apparatuses containing conducting fluids subject to fast helical flows. Here we propose that electronic solid-state materials-in particular, hydrodynamic metals-may serve as an alternative platform to observe some aspects of the dynamo effect. Motivated by recent experimental developments, this Letter focuses on hydrodynamic Weyl semimetals, where the dominant scattering mechanism is due to interactions. We derive Navier-Stokes equations along with equations of magnetohydrodynamics that describe the transport of a Weyl electron-hole plasma appropriate in this regime. We estimate the hydrodynamic and magnetic Reynolds numbers for this system. The latter is a key figure of merit of the dynamo mechanism. We show that it can be relatively large to enable observation of the dynamo-induced magnetic field bootstrap in an experiment. Finally, we generalize the simplest dynamo instability model-the Ponomarenko dynamo-to the case of a hydrodynamic Weyl semimetal and show that the chiral anomaly term reduces the threshold magnetic Reynolds number for the dynamo instability.}, } @article {pmid30404389, year = {2016}, author = {Zhou, T and Wang, H and Shi, L and Liu, Z and Joo, SW}, title = {An Enhanced Electroosmotic Micromixer with an Efficient Asymmetric Lateral Structure.}, journal = {Micromachines}, volume = {7}, number = {12}, pages = {}, doi = {10.3390/mi7120218}, pmid = {30404389}, issn = {2072-666X}, abstract = {Homogeneous and rapid mixing in microfluidic devices is difficult to accomplish, owing to the low Reynolds number associated with most flows in microfluidic channels. Here, an efficient electroosmotic micromixer based on a carefully designed lateral structure is demonstrated. The electroosmotic flow in this mixer with an asymmetrical structure induces enhanced disturbance in the micro channel, helping the fluid streams' folding and stretching, thereby enabling appreciable mixing. Quantitative analysis of the mixing efficiency with respect to the potential applied and the flow rate suggests that the electroosmotic microfluidic mixer developed in the present work can achieve efficient mixing with low applied potential.}, } @article {pmid30404361, year = {2016}, author = {Salieb-Beugelaar, GB and Gonçalves, D and Wolf, MP and Hunziker, P}, title = {Microfluidic 3D Helix Mixers.}, journal = {Micromachines}, volume = {7}, number = {10}, pages = {}, doi = {10.3390/mi7100189}, pmid = {30404361}, issn = {2072-666X}, support = {NRP62//Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung/ ; 160178//Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung/ ; }, abstract = {Polymeric microfluidic systems are well suited for miniaturized devices with complex functionality, and rapid prototyping methods for 3D microfluidic structures are increasingly used. Mixing at the microscale and performing chemical reactions at the microscale are important applications of such systems and we therefore explored feasibility, mixing characteristics and the ability to control a chemical reaction in helical 3D channels produced by the emerging thread template method. Mixing at the microscale is challenging because channel size reduction for improving solute diffusion comes at the price of a reduced Reynolds number that induces a strictly laminar flow regime and abolishes turbulence that would be desired for improved mixing. Microfluidic 3D helix mixers were rapidly prototyped in polydimethylsiloxane (PDMS) using low-surface energy polymeric threads, twisted to form 2-channel and 3-channel helices. Structure and flow characteristics were assessed experimentally by microscopy, hydraulic measurements and chromogenic reaction, and were modeled by computational fluid dynamics. We found that helical 3D microfluidic systems produced by thread templating allow rapid prototyping, can be used for mixing and for controlled chemical reaction with two or three reaction partners at the microscale. Compared to the conventional T-shaped microfluidic system used as a control device, enhanced mixing and faster chemical reaction was found to occur due to the combination of diffusive mixing in small channels and flow folding due to the 3D helix shape. Thus, microfluidic 3D helix mixers can be rapidly prototyped using the thread template method and are an attractive and competitive method for fluid mixing and chemical reactions at the microscale.}, } @article {pmid30404299, year = {2016}, author = {Lee, SJ and Kwon, K and Jeon, TJ and Kim, SM and Kim, D}, title = {Quantification of Vortex Generation Due to Non-Equilibrium Electrokinetics at the Micro/Nanochannel Interface: Particle Tracking Velocimetry.}, journal = {Micromachines}, volume = {7}, number = {7}, pages = {}, doi = {10.3390/mi7070127}, pmid = {30404299}, issn = {2072-666X}, support = {NRF 2014R1A2A2A01003618//National Research Foundation of Korea/ ; NRF-2016R1A2B4006987//National Research Foundation of Korea/ ; }, abstract = {We describe a quantitative study of vortex generation due to non-equilibrium electrokinetics near a micro/nanochannel interface. The microfluidic device is comprised of a microchannel with a set of nanochannels. These perm-selective nanochannels induce flow instability and thereby produce strong vortex generation. We performed tracking visualization of fluorescent microparticles to obtain velocity fields. Particle tracking enables the calculation of an averaged velocity field and the velocity fluctuations. We characterized the effect of applied voltages and electrolyte concentrations on vortex formation. The experimental results show that an increasing voltage or decreasing concentration results in a larger vortex region and a strong velocity fluctuation. We calculate the normalized velocity fluctuation-whose meaning is comparable to turbulent intensity-and we found that it is as high as 0.12. This value is indicative of very efficient mixing, albeit with a small Reynolds number.}, } @article {pmid30404283, year = {2016}, author = {Lee, SJ and Jeon, TJ and Kim, SM and Kim, D}, title = {Quantification of Vortex Generation Due to Non-Equilibrium Electrokinetics at the Micro/Nanochannel Interface: Spectral Analysis.}, journal = {Micromachines}, volume = {7}, number = {7}, pages = {}, doi = {10.3390/mi7070109}, pmid = {30404283}, issn = {2072-666X}, support = {2009-0083501//National Research Foundation of Korea/ ; NRF-2012-0009578//National Research Foundation of Korea/ ; Research Grant//Inha University/ ; }, abstract = {We report on our investigation of a low Reynolds number non-equilibrium electrokinetic flow in a micro/nanochannel platform. Non-equilibrium electrokinetic phenomena include so-called concentration polarization in a moderate electric field and vortex formation in a high electric field. We conducted a spectral analysis of non-equilibrium electrokinetic vortices at a micro/nanochannel interface. We found that periodic vortices are formed while the frequency varies with the applied voltages and solution concentrations. At a frequency as high as 60 Hz, vortex generation was obtained with the strongest electric field and the lowest concentration. The power spectra show increasing frequency with increasing voltage or decreasing concentration. We expect that our spectral analysis results will be useful for micromixer developers in the micromachine research field.}, } @article {pmid30400469, year = {2017}, author = {Afzal, MJ and Tayyaba, S and Ashraf, MW and Hossain, MK and Uddin, MJ and Afzulpurkar, N}, title = {Simulation, Fabrication and Analysis of Silver Based Ascending Sinusoidal Microchannel (ASMC) for Implant of Varicose Veins.}, journal = {Micromachines}, volume = {8}, number = {9}, pages = {}, doi = {10.3390/mi8090278}, pmid = {30400469}, issn = {2072-666X}, abstract = {Bioengineered veins can benefit humans needing bypass surgery, dialysis, and now, in the treatment of varicose veins. The implant of this vein in varicose veins has significant advantages over the conventional treatment methods. Deep vein thrombosis (DVT), vein patch repair, pulmonary embolus, and tissue-damaging problems can be solved with this implant. Here, the authors have proposed biomedical microdevices as an alternative for varicose veins. MATLAB and ANSYS Fluent have been used for simulations of blood flow for bioengineered veins. The silver based microchannel has been fabricated by using a micromachining process. The dimensions of the silver substrates are 51 mm, 25 mm, and 1.1 mm, in length, width, and depth respectively. The dimensions of microchannels grooved in the substrates are 0.9 mm in width and depth. The boundary conditions for pressure and velocity were considered, from 1.0 kPa to 1.50 kPa, and 0.02 m/s to 0.07 m/s, respectively. These are the actual values of pressure and velocity in varicose veins. The flow rate of 5.843 (0.1 nL/s) and velocity of 5.843 cm/s were determined at Reynolds number 164.88 in experimental testing. The graphs and results from simulations and experiments are in close agreement. These microchannels can be inserted into varicose veins as a replacement to maintain the excellent blood flow in human legs.}, } @article {pmid30400388, year = {2017}, author = {Wang, Q and Yuan, D and Li, W}, title = {Analysis of Hydrodynamic Mechanism on Particles Focusing in Micro-Channel Flows.}, journal = {Micromachines}, volume = {8}, number = {7}, pages = {}, doi = {10.3390/mi8070197}, pmid = {30400388}, issn = {2072-666X}, abstract = {In this paper, the hydrodynamic mechanism of moving particles in laminar micro-channel flows was numerically investigated. A hydrodynamic criterion was proposed to determine whether particles in channel flows can form a focusing pattern or not. A simple formula was derived to demonstrate how the focusing position varies with Reynolds number and particle size. Based on this proposed criterion, a possible hydrodynamic mechanism was discussed as to why the particles would not be focused if their sizes were too small or the channel Reynolds number was too low. The Re-λ curve (Re, λ respectively represents the channel-based Reynolds number and the particle's diameter scaled by the channel) was obtained using the data fitting with a least square method so as to obtain a parameter range of the focusing pattern. In addition, the importance of the particle rotation to the numerical modeling for the focusing of particles was discussed in view of the hydrodynamics. This research is expected to deepen the understanding of the particle transport phenomena in bounded flow, either in micro or macro fluidic scope.}, } @article {pmid30400531, year = {2017}, author = {Guo, X and Qi, H}, title = {Analytical Solution of Electro-Osmotic Peristalsis of Fractional Jeffreys Fluid in a Micro-Channel.}, journal = {Micromachines}, volume = {8}, number = {12}, pages = {}, doi = {10.3390/mi8120341}, pmid = {30400531}, issn = {2072-666X}, support = {11402108//National Natural Science Foundation of China/ ; 11672163//National Natural Science Foundation of China/ ; 11571157//National Natural Science Foundation of China/ ; }, abstract = {The electro-osmotic peristaltic flow of a viscoelastic fluid through a cylindrical micro-channel is studied in this paper. The fractional Jeffreys constitutive model, including the relaxation time and retardation time, is utilized to describe the viscoelasticity of the fluid. Under the assumptions of long wavelength, low Reynolds number, and Debye-Hückel linearization, the analytical solutions of pressure gradient, stream function and axial velocity are explored in terms of Mittag-Leffler function by Laplace transform method. The corresponding solutions of fractional Maxwell fluid and generalized second grade fluid are also obtained as special cases. The numerical analysis of the results are depicted graphically, and the effects of electro-osmotic parameter, external electric field, fractional parameters and viscoelastic parameters on the peristaltic flow are discussed.}, } @article {pmid30397239, year = {2018}, author = {Nourazar, SS and Nazari-Golshan, A and Soleymanpour, F}, title = {On the expedient solution of the magneto-hydrodynamic Jeffery-Hamel flow of Casson fluid.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {16358}, doi = {10.1038/s41598-018-34778-w}, pmid = {30397239}, issn = {2045-2322}, abstract = {The equation of magneto-hydrodynamic Jeffery-Hamel flow of non-Newtonian Casson fluid in a stretching/shrinking convergent/divergent channel is derived and solved using a new modified Adomian decomposition method (ADM). So far in all problems where semi-analytical methods are used the boundary conditions are not satisfied completely. In the present research, a hybrid of the Fourier transform and the Adomian decomposition method (FTADM), is presented in order to incorporate all boundary conditions into our solution of magneto-hydrodynamic Jeffery-Hamel flow of non-Newtonian Casson fluid in a stretching/shrinking convergent/divergent channel flow. The effects of various emerging parameters such as channel angle, stretching/shrinking parameter, Casson fluid parameter, Reynolds number and Hartmann number on velocity profile are considered. The results using the FTADM are compared with the results of ADM and numerical Range-Kutta fourth-order method. The comparison reveals that, for the same number of components of the recursive sequences over a wide range of spatial domain, the relative errors associated with the new method, FTADM, are much less than the ADM. The results of the new method show that the method is an accurate and expedient approximate analytic method in solving the third-order nonlinear equation of Jeffery-Hamel flow of non-Newtonian Casson fluid.}, } @article {pmid30393471, year = {2018}, author = {Shahzad, K and Aeken, WV and Mottaghi, M and Kamyab, VK and Kuhn, S}, title = {Aggregation and clogging phenomena of rigid microparticles in microfluidics: Comparison of a discrete element method (DEM) and CFD-DEM coupling method.}, journal = {Microfluidics and nanofluidics}, volume = {22}, number = {9}, pages = {104}, doi = {10.1007/s10404-018-2124-7}, pmid = {30393471}, issn = {1613-4982}, abstract = {We developed a numerical tool to investigate the phenomena of aggregation and clogging of rigid microparticles suspended in a Newtonian fluid transported through a straight microchannel. In a first step, we implement a time-dependent one-way coupling Discrete Element Method (DEM) technique to simulate the movement and effect of adhesion on rigid microparticles in two- and three-dimensional computational domains. The Johnson-Kendall-Roberts (JKR) theory of adhesion is applied to investigate the contact mechanics of particle-particle and particle-wall interactions. Using the one-way coupled solver, the agglomeration, aggregation and deposition behavior of the microparticles is studied by varying the Reynolds number and the particle adhesion. In a second step, we apply a two-way coupling CFD-DEM approach, which solves the equation of motion for each particle, and transfers the force field corresponding to particle-fluid interactions to the CFD toolbox OpenFOAM. Results for the one-way (DEM) and two-way (CFD-DEM) coupling techniques are compared in terms of aggregate size, aggregate percentages, spatial and temporal evaluation of aggregates in 2D and 3D. We conclude that two-way coupling is the more realistic approach, which can accurately capture the particle-fluid dynamics in microfluidic applications.}, } @article {pmid30393363, year = {2018}, author = {Ma, N and Duan, Z and Ma, H and Su, L and Liang, P and Ning, X and He, B and Zhang, X}, title = {Lattice Boltzmann Simulation of the Hydrodynamic Entrance Region of Rectangular Microchannels in the Slip Regime.}, journal = {Micromachines}, volume = {9}, number = {2}, pages = {}, doi = {10.3390/mi9020087}, pmid = {30393363}, issn = {2072-666X}, abstract = {Developing a three-dimensional laminar flow in the entrance region of rectangular microchannels has been investigated in this paper. When the hydrodynamic development length is the same magnitude as the microchannel length, entrance effects have to be taken into account, especially in relatively short ducts. Simultaneously, there are a variety of non-continuum or rarefaction effects, such as velocity slip and temperature jump. The available data in the literature appearing on this issue is quite limited, the available study is the semi-theoretical approximate model to predict pressure drop of developing slip flow in rectangular microchannels with different aspect ratios. In this paper, we apply the lattice Boltzmann equation method (LBE) to investigate the developing slip flow through a rectangular microchannel. The effects of the Reynolds number (1 < Re < 1000), channel aspect ratio (0 < ε < 1), and Knudsen number (0.001 < Kn < 0.1) on the dimensionless hydrodynamic entrance length, and the apparent friction factor, and Reynolds number product, are examined in detail. The numerical solution of LBM can recover excellent agreement with the available data in the literature, which proves its accuracy in capturing fundamental fluid characteristics in the slip-flow regime.}, } @article {pmid30393335, year = {2018}, author = {Afzal, MJ and Ashraf, MW and Tayyaba, S and Hossain, MK and Afzulpurkar, N}, title = {Sinusoidal Microchannel with Descending Curves for Varicose Veins Implantation.}, journal = {Micromachines}, volume = {9}, number = {2}, pages = {}, doi = {10.3390/mi9020059}, pmid = {30393335}, issn = {2072-666X}, abstract = {Approximately 26% of adult people, mostly females, are affected by varicose veins in old age. It is a common reason for distress, loss of efficiency, and worsening living conditions. Several traditional treatment techniques (sclerotherapy and foam sclerotherapy of large veins, laser surgeries and radiofrequency ablation, vein ligation and stripping, ambulatory phlebectomy, and endoscopic vein surgery) have failed to handle this disease effectively. Herein, authors have presented an alternative varicose vein implant method-the descending sinusoidal microchannel (DSMC). DSMC was simulated by Fuzzy logic MATLAB (The MathWorks, Natick, MA, USA) and ANSYS (ANSYS 18.2, perpetual license purchased by Ibadat Education Trust, The University of Lahore, Pakistan) with real and actual conditions. After simulations of DSMC, fabrication and testing were performed. The silver DSMC was manufactured by utilizing a micromachining procedure. The length, width, and depth of the silver substrate were 51 mm, 25 mm, and 1.1 mm, respectively. The measurements of the DSMC channel in the silver wafer substrate were 0.9 mm in width and 0.9 mm in depth. The three descending curves of the DSMC were 7 mm, 6 mm, and 5 mm in height. For pressure, actual conditions were carefully taken as 1.0 kPa to 1.5 kPa for varicose veins. For velocity, actual conditions were carefully taken as 0.02 m/s to 0.07 m/s for these veins. These are real and standard values used in simulations and experiments. At Reynolds number 323, the flow rate and velocity were determined as 1001.0 (0.1 nL/s), 11.4 cm/s and 1015.3 (0.1 nL/s), 12.19 cm/s by MATLAB (The MathWorks, Natick, MA, USA) and ANSYS simulations, respectively. The flow rate and velocity were determined to be 995.3 (0.1 nL/s) and 12.2 cm/s, respectively, at the same Reynolds number (323) in the experiment. Moreover, the Dean number was also calculated to observe Dean vortices. All simulated and experimental results were in close agreement. Consequently, DSMC can be implanted in varicose veins as a new treatment to preserve excellent blood flow in human legs from the original place to avoid tissue damage and other problems.}, } @article {pmid30389406, year = {2018}, author = {Amiri Delouei, A and Sajjadi, H and Mohebbi, R and Izadi, M}, title = {Experimental study on inlet turbulent flow under ultrasonic vibration: Pressure drop and heat transfer enhancement.}, journal = {Ultrasonics sonochemistry}, volume = {}, number = {}, pages = {}, doi = {10.1016/j.ultsonch.2018.10.032}, pmid = {30389406}, issn = {1873-2828}, abstract = {This experimental study examines the impact of ultrasonic vibration on pressure drop and heat transfer enhancement of inlet turbulent flows. A stainless steel tube connected to an ultrasonic transducer and immersed in a constant temperature two-phase fluid was considered as the test section. Regarding the designed configuration, the ultrasonic transducer utilized had an acoustic frequency of 28 kHz and two different power levels of 75 W and 100 W. The experiments were conducted for different ultrasonic power levels, inlet temperatures, and flow rates. The accuracy of measurements was successfully validated via the existing empirical correlations. The results indicate that the effect of ultrasonic vibration on pressure drop and heat transfer enhancement diminishes with the growth of both Reynolds number and inlet temperature. Based on previously reported results on inlet flows with a laminar flow regime, the effect of ultrasonic vibration is very trivial in current turbulent inlet flows (up to 7.28% for heat convection enhancement). The results of the present study will be beneficial for future investigations on designing vibrating heat exchangers.}, } @article {pmid30387646, year = {2018}, author = {Falkovich, G and Vladimirova, N}, title = {Turbulence Appearance and Nonappearance in Thin Fluid Layers.}, journal = {Physical review letters}, volume = {121}, number = {16}, pages = {164501}, doi = {10.1103/PhysRevLett.121.164501}, pmid = {30387646}, issn = {1079-7114}, abstract = {Flows in fluid layers are ubiquitous in industry, geophysics, and astrophysics. Large-scale flows in thin layers can be considered two dimensional with bottom friction added. Here we find that the properties of such flows depend dramatically on the way they are driven. We argue that a wall-driven (Couette) flow cannot sustain turbulence, no matter how small the viscosity and friction. Direct numerical simulations (DNSs) up to the Reynolds number Re=10^{6} confirm that all perturbations die in a plane Couette flow. On the contrary, for sufficiently small viscosity and friction, perturbations destroy the pressure-driven laminar (Poiseuille) flow. What appears instead is a traveling wave in the form of a jet slithering between wall vortices. For 5×10^{3}

METHODS: For steady flow the analytical approach has been taken to obtain the exact solution. Regular perturbation expansion method has been used to solve the governing equations for pulsatile flow up to first order of approximation by assuming the pulsatile Reynolds number to be very small (much less than unity).

RESULTS: Flow rate, wall shear stress and velocity profile have been graphically analyzed and compared with constant viscosity model. A noteworthy observation of the present study is that rise in viscosity index leads to decay in velocity, velocity of plug flow region, flow rate while flow resistance increases with rising viscosity index (m). The results for Power-law fluid (PL), Bingham-plastic fluid (BP), Newtonian fluid (NF) are found as special cases from this model. Like the constant viscosity model, it has been also observed that the velocity, flow rate and plug core velocity of two-fluid model are higher than the single-fluid model for variable viscosity.

CONCLUSIONS: The two-phase fluid model is more significant than the single-fluid model. Effect of viscosity parameter on various hemodynamical quantities has been obtained. It is also concluded that a rising viscosity parameter (varying nature of viscosity) significantly distinguishes the single and two-fluid models in terms of changes in blood flow resistance. The outcome of present study may leave a significant impact on analyzing blood flow through small blood vessels with constriction, where correct measurement of flow rate and flow resistance for medical treatment is very important.}, } @article {pmid30297857, year = {2018}, author = {Karathanassis, IK and Trickett, K and Koukouvinis, P and Wang, J and Barbour, R and Gavaises, M}, title = {Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {14968}, doi = {10.1038/s41598-018-32996-w}, pmid = {30297857}, issn = {2045-2322}, abstract = {The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0-7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice.}, } @article {pmid30280982, year = {2018}, author = {de Matos, DB and Barbosa, MPR and Leite, OM and Steter, JR and Lima, NS and Torres, NH and Marques, MN and de Alsina, OLS and Cavalcanti, EB}, title = {Characterization of a tubular electrochemical reactor for the degradation of the commercial diuron herbicide.}, journal = {Environmental technology}, volume = {}, number = {}, pages = {1-48}, doi = {10.1080/09593330.2018.1531941}, pmid = {30280982}, issn = {0959-3330}, abstract = {After designing and constructing an electrochemical reactor with concentric electrodes and tangential feed (RECT), it is necessary to characterize it and to study its performance. The experimental study of the residence time distribution (RTD) was conducted for flow rates of 2.78 × 10-6 m3 s-1, 8.33 × 10-6 m3 s-1 and 13.9 × 10-6 m3 s-1. According to the values obtained from the Pe number (0.67 to 1.52), the RECT fits as tubular with great dispersion. The determined empirical correlation (Sh = 18.16 Re0.50 Sc0.33) showed a laminar flow behavior in the range of Reynolds number (Re) between 23 and 117. In order to use RECT in effluent treatment, an electrochemical oxidation study of the Diuron model molecule (Nortox®) was performed to analyze reactor performance in a closed system with total reflux. A decay kinetics of pseudo-first order was associated with the decay of the concentration of diuron and 30% mineralization in 180 min of process were obtained, having a total volume of 4 × 10-3 m3 and an initial concentration of commercial Diuron in 215.83 mg dm-3. Eleven by-products were identified by HPLC-MS analysis and, from this, it was possible to propose a route of degradation of the diuron. From these observations, it can be inferred that the studied electrochemical reactor had applicability in the degradation of recalcitrant compounds, as is the case of commercial diuron. Make some changes in the electrochemical reactor studied and other advanced oxidative processes, such as electro-Fenton, can be associated with the studied system to achieve a better conversion efficiency.}, } @article {pmid30271109, year = {2016}, author = {Poroseva, SV and Colmenares F, JD and Murman, SM}, title = {On the accuracy of RANS simulations with DNS data.}, journal = {Physics of fluids (Woodbury, N.Y. : 1994)}, volume = {28}, number = {11}, pages = {}, doi = {10.1063/1.4966639}, pmid = {30271109}, issn = {1070-6631}, abstract = {Simulation results conducted for incompressible planar wall-bounded turbulent flows with the Reynolds-Averaged Navier-Stokes (RANS) equations with no modeling involved are presented. Instead, all terms but the molecular diffusion are represented by the data from direct numerical simulation (DNS). In simulations, the transport equations for velocity moments through the second order (and the fourth order where the data are available) are solved in a zero-pressure gradient boundary layer over a flat plate and in a fully-developed channel flow in a wide range of Reynolds numbers using DNS data from Sillero et al. (2013), Lee & Moser (2015), and Jeyapaul et al. (2015). The results obtained demonstrate that DNS data are the significant and dominant source of uncertainty in such simulations (hereafter, RANS-DNS simulations). Effects of the Reynolds number, flow geometry, and the velocity moment order as well as an uncertainty quantification technique used to collect the DNS data on the results of RANS-DNS simulations are analyzed. New criteria for uncertainty quantification in statistical data collected from DNS are proposed to guarantee the data accuracy sufficient for their use in RANS equations and for the turbulence model validation.}, } @article {pmid30245535, year = {2014}, author = {Poroseva, SV}, title = {The Effect of a Pressure-Containing Correlation Model on Near-Wall Flow Simulations with RST Models.}, journal = {Journal of fluids engineering}, volume = {136}, number = {6}, pages = {}, doi = {10.1115/1.4025936}, pmid = {30245535}, issn = {0098-2202}, abstract = {It is accustomed to think that turbulence models based on solving the Reynolds-Averaged Navier-Stokes equations require empirical functions to accurately reproduce the behavior of flow characteristics of interest, particularly near a wall. The current paper analyzes how choosing a model for pressure-strain correlations in second-order closures affects the need for introducing empirical functions in model equations to reproduce the flow behavior near a wall correctly. An axially-rotating pipe flow is used as a test flow for the analysis. Results of simulations demonstrate that by using more physics-based models to represent pressure-strain correlations, one can eliminate wall functions associated with such models. The higher the Reynolds number or the strength of imposed rotation on a flow, the less need there is for empirical functions regardless of the choice of a pressure-strain correlation model.}, } @article {pmid30242545, year = {2018}, author = {Waldrop, LD and He, Y and Khatri, S}, title = {What Can Computational Modeling Tell Us about the Diversity of Odor-Capture Structures in the Pancrustacea?.}, journal = {Journal of chemical ecology}, volume = {}, number = {}, pages = {}, doi = {10.1007/s10886-018-1017-2}, pmid = {30242545}, issn = {1573-1561}, support = {1505061//Division of Physics/ ; TG-CDA160015//Extreme Science and Engineering Discovery Environment/ ; TG-BIO170090//Extreme Science and Engineering Discovery Environment/ ; }, abstract = {A major transition in the history of the Pancrustacea was the invasion of several lineages of these animals onto land. We investigated the functional performance of odor-capture organs, antennae with olfactory sensilla arrays, through the use of a computational model of advection and diffusion of odorants to olfactory sensilla while varying three parameters thought to be important to odor capture (Reynolds number, gap-width-to-sensillum-diameter ratio, and angle of the sensilla array with respect to oncoming flow). We also performed a sensitivity analysis on these parameters using uncertainty quantification to analyze their relative contributions to odor-capture performance. The results of this analysis indicate that odor capture in water and in air are fundamentally different. Odor capture in water and leakiness of the array are highly sensitive to Reynolds number and moderately sensitive to angle, whereas odor capture in air is highly sensitive to gap widths between sensilla and moderately sensitive to angle. Leakiness is not a good predictor of odor capture in air, likely due to the relative importance of diffusion to odor transport in air compared to water. We also used the sensitivity analysis to make predictions about morphological and kinematic diversity in extant groups of aquatic and terrestrial crustaceans. Aquatic crustaceans will likely exhibit denser arrays and induce flow within the arrays, whereas terrestrial crustaceans will rely on more sparse arrays with wider gaps and little-to-no animal-induced currents.}, } @article {pmid30213252, year = {2016}, author = {Tripathi, D and Akbar, NS and Khan, ZH and Bég, OA}, title = {Peristaltic transport of bi-viscosity fluids through a curved tube: A mathematical model for intestinal flow.}, journal = {Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine}, volume = {230}, number = {9}, pages = {817-828}, doi = {10.1177/0954411916658318}, pmid = {30213252}, issn = {2041-3033}, abstract = {The human intestinal tract is a long, curved tube constituting the final section of the digestive system in which nutrients and water are mostly absorbed. Motivated by the dynamics of chyme in the intestine, a mathematical model is developed to simulate the associated transport phenomena via peristaltic transport. Rheology of chyme is modelled using the Nakamura-Sawada bi-viscosity non-Newtonian formulation. The intestinal tract is considered as a curved tube geometric model. Low Reynolds number (creeping hydrodynamics) and long wavelength approximations are taken into consideration. Analytical solutions of the moving boundary value problem are derived for velocity field, pressure gradient and pressure rise. Streamline flow visualization is achieved with Mathematica symbolic software. Peristaltic pumping phenomenon and trapping of the bolus are also examined. The influence of curvature parameter, apparent viscosity coefficient (rheological parameter) and volumetric flow rate on flow characteristics is described. Validation of analytical solutions is achieved with a MAPLE17 numerical quadrature algorithm. The work is relevant to improving understanding of gastric hydrodynamics and provides a benchmark for further computational fluid dynamic simulations.}, } @article {pmid30212772, year = {2018}, author = {Fu, Q and Chen, H and Liao, Q and Huang, Y and Xia, A and Zhu, X and Xiao, C and Reungsang, A and Liu, Z}, title = {Drag reduction and shear-induced cells migration behavior of microalgae slurry in tube flow.}, journal = {Bioresource technology}, volume = {270}, number = {}, pages = {38-45}, doi = {10.1016/j.biortech.2018.08.133}, pmid = {30212772}, issn = {1873-2976}, abstract = {To optimize the designing of microalgae slurry pumping system and enhance the efficiency of microalgae products production, the flow characteristics of microalgae slurries (Chlorella pyrenoidosa) in tube flow were for the first time investigated combining experiments and numerical simulation. The drag reduction behavior of microalgae slurry in the fully developed laminar flow regime was studied. In addition, the transition Reynolds number of microalgae slurries from laminar flow to turbulent flow was about 1000-1300, which was similar to the expression of two-phase flow. To provide a further understanding of flow feature of microalgae slurries in tube, a two-phase mixture model was proposed by considering the heterogeneity of concentration due to the shear-induced microalgae cells migration behavior. Simulation results revealed that the heterogeneous distribution of concentration was affected by average velocity and volume fraction of microalgae slurries, significantly affecting the flow resistance and flow stability of microalgae slurry in the tube flow.}, } @article {pmid30211982, year = {2018}, author = {Bergersen, AW and Mortensen, M and Valen-Sendstad, K}, title = {The FDA Nozzle Benchmark: In Theory There Is No Difference Between Theory and Practice, But in Practice There Is.}, journal = {International journal for numerical methods in biomedical engineering}, volume = {}, number = {}, pages = {}, doi = {10.1002/cnm.3150}, pmid = {30211982}, issn = {2040-7947}, abstract = {The utility of flow simulations relies on the robustness of computational fluid dynamics (CFD) solvers and reproducibility of results. The aim of this study was to validate the Oasis CFD solver against in-vitro experimental measurements of jet breakdown location from the FDA nozzle benchmark at Reynolds number 3500, which is in the particularly-challenging transitional regime. Simulations were performed on meshes consisting of 5, 10, 17 and 28 million (M) tetrahedra, with Δt = 10-5 seconds. The 5 and 10M simulation jets broke down in reasonable agreement with the experiments. However, the 17 and 28M simulation jets broke down further downstream. But which of our simulations are 'correct'? From a theoretical point of view, they are all wrong because the jet should not break down in the absence of disturbances. The geometry is axisymmetric with no geometrical features that can generate angular velocities. A stable flow was supported by linear stability analysis. From a physical point of view, a finite amount of 'noise' will always be present in experiments, which lowers transition point. To replicate noise numerically, we prescribed minor random angular velocities (∼0.31%), much smaller than the reported flow asymmetry (∼3%) and model accuracy (∼1%), at the inlet of the 17M simulation, which shifted the jet breakdown location closer to the measurements. Hence, the high-resolution simulations and 'noise' experiment can potentially explain discrepancies in transition between sometimes 'sterile' CFD and inherently noisy 'ground truth' experiments. Thus, we have shown that numerical simulations can agree with experiments, but for the wrong reasons.}, } @article {pmid30200611, year = {2018}, author = {Wan, G and Jin, C and Trase, I and Zhao, S and Chen, Z}, title = {Helical Structures Mimicking Chiral Seedpod Opening and Tendril Coiling.}, journal = {Sensors (Basel, Switzerland)}, volume = {18}, number = {9}, pages = {}, doi = {10.3390/s18092973}, pmid = {30200611}, issn = {1424-8220}, support = {Dartmouth College//Startup fund from Thayer School of Engineering at Dartmouth College/ ; Branco Weiss-Society in Science fellowship//Branco Weiss-Society in Science fellowship (administered by ETH Zürich)/ ; }, abstract = {Helical structures are ubiquitous in natural and engineered systems across multiple length scales. Examples include DNA molecules, plants' tendrils, sea snails' shells, and spiral nanoribbons. Although this symmetry-breaking shape has shown excellent performance in elastic springs or propulsion generation in a low-Reynolds-number environment, a general principle to produce a helical structure with programmable geometry regardless of length scales is still in demand. In recent years, inspired by the chiral opening of Bauhinia variegata's seedpod and the coiling of plant's tendril, researchers have made significant breakthroughs in synthesizing state-of-the-art 3D helical structures through creating intrinsic curvatures in 2D rod-like or ribbon-like precursors. The intrinsic curvature results from the differential response to a variety of external stimuli of functional materials, such as hydrogels, liquid crystal elastomers, and shape memory polymers. In this review, we give a brief overview of the shape transformation mechanisms of these two plant's structures and then review recent progress in the fabrication of biomimetic helical structures that are categorized by the stimuli-responsive materials involved. By providing this survey on important recent advances along with our perspectives, we hope to solicit new inspirations and insights on the development and fabrication of helical structures, as well as the future development of interdisciplinary research at the interface of physics, engineering, and biology.}, } @article {pmid30194679, year = {2018}, author = {Daddi-Moussa-Ider, A and Löwen, H and Gekle, S}, title = {Creeping motion of a solid particle inside a spherical elastic cavity⋆.}, journal = {The European physical journal. E, Soft matter}, volume = {41}, number = {9}, pages = {104}, doi = {10.1140/epje/i2018-11715-7}, pmid = {30194679}, issn = {1292-895X}, abstract = {On the basis of the linear hydrodynamic equations, we present an analytical theory for the low-Reynolds-number motion of a solid particle moving inside a larger spherical elastic cavity which can be seen as a model system for a fluid vesicle. In the particular situation where the particle is concentric with the cavity, we use the stream function technique to find exact analytical solutions of the fluid motion equations on both sides of the elastic cavity. In this particular situation, we find that the solution of the hydrodynamic equations is solely determined by membrane shear properties and that bending does not play a role. For an arbitrary position of the solid particle within the spherical cavity, we employ the image solution technique to compute the axisymmetric flow field induced by a point force (Stokeslet). We then obtain analytical expressions of the leading-order mobility function describing the fluid-mediated hydrodynamic interactions between the particle and the confining elastic cavity. In the quasi-steady limit of vanishing frequency, we find that the particle self-mobility function is higher than that predicted inside a rigid no-slip cavity. Considering the cavity motion, we find that the pair-mobility function is determined only by membrane shear properties. Our analytical predictions are supplemented and validated by fully resolved boundary integral simulations where a very good agreement is obtained over the whole range of applied forcing frequencies.}, } @article {pmid30174548, year = {2017}, author = {Oosterhuis, JP and Verbeek, AA and Bühler, S and Wilcox, D and van der Meer, TH}, title = {Flow Separation and Turbulence in Jet Pumps for Thermoacoustic Applications.}, journal = {Flow, turbulence and combustion}, volume = {98}, number = {1}, pages = {311-326}, doi = {10.1007/s10494-016-9731-8}, pmid = {30174548}, issn = {1573-1987}, abstract = {The effect of flow separation and turbulence on the performance of a jet pump in oscillatory flows is investigated. A jet pump is a static device whose shape induces asymmetric hydrodynamic end effects when placed in an oscillatory flow. This will result in a time-averaged pressure drop which can be used to suppress acoustic streaming in closed-loop thermoacoustic devices. An experimental setup is used to measure the time-averaged pressure drop as well as the acoustic power dissipation across two different jet pump geometries in a pure oscillatory flow. The results are compared against published numerical results where flow separation was found to have a negative effect on the jet pump performance in a laminar flow. Using hot-wire anemometry the onset of flow separation is determined experimentally and the applicability of a critical Reynolds number for oscillatory pipe flows is confirmed for jet pump applications. It is found that turbulence can lead to a reduction of flow separation and hence, to an improvement in jet pump performance compared to laminar oscillatory flows.}, } @article {pmid30139156, year = {1993}, author = {Andersen, MC}, title = {DIASPORE MORPHOLOGY AND SEED DISPERSAL IN SEVERAL WIND-DISPERSED ASTERACEAE.}, journal = {American journal of botany}, volume = {80}, number = {5}, pages = {487-492}, doi = {10.1002/j.1537-2197.1993.tb13830.x}, pmid = {30139156}, issn = {1537-2197}, abstract = {I made measurements of morphology and settling velocity on seeds of 19 species of wind-dispersed Asteraceae. From the morphological measurements I calculated Reynolds numbers and approximate plume loadings for the species. Diaspore settling velocity increases linearly with the square root of plume loading. This relationship varies among species and among subfamilies, but not among life history types. Reynolds number is highly variable among subfamilies, less so within subfamilies. Diaspores with beaked achenes have significantly lower settling velocities than diaspores with unbeaked achenes, even though beaked and unbeaked achenes do not differ in plume loading or in Reynolds number. Reynolds numbers of all diaspores examined are well above the range in which Stokes' Law applies. I recommend that the use of formulae based on Stokes' Law be curtailed in studies of the relationship between plume loading and settling velocity. The results suggest that many seed characters may have evolved due to selection on dispersal ability. This is in spite of phyletic constraints on morphology reflected in the relative uniformity of Reynolds numbers within subfamilies.}, } @article {pmid30140921, year = {2018}, author = {Molony, D and Park, J and Zhou, L and Fleischer, C and Sun, HY and Hu, X and Oshinski, J and Samady, H and Giddens, DP and Rezvan, A}, title = {Bulk Flow and Near Wall Hemodynamics of the Rabbit Aortic Arch: A 4D PC-MRI Derived CFD Study.}, journal = {Journal of biomechanical engineering}, volume = {}, number = {}, pages = {}, doi = {10.1115/1.4041222}, pmid = {30140921}, issn = {1528-8951}, abstract = {Animal models offer a flexible experimental environment for studying atherosclerosis. The mouse is the most commonly used animal, however, the underlying hemodynamics in larger animals such as the rabbit are far closer to that of humans. The aortic arch is a vessel with complex helical flow and highly heterogeneous shear stress patterns which may influence where atherosclerotic lesions form. A better understanding of intra-species flow variation and the impact of geometry on flow may improve our understanding of where disease forms. In this work we use Magnetic Resonance Angiography (MRA) and 4D Phase contrast magnetic resonance imaging (PC-MRI) to image and measure blood velocity in the rabbit aortic arch. Measured flow rates from the PC-MRI were used as boundary conditions in computational fluid dynamics models of the arches. Helical flow, cross flow index (CFI) and time-averaged wall shear stress (TAWSS) were determined from the simulated flow field. Both traditional geometric metrics and shape modes derived from statistical shape analysis were analyzed with respect to flow helicity. High CFI and low TAWSS were found to co-localize in the ascending aorta and to a lesser extent on the inner curvature of the aortic arch. The Reynolds number was linearly associated with an increase in helical flow intensity (R=0.85, p<.05). Both traditional and statistical shape analysis correlated with increased helical flow symmetry. However, a stronger correlation was obtained from the statistical shape analysis demonstrating its potential for discerning the role of shape in hemodynamic studies.}, } @article {pmid30136131, year = {2018}, author = {Krastev, VK and Amati, G and Succi, S and Falcucci, G}, title = {On the effects of surface corrugation on the hydrodynamic performance of cylindrical rigid structures.}, journal = {The European physical journal. E, Soft matter}, volume = {41}, number = {8}, pages = {95}, doi = {10.1140/epje/i2018-11703-y}, pmid = {30136131}, issn = {1292-895X}, abstract = {In this work, we perform fully three-dimensional numerical simulations of the flow field surrounding cylindrical structures characterized by different types of corrugated surface. The simulations are carried out using the Lattice Boltzmann Method (LBM), considering a flow regime with a Reynolds number [Formula: see text]. The fluid-dynamic wake structure and stability are investigated by means of PSD analyses of the velocity components and by visual inspection of the vortical coherent structure evolution. Moreover, the energy dissipation of the flow is assessed by considering an equivalent discharge coefficient [Formula: see text], which measures the total pressure losses of the flow moving around the various layout under investigation. Outcomes from our study demonstrate that the helical ridges augment energy dissipation, but might also have a role in the passive control of the characteristic frequencies of the unsteady wake flow.}, } @article {pmid30132443, year = {2018}, author = {Lee, YJ and Lua, KB}, title = {Wing-wake interaction: comparison of two- and three-dimensional flapping wings in hover.}, journal = {Bioinspiration & biomimetics}, volume = {}, number = {}, pages = {}, doi = {10.1088/1748-3190/aadc31}, pmid = {30132443}, issn = {1748-3190}, abstract = {The wing-wake interaction of flapping wings in hover has been investigated, with a focus on the difference in wing-wake interaction between two-dimensional (2D) and three-dimensional (3D) flapping wings. Numerical simulations are conducted at Reynolds number of 100, and the flapping configurations are divided into the 2D, quasi-3D, and 3D categories. Variations of aspect ratio and Rossby number allow the flapping configuration to morph gradually between categories. The wing-wake interaction mechanisms are identified and the effect of three-dimensionality on these mechanisms is discussed. Three-dimensionality affects wing-wake interaction through four primary aerodynamic mechanisms, namely, induced jet, downwash/upwash, leading-edge vortex (LEV) shedding due to vortex pairing, and the formation of a closely attached LEV. The first two mechanisms are well-established in literature. On the LEV shedding mechanism, it is revealed that the interaction between the LEV and the residue vortex from the previous stroke plays an important role in the early vortex shedding of 2D flapping wings. This effect diminishes with increasing three-dimensionality. On the mechanism of the closely attached LEV, the wake encourages the formation of a LEV that is closely attached to the wing's top surface, which is beneficial to lift generation. This closely attached LEV mechanism accounts for most of the lift enhancement that arises from wake effects. Three-dimensionality alters the efficacy of the different aerodynamic mechanisms. Consequently, the dual peak lift coefficient pattern typically seen on 2D flapping wings transforms into the single peak lift coefficient pattern of the 3D flapping wing. It is also demonstrated that mean lift enhancement due to wing-wake interaction diminishes rapidly when three-dimensionality is introduced. Results suggest that, for wings with parameters close to those of natural flyers, wing-wake interaction yields marginal lift enhancement and a small increase in energy consumption.}, } @article {pmid30132198, year = {2018}, author = {Espeso, DR and Martínez-García, E and Carpio, A and de Lorenzo, V}, title = {Dynamics of Pseudomonas putida biofilms in an upscale experimental framework.}, journal = {Journal of industrial microbiology & biotechnology}, volume = {}, number = {}, pages = {}, doi = {10.1007/s10295-018-2070-0}, pmid = {30132198}, issn = {1476-5535}, support = {ERC-2012-ADG-322797//European Research Council/International ; EU-H2020-BIOTEC-2014-2015-6335536//Horizon 2020 Framework Programme/ ; H2020-FET-OPEN-RIA-2017-1-766975//Horizon 2020 Framework Programme/ ; }, abstract = {Exploitation of biofilms for industrial processes requires them to adopt suitable physical structures for rendering them efficient and predictable. While hydrodynamics could be used to control material features of biofilms of the platform strain Pseudomonas putida KT2440 there is a dearth of experimental data on surface-associated growth behavior in such settings. Millimeter scale biofilm patterns formed by its parental strain P. putida mt-2 under different Reynolds numbers (Re) within laminar regime were analyzed using an upscale experimental continuous cultivation assembly. A tile-scan image acquisition process combined with a customized image analysis revealed patterns of dense heterogeneous structures at Re = 1000, but mostly flattened coverings sparsely patched for Re < 400. These results not only fix the somewhat narrow hydrodynamic regime under which P. putida cells form stable coatings on surfaces destined for large-scale processes, but also provide useful sets of parameters for engineering catalytic biofilms based on this important bacterium as a cell factory.}, } @article {pmid30123895, year = {2018}, author = {Lee, J and Estlack, Z and Somaweera, H and Wang, X and Lacerda, CMR and Kim, J}, title = {A microfluidic cardiac flow profile generator for studying the effect of shear stress on valvular endothelial cells.}, journal = {Lab on a chip}, volume = {}, number = {}, pages = {}, doi = {10.1039/c8lc00545a}, pmid = {30123895}, issn = {1473-0189}, abstract = {To precisely investigate the mechanobiological responses of valvular endothelial cells, we developed a microfluidic flow profile generator using a pneumatically-actuated micropump consisting of microvalves of various sizes. By controlling the closing pressures and the actuation times of these microvalves, we modulated the magnitude and frequency of the shear stress to mimic mitral and aortic inflow profiles with frequencies in the range of 0.8-2 Hz and shear stresses up to 20 dyn cm-2. To demonstrate this flow profile generator, aortic inflow with an average of 5.9 dyn cm-2 shear stress at a frequency of 1.2 Hz with a Reynolds number of 2.75, a Womersley number of 0.27, and an oscillatory shear index (OSI) value of 0.2 was applied to porcine aortic valvular endothelial cells (PAVECs) for mechanobiological studies. The cell alignment, cell elongation, and alpha-smooth muscle actin (αSMA) expression of PAVECs under perfusion, steady flow, and aortic inflow conditions were analyzed to determine their shear-induced cell migration and trans-differentiation. In this morphological and immunocytochemical study, we found that the PAVECs elongated and aligned themselves perpendicular to the directions of the steady flow and the aortic inflow. In contrast, under perfusion with a fluidic shear stress of 0.47 dyn cm-2, the PAVECs elongated and aligned themselves parallel to the direction of flow. The PAVECs exposed to the aortic inflow upregulated their αSMA-protein expression to a greater degree than those exposed to perfusion and steady flow. By comparing these results to those of previous studies of pulsatile flow, we also found that the ratio of positive to negative shear stress plays an important role in determining PAVECs' trans-differentiation and adaptation to flow. This microfluidic cardiac flow profile generator will enable future valvular mechanobiological studies to determine the roles of magnitude and frequency of shear stresses.}, } @article {pmid30119494, year = {2018}, author = {Gao, J and Katz, J}, title = {Self-calibrated microscopic dual-view tomographic holography for 3D flow measurements.}, journal = {Optics express}, volume = {26}, number = {13}, pages = {16708-16725}, pmid = {30119494}, issn = {1094-4087}, abstract = {This paper introduces the application of microscopic dual-view tomographic holography (M-DTH) to measure the 3D position and motion of micro-particles located in dense suspensions. Pairing of elongated traces of the same particle in the two inclined reconstructed fields requires precise matching of the entire sample volume that accounts for the inherent distortions in each view. It is achieved by an iterative volumetric self-calibration method, consisting of mapping one view onto the next, dividing the sample volume into slabs, and cross-correlating the two views. Testing of the procedures using synthetic particle fields with imposed distortion and realistic errors in particle locations shows that the self-calibration method achieves a 3D uncertainty of about 1µm, a third of the particle diameter. Multiplying the corrected intensity fields is used for truncating the elongated traces, whose centers are located within 1µm of the exact value. Without correction, only a small fraction of the traces even overlap. The distortion correction also increases the number of intersecting traces in experimental data along with their intensity. Application of this method for 3D velocity measurements is based on the centroids of the truncated/shortened particle traces. Matching of these traces in successive fields is guided by several criteria, including results of volumetric cross-correlation of the multiplied intensity fields. The resulting 3D velocity distribution is substantially more divergence-free, i.e., satisfies conservation of mass, compared to analysis performed using single-view data. Sample application of the new method shows the 3D flow structure around a pair of cubic roughness elements embedded in the inner part of a high Reynolds number turbulent boundary layer.}, } @article {pmid30118276, year = {2018}, author = {Mathai, V and Huisman, SG and Sun, C and Lohse, D and Bourgoin, M}, title = {Dispersion of Air Bubbles in Isotropic Turbulence.}, journal = {Physical review letters}, volume = {121}, number = {5}, pages = {054501}, doi = {10.1103/PhysRevLett.121.054501}, pmid = {30118276}, issn = {1079-7114}, abstract = {Bubbles play an important role in the transport of chemicals and nutrients in many natural and industrial flows. Their dispersion is crucial to understanding the mixing processes in these flows. Here we report on the dispersion of millimetric air bubbles in a homogeneous and isotropic turbulent flow with a Taylor Reynolds number from 110 to 310. We find that the mean squared displacement (MSD) of the bubbles far exceeds that of fluid tracers in turbulence. The MSD shows two regimes. At short times, it grows ballistically (∝τ^{2} ), while at larger times, it approaches the diffusive regime where the MSD∝τ. Strikingly, for the bubbles, the ballistic-to-diffusive transition occurs one decade earlier than for the fluid. We reveal that both the enhanced dispersion and the early transition to the diffusive regime can be traced back to the unsteady wake-induced motion of the bubbles. Further, the diffusion transition for bubbles is not set by the integral timescale of the turbulence (as it is for fluid tracers and microbubbles), but instead, by a timescale of eddy crossing of the rising bubbles. The present findings provide a Lagrangian perspective towards understanding mixing in turbulent bubbly flows.}, } @article {pmid30118271, year = {2018}, author = {Oettinger, D and Ault, JT and Stone, HA and Haller, G}, title = {Invisible Anchors Trap Particles in Branching Junctions.}, journal = {Physical review letters}, volume = {121}, number = {5}, pages = {054502}, doi = {10.1103/PhysRevLett.121.054502}, pmid = {30118271}, issn = {1079-7114}, abstract = {We combine numerical simulations and an analytic approach to show that the capture of finite, inertial particles during flow in branching junctions is due to invisible, anchor-shaped three-dimensional flow structures. These Reynolds-number-dependent anchors define trapping regions that confine particles to the junction. For a wide range of Stokes numbers, these structures occupy a large part of the flow domain. For flow in a V-shaped junction, at a critical Stokes number, we observe a topological transition due to the merger of two anchors into one. From a stability analysis, we identify the parameter region of particle sizes and densities where capture due to anchors occurs.}, } @article {pmid30117966, year = {2018}, author = {Karaminejad, S and Askari, MH and Ashjaee, M}, title = {Temperature field investigation of hydrogen/air and syngas/air axisymmetric laminar flames using Mach-Zehnder interferometry.}, journal = {Applied optics}, volume = {57}, number = {18}, pages = {5057-5067}, pmid = {30117966}, issn = {1539-4522}, abstract = {In this study, the optical method of Mach-Zehnder interferometry (MZI) is utilized in order to explore the flame structure and temperature field of syngas/air and hydrogen/air flames. Two axisymmetric burners with inner diameters of 4 mm and 6 mm are used for temperature field measurement of hydrogen and syngas, respectively. The effects of fuel composition, equivalence ratio, and Reynolds number (Re) are investigated at ambient condition (P=0.87 bar, T=300 K). Three different H2/CO fuel compositions with hydrogen fractions of 30%, 50%, and 100% are studied. Temperature profiles are reported at four different sections above the burner tip. Measured temperatures using the interferometry method are compared with thermocouple data and good agreement between them is observed. The results obtained in this investigation indicated that the MZI can be applied for accurate determination of flame front and temperature field, especially for high-temperature flames where other methods cannot be properly utilized. Analyses of the data reduction method revealed that the exact determination of the refractive index distribution and reference temperature is critical for accurate determination of the temperature field. The results indicated that by increasing the Re, the maximum flame temperature is enhanced. Increasing the equivalence ratio leads to expansion of the flame radial distribution (at the same distance from the burner tip). At higher distances from the burner tip, temperature increases uniformly from the flame boundary toward the flame axis, while at lower heights it shows reduction at the burner axis. By increasing the CO content of fuel, the maximum flame temperature increases at all equivalence ratios except at the stoichiometric condition, where SH100 illustrates the highest maximum flame temperature.}, } @article {pmid30109056, year = {2018}, author = {Bhat, SS and Zhao, J and Sheridan, J and Hourigan, K and Thompson, MC}, title = {The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size.}, journal = {Royal Society open science}, volume = {5}, number = {7}, pages = {172197}, doi = {10.1098/rsos.172197}, pmid = {30109056}, issn = {2054-5703}, abstract = {Stable attachment of a leading-edge vortex (LEV) plays a key role in generating the high lift on rotating wings with a central body. The central body size can affect the LEV structure broadly in two ways. First, an overall change in the size changes the Reynolds number, which is known to have an influence on the LEV structure. Second, it may affect the Coriolis acceleration acting across the wing, depending on the wing-offset from the axis of rotation. To investigate this, the effects of Reynolds number and the wing-offset are independently studied for a rotating wing. The three-dimensional LEV structure is mapped using a scanning particle image velocimetry technique. The rapid acquisition of images and their correlation are carefully validated. The results presented in this paper show that the LEV structure changes mainly with the Reynolds number. The LEV-split is found to be only minimally affected by changing the central body radius in the range of small offsets, which interestingly includes the range for most insects. However, beyond this small offset range, the LEV-split is found to change dramatically.}, } @article {pmid30089029, year = {2018}, author = {Gilmer, GG and Deshpande, V and Chou, CL and Knepper, MA}, title = {Flow Resistance along the Rat Renal Tubule.}, journal = {American journal of physiology. Renal physiology}, volume = {}, number = {}, pages = {}, doi = {10.1152/ajprenal.00219.2018}, pmid = {30089029}, issn = {1522-1466}, abstract = {The Reynolds number in the renal tubule is extremely low, consistent with laminar flow. Consequently, luminal flow can be described by the Hagen-Poiseuille laminar flow equation. This equation calculates the volumetric flow rate from values of the axial pressure gradient and flow resistance, which is dependent on the length and diameter of each renal tubule segment. Our goal was to calculate the pressure drop along each segment of the renal tubule and determine the points of highest resistance. When the Hagen-Poiseuille equation was used for rat superficial nephrons based on known flow rates, tubule lengths, and diameters for each renal tubule segment, it was found that maximum pressure drop occurred in two segments: the thin descending limbs of Henle and the inner medullary collecting ducts. The high resistance in the thin descending limbs is due to their small diameters. The steep pressure drop observed in the inner medullary collecting ducts is due to the convergent structure of the tubules, which channels flow into fewer and fewer tubules toward the papillary tip. For short-looped nephrons, the calculated glomerular capsular pressure matched measured values, even with the high collecting duct flow rates seen in water diuresis, providing that tubule compliance was taken into account. In long-looped nephrons, the greater length of thin limb segments is compensated for by a larger luminal diameter. Simulation of the effect of proximal diuretics, viz. acetazolamide or SGLT2-inhibitors, predicts a substantial back pressure in Bowman's capsule, which may contribute to observed decreases in glomerular filtration rate.}, } @article {pmid30088905, year = {2018}, author = {Mateos-Maroto, A and Guerrero-Martínez, A and Rubio, RG and Ortega, F and Martinez-Pedrero, F}, title = {Magnetic Biohybrid Vesicles Transported by an Internal Propulsion Mechanism.}, journal = {ACS applied materials & interfaces}, volume = {}, number = {}, pages = {}, doi = {10.1021/acsami.8b09862}, pmid = {30088905}, issn = {1944-8252}, abstract = {Some biological microorganisms can crawl or swim due to coordinated motions of their cytoskeleton or the flagella located inside their bodies, which push the cells forward through intracellular forces. To date, there is no demonstration of a biomimetic self-propelled swimmer operating at a low Reynolds number due to internal movements within an enclosing membrane. Here, we report lipid vesicles and other more complex self-assembled biohybrid structures able to propel due to the advection flows generated by the actuated rotation of the superparamagnetic particles they contain. The capsules separate the confined substances from the outside environment, protecting them from the surroundings. The proposed swimming and release strategies, based on near infrared laser pulse-triggered destabilization of the phospholipid membranes, open new possibilities for the on-command transport of minute quantities of drugs, fluids or nano-objects. The lipid membranes protect the confined substances from the outside environment during transportation, thus enabling to work in physiological conditions.}, } @article {pmid30083106, year = {2018}, author = {Vidal, EAG and Zeidberg, LD and Buskey, EJ}, title = {Development of Swimming Abilities in Squid Paralarvae: Behavioral and Ecological Implications for Dispersal.}, journal = {Frontiers in physiology}, volume = {9}, number = {}, pages = {954}, doi = {10.3389/fphys.2018.00954}, pmid = {30083106}, issn = {1664-042X}, abstract = {This study investigates the development of swimming abilities and its relationship with morphology, growth, and nourishment of reared Doryteuthis opalescens paralarvae from hatching to 60 days of age. Paralarvae (2.5-11 mm mantle length - ML) were videotaped, and their behavior quantified throughout development using computerized motion analysis. Hatchlings swim dispersed maintaining large nearest neighbor distances (NND, 8.7 ML), with swimming speeds (SS) of 3-8 mm s-1 and paths with long horizontal displacements, resulting in high net to gross displacement ratios (NGDR). For 15-day-old paralarvae, swimming paths are more consistent between jets, growth of fins, length, and mass increases. The swimming pattern of 18-day-old paralarvae starved for 72 h exhibited a significant reduction in mean SS and inability to perform escape jets. A key morphological, behavioral, and ecological transition occurs at about 6 mm ML (>35-day old), when there is a clear change in body shape, swimming performance, and behavior, paths are more regularly repeated and directional swimming is evident, suggesting that morphological changes incur in swimming performance. These squid are able to perform sustained swimming and hover against a current at significantly closer NND (2.0 ML), as path displacement is reduced and maneuverability increases. As paralarvae reach 6-7 mm ML, they are able to attain speeds up to 562 mm s-1 and to form schools. Social feeding interactions (kleptoparasitism) are often observed prior to the formation of schools. Schools are always formed within areas of high flow gradient in the tanks and are dependent on squid size and current speed. Fin development is a requisite for synchronized and maneuverable swimming of schooling early juveniles. Although average speeds of paralarvae are within intermediate Reynolds numbers (Re < 100), they make the transition to the inertia-dominated realm during escape jets of high propulsion (Re > 3200), transitioning from plankton to nekton after their first month of life. The progressive development of swimming capabilities and social interactions enable juvenile squid to school, while also accelerates learning, orientation and cognition. These observations indicate that modeling of the lifecycle should include competency to exert influence over small currents and dispersal patterns after the first month of life.}, } @article {pmid30072230, year = {2018}, author = {Gritti, F}, title = {High-resolution turbulent flow chromatography.}, journal = {Journal of chromatography. A}, volume = {}, number = {}, pages = {}, doi = {10.1016/j.chroma.2018.07.059}, pmid = {30072230}, issn = {1873-3778}, abstract = {The resolution power of turbulent flow chromatography using carbon dioxide as the mobile phase and coated (crosslinked methyl phenyl polysiloxane) open tube columns (OTCs) as the stationary phase was investigated under retentive conditions (0

MATERIALS AND METHODS: This study presents an experimental observation of a simplified Weibel-based model of the human trachea and bronchi with cartilaginous rings. A transparent model and refractive index-matching methods were used to observe the flow, particularly near the wall. The flow was seeded with tracers to perform particle image velocimetry and particle tracking velocimetry to quantify the effect the rings have on the flow near the trachea and bronchi walls. The experiments were carried out with a flow rate comparable with a resting state (trachea-based Reynolds number of ReD = 2650).

RESULTS: The results present a previously unknown phenomenon in the cavities between the cartilaginous rings: a small recirculation is observed in the upstream side of the cavities throughout the trachea. This recirculation is due to the adverse pressure gradient created by the expansion, which traps particles within the ring cavity, thus affecting the treatment of patients suffering from lung disease and other respiratory conditions.

CONCLUSIONS: The detection of recirculation zones in the cartilage ring cavities sheds light on the particle deposition mechanism and helps explain results from previous studies that have observed an enhancement of particle deposition in models with cartilage rings. These results bring to light the importance of including cartilage rings in experimental, numerical, and theoretical models to better understand particle deposition in the trachea and bronchi. In addition, the results provide scientists and medical staff with new insights for improving drug delivery.}, } @article {pmid29786774, year = {2018}, author = {Bordones, AD and Leroux, M and Kheyfets, VO and Wu, YA and Chen, CY and Finol, EA}, title = {Computational Fluid Dynamics Modeling of the Human Pulmonary Arteries with Experimental Validation.}, journal = {Annals of biomedical engineering}, volume = {}, number = {}, pages = {}, doi = {10.1007/s10439-018-2047-1}, pmid = {29786774}, issn = {1573-9686}, support = {R01 HL121293/HL/NHLBI NIH HHS/United States ; 14GRNT19020017//American Heart Association/ ; R01HL121293//National Institutes of Health/ ; }, abstract = {Pulmonary hypertension (PH) is a chronic progressive disease characterized by elevated pulmonary arterial pressure, caused by an increase in pulmonary arterial impedance. Computational fluid dynamics (CFD) can be used to identify metrics representative of the stage of PH disease. However, experimental validation of CFD models is often not pursued due to the geometric complexity of the model or uncertainties in the reproduction of the required flow conditions. The goal of this work is to validate experimentally a CFD model of a pulmonary artery phantom using a particle image velocimetry (PIV) technique. Rapid prototyping was used for the construction of the patient-specific pulmonary geometry, derived from chest computed tomography angiography images. CFD simulations were performed with the pulmonary model with a Reynolds number matching those of the experiments. Flow rates, the velocity field, and shear stress distributions obtained with the CFD simulations were compared to their counterparts from the PIV flow visualization experiments. Computationally predicted flow rates were within 1% of the experimental measurements for three of the four branches of the CFD model. The mean velocities in four transversal planes of study were within 5.9 to 13.1% of the experimental mean velocities. Shear stresses were qualitatively similar between the two methods with some discrepancies in the regions of high velocity gradients. The fluid flow differences between the CFD model and the PIV phantom are attributed to experimental inaccuracies and the relative compliance of the phantom. This comparative analysis yielded valuable information on the accuracy of CFD predicted hemodynamics in pulmonary circulation models.}, } @article {pmid29776113, year = {2018}, author = {Zhu, B and Ji, Z and Lou, Z and Qian, P}, title = {Torque scaling in small-gap Taylor-Couette flow with smooth or grooved wall.}, journal = {Physical review. E}, volume = {97}, number = {3-1}, pages = {033110}, doi = {10.1103/PhysRevE.97.033110}, pmid = {29776113}, issn = {2470-0053}, abstract = {The torque in the Taylor-Couette flow for radius ratios η≥0.97, with smooth or grooved wall static outer cylinders, is studied experimentally, with the Reynolds number of the inner cylinder reaching up to Re_{i} =2×10^{5}, corresponding to the Taylor number up to Ta=5×10^{10} . The grooves are perpendicular to the mean flow, and similar to the structure of a submersible motor stator. It is found that the dimensionless torque G, at a given Re_{i} and η, is significantly greater for grooved cases than smooth cases. We compare our experimental torques for the smooth cases to the fit proposed by Wendt [F. Wendt, Ing.-Arch. 4, 577 (1993)10.1007/BF02084936] and the fit proposed by Bilgen and Boulos [E. Bilgen and R. Boulos, J Fluids Eng. 95, 122 (1973)10.1115/1.3446944], which shows both fits are outside their range for small gaps. Furthermore, an additional dimensionless torque (angular velocity flux) Nu_{ω} in the smooth cases exhibits an effective scaling of Nu_{ω} ∼Ta^{0.39} in the ultimate regime, which occurs at a lower Taylor number, Ta≈3.5×10^{7}, than the well-explored η=0.714 case (at Ta≈3×10^{8} ). The same effective scaling exponent, 0.39, is also evident in the grooved cases, but for η=0.97 and 0.985, there is a peak before this exponent appears.}, } @article {pmid29776082, year = {2018}, author = {Liang, H and Xu, J and Chen, J and Wang, H and Chai, Z and Shi, B}, title = {Phase-field-based lattice Boltzmann modeling of large-density-ratio two-phase flows.}, journal = {Physical review. E}, volume = {97}, number = {3-1}, pages = {033309}, doi = {10.1103/PhysRevE.97.033309}, pmid = {29776082}, issn = {2470-0053}, abstract = {In this paper, we present a simple and accurate lattice Boltzmann (LB) model for immiscible two-phase flows, which is able to deal with large density contrasts. This model utilizes two LB equations, one of which is used to solve the conservative Allen-Cahn equation, and the other is adopted to solve the incompressible Navier-Stokes equations. A forcing distribution function is elaborately designed in the LB equation for the Navier-Stokes equations, which make it much simpler than the existing LB models. In addition, the proposed model can achieve superior numerical accuracy compared with previous Allen-Cahn type of LB models. Several benchmark two-phase problems, including static droplet, layered Poiseuille flow, and spinodal decomposition are simulated to validate the present LB model. It is found that the present model can achieve relatively small spurious velocity in the LB community, and the obtained numerical results also show good agreement with the analytical solutions or some available results. Lastly, we use the present model to investigate the droplet impact on a thin liquid film with a large density ratio of 1000 and the Reynolds number ranging from 20 to 500. The fascinating phenomena of droplet splashing is successfully reproduced by the present model and the numerically predicted spreading radius exhibits to obey the power law reported in the literature.}, } @article {pmid29776043, year = {2018}, author = {Oyama, N and Teshigawara, K and Molina, JJ and Yamamoto, R and Taniguchi, T}, title = {Reynolds-number-dependent dynamical transitions on hydrodynamic synchronization modes of externally driven colloids.}, journal = {Physical review. E}, volume = {97}, number = {3-1}, pages = {032611}, doi = {10.1103/PhysRevE.97.032611}, pmid = {29776043}, issn = {2470-0053}, abstract = {The collective dynamics of externally driven N_{p} -colloidal systems (1≤N_{p} ≤4) in a confined viscous fluid have been investigated using three-dimensional direct numerical simulations with fully resolved hydrodynamics. The dynamical modes of collective particle motion are studied by changing the particle Reynolds number as determined by the strength of the external driving force and the confining wall distance. For a system with N_{p} =3, we found that at a critical Reynolds number a dynamical mode transition occurs from the doublet-singlet mode to the triplet mode, which has not been reported experimentally. The dynamical mode transition was analyzed in detail from the following two viewpoints: (1) spectrum analysis of the time evolution of a tagged particle velocity and (2) the relative acceleration of the doublet cluster with respect to the singlet particle. For a system with N_{p} =4, we found similar dynamical mode transitions from the doublet-singlet-singlet mode to the triplet-singlet mode and further to the quartet mode.}, } @article {pmid29772223, year = {2018}, author = {Markwalter, CE and Prud'homme, RK}, title = {Design of a Small-Scale Multi-Inlet Vortex Mixer for Scalable Nanoparticle Production and Application to the Encapsulation of Biologics by Inverse Flash NanoPrecipitation.}, journal = {Journal of pharmaceutical sciences}, volume = {}, number = {}, pages = {}, doi = {10.1016/j.xphs.2018.05.003}, pmid = {29772223}, issn = {1520-6017}, abstract = {Flash NanoPrecipitation is a scalable approach to generate polymeric nanoparticles using rapid micromixing in specially designed geometries such as a confined impinging jets mixer or a Multi-Inlet Vortex Mixer (MIVM). A major limitation of formulation screening using the MIVM is that a single run requires tens of milligrams of the therapeutic. To overcome this, we have developed a scaled-down version of the MIVM, requiring as little as 0.2 mg of therapeutic, for formulation screening. The redesigned mixer can then be attached to pumps for scale-up of the identified formulation. It was shown that Reynolds number allowed accurate scaling between the 2 MIVM designs. The utility of the small-scale MIVM for formulation development was demonstrated through the encapsulation of a number of hydrophilic macromolecules using inverse Flash NanoPrecipitation with target loadings as high as 50% by mass.}, } @article {pmid29758688, year = {2018}, author = {Sanjeevi, SKP and Zarghami, A and Padding, JT}, title = {Choice of no-slip curved boundary condition for lattice Boltzmann simulations of high-Reynolds-number flows.}, journal = {Physical review. E}, volume = {97}, number = {4-1}, pages = {043305}, doi = {10.1103/PhysRevE.97.043305}, pmid = {29758688}, issn = {2470-0053}, abstract = {Various curved no-slip boundary conditions available in literature improve the accuracy of lattice Boltzmann simulations compared to the traditional staircase approximation of curved geometries. Usually, the required unknown distribution functions emerging from the solid nodes are computed based on the known distribution functions using interpolation or extrapolation schemes. On using such curved boundary schemes, there will be mass loss or gain at each time step during the simulations, especially apparent at high Reynolds numbers, which is called mass leakage. Such an issue becomes severe in periodic flows, where the mass leakage accumulation would affect the computed flow fields over time. In this paper, we examine mass leakage of the most well-known curved boundary treatments for high-Reynolds-number flows. Apart from the existing schemes, we also test different forced mass conservation schemes and a constant density scheme. The capability of each scheme is investigated and, finally, recommendations for choosing a proper boundary condition scheme are given for stable and accurate simulations.}, } @article {pmid29758634, year = {2018}, author = {Mahalinkam, R and Gong, F and Khair, AS}, title = {Reduced-order model for inertial locomotion of a slender swimmer.}, journal = {Physical review. E}, volume = {97}, number = {4-1}, pages = {043102}, doi = {10.1103/PhysRevE.97.043102}, pmid = {29758634}, issn = {2470-0053}, abstract = {The inertial locomotion of an elongated model swimmer in a Newtonian fluid is quantified, wherein self-propulsion is achieved via steady tangential surface treadmilling. The swimmer has a length 2l and a circular cross section of longitudinal profile aR(z), where a is the characteristic width of the cross section, R(z) is a dimensionless shape function, and z is a dimensionless coordinate, normalized by l, along the centerline of the body. It is assumed that the swimmer is slender, ε=a/l≪1. Hence, we utilize slender-body theory to analyze the Navier-Stokes equations that describe the flow around the swimmer. Therefrom, we compute an asymptotic approximation to the swimming speed, U, as U/u_{s} =1-β[V(Re)-1/2∫_{-1} ^{1} zlnR(z)dz]/ln(1/ε)+O[1/ln^{2} (1/ε)], where u_{s} is the characteristic speed of the surface treadmilling, Re is the Reynolds number based on the body length, and β is a dimensionless parameter that differentiates between "pusher" (propelled from the rear, β<0) and "puller" (propelled from the front, β>0) -type swimmers. The function V(Re) increases monotonically with increasing Re; hence, fluid inertia causes an increase (decrease) in the swimming speed of a pusher (puller). Next, we demonstrate that the power expenditure of the swimmer increases monotonically with increasing Re. Further, the power expenditures of a puller and pusher with the same value of |β| are equal. Therefore, pushers are superior in inertial locomotion as compared to pullers, in that they achieve a faster swimming speed for the same power expended. Finally, it is demonstrated that the flow structure predicted from our reduced-order model is consistent with that from direct numerical simulation of swimmers at intermediate Re.}, } @article {pmid29757157, year = {2018}, author = {Daddi-Moussa-Ider, A and Lisicki, M and Mathijssen, AJTM and Hoell, C and Goh, S and Bławzdziewicz, J and Menzel, AM and Löwen, H}, title = {State diagram of a three-sphere microswimmer in a channel.}, journal = {Journal of physics. Condensed matter : an Institute of Physics journal}, volume = {30}, number = {25}, pages = {254004}, doi = {10.1088/1361-648X/aac470}, pmid = {29757157}, issn = {1361-648X}, abstract = {Geometric confinements are frequently encountered in soft matter systems and in particular significantly alter the dynamics of swimming microorganisms in viscous media. Surface-related effects on the motility of microswimmers can lead to important consequences in a large number of biological systems, such as biofilm formation, bacterial adhesion and microbial activity. On the basis of low-Reynolds-number hydrodynamics, we explore the state diagram of a three-sphere microswimmer under channel confinement in a slit geometry and fully characterize the swimming behavior and trajectories for neutral swimmers, puller- and pusher-type swimmers. While pushers always end up trapped at the channel walls, neutral swimmers and pullers may further perform a gliding motion and maintain a stable navigation along the channel. We find that the resulting dynamical system exhibits a supercritical pitchfork bifurcation in which swimming in the mid-plane becomes unstable beyond a transition channel height while two new stable limit cycles or fixed points that are symmetrically disposed with respect to the channel mid-height emerge. Additionally, we show that an accurate description of the averaged swimming velocity and rotation rate in a channel can be captured analytically using the method of hydrodynamic images, provided that the swimmer size is much smaller than the channel height.}, } @article {pmid29749100, year = {2018}, author = {Tottori, S and Nelson, BJ}, title = {Controlled Propulsion of Two-Dimensional Microswimmers in a Precessing Magnetic Field.}, journal = {Small (Weinheim an der Bergstrasse, Germany)}, volume = {14}, number = {24}, pages = {e1800722}, doi = {10.1002/smll.201800722}, pmid = {29749100}, issn = {1613-6829}, abstract = {Magnetically actuated micro-/nanoswimmers can potentially be used in noninvasive biomedical applications, such as targeted drug delivery and micromanipulation. Herein, two-dimensional (2D) rigid ferromagnetic microstructures are shown to be capable of propelling themselves in three dimensions at low Reynolds numbers in a precessing field. Importantly, the above propulsion relies neither on soft structure deformation nor on the geometrical chirality of swimmers, but is rather driven by the dynamic chirality generated by field precession, which allows an almost unconstrained choice of materials and fabrication methods. Therefore, the swimming performance is systematically investigated as a function of precession angle and geometric design. One disadvantage of the described propulsion method is that the fabricated 2D swimmers are achiral, which means that the forward/backward swimming direction cannot be controlled. However, it has been found that asymmetric 2D swimmers always propel themselves toward their longer arm, which implies that dynamic chirality can be constrained to be either right-handed or left-handed by permanent magnetization. Thus, the simplicity of fabrication and possibility of dynamic chirality control make the developed method ideal for applications and fundamental studies that require a large number of swimmers.}, } @article {pmid29745778, year = {2018}, author = {Perrin, A and Herbelin, P and Jorand, FPA and Skali-Lami, S and Mathieu, L}, title = {Design of a rotating disk reactor to assess the colonization of biofilms by free-living amoebae under high shear rates.}, journal = {Biofouling}, volume = {34}, number = {4}, pages = {368-377}, doi = {10.1080/08927014.2018.1444756}, pmid = {29745778}, issn = {1029-2454}, abstract = {The present study was aimed at designing and optimizing a rotating disk reactor simulating high hydrodynamic shear rates (γ), which are representative of cooling circuits. The characteristics of the hydrodynamic conditions in the reactor and the complex approach used to engineer it are described. A 60 l tank was filled with freshwater containing free-living amoebae (FLA) and bacteria. Adhesion of the bacteria and formation of a biofilm on the stainless steel coupons were observed. FLA were able to establish in these biofilms under γ as high as 85,000 s-1. Several physical mechanisms (convection, diffusion, sedimentation) could explain the accumulation of amoeboid cells on surfaces, but further research is required to fully understand and model the fine mechanisms governing such transport under γ similar to those encountered in the industrial environment. This technological advance may enable research into these topics.}, } @article {pmid29744606, year = {2018}, author = {Zhang, S and Luo, X and Cai, Z}, title = {Three-dimensional flows in a hyperelastic vessel under external pressure.}, journal = {Biomechanics and modeling in mechanobiology}, volume = {}, number = {}, pages = {}, doi = {10.1007/s10237-018-1022-y}, pmid = {29744606}, issn = {1617-7940}, support = {EP/N014642/1//Engineering and Physical Sciences Research Council/ ; 11172200//National Natural Science Foundation of China/ ; 2013CB035042//National Basic Research Program of China/ ; RF-2015-510//Leverhulme Trust/ ; }, abstract = {We study the collapsible behaviour of a vessel conveying viscous flows subject to external pressure, a scenario that could occur in many physiological applications. The vessel is modelled as a three-dimensional cylindrical tube of nonlinear hyperelastic material. To solve the fully coupled fluid-structure interaction, we have developed a novel approach based on the Arbitrary Lagrangian-Eulerian (ALE) method and the frontal solver. The method of rotating spines is used to enable an automatic mesh adaptation. The numerical code is verified extensively with published results and those obtained using the commercial packages in simpler cases, e.g. ANSYS for the structure with the prescribed flow, and FLUENT for the fluid flow with prescribed structure deformation. We examine three different hyperelastic material models for the tube for the first time in this context and show that at the small strain, all three material models give similar results. However, for the large strain, results differ depending on the material model used. We further study the behaviour of the tube under a mode-3 buckling and reveal its complex flow patterns under various external pressures. To understand these flow patterns, we show how energy dissipation is associated with the boundary layers created at the narrowest collapsed section of the tube, and how the transverse flow forms a virtual sink to feed a strong axial jet. We found that the energy dissipation associated with the recirculation does not coincide with the flow separation zone itself, but overlaps with the streamlines that divide the three recirculation zones. Finally, we examine the bifurcation diagrams for both mode-3 and mode-2 collapses and reveal that multiple solutions exist for a range of the Reynolds number. Our work is a step towards modelling more realistic physiological flows in collapsible arteries and veins.}, } @article {pmid29732048, year = {2018}, author = {Zhou, Y and Lee, C and Wang, J}, title = {The Computational Fluid Dynamics Analyses on Hemodynamic Characteristics in Stenosed Arterial Models.}, journal = {Journal of healthcare engineering}, volume = {2018}, number = {}, pages = {4312415}, doi = {10.1155/2018/4312415}, pmid = {29732048}, issn = {2040-2295}, abstract = {Arterial stenosis plays an important role in the progressions of thrombosis and stroke. In the present study, a standard axisymmetric tube model of the stenotic artery is introduced and the degree of stenosis η is evaluated by the area ratio of the blockage to the normal vessel. A normal case (η = 0) and four stenotic cases of η = 0.25, 0.5, 0.625, and 0.75 with a constant Reynolds number of 300 are simulated by computational fluid dynamics (CFD), respectively, with the Newtonian and Carreau models for comparison. Results show that for both models, the poststenotic separation vortex length increases exponentially with the growth of stenosis degree. However, the vortex length of the Carreau model is shorter than that of the Newtonian model. The artery narrowing accelerates blood flow, which causes high blood pressure and wall shear stress (WSS). The pressure drop of the η = 0.75 case is nearly 8 times that of the normal value, while the WSS peak at the stenosis region of η = 0.75 case even reaches up to 15 times that of the normal value. The present conclusions are of generality and contribute to the understanding of the dynamic mechanisms of artery stenosis diseases.}, } @article {pmid29729406, year = {2018}, author = {García-Salazar, G and de la Luz Zambrano-Zaragoza, M and Quintanar-Guerrero, D}, title = {Preparation of nanodispersions by solvent displacement using the Venturi tube.}, journal = {International journal of pharmaceutics}, volume = {545}, number = {1-2}, pages = {254-260}, doi = {10.1016/j.ijpharm.2018.05.005}, pmid = {29729406}, issn = {1873-3476}, abstract = {The Venturi tube (VT) is an apparatus that produces turbulence which is taken advantage of to produce nanoparticles (NP) by solvent displacement. The objective of this study was to evaluate the potential of this device for preparing NP of poly-ε-caprolactone. Response Surface Methodology was used to determine the effect of the operating conditions and optimization. The NP produced by VT were characterized by Dynamic Light-Scattering to determine their particle size distribution (PS) and polydispersity index (PDI). Results showed that the Reynolds number (Re) has a strong effect on both PS and process yield (PY).The turbulence regime is key to the efficient formation of NP. The optimal conditions for obtaining NP were a polymer concentration of 1.6 w/v, a recirculation rate of 4.8 L/min, and a stabilizer concentration of 1.1 w/v. The predicted response of the PY was 99.7%, with a PS of 333 nm, and a PDI of 0.2. Maintaining the same preparation conditions will make it possible to obtain NP using other polymers with similar properties. Our results show that VT is a reproducible and versatile method for manufacturing NP, and so may be a feasible method for industrial-scale nanoprecipitation production.}, } @article {pmid29710732, year = {2018}, author = {Sandeep, N and Kumaran, G and Saleem, S}, title = {The influence of cross diffusion on magnetohydrodynamic flow of Carreau liquid in the presence of buoyancy force.}, journal = {Journal of integrative neuroscience}, volume = {}, number = {}, pages = {}, doi = {10.3233/JIN-180086}, pmid = {29710732}, issn = {0219-6352}, abstract = {The flow of magnetohydrodynamic Carreau liquid with the Brownian moment, thermophoresis and cross diffusion effects is investigated numerically. The buoyancy persuades on the flow is contemplated in such a way that the surface is neither perpendicular/horizontal nor wedge/cone. This is very helpful in the design of jet-engine. The equations govern the flow are transmuted using acceptable similarity variables and numerically solved by recruiting Runge-Kutta based Newtons method. The graphical results are obtained to discuss the stimulus of flow, thermal and concentration fields for different parameters of interest. The wall friction, local Nusselt and Sherwood numbers are examined with the assistance of tables. It is noticed that the parabolic flow is controlled by the buoyant forces developed by the temperature difference. Since the flow is laminar, the Reynolds number considered as <1000. This study has applicable in man-made products and various industries like pumps and oil purification, petroleum production, power engineering and chemical engineering processes.}, } @article {pmid29708343, year = {2018}, author = {Gao, S and Liao, Q and Liu, W and Liu, Z}, title = {Nanodroplets Impact on Rough Surfaces: A Simulation and Theoretical Study.}, journal = {Langmuir : the ACS journal of surfaces and colloids}, volume = {34}, number = {20}, pages = {5910-5917}, doi = {10.1021/acs.langmuir.8b00480}, pmid = {29708343}, issn = {1520-5827}, abstract = {Impact of droplets is widespread in life, and modulating the dynamics of impinging droplets is a significant problem in production. However, on textured surfaces, the micromorphologic change and mechanism of impinging nanodroplets are not well-understood; furthermore, the accuracy of the theoretical model for nanodroplets needs to be improved. Here, considering the great challenge of conducting experiments on nanodroplets, a molecular dynamics simulation is performed to visualize the impact process of nanodroplets on nanopillar surfaces. Compared with macroscale droplets, apart from the similar relation of restitution coefficient with the Weber number, we found some distinctive results: the maximum spreading time is described as a power law of impact velocity, and the relation of maximum spreading factor with impact velocity or the Reynolds number is exponential. Moreover, the roughness of substrates plays a prominent role in the dynamics of impact nanodroplets, and on surfaces with lower solid fraction, the lower attraction force induces an easier rebound of impact nanodroplets. At last, on the basis of the energy balance, through modifying the estimation of viscous dissipation and surface energy terms, we proposed an improved model for the maximum spreading factor, which shows greater accuracy for nanodroplets, especially in the low-to-moderate velocity range. The outcome of this study demonstrates that a distinctive dynamical behavior of impinging nanodroplets, the fundamental insight, and more accurate prediction are very useful in the improvement of the hydrodynamic behavior of the nanodroplets.}, } @article {pmid29670984, year = {2018}, author = {Vilfan, M and Osterman, N and Vilfan, A}, title = {Magnetically driven omnidirectional artificial microswimmers.}, journal = {Soft matter}, volume = {14}, number = {17}, pages = {3415-3422}, doi = {10.1039/c8sm00230d}, pmid = {29670984}, issn = {1744-6848}, abstract = {We present an experimental realisation of two new artificial microswimmers that swim at low Reynolds number. The swimmers are externally driven with a periodically modulated magnetic field that induces an alternating attractive/repulsive interaction between the swimmer parts. The field sequence also modulates the drag on the swimmer components, making the working cycle non-reciprocal. The resulting net translational displacement leads to velocities of up to 2 micrometers per second. The swimmers can be made omnidirectional, meaning that the same magnetic field sequence can drive swimmers in any direction in the sample plane. Although the direction of their swimming is determined by the momentary orientation of the swimmer, their motion can be guided by solid boundaries. We demonstrate their omnidirectionality by letting them travel through a circular microfluidic channel. We use simple scaling arguments as well as more detailed numerical simulations to explain the measured velocity as a function of the actuation frequency.}, } @article {pmid29670148, year = {2018}, author = {Govindarajan, V and Mousel, J and Udaykumar, HS and Vigmostad, SC and McPherson, DD and Kim, H and Chandran, KB}, title = {Synergy between Diastolic Mitral Valve Function and Left Ventricular Flow Aids in Valve Closure and Blood Transport during Systole.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {6187}, doi = {10.1038/s41598-018-24469-x}, pmid = {29670148}, issn = {2045-2322}, abstract = {Highly resolved three-dimensional (3D) fluid structure interaction (FSI) simulation using patient-specific echocardiographic data can be a powerful tool for accurately and thoroughly elucidating the biomechanics of mitral valve (MV) function and left ventricular (LV) fluid dynamics. We developed and validated a strongly coupled FSI algorithm to fully characterize the LV flow field during diastolic MV opening under physiologic conditions. Our model revealed that distinct MV deformation and LV flow patterns developed during different diastolic stages. A vortex ring that strongly depended on MV deformation formed during early diastole. At peak E wave, the MV fully opened, with a local Reynolds number of ~5500, indicating that the flow was in the laminar-turbulent transitional regime. Our results showed that during diastasis, the vortex structures caused the MV leaflets to converge, thus increasing mitral jet's velocity. The vortex ring became asymmetrical, with the vortex structures on the anterior side being larger than on the posterior side. During the late diastolic stages, the flow structures advected toward the LV outflow tract, enhancing fluid transport to the aorta. This 3D-FSI study demonstrated the importance of leaflet dynamics, their effect on the vortex ring, and their influence on MV function and fluid transport within the LV during diastole.}, } @article {pmid29657749, year = {2018}, author = {Li, H and Guo, S}, title = {Aerodynamic efficiency of a bioinspired flapping wing rotor at low Reynolds number.}, journal = {Royal Society open science}, volume = {5}, number = {3}, pages = {171307}, doi = {10.1098/rsos.171307}, pmid = {29657749}, issn = {2054-5703}, abstract = {This study investigates the aerodynamic efficiency of a bioinspired flapping wing rotor kinematics which combines an active vertical flapping motion and a passive horizontal rotation induced by aerodynamic thrust. The aerodynamic efficiencies for producing both vertical lift and horizontal thrust of the wing are obtained using a quasi-steady aerodynamic model and two-dimensional (2D) CFD analysis at Reynolds number of 2500. The calculated efficiency data show that both efficiencies (propulsive efficiency-ηp, and efficiency for producing lift-Pf) of the wing are optimized at Strouhal number (St) between 0.1 and 0.5 for a range of wing pitch angles (upstroke angle of attack αu less than 45°); the St for high Pf (St = 0.1 ∼ 0.3) is generally lower than for high ηp (St = 0.2 ∼ 0.5), while the St for equilibrium rotation states lies between the two. Further systematic calculations show that the natural equilibrium of the passive rotating wing automatically converges to high-efficiency states: above 85% of maximum Pf can be obtained for a wide range of prescribed wing kinematics. This study provides insight into the aerodynamic efficiency of biological flyers in cruising flight, as well as practical applications for micro air vehicle design.}, } @article {pmid29648545, year = {2018}, author = {Wang, C and Tang, H}, title = {Enhancement of aerodynamic performance of a heaving airfoil using synthetic-jet based active flow control.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {4}, pages = {046005}, doi = {10.1088/1748-3190/aabdb9}, pmid = {29648545}, issn = {1748-3190}, abstract = {In this study, we explore the use of synthetic jet (SJ) in manipulating the vortices around a rigid heaving airfoil, so as to enhance its aerodynamic performance. The airfoil heaves at two fixed pitching angles, with the Strouhal number, reduced frequency and Reynolds number chosen as St = 0.3, k = 0.25 and Re = 100, respectively, all falling in the ranges for natural flyers. As such, the vortex force plays a dominant role in determining the airfoil's aerodynamic performance. A pair of in-phase SJs is implemented on the airfoil's upper and lower surfaces, operating with the same strength but in opposite directions. Such a fluid-structure interaction problem is numerically solved using a lattice Boltzmann method based numerical framework. It is found that, as the airfoil heaves with zero pitching angle, its lift and drag can be improved concurrently when the SJ phase angle [Formula: see text] relative to the heave motion varies between [Formula: see text] and [Formula: see text]. But this concurrent improvement does not occur as the airfoil heaves with [Formula: see text] pitching angle. Detailed inspection of the vortex evolution and fluid stress over the airfoil surface reveals that, if at good timing, the suction and blowing strokes of the SJ pair can effectively delay or promote the shedding of leading edge vortices, and mitigate or even eliminate the generation of trailing edge vortices, so as to enhance the airfoil's aerodynamic performance. Based on these understandings, an intermittent operation of the SJ pair is then proposed to realize concurrent lift and drag improvement for the heaving airfoil with [Formula: see text] pitching angle.}, } @article {pmid29633848, year = {2018}, author = {Bordbar, A and Taassob, A and Khojasteh, D and Marengo, M and Kamali, R}, title = {Maximum Spreading and Rebound of a Droplet Impacting onto a Spherical Surface at Low Weber Numbers.}, journal = {Langmuir : the ACS journal of surfaces and colloids}, volume = {34}, number = {17}, pages = {5149-5158}, doi = {10.1021/acs.langmuir.8b00625}, pmid = {29633848}, issn = {1520-5827}, abstract = {The spreading and rebound patterns of low-viscous droplets upon impacting spherical solid surfaces are investigated numerically. The studied cases consider a droplet impinging onto hydrophobic and superhydrophobic surfaces with various parameters varied throughout the study, and their effects on the postimpingement behavior are discussed. These parameters include impact Weber number (through varying the surface tension and impingement velocity), the size ratio of the droplet to the solid surface, and the surface contact angle. According to the findings, the maximum spreading diameter increases with the impact velocity, with an increase of the sphere diameter, with a lower surface wettability, and with a lower surface tension. Typical outcomes of the impact include (1) complete rebound, (2) splash, and (3) a final deposition stage after a series of spreading and recoiling phases. Finally, a novel, practical model is proposed, which can reasonably predict the maximum deformation of low Reynolds number impact of droplets onto hydrophobic or superhydrophobic spherical solid surfaces.}, } @article {pmid29608886, year = {2018}, author = {Hopgood, M and Reynolds, G and Barker, R}, title = {Using Computational Fluid Dynamics to Compare Shear Rate and Turbulence in the TIM-Automated Gastric Compartment With USP Apparatus II.}, journal = {Journal of pharmaceutical sciences}, volume = {107}, number = {7}, pages = {1911-1919}, doi = {10.1016/j.xphs.2018.03.019}, pmid = {29608886}, issn = {1520-6017}, abstract = {We use computational fluid dynamics to compare the shear rate and turbulence in an advanced in vitro gastric model (TIMagc) during its simulation of fasted state Migrating Motor Complex phases I and II, with the United States Pharmacopeia paddle dissolution apparatus II (USPII). A specific focus is placed on how shear rate in these apparatus affects erosion-based solid oral dosage forms. The study finds that tablet surface shear rates in TIMagc are strongly time dependant and fluctuate between 0.001 and 360 s-1. In USPII, tablet surface shear rates are approximately constant for a given paddle speed and increase linearly from 9 s-1 to 36 s-1 as the paddle speed is increased from 25 to 100 rpm. A strong linear relationship is observed between tablet surface shear rate and tablet erosion rate in USPII, whereas TIMagc shows highly variable behavior. The flow regimes present in each apparatus are compared to in vivo predictions using Reynolds number analysis. Reynolds numbers for flow in TIMagc lie predominantly within the predicted in vivo bounds (0.01-30), whereas Reynolds numbers for flow in USPII lie above the predicted upper bound when operating with paddle speeds as low as 25 rpm (33).}, } @article {pmid29596861, year = {2018}, author = {Prakash, J and Ramesh, K and Tripathi, D and Kumar, R}, title = {Numerical simulation of heat transfer in blood flow altered by electroosmosis through tapered micro-vessels.}, journal = {Microvascular research}, volume = {118}, number = {}, pages = {162-172}, doi = {10.1016/j.mvr.2018.03.009}, pmid = {29596861}, issn = {1095-9319}, abstract = {A numerical simulation is presented to study the heat and flow characteristics of blood flow altered by electroosmosis through the tapered micro-vessels. Blood is assumed as non-Newtonian (micropolar) nanofluids. The flow regime is considered as asymmetric diverging (tapered) microchannel for more realistic micro-vessels which is produced by choosing the peristaltic wave train on the walls to have different amplitudes and phase. The Rosseland approximation is employed to model the radiation heat transfer and temperatures of the walls are presumed constants. The mathematical formulation of the present problem is simplified under the long-wavelength, low-Reynolds number and Debye-Hückel linearization approximations. The influence of various dominant physical parameters are discussed for axial velocity, microrotation distribution, thermal temperature distribution and nanoparticle volume fraction field. However, our foremost emphasis is to determine the effects of thermal radiation and coupling number on the axial velocity and microrotation distribution beneath electroosmotic environment. This analysis places a significant observation on the thermal radiation and coupling number which plays an influential role in hearten fluid velocity. This study is encouraged by exploring the nanofluid-dynamics in peristaltic transport as symbolized by heat transport in biological flows and also in novel pharmacodynamics pumps and gastro-intestinal motility enhancement.}, } @article {pmid29594300, year = {2018}, author = {Jing, H and Das, S}, title = {Theory of diffusioosmosis in a charged nanochannel.}, journal = {Physical chemistry chemical physics : PCCP}, volume = {20}, number = {15}, pages = {10204-10212}, doi = {10.1039/c8cp01091a}, pmid = {29594300}, issn = {1463-9084}, abstract = {We probe the diffusioosmotic transport in a charged nanofluidic channel in the presence of an applied tangential salt concentration gradient. Ionic salt gradient driven diffusioosmosis or ionic diffusioosmosis (IDO) is characterized by the generation of an induced tangential electric field and a diffusioosmotic velocity (DOSV) that is a combination of an electroosmotic velocity (EOSV) triggered by this electric field and a chemiosmotic velocity (COSV) triggered by an induced tangential pressure gradient. We explain that unlike the existing theories on IDO, it is more appropriate to apply the zero net current conditions (formulation F2) and not more restrictive zero net local flux conditions (formulation F1) particularly for the case where one considers a nanochannel connected to two reservoirs. We pinpoint limitations in the existing literature in correctly predicting the diffusioosmotic behavior even for the case where formulation F1 is used. We address these limitations and establish that (a) the induced electric field is an interplay of the differences in ionic diffusivity, the EDL-induced imbalance in ion concentrations, and the advection effects, (b) formulation F1 may overpredict or underpredict the electric field and the EOSV leading to an overprediction/underprediction of the DOSV and (c) formulation F2 demonstrates remarkable fluid physics of localized backflows owing to a dominant local influence of the COSV, which is missed by formulation F1. We anticipate that our theory will provide the first rigorous understanding of nanofluidic IDO with applications in multiple areas of low Reynolds number transport such as biofluidics, microfluidic separation, and colloidal transport.}, } @article {pmid29589515, year = {2018}, author = {Rigatelli, G and Zuin, M and Dell'Avvocata, F and Nguyen, T}, title = {Rheolytic effects of left main mid-shaft/distal stenting: a computational flow dynamic analysis.}, journal = {Therapeutic advances in cardiovascular disease}, volume = {12}, number = {6}, pages = {161-168}, doi = {10.1177/1753944718765734}, pmid = {29589515}, issn = {1753-9455}, abstract = {Background The aim of this study was to evaluate the rheolytic effects of stenting a mid-shaft/distal left main coronary artery (LMCA) lesion with and without ostial coverage. Stenting of the LMCA has emerged as a valid alternative in place of traditional coronary bypass graft surgery. However, in case of mid-shaft/distal lesion, there is no consensus regarding the extension of the strut coverage up to the ostium or to stent only the culprit lesion. Methods We reconstructed a left main-left descending coronary artery (LM-LCA)-left circumflex (LCX) bifurcation after analysing 100 consecutive patients (mean age 71.4 ± 9.3, 49 males) with LM mid-shaft/distal disease. The mean diameter of proximal LM, left anterior descending (LAD) and LCX, evaluated with quantitative coronary angiography (QCA) was 4.62 ± 0.86 mm, 3.31 ± 0.92 mm, and 2.74 ± 0.93 mm, respectively. For the stent simulation, a third-generation, everolimus-eluting stent was virtually reconstructed. Results After virtual stenting, the net area averaged wall shear stress (WSS) of the model and the WSS at the LCA-LCX bifurcation resulted higher when the stent covered the culprit mid-shaft lesion only compared with the extension of the stent covering the ostium (3.68 versus 2.06 Pa, p = 0.01 and 3.97 versus 1.98 Pa, p < 0.001, respectively. Similarly, the static pressure and the Reynolds number were significantly higher after stent implantation covering up the ostium. At the ostium, the flow resulted more laminar when stenting only the mid-shaft lesion than including the ostium. Conclusions Although these findings cannot be translated directly into real practice our brief study suggests that stenting lesion 1:1 or extending the stent to cover the LM ostium impacts differently the rheolytic properties of LMCA bifurcation with potential insights for restenosis or thrombosis.}, } @article {pmid29584651, year = {2018}, author = {Xi, J and Hu, Q and Zhao, L and Si, XA}, title = {Molecular Binding Contributes to Concentration Dependent Acrolein Deposition in Rat Upper Airways: CFD and Molecular Dynamics Analyses.}, journal = {International journal of molecular sciences}, volume = {19}, number = {4}, pages = {}, doi = {10.3390/ijms19040997}, pmid = {29584651}, issn = {1422-0067}, abstract = {Existing in vivo experiments show significantly decreased acrolein uptake in rats with increasing inhaled acrolein concentrations. Considering that high-polarity chemicals are prone to bond with each other, it is hypothesized that molecular binding between acrolein and water will contribute to the experimentally observed deposition decrease by decreasing the effective diffusivity. The objective of this study is to quantify the probability of molecular binding for acrolein, as well as its effects on acrolein deposition, using multiscale simulations. An image-based rat airway geometry was used to predict the transport and deposition of acrolein using the chemical species model. The low Reynolds number turbulence model was used to simulate the airflows. Molecular dynamic (MD) simulations were used to study the molecular binding of acrolein in different media and at different acrolein concentrations. MD results show that significant molecular binding can happen between acrolein and water molecules in human and rat airways. With 72 acrolein embedded in 800 water molecules, about 48% of acrolein compounds contain one hydrogen bond and 10% contain two hydrogen bonds, which agreed favorably with previous MD results. The percentage of hydrogen-bonded acrolein compounds is higher at higher acrolein concentrations or in a medium with higher polarity. Computational dosimetry results show that the size increase caused by the molecular binding reduces the effective diffusivity of acrolein and lowers the chemical deposition onto the airway surfaces. This result is consistent with the experimentally observed deposition decrease at higher concentrations. However, this size increase can only explain part of the concentration-dependent variation of the acrolein uptake and acts as a concurrent mechanism with the uptake-limiting tissue ration rate. Intermolecular interactions and associated variation in diffusivity should be considered in future dosimetry modeling of high-polarity chemicals such as acrolein.}, } @article {pmid29557508, year = {2018}, author = {Gupta, A and Clercx, HJH and Toschi, F}, title = {Computational study of radial particle migration and stresslet distributions in particle-laden turbulent pipe flow.}, journal = {The European physical journal. E, Soft matter}, volume = {41}, number = {3}, pages = {34}, doi = {10.1140/epje/i2018-11638-3}, pmid = {29557508}, issn = {1292-895X}, abstract = {Particle-laden turbulent flows occur in a variety of industrial applications as well as in naturally occurring flows. While the numerical simulation of such flows has seen significant advances in recent years, it still remains a challenging problem. Many studies investigated the rheology of dense suspensions in laminar flows as well as the dynamics of point-particles in turbulence. Here we employ a fully-resolved numerical simulation based on a lattice Boltzmann scheme, to investigate turbulent flow with large neutrally buoyant particles in a pipe flow at low Reynolds number and in dilute regimes. The energy input is kept fixed resulting in a Reynolds number based on the friction velocity around 250. Two different particle radii were used giving a particle-pipe diameter ratio of 0.05 and 0.075. The number of particles is kept constant resulting in a volume fraction of 0.54% and 1.83%, respectively. We investigated Eulerian and Lagrangian statistics along with the stresslet exerted by the fluid on the spherical particles. It was observed that the high particle-to-fluid slip velocity close to the wall corresponds locally to events of high energy dissipation, which are not present in the single-phase flow. The migration of particles from the inner to the outer region of the pipe, the dependence of the stresslet on the particle radial positions and a proxy for the fragmentation rate of the particles computed using the stresslet have been investigated.}, } @article {pmid29557347, year = {2018}, author = {Bayiz, Y and Ghanaatpishe, M and Fathy, H and Cheng, B}, title = {Hovering efficiency comparison of rotary and flapping flight for rigid rectangular wings via dimensionless multi-objective optimization.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {4}, pages = {046002}, doi = {10.1088/1748-3190/aab801}, pmid = {29557347}, issn = {1748-3190}, abstract = {In this work, a multi-objective optimization framework is developed for optimizing low Reynolds number ([Formula: see text]) hovering flight. This framework is then applied to compare the efficiency of rigid revolving and flapping wings with rectangular shape under varying [Formula: see text] and Rossby number ([Formula: see text], or aspect ratio). The proposed framework is capable of generating sets of optimal solutions and Pareto fronts for maximizing the lift coefficient and minimizing the power coefficient in dimensionless space, explicitly revealing the trade-off between lift generation and power consumption. The results indicate that revolving wings are more efficient when the required average lift coefficient [Formula: see text] is low (<1 for [Formula: see text] and <1.6 for [Formula: see text]), while flapping wings are more efficient in achieving higher [Formula: see text]. With the dimensionless power loading as the single-objective performance measure to be maximized, rotary flight is more efficient than flapping wings for [Formula: see text] regardless of the amount of energy storage assumed in the flapping wing actuation mechanism, while flapping flight is more efficient for [Formula: see text]. It is observed that wings with low [Formula: see text] perform better when higher [Formula: see text] is needed, whereas higher [Formula: see text] cases are more efficient at [Formula: see text] regions. However, for the selected geometry and [Formula: see text], the efficiency is weakly dependent on [Formula: see text] when the dimensionless power loading is maximized.}, } @article {pmid29555806, year = {2018}, author = {Margolin, LG}, title = {Scale matters.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {376}, number = {2118}, pages = {}, doi = {10.1098/rsta.2017.0235}, pmid = {29555806}, issn = {1471-2962}, abstract = {The applicability of Navier-Stokes equations is limited to near-equilibrium flows in which the gradients of density, velocity and energy are small. Here I propose an extension of the Chapman-Enskog approximation in which the velocity probability distribution function (PDF) is averaged in the coordinate phase space as well as the velocity phase space. I derive a PDF that depends on the gradients and represents a first-order generalization of local thermodynamic equilibrium. I then integrate this PDF to derive a hydrodynamic model. I discuss the properties of that model and its relation to the discrete equations of computational fluid dynamics.This article is part of the theme issue 'Hilbert's sixth problem'.}, } @article {pmid29553536, year = {2018}, author = {Agudo, JR and Han, J and Park, J and Kwon, S and Loekman, S and Luzi, G and Linderberger, C and Delgado, A and Wierschem, A}, title = {Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions.}, journal = {Journal of visualized experiments : JoVE}, volume = {}, number = {132}, pages = {}, doi = {10.3791/57238}, pmid = {29553536}, issn = {1940-087X}, mesh = {Computer Simulation ; Motion ; Nonlinear Dynamics ; *Stress, Mechanical ; }, abstract = {Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.}, } @article {pmid29548174, year = {2018}, author = {Kawamura, Y and Tsubaki, R}, title = {Phase reduction approach to elastohydrodynamic synchronization of beating flagella.}, journal = {Physical review. E}, volume = {97}, number = {2-1}, pages = {022212}, doi = {10.1103/PhysRevE.97.022212}, pmid = {29548174}, issn = {2470-0053}, abstract = {We formulate a theory for the phase reduction of a beating flagellum. The theory enables us to describe the dynamics of a beating flagellum in a systematic manner using a single variable called the phase. The theory can also be considered as a phase reduction method for the limit-cycle solutions in infinite-dimensional dynamical systems, namely, the limit-cycle solutions to partial differential equations representing beating flagella. We derive the phase sensitivity function, which quantifies the phase response of a beating flagellum to weak perturbations applied at each point and at each time. Using the phase sensitivity function, we analyze the phase synchronization between a pair of beating flagella through hydrodynamic interactions at a low Reynolds number.}, } @article {pmid29548159, year = {2018}, author = {Rolland, J}, title = {Extremely rare collapse and build-up of turbulence in stochastic models of transitional wall flows.}, journal = {Physical review. E}, volume = {97}, number = {2-1}, pages = {023109}, doi = {10.1103/PhysRevE.97.023109}, pmid = {29548159}, issn = {2470-0053}, abstract = {This paper presents a numerical and theoretical study of multistability in two stochastic models of transitional wall flows. An algorithm dedicated to the computation of rare events is adapted on these two stochastic models. The main focus is placed on a stochastic partial differential equation model proposed by Barkley. Three types of events are computed in a systematic and reproducible manner: (i) the collapse of isolated puffs and domains initially containing their steady turbulent fraction; (ii) the puff splitting; (iii) the build-up of turbulence from the laminar base flow under a noise perturbation of vanishing variance. For build-up events, an extreme realization of the vanishing variance noise pushes the state from the laminar base flow to the most probable germ of turbulence which in turn develops into a full blown puff. For collapse events, the Reynolds number and length ranges of the two regimes of collapse of laminar-turbulent pipes, independent collapse or global collapse of puffs, is determined. The mean first passage time before each event is then systematically computed as a function of the Reynolds number r and pipe length L in the laminar-turbulent coexistence range of Reynolds number. In the case of isolated puffs, the faster-than-linear growth with Reynolds number of the logarithm of mean first passage time T before collapse is separated in two. One finds that ln(T)=A_{p} r-B_{p}, with A_{p} and B_{p} positive. Moreover, A_{p} and B_{p} are affine in the spatial integral of turbulence intensity of the puff, with the same slope. In the case of pipes initially containing the steady turbulent fraction, the length L and Reynolds number r dependence of the mean first passage time T before collapse is also separated. The author finds that T≍exp[L(Ar-B)] with A and B positive. The length and Reynolds number dependence of T are then discussed in view of the large deviations theoretical approaches of the study of mean first passage times and multistability, where ln(T) in the limit of small variance noise is studied. Two points of view, local noise of small variance and large length, can be used to discuss the exponential dependence in L of T. In particular, it is shown how a T≍exp[L(A^{'} R-B^{'} )] can be derived in a conceptual two degrees of freedom model of a transitional wall flow proposed by Dauchot and Manneville. This is done by identifying a quasipotential in low variance noise, large length limit. This pinpoints the physical effects controlling collapse and build-up trajectories and corresponding passage times with an emphasis on the saddle points between laminar and turbulent states. This analytical analysis also shows that these effects lead to the asymmetric probability density function of kinetic energy of turbulence.}, } @article {pmid29548094, year = {2018}, author = {Nemoto, T and Alexakis, A}, title = {Method to measure efficiently rare fluctuations of turbulence intensity for turbulent-laminar transitions in pipe flows.}, journal = {Physical review. E}, volume = {97}, number = {2-1}, pages = {022207}, doi = {10.1103/PhysRevE.97.022207}, pmid = {29548094}, issn = {2470-0053}, abstract = {The fluctuations of turbulence intensity in a pipe flow around the critical Reynolds number is difficult to study but important because they are related to turbulent-laminar transitions. We here propose a rare-event sampling method to study such fluctuations in order to measure the time scale of the transition efficiently. The method is composed of two parts: (i) the measurement of typical fluctuations (the bulk part of an accumulative probability function) and (ii) the measurement of rare fluctuations (the tail part of the probability function) by employing dynamics where a feedback control of the Reynolds number is implemented. We apply this method to a chaotic model of turbulent puffs proposed by Barkley and confirm that the time scale of turbulence decay increases super exponentially even for high Reynolds numbers up to Re =2500, where getting enough statistics by brute-force calculations is difficult. The method uses a simple procedure of changing Reynolds number that can be applied even to experiments.}, } @article {pmid29542932, year = {2018}, author = {Song, F and Ju, D and Gu, F and Liu, Y and Ji, Y and Ren, Y and He, X and Sha, B and Li, BQ and Yang, Q}, title = {Parametric Study on Electric Field-Induced Micro-/Nanopatterns in Thin Polymer Films.}, journal = {Langmuir : the ACS journal of surfaces and colloids}, volume = {34}, number = {14}, pages = {4188-4198}, doi = {10.1021/acs.langmuir.8b00007}, pmid = {29542932}, issn = {1520-5827}, abstract = {Electric field-induced micro-/nanopatterns in thin polymer films, sometimes referred as electrohydrodynamic patterning, is a promising technique to fabricate micro-/nanostructures. Extensive attention has been attracted because of its advantages in microcontact (easy demolding) and low cost. Although considerable work has been done on this technique, including both experimental and theoretical ones, there still appears a requirement for understanding the mechanism of electrohydrodynamic patterning. Thus, we systematically studied the effect of different parameters on electrohydrodynamic patterning with a numerical phase field model. Previous researchers usually employed lubrication approximation (i.e., long-wave approximation) to simplify the numerical model. However, this approximation would lose its validity if the structure height is on the same scale or larger than the wavelength, which occurs in most cases. Thus, we abandoned the lubrication approximation and solved the full governing equations for fluid flow and electric field. In this model, the deformation of polymer film is described by the phase field model. As to the electric field, the leaky dielectric model is adopted in which both electrical permittivity and conductivity are considered. The fluid flow together with electric field is coupled together in the framework of phase field. By this model, the effect of physical parameters, such as external voltage, template structure height, and polymer conductivity, is studied in detail. After that, the governing equations are nondimesionalized to analyze the relationship between different parameters. A dimensionless parameter, electrical Reynolds number ER, is defined, for which, a large value would simplify the electric field to perfect dielectric model and a small value leads it to steady leaky model. These findings and results may enhance our understanding of electrohydrodynamic patterning and may be a meaningful guide for experiments.}, } @article {pmid29542796, year = {2018}, author = {Du, D and Hilou, E and Biswal, SL}, title = {Reconfigurable paramagnetic microswimmers: Brownian motion affects non-reciprocal actuation.}, journal = {Soft matter}, volume = {14}, number = {18}, pages = {3463-3470}, doi = {10.1039/c8sm00069g}, pmid = {29542796}, issn = {1744-6848}, abstract = {Swimming at low Reynolds number is typically dominated by a large viscous drag, therefore microscale swimmers require non-reciprocal body deformation to generate locomotion. Purcell described a simple mechanical swimmer at the microscale consisting of three rigid components connected together with two hinges. Here we present a simple microswimmer consisting of two rigid paramagnetic particles with different sizes. When placed in an eccentric magnetic field, this simple microswimmer exhibits non-reciprocal body motion and its swimming locomotion can be directed in a controllable manner. Additional components can be added to create a multibody microswimmer, whereby the particles act cooperatively and translate in a given direction. For some multibody swimmers, the stochastic thermal forces fragment the arm, which therefore modifies the swimming strokes and changes the locomotive speed. This work offers insight into directing the motion of active systems with novel time-varying magnetic fields. It also reveals that Brownian motion not only affects the locomotion of reciprocal swimmers that are subject to the Scallop theorem, but also affects that of non-reciprocal swimmers.}, } @article {pmid29542373, year = {2018}, author = {Oota-Ishigaki, A and Masuzawa, T and Nagayama, K}, title = {Analysis of the effect of the size of three-dimensional micro-geometric structures on physical adhesion phenomena using microprint technique.}, journal = {The International journal of artificial organs}, volume = {41}, number = {5}, pages = {277-283}, doi = {10.1177/0391398818763264}, pmid = {29542373}, issn = {1724-6040}, abstract = {Thrombus formation on biomaterial surfaces with microstructures is complex and not fully understood. We have studied the micro-secondary flow around microstructures that causes components of blood to adhere physically in a low Reynolds number region. The purpose of this study was to investigate the effect of micro-column size on the adhesion phenomena and show a quantitative relationship between the micro-secondary flow and physical adhesion phenomena, considering microstructures of various sizes. The flow simulation and quantitative assessment of adhesion rates around micro-columns was conducted using four sizes of micro-columns. This study also calculated the vectors of micro-secondary flow and average shear rate around a micro-column using a computational fluid dynamics analysis. The simulation showed the micro-secondary flow toward the bottom surface at upstream side and low shear rate distribution generated around a micro-column. Furthermore, physical adhesion tests were conducted using microbeads and a perfusion circuit to examine the size effect of the micro-columns on the physical adhesion. The results showed that the average adhesion rate around the micro-column increases with the associated size increase of the micro-column. Our results indicate that quantification of micro-secondary flow on a material surface with microstructures of several sizes and shapes (such as in a rough surface) is important for the evaluation of the adhesion phenomenon even though the surface roughness value on the material surface is small.}, } @article {pmid29535341, year = {2018}, author = {Rubol, S and Ling, B and Battiato, I}, title = {Universal scaling-law for flow resistance over canopies with complex morphology.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {4430}, doi = {10.1038/s41598-018-22346-1}, pmid = {29535341}, issn = {2045-2322}, abstract = {Flow resistance caused by vegetation is a key parameter to properly assess flood management and river restoration. However, quantifying the friction factor or any of its alternative metrics, e.g. the drag coefficient, in canopies with complex geometry has proven elusive. We explore the effect of canopy morphology on vegetated channels flow structure and resistance by treating the canopy as a porous medium characterized by an effective permeability, a property that describes the ease with which water can flow through the canopy layer. We employ a two-domain model for flow over and within the canopy, which couples the log-law in the free layer to the Darcy-Brinkman equation in the vegetated layer. We validate the model analytical solutions for the average velocity profile within and above the canopy, the volumetric discharge and the friction factor against data collected across a wide range of canopy morphologies encountered in riverine systems. Results indicate agreement between model predictions and data for both simple and complex plant morphologies. For low submergence canopies, we find a universal scaling law that relates friction factor with canopy permeability and a rescaled bulk Reynolds number. This provides a valuable tool to assess habitats sustainability associated with hydro-dynamical conditions.}, } @article {pmid29527067, year = {2017}, author = {Yang, J and Wang, X and Krane, M and Zhang, LT}, title = {Fully-coupled aeroelastic simulation with fluid compressibility - For application to vocal fold vibration.}, journal = {Computer methods in applied mechanics and engineering}, volume = {315}, number = {}, pages = {584-606}, doi = {10.1016/j.cma.2016.11.010}, pmid = {29527067}, issn = {0045-7825}, support = {R01 DC005642/DC/NIDCD NIH HHS/United States ; }, abstract = {In this study, a fully-coupled fluid-structure interaction model is developed for studying dynamic interactions between compressible fluid and aeroelastic structures. The technique is built based on the modified Immersed Finite Element Method (mIFEM), a robust numerical technique to simulate fluid-structure interactions that has capabilities to simulate high Reynolds number flows and handles large density disparities between the fluid and the solid. For accurate assessment of this intricate dynamic process between compressible fluid, such as air and aeroelastic structures, we included in the model the fluid compressibility in an isentropic process and a solid contact model. The accuracy of the compressible fluid solver is verified by examining acoustic wave propagations in a closed and an open duct, respectively. The fully-coupled fluid-structure interaction model is then used to simulate and analyze vocal folds vibrations using compressible air interacting with vocal folds that are represented as layered viscoelastic structures. Using physiological geometric and parametric setup, we are able to obtain a self-sustained vocal fold vibration with a constant inflow pressure. Parametric studies are also performed to study the effects of lung pressure and vocal fold tissue stiffness in vocal folds vibrations. All the case studies produce expected airflow behavior and a sustained vibration, which provide verification and confidence in our future studies of realistic acoustical studies of the phonation process.}, } @article {pmid29513310, year = {2018}, author = {Marson, RL and Huang, Y and Huang, M and Fu, T and Larson, RG}, title = {Inertio-capillary cross-streamline drift of droplets in Poiseuille flow using dissipative particle dynamics simulations.}, journal = {Soft matter}, volume = {14}, number = {12}, pages = {2267-2280}, doi = {10.1039/c7sm02294h}, pmid = {29513310}, issn = {1744-6848}, abstract = {We find using dissipative particle dynamics (DPD) simulations that a deformable droplet sheared in a narrow microchannel migrates to steady-state position that depends upon the dimensionless particle capillary number , which controls the droplet deformability (with Vmax the centerline velocity, μf the fluid viscosity, Γ the surface tension, R the droplet radius, and H the gap), the droplet (particle) Reynolds number , which controls inertia, where ρ is the fluid density, as well as on the viscosity ratio of the droplet to the suspending fluid κ = μd/μf. We find that when the Ohnesorge number is around 0.06, so that inertia is stronger than capillarity, at small capillary number Cap < 0.1, the droplet migrates to a position close to that observed for hard spheres by Segre and Silberberg, around 60% of the distance from the centerline to the wall, while for increasing Cap the droplet steady-state position moves smoothly towards the centerline, reaching around 20% of the distance from centerline to the wall when Cap reaches 0.5 or so. For higher Oh, the droplet position is much less sensitive to Cap, and remains at around 30% of the distance from centerline to the wall over the whole accessible range of Cap. The results are insensitive to viscosity ratios from unity to the highest value studied here, around 13, and the drift towards the centerline for increasing Cap is observed for ratios of droplet diameter to gap size ranging from 0.1 to 0.3. We also find consistency between our predictions and existing perturbation theory for small droplet or particle size, as well as with experimental data. Additionally, we assess the accuracy of the DPD method and conclude that with current computer resources and methods DPD is not readily able to predict cross-stream-line drift for small particle Reynolds number (much less than unity), or for droplets that are less than one tenth the gap size, owing to excessive noise and inadequate numbers of DPD particles per droplet.}, } @article {pmid29512658, year = {2018}, author = {Hadikhani, P and Hashemi, SMH and Balestra, G and Zhu, L and Modestino, MA and Gallaire, F and Psaltis, D}, title = {Inertial manipulation of bubbles in rectangular microfluidic channels.}, journal = {Lab on a chip}, volume = {18}, number = {7}, pages = {1035-1046}, doi = {10.1039/c7lc01283g}, pmid = {29512658}, issn = {1473-0189}, abstract = {Inertial microfluidics is an active field of research that deals with crossflow positioning of the suspended entities in microflows. Until now, the majority of the studies have focused on the behavior of rigid particles in order to provide guidelines for microfluidic applications such as sorting and filtering. Deformable entities such as bubbles and droplets are considered in fewer studies despite their importance in multiphase microflows. In this paper, we show that the trajectory of bubbles flowing in rectangular and square microchannels can be controlled by tuning the balance of forces acting on them. A T-junction geometry is employed to introduce bubbles into a microchannel and analyze their lateral equilibrium position in a range of Reynolds (1 < Re < 40) and capillary numbers (0.1 < Ca < 1). We find that the Reynolds number (Re), the capillary number (Ca), the diameter of the bubble (D[combining macron]), and the aspect ratio of the channel are the influential parameters in this phenomenon. For instance, at high Re, the flow pushes the bubble towards the wall while large Ca or D[combining macron] moves the bubble towards the center. Moreover, in the shallow channels, having aspect ratios higher than one, the bubble moves towards the narrower sidewalls. One important outcome of this study is that the equilibrium position of bubbles in rectangular channels is different from that of solid particles. The experimental observations are in good agreement with the performed numerical simulations and provide insights into the dynamics of bubbles in laminar flows which can be utilized in the design of flow based multiphase flow reactors.}, } @article {pmid29505991, year = {2018}, author = {Martínez-Pedrero, F and Tierno, P}, title = {Advances in colloidal manipulation and transport via hydrodynamic interactions.}, journal = {Journal of colloid and interface science}, volume = {519}, number = {}, pages = {296-311}, doi = {10.1016/j.jcis.2018.02.062}, pmid = {29505991}, issn = {1095-7103}, abstract = {In this review article, we highlight many recent advances in the field of micromanipulation of colloidal particles using hydrodynamic interactions (HIs), namely solvent mediated long-range interactions. At the micrsocale, the hydrodynamic laws are time reversible and the flow becomes laminar, features that allow precise manipulation and control of colloidal matter. We focus on different strategies where externally operated microstructures generate local flow fields that induce the advection and motion of the surrounding components. In addition, we review cases where the induced flow gives rise to hydrodynamic bound states that may synchronize during the process, a phenomenon essential in different systems such as those that exhibit self-assembly and swarming.}, } @article {pmid29497820, year = {2018}, author = {Wang, Q and Othmer, HG}, title = {Analysis of a model microswimmer with applications to blebbing cells and mini-robots.}, journal = {Journal of mathematical biology}, volume = {76}, number = {7}, pages = {1699-1763}, doi = {10.1007/s00285-018-1225-y}, pmid = {29497820}, issn = {1432-1416}, support = {DMS 0817529//National Science Foundation/ ; 1311974//National Science Foundation/ ; DMS1562176//National Science Foundation/ ; R01GM107264//National Institutes of Health/ ; }, abstract = {Recent research has shown that motile cells can adapt their mode of propulsion depending on the environment in which they find themselves. One mode is swimming by blebbing or other shape changes, and in this paper we analyze a class of models for movement of cells by blebbing and of nano-robots in a viscous fluid at low Reynolds number. At the level of individuals, the shape changes comprise volume exchanges between connected spheres that can control their separation, which are simple enough that significant analytical results can be obtained. Our goal is to understand how the efficiency of movement depends on the amplitude and period of the volume exchanges when the spheres approach closely during a cycle. Previous analyses were predicated on wide separation, and we show that the speed increases significantly as the separation decreases due to the strong hydrodynamic interactions between spheres in close proximity. The scallop theorem asserts that at least two degrees of freedom are needed to produce net motion in a cyclic sequence of shape changes, and we show that these degrees can reside in different swimmers whose collective motion is studied. We also show that different combinations of mode sharing can lead to significant differences in the translation and performance of pairs of swimmers.}, } @article {pmid29487930, year = {2018}, author = {Bao, L and Spandan, V and Yang, Y and Dyett, B and Verzicco, R and Lohse, D and Zhang, X}, title = {Flow-induced dissolution of femtoliter surface droplet arrays.}, journal = {Lab on a chip}, volume = {18}, number = {7}, pages = {1066-1074}, doi = {10.1039/c7lc01321c}, pmid = {29487930}, issn = {1473-0189}, abstract = {The dissolution of liquid nanodroplets is a crucial step in many applied processes, such as separation and dispersion in the food industry, crystal formation of pharmaceutical products, concentrating and analysis in medical diagnosis, and drug delivery in aerosols. In this work, using both experiments and numerical simulations, we quantitatively study the dissolution dynamics of femtoliter surface droplets in a highly ordered array under a uniform flow. Our results show that the dissolution of femtoliter droplets strongly depends on their spatial positions relative to the flow direction, drop-to-drop spacing in the array, and the imposed flow rate. In some particular cases, the droplet at the edge of the array can dissolve about 30% faster than the ones located near the centre. The dissolution rate of the droplet increases by 60% as the inter-droplet spacing is increased from 2.5 μm to 20 μm. Moreover, the droplets close to the front of the flow commence to shrink earlier than those droplets in the center of the array. The average dissolution rate is faster for the faster flow. As a result, the dissolution time (Ti) decreases with the Reynolds number (Re) of the flow as Ti ∝ Re-3/4. The experimental results are in good agreement with the numerical simulations where the advection-diffusion equation for the concentration field is solved and the concentration gradient on the surface of the drop is computed. The findings suggest potential approaches to manipulate nanodroplet sizes in droplet arrays simply by dissolution controlled by an external flow. The obtained droplets with varying curvatures may serve as templates for generating multifocal microlenses in one array.}, } @article {pmid29481162, year = {2018}, author = {Nissan, A and Berkowitz, B}, title = {Inertial Effects on Flow and Transport in Heterogeneous Porous Media.}, journal = {Physical review letters}, volume = {120}, number = {5}, pages = {054504}, doi = {10.1103/PhysRevLett.120.054504}, pmid = {29481162}, issn = {1079-7114}, abstract = {We investigate the effects of high fluid velocities on flow and tracer transport in heterogeneous porous media. We simulate fluid flow and advective transport through two-dimensional pore-scale matrices with varying structural complexity. As the Reynolds number increases, the flow regime transitions from linear to nonlinear; this behavior is controlled by the medium structure, where higher complexity amplifies inertial effects. The result is, nonintuitively, increased homogenization of the flow field, which leads in the context of conservative chemical transport to less anomalous behavior. We quantify the transport patterns via a continuous time random walk, using the spatial distribution of the kinetic energy within the fluid as a characteristic measure.}, } @article {pmid29481155, year = {2018}, author = {Cerbus, RT and Liu, CC and Gioia, G and Chakraborty, P}, title = {Laws of Resistance in Transitional Pipe Flows.}, journal = {Physical review letters}, volume = {120}, number = {5}, pages = {054502}, doi = {10.1103/PhysRevLett.120.054502}, pmid = {29481155}, issn = {1079-7114}, abstract = {As everyone knows who has opened a kitchen faucet, pipe flow is laminar at low flow velocities and turbulent at high flow velocities. At intermediate velocities, there is a transition wherein plugs of laminar flow alternate along the pipe with "flashes" of a type of fluctuating, nonlaminar flow that remains poorly understood. In the 19th century, Osborne Reynolds sought to connect these states of flow with quantitative "laws of resistance," whereby the fluid friction is determined as a function of the Reynolds number. While he succeeded for laminar and turbulent flows, the laws for transitional flows eluded him and remain unknown to this day. By properly distinguishing between laminar plugs and flashes in the transitional regime, we show experimentally and numerically that the law of resistance for laminar plugs corresponds to the laminar law and the law of resistance for flashes is identical to that of turbulence.}, } @article {pmid29480329, year = {2018}, author = {Wu, H and de León, MAP and Othmer, HG}, title = {Getting in shape and swimming: the role of cortical forces and membrane heterogeneity in eukaryotic cells.}, journal = {Journal of mathematical biology}, volume = {}, number = {}, pages = {}, doi = {10.1007/s00285-018-1223-0}, pmid = {29480329}, issn = {1432-1416}, support = {R01 GM029123/GM/NIGMS NIH HHS/United States ; DMS 0817529//National Science Foundation/ ; 1311974//National Science Foundation/ ; #54-CA-210190//National Institutes of Health/ ; }, abstract = {Recent research has shown that motile cells can adapt their mode of propulsion to the mechanical properties of the environment in which they find themselves-crawling in some environments while swimming in others. The latter can involve movement by blebbing or other cyclic shape changes, and both highly-simplified and more realistic models of these modes have been studied previously. Herein we study swimming that is driven by membrane tension gradients that arise from flows in the actin cortex underlying the membrane, and does not involve imposed cyclic shape changes. Such gradients can lead to a number of different characteristic cell shapes, and our first objective is to understand how different distributions of membrane tension influence the shape of cells in an inviscid quiescent fluid. We then analyze the effects of spatial variation in other membrane properties, and how they interact with tension gradients to determine the shape. We also study the effect of fluid-cell interactions and show how tension leads to cell movement, how the balance between tension gradients and a variable bending modulus determine the shape and direction of movement, and how the efficiency of movement depends on the properties of the fluid and the distribution of tension and bending modulus in the membrane.}, } @article {pmid29466251, year = {2018}, author = {Arranz, G and Moriche, M and Uhlmann, M and Flores, O and García-Villalba, M}, title = {Kinematics and dynamics of the auto-rotation of a model winged seed.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {3}, pages = {036011}, doi = {10.1088/1748-3190/aab144}, pmid = {29466251}, issn = {1748-3190}, abstract = {Numerical simulations of the auto-rotation of a model winged seed are presented. The calculations are performed by solving simultaneously the Navier-Stokes equations for the flow surrounding the seed and the rigid-body equations for the motion of the seed. The Reynolds number based on the descent speed and a characteristic chord length is varied in the range 80-240. Within this range, the seed attains an asymptotic state with finite amplitude auto-rotation, while for smaller values of the Reynolds number no auto-rotation is observed. The motion of the seed is characterized by the coning and pitch angles, the angular velocity and the horizontal translation of the seed. The values obtained for these quantities are qualitatively similar to those reported in the literature in experiments with real winged seeds. When increasing the Reynolds number, the seed tends to rotate at higher speeds, with less inclination with respect to the horizontal plane, and with a larger translation velocity. With respect to the aerodynamic forces, it is observed that, with increasing Reynolds number, the horizontal components decrease in magnitude while the vertical component increases. The force distribution along the wing span is characterized using both global and local characteristic speeds and chord lengths for the non-dimensionalisation of the force coefficients. It is found that the vertical component does not depend on the Reynolds number when using local scaling, while the chordwise component of the force does.}, } @article {pmid29462624, year = {2018}, author = {Ishimoto, K and Gadêlha, H and Gaffney, EA and Smith, DJ and Kirkman-Brown, J}, title = {Human sperm swimming in a high viscosity mucus analogue.}, journal = {Journal of theoretical biology}, volume = {446}, number = {}, pages = {1-10}, doi = {10.1016/j.jtbi.2018.02.013}, pmid = {29462624}, issn = {1095-8541}, abstract = {Remarkably, mammalian sperm maintain a substantive proportion of their progressive swimming speed within highly viscous fluids, including those of the female reproductive tract. Here, we analyse the digital microscopy of a human sperm swimming in a highly viscous, weakly elastic mucus analogue. We exploit principal component analysis to simplify its flagellar beat pattern, from which boundary element calculations are used to determine the time-dependent flow field around the sperm cell. The sperm flow field is further approximated in terms of regularised point forces, and estimates of the mechanical power consumption are determined, for comparison with analogous low viscosity media studies. This highlights extensive differences in the structure of the flows surrounding human sperm in different media, indicating how the cell-cell and cell-boundary hydrodynamic interactions significantly differ with the physical microenvironment. The regularised point force decomposition also provides cell-level information that may ultimately be incorporated into sperm population models. We further observe indications that the core feature in explaining the effectiveness of sperm swimming in high viscosity media is the loss of cell yawing, which is related with a greater density of regularised point force singularities along the axis of symmetry of the flagellar beat to represent the flow field. In turn this implicates a reduction of the wavelength of the distal beat pattern - and hence dynamical wavelength selection of the flagellar beat - as the dominant feature governing the effectiveness of sperm swimming in highly viscous media.}, } @article {pmid29454239, year = {2018}, author = {Ren, LF and Adeel, M and Li, J and Xu, C and Xu, Z and Zhang, X and Shao, J and He, Y}, title = {Phenol separation from phenol-laden saline wastewater by membrane aromatic recovery system-like membrane contactor using superhydrophobic/organophilic electrospun PDMS/PMMA membrane.}, journal = {Water research}, volume = {135}, number = {}, pages = {31-43}, doi = {10.1016/j.watres.2018.02.011}, pmid = {29454239}, issn = {1879-2448}, abstract = {Phenol recovery from phenol-laden saline wastewater plays an important role in the waste reclamation and pollution control. A membrane aromatic recovery system-like membrane contactor (MARS-like membrane contactor) was set up in this study using electrospun polydimethylsiloxane/polymethyl methacrylate (PDMS/PMMA) membrane with 0.0048 m2 effective area to separate phenol from saline wastewater. Phenol and water contact angles of 0° and 162° were achieved on this membrane surface simultaneously, indicating its potential in the separation of phenol and water-soluble salt. Feed solution (500 mL) of 0.90 L/h and receiving solution (500 mL) of 1.26 L/h were investigated to be the optimum conditions for phenol separation, which corresponds to the employed Reynolds number of 14.6 and 20.5. During 108-h continuous separation for feed solution (2.0 g/L phenol, 10.0 g/L NaCl) under room temperature (20 °C), 42.6% of phenol was recycled in receiving solution with a salt rejection of 99.95%. Meanwhile, the mean phenol mass transfer coefficient (Kov) was 6.7 × 10-7 m s-1. As a membrane-based process, though the permeated phenol increased with the increase of phenol concentration in feed solution, the phenol recovery ratio was determined by the membrane properties rather than the pollutant concentrations. Phenol was found to permeate this membrane via adsorption, diffusion and desorption, and therefore, the membrane fouling generated from pore blockage in other membrane separation processes was totally avoided.}, } @article {pmid29448779, year = {2018}, author = {Fang, C and Wu, X and Yang, F and Qiao, R}, title = {Flow of quasi-two dimensional water in graphene channels.}, journal = {The Journal of chemical physics}, volume = {148}, number = {6}, pages = {064702}, doi = {10.1063/1.5017491}, pmid = {29448779}, issn = {1089-7690}, abstract = {When liquids confined in slit channels approach a monolayer, they become two-dimensional (2D) fluids. Using molecular dynamics simulations, we study the flow of quasi-2D water confined in slit channels featuring pristine graphene walls and graphene walls with hydroxyl groups. We focus on to what extent the flow of quasi-2D water can be described using classical hydrodynamics and what are the effective transport properties of the water and the channel. First, the in-plane shearing of quasi-2D water confined between pristine graphene can be described using the classical hydrodynamic equation, and the viscosity of the water is ∼50% higher than that of the bulk water in the channel studied here. Second, the flow of quasi-2D water around a single hydroxyl group is perturbed at a position of tens of cluster radius from its center, as expected for low Reynolds number flows. Even though water is not pinned at the edge of the hydroxyl group, the hydroxyl group screens the flow greatly, with a single, isolated hydroxyl group rendering drag similar to ∼90 nm2 pristine graphene walls. Finally, the flow of quasi-2D water through graphene channels featuring randomly distributed hydroxyl groups resembles the fluid flow through porous media. The effective friction factor of the channel increases linearly with the hydroxyl groups' area density up to 0.5 nm-2 but increases nonlinearly at higher densities. The effective friction factor of the channel can be fitted to a modified Carman equation at least up to a hydroxyl area density of 2.0 nm-2. These findings help understand the liquid transport in 2D material-based nanochannels for applications including desalination.}, } @article {pmid29445037, year = {2018}, author = {Godoy-Diana, R and Thiria, B}, title = {On the diverse roles of fluid dynamic drag in animal swimming and flying.}, journal = {Journal of the Royal Society, Interface}, volume = {15}, number = {139}, pages = {}, doi = {10.1098/rsif.2017.0715}, pmid = {29445037}, issn = {1742-5662}, abstract = {Questions of energy dissipation or friction appear immediately when addressing the problem of a body moving in a fluid. For the most simple problems, involving a constant steady propulsive force on the body, a straightforward relation can be established balancing this driving force with a skin friction or form drag, depending on the Reynolds number and body geometry. This elementary relation closes the full dynamical problem and sets, for instance, average cruising velocity or energy cost. In the case of finite-sized and time-deformable bodies though, such as flapping flyers or undulatory swimmers, the comprehension of driving/dissipation interactions is not straightforward. The intrinsic unsteadiness of the flapping and deforming animal bodies complicates the usual application of classical fluid dynamic forces balance. One of the complications is because the shape of the body is indeed changing in time, accelerating and decelerating perpetually, but also because the role of drag (more specifically the role of the local drag) has two different facets, contributing at the same time to global dissipation and to driving forces. This causes situations where a strong drag is not necessarily equivalent to inefficient systems. A lot of living systems are precisely using strong sources of drag to optimize their performance. In addition to revisiting classical results under the light of recent research on these questions, we discuss in this review the crucial role of drag from another point of view that concerns the fluid-structure interaction problem of animal locomotion. We consider, in particular, the dynamic subtleties brought by the quadratic drag that resists transverse motions of a flexible body or appendage performing complex kinematics, such as the phase dynamics of a flexible flapping wing, the propagative nature of the bending wave in undulatory swimmers, or the surprising relevance of drag-based resistive thrust in inertial swimmers.}, } @article {pmid29435817, year = {2018}, author = {Walait, A and Siddiqui, AM and Rana, MA}, title = {Analysis of a self-propelling sheet with heat transfer through non-isothermal fluid in an inclined human cervical canal.}, journal = {Journal of biological physics}, volume = {}, number = {}, pages = {}, doi = {10.1007/s10867-018-9481-z}, pmid = {29435817}, issn = {1573-0689}, abstract = {The present theoretical analysis deals with biomechanics of the self-propulsion of a swimming sheet with heat transfer through non-isothermal fluid filling an inclined human cervical canal. Partial differential equations arising from the mathematical modeling of the proposed model are solved analytically. Flow variables like pressure gradient, propulsive velocity, fluid velocity, time mean flow rate, fluid temperature, and heat-transfer coefficients are analyzed for the pertinent parameters. Striking features of the pumping characteristics are explored. Propulsive velocity of the swimming sheet becomes faster for lower Froude number, higher Reynolds number, and for a vertical channel. Temperature and peak value of the heat-transfer coefficients below the swimming sheet showed an increase by the increment of Brinkmann number, inclination, pressure difference over wavelength, and Reynolds number whereas these quantities decrease with increasing Froude number. Aforesaid parameters have shown opposite effects on the peak value of the heat-transfer coefficients below and above the swimming sheet. Relevance of the current results to the spermatozoa transport with heat transfer through non-isothermal cervical mucus filling an inclined human cervical canal is also explored.}, } @article {pmid29429719, year = {2018}, author = {Dehbani, M and Rahimi, M}, title = {Introducing ultrasonic falling film evaporator for moderate temperature evaporation enhancement.}, journal = {Ultrasonics sonochemistry}, volume = {42}, number = {}, pages = {689-696}, doi = {10.1016/j.ultsonch.2017.12.016}, pmid = {29429719}, issn = {1873-2828}, abstract = {In the present study, Ultrasonic Falling Film (USFF), as a novel technique has been proposed to increase the evaporation rate of moderate temperature liquid film. It is a proper method for some applications which cannot be performed at high temperature, such as foodstuff industry, due to their sensitivity to high temperatures. Evaporation rate of sodium chloride solution from an USFF on an inclined flat plate compared to that for Falling Film without ultrasonic irradiation (FF) at various temperatures was investigated. The results revealed that produced cavitation bubbles have different effects on evaporation rate at different temperatures. At lower temperatures, size fluctuation and collapse of bubbles and in consequence induced physical effects of cavitation bubbles resulted in more turbulency and evaporation rate enhancement. At higher temperatures, the behavior was different. Numerous created bubbles joined together and cover the plate surface, so not only decreased the ultrasound vibrations but also reduced the evaporation rate in comparison with FF. The highest evaporation rate enhancement of 353% was obtained at 40 °C at the lowest Reynolds number of 250. In addition, the results reveal that at temperature of 40 °C, USFF has the highest efficiency compared to FF.}, } @article {pmid29420569, year = {2018}, author = {Tang, H and Xu, L and Hu, F}, title = {Hydrodynamic characteristics of knotted and knotless purse seine netting panels as determined in a flume tank.}, journal = {PloS one}, volume = {13}, number = {2}, pages = {e0192206}, doi = {10.1371/journal.pone.0192206}, pmid = {29420569}, issn = {1932-6203}, mesh = {*Hydrodynamics ; Models, Theoretical ; *Nylons ; }, abstract = {Nylon (PA) netting is widely used in purse seines and other fishing gears due to its high strength and good sinking performance. However, hydrodynamic properties of nylon netting of different characteristics are poorly understood. This study investigated hydrodynamic characteristics of nylon netting of different knot types and solidity ratios under different attack angles and flow velocities. It was found that the hydrodynamic coefficient of netting panels was related to Reynolds number, solidity ratio, attack angle, knot type and twine construction. The solidity ratio was found to positively correlate with drag coefficient when the netting was normal to the flow (CD90), but not the case when the netting was parallel to the flow (CD0). For netting panels inclined to the flow, the inclined drag coefficient had a negative relationship with the solidity ratio for attack angles between 0° and 50°, but a positive relationship for attack angles between 50° and 90°. The lift coefficient increased with the attack angle, reaching the culminating point at an attack angle of 50°, before subsequent decline. We found that the drag generated by knot accounted for 15-25% of total drag, and the knotted netting with higher solidity ratio exhibited a greater CD0, but it was not the case for the knotless netting. Compared to knotless polyethylene (PE) netting, the drag coefficients of knotless PA netting were dominant at higher Reynolds number (Re>2200).}, } @article {pmid29407276, year = {2018}, author = {Mojahed, A and Rajabi, M}, title = {Self-motile swimmers: Ultrasound driven spherical model.}, journal = {Ultrasonics}, volume = {86}, number = {}, pages = {1-5}, doi = {10.1016/j.ultras.2018.01.006}, pmid = {29407276}, issn = {1874-9968}, abstract = {The concept of ultrasound acoustic driven self-motile swimmers which is the source of autonomous propulsion is the acoustic field generated by the swimmer due to the partial oscillation of its surface is investigated. Limiting the subject to a body with simple spherical geometry, it is analytically shown that the generated acoustic radiation force due to induction by asymmetric acoustic field in host medium is non-zero, which propels the device. Assuming low Reynolds number condition, the frequency-dependent swimming velocity is calculated as a function of design parameters and optimum operating condition is obtained. The proposed methodology will open a new path towards the micro- or molecular-sized self propulsive machines or mechanism with great applications in engineering, medicine and biology.}, } @article {pmid29404782, year = {2018}, author = {Huang, JJ and Wu, J and Huang, H}, title = {An alternative method to implement contact angle boundary condition and its application in hybrid lattice-Boltzmann finite-difference simulations of two-phase flows with immersed surfaces.}, journal = {The European physical journal. E, Soft matter}, volume = {41}, number = {2}, pages = {17}, doi = {10.1140/epje/i2018-11622-y}, pmid = {29404782}, issn = {1292-895X}, abstract = {We propose an alternative method to implement the contact angle boundary condition on a solid wall and apply it in hybrid lattice-Boltzmann finite-difference simulations of two-phase flows with immersed surfaces in which the flow equations are solved by the lattice-Boltzmann method and the interface equations are solved by the finite-difference method. Using the hyperbolic tangent profile of the order parameter across an interface in phase-field theory, we were able to obtain its unknown value at a ghost point from the information at only one point in the fluid domain. This is in contrast with existing approaches relying on interpolations involving several points. The special feature allows it to be more easily implemented on immersed surfaces cutting through the grid lines. It was properly incorporated into the framework of the hybrid lattice-Boltzmann finite-difference simulation, and applied successfully for several problems with different levels of complexity. First, the equilibrium shapes of a droplet on a sphere with different contact angles and radii were studied under cylindrical geometry and a good agreement with theoretical predictions was found. Preliminary studies on a three-dimensional droplet spreading on a sphere were also performed and the results agreed well with the corresponding axisymmetric results. Second, the spreading of a two-dimensional drop on an embedded inclined wall with a given contact angle was investigated and the results matched those on a flat wall at the domain boundary under the same condition. Third, capillary filling in a cylindrical tube was studied and the speed of the interface in the tube was found to follow Washburn's law. Fourth, a droplet impacting on a sphere was investigated and several different outcomes were captured depending on the Reynolds number, the viscosity ratio, and the wettability and radius of the sphere. Finally, the proposed method was shown to be capable of studying even more complicated problems involving the interaction between a droplet and multiple objects of different sizes and contact angles.}, } @article {pmid29395261, year = {2018}, author = {Mehrabian, S and Letendre, F and Cameron, CB}, title = {The mechanisms of filter feeding on oil droplets: Theoretical considerations.}, journal = {Marine environmental research}, volume = {135}, number = {}, pages = {29-42}, doi = {10.1016/j.marenvres.2018.01.006}, pmid = {29395261}, issn = {1879-0291}, abstract = {Filter feeding animals capture food particles and oil droplets from the fluid environment using cilia or appendages composed of arrays of fibers. Here we review the theoretical models that have provided a foundation for observations on the efficiency of particle capture. We then provide the mathematical theoretical framework to characterize the efficient filtration of oil droplets. In the aquatic and marine environments oil droplets are released from the decay of organisms or as hydrocarbons. Droplet size and flow velocity, oil-to-water viscosity ratio, oil-water interfacial tension, oil and water density difference, and the surface wettability, or surface texture, of the filter fiber are the key parameters for oil droplet capture. Following capture, capillary force maintains the droplet at its location due to the oil-water interfacial tension. If the oil-coated fiber is subject to any external force such as viscous or gravitational forces, it may deform and separate from the fiber and re-enter the fluid stream. We show oil droplet capture in Daphnia and the barnacle Balanus glandula, and outline some of the ecological unknowns regarding oil capture in the oceans. Awareness of these mechanisms and their interrelationships will provide a foundation for investigations into the efficiency of various modes of filter feeding on oil droplets.}, } @article {pmid29390777, year = {2018}, author = {Karthik, K and Vengadesan, S and Bhattacharyya, SK}, title = {Prediction of flow induced sound generated by cross flow past finite length circular cylinders.}, journal = {The Journal of the Acoustical Society of America}, volume = {143}, number = {1}, pages = {260}, doi = {10.1121/1.5021243}, pmid = {29390777}, issn = {1520-8524}, abstract = {The paper presents aeroacoustic results for the flow around finite-length circular cylinders at Reynolds number 84 770 for various length-to-diameter (L/D) ratios (= 3, 9, 20, 25, 30, and 35). The incompressible Navier-Stokes equations are solved using the large eddy simulation model of turbulence followed by acoustic predictions in the far field using Ffwocs Williams and Hawkings method. The comparisons of numerical and anechoic wind tunnel measurements show good agreement in terms of the aerodynamic forces and acoustic parameters such as tonal frequency, tonal sound pressure level, and overall sound pressure level. The cylinder L/D ratio was observed to be a significant parameter that controls vortex shedding and consequently the flow induced sound generation.}, } @article {pmid29390714, year = {2018}, author = {Mastracci, B and Guo, W}, title = {An apparatus for generation and quantitative measurement of homogeneous isotropic turbulence in He ii.}, journal = {The Review of scientific instruments}, volume = {89}, number = {1}, pages = {015107}, doi = {10.1063/1.4997735}, pmid = {29390714}, issn = {1089-7623}, abstract = {The superfluid phase of helium-4, known as He ii, exhibits extremely small kinematic viscosity and may be a useful tool for economically producing and studying high Reynolds number turbulent flow. Such applications are not currently possible because a comprehensive understanding of the complex two-fluid behavior of He ii is lacking. This situation could be remedied by a systematic investigation of simple, well controlled turbulence that can be directly compared with theoretical models. To this end, we have developed a new apparatus that combines flow visualization with second sound attenuation to study turbulence in the wake of a mesh grid towed through a He ii filled channel. One of three mesh grids (mesh number M = 3, 3.75, or 5 mm) can be pulled at speeds between 0.1 and 60 cm/s through a cast acrylic flow channel which has a 16 mm × 16 mm cross section and measures 330 mm long. The motion of solidified deuterium tracer particles, with diameter of the order 1 μm, in the resulting flow is captured by a high speed camera, and a particle tracking velocimetry algorithm resolves the Lagrangian particle trajectories through the turbulent flow field. A pair of oscillating superleak second sound transducers installed in the channel allows complementary measurement of vortex line density in the superfluid throughout the turbulent decay process. Success in early experiments demonstrates the effectiveness of both probes, and preliminary analysis of the data shows that both measurements strongly correlate with each other. Further investigations will provide comprehensive information that can be used to address open questions about turbulence in He ii and move toward the application of this fluid to high Reynolds number fluid research.}, } @article {pmid29390681, year = {2018}, author = {Sajjadi, S and Buelna, X and Eloranta, J}, title = {Application of time-resolved shadowgraph imaging and computer analysis to study micrometer-scale response of superfluid helium.}, journal = {The Review of scientific instruments}, volume = {89}, number = {1}, pages = {013102}, doi = {10.1063/1.5002564}, pmid = {29390681}, issn = {1089-7623}, abstract = {Application of inexpensive light emitting diodes as backlight sources for time-resolved shadowgraph imaging is demonstrated. The two light sources tested are able to produce light pulse sequences in the nanosecond and microsecond time regimes. After determining their time response characteristics, the diodes were applied to study the gas bubble formation around laser-heated copper nanoparticles in superfluid helium at 1.7 K and to determine the local cavitation bubble dynamics around fast moving metal micro-particles in the liquid. A convolutional neural network algorithm for analyzing the shadowgraph images by a computer is presented and the method is validated against the results from manual image analysis. The second application employed the red-green-blue light emitting diode source that produces light pulse sequences of the individual colors such that three separate shadowgraph frames can be recorded onto the color pixels of a charge-coupled device camera. Such an image sequence can be used to determine the moving object geometry, local velocity, and acceleration/deceleration. These data can be used to calculate, for example, the instantaneous Reynolds number for the liquid flow around the particle. Although specifically demonstrated for superfluid helium, the technique can be used to study the dynamic response of any medium that exhibits spatial variations in the index of refraction.}, } @article {pmid29388556, year = {2018}, author = {Lynch, M and Mandadzhiev, B and Wissa, A}, title = {Bioinspired wingtip devices: a pathway to improve aerodynamic performance during low Reynolds number flight.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {3}, pages = {036003}, doi = {10.1088/1748-3190/aaac53}, pmid = {29388556}, issn = {1748-3190}, abstract = {Birds are highly capable and maneuverable fliers, traits not currently shared with current small unmanned aerial vehicles. They are able to achieve these flight capabilities by adapting the shape of their wings during flight in a variety of complex manners. One feature of bird wings, the primary feathers, separate to form wingtip gaps at the distal end of the wing. This paper presents bio-inspired wingtip devices with varying wingtip gap sizes, defined as the chordwise distance between wingtip devices, for operation in low Reynolds number conditions of Re = 100 000, where many bird species operate. Lift and drag data was measured for planar and nonplanar wingtip devices with the total wingtip gap size ranging from 0% to 40% of the wing's mean chord. For a planar wing with a gap size of 20%, the mean coefficient of lift in the pre-stall region is increased by 7.25%, and the maximum coefficient of lift is increased by 5.6% compared to a configuration with no gaps. The nonplanar wingtip device was shown to reduce the induced drag. The effect of wingtip gap sizes is shown to be independent of the planarity/nonplanarity of the wingtip device, thereby allowing designers to decouple the wingtip parameters to tune the desired lift and drag produced.}, } @article {pmid29377529, year = {2018}, author = {Lee, J and Burns, MA}, title = {One-Way Particle Transport Using Oscillatory Flow in Asymmetric Traps.}, journal = {Small (Weinheim an der Bergstrasse, Germany)}, volume = {14}, number = {9}, pages = {}, doi = {10.1002/smll.201702724}, pmid = {29377529}, issn = {1613-6829}, abstract = {One challenge of integrating of passive, microparticles manipulation techniques into multifunctional microfluidic devices is coupling the continuous-flow format of most systems with the often batch-type operation of particle separation systems. Here, a passive fluidic technique-one-way particle transport-that can conduct microparticle operations in a closed fluidic circuit is presented. Exploiting pass/capture interactions between microparticles and asymmetric traps, this technique accomplishes a net displacement of particles in an oscillatory flow field. One-way particle transport is achieved through four kinds of trap-particle interactions: mechanical capture of the particle, asymmetric interactions between the trap and the particle, physical collision of the particle with an obstacle, and lateral shift of the particle into a particle-trapping stream. The critical dimensions for those four conditions are found by numerically solving analytical mass balance equations formulated using the characteristics of the flow field in periodic obstacle arrays. Visual observation of experimental trap-particle dynamics in low Reynolds number flow (<0.01) confirms the validity of the theoretical predictions. This technique can transport hundreds of microparticles across trap rows in only a few fluid oscillations (<500 ms per oscillation) and separate particles by their size differences.}, } @article {pmid29376688, year = {2018}, author = {Berera, A and Ho, RDJG}, title = {Chaotic Properties of a Turbulent Isotropic Fluid.}, journal = {Physical review letters}, volume = {120}, number = {2}, pages = {024101}, doi = {10.1103/PhysRevLett.120.024101}, pmid = {29376688}, issn = {1079-7114}, abstract = {By tracking the divergence of two initially close trajectories in phase space in an Eulerian approach to forced turbulence, the relation between the maximal Lyapunov exponent λ and the Reynolds number Re is measured using direct numerical simulations, performed on up to 2048^{3} collocation points. The Lyapunov exponent is found to solely depend on the Reynolds number with λ∝Re^{0.53} and that after a transient period the divergence of trajectories grows at the same rate at all scales. Finally a linear divergence is seen that is dependent on the energy forcing rate. Links are made with other chaotic systems.}, } @article {pmid29376342, year = {2018}, author = {Lee, D and Nam, SM and Kim, JA and Di Carlo, D and Lee, W}, title = {Active Control of Inertial Focusing Positions and Particle Separations Enabled by Velocity Profile Tuning with Coflow Systems.}, journal = {Analytical chemistry}, volume = {90}, number = {4}, pages = {2902-2911}, doi = {10.1021/acs.analchem.7b05143}, pmid = {29376342}, issn = {1520-6882}, abstract = {Inertial microfluidics has drawn much attention not only for its diverse applications but also for counterintuitive new fluid dynamic behaviors. Inertial focusing positions are determined by two lift forces, that is, shear gradient and wall-induced lift forces, that are generally known to be opposite in direction in the flow through a channel. However, the direction of shear gradient lift force can be reversed if velocity profiles are shaped properly. We used coflows of two liquids with different viscosities to produce complex velocity profiles that lead to inflection point focusing and alteration of inertial focusing positions; the number and the locations of focusing positions could be actively controlled by tuning flow rates and viscosities of the liquids. Interestingly, 3-inlet coflow systems showed focusing mode switching between inflection point focusing and channel face focusing depending on Reynolds number and particle size. The focusing mode switching occurred at a specific size threshold, which was easily adjustable with the viscosity ratio of the coflows. This property led to different-sized particles focusing at completely different focusing positions and resulted in highly efficient particle separation of which the separation threshold was tunable. Passive separation techniques, including inertial microfluidics, generally have a limitation in the control of separation parameters. Coflow systems can provide a simple and versatile platform for active tuning of velocity profiles and subsequent inertial focusing characteristics, which was demonstrated by active control of the focusing mode using viscosity ratio tuning and temperature changes of the coflows.}, } @article {pmid29372888, year = {2018}, author = {Fu, J and Liu, X and Shyy, W and Qiu, H}, title = {Effects of flexibility and aspect ratio on the aerodynamic performance of flapping wings.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {3}, pages = {036001}, doi = {10.1088/1748-3190/aaaac1}, pmid = {29372888}, issn = {1748-3190}, abstract = {In the current study, we experimentally investigated the flexibility effects on the aerodynamic performance of flapping wings and the correlation with aspect ratio at angle of attack α = 45°. The Reynolds number based on the chord length and the wing tip velocity is maintained at Re = 5.3 × 103. Our result for compliant wings with an aspect ratio of 4 shows that wing flexibility can offer improved aerodynamic performance compared to that of a rigid wing. Flexible wings are found to offer higher lift-to-drag ratios; in particular, there is significant reduction in drag with little compromise in lift. The mechanism of the flexibility effects on the aerodynamic performance is addressed by quantifying the aerodynamic lift and drag forces, the transverse displacement on the wings and the flow field around the wings. The regime of the effective stiffness that offers improved aerodynamic performance is quantified in a range of about 0.5-10 and it matches the stiffness of insect wings with similar aspect ratios. Furthermore, we find that the aspect ratio of the wing is the predominant parameter determining the flexibility effects of compliant wings. Compliant wings with an aspect ratio of two do not demonstrate improved performance compared to their rigid counterparts throughout the entire stiffness regime investigated. The correlation between wing flexibility effects and the aspect ratio is supported by the stiffness of real insect wings.}, } @article {pmid29367420, year = {2018}, author = {Bocanegra Evans, H and Hamed, AM and Gorumlu, S and Doosttalab, A and Aksak, B and Chamorro, LP and Castillo, L}, title = {Engineered bio-inspired coating for passive flow control.}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {115}, number = {6}, pages = {1210-1214}, doi = {10.1073/pnas.1715567115}, pmid = {29367420}, issn = {1091-6490}, abstract = {Flow separation and vortex shedding are some of the most common phenomena experienced by bluff bodies under relative motion with the surrounding medium. They often result in a recirculation bubble in regions with adverse pressure gradient, which typically reduces efficiency in vehicles and increases loading on structures. Here, the ability of an engineered coating to manipulate the large-scale recirculation region was tested in a separated flow at moderate momentum thickness Reynolds number, [Formula: see text] We show that the coating, composed of uniformly distributed cylindrical pillars with diverging tips, successfully reduces the size of, and shifts downstream, the separation bubble. Despite the so-called roughness parameter, [Formula: see text], falling within the hydrodynamic smooth regime, the coating is able to modulate the large-scale recirculating motion. Remarkably, this modulation does not induce noticeable changes in the near-wall turbulence levels. Supported with experimental data and theoretical arguments based on the averaged equations of motion, we suggest that the inherent mechanism responsible for the bubble modulation is essentially unsteady suction and blowing controlled by the increasing cross-section of the tips. The coating can be easily fabricated and installed and works under dry and wet conditions, increasing its potential impact on a diverse range of applications.}, } @article {pmid29347793, year = {2017}, author = {Zhao, Y and Tao, J and Xiong, X}, title = {Instabilities of an annulus flow between rotating cylinders in a helical magnetic field.}, journal = {Physical review. E}, volume = {96}, number = {5-1}, pages = {053101}, doi = {10.1103/PhysRevE.96.053101}, pmid = {29347793}, issn = {2470-0053}, abstract = {The stabilities of an annulus flow between the rotating inner and outer cylinders with an external helical magnetic field are studied by using the quasistatic approximation. It is shown numerically that for the spiral base flow with a zero axial pressure gradient, the helical magnetic field yields a helical traveling wave at a critical Reynolds number. This wave mode is revealed to be the most unstable mode by linear stability analysis. At higher Reynolds numbers, the first wave mode is superposed by a second antisymmetric helical wave mode, which travels with a higher phase velocity than the first mode. When the Reynolds number is increased further, the flow becomes turbulent, but the key features of the flow structure are still dominated by the first and the second wave modes. Furthermore, when a finite axial pressure gradient is applied to guarantee a zero axial flow rate, the annulus flow is found to be more unstable than the case with zero axial pressure gradient.}, } @article {pmid29347758, year = {2017}, author = {Elperin, T and Kleeorin, N and Liberman, M and Lipatnikov, AN and Rogachevskii, I and Yu, R}, title = {Turbulent diffusion of chemically reacting flows: Theory and numerical simulations.}, journal = {Physical review. E}, volume = {96}, number = {5-1}, pages = {053111}, doi = {10.1103/PhysRevE.96.053111}, pmid = {29347758}, issn = {2470-0053}, abstract = {The theory of turbulent diffusion of chemically reacting gaseous admixtures developed previously [T. Elperin et al., Phys. Rev. E 90, 053001 (2014)PLEEE81539-375510.1103/PhysRevE.90.053001] is generalized for large yet finite Reynolds numbers and the dependence of turbulent diffusion coefficient on two parameters, the Reynolds number and Damköhler number (which characterizes a ratio of turbulent and reaction time scales), is obtained. Three-dimensional direct numerical simulations (DNSs) of a finite-thickness reaction wave for the first-order chemical reactions propagating in forced, homogeneous, isotropic, and incompressible turbulence are performed to validate the theoretically predicted effect of chemical reactions on turbulent diffusion. It is shown that the obtained DNS results are in good agreement with the developed theory.}, } @article {pmid29347655, year = {2017}, author = {Plan, ELCVM and Musacchio, S and Vincenzi, D}, title = {Emergence of chaos in a viscous solution of rods.}, journal = {Physical review. E}, volume = {96}, number = {5-1}, pages = {053108}, doi = {10.1103/PhysRevE.96.053108}, pmid = {29347655}, issn = {2470-0053}, abstract = {It is shown that the addition of small amounts of microscopic rods in a viscous fluid at low Reynolds number causes a significant increase of the flow resistance. Numerical simulations of the dynamics of the solution reveal that this phenomenon is associated to a transition from laminar to chaotic flow. Polymer stresses give rise to flow instabilities which, in turn, perturb the alignment of the rods. This coupled dynamics results in the activation of a wide range of scales, which enhances the mixing efficiency of viscous flows.}, } @article {pmid29347433, year = {2017}, author = {Fouxon, I and Ge, Z and Brandt, L and Leshansky, A}, title = {Integral representation of channel flow with interacting particles.}, journal = {Physical review. E}, volume = {96}, number = {6-1}, pages = {063110}, doi = {10.1103/PhysRevE.96.063110}, pmid = {29347433}, issn = {2470-0053}, abstract = {We construct a boundary integral representation for the low-Reynolds-number flow in a channel in the presence of freely suspended particles (or droplets) of arbitrary size and shape. We demonstrate that lubrication theory holds away from the particles at horizontal distances exceeding the channel height and derive a multipole expansion of the flow which is dipolar to the leading approximation. We show that the dipole moment of an arbitrary particle is a weighted integral of the stress and the flow at the particle surface, which can be determined numerically. We introduce the equation of motion that describes hydrodynamic interactions between arbitrary, possibly different, distant particles, with interactions determined by the product of the mobility matrix and the dipole moment. Further, the problem of three identical interacting spheres initially aligned in the streamwise direction is considered and the experimentally observed "pair exchange" phenomenon is derived analytically and confirmed numerically. For nonaligned particles, we demonstrate the formation of a configuration with one particle separating from a stable pair. Our results suggest that in a dilute initially homogenous particulate suspension flowing in a channel the particles will eventually separate into singlets and pairs.}, } @article {pmid29347297, year = {2017}, author = {Hiruta, Y and Toh, S}, title = {Intermittent direction reversals of moving spatially localized turbulence observed in two-dimensional Kolmogorov flow.}, journal = {Physical review. E}, volume = {96}, number = {6-1}, pages = {063112}, doi = {10.1103/PhysRevE.96.063112}, pmid = {29347297}, issn = {2470-0053}, abstract = {We have found that in two-dimensional Kolmogorov flow a spatially localized turbulent state (SLT) exists stably and travels with a constant speed on average switching the moving direction randomly and intermittently for moderate values of the control parameters: Reynolds number and the flow rate. We define the coarse-grained position and velocity of an SLT and separate the motion of the SLT from its internal turbulent dynamics by introducing a co-moving frame. The switching process of an SLT represented by the coarse-grained velocity seems to be a random telegraph signal. Focusing on the asymmetry of the internal turbulence we introduce two coarse-grained variables characterizing the internal dynamics. These quantities follow the switching process reasonably. This suggests that the twin attracting invariant sets each of which corresponds to a one-way traveling SLT are embedded in the attractor of the moving SLT and the connection of the two sets is too complicated to be represented by a few degrees of freedom but the motion of an SLT is correlated with the internal turbulent dynamics.}, } @article {pmid29347290, year = {2017}, author = {Schilling, O and Mueschke, NJ}, title = {Turbulent transport and mixing in transitional Rayleigh-Taylor unstable flow: A priori assessment of gradient-diffusion and similarity modeling.}, journal = {Physical review. E}, volume = {96}, number = {6-1}, pages = {063111}, doi = {10.1103/PhysRevE.96.063111}, pmid = {29347290}, issn = {2470-0053}, abstract = {Data from a 1152×760×1280 direct numerical simulation [N. J. Mueschke and O. Schilling, Phys. Fluids 21, 014106 (2009)PHFLE61070-663110.1063/1.3064120] of a Rayleigh-Taylor mixing layer modeled after a small-Atwood-number water-channel experiment is used to investigate the validity of gradient diffusion and similarity closures a priori. The budgets of the mean flow, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate transport equations across the mixing layer were previously analyzed [O. Schilling and N. J. Mueschke, Phys. Fluids 22, 105102 (2010)PHFLE61070-663110.1063/1.3484247] at different evolution times to identify the most important transport and mixing mechanisms. Here a methodology is introduced to systematically estimate model coefficients as a function of time in the closures of the dynamically significant terms in the transport equations by minimizing the L_{2} norm of the difference between the model and correlations constructed using the simulation data. It is shown that gradient-diffusion and similarity closures used for the turbulent kinetic energy K, turbulent kinetic energy dissipation rate ε, heavy-fluid mass fraction variance S, and heavy-fluid mass fraction variance dissipation rate χ equations capture the shape of the exact, unclosed profiles well over the nonlinear and turbulent evolution regimes. Using order-of-magnitude estimates [O. Schilling and N. J. Mueschke, Phys. Fluids 22, 105102 (2010)PHFLE61070-663110.1063/1.3484247] for the terms in the exact transport equations and their closure models, it is shown that several of the standard closures for the turbulent production and dissipation (destruction) must be modified to include Reynolds-number scalings appropriate for Rayleigh-Taylor flow at small to intermediate Reynolds numbers. The late-time, large Reynolds number coefficients are determined to be different from those used in shear flow applications and largely adopted in two-equation Reynolds-averaged Navier-Stokes (RANS) models of Rayleigh-Taylor turbulent mixing. In addition, it is shown that the predictions of the Boussinesq model for the Reynolds stress agree better with the data when additional buoyancy-related terms are included. It is shown that an unsteady RANS paradigm is needed to predict the transitional flow dynamics from early evolution times, analogous to the small Reynolds number modifications in RANS models of wall-bounded flows in which the production-to-dissipation ratio is far from equilibrium. Although the present study is specific to one particular flow and one set of initial conditions, the methodology could be applied to calibrations of other Rayleigh-Taylor flows with different initial conditions (which may give different results during the early-time, transitional flow stages, and perhaps asymptotic stage). The implications of these findings for developing high-fidelity eddy viscosity-based turbulent transport and mixing models of Rayleigh-Taylor turbulence are discussed.}, } @article {pmid29347258, year = {2017}, author = {Adebayo, I and Xie, Z and Che, Z and Matar, OK}, title = {Doubly excited pulse waves on thin liquid films flowing down an inclined plane: An experimental and numerical study.}, journal = {Physical review. E}, volume = {96}, number = {1-1}, pages = {013118}, doi = {10.1103/PhysRevE.96.013118}, pmid = {29347258}, issn = {2470-0053}, abstract = {The interaction patterns between doubly excited pulse waves on thin liquid films flowing down an inclined plane are studied both experimentally and numerically. The effect of varying the film flow rate, interpulse interval, and substrate inclination angle on the pulse interaction patterns is examined. Our results show that different interaction patterns exist for these binary pulses, which include solitary wave behavior, partial or complete pulse coalescence, and pulse noncoalescence. A regime map of these patterns is plotted for each inclination angle examined, parametrized by the film Reynolds number and interpulse interval. Finally, the individual effect of the system parameters mentioned above on the coalescence distance of binary pulses in the "complete pulse coalescence" mode is studied; the results are compared to numerical simulations of the two-dimensional Navier-Stokes equations yielding good agreement.}, } @article {pmid29347222, year = {2017}, author = {Huang, Y and Wang, L and Schmitt, FG and Zheng, X and Jiang, N and Liu, Y}, title = {Extremal-point density of scaling processes: From fractional Brownian motion to turbulence in one dimension.}, journal = {Physical review. E}, volume = {96}, number = {1-1}, pages = {012215}, doi = {10.1103/PhysRevE.96.012215}, pmid = {29347222}, issn = {2470-0053}, abstract = {In recent years several local extrema-based methodologies have been proposed to investigate either the nonlinear or the nonstationary time series for scaling analysis. In the present work, we study systematically the distribution of the local extrema for both synthesized scaling processes and turbulent velocity data from experiments. The results show that for the fractional Brownian motion (fBm) without intermittency correction the measured extremal-point-density (EPD) agrees well with a theoretical prediction. For a multifractal random walk (MRW) with the lognormal statistics, the measured EPD is independent of the intermittency parameter μ, suggesting that the intermittency correction does not change the distribution of extremal points but changes the amplitude. By introducing a coarse-grained operator, the power-law behavior of these scaling processes is then revealed via the measured EPD for different scales. For fBm the scaling exponent ξ(H) is found to be ξ(H)=H, where H is Hurst number, while for MRW ξ(μ) shows a linear relation with the intermittency parameter μ. Such EPD approach is further applied to the turbulent velocity data obtained from a wind tunnel flow experiment with the Taylor scale λ-based Reynolds number Re_{λ} =720, and a turbulent boundary layer with the momentum thickness θ based Reynolds number Re_{θ} =810. A scaling exponent ξ≃0.37 is retrieved for the former case. For the latter one, the measured EPD shows clearly four regimes, which agrees well with the corresponding sublayer structures inside the turbulent boundary layer.}, } @article {pmid29347180, year = {2017}, author = {Saito, S and Abe, Y and Koyama, K}, title = {Lattice Boltzmann modeling and simulation of liquid jet breakup.}, journal = {Physical review. E}, volume = {96}, number = {1-1}, pages = {013317}, doi = {10.1103/PhysRevE.96.013317}, pmid = {29347180}, issn = {2470-0053}, abstract = {A three-dimensional color-fluid lattice Boltzmann model for immiscible two-phase flows is developed in the framework of a three-dimensional 27-velocity (D3Q27) lattice. The collision operator comprises the D3Q27 versions of three suboperators: a multiple-relaxation-time (MRT) collision operator, a generalized Liu-Valocchi-Kang perturbation operator, and a Latva-Kokko-Rothman recoloring operator. A D3Q27 version of an enhanced equilibrium distribution function is also incorporated into this model to improve the Galilean invariance. Three types of numerical tests, namely, a static droplet, an oscillating droplet, and the Rayleigh-Taylor instability, show a good agreement with analytical solutions and numerical simulations. Following these numerical tests, this model is applied to liquid-jet-breakup simulations. The simulation conditions are matched to the conditions of the previous experiments. In this case, numerical stability is maintained throughout the simulation, although the kinematic viscosity for the continuous phase is set as low as 1.8×10^{-4}, in which case the corresponding Reynolds number is 3.4×10^{3} ; the developed lattice Boltzmann model based on the D3Q27 lattice enables us to perform the simulation with parameters directly matched to the experiments. The jet's liquid column transitions from an asymmetrical to an axisymmetrical shape, and entrainment occurs from the side of the jet. The measured time history of the jet's leading-edge position shows a good agreement with the experiments. Finally, the reproducibility of the regime map for liquid-liquid systems is assessed. The present lattice Boltzmann simulations well reproduce the characteristics of predicted regimes, including varicose breakup, sinuous breakup, and atomization.}, } @article {pmid29346972, year = {2017}, author = {Coreixas, C and Wissocq, G and Puigt, G and Boussuge, JF and Sagaut, P}, title = {Recursive regularization step for high-order lattice Boltzmann methods.}, journal = {Physical review. E}, volume = {96}, number = {3-1}, pages = {033306}, doi = {10.1103/PhysRevE.96.033306}, pmid = {29346972}, issn = {2470-0053}, abstract = {A lattice Boltzmann method (LBM) with enhanced stability and accuracy is presented for various Hermite tensor-based lattice structures. The collision operator relies on a regularization step, which is here improved through a recursive computation of nonequilibrium Hermite polynomial coefficients. In addition to the reduced computational cost of this procedure with respect to the standard one, the recursive step allows to considerably enhance the stability and accuracy of the numerical scheme by properly filtering out second- (and higher-) order nonhydrodynamic contributions in under-resolved conditions. This is first shown in the isothermal case where the simulation of the doubly periodic shear layer is performed with a Reynolds number ranging from 10^{4} to 10^{6}, and where a thorough analysis of the case at Re=3×10^{4} is conducted. In the latter, results obtained using both regularization steps are compared against the Bhatnagar-Gross-Krook LBM for standard (D2Q9) and high-order (D2V17 and D2V37) lattice structures, confirming the tremendous increase of stability range of the proposed approach. Further comparisons on thermal and fully compressible flows, using the general extension of this procedure, are then conducted through the numerical simulation of Sod shock tubes with the D2V37 lattice. They confirm the stability increase induced by the recursive approach as compared with the standard one.}, } @article {pmid29346921, year = {2017}, author = {Maleki, M and Martinuzzi, RJ and Herzog, W and Federico, S}, title = {Orthotropic hydraulic permeability of arrays of parallel cylinders.}, journal = {Physical review. E}, volume = {96}, number = {3-1}, pages = {033112}, doi = {10.1103/PhysRevE.96.033112}, pmid = {29346921}, issn = {2470-0053}, abstract = {Approximate analytical methods are presented to calculate the overall orthotropic hydraulic permeability of a flow with low Reynolds number, passing through a bundle of parallel circular cylinders. Two particular distributions are considered: (i) arrays with ordered rectangular lattices and (ii) irregular nonrandom distributions for which the unit cell cross sections are elliptical. The standard unit cell models, originally developed by Happel and Kuwabara for a random distribution of cylinders, are adapted to the case of nonrandom distributions. The drag force on a representative cylinder in a direction perpendicular to its axis is obtained based on the standard unit cell model: the actual unit cell of rectangular or elliptical cross section is replaced with an "equivalent" cylindrical unit cell of diameter equal to the maximum width of the actual unit cell. Using the obtained drag forces and referring back to the original geometry of the unit cell, closed-form approximate expressions for the overall permeabilities in the perpendicular directions are obtained. Numerical comparisons with more sophisticated approaches confirm the good efficiency of the presented approach, especially in the range of low solid volume fraction, i.e., of high porosity. Previous studies have revealed that, for the parallel fluid flow, the variation of permeability with aspect ratio (or in general the lateral arrangement) of parallel cylinders is generally weak. These observations suggest that Happel's model for parallel permeability in a random distribution of cylinders could be a good approximation for parallel permeabilities in nonrandom distributions with the same volume fraction.}, } @article {pmid29346864, year = {2017}, author = {Yu, Z and Lin, Z and Shao, X and Wang, LP}, title = {Effects of particle-fluid density ratio on the interactions between the turbulent channel flow and finite-size particles.}, journal = {Physical review. E}, volume = {96}, number = {3-1}, pages = {033102}, doi = {10.1103/PhysRevE.96.033102}, pmid = {29346864}, issn = {2470-0053}, abstract = {A parallel direct-forcing fictitious domain method is employed to perform fully resolved numerical simulations of turbulent channel flow laden with finite-size particles. The effects of the particle-fluid density ratio on the turbulence modulation in the channel flow are investigated at the friction Reynolds number of 180, the particle volume fraction of 0.84%, and the particle-fluid density ratio ranging from 1 to 104.2. The results show that the variation of the flow drag with the particle-fluid density ratio is not monotonic, with a larger flow drag for the density ratio of 10.42, compared to those of unity and 104.2. A significant drag reduction by the particles is observed for large particle-fluid density ratios during the transient stage, but not at the statistically stationary stage. The intensity of particle velocity fluctuations generally decreases with increasing particle inertia, except that the particle streamwise root-mean-square velocity and streamwise-transverse velocity correlation in the near-wall region are largest at the density ratio of the order of 10. The averaged momentum equations are derived with the spatial averaging theorem and are used to analyze the mechanisms for the effects of the particles on the flow drag. The results indicate that the drag-reduction effect due to the decrease in the fluid Reynolds shear stress is counteracted by the drag-enhancement effect due to the increase in the total particle stress or the interphase drag force for the large particle-inertia case. The sum of the total Reynolds stress and particle inner stress contributions to the flow drag is largest at the density ratio of the order of 10, which is the reason for the largest flow drag at this density ratio. The interphase drag force obtained from the averaged momentum equation (the balance theory) is significantly smaller than (but agrees qualitatively with) that from the empirical drag formula based on the phase-averaged slip velocity for large density ratios. For the neutrally buoyant case, the balance theory predicts a positive interphase force on the particles arising from the negative gradient of the particle inner stress, which cannot be predicted by the drag formula based on the phase-averaged slip velocity. In addition, our results show that both particle collision and particle-turbulence interaction play roles in the formation of the inhomogeneous distribution of the particles at the density ratio of the order of 10.}, } @article {pmid29343852, year = {2018}, author = {Hamilton, JK and Bryan, MT and Gilbert, AD and Ogrin, FY and Myers, TO}, title = {A new class of magnetically actuated pumps and valves for microfluidic applications.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {933}, doi = {10.1038/s41598-018-19506-8}, pmid = {29343852}, issn = {2045-2322}, abstract = {We propose a new class of magnetically actuated pumps and valves that could be incorporated into microfluidic chips with no further external connections. The idea is to repurpose ferromagnetic low Reynolds number swimmers as devices capable of generating fluid flow, by restricting the swimmers' translational degrees of freedom. We experimentally investigate the flow structure generated by a pinned swimmer in different scenarios, such as unrestricted flow around it as well as flow generated in straight, cross-shaped, Y-shaped and circular channels. This demonstrates the feasibility of incorporating the device into a channel and its capability of acting as a pump, valve and flow splitter. Different regimes could be selected by tuning the frequency and amplitude of the external magnetic field driving the swimmer, or by changing the channel orientation with respect to the field. This versatility endows the device with varied functionality which, together with the robust remote control and reproducibility, makes it a promising candidate for several applications.}, } @article {pmid29341736, year = {2017}, author = {Yakhot, V and Donzis, D}, title = {Emergence of Multiscaling in a Random-Force Stirred Fluid.}, journal = {Physical review letters}, volume = {119}, number = {4}, pages = {044501}, doi = {10.1103/PhysRevLett.119.044501}, pmid = {29341736}, issn = {1079-7114}, abstract = {We consider the transition to strong turbulence in an infinite fluid stirred by a Gaussian random force. The transition is defined as a first appearance of anomalous scaling of normalized moments of velocity derivatives (dissipation rates) emerging from the low-Reynolds-number Gaussian background. It is shown that, due to multiscaling, strongly intermittent rare events can be quantitatively described in terms of an infinite number of different "Reynolds numbers" reflecting a multitude of anomalous scaling exponents. The theoretically predicted transition disappears at R_{λ} ≤3. The developed theory is in quantitative agreement with the outcome of large-scale numerical simulations.}, } @article {pmid29333202, year = {2017}, author = {Raffiee, AH and Dabiri, S and Ardekani, AM}, title = {Elasto-inertial migration of deformable capsules in a microchannel.}, journal = {Biomicrofluidics}, volume = {11}, number = {6}, pages = {064113}, doi = {10.1063/1.5004572}, pmid = {29333202}, issn = {1932-1058}, abstract = {In this paper, we study the dynamics of deformable cells in a channel flow of Newtonian and polymeric fluids and unravel the effects of deformability, elasticity, inertia, and size on the cell motion. We investigate the role of polymeric fluids on the cell migration behavior and the performance of inertial microfluidic devices. Our results show that the equilibrium position of the cell is on the channel diagonal, in contrast to that of rigid particles, which is on the center of the channel faces for the same range of Reynolds number. A constant-viscosity polymeric fluid, modeled using an Oldroyd-B constitutive equation, drives the cells toward the channel centerline, while a shear-thinning polymeric fluid, modeled using a Giesekus constitutive equation, pushes the cells toward the channel wall. The findings of this paper suggest that the addition of polymers in microfluidic devices can be used to enhance the throughput of cell focusing and separation devices at a low cost. This study provides an insight on the role of rheological properties of the fluid and the ways that they can be tuned to control the focal position of the cells.}, } @article {pmid29323150, year = {2018}, author = {Mrokowska, MM}, title = {Stratification-induced reorientation of disk settling through ambient density transition.}, journal = {Scientific reports}, volume = {8}, number = {1}, pages = {412}, doi = {10.1038/s41598-017-18654-7}, pmid = {29323150}, issn = {2045-2322}, abstract = {Settling due to gravity force is a basic transport mechanism of solid particles in fluids in the Earth. A large portion of particles occurring in nature and used in technical applications are non-spherical. Settling of particles is usually studied in homogeneous ambient conditions, however, stratification is inherent of natural fluids. It has been acknowledged that stratification modifies the velocity of settling spheres and amorphous aggregates. However, the effect of particle shape on the dynamics of settling through density-stratified ambient fluid has not been recognized well enough. Here I show experimental evidence that continuous density transition markedly modifies the settling dynamics of a disk in terms of settling velocity and orientation of a particle. Settling dynamics of a disk are more complex than dynamics of spheres and aggregates studied previously. I found that in a two-layer ambient with density transition, a disk settling in a low Reynolds number regime undergoes five phases of settling with the orientation varying from horizontal to vertical, and it may achieve two local minimum settling velocities in the density transition layer. Moreover, I found that the settling dynamics depends on a density difference between upper and lower homogeneous layers, stratification strength and thickness of density transition.}, } @article {pmid29291432, year = {2018}, author = {Ranjit, NK and Shit, GC and Tripathi, D}, title = {Joule heating and zeta potential effects on peristaltic blood flow through porous micro vessels altered by electrohydrodynamic.}, journal = {Microvascular research}, volume = {117}, number = {}, pages = {74-89}, doi = {10.1016/j.mvr.2017.12.004}, pmid = {29291432}, issn = {1095-9319}, abstract = {In most of the medical therapies, electromagnetic field plays important role to modulate the blood flow and to reduce the pain of human body. With this fact, this paper presents a mathematical model to study the peristaltic blood flow through porous microvessels in the presence of electrohydrodynamics. The effects of Joule heating and different zeta potential are also considered. Darcy law is employed for porous medium. The mathematical analysis is carried out in the form of electroosmosis, flow analysis and heat transfer analysis. Velocity slip conditions are imposed to solve momentum equation and thermal energy equation. Time dependent volumetric flow rate is considered which varies exponentially. Closed form solutions for potential function is obtained under Debye-Hückel approximation and velocity and temperature fields are obtained under low Reynolds number and large wavelength approximations. The influence of Hartmann number, electroosmotic parameter, slip parameters, Zeta potential, and couple stress parameter on flow characteristics, pumping characteristics and trapping phenomenon is computed. The effects of thermal slip parameters, Joule heating parameter, and Brinkman number on heat transfer characteristics are also presented graphically. Finally, the effect of Brinkman number on a graph between Nusselt number and Joule heating parameter is examined.}, } @article {pmid29289052, year = {2017}, author = {Hamilton, E and Bruot, N and Cicuta, P}, title = {The chimera state in colloidal phase oscillators with hydrodynamic interaction.}, journal = {Chaos (Woodbury, N.Y.)}, volume = {27}, number = {12}, pages = {123108}, doi = {10.1063/1.4989466}, pmid = {29289052}, issn = {1089-7682}, abstract = {The chimera state is the incongruous situation where coherent and incoherent populations coexist in sets of identical oscillators. Using driven non-linear oscillators interacting purely through hydrodynamic forces at low Reynolds number, previously studied as a simple model of motile cilia supporting waves, we find concurrent incoherent and synchronised subsets in small arrays. The chimeras seen in simulation display a "breathing" aspect, reminiscent of uniformly interacting phase oscillators. In contrast to other systems where chimera has been observed, this system has a well-defined interaction metric, and we know that the emergent dynamics inherit the symmetry of the underlying Oseen tensor eigenmodes. The chimera state can thus be connected to a superposition of eigenstates, whilst considering the mean interaction strength within and across subsystems allows us to make a connection to more generic (and abstract) chimeras in populations of Kuramoto phase oscillators. From this work, we expect the chimera state to emerge in experimental observations of oscillators coupled through hydrodynamic forces.}, } @article {pmid29286796, year = {2017}, author = {Djellouli, A and Marmottant, P and Djeridi, H and Quilliet, C and Coupier, G}, title = {Buckling Instability Causes Inertial Thrust for Spherical Swimmers at All Scales.}, journal = {Physical review letters}, volume = {119}, number = {22}, pages = {224501}, doi = {10.1103/PhysRevLett.119.224501}, pmid = {29286796}, issn = {1079-7114}, abstract = {Microswimmers, and among them aspirant microrobots, generally have to cope with flows where viscous forces are dominant, characterized by a low Reynolds number (Re). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities. Here, we propose a swimming mechanism which uses the buckling instability triggered by pressure waves to propel a spherical, hollow shell. With a macroscopic experimental model, we show that a net displacement is produced at all Re regimes. An optimal displacement caused by nontrivial history effects is reached at intermediate Re. We show that, due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high-frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm/s for a remote-controlled microrobot, a suitable value for biological applications such as drug delivery.}, } @article {pmid29286719, year = {2017}, author = {Atif, M and Kolluru, PK and Thantanapally, C and Ansumali, S}, title = {Essentially Entropic Lattice Boltzmann Model.}, journal = {Physical review letters}, volume = {119}, number = {24}, pages = {240602}, doi = {10.1103/PhysRevLett.119.240602}, pmid = {29286719}, issn = {1079-7114}, abstract = {The entropic lattice Boltzmann model (ELBM), a discrete space-time kinetic theory for hydrodynamics, ensures nonlinear stability via the discrete time version of the second law of thermodynamics (the H theorem). Compliance with the H theorem is numerically enforced in this methodology and involves a search for the maximal discrete path length corresponding to the zero dissipation state by iteratively solving a nonlinear equation. We demonstrate that an exact solution for the path length can be obtained by assuming a natural criterion of negative entropy change, thereby reducing the problem to solving an inequality. This inequality is solved by creating a new framework for construction of Padé approximants via quadrature on appropriate convex function. This exact solution also resolves the issue of indeterminacy in case of nonexistence of the entropic involution step. Since our formulation is devoid of complex mathematical library functions, the computational cost is drastically reduced. To illustrate this, we have simulated a model setup of flow over the NACA-0012 airfoil at a Reynolds number of 2.88×10^{6} .}, } @article {pmid29271639, year = {2018}, author = {Kim, JA and Lee, JR and Je, TJ and Jeon, EC and Lee, W}, title = {Size-Dependent Inertial Focusing Position Shift and Particle Separations in Triangular Microchannels.}, journal = {Analytical chemistry}, volume = {90}, number = {3}, pages = {1827-1835}, doi = {10.1021/acs.analchem.7b03851}, pmid = {29271639}, issn = {1520-6882}, abstract = {A recent study of inertial microfluidics within nonrectangular cross-section channels showed that the inertial focusing positions changes with cross-sectional shapes; therefore, the cross-sectional shape can be a useful control parameter for microfluidic particle manipulations. Here, we conducted detail investigation on unique focusing position shift phenomena, which occurs strongly in channels with the cross-sectional shape of the isosceles right triangle. The top focusing positions shift along the channel walls to the direction away from the apex with increasing Reynolds number and decreasing particle size. A larger particle with its center further away from the side walls experiences shear gradient lift toward the apex, which leads to an opposite result with changes of Reynolds and particle size. The focusing position shift and the subsequent stabilization of corner focusing lead to changes in the number of focusing positions, which enables a novel method for microparticle separations with high efficiency (>95%) and resolution (<2 μm). The separation method based on equilibrium focusing; therefore, the operation is simple and no complex separation optimization is needed. Moreover, the separation threshold can be easily modulated with flow rate adjustment. Rare cell separation from blood cell was successfully demonstrated with spiked MCF-7 cells in blood by achieving the yield of ∼95% and the throughput of ∼106 cells/min.}, } @article {pmid29264667, year = {2018}, author = {Oakes, JM and Roth, SC and Shadden, SC}, title = {Airflow Simulations in Infant, Child, and Adult Pulmonary Conducting Airways.}, journal = {Annals of biomedical engineering}, volume = {46}, number = {3}, pages = {498-512}, doi = {10.1007/s10439-017-1971-9}, pmid = {29264667}, issn = {1573-9686}, abstract = {The airway structure continuously evolves from birth to adulthood, influencing airflow dynamics and respiratory mechanics. We currently know very little about how airflow patterns change throughout early life and its impact on airway resistance, namely because of experimental limitations. To uncover differences in respiratory dynamics between age groups, we performed subject-specific airflow simulations in an infant, child, and adult conducting airways. Airflow throughout the respiration cycle was calculated by coupling image-based models of the conducting airways to the global respiratory mechanics, where flow was driven by a pressure differential. Trachea diameter was 19, 9, and 4.5 mm for the adult (36 years, female), child (6 years, male), and infant (0.25 years, female), respectively. Mean Reynolds number within the trachea was nearly the same for each subject (1100) and Womersley number was above unity for all three subjects and largest for the adult, highlighting the significance of transient effects. In general, air speeds and airway resistances within the conducting airways were inversely correlated with age; the 3D pressure drop was highest in the infant model. These simulations provide new insight into age-dependent flow dynamics throughout the respiration cycle within subject-specific airways.}, } @article {pmid29258534, year = {2017}, author = {Sass, LR and Khani, M and Natividad, GC and Tubbs, RS and Baledent, O and Martin, BA}, title = {A 3D subject-specific model of the spinal subarachnoid space with anatomically realistic ventral and dorsal spinal cord nerve rootlets.}, journal = {Fluids and barriers of the CNS}, volume = {14}, number = {1}, pages = {36}, doi = {10.1186/s12987-017-0085-y}, pmid = {29258534}, issn = {2045-8118}, support = {1R44MH112210-01A1//National Institute of Mental Health/ ; 4U54GM104944-04TBD//National Institute of General Medical Sciences/ ; P20GM1033408//National Institute of General Medical Sciences/ ; P20 GM103408/GM/NIGMS NIH HHS/United States ; Vandal Ideas Project//University of Idaho/ ; R44 MH112210/MH/NIMH NIH HHS/United States ; U54 GM104944/GM/NIGMS NIH HHS/United States ; }, mesh = {Adult ; Female ; Humans ; Imaging, Three-Dimensional ; Magnetic Resonance Imaging ; *Models, Anatomic ; *Models, Neurological ; Spinal Cord/*anatomy & histology/diagnostic imaging ; Spinal Nerve Roots/*anatomy & histology/diagnostic imaging ; Subarachnoid Space/*anatomy & histology/diagnostic imaging ; Young Adult ; }, abstract = {BACKGROUND: The spinal subarachnoid space (SSS) has a complex 3D fluid-filled geometry with multiple levels of anatomic complexity, the most salient features being the spinal cord and dorsal and ventral nerve rootlets. An accurate anthropomorphic representation of these features is needed for development of in vitro and numerical models of cerebrospinal fluid (CSF) dynamics that can be used to inform and optimize CSF-based therapeutics.

METHODS: A subject-specific 3D model of the SSS was constructed based on high-resolution anatomic MRI. An expert operator completed manual segmentation of the CSF space with detailed consideration of the anatomy. 31 pairs of semi-idealized dorsal and ventral nerve rootlets (NR) were added to the model based on anatomic reference to the magnetic resonance (MR) imaging and cadaveric measurements in the literature. Key design criteria for each NR pair included the radicular line, descending angle, number of NR, attachment location along the spinal cord and exit through the dura mater. Model simplification and smoothing was performed to produce a final model with minimum vertices while maintaining minimum error between the original segmentation and final design. Final model geometry and hydrodynamics were characterized in terms of axial distribution of Reynolds number, Womersley number, hydraulic diameter, cross-sectional area and perimeter.

RESULTS: The final model had a total of 139,901 vertices with a total CSF volume within the SSS of 97.3 cm3. Volume of the dura mater, spinal cord and NR was 123.1, 19.9 and 5.8 cm3. Surface area of these features was 318.52, 112.2 and 232.1 cm2 respectively. Maximum Reynolds number was 174.9 and average Womersley number was 9.6, likely indicating presence of a laminar inertia-dominated oscillatory CSF flow field.

CONCLUSIONS: This study details an anatomically realistic anthropomorphic 3D model of the SSS based on high-resolution MR imaging of a healthy human adult female. The model is provided for re-use under the Creative Commons Attribution-ShareAlike 4.0 International license (CC BY-SA 4.0) and can be used as a tool for development of in vitro and numerical models of CSF dynamics for design and optimization of intrathecal therapeutics.}, } @article {pmid29244030, year = {2018}, author = {Shidhore, TC and Christov, IC}, title = {Static response of deformable microchannels: a comparative modelling study.}, journal = {Journal of physics. Condensed matter : an Institute of Physics journal}, volume = {30}, number = {5}, pages = {054002}, doi = {10.1088/1361-648X/aaa226}, pmid = {29244030}, issn = {1361-648X}, abstract = {We present a comparative modelling study of fluid-structure interactions in microchannels. Through a mathematical analysis based on plate theory and the lubrication approximation for low-Reynolds-number flow, we derive models for the flow rate-pressure drop relation for long shallow microchannels with both thin and thick deformable top walls. These relations are tested against full three-dimensional two-way-coupled fluid-structure interaction simulations. Three types of microchannels, representing different elasticity regimes and having been experimentally characterized previously, are chosen as benchmarks for our theory and simulations. Good agreement is found in most cases for the predicted, simulated and measured flow rate-pressure drop relationships. The numerical simulations performed allow us to also carefully examine the deformation profile of the top wall of the microchannel in any cross section, showing good agreement with the theory. Specifically, the prediction that span-wise displacement in a long shallow microchannel decouples from the flow-wise deformation is confirmed, and the predicted scaling of the maximum displacement with the hydrodynamic pressure and the various material and geometric parameters is validated.}, } @article {pmid29239446, year = {2018}, author = {Denn, MM and Morris, JF and Bonn, D}, title = {Shear thickening in concentrated suspensions of smooth spheres in Newtonian suspending fluids.}, journal = {Soft matter}, volume = {14}, number = {2}, pages = {170-184}, doi = {10.1039/c7sm00761b}, pmid = {29239446}, issn = {1744-6848}, abstract = {Shear thickening is a phenomenon in which the viscosity of a suspension increases with increasing stress or shear rate, sometimes in a discontinuous fashion. While the phenomenon, when observed in suspensions of corn starch in water, or Oobleck, is popular as a science experiment for children, shear thickening is actually of considerable importance for technological applications and exhibited by far simpler systems. Concentrated suspensions of smooth hard spheres will exhibit shear thickening, and understanding this behavior has required a fundamental change in the paradigm of describing low-Reynolds-number solid-fluid flows, in which contact forces have traditionally been absent. Here, we provide an overview of our understanding of shear thickening and the methods that have been developed to describe it, as well as outstanding questions.}, } @article {pmid29219534, year = {2017}, author = {Reeves, MT and Billam, TP and Yu, X and Bradley, AS}, title = {Enstrophy Cascade in Decaying Two-Dimensional Quantum Turbulence.}, journal = {Physical review letters}, volume = {119}, number = {18}, pages = {184502}, doi = {10.1103/PhysRevLett.119.184502}, pmid = {29219534}, issn = {1079-7114}, abstract = {We report evidence for an enstrophy cascade in large-scale point-vortex simulations of decaying two-dimensional quantum turbulence. Devising a method to generate quantum vortex configurations with kinetic energy narrowly localized near a single length scale, the dynamics are found to be well characterized by a superfluid Reynolds number Re_{s} that depends only on the number of vortices and the initial kinetic energy scale. Under free evolution the vortices exhibit features of a classical enstrophy cascade, including a k^{-3} power-law kinetic energy spectrum, and constant enstrophy flux associated with inertial transport to small scales. Clear signatures of the cascade emerge for N≳500 vortices. Simulating up to very large Reynolds numbers (N=32 768 vortices), additional features of the classical theory are observed: the Kraichnan-Batchelor constant is found to converge to C^{'} ≈1.6, and the width of the k^{-3} range scales as Re_{s} ^{1/2} .}, } @article {pmid29219520, year = {2017}, author = {Matsunaga, D and Meng, F and Zöttl, A and Golestanian, R and Yeomans, JM}, title = {Focusing and Sorting of Ellipsoidal Magnetic Particles in Microchannels.}, journal = {Physical review letters}, volume = {119}, number = {19}, pages = {198002}, doi = {10.1103/PhysRevLett.119.198002}, pmid = {29219520}, issn = {1079-7114}, abstract = {We present a simple method to control the position of ellipsoidal magnetic particles in microchannel Poiseuille flow at low Reynolds number using a static uniform magnetic field. The magnetic field is utilized to pin the particle orientation, and the hydrodynamic interactions between ellipsoids and channel walls allow control of the transverse position of the particles. We employ a far-field hydrodynamic theory and simulations using the boundary element method and Brownian dynamics to show how magnetic particles can be focused and segregated by size and shape. This is of importance for particle manipulation in lab-on-a-chip devices.}, } @article {pmid29201603, year = {2017}, author = {Wang, Q and Yang, L and Yu, J and Zhang, L}, title = {Characterizing dynamic behaviors of three-particle paramagnetic microswimmer near a solid surface.}, journal = {Robotics and biomimetics}, volume = {4}, number = {1}, pages = {20}, doi = {10.1186/s40638-017-0076-0}, pmid = {29201603}, issn = {2197-3768}, abstract = {Particle-based magnetically actuated microswimmers have the potential to act as microrobotic tools for biomedical applications. In this paper, we report the dynamic behaviors of a three-particle paramagnetic microswimmer. Actuated by a rotating magnetic field with different frequencies, the microswimmer exhibits simple rotation and propulsion. When the input frequency is below 8 Hz, it exhibits simple rotation on the substrate, whereas it shows propulsion with varied poses when subjected to a frequency between 8 and 15 Hz. Furthermore, a solid surface that enhances swimming velocity was observed as the microswimmer is actuated near a solid surface. Our simulation results testify that the surface-enhanced swimming near a solid surface is because of the induced pressure difference in the surrounding fluid of the microagent.}, } @article {pmid29195661, year = {2018}, author = {Tarafder, A}, title = {A study on the onset of turbulent conditions with supercritical fluid chromatography mobile-phases.}, journal = {Journal of chromatography. A}, volume = {1532}, number = {}, pages = {182-190}, doi = {10.1016/j.chroma.2017.11.056}, pmid = {29195661}, issn = {1873-3778}, mesh = {Carbon Dioxide/chemistry ; Chromatography, Supercritical Fluid/*methods ; Pressure ; *Rheology ; Solvents/*chemistry ; Temperature ; Viscosity ; }, abstract = {Following a recent publication [1], the topic of turbulent flow in SFC has generated both interest and questions. Liquid-like density, coupled with significantly low viscosity of CO2-based mobile-phases may result in high Reynolds number (Re) - higher than what represents laminar flow conditions, reaching the so-called turbulent regions. Although such turbulent flows can form only in the connecting tubings, thus not directly affecting the chromatographic process, it is important to know under many situations, whether the flow inside the tubing is laminar or turbulent. In this report a comprehensive guideline to identify the possibilities of turbulent flow conditions is provided through a series of charts. Flow properties depend on state conditions (composition, pressure and temperature) and also on the tubing material and geometry. Here guidelines to detect the onset of turbulent conditions is provided for cylindrical stainless-steel tubings of different internal diameters (i.d.) under a wide range of SFC mobile-phase conditions.}, } @article {pmid29195464, year = {2017}, author = {Zhao, S and Cheng, E and Qiu, X and Burnett, I and Liu, JC}, title = {Wind noise spectra in small Reynolds number turbulent flows.}, journal = {The Journal of the Acoustical Society of America}, volume = {142}, number = {5}, pages = {3227}, doi = {10.1121/1.5012740}, pmid = {29195464}, issn = {1520-8524}, abstract = {Wind noise spectra caused by wind from fans in indoor environments have been found to be different from those measured in outdoor atmospheric conditions. Although many models have been developed to predict outdoor wind noise spectra under the assumption of large Reynolds number [Zhao, Cheng, Qiu, Burnett, and Liu (2016). J. Acoust. Soc. Am. 140, 4178-4182, and the references therein], they cannot be applied directly to the indoor situations because the Reynolds number of wind from fans in indoor environments is usually much smaller than that experienced in atmospheric turbulence. This paper proposes a pressure structure function model that combines the energy-containing and dissipation ranges so that the pressure spectrum for small Reynolds number turbulent flows can be calculated. The proposed pressure structure function model is validated with the experimental results in the literature, and then the obtained pressure spectrum is verified with the numerical simulation and experiment results. It is demonstrated that the pressure spectrum obtained from the proposed pressure structure function model can be utilized to estimate wind noise spectra caused by turbulent flows with small Reynolds numbers.}, } @article {pmid29183943, year = {2017}, author = {Boselli, F and Steed, E and Freund, JB and Vermot, J}, title = {Anisotropic shear stress patterns predict the orientation of convergent tissue movements in the embryonic heart.}, journal = {Development (Cambridge, England)}, volume = {144}, number = {23}, pages = {4322-4327}, doi = {10.1242/dev.152124}, pmid = {29183943}, issn = {1477-9129}, mesh = {Animals ; Anisotropy ; Biomechanical Phenomena ; Endocardial Cushions/cytology/embryology ; Endothelial Cells/cytology/physiology ; Erythrocytes/physiology ; Heart/*embryology ; Hemodynamics ; Hydrodynamics ; Imaging, Three-Dimensional ; *Models, Cardiovascular ; Organogenesis/physiology ; Shear Strength ; Stress, Mechanical ; Zebrafish/*embryology ; }, abstract = {Myocardial contractility and blood flow provide essential mechanical cues for the morphogenesis of the heart. In general, endothelial cells change their migratory behavior in response to shear stress patterns, according to flow directionality. Here, we assessed the impact of shear stress patterns and flow directionality on the behavior of endocardial cells, the specialized endothelial cells of the heart. At the early stages of zebrafish heart valve formation, we show that endocardial cells are converging to the valve-forming area and that this behavior depends upon mechanical forces. Quantitative live imaging and mathematical modeling allow us to correlate this tissue convergence with the underlying flow forces. We predict that tissue convergence is associated with the direction of the mean wall shear stress and of the gradient of harmonic phase-averaged shear stresses, which surprisingly do not match the overall direction of the flow. This contrasts with the usual role of flow directionality in vascular development and suggests that the full spatial and temporal complexity of the wall shear stress should be taken into account when studying endothelial cell responses to flow in vivo.}, } @article {pmid29181289, year = {2017}, author = {Li, K and Jing, D and Hu, J and Ding, X and Yao, Z}, title = {Numerical investigation of the tribological performance of micro-dimple textured surfaces under hydrodynamic lubrication.}, journal = {Beilstein journal of nanotechnology}, volume = {8}, number = {}, pages = {2324-2338}, doi = {10.3762/bjnano.8.232}, pmid = {29181289}, issn = {2190-4286}, abstract = {Surface texturing is an important approach for controlling the tribological behavior of friction pairs used in mechanical and biological engineering. In this study, by utilizing the method of three-dimensional computational fluid dynamics (CFD) simulation, the lubrication model of a friction pair with micro-dimple array was established based on the Navier-Stokes equations. The typical pressure distribution of the lubricant film was analyzed. It was found that a positive hydrodynamic pressure is generated in the convergent part of the micro-dimple, while a negative hydrodynamic pressure is generated in the divergent part. With suitable parameters, the total integration of the pressure is positive, which can increase the load-carrying capacity of a friction pair. The effects of the micro-dimple parameters as well as fluid properties on tribological performance were investigated. It was concluded that under the condition of hydrodynamic lubrication, the main mechanism for the improvement in the tribological performance is the combined effects of wedging and recirculation. Within the range of parameters investigated in this study, the optimum texture density is 13%, while the optimum aspect ratio varies with the Reynolds number. For a given Reynolds number, there exists a combination of texture density and aspect ratio at which the optimum tribological performance could be obtained. Conclusions from this study could be helpful for the design of texture parameters in mechanical friction components and even in artificial joints.}, } @article {pmid29178057, year = {2017}, author = {Goldobin, DS}, title = {Existence of the passage to the limit of an inviscid fluid.}, journal = {The European physical journal. E, Soft matter}, volume = {40}, number = {11}, pages = {103}, doi = {10.1140/epje/i2017-11594-4}, pmid = {29178057}, issn = {1292-895X}, abstract = {In the dynamics of a viscous fluid, the case of vanishing kinematic viscosity is actually equivalent to the Reynolds number tending to infinity. Hence, in the limit of vanishing viscosity the fluid flow is essentially turbulent. On the other hand, the Euler equation, which is conventionally adopted for the description of the flow of an inviscid fluid, does not possess proper turbulent behaviour. This raises the question of the existence of the passage to the limit of an inviscid fluid for real low-viscosity fluids. To address this question, one should employ the theory of turbulent boundary layer near an inflexible boundary (e.g., rigid wall). On the basis of this theory, one can see how the solutions to the Euler equation become relevant for the description of the flow of low-viscosity fluids, and obtain the small parameter quantifying accuracy of this description for real fluids.}, } @article {pmid29167371, year = {2017}, author = {De Canio, G and Lauga, E and Goldstein, RE}, title = {Spontaneous oscillations of elastic filaments induced by molecular motors.}, journal = {Journal of the Royal Society, Interface}, volume = {14}, number = {136}, pages = {}, doi = {10.1098/rsif.2017.0491}, pmid = {29167371}, issn = {1742-5662}, abstract = {It is known from the wave-like motion of microtubules in motility assays that the piconewton forces that motors produce can be sufficient to bend the filaments. In cellular phenomena such as cytosplasmic streaming, molecular motors translocate along cytoskeletal filaments, carrying cargo which entrains fluid. When large numbers of such forced filaments interact through the surrounding fluid, as in particular stages of oocyte development in Drosophila melanogaster, complex dynamics are observed, but the detailed mechanics underlying them has remained unclear. Motivated by these observations, we study here perhaps the simplest model for these phenomena: an elastic filament, pinned at one end, acted on by a molecular motor treated as a point force. Because the force acts tangential to the filament, no matter what its shape, this 'follower-force' problem is intrinsically non-variational, and thereby differs fundamentally from Euler buckling, where the force has a fixed direction, and which, in the low-Reynolds-number regime, ultimately leads to a stationary, energy-minimizing shape. Through a combination of linear stability theory, analytical study of a solvable simplified 'two-link' model and numerical studies of the full elastohydrodynamic equations of motion, we elucidate the Hopf bifurcation that occurs with increasing forcing of a filament, leading to flapping motion analogous to the high-Reynolds-number oscillations of a garden hose with a free end.}, } @article {pmid29144304, year = {2017}, author = {Wang, Z and Dong, W and Hu, X and Sun, T and Wang, T and Sun, Y}, title = {Low energy consumption vortex wave flow membrane bioreactor.}, journal = {Water science and technology : a journal of the International Association on Water Pollution Research}, volume = {76}, number = {9-10}, pages = {2465-2472}, doi = {10.2166/wst.2017.400}, pmid = {29144304}, issn = {0273-1223}, mesh = {Bioreactors ; Membranes, Artificial ; Waste Water/*chemistry ; Water Pollutants, Chemical/chemistry ; Water Purification/instrumentation/*methods ; }, abstract = {In order to reduce the energy consumption and membrane fouling of the conventional membrane bioreactor (MBR), a kind of low energy consumption vortex wave flow MBR was exploited based on the combination of biofilm process and membrane filtration process, as well as the vortex wave flow technique. The experimental results showed that the vortex wave flow state in the membrane module could be formed when the Reynolds number (Re) of liquid was adjusted between 450 and 1,050, and the membrane flux declined more slowly in the vortex wave flow state than those in the laminar flow state and turbulent flow state. The MBR system was used to treat domestic wastewater under the condition of vortex wave flow state for 30 days. The results showed that the removal efficiency for CODcr and NH3-N was 82% and 98% respectively, and the permeate quality met the requirement of 'Water quality standard for urban miscellaneous water consumption (GB/T 18920-2002)'. Analysis of the energy consumption of the MBR showed that the average energy consumption was 1.90 ± 0.55 kWh/m3 (permeate), which was only two thirds of conventional MBR energy consumption.}, } @article {pmid29131890, year = {2018}, author = {Gamage, PPT and Khalili, F and Khurshidul Azad, MD and Mansy, HA}, title = {Modeling Inspiratory Flow in a Porcine Lung Airway.}, journal = {Journal of biomechanical engineering}, volume = {140}, number = {6}, pages = {}, doi = {10.1115/1.4038431}, pmid = {29131890}, issn = {1528-8951}, abstract = {Inspiratory flow in a multigeneration pig lung airways was numerically studied at a steady inlet flow rate of 3.2 × 10-4 m3/s corresponding to a Reynolds number of 1150 in the trachea. The model was validated by comparing velocity distributions with previous measurements and simulations in simplified airway geometries. Simulation results provided detailed maps of the axial and secondary flow patterns at different cross sections of the airway tree. The vortex core regions in the airways were visualized using absolute helicity values and suggested the presence of secondary flow vortices where two counter-rotating vortices were observed at the main bifurcation and in many other bifurcations. Both laminar and turbulent flows were considered. Results showed that axial and secondary flows were comparable in the laminar and turbulent cases. Turbulent kinetic energy (TKE) vanished in the more distal airways, which indicates that the flow in these airways approaches laminar flow conditions. The simulation results suggested viscous pressure drop values comparable to earlier studies. The monopodial asymmetric nature of airway branching in pigs resulted in airflow patterns that are different from the less asymmetric human airways. The major daughters of the pig airways tended to have high airflow ratios, which may lead to different particle distribution and sound generation patterns. These differences need to be taken into consideration when interpreting the results of animal studies involving pigs before generalizing these results to humans.}, } @article {pmid29128680, year = {2018}, author = {Han, X and Fang, H and He, G and Reible, D}, title = {Effects of roughness and permeability on solute transfer at the sediment water interface.}, journal = {Water research}, volume = {129}, number = {}, pages = {39-50}, doi = {10.1016/j.watres.2017.10.049}, pmid = {29128680}, issn = {1879-2448}, abstract = {Understanding the mechanisms of solute transfer across the sediment-water interface plays a crucial role in water quality prediction and management. In this study, different arranged particles are used to form typical rough and permeable beds. Large Eddy Simulation (LES) is used to model the solute transfer from the overlying water to sediment beds. Three rough wall turbulence regimes, i.e., smooth, transitional and rough regime, are separately considered and the effects of bed roughness on solute transfer are quantitatively analyzed. Results show that the classic laws related to Schmidt numbers can well reflect the solute transfer under the smooth regime with small roughness Reynolds numbers. Under the transitional regime, the solute transfer coefficient (KL+) is enhanced and the effect of Schmidt number is weakened by increasing roughness Reynolds number. Under the rough regime, the solute transfer is suppressed by the transition layer (Brinkman layer) and controlled by the bed permeability. Moreover, it is found that water depth, friction velocity and bed permeability can be used to estimate the solute transfer velocity (KL) under the completely rough regime.}, } @article {pmid29117039, year = {2017}, author = {Korakianitis, T and Rezaienia, MA and Paul, GM and Avital, EJ and Rothman, MT and Mozafari, S}, title = {Optimization of Axial Pump Characteristic Dimensions and Induced Hemolysis for Mechanical Circulatory Support Devices.}, journal = {ASAIO journal (American Society for Artificial Internal Organs : 1992)}, volume = {}, number = {}, pages = {}, doi = {10.1097/MAT.0000000000000719}, pmid = {29117039}, issn = {1538-943X}, abstract = {The application of axial pumps as ventricular assist devices (VADs) requires significant modifications to the size and characteristics of industrial pumps due to the difference in flow fields of industrial and medical pumps. Industrial pumps operate in the region of Reynolds number Re = 108, whereas axial blood pumps operate in Re < 10. The common pump design technique is to rely on the performance of previously designed pumps using the concept of fluid dynamic similarity. Such data are available for industrial pumps as specific speed-specific diameter (ns-ds) graphs. The difference between the flow fields of industrial and medical pumps makes the industrial ns-ds graphs unsuitable for medical pumps and consequently several clinically available axial blood pumps operate with low efficiencies. In this article, numerical and experimental techniques were used to design 62 axial pump impellers with different design characteristics suitable for VADs and mechanical circulatory support devices (MCSDs). The impellers were manufactured and experimentally tested in various operating conditions of flow, pressure, and rotational speed. The hemocompatibility of the impellers was numerically investigated by modeling shear stress and hemolysis. The highest efficiency of each pump impeller was plotted on an ns-ds diagram. The nondimensional results presented in this article enable preliminary design of efficient and hemocompatible axial flow pumps for VADs and MCSDs.}, } @article {pmid29104418, year = {2017}, author = {Luo, K and Hu, C and Wu, F and Fan, J}, title = {Direct numerical simulation of turbulent boundary layer with fully resolved particles at low volume fraction.}, journal = {Physics of fluids (Woodbury, N.Y. : 1994)}, volume = {29}, number = {5}, pages = {053301}, doi = {10.1063/1.4982233}, pmid = {29104418}, issn = {1070-6631}, } @article {pmid29061390, year = {2017}, author = {Watson, DJ and Sazonov, I and Zawieja, DC and Moore, JE and van Loon, R}, title = {Integrated geometric and mechanical analysis of an image-based lymphatic valve.}, journal = {Journal of biomechanics}, volume = {64}, number = {}, pages = {172-179}, doi = {10.1016/j.jbiomech.2017.09.040}, pmid = {29061390}, issn = {1873-2380}, support = {U01 HL123420/HL/NHLBI NIH HHS/United States ; }, mesh = {Algorithms ; Biomechanical Phenomena ; Compliance ; Computer Simulation ; Humans ; Image Processing, Computer-Assisted ; Lymph/physiology ; Lymphatic Vessels/anatomy & histology/*physiology ; Microscopy, Confocal ; Models, Biological ; Pressure ; }, abstract = {Lymphatic valves facilitate the lymphatic system's role in maintaining fluid homeostasis. Malformed valves are found in several forms of primary lymphœdema, resulting in incurable swelling of the tissues and immune dysfunction. Their experimental study is complicated by their small size and operation in low pressure and low Reynolds number environments. Mathematical models of these structures can give insight and complement experimentation. In this work, we present the first valve geometry reconstructed from confocal imagery and used in the construction of a subject-specific model in a closing mode. A framework is proposed whereby an image is converted into a valve model. An FEA study was performed to identify the significance of the shear modulus, the consequences of smoothing the leaflet surface and the effect of wall motion on valve behaviour. Smoothing is inherent to any analysis from imagery. The nature of the image, segmentation and meshing all cause attenuation of high-frequency features. Smoothing not only causes loss of surface area but also the loss of high-frequency geometric features which may reduce stiffness. This work aimed to consider these effects and inform studies by taking a manual reconstruction and through manifold harmonic analysis, attenuating higher frequency features to replicate lower resolution images or lower degree-of-freedom reconstructions. In conclusion, two metrics were considered: trans-valvular pressure required to close the valve, ΔPc, and the retrograde volume displacement after closure. The higher ΔPc, the greater the volume of lymph that will pass through the valve during closure. Retrograde volume displacement after closure gives a metric of compliance of the valve and for the quality of the valve seal. In the case of the image-specific reconstructed valve, removing features with a wavelength longer than four μm caused changes in ΔPc. Varying the shear modulus from 10 kPa to 60 kPa caused a 3.85-fold increase in the retrograde volume displaced. The inclusion of a non-rigid wall caused ΔPc to increase from 1.56 to 2.52 cmH2O.}, } @article {pmid29054787, year = {2018}, author = {Chen, Y and Li, Y and Valocchi, AJ and Christensen, KT}, title = {Lattice Boltzmann simulations of liquid CO2 displacing water in a 2D heterogeneous micromodel at reservoir pressure conditions.}, journal = {Journal of contaminant hydrology}, volume = {212}, number = {}, pages = {14-27}, doi = {10.1016/j.jconhyd.2017.09.005}, pmid = {29054787}, issn = {1873-6009}, abstract = {We employed the color-fluid lattice Boltzmann multiphase model to simulate liquid CO2 displacing water documented in experiments in a 2D heterogeneous micromodel at reservoir pressure conditions. The main purpose is to investigate whether lattice Boltzmann simulation can reproduce the CO2 invasion patterns observed in these experiments for a range of capillary numbers. Although the viscosity ratio used in the simulation matches the experimental conditions, the viscosity of the fluids in the simulation is higher than that of the actual fluids used in the experiments. Doing so is required to enhance numerical stability, and is a common strategy employed in the literature when using the lattice Boltzmann method to simulate CO2 displacing water. The simulations reproduce qualitatively similar trends of changes in invasion patterns as the capillary number is increased. However, the development of secondary CO2 pathways, a key feature of the invasion patterns in the simulations and experiments, is found to occur at a much higher capillary number in the simulations compared with the experiments. Additional numerical simulations were conducted to investigate the effect of the absolute value of viscosity on the invasion patterns while maintaining the viscosity ratio and capillary number fixed. These results indicate that the use of a high viscosity (which significantly reduces the inertial effect in the simulations) suppresses the development of secondary CO2 pathways, leading to a different fluid distribution compared with corresponding experiments at the same capillary number. Therefore, inertial effects are not negligible in drainage process with liquid CO2 and water despite the low Reynolds number based on the average velocity, as the local velocity can be much higher due to Haines jump events. These higher velocities, coupled with the low viscosity of CO2, further amplifies the inertial effect. Therefore, we conclude that caution should be taken when using proxy fluids that only rely on the capillary number and viscosity ratio in both experiment and simulation.}, } @article {pmid29052556, year = {2017}, author = {Chen, D and Kolomenskiy, D and Nakata, T and Liu, H}, title = {Forewings match the formation of leading-edge vortices and dominate aerodynamic force production in revolving insect wings.}, journal = {Bioinspiration & biomimetics}, volume = {13}, number = {1}, pages = {016009}, doi = {10.1088/1748-3190/aa94d7}, pmid = {29052556}, issn = {1748-3190}, abstract = {In many flying insects, forewings and hindwings are coupled mechanically to achieve flapping flight synchronously while being driven by action of the forewings. How the forewings and hindwings as well as their morphologies contribute to aerodynamic force production and flight control remains unclear. Here we address the point that the forewings can produce most of the aerodynamic forces even with the hindwings removed through a computational fluid dynamic study of three revolving insect wing models, which are identical to the wing morphologies and Reynolds numbers of hawkmoth (Manduca sexta), bumblebee (Bombus ignitus) and fruitfly (Drosophila melanogaster). We find that the forewing morphologies match the formation of leading-edge vortices (LEV) and are responsible for generating sufficient lift forces at the mean angles of attack and the Reynolds numbers where the three representative insects fly. The LEV formation and pressure loading keep almost unchanged with the hindwing removed, and even lead to some improvement in power factor and aerodynamic efficiency. Moreover, our results indicate that the size and strength of the LEVs can be well quantified with introduction of a conical LEV angle, which varies remarkably with angles of attack and Reynolds numbers but within the forewing region while showing less sensitivity to the wing morphologies. This implies that the forewing morphology very likely plays a dominant role in achieving low-Reynolds number aerodynamic performance in natural flyers as well as in revolving and/or flapping micro air vehicles.}, } @article {pmid29036075, year = {2017}, author = {Prasad, S}, title = {Extended Taylor frozen-flow hypothesis and statistics of optical phase in aero-optics.}, journal = {Journal of the Optical Society of America. A, Optics, image science, and vision}, volume = {34}, number = {6}, pages = {931-942}, pmid = {29036075}, issn = {1520-8532}, abstract = {We present an extended Taylor frozen-flow model for the statistics of the spatiotemporal disturbances of the index of refraction of air and the phase of an optical beam propagated through the turbulent boundary and shear layers in a high-Reynolds-number flow. By incorporating rapid random fluctuations of the flow velocity about a mean convection velocity and an anisotropic spatial power spectrum for the index of refraction, we calculate both the short-delay temporal structure function and the power spectral density of these disturbances. We discuss the predicted scaling behaviors for these quantities in the context of existing experimental observations, showing specifically the agreement of these predictions with some optical phase data obtained by the Airborne Aero-Optical Laboratory.}, } @article {pmid29027922, year = {2017}, author = {Song, Y and Zhao, K and Zuo, J and Wang, C and Li, Y and Miao, X and Zhao, X}, title = {The Detection of Water Flow in Rectangular Microchannels by Terahertz Time Domain Spectroscopy.}, journal = {Sensors (Basel, Switzerland)}, volume = {17}, number = {10}, pages = {}, doi = {10.3390/s17102330}, pmid = {29027922}, issn = {1424-8220}, abstract = {Flow characteristics of water were tested in a rectangular microchannel for Reynolds number (Re) between 0 and 446 by terahertz time domain spectroscopy (THz-TDS). Output THz peak trough intensities and the calculated absorbances of the flow were analyzed theoretically. The results show a rapid change for Re < 250 and a slow change as Re increases, which is caused by the early transition from laminar to transition flow beginning nearly at Re = 250. Then this finding is confirmed in the plot of the flow resistant. Our results demonstrate that the THz-TDS could be a valuable tool to monitor and character the flow performance in microscale structures.}, } @article {pmid29027748, year = {2018}, author = {Middleton, K and Kondiboyina, A and Borrett, M and Cui, Y and Mei, X and You, L}, title = {Microfluidics approach to investigate the role of dynamic similitude in osteocyte mechanobiology.}, journal = {Journal of orthopaedic research : official publication of the Orthopaedic Research Society}, volume = {36}, number = {2}, pages = {663-671}, doi = {10.1002/jor.23773}, pmid = {29027748}, issn = {1554-527X}, abstract = {Fluid flow is an important regulator of cell function and metabolism in many tissues. Fluid shear stresses have been used to level the mechanical stimuli applied in vitro with what occurs in vivo. However, these experiments often lack dynamic similarity, which is necessary to ensure the validity of the model. For interstitial fluid flow, the major requirement for dynamic similarity is the Reynolds number (Re), the ratio of inertial to viscous forces, is the same between the system and model. To study the necessity of dynamic similarity for cell mechanotransduction studies, we investigated the response of osteocyte-like MLO-Y4 cells to different Re flows at the same level of fluid shear stress. Osteocytes were chosen for this study as flows applied in vitro and in vivo have Re that are orders of magnitude different. We hypothesize that osteocytes' response to fluid flow is Re dependent. We observed that cells exposed to lower and higher Re flows developed rounded and triangular morphologies, respectively. Lower Re flows also reduced apoptosis rates compared to higher Re flows. Furthermore, MLO-Y4 cells exposed to higher Re flows had stronger calcium responses compared to lower Re flows. However, by also controlling for flow rate, the lower Re flows induced a stronger calcium response; while degradation of components of the osteocyte glycocalyx reversed this effect. This work suggests that osteocytes are highly sensitive to differences in Re, independent of just shear stresses, supporting the need for improved in vitro flow platforms that better recapitulate the physiological environment. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:663-671, 2018.}, } @article {pmid28989310, year = {2017}, author = {Manikantan, H and Squires, TM}, title = {Irreversible particle motion in surfactant-laden interfaces due to pressure-dependent surface viscosity.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {473}, number = {2205}, pages = {20170346}, doi = {10.1098/rspa.2017.0346}, pmid = {28989310}, issn = {1364-5021}, abstract = {The surface shear viscosity of an insoluble surfactant monolayer often depends strongly on its surface pressure. Here, we show that a particle moving within a bounded monolayer breaks the kinematic reversibility of low-Reynolds-number flows. The Lorentz reciprocal theorem allows such irreversibilities to be computed without solving the full nonlinear equations, giving the leading-order contribution of surface pressure-dependent surface viscosity. In particular, we show that a disc translating or rotating near an interfacial boundary experiences a force in the direction perpendicular to that boundary. In unbounded monolayers, coupled modes of motion can also lead to non-intuitive trajectories, which we illustrate using an interfacial analogue of the Magnus effect. This perturbative approach can be extended to more complex geometries, and to two-dimensional suspensions more generally.}, } @article {pmid28982172, year = {2017}, author = {Khan, NB and Ibrahim, Z and Nguyen, LTT and Javed, MF and Jameel, M}, title = {Numerical investigation of the vortex-induced vibration of an elastically mounted circular cylinder at high Reynolds number (Re = 104) and low mass ratio using the RANS code.}, journal = {PloS one}, volume = {12}, number = {10}, pages = {e0185832}, doi = {10.1371/journal.pone.0185832}, pmid = {28982172}, issn = {1932-6203}, mesh = {*Elasticity ; Models, Theoretical ; *Vibration ; }, abstract = {This study numerically investigates the vortex-induced vibration (VIV) of an elastically mounted rigid cylinder by using Reynolds-averaged Navier-Stokes (RANS) equations with computational fluid dynamic (CFD) tools. CFD analysis is performed for a fixed-cylinder case with Reynolds number (Re) = 104 and for a cylinder that is free to oscillate in the transverse direction and possesses a low mass-damping ratio and Re = 104. Previously, similar studies have been performed with 3-dimensional and comparatively expensive turbulent models. In the current study, the capability and accuracy of the RANS model are validated, and the results of this model are compared with those of detached eddy simulation, direct numerical simulation, and large eddy simulation models. All three response branches and the maximum amplitude are well captured. The 2-dimensional case with the RANS shear-stress transport k-w model, which involves minimal computational cost, is reliable and appropriate for analyzing the characteristics of VIV.}, } @article {pmid28981539, year = {2017}, author = {Islam, S and Nazeer, G and Ying, ZC and Islam, Z and Manzoor, R}, title = {Transitions in the flow patterns and aerodynamic characteristics of the flow around staggered rows of cylinders.}, journal = {PloS one}, volume = {12}, number = {10}, pages = {e0184169}, doi = {10.1371/journal.pone.0184169}, pmid = {28981539}, issn = {1932-6203}, mesh = {Computer Simulation ; *Models, Theoretical ; *Physical Phenomena ; }, abstract = {A two-dimensional numerical study of flow across rows of identical square cylinders arranged in staggered fashion is carried out. This study will unreveal complex flow physics depending upon the Reynolds number (Re) and gap spacing (g) between the cylinders. The combined effect of Reynolds number and gap spacing on the flow physics around staggered rows of cylinders are numerically studied for 20 ≤ Re ≤ 140 and 1 ≤ g ≤ 6. We use the lattice Boltzmann method for numerical computations. It is found that with increase in gap spacing between the cylinders the critical Reynolds number for the onset of vortex shedding also increases. We observed a strong effect of Reynolds number at g = 2 and 4. Secondary cylinder interaction frequency disappears for large Reynolds number at g = 6 and 5 and the flow around cylinders are fully dominated by the primary vortex shedding frequency. This ensures that at large gap spacing with an increase in the Reynolds number the wakes interaction between and behind the cylinders is weaken. Furthermore, it also ensures that the wake interaction behind the cylinders is strongly influenced by the jets in the gap spacing between the cylinders. We also found that g = 2 is the critical gap spacing for flow across rows of staggered square cylinders for the considered range of Reynolds number. Depending on the Reynolds number we observed; synchronous, quasi-periodic-I, quasi-periodic-II, and chaotic flow patterns. In synchronous flow pattern, an in-phase and anti-phase characteristics of consecutive cylinders has been observed. The important physical parameters are also analyzed and discussed in detail.}, } @article {pmid28949715, year = {2017}, author = {Boffetta, G and Musacchio, S}, title = {Chaos and Predictability of Homogeneous-Isotropic Turbulence.}, journal = {Physical review letters}, volume = {119}, number = {5}, pages = {054102}, doi = {10.1103/PhysRevLett.119.054102}, pmid = {28949715}, issn = {1079-7114}, abstract = {We study the chaoticity and the predictability of a turbulent flow on the basis of high-resolution direct numerical simulations at different Reynolds numbers. We find that the Lyapunov exponent of turbulence, which measures the exponential separation of two initially close solutions of the Navier-Stokes equations, grows with the Reynolds number of the flow, with an anomalous scaling exponent, larger than the one obtained on dimensional grounds. For large perturbations, the error is transferred to larger, slower scales, where it grows algebraically generating an "inverse cascade" of perturbations in the inertial range. In this regime, our simulations confirm the classical predictions based on closure models of turbulence. We show how to link chaoticity and predictability of a turbulent flow in terms of a finite size extension of the Lyapunov exponent.}, } @article {pmid28905060, year = {2017}, author = {Baber, R and Mazzei, L and Thanh, NTK and Gavriilidis, A}, title = {An engineering approach to synthesis of gold and silver nanoparticles by controlling hydrodynamics and mixing based on a coaxial flow reactor.}, journal = {Nanoscale}, volume = {9}, number = {37}, pages = {14149-14161}, doi = {10.1039/c7nr04962e}, pmid = {28905060}, issn = {2040-3372}, abstract = {In this work we present a detailed study of flow technology approaches that could open up new possibilities for nanoparticle synthesis. The synthesis of gold and silver nanoparticles (NPs) in a flow device based on a coaxial flow reactor (CFR) was investigated. The CFR comprised of an outer glass tube of 2 mm inner diameter (I.D.) and an inner glass tube whose I.D. varied between 0.142 and 0.798 mm. A split and recombine (SAR) mixer and coiled flow inverter (CFI) were further employed to alter the mixing conditions after the CFR. The 'Turkevich' method was used to synthesize gold NPs, with a CFR followed by a CFI. This assembly allows control over nucleation and growth through variation of residence time. Increasing the total flow rate from 0.25 ml min-1 to 3 ml min-1 resulted initially in a constant Au NP size, and beyond 1 ml min-1 to a size increase of Au NPs from 17.9 ± 2.1 nm to 23.9 ± 4.7 nm. The temperature was varied between 60-100 °C and a minimum Au NP size of 17.9 ± 2.1 nm was observed at 80 °C. Silver NPs were synthesized in a CFR followed by a SAR mixer, using sodium borohydride to reduce silver nitrate in the presence of trisodium citrate. The SAR mixer provided an enhancement of the well-controlled laminar mixing in the CFR. Increasing silver nitrate concentration resulted in a decrease in Ag NP size from 5.5 ± 2.4 nm to 3.4 ± 1.4 nm. Different hydrodynamic conditions were studied in the CFR operated in isolation for silver NP synthesis. Increasing the Reynolds number from 132 to 530 in the inner tube created a vortex flow resulting in Ag NPs in the size range between 5.9 ± 1.5 nm to 7.7 ± 3.4 nm. Decreasing the inner tube I.D. from 0.798 mm to 0.142 mm resulted in a decrease in Ag NP size from 10.5 ± 4.0 nm to 4.7 ± 1.4 nm. Thus, changing the thickness of the inner stream enabled control over size of the Ag NPs.}, } @article {pmid28878968, year = {2017}, author = {Muir, RE and Arredondo-Galeana, A and Viola, IM}, title = {The leading-edge vortex of swift wing-shaped delta wings.}, journal = {Royal Society open science}, volume = {4}, number = {8}, pages = {170077}, doi = {10.1098/rsos.170077}, pmid = {28878968}, issn = {2054-5703}, abstract = {Recent investigations on the aerodynamics of natural fliers have illuminated the significance of the leading-edge vortex (LEV) for lift generation in a variety of flight conditions. A well-documented example of an LEV is that generated by aircraft with highly swept, delta-shaped wings. While the wing aerodynamics of a manoeuvring aircraft, a bird gliding and a bird in flapping flight vary significantly, it is believed that this existing knowledge can serve to add understanding to the complex aerodynamics of natural fliers. In this investigation, a model non-slender delta-shaped wing with a sharp leading edge is tested at low Reynolds number, along with a delta wing of the same design, but with a modified trailing edge inspired by the wing of a common swift Apus apus. The effect of the tapering swift wing on LEV development and stability is compared with the flow structure over the unmodified delta wing model through particle image velocimetry. For the first time, a leading-edge vortex system consisting of a dual or triple LEV is recorded on a swift wing-shaped delta wing, where such a system is found across all tested conditions. It is shown that the spanwise location of LEV breakdown is governed by the local chord rather than Reynolds number or angle of attack. These findings suggest that the trailing-edge geometry of the swift wing alone does not prevent the common swift from generating an LEV system comparable with that of a delta-shaped wing.}, } @article {pmid28869874, year = {2017}, author = {Xiang, J and Liu, K and Li, D and Du, J}, title = {Effects of micro-structure on aerodynamics of Coccinella septempunctata elytra (ladybird) in forward flight as assessed via electron microscopy.}, journal = {Micron (Oxford, England : 1993)}, volume = {102}, number = {}, pages = {21-34}, doi = {10.1016/j.micron.2017.08.003}, pmid = {28869874}, issn = {1878-4291}, abstract = {The effects of micro-structure on aerodynamics of Coccinella septempunctata (Coleoptera: Coccinellidae) elytra in forward flight were investigated. The micro-structure was examined by a scanning electron microscope and a digital microscope. Based on the experimental results, five elytron models were constructed to separately investigate the effects of the camber and the local corrugation in both leading edge and trailing edge on aerodynamics. Computational fluid dynamic simulations of five elytron models were conducted by solving the Reynolds-Averaged Navier-Stokes equations with the Reynolds number of 245. The results show that camber and the local corrugation in the leading edge play significant roles in improving the aerodynamic performance, while the local corrugation in the trailing edge has little effect on aerodynamics.}, } @article {pmid28862154, year = {2017}, author = {Chung, EG and Ryu, S}, title = {Stalk-length-dependence of the contractility of Vorticella convallaria.}, journal = {Physical biology}, volume = {14}, number = {6}, pages = {066002}, doi = {10.1088/1478-3975/aa89b8}, pmid = {28862154}, issn = {1478-3975}, mesh = {Biomechanical Phenomena ; Calcium/*metabolism ; Energy Metabolism ; Oligohymenophorea/*physiology ; Stress, Mechanical ; }, abstract = {Vorticella convallaria is a sessile protozoan of which the spasmoneme contracts on a millisecond timescale. Because this contraction is induced and powered by the binding of calcium ions (Ca2+), the spasmoneme showcases Ca2+-powered cellular motility. Because the isometric tension of V. convallaria increases linearly with its stalk length, it is hypothesized that the contractility of V. convallaria during unhindered contraction depends on the stalk length. In this study, the contractile force and energetics of V. convallaria cells of different stalk lengths were evaluated using a fluid dynamic drag model which accounts for the unsteadiness and finite Reynolds number of the water flow caused by contracting V. convallaria and the wall effect of the no-slip substrate. It was found that the contraction displacement, peak contraction speed, peak contractile force, total mechanical work, and peak power depended on the stalk length. The observed stalk-length-dependencies were simulated using a damped spring model, and the model estimated that the average spring constant of the contracting stalk was 1.34 nN µm-1. These observed length-dependencies of Vorticella's key contractility parameters reflect the biophysical mechanism of the spasmonemal contraction, and thus they should be considered in developing a theoretical model of the Vorticella spasmoneme.}, } @article {pmid28852432, year = {2017}, author = {Malvar, S and Gontijo, RG and Carmo, BS and Cunha, FR}, title = {On the kinematics-wave motion of living particles in suspension.}, journal = {Biomicrofluidics}, volume = {11}, number = {4}, pages = {044112}, doi = {10.1063/1.4997715}, pmid = {28852432}, issn = {1932-1058}, abstract = {This work presents theoretical and experimental analyses on the kinematics-wave motion of suspended active particles in a biological fluid. The fluid is an active suspension of nematodes immersed in a gel-like biological structure, moving at a low Reynolds number. The nematode chosen for the study is Caenorhabditis elegans. Its motion is subjected to the time reversibility of creeping flows. We investigate how this worm reacts to this reversibility condition in order to break the flow symmetry and move in the surrounding fluid. We show that the relationship between the length of an individual nematode and the wavelength of its motion is linear and can be fitted by a theoretical prediction proposed in this work. We provide a deep discussion regarding the propulsion mechanics based on a scaling analysis that identifies three major forces acting on an individual nematode. These forces are a viscous force, a yield stress force due to gelification of agar molecules in the gel-like medium, and a bending force associated with the muscular tension imposed by the nematodes in the medium. By the scalings, we identify the most relevant physical parameters of the nematode's motion. In order to examine and quantify the motion, dynamical system tools such as FFT are used in the present analysis. The motion characterization is performed by examining (or studying) two different populations: (i) in the absence of food with starving nematodes and (ii) with well-fed nematodes. In addition, several kinematic quantities of the head, center of mass, and tail for a sample of nematodes are also investigated: their slip velocities, wavelengths, trajectories, frequency spectra, and mean curvatures. The main findings of this work are the confirmation of a linear relationship between the nematode's physical length and its motion wavelength, the identification of secondary movements in high frequencies that helps breaking the time-reversibility in which the worms are bonded, and the observation and interpretation of a systematic difference between the individual motion of well-fed and starving nematodes.}, } @article {pmid28850622, year = {2017}, author = {Aftab, SMA and Ahmad, KA}, title = {CFD study on NACA 4415 airfoil implementing spherical and sinusoidal Tubercle Leading Edge.}, journal = {PloS one}, volume = {12}, number = {8}, pages = {e0183456}, doi = {10.1371/journal.pone.0183456}, pmid = {28850622}, issn = {1932-6203}, mesh = {Animal Structures/*anatomy & histology ; Animals ; *Computer Simulation ; *Humpback Whale ; *Models, Biological ; }, abstract = {The Humpback whale tubercles have been studied for more than a decade. Tubercle Leading Edge (TLE) effectively reduces the separation bubble size and helps in delaying stall. They are very effective in case of low Reynolds number flows. The current Computational Fluid Dynamics (CFD) study is on NACA 4415 airfoil, at a Reynolds number 120,000. Two TLE shapes are tested on NACA 4415 airfoil. The tubercle designs implemented on the airfoil are sinusoidal and spherical. A parametric study is also carried out considering three amplitudes (0.025c, 0.05c and 0.075c), the wavelength (0.25c) is fixed. Structured mesh is utilized to generate grid and Transition SST turbulence model is used to capture the flow physics. Results clearly show spherical tubercles outperform sinusoidal tubercles. Furthermore experimental study considering spherical TLE is carried out at Reynolds number 200,000. The experimental results show that spherical TLE improve performance compared to clean airfoil.}, } @article {pmid28837786, year = {2017}, author = {Narla, VK and Prasad, KM and Ramana Murthy, JV}, title = {Time-dependent peristaltic analysis in a curved conduit: Application to chyme movement through intestine.}, journal = {Mathematical biosciences}, volume = {293}, number = {}, pages = {21-28}, doi = {10.1016/j.mbs.2017.08.005}, pmid = {28837786}, issn = {1879-3134}, mesh = {*Gastrointestinal Contents ; *Gastrointestinal Transit ; Intestines/*metabolism ; Magnetic Fields ; *Models, Biological ; *Peristalsis ; Time Factors ; Viscosity ; }, abstract = {A theoretical model of time-dependent peristaltic viscous fluid flow through a curved channel in the presence of an applied magnetic field is investigated. The results for stream function, pressure distribution and mechanical efficiency are obtained under the assumptions of long wavelength and low Reynolds number approximation. Pressure fluctuations due to an integral and a non-integral number of waves along the channel length are discussed under influence of channel curvature and magnetic parameter. Two inherent characteristics of peristaltic flow regimes (trapping and reflux) are discussed numerically. The mechanical efficiency of curved magnetohydrodynamic peristaltic pumping is also examined. The magnitude of pressure increases with an increasing channel curvature and magnetic parameter. Reflex phenomenon is analyzed in the Lagrangian frame of reference. It is observed that reflex in the curved channel is higher than in the straight channel. The trapped fluid in a curved channel is studied in the Eulerian frame of reference and it contains two asymmetric boluses. The size of the lower bolus grows and the upper bolus decreases with increasing effect of magnetic strength. Pumping efficiency of the peristaltic pump is low for curved channel flow than for straight channel flow. Also, the pumping efficiency is comparatively low at the high effect of the magnetic parameter.}, } @article {pmid28836761, year = {2017}, author = {de Anda, J and Lee, EY and Lee, CK and Bennett, RR and Ji, X and Soltani, S and Harrison, MC and Baker, AE and Luo, Y and Chou, T and O'Toole, GA and Armani, AM and Golestanian, R and Wong, GCL}, title = {High-Speed "4D" Computational Microscopy of Bacterial Surface Motility.}, journal = {ACS nano}, volume = {11}, number = {9}, pages = {9340-9351}, doi = {10.1021/acsnano.7b04738}, pmid = {28836761}, issn = {1936-086X}, support = {R37 AI083256/AI/NIAID NIH HHS/United States ; T32 GM008042/GM/NIGMS NIH HHS/United States ; U54 CA193417/CA/NCI NIH HHS/United States ; R01 AI102584/AI/NIAID NIH HHS/United States ; T32 GM008185/GM/NIGMS NIH HHS/United States ; }, abstract = {Bacteria exhibit surface motility modes that play pivotal roles in early-stage biofilm community development, such as type IV pili-driven "twitching" motility and flagellum-driven "spinning" and "swarming" motility. Appendage-driven motility is controlled by molecular motors, and analysis of surface motility behavior is complicated by its inherently 3D nature, the speed of which is too fast for confocal microscopy to capture. Here, we combine electromagnetic field computation and statistical image analysis to generate 3D movies close to a surface at 5 ms time resolution using conventional inverted microscopes. We treat each bacterial cell as a spherocylindrical lens and use finite element modeling to solve Maxwell's equations and compute the diffracted light intensities associated with different angular orientations of the bacterium relative to the surface. By performing cross-correlation calculations between measured 2D microscopy images and a library of computed light intensities, we demonstrate that near-surface 3D movies of Pseudomonas aeruginosa translational and rotational motion are possible at high temporal resolution. Comparison between computational reconstructions and detailed hydrodynamic calculations reveals that P. aeruginosa act like low Reynolds number spinning tops with unstable orbits, driven by a flagellum motor with a torque output of ∼2 pN μm. Interestingly, our analysis reveals that P. aeruginosa can undergo complex flagellum-driven dynamical behavior, including precession, nutation, and an unexpected taxonomy of surface motility mechanisms, including upright-spinning bacteria that diffuse laterally across the surface, and horizontal bacteria that follow helicoidal trajectories and exhibit superdiffusive movements parallel to the surface.}, } @article {pmid28815228, year = {2017}, author = {Kim, J and Hong, SO and Shim, TS and Kim, JM}, title = {Inertio-elastic flow instabilities in a 90° bent microchannel.}, journal = {Soft matter}, volume = {13}, number = {34}, pages = {5656-5664}, doi = {10.1039/c7sm01355h}, pmid = {28815228}, issn = {1744-6848}, abstract = {Biological samples having viscoelastic properties are frequently tested using microfluidic devices. In addition, viscoelastic fluids such as polymer solutions have been used as a suspending medium to spatially focus particles in microchannels. The occurrence of flow instability even at low Reynolds number is a unique property of viscoelastic fluids. In this study, we report the instability in viscoelastic flow for a channel having a 90° bent geometry, which is a characteristic of many microfluidic devices. Interestingly, we observed that the flow instability in aqueous poly(ethylene oxide) (PEO) solution occurs when the concentration of PEO is as low as 50 ppm. We systematically investigated the effects of the polymer concentration, flow rate, and elasticity number on the flow instability. The results show that the flow is stabilized in shear-thinning fluids, whereas the flow instability is amplified when both elastic and inertial effects are pronounced. We believe that this study is useful to design microfluidic devices such as cell-deformability measurement devices based on viscoelastic particle focusing.}, } @article {pmid28809710, year = {2017}, author = {Zhao, Q and Yan, S and Yuan, D and Zhang, J and Du, H and Alici, G and Li, W}, title = {Double-Mode Microparticle Manipulation by Tunable Secondary Flow in Microchannel With Arc-Shaped Groove Arrays.}, journal = {IEEE transactions on biomedical circuits and systems}, volume = {11}, number = {6}, pages = {1406-1412}, doi = {10.1109/TBCAS.2017.2722012}, pmid = {28809710}, issn = {1940-9990}, abstract = {In this paper, we proposed a microparticle manipulation approach, by which particles are able to be guided to different equilibrium positions through modulating the Reynolds number. In the microchannel with arc-shaped groove arrays, secondary flow vortex arisen due to the pressure gradient varies in the aspects of both magnitude and shape with the increase of Reynolds number. And the variation of secondary flow vortex brings about different focusing modes of microparticles in the microchannel. We investigated the focusing phenomenon experimentally and analyzed the mechanism through numerical simulations. At a high Reynolds number (Re = 127.27), the geometry-induced secondary flow rotates constantly along a direction, and most particles are guided to the equilibrium position near one side of the microchannel. However, at a low Reynolds number (Re = 2.39), the shapes of geometry-induced secondary flow vortices are obviously different, forming a variant Dean-like vortex that consists of two asymmetric counter-rotating streams in cross sections of the straight channel. Because of the periodical effects, suspended particles are concentrated at another equilibrium position on the opposite side of the microchannel. Meanwhile, the effects of particle size influence both the focusing position and quality in regimes.}, } @article {pmid28805871, year = {2017}, author = {Hanasoge, S and Ballard, M and Hesketh, PJ and Alexeev, A}, title = {Asymmetric motion of magnetically actuated artificial cilia.}, journal = {Lab on a chip}, volume = {17}, number = {18}, pages = {3138-3145}, doi = {10.1039/c7lc00556c}, pmid = {28805871}, issn = {1473-0189}, abstract = {Most microorganisms use hair-like cilia with asymmetric beating to perform vital bio-physical processes. In this paper, we demonstrate a novel fabrication method for creating magnetic artificial cilia capable of such a biologically inspired asymmetric beating pattern essential for inducing microfluidic transport at low Reynolds number. The cilia are fabricated using a lithographic process in conjunction with deposition of magnetic nickel-iron permalloy to create flexible filaments that can be manipulated by varying an external magnetic field. A rotating permanent magnet is used to actuate the cilia. We examine the kinematics of a cilium and demonstrate that the cilium motion is defined by an interplay among elastic, magnetic, and viscous forces. Specifically, the forward stroke is induced by the rotation of the magnet which bends the cilium, whereas the recovery stroke is defined by the straightening of the deformed cilium, releasing accumulated elastic potential energy. This difference in dominating forces acting during the forward stroke and the recovery stroke leads to an asymmetric beating pattern of the cilium. Such magnetic cilia can find applications in microfluidic pumping, mixing, and other fluid handling processes.}, } @article {pmid28772551, year = {2017}, author = {Ng, WL and Yeong, WY and Naing, MW}, title = {Polyvinylpyrrolidone-Based Bio-Ink Improves Cell Viability and Homogeneity during Drop-On-Demand Printing.}, journal = {Materials (Basel, Switzerland)}, volume = {10}, number = {2}, pages = {}, doi = {10.3390/ma10020190}, pmid = {28772551}, issn = {1996-1944}, abstract = {Drop-on-demand (DOD) bioprinting has attracted huge attention for numerous biological applications due to its precise control over material volume and deposition pattern in a contactless printing approach. 3D bioprinting is still an emerging field and more work is required to improve the viability and homogeneity of printed cells during the printing process. Here, a general purpose bio-ink was developed using polyvinylpyrrolidone (PVP) macromolecules. Different PVP-based bio-inks (0%-3% w/v) were prepared and evaluated for their printability; the short-term and long-term viability of the printed cells were first investigated. The Z value of a bio-ink determines its printability; it is the inverse of the Ohnesorge number (Oh), which is the ratio between the Reynolds number and a square root of the Weber number, and is independent of the bio-ink velocity. The viability of printed cells is dependent on the Z values of the bio-inks; the results indicated that the cells can be printed without any significant impairment using a bio-ink with a threshold Z value of ≤9.30 (2% and 2.5% w/v). Next, the cell output was evaluated over a period of 30 min. The results indicated that PVP molecules mitigate the cell adhesion and sedimentation during the printing process; the 2.5% w/v PVP bio-ink demonstrated the most consistent cell output over a period of 30 min. Hence, PVP macromolecules can play a critical role in improving the cell viability and homogeneity during the bioprinting process.}, } @article {pmid28754380, year = {2018}, author = {Grosjean, G and Hubert, M and Vandewalle, N}, title = {Magnetocapillary self-assemblies: Locomotion and micromanipulation along a liquid interface.}, journal = {Advances in colloid and interface science}, volume = {255}, number = {}, pages = {84-93}, doi = {10.1016/j.cis.2017.07.019}, pmid = {28754380}, issn = {1873-3727}, abstract = {This paper presents an overview and discussion of magnetocapillary self-assemblies. New results are presented, in particular concerning the possible development of future applications. These self-organizing structures possess the notable ability to move along an interface when powered by an oscillatory, uniform magnetic field. The system is constructed as follows. Soft magnetic particles are placed on a liquid interface, and submitted to a magnetic induction field. An attractive force due to the curvature of the interface around the particles competes with an interaction between magnetic dipoles. Ordered structures can spontaneously emerge from these conditions. Furthermore, time-dependent magnetic fields can produce a wide range of dynamic behaviours, including non-time-reversible deformation sequences that produce translational motion at low Reynolds number. In other words, due to a spontaneous breaking of time-reversal symmetry, the assembly can turn into a surface microswimmer. Trajectories have been shown to be precisely controllable. As a consequence, this system offers a way to produce microrobots able to perform different tasks. This is illustrated in this paper by the capture, transport and release of a floating cargo, and the controlled mixing of fluids at low Reynolds number.}, } @article {pmid28749743, year = {2017}, author = {Morley, ST and Walsh, MT and Newport, DT}, title = {Opportunities for Studying the Hydrodynamic Context for Breast Cancer Cell Spread Through Lymph Flow.}, journal = {Lymphatic research and biology}, volume = {15}, number = {3}, pages = {204-219}, doi = {10.1089/lrb.2017.0005}, pmid = {28749743}, issn = {1557-8585}, abstract = {The lymphatic system serves as the primary route for the metastatic spread of breast cancer cells (BCCs). A scarcity of information exists with regard to the advection of BCCs in lymph flow and a fundamental understanding of the response of BCCs to the forces in the lymphatics needs to be established. This review summarizes the flow environment metastatic BCCs are exposed to in the lymphatics. Special attention is paid to the behavior of cells/particles in microflows in an attempt to elucidate the behavior of BCCs under lymph flow conditions (Reynolds number <1).}, } @article {pmid28732945, year = {2017}, author = {Bulliard-Sauret, O and Ferrouillat, S and Vignal, L and Memponteil, A and Gondrexon, N}, title = {Heat transfer enhancement using 2MHz ultrasound.}, journal = {Ultrasonics sonochemistry}, volume = {39}, number = {}, pages = {262-271}, doi = {10.1016/j.ultsonch.2017.04.021}, pmid = {28732945}, issn = {1873-2828}, abstract = {The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power=110W and supplied thermal power=450W). In the same time cavitational activity due to 2MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.}, } @article {pmid28726415, year = {2017}, author = {Zhao, X and Dey, KK and Jeganathan, S and Butler, PJ and Córdova-Figueroa, UM and Sen, A}, title = {Enhanced Diffusion of Passive Tracers in Active Enzyme Solutions.}, journal = {Nano letters}, volume = {17}, number = {8}, pages = {4807-4812}, doi = {10.1021/acs.nanolett.7b01618}, pmid = {28726415}, issn = {1530-6992}, abstract = {Colloidal suspensions containing microscopic swimmers have been the focus of recent studies aimed at understanding the principles of energy transfer in fluidic media at low Reynolds number conditions. Going down in scale, active enzymes have been shown to be force-generating, nonequilibrium systems, thus offering opportunity to examine energy transfer at the ultralow Reynolds number regime. By monitoring the change of diffusion of inert tracers dispersed in active enzyme solutions, we demonstrate that the nature of energy transfer in these systems is similar to that reported for larger microscopic active systems, despite the large differences in scale, modes of energy transduction, and propulsion. Additionally, even an enzyme that catalyzes an endothermic reaction behaves analogously, suggesting that heat generation is not the primary factor for the observed enhanced tracer diffusion. Our results provide new insights into the mechanism of energy transfer at the molecular level.}, } @article {pmid28725740, year = {2016}, author = {Darr, S and Dong, J and Glikin, N and Hartwig, J and Majumdar, A and Leclair, A and Chung, J}, title = {The effect of reduced gravity on cryogenic nitrogen boiling and pipe chilldown.}, journal = {NPJ microgravity}, volume = {2}, number = {}, pages = {16033}, doi = {10.1038/npjmgrav.2016.33}, pmid = {28725740}, issn = {2373-8065}, abstract = {Manned deep space exploration will require cryogenic in-space propulsion. Yet, accurate prediction of cryogenic pipe flow boiling heat transfer is lacking, due to the absence of a cohesive reduced gravity data set covering the expected flow and thermodynamic parameter ranges needed to validate cryogenic two-phase heat transfer models. This work provides a wide range of cryogenic chilldown data aboard an aircraft flying parabolic trajectories to simulate reduced gravity. Liquid nitrogen is used to quench a 1.27 cm diameter tube from room temperature. The pressure, temperature, flow rate, and inlet conditions are reported from 10 tests covering liquid Reynolds number from 2,000 to 80,000 and pressures from 80 to 810 kPa. Corresponding terrestrial gravity tests were performed in upward, downward, and horizontal flow configurations to identify gravity and flow direction effects on chilldown. Film boiling heat transfer was lessened by up to 25% in reduced gravity, resulting in longer time and more liquid to quench the pipe to liquid temperatures. Heat transfer was enhanced by increasing the flow rate, and differences between reduced and terrestrial gravity diminished at high flow rates. The new data set will enable the development of accurate and robust heat transfer models of cryogenic pipe chilldown in reduced gravity.}, } @article {pmid28709337, year = {2017}, author = {Gürcan, ÖD}, title = {Nested polyhedra model of turbulence.}, journal = {Physical review. E}, volume = {95}, number = {6-1}, pages = {063102}, doi = {10.1103/PhysRevE.95.063102}, pmid = {28709337}, issn = {2470-0053}, abstract = {A discretization of the wave-number space is proposed, using nested polyhedra, in the form of alternating dodecahedra and icosahedra that are self-similarly scaled. This particular choice allows the possibility of forming triangles using only discretized wave vectors when the scaling between two consecutive dodecahedra is equal to the golden ratio and the icosahedron between the two dodecahedra is the dual of the inner dodecahedron. Alternatively, the same discretization can be described as a logarithmically spaced (with a scaling equal to the golden ratio), nested dodecahedron-icosahedron compounds. A wave vector which points from the origin to a vertex of such a mesh, can always find two other discretized wave vectors that are also on the vertices of the mesh (which is not true for an arbitrary mesh). Thus, the nested polyhedra grid can be thought of as a reduction (or decimation) of the Fourier space using a particular set of self-similar triads arranged approximately in a spherical form. For each vertex (i.e., discretized wave vector) in this space, there are either 9 or 15 pairs of vertices (i.e., wave vectors) with which the initial vertex can interact to form a triangle. This allows the reduction of the convolution integral in the Navier-Stokes equation to a sum over 9 or 15 interaction pairs, transforming the equation in Fourier space to a network of "interacting" nodes that can be constructed as a numerical model, which evolves each component of the velocity vector on each node of the network. This model gives the usual Kolmogorov spectrum of k^{-5/3} . Since the scaling is logarithmic, and the number of nodes for each scale is constant, a very large inertial range (i.e., a very high Reynolds number) with a much lower number of degrees of freedom can be considered. Incidentally, by assuming isotropy and a certain relation between the phases, the model can be used to systematically derive shell models.}, } @article {pmid28709335, year = {2017}, author = {Dorschner, B and Chikatamarla, SS and Karlin, IV}, title = {Entropic multirelaxation-time lattice Boltzmann method for moving and deforming geometries in three dimensions.}, journal = {Physical review. E}, volume = {95}, number = {6-1}, pages = {063306}, doi = {10.1103/PhysRevE.95.063306}, pmid = {28709335}, issn = {2470-0053}, abstract = {Entropic lattice Boltzmann methods have been developed to alleviate intrinsic stability issues of lattice Boltzmann models for under-resolved simulations. Its reliability in combination with moving objects was established for various laminar benchmark flows in two dimensions in our previous work [B. Dorschner, S. Chikatamarla, F. Bösch, and I. Karlin, J. Comput. Phys. 295, 340 (2015)JCTPAH0021-999110.1016/j.jcp.2015.04.017] as well as for three-dimensional one-way coupled simulations of engine-type geometries in B. Dorschner, F. Bösch, S. Chikatamarla, K. Boulouchos, and I. Karlin [J. Fluid Mech. 801, 623 (2016)JFLSA70022-112010.1017/jfm.2016.448] for flat moving walls. The present contribution aims to fully exploit the advantages of entropic lattice Boltzmann models in terms of stability and accuracy and extends the methodology to three-dimensional cases, including two-way coupling between fluid and structure and then turbulence and deforming geometries. To cover this wide range of applications, the classical benchmark of a sedimenting sphere is chosen first to validate the general two-way coupling algorithm. Increasing the complexity, we subsequently consider the simulation of a plunging SD7003 airfoil in the transitional regime at a Reynolds number of Re=40000 and, finally, to access the model's performance for deforming geometries, we conduct a two-way coupled simulation of a self-propelled anguilliform swimmer. These simulations confirm the viability of the new fluid-structure interaction lattice Boltzmann algorithm to simulate flows of engineering relevance.}, } @article {pmid28698630, year = {2017}, author = {Castillo-Orozco, E and Kar, A and Kumar, R}, title = {Electrospray mode transition of microdroplets with semiconductor nanoparticle suspension.}, journal = {Scientific reports}, volume = {7}, number = {1}, pages = {5144}, doi = {10.1038/s41598-017-05175-6}, pmid = {28698630}, issn = {2045-2322}, abstract = {Electrosprays operate in several modes depending on the flow rate and electric potential. This allows the deposition of droplets containing nanoparticles into discrete nanodot arrays to fabricate various electronic devices. In this study, seven different suspensions with varying properties were investigated. In the dripping mode, the normalized dropsize decreases linearly with electric capillary number, Ca e , (ratio of electric to surface tension forces) up to Ca e ≈ 1.0. The effect of viscous forces is found to be negligible in the dripping mode since the capillary number is small. For flow rates with low Reynolds number, the mode changes to microdripping mode, and then to a planar oscillating microdripping mode as Ca e increases. The normalized dropsize remains nearly constant at 0.07 for Ca e > 3.3. The microdripping mode which is important for depositing discrete array of nanodots is found to occur in the range, 2 ≤ Ca e ≤ 2.5. The droplet frequency increases steadily from dripping to microdripping mode, but stays roughly constant in the oscillating microdripping mode. This work provides a physical basis by which the flow rate and the voltage can be chosen for any nanosuspension to precisely operate in the microdripping mode at a predetermined dropsize and droplet frequency.}, } @article {pmid28690413, year = {2017}, author = {Marques, F and Meseguer, A and Mellibovsky, F and Weidman, PD}, title = {Extensional channel flow revisited: a dynamical systems perspective.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {473}, number = {2202}, pages = {20170151}, doi = {10.1098/rspa.2017.0151}, pmid = {28690413}, issn = {1364-5021}, abstract = {Extensional self-similar flows in a channel are explored numerically for arbitrary stretching-shrinking rates of the confining parallel walls. The present analysis embraces time integrations, and continuations of steady and periodic solutions unfolded in the parameter space. Previous studies focused on the analysis of branches of steady solutions for particular stretching-shrinking rates, although recent studies focused also on the dynamical aspects of the problems. We have adopted a dynamical systems perspective, analysing the instabilities and bifurcations the base state undergoes when increasing the Reynolds number. It has been found that the base state becomes unstable for small Reynolds numbers, and a transitional region including complex dynamics takes place at intermediate Reynolds numbers, depending on the wall acceleration values. The base flow instabilities are constitutive parts of different codimension-two bifurcations that control the dynamics in parameter space. For large Reynolds numbers, the restriction to self-similarity results in simple flows with no realistic behaviour, but the flows obtained in the transition region can be a valuable tool for the understanding of the dynamics of realistic Navier-Stokes solutions.}, } @article {pmid28688478, year = {2017}, author = {Akbar, NS and Butt, AW and Tripathi, D}, title = {Biomechanically driven unsteady non-uniform flow of Copper water and Silver water nanofluids through finite length channel.}, journal = {Computer methods and programs in biomedicine}, volume = {146}, number = {}, pages = {1-9}, doi = {10.1016/j.cmpb.2017.04.016}, pmid = {28688478}, issn = {1872-7565}, mesh = {Copper/*analysis ; Nanostructures/*analysis ; Pressure ; *Rheology ; Silver/*analysis ; Water/*analysis ; }, abstract = {BACKGROUND AND OBJECTIVES: This paper aims to investigate the unsteady flow of two types of nanofluids i.e Copper water nanofluids and Silver water nanofluids) through finite length non-uniform channel driven by peristaltic sinusoidal wave propagations.

METHODS: The governing equations are reduced in linear form using dimensional analysis and considering the low Reynolds number and large wavelength approximations. The time dependent temperature field, axial velocity, transverse velocity and pressure difference are obtained analytically in closed form solution. Trapping phenomenon is also discussed with the help of contour plots of stream function. A comparative study of pure water (Newtonian fluid), Copper water nanofluids and Silver water nanofluids under the influence of relevant physical parameters is made in graphical form and also discussed. The effects of absorption parameter and Grashof number on velocity profiles, temperature profiles and pressure distribution along the length of channel are examined.

RESULTS CONCLUSIONS: The computational results reveal that the velocity profile is maximum for Silver water nanofluids however, it is least for Copper water nanofluids. It is also concluded the temperature profile is more for pure water in comparison to Silver water and Copper water nanofluids. This model is applicable to design, micro-peristaltic pumps which help in Nanoparticle-based targeted drug delivery and to transport the sensitive or corrosive fluids, sanitary fluids, slurries and noxious fluids in nuclear industry.}, } @article {pmid28670352, year = {2017}, author = {Zhou, J and Ryu, S and Admiraal, D}, title = {Flow and transport effect caused by the stalk contraction cycle of Vorticella convallaria.}, journal = {Biomicrofluidics}, volume = {11}, number = {3}, pages = {034119}, doi = {10.1063/1.4985654}, pmid = {28670352}, issn = {1932-1058}, abstract = {Vorticella convallaria is a protozoan attached to a substrate by a stalk which can contract in less than 10 ms, translating the zooid toward the substrate with a maximum Reynolds number of ∼1. Following contraction, the stalk slowly relaxes, moving the zooid away from the substrate, which results in creeping flow. Although Vorticella has long been believed to contract to evade danger, it has been suggested that its stalk may contract to enhance food transport near the substrate. To elucidate how Vorticella utilizes its contraction-relaxation cycle, we investigated water flow caused by the cycle, using a computational fluid dynamics model validated with an experimental scale model and particle tracking velocimetry. The simulated flow was visualized and analyzed by tracing virtual particles around the Vorticella. It is observed that one cycle can displace particles up to ∼190 μm with the maximum net vertical displacement of 3-4 μm and that the net transport effect becomes more evident over repeated cycles. This transport effect appears to be due to asymmetry of the contraction and relaxation phases of the flow field, and it can be more effective on motile food particles than non-motile ones. Therefore, our Vorticella model enabled investigating the fluid dynamics principle and ecological role of the transport effects of Vorticella's stalk contraction.}, } @article {pmid28665292, year = {2017}, author = {Kazemi, A and Van de Riet, K and Curet, OM}, title = {Hydrodynamics of mangrove-type root models: the effect of porosity, spacing ratio and flexibility.}, journal = {Bioinspiration & biomimetics}, volume = {12}, number = {5}, pages = {056003}, doi = {10.1088/1748-3190/aa7ccf}, pmid = {28665292}, issn = {1748-3190}, mesh = {*Biomimetic Materials ; Ecosystem ; Equipment Design ; *Hydrodynamics ; Plant Roots/*anatomy & histology/*physiology ; *Porosity ; Rhizophoraceae/*anatomy & histology/*physiology ; }, abstract = {Mangrove trees play a prominent role in coastal tropic and subtropical regions, providing habitats for many organisms and protecting shorelines against high energy flows. In particular, the species Rhizophora mangle (red mangrove) exhibits complex cluster roots interacting with different hydrological flow conditions. To better understand the resilience of mangrove trees, we modeled the roots as a collection of cylinders with a circular pattern subject to unidirectional flow. We investigated the effect of porosity and spacing ratio between roots by varying both the diameter of the patch, D, and inset cylinders, d. In addition, we modeled hanging roots of red mangroves as cantilevered rigid cylinders on a hinge. Force and velocity measurements were performed in a water tunnel (Reynolds numbers from 2200 to 11 000). Concurrently, we performed 2D flow visualization using a flowing soap film. We found that the frequency of the vortex shedding increases as the diameter of the small cylinders decreases while the patch diameter is constant, therefore increasing the Strouhal number, [Formula: see text]. By comparing the change of Strouhal numbers with a single solid cylinder, we introduced a new length scale, the effective diameter. The effective diameter of the patch decreases as the porosity increases. In addition, we found that patch drag scales linearly with the patch diameter but decreases linearly as the spacing ratio increases. After a spacing ratio of ([Formula: see text]), the force scales linearly with the free stream velocity, and the mean velocity behind the patch is independent of the Reynolds number and the patch effect disappears. For flexible cylinders, we found that a decrease in stiffness increases both patch drag and the wake deficit behind the patch in a similar fashion as increasing the blockage of the patch. This information has the potential to help in the development of methods to design resilient bio-inspired coastline structures.}, } @article {pmid28658586, year = {2017}, author = {Qamar, A and Warnez, M and Valassis, DT and Guetzko, ME and Bull, JL}, title = {Small-bubble transport and splitting dynamics in a symmetric bifurcation.}, journal = {Computer methods in biomechanics and biomedical engineering}, volume = {20}, number = {11}, pages = {1182-1194}, doi = {10.1080/10255842.2017.1340466}, pmid = {28658586}, issn = {1476-8259}, mesh = {Arteries/physiology ; Embolization, Therapeutic ; Friction ; Humans ; *Microbubbles ; *Models, Theoretical ; Numerical Analysis, Computer-Assisted ; Skin ; Stress, Mechanical ; }, abstract = {Simulations of small bubbles traveling through symmetric bifurcations are conducted to garner information pertinent to gas embolotherapy, a potential cancer treatment. Gas embolotherapy procedures use intra-arterial bubbles to occlude tumor blood supply. As bubbles pass through bifurcations in the blood stream nonhomogeneous splitting and undesirable bioeffects may occur. To aid development of gas embolotherapy techniques, a volume of fluid method is used to model the splitting process of gas bubbles passing through artery and arteriole bifurcations. The model reproduces the variety of splitting behaviors observed experimentally, including the bubble reversal phenomenon. Splitting homogeneity and maximum shear stress along the vessel walls is predicted over a variety of physical parameters. Small bubbles, having initial length less than twice the vessel diameter, were found unlikely to split in the presence of gravitational asymmetry. Maximum shear stresses were found to decrease exponentially with increasing Reynolds number. Vortex-induced shearing near the bifurcation is identified as a possible mechanism for endothelial cell damage.}, } @article {pmid28658585, year = {2017}, author = {Qamar, A and Bull, JL}, title = {Transport and flow characteristics of an oscillating cylindrical fiber for total artificial lung application.}, journal = {Computer methods in biomechanics and biomedical engineering}, volume = {20}, number = {11}, pages = {1195-1211}, doi = {10.1080/10255842.2017.1340467}, pmid = {28658585}, issn = {1476-8259}, mesh = {*Artificial Organs ; Biological Transport ; Humans ; Numerical Analysis, Computer-Assisted ; Reproducibility of Results ; *Rheology ; Stress, Mechanical ; Time Factors ; }, abstract = {Mass transport and fluid dynamics characteristics in the vicinity of an oscillating cylindrical fiber with an imposed pulsatile inflow condition are computationally investigated in the present study. The work is motivated by a recently proposed design modification to the Total Artificial Lung (TAL) device, which is expected to provide better gas exchange. Navier-Stokes computations, coupled with convection-diffusion equation are performed to assess flow dynamics and mass transport behavior around the oscillating fiber. The oscillations and the pulsatile free stream velocity are represented by two sinusoidal functions. The resulting non-dimensional parameters are Keulegan-Carpenter number (KC), Schmidt number (Sc), Reynolds number (Re), pulsatile inflow amplitude ([Formula: see text]), and amplitude of cylinder oscillation ([Formula: see text]). Results are computed for [Formula: see text], Sc = 1000, Re = 5 and 10, [Formula: see text] and 0.7 and 0.25 [Formula: see text][Formula: see text][Formula: see text] 5.25. The pulsatile inflow parameters correspond to the flow velocities found in human pulmonary artery while matching the operating TAL Reynolds number. Mass transport from the surface of the cylinder to the bulk fluid is found to be primarily dependent on the size of surface vortices created by the movement of the cylinder. Time-averaged surface Sherwood number (Sh) is dependent on the amplitude and KC of cylinder oscillation. Compared to the fixed cylinder case, a significant gain up to 380% in Sh is achieved by oscillating the cylinder even at the small displacement amplitude (AD = 0.75D). Moreover, with decrease in KC the oscillating cylinder exhibits a lower drag amplitude compared with the fixed cylinder case. Inflow pulsation amplitude has minor effects on the mass transport characteristics. However, an increase in [Formula: see text] results in an increase in the amplitude of the periodic drag force on the cylinder. This rise in the drag amplitude is similar to that measured for the fixed cylinder case. Quantifications of shear stress distribution in the bulk fluid suggest that the physiological concerns of platelet activation and injury to red blood cells due to cylinder oscillation are negligible.}, } @article {pmid28652683, year = {2017}, author = {Qi, TY and Liu, C and Ni, MJ and Yang, JC}, title = {The linear stability of Hunt-Rayleigh-Bénard flow.}, journal = {Physics of fluids (Woodbury, N.Y. : 1994)}, volume = {29}, number = {6}, pages = {064103}, doi = {10.1063/1.4984842}, pmid = {28652683}, issn = {1070-6631}, abstract = {The stability of a pressure driven flow in a duct heated from below and subjected to a vertical magnetic field (Hunt-Rayleigh-Bénard flow) is studied. We use the Chebyshev collocation approach to solve the eigenvalue problem for the small-amplitude perturbations. It is demonstrated that the magnetic field can stabilize the flow, while the temperature field can disturb the flow. There exists a threshold for the Hartmann number below which the growth rate changes with the Prandtl number non-monotonously (first increases and then decreases) with a critical Prandtl number for the maximum growth rate. By comparing the [Formula: see text] neutral curves at different Rayleigh numbers, we find that the critical Reynolds number decreases with the increase in the Rayleigh number, which has an obvious influence on the long-wave instability and a little influence on the short-wave instability. The dominant mode of the long-wave instability changes from the boundary layer instability to the inflectional instability with the increase in the growth rate, which forms a new flow map. We also compare the [Formula: see text] curves and find that the critical Rayleigh number decreases with the increase in the Reynolds number. The obtained results gain an insight into the flow stability affected by the temperature field and the magnetic field.}, } @article {pmid28619665, year = {2017}, author = {Tripathi, D and Borode, A and Jhorar, R and Bég, OA and Tiwari, AK}, title = {Computer modelling of electro-osmotically augmented three-layered microvascular peristaltic blood flow.}, journal = {Microvascular research}, volume = {114}, number = {}, pages = {65-83}, doi = {10.1016/j.mvr.2017.06.004}, pmid = {28619665}, issn = {1095-9319}, mesh = {Animals ; Biomimetics/methods ; Blood Viscosity ; *Computer Simulation ; *Electroosmosis ; Humans ; *Microcirculation ; Microvessels/anatomy & histology/*physiology ; *Models, Cardiovascular ; *Pulsatile Flow ; Time Factors ; }, abstract = {A theoretical study is presented here for the electro-osmosis modulated peristaltic three-layered capillary flow of viscous fluids with different viscosities in the layers. The layers considered here are the core layer, the intermediate layer and the peripheral layer. The analysis has been carried out under a number of physical restrictions viz. Debye-Hückel linearization (i.e. wall zeta potential ≤25mV) is assumed sufficiently small, thin electric double layer limit (i.e. the peripheral layer is much thicker than the electric double layer thickness), low Reynolds number and large wavelength approximations. A non-dimensional analysis is used to linearize the boundary value problem. Fluid-fluid interfaces, peristaltic pumping characteristics, and trapping phenomenon are simulated. Present study also evaluates the responses of interface, pressure rise, time-averaged volume flow rate, maximum pressure rise, and the influence of Helmholtz-Smoluchowski velocity on the mechanical efficiency (with two different cases of the viscosity of fluids between the intermediate and the peripheral layer). Trapping phenomenon along with bolus dynamics evolution with thin EDL effects are analyzed. The findings of this study may ultimately be useful to control the microvascular flow during the fractionation of blood into plasma (in the peripheral layer), buffy coat (intermediate layer) and erythrocytes (core layer). This work may also contributes in electrophoresis, hematology, electrohydrodynamic therapy and, design and development of biomimetic electro-osmotic pumps.}, } @article {pmid28618644, year = {2017}, author = {Wang, L and Huang, Y}, title = {Intrinsic flow structure and multifractality in two-dimensional bacterial turbulence.}, journal = {Physical review. E}, volume = {95}, number = {5-1}, pages = {052215}, doi = {10.1103/PhysRevE.95.052215}, pmid = {28618644}, issn = {2470-0053}, abstract = {The active interaction between the bacteria and fluid generates turbulent structures even at zero Reynolds number. The velocity of such a flow obtained experimentally has been quantitatively investigated based on streamline segment analysis. There is a clear transition at about 16 times the organism body length separating two different scale regimes, which may be attributed to the different influence of the viscous effect. Surprisingly the scaling extracted from the streamline segment indicates the existence of scale similarity even at the zero Reynolds number limit. Moreover, the multifractal feature can be quantitatively described via a lognormal formula with the Hurst number H=0.76 and the intermittency parameter μ=0.20, which is coincidentally in agreement with the three-dimensional hydrodynamic turbulence result. The direction of cascade is measured via the filter-space technique. An inverse energy cascade is confirmed. For the enstrophy, a forward cascade is observed when r/R≤3, and an inverse one is observed when r/R>3, where r and R are the separation distance and the bacteria body size, respectively. Additionally, the lognormal statistics is verified for the coarse-grained energy dissipation and enstrophy, which supports the lognormal formula to fit the measured scaling exponent.}, } @article {pmid28618575, year = {2017}, author = {True, AC and Crimaldi, JP}, title = {Hydrodynamics of viscous inhalant flows.}, journal = {Physical review. E}, volume = {95}, number = {5-1}, pages = {053107}, doi = {10.1103/PhysRevE.95.053107}, pmid = {28618575}, issn = {2470-0053}, abstract = {Inhalant flows draw fluid into an orifice from a reservoir and are ubiquitous in engineering and biology. Surprisingly, there is a lack of quantitative information on viscous inhalant flows. We consider here laminar flows (Reynolds number Re≤100) developing after impulsive inhalation begins. We implement finite element simulations of flows with varying Re and extraction height h (orifice height above a bottom bed). Numerical results are experimentally validated using particle image velocimetry measurements in a physical model for a representative flow case in the middle of the Re-h parameter space. We use two metrics to characterize the flow in space and time: regions of influence (ROIs), which describe the spatial extent of the flow field, and inhalation volumes, which describe the initial distribution of inhaled fluid. The transient response for all Re features an inviscid sinklike component at early times followed by a viscous diffusive component. At lower Re, diffusion entrains an increasing volume of fluid over time, enlarging the ROI indefinitely. In some geometries, these flows spatially bifurcate, with some fluid being inhaled through the orifice and some bypassing into recirculation. At higher Re, inward advection dominates outward viscous diffusion and the flow remains trapped in a sinklike state. Both ROIs and inhalation volumes are strongly dependent on Re and extraction height, suggesting that organisms or engineers could tune these parameters to achieve specific inhalation criteria.}, } @article {pmid28618505, year = {2017}, author = {Puljiz, M and Menzel, AM}, title = {Forces and torques on rigid inclusions in an elastic environment: Resulting matrix-mediated interactions, displacements, and rotations.}, journal = {Physical review. E}, volume = {95}, number = {5-1}, pages = {053002}, doi = {10.1103/PhysRevE.95.053002}, pmid = {28618505}, issn = {2470-0053}, abstract = {Embedding rigid inclusions into elastic matrix materials is a procedure of high practical relevance, for instance, for the fabrication of elastic composite materials. We theoretically analyze the following situation. Rigid spherical inclusions are enclosed by a homogeneous elastic medium under stick boundary conditions. Forces and torques are directly imposed from outside onto the inclusions or are externally induced between them. The inclusions respond to these forces and torques by translations and rotations against the surrounding elastic matrix. This leads to elastic matrix deformations, and in turn results in mutual long-ranged matrix-mediated interactions between the inclusions. Adapting a well-known approach from low-Reynolds-number hydrodynamics, we explicitly calculate the displacements and rotations of the inclusions from the externally imposed or induced forces and torques. Analytical expressions are presented as a function of the inclusion configuration in terms of displaceability and rotateability matrices. The role of the elastic environment is implicitly included in these relations. That is, the resulting expressions allow a calculation of the induced displacements and rotations directly from the inclusion configuration, without having to explicitly determine the deformations of the elastic environment. In contrast to the hydrodynamic case, compressibility of the surrounding medium is readily taken into account. We present the complete derivation based on the underlying equations of linear elasticity theory. In the future, the method will, for example, be helpful to characterize the behavior of externally tunable elastic composite materials, to accelerate numerical approaches, as well as to improve the quantitative interpretation of microrheological results.}, } @article {pmid28618504, year = {2017}, author = {Altmeyer, S and Lueptow, RM}, title = {Wave propagation reversal for wavy vortices in wide-gap counter-rotating cylindrical Couette flow.}, journal = {Physical review. E}, volume = {95}, number = {5-1}, pages = {053103}, doi = {10.1103/PhysRevE.95.053103}, pmid = {28618504}, issn = {2470-0053}, abstract = {We present a numerical study of wavy supercritical cylindrical Couette flow between counter-rotating cylinders in which the wavy pattern propagates either prograde with the inner cylinder or retrograde opposite the rotation of the inner cylinder. The wave propagation reversals from prograde to retrograde and vice versa occur at distinct values of the inner cylinder Reynolds number when the associated frequency of the wavy instability vanishes. The reversal occurs for both twofold and threefold symmetric wavy vortices. Moreover, the wave propagation reversal only occurs for sufficiently strong counter-rotation. The flow pattern reversal appears to be intrinsic in the system as either periodic boundary conditions or fixed end wall boundary conditions for different system sizes always result in the wave propagation reversal. We present a detailed bifurcation sequence and parameter space diagram with respect to retrograde behavior of wavy flows. The retrograde propagation of the instability occurs when the inner Reynolds number is about two times the outer Reynolds number. The mechanism for the retrograde propagation is associated with the inviscidly unstable region near the inner cylinder and the direction of the global average azimuthal velocity. Flow dynamics, spatio-temporal behavior, global mean angular velocity, and torque of the flow with the wavy pattern are explored.}, } @article {pmid28613306, year = {2017}, author = {Khojah, R and Stoutamore, R and Di Carlo, D}, title = {Size-tunable microvortex capture of rare cells.}, journal = {Lab on a chip}, volume = {17}, number = {15}, pages = {2542-2549}, doi = {10.1039/c7lc00355b}, pmid = {28613306}, issn = {1473-0189}, abstract = {Inertial separation of particles and cells based on their size has advanced significantly over the last decade. However, size-based inertial separation methods require precise tuning of microfluidic device geometries to adjust the separation size of particles or cells. Here, we show a passive capture method that targets a wide size range of cells by controlling the flow conditions in a single device geometry. This multimodal capture device is designed to generate laminar vortices in lateral cavities that branch from long rectangular channels. Micro-vortices generated at lower Reynolds numbers capture and stabilize large particles in equilibrium orbits or limit cycles near the vortex core. Other smaller particles or cells orbit near the vortex boundaries and they are susceptible to exiting the cavity flow. In the same cavity, however, at higher Reynolds number, we observe small particles migrating inward. This evolution in limit cycle trajectories led to a corresponding evolution in the average size of captured particles, indicating that the outermost orbits are less stable. We identify three phases of capture as a function of Reynolds number that give rise to unique particle orbit trajectories. Flow-based switching overcomes a major engineering challenge to automate capture and release of polydisperse cell subpopulations. The approach can expand clinical applications of label free trapping in isolating and processing a larger subset of rare cells like circulating tumor cells (CTCs) from blood and other body fluids.}, } @article {pmid28613157, year = {2017}, author = {Ferreira, RR and Vilfan, A and Jülicher, F and Supatto, W and Vermot, J}, title = {Physical limits of flow sensing in the left-right organizer.}, journal = {eLife}, volume = {6}, number = {}, pages = {}, doi = {10.7554/eLife.25078}, pmid = {28613157}, issn = {2050-084X}, mesh = {Animals ; *Body Patterning ; Cilia/*physiology ; Embryo, Nonmammalian/cytology/*physiology ; Functional Laterality ; Gene Expression Regulation, Developmental ; Hydrodynamics ; Signal Transduction ; Zebrafish/embryology/*physiology ; Zebrafish Proteins/metabolism ; }, abstract = {Fluid flows generated by motile cilia are guiding the establishment of the left-right asymmetry of the body in the vertebrate left-right organizer. Competing hypotheses have been proposed: the direction of flow is sensed either through mechanosensation, or via the detection of chemical signals transported in the flow. We investigated the physical limits of flow detection to clarify which mechanisms could be reliably used for symmetry breaking. We integrated parameters describing cilia distribution and orientation obtained in vivo in zebrafish into a multiscale physical study of flow generation and detection. Our results show that the number of immotile cilia is too small to ensure robust left and right determination by mechanosensing, given the large spatial variability of the flow. However, motile cilia could sense their own motion by a yet unknown mechanism. Finally, transport of chemical signals by the flow can provide a simple and reliable mechanism of asymmetry establishment.}, } @article {pmid28582613, year = {2017}, author = {Chu, X and Yu, X and Greenstein, J and Aydin, F and Uppaladadium, G and Dutt, M}, title = {Flow-Induced Shape Reconfiguration, Phase Separation, and Rupture of Bio-Inspired Vesicles.}, journal = {ACS nano}, volume = {11}, number = {7}, pages = {6661-6671}, doi = {10.1021/acsnano.7b00753}, pmid = {28582613}, issn = {1936-086X}, abstract = {The structural integrity of red blood cells and drug delivery carriers through blood vessels is dependent upon their ability to adapt their shape during their transportation. Our goal is to examine the role of the composition of bio-inspired multicomponent and hairy vesicles on their shape during their transport through in a channel. Through the dissipative particle dynamics simulation technique, we apply Poiseuille flow in a cylindrical channel. We investigate the effect of flow conditions and concentration of key molecular components on the shape, phase separation, and structural integrity of the bio-inspired multicomponent and hairy vesicles. Our results show the Reynolds number and molecular composition of the vesicles impact their flow-induced deformation, phase separation on the outer monolayer due to the Marangoni effect, and rupture. The findings from this study could be used to enhance the design of drug delivery and tissue engineering systems.}, } @article {pmid28577022, year = {2017}, author = {Huang, Y and Wang, HL and Chen, YQ and Zhang, YH and Yang, Q and Bai, ZS and Ma, L}, title = {Liquid-liquid extraction intensification by micro-droplet rotation in a hydrocyclone.}, journal = {Scientific reports}, volume = {7}, number = {1}, pages = {2678}, doi = {10.1038/s41598-017-02732-x}, pmid = {28577022}, issn = {2045-2322}, abstract = {The previous literature reports that using a hydrocyclone as an extractor intensifies the mass transfer and largely reduces the consumption of extractant from 1800-2000 kg h-1 to 30-90 kg h-1. However, the intensification mechanism has not been clear. This paper presents experimental and numerical methods to study the multi-scale motion of particles in hydrocyclones. In addition to the usually considered translational behavior, the high-speed rotation of dispersed micro-spheres caused by the anisotropic swirling shear flow is determined. The rotation speeds of the tested micro-spheres are above 1000 rad s-1, which are much larger than the instantaneous rotation speed in isotropic turbulence. Due to the conical structure of a hydrocyclone, the rotation speed maintains stability along the axial direction. Numerical results show that the particle Reynolds number of micro-droplets in a hydrocyclone is equal to that in conventional extractors, but the particles have high rotation speeds of up to 10,000 rad s-1 and long mixing lengths of more than 1000 mm. Both the rotation of micro-droplets along the spiral trajectories and the intense eddy diffusion in a hydrocyclone contribute to the extraction intensification.}, } @article {pmid28573002, year = {2017}, author = {Petford, N and Mirhadizadeh, S}, title = {Image-based modelling of lateral magma flow: the Basement Sill, Antarctica.}, journal = {Royal Society open science}, volume = {4}, number = {5}, pages = {161083}, doi = {10.1098/rsos.161083}, pmid = {28573002}, issn = {2054-5703}, abstract = {The McMurdo Dry Valleys magmatic system, Antarctica, provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle microstructure of a congested magma slurry. We simulated the flow regime in two and three dimensions using numerical models built on a finite-element mesh derived from field data. The model captures the flow behaviour of the Basement Sill magma over a viscosity range of 1-104 Pa s where the higher end (greater than or equal to 102 Pa s) corresponds to a magmatic slurry with crystal fractions varying between 30 and 70%. A novel feature of the model is the discovery of transient, low viscosity (less than or equal to 50 Pa s) high Reynolds number eddies formed along undulating contacts at the floor and roof of the intrusion. Numerical tracing of particle orbits implies crystals trapped in eddies segregate according to their mass density. Recovered shear strain rates (10-3-10-5 s-1) at viscosities equating to high particle concentrations (around more than 40%) in the Sill interior point to shear-thinning as an explanation for some types of magmatic layering there. Model transport rates for the Sill magmas imply a maximum emplacement time of ca 105 years, consistent with geochemical evidence for long-range lateral flow. It is a theoretically possibility that fast-flowing magma on a continental scale will be susceptible to planetary-scale rotational forces.}, } @article {pmid28555615, year = {2017}, author = {Stieger, T and Agha, H and Schoen, M and Mazza, MG and Sengupta, A}, title = {Hydrodynamic cavitation in Stokes flow of anisotropic fluids.}, journal = {Nature communications}, volume = {8}, number = {}, pages = {15550}, doi = {10.1038/ncomms15550}, pmid = {28555615}, issn = {2041-1723}, abstract = {Cavitation, the nucleation of vapour in liquids, is ubiquitous in fluid dynamics, and is often implicated in a myriad of industrial and biomedical applications. Although extensively studied in isotropic liquids, corresponding investigations in anisotropic liquids are largely lacking. Here, by combining liquid crystal microfluidic experiments, nonequilibrium molecular dynamics simulations and theoretical arguments, we report flow-induced cavitation in an anisotropic fluid. The cavitation domain nucleates due to sudden pressure drop upon flow past a cylindrical obstacle within a microchannel. For an anisotropic fluid, the inception and growth of the cavitation domain ensued in the Stokes regime, while no cavitation was observed in isotropic liquids flowing under similar hydrodynamic parameters. Using simulations we identify a critical value of the Reynolds number for cavitation inception that scales inversely with the order parameter of the fluid. Strikingly, the critical Reynolds number for anisotropic fluids can be 50% lower than that of isotropic fluids.}, } @article {pmid28529384, year = {2017}, author = {Rosti, ME and Kamps, L and Bruecker, C and Omidyeganeh, M and Pinelli, A}, title = {The PELskin project-part V: towards the control of the flow around aerofoils at high angle of attack using a self-activated deployable flap.}, journal = {Meccanica}, volume = {52}, number = {8}, pages = {1811-1824}, doi = {10.1007/s11012-016-0524-x}, pmid = {28529384}, issn = {0025-6455}, abstract = {During the flight of birds, it is often possible to notice that some of the primaries and covert feathers on the upper side of the wing pop-up under critical flight conditions, such as the landing approach or when stalking their prey (see Fig. 1) . It is often conjectured that the feathers pop up plays an aerodynamic role by limiting the spread of flow separation . A combined experimental and numerical study was conducted to shed some light on the physical mechanism determining the feathers self actuation and their effective role in controlling the flow field in nominally stalled conditions. In particular, we have considered a NACA0020 aerofoil, equipped with a flexible flap at low chord Reynolds numbers. A parametric study has been conducted on the effects of the length, natural frequency, and position of the flap. A configuration with a single flap hinged on the suction side at 70 % of the chord size c (from the leading edge), with a length of [Formula: see text] matching the shedding frequency of vortices at stall condition has been found to be optimum in delivering maximum aerodynamic efficiency and lift gains. Flow evolution both during a ramp-up motion (incidence angle from [Formula: see text] to [Formula: see text] with a reduced frequency of [Formula: see text], [Formula: see text] being the free stream velocity magnitude), and at a static stalled condition ([Formula: see text]) were analysed with and without the flap. A significant increase of the mean lift after a ramp-up manoeuvre is observed in presence of the flap. Stall dynamics (i.e., lift overshoot and oscillations) are altered and the simulations reveal a periodic re-generation cycle composed of a leading edge vortex that lift the flap during his passage, and an ejection generated by the relaxing of the flap in its equilibrium position. The flap movement in turns avoid the interaction between leading and trailing edge vortices when lift up and push the trailing edge vortex downstream when relaxing back. This cyclic behaviour is clearly shown by the periodic variation of the lift about the average value, and also from the periodic motion of the flap. A comparison with the experiments shows a similar but somewhat higher non-dimensional frequency of the flap oscillation. By assuming that the cycle frequency scales inversely with the boundary layer thickness, one can explain the higher frequencies observed in the experiments which were run at a Reynolds number about one order of magnitude higher than in the simulations. In addition, in experiments the periodic re-generation cycle decays after 3-4 periods ultimately leading to the full stall of the aerofoil. In contrast, the 2D simulations show that the cycle can become self-sustained without any decay when the flap parameters are accurately tuned.}, } @article {pmid28508858, year = {2017}, author = {Doostmohammadi, A and Shendruk, TN and Thijssen, K and Yeomans, JM}, title = {Onset of meso-scale turbulence in active nematics.}, journal = {Nature communications}, volume = {8}, number = {}, pages = {15326}, doi = {10.1038/ncomms15326}, pmid = {28508858}, issn = {2041-1723}, support = {291234//European Research Council/International ; }, abstract = {Meso-scale turbulence is an innate phenomenon, distinct from inertial turbulence, that spontaneously occurs at low Reynolds number in fluidized biological systems. This spatiotemporal disordered flow radically changes nutrient and molecular transport in living fluids and can strongly affect the collective behaviour in prominent biological processes, including biofilm formation, morphogenesis and cancer invasion. Despite its crucial role in such physiological processes, understanding meso-scale turbulence and any relation to classical inertial turbulence remains obscure. Here we show how the motion of active matter along a micro-channel transitions to meso-scale turbulence through the evolution of locally disordered patches (active puffs) from an ordered vortex-lattice flow state. We demonstrate that the stationary critical exponents of this transition to meso-scale turbulence in a channel coincide with the directed percolation universality class. This finding bridges our understanding of the onset of low-Reynolds-number meso-scale turbulence and traditional scale-invariant turbulence in confinement.}, } @article {pmid28505752, year = {2017}, author = {Tasaka, Y and Iima, M}, title = {Surface switching statistics of rotating fluid: Disk-rim gap effects.}, journal = {Physical review. E}, volume = {95}, number = {4-1}, pages = {043113}, doi = {10.1103/PhysRevE.95.043113}, pmid = {28505752}, issn = {2470-0053}, abstract = {We examined the influence of internal noise on the irregular switching of the shape of the free surface of fluids in an open cylindrical vessel driven by a bottom disk rotating at constant speed [Suzuki, Iima, and Hayase, Phys. Fluids 18, 101701 (2006)PHFLE61070-663110.1063/1.2359740]. A slight increase in the disk-rim gap (less than 3% of the disk radius) was established experimentally to cause significant changes in this system, specifically, frequent appearance of the surface descending event connecting a nonaxisymmetric shape in strong mixing flow (turbulent flow) and an axisymmetric shape in laminar flow, as well as a shift in critical Reynolds number that define the characteristic states. The physical mechanism underlying the change is analyzed in terms of flow characteristics in the disk-rim gap, which acts as a noise source, and a mathematical model established from measurements of the surface height fluctuations with noise term.}, } @article {pmid28479086, year = {2017}, author = {Moroni, M and Lupo, E and La Marca, F}, title = {Hydraulic separation of plastic wastes: Analysis of liquid-solid interaction.}, journal = {Waste management (New York, N.Y.)}, volume = {66}, number = {}, pages = {13-22}, doi = {10.1016/j.wasman.2017.04.045}, pmid = {28479086}, issn = {1879-2456}, mesh = {*Plastics ; Polymers ; *Recycling ; Refuse Disposal ; }, abstract = {The separation of plastic wastes in mechanical recycling plants is the process that ensures high-quality secondary raw materials. An innovative device employing a wet technology for particle separation is presented in this work. Due to the combination of the characteristic flow pattern developing within the apparatus and density, shape and size differences among two or more polymers, it allows their separation into two products, one collected within the instrument and the other one expelled through its outlet ducts. The kinematic investigation of the fluid flowing within the apparatus seeded with a passive tracer was conducted via image analysis for different hydraulic configurations. The two-dimensional turbulent kinetic energy results strictly connected to the apparatus separation efficacy. Image analysis was also employed to study the behaviour of mixtures of passive tracer and plastic particles with different physical characteristics in order to understand the coupling regime between fluid and solid phases. The two-dimensional turbulent kinetic energy analysis turned out to be fundamental to this aim. For the tested operating conditions, two-way coupling takes place, i.e., the fluid exerts an influence on the plastic particle and the opposite occurs too. Image analysis confirms the outcomes from the investigation of the two-phase flow via non-dimensional numbers (particle Reynolds number, Stokes number and solid phase volume fraction).}, } @article {pmid28470538, year = {2017}, author = {Hosseinzadegan, H and Tafti, DK}, title = {Prediction of Thrombus Growth: Effect of Stenosis and Reynolds Number.}, journal = {Cardiovascular engineering and technology}, volume = {8}, number = {2}, pages = {164-181}, doi = {10.1007/s13239-017-0304-3}, pmid = {28470538}, issn = {1869-4098}, mesh = {Algorithms ; Heparin/*metabolism ; Humans ; Models, Theoretical ; Platelet Activation ; Platelet Adhesiveness ; Platelet Aggregation ; Shear Strength ; Thrombosis/*physiopathology ; }, abstract = {Shear stresses play a major role in platelet-substrate interactions and thrombus formation and growth in blood flow, where under both pathological and physiological conditions platelet adhesion and accumulation occur. In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is presented. The model was first verified under three different shear conditions and at two heparin levels. Three-dimensional simulations were then carried out to evaluate the performance of the model for severely damaged (stripped) aortas with mild and severe stenosis degrees in laminar flow regime. For these cases, linear shear-dependent functions were developed for platelet-surface and platelet-platelet adhesion rates. It was confirmed that the platelet adhesion rate is not only a function of Reynolds number (or wall shear rate) but also the stenosis severity of the vessel. General correlations for adhesion rates of platelets as functions of stenosis and Reynolds number were obtained based on these cases. Finally using the new platelet adhesion rates, the model was applied to different experimental systems and shown to agree well with measured platelet deposition.}, } @article {pmid28462417, year = {2017}, author = {Khani, M and Xing, T and Gibbs, C and Oshinski, JN and Stewart, GR and Zeller, JR and Martin, BA}, title = {Nonuniform Moving Boundary Method for Computational Fluid Dynamics Simulation of Intrathecal Cerebrospinal Flow Distribution in a Cynomolgus Monkey.}, journal = {Journal of biomechanical engineering}, volume = {139}, number = {8}, pages = {}, doi = {10.1115/1.4036608}, pmid = {28462417}, issn = {1528-8951}, support = {P20 GM103408/GM/NIGMS NIH HHS/United States ; R44 MH112210/MH/NIMH NIH HHS/United States ; U54 GM104944/GM/NIGMS NIH HHS/United States ; }, mesh = {Animals ; Cerebrospinal Fluid/diagnostic imaging/*physiology ; *Computer Simulation ; *Hydrodynamics ; Macaca fascicularis ; Magnetic Resonance Imaging ; Male ; }, abstract = {A detailed quantification and understanding of cerebrospinal fluid (CSF) dynamics may improve detection and treatment of central nervous system (CNS) diseases and help optimize CSF system-based delivery of CNS therapeutics. This study presents a computational fluid dynamics (CFD) model that utilizes a nonuniform moving boundary approach to accurately reproduce the nonuniform distribution of CSF flow along the spinal subarachnoid space (SAS) of a single cynomolgus monkey. A magnetic resonance imaging (MRI) protocol was developed and applied to quantify subject-specific CSF space geometry and flow and define the CFD domain and boundary conditions. An algorithm was implemented to reproduce the axial distribution of unsteady CSF flow by nonuniform deformation of the dura surface. Results showed that maximum difference between the MRI measurements and CFD simulation of CSF flow rates was <3.6%. CSF flow along the entire spine was laminar with a peak Reynolds number of ∼150 and average Womersley number of ∼5.4. Maximum CSF flow rate was present at the C4-C5 vertebral level. Deformation of the dura ranged up to a maximum of 134 μm. Geometric analysis indicated that total spinal CSF space volume was ∼8.7 ml. Average hydraulic diameter, wetted perimeter, and SAS area were 2.9 mm, 37.3 mm and 27.24 mm2, respectively. CSF pulse wave velocity (PWV) along the spine was quantified to be 1.2 m/s.}, } @article {pmid28452782, year = {2017}, author = {Carrera, L and Springer, F and Lipeme-Kouyi, G and Buffiere, P}, title = {Sulfide emissions in sewer networks: focus on liquid to gas mass transfer coefficient.}, journal = {Water science and technology : a journal of the International Association on Water Pollution Research}, volume = {75}, number = {7-8}, pages = {1899-1908}, doi = {10.2166/wst.2017.070}, pmid = {28452782}, issn = {0273-1223}, mesh = {Gases/chemistry ; Gravitation ; Models, Theoretical ; Phase Transition ; Sewage/*chemistry ; Sulfides/*chemistry ; Water Pollutants, Chemical/*chemistry ; }, abstract = {H2S emission dynamics in sewers are conditioned by the mass transfer coefficient at the interface. This work aims at measuring the variation of the mass transfer coefficient with the hydraulic characteristics, with the objective of estimating H2S emission in gravity pipes, and collecting data to establish models independent of the system geometry. The ratio between the H2S and O2 mass transfer coefficient was assessed in an 8 L mixed reactor under different experimental conditions. Then, oxygen mass transfer measurements were performed in a 10 m long gravity pipe. The following ranges of experimental conditions were investigated: velocity flow [0-0.61 m.s-1], Reynolds number [0-23,333]. The hydrodynamic parameters at the liquid/gas interface were calculated by computational fluid dynamics (CFD). In the laboratory-scale reactor, the O2 mass transfer coefficient was found to depend on the stirring rate (rph) as follows: KL,O2 = 0.016 + 0.025 N3.85. A KL,H2S/KL,O2 ratio of 0.64 ± 0.24 was found, in accordance with previously published data. CFD results helped in refining this correlation: the mass transfer coefficient depends on the local interface velocity ui (m.h-1): KL,O2 = 0.016 + 1.02 × 10-5 ui3.85 In the gravity pipe device, KL,O2 also exponentially increased with the mean flow velocity. These trends were found to be consistent with the increasing level of turbulence.}, } @article {pmid28446697, year = {2017}, author = {China, V and Levy, L and Liberzon, A and Elmaliach, T and Holzman, R}, title = {Hydrodynamic regime determines the feeding success of larval fish through the modulation of strike kinematics.}, journal = {Proceedings. Biological sciences}, volume = {284}, number = {1853}, pages = {}, doi = {10.1098/rspb.2017.0235}, pmid = {28446697}, issn = {1471-2954}, mesh = {Animals ; Biomechanical Phenomena ; *Feeding Behavior ; Hydrodynamics ; Larva/physiology ; Sea Bream/*physiology ; }, abstract = {Larval fishes experience extreme mortality rates, with 99% of a cohort perishing within days after starting to actively feed. While recent evidence suggests that hydrodynamic factors contribute to constraining larval feeding during early ontogeny, feeding is a complex process that involves numerous interacting behavioural and biomechanical components. How these components change throughout ontogeny and how they contribute to feeding remain unclear. Using 339 observations of larval feeding attempts, we quantified the effects of morphological and behavioural traits on feeding success of Sparus aurata larvae during early ontogeny. Feeding success was determined using high-speed videography, under both natural and increased water viscosity treatments. Successful strikes were characterized by Reynolds numbers that were an order of magnitude higher than those of failed strikes. The pattern of increasing strike success with increasing age was driven by the ontogeny of traits that facilitate the transition to higher Reynolds numbers. Hence, the physical growth of a larva plays an important role in its transition to a hydrodynamic regime of higher Reynolds numbers, in which suction feeding is more effective.}, } @article {pmid28443874, year = {2017}, author = {Schaaf, C and Stark, H}, title = {Inertial migration and axial control of deformable capsules.}, journal = {Soft matter}, volume = {13}, number = {19}, pages = {3544-3555}, doi = {10.1039/c7sm00339k}, pmid = {28443874}, issn = {1744-6848}, abstract = {The mechanical deformability of single cells is an important indicator for various diseases such as cancer, blood diseases and inflammation. Lab-on-a-chip devices allow to separate such cells from healthy cells using hydrodynamic forces. We perform hydrodynamic simulations based on the lattice-Boltzmann method and study the behavior of an elastic capsule in a microfluidic channel flow in the inertial regime. While inertial lift forces drive the capsule away from the channel center, its deformability favors migration in the opposite direction. Balancing both migration mechanisms, a deformable capsule assembles at a specific equilibrium distance depending on its size and deformability. We find that this equilibrium distance is nearly independent of the channel Reynolds number and falls on a single master curve when plotted versus the Laplace number. We identify a similar master curve for varying particle radius. In contrast, the actual deformation of a capsule strongly depends on the Reynolds number. The lift-force profiles behave in a similar manner as those for rigid particles. Using the Saffman effect, the capsule's equilibrium position can be controlled by an external force along the channel axis. While rigid particles move to the center when slowed down, very soft capsules show the opposite behavior. Interestingly, for a specific control force particles are focused on the same equilibrium position independent of their deformability.}, } @article {pmid28434714, year = {2017}, author = {Gritti, F and Fogwill, M}, title = {Speed-resolution advantage of turbulent supercritical fluid chromatography in open tubular columns: II - Theoretical and experimental evidences.}, journal = {Journal of chromatography. A}, volume = {1501}, number = {}, pages = {142-150}, doi = {10.1016/j.chroma.2017.04.032}, pmid = {28434714}, issn = {1873-3778}, mesh = {Benzene Derivatives/chemistry ; Carbon Dioxide/chemistry ; Chromatography, Supercritical Fluid/*instrumentation/methods ; Kinetics ; Models, Theoretical ; Pressure ; Silicon Dioxide/chemistry ; Temperature ; }, abstract = {The potential advantage of turbulent supercritical fluid chromatography (TSFC) in open tubular columns (OTC) was evaluated on both theoretical and practical viewpoints. First, the dispersion model derived by Golay in 1958 and recently extended from laminar to turbulent flow regime is used for the predictions of the speed-resolution performance in TSFC. The average dispersion coefficient of matter in the turbulent flow regime was taken from the available experimental data over a range of Reynolds number from 2000 to 6000. Kinetic plots are built at constant pressure drop (ΔP=4500psi) and Schmidt number (Sc=15) for four inner diameters (10, 30, 100, and 300μm) of the OTC and for three retention factors (0, 1, and 10). Accordingly, in turbulent flow regime, for a Reynolds number of 4000 and a retention factor of 1 (the stationary film thickness is assumed to be negligible with respect to the OTC diameter), the theory projects that a 300μm i.d. OTC has the same speed-resolution power (200,000 theoretical plates; 2.4min hold-up time) as that of a 10μm i.d. OTC operated in laminar flow regime. Secondly, the experimental plate heights of n-butylbenzene are measured in laminar and turbulent flow regimes for a 180μm×4.8m fused silica capillary column using pure carbon dioxide as the mobile phase. The back pressure regulator was set at 1500psi, the temperature was uniform at 297K, and the flow rate was increased step-wise from 0.50 to 3.60mL/min so that the experimental Reynolds number increases from 700 to 5400. The experiments are in good agreement with the plate heights projected in TSFC at high flow rates and with those expected at low flow rates in a laminar flow regime.}, } @article {pmid28429018, year = {2017}, author = {Menzel, AM}, title = {Force-induced elastic matrix-mediated interactions in the presence of a rigid wall.}, journal = {Soft matter}, volume = {13}, number = {18}, pages = {3373-3384}, doi = {10.1039/c7sm00459a}, pmid = {28429018}, issn = {1744-6848}, abstract = {We consider a soft elastic matrix that contains particulate inclusions and is bounded by a rigid wall, e.g., the substrate. Such a situation arises in elastic composite materials. They may serve as soft actuators when forces are imposed on or induced between the embedded particles. We investigate how the presence of the rigid wall affects the interactions between the inclusions. First, for no-slip boundary conditions, we transfer Blake's derivation of a corresponding Green's function from low-Reynolds-number hydrodynamics to the linearly elastic case. Then, we list the general expressions to describe the situation for point-like particles in the presence of no-slip and free-slip surface conditions. To compare the effect of the different surface conditions to each other and to the bulk behavior, we address the example situation of pairwise interactions between two embedded particles. The axis through both particle centers is either aligned parallel or perpendicular to the surface. Our results suggest that walls with free-slip surface conditions are preferred when they serve as substrates for soft actuators made from elastic composite materials. As we further demonstrate, the presence of a rigid wall can qualitatively change the interactions between the inclusions. In effect, it can switch attractive interactions into repulsive ones (and vice versa). It should be straightforward to observe the effects in future experiments and to combine our results, e.g., with the modeling of biological cells and tissue on rigid surfaces.}, } @article {pmid28415285, year = {2017}, author = {Fan, WL and Pak, OS and Sandoval, M}, title = {Ellipsoidal Brownian self-driven particles in a magnetic field.}, journal = {Physical review. E}, volume = {95}, number = {3-1}, pages = {032605}, doi = {10.1103/PhysRevE.95.032605}, pmid = {28415285}, issn = {2470-0053}, abstract = {We study the two-dimensional Brownian dynamics of an ellipsoidal paramagnetic microswimmer moving at a low Reynolds number and subject to a magnetic field. Its corresponding mean-square displacement, showing the effect of a particles's shape, activity, and magnetic field on the microswimmer's diffusion, is analytically obtained. Comparison between analytical and computational results shows good agreement. In addition, the effect of self-propulsion on the transition time from anisotropic to isotropic diffusion of the ellipse is investigated.}, } @article {pmid28415282, year = {2017}, author = {Shen, X and Marcos, and Fu, HC}, title = {Traction reveals mechanisms of wall effects for microswimmers near boundaries.}, journal = {Physical review. E}, volume = {95}, number = {3-1}, pages = {033105}, doi = {10.1103/PhysRevE.95.033105}, pmid = {28415282}, issn = {2470-0053}, abstract = {The influence of a plane boundary on low-Reynolds-number swimmers has frequently been studied using image systems for flow singularities. However, the boundary effect can also be expressed using a boundary integral representation over the traction on the boundary. We show that examining the traction pattern on the boundary caused by a swimmer can yield physical insights into determining when far-field multipole models are accurate. We investigate the swimming velocities and the traction of a three-sphere swimmer initially placed parallel to an infinite planar wall. In the far field, the instantaneous effect of the wall on the swimmer is well approximated by that of a multipole expansion consisting of a force dipole and a force quadrupole. On the other hand, the swimmer close to the wall must be described by a system of singularities reflecting its internal structure. We show that these limits and the transition between them can be independently identified by examining the traction pattern on the wall, either using a quantitative correlation coefficient or by visual inspection. Last, we find that for nonconstant propulsion, correlations between swimming stroke motions and internal positions are important and not captured by time-averaged traction on the wall, indicating that care must be taken when applying multipole expansions to study boundary effects in cases of nonconstant propulsion.}, } @article {pmid28413338, year = {2017}, author = {Aurnou, JM and King, EM}, title = {The cross-over to magnetostrophic convection in planetary dynamo systems.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {473}, number = {2199}, pages = {20160731}, doi = {10.1098/rspa.2016.0731}, pmid = {28413338}, issn = {1364-5021}, } @article {pmid28413332, year = {2017}, author = {Ferrari, A}, title = {Fluid dynamics of acoustic and hydrodynamic cavitation in hydraulic power systems.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {473}, number = {2199}, pages = {20160345}, doi = {10.1098/rspa.2016.0345}, pmid = {28413332}, issn = {1364-5021}, abstract = {Cavitation is the transition from a liquid to a vapour phase, due to a drop in pressure to the level of the vapour tension of the fluid. Two kinds of cavitation have been reviewed here: acoustic cavitation and hydrodynamic cavitation. As acoustic cavitation in engineering systems is related to the propagation of waves through a region subjected to liquid vaporization, the available expressions of the sound speed are discussed. One of the main effects of hydrodynamic cavitation in the nozzles and orifices of hydraulic power systems is a reduction in flow permeability. Different discharge coefficient formulae are analysed in this paper: the Reynolds number and the cavitation number result to be the key fluid dynamical parameters for liquid and cavitating flows, respectively. The latest advances in the characterization of different cavitation regimes in a nozzle, as the cavitation number reduces, are presented. The physical cause of choked flows is explained, and an analogy between cavitation and supersonic aerodynamic flows is proposed. The main approaches to cavitation modelling in hydraulic power systems are also reviewed: these are divided into homogeneous-mixture and two-phase models. The homogeneous-mixture models are further subdivided into barotropic and baroclinic models. The advantages and disadvantages of an implementation of the complete Rayleigh-Plesset equation are examined.}, } @article {pmid28399142, year = {2017}, author = {Potvin, J and Werth, AJ}, title = {Oral cavity hydrodynamics and drag production in Balaenid whale suspension feeding.}, journal = {PloS one}, volume = {12}, number = {4}, pages = {e0175220}, doi = {10.1371/journal.pone.0175220}, pmid = {28399142}, issn = {1932-6203}, mesh = {Animals ; Balaenoptera/*physiology ; *Feeding Behavior ; *Hydrodynamics ; Mouth/*physiology ; }, abstract = {Balaenid whales feed on large aggregates of small and slow-moving prey (predominantly copepods) through a filtration process enabled by baleen. These whales exhibit continuous filtration, namely, with the mouth kept partially opened and the baleen exposed to oncoming prey-laden waters while fluking. The process is an example of crossflow filtration (CFF) in which most of the particulates (prey) are separated from the substrate (water) without ever coming into contact with the filtering surface (baleen). This paper discusses the simulation of baleen filtration hydrodynamics based on a type of hydraulic circuit modeling commonly used in microfluidics, but adapted to the much higher Reynolds number flows typical of whale hydrodynamics. This so-called Baleen Hydraulic Circuit (BHC) model uses as input the basic characteristics of the flows moving through a section of baleen observed in a previous flume study by the authors. The model has low-spatial resolution but incorporates the effects of fluid viscosity, which doubles or more a whale's total body drag in comparison to non-feeding travel. Modeling viscous friction is crucial here since exposing the baleen system to the open ocean ends up tripling a whale's total wetted surface area. Among other findings, the BHC shows how CFF is enhanced by a large filtration surface and hence large body size; how it is carried out via the establishment of rapid anteroposterior flows transporting most of the prey-water slurry towards the oropharyngeal wall; how slower intra-baleen flows manage to transfer most of the substrate out of the mouth, all the while contributing only a fraction to overall oral cavity drag; and how these anteroposterior and intra-baleen flows lose speed as they approach the oropharyngeal wall.}, } @article {pmid28397936, year = {2017}, author = {Reigh, SY and Zhu, L and Gallaire, F and Lauga, E}, title = {Swimming with a cage: low-Reynolds-number locomotion inside a droplet.}, journal = {Soft matter}, volume = {13}, number = {17}, pages = {3161-3173}, doi = {10.1039/c6sm01636g}, pmid = {28397936}, issn = {1744-6848}, abstract = {Inspired by recent experiments using synthetic microswimmers to manipulate droplets, we investigate the low-Reynolds-number locomotion of a model swimmer (a spherical squirmer) encapsulated inside a droplet of a comparable size in another viscous fluid. Meditated solely by hydrodynamic interactions, the encaged swimmer is seen to be able to propel the droplet, and in some situations both remain in a stable co-swimming state. The problem is tackled using both an exact analytical theory and a numerical implementation based on a boundary element method, with a particular focus on the kinematics of the co-moving swimmer and the droplet in a concentric configuration, and we obtain excellent quantitative agreement between the two. The droplet always moves slower than a swimmer which uses purely tangential surface actuation but when it uses a particular combination of tangential and normal actuations, the squirmer and droplet are able to attain the same velocity and stay concentric for all times. We next employ numerical simulations to examine the stability of their concentric co-movement, and highlight several stability scenarios depending on the particular gait adopted by the swimmer. Furthermore, we show that the droplet reverses the nature of the far-field flow induced by the swimmer: a droplet cage turns a pusher swimmer into a puller, and vice versa. Our work sheds light on the potential development of droplets as self-contained carriers of both chemical content and self-propelled devices for controllable and precise drug deliveries.}, } @article {pmid28388112, year = {2017}, author = {Dyer, OT and Ball, RC}, title = {Wavelet Monte Carlo dynamics: A new algorithm for simulating the hydrodynamics of interacting Brownian particles.}, journal = {The Journal of chemical physics}, volume = {146}, number = {12}, pages = {124111}, doi = {10.1063/1.4978808}, pmid = {28388112}, issn = {1089-7690}, abstract = {We develop a new algorithm for the Brownian dynamics of soft matter systems that evolves time by spatially correlated Monte Carlo moves. The algorithm uses vector wavelets as its basic moves and produces hydrodynamics in the low Reynolds number regime propagated according to the Oseen tensor. When small moves are removed, the correlations closely approximate the Rotne-Prager tensor, itself widely used to correct for deficiencies in Oseen. We also include plane wave moves to provide the longest range correlations, which we detail for both infinite and periodic systems. The computational cost of the algorithm scales competitively with the number of particles simulated, N, scaling as N In N in homogeneous systems and as N in dilute systems. In comparisons to established lattice Boltzmann and Brownian dynamics algorithms, the wavelet method was found to be only a factor of order 1 times more expensive than the cheaper lattice Boltzmann algorithm in marginally semi-dilute simulations, while it is significantly faster than both algorithms at large N in dilute simulations. We also validate the algorithm by checking that it reproduces the correct dynamics and equilibrium properties of simple single polymer systems, as well as verifying the effect of periodicity on the mobility tensor.}, } @article {pmid28373015, year = {2017}, author = {Miguel, AF}, title = {Penetration of inhaled aerosols in the bronchial tree.}, journal = {Medical engineering & physics}, volume = {44}, number = {}, pages = {25-31}, doi = {10.1016/j.medengphy.2017.03.004}, pmid = {28373015}, issn = {1873-4030}, mesh = {Aerosols ; Bronchi/*anatomy & histology/*metabolism/physiology ; Humans ; *Inhalation ; *Models, Anatomic ; Particle Size ; }, abstract = {It has long been recognized that the pattern of particle deposition in the respiratory tree affects how far aerosols penetrate into the deeper zones of the arterial tree, and hence contribute to either their pathogenic potential or therapeutic benefit. In this paper, we introduce an anatomically-inspired model of the human respiratory tree featuring the generations 0-7 in the Weibel model of respiratory tree (i.e., the conducting zone). This model is used to study experimentally the dynamics of inhaled aerosol particles (0.5-20µm aerodynamic diameter), in terms of the penetration fraction of particles (i.e., the fraction of inflowing particles that leave the flow system) during typical breathing patterns. Our study underline important modifications in the penetration patterns for coarse particles compared to fine particles. Our experiments suggest a significant decrease of particle penetration for large-sized particles and higher respiratory frequencies. Dimensionless numbers are also introduced to further understand the particle penetration into the respiratory tree. A decline is seen in the penetration fraction with decreasing Reynolds number and increasing Stokes number. A simple conceptual framework is presented to provide additional insights into the findings obtained.}, } @article {pmid28364770, year = {2017}, author = {Smaoui, N and Zribi, M}, title = {On the control of the chaotic attractors of the 2-d Navier-Stokes equations.}, journal = {Chaos (Woodbury, N.Y.)}, volume = {27}, number = {3}, pages = {033111}, doi = {10.1063/1.4978682}, pmid = {28364770}, issn = {1089-7682}, abstract = {The control problem of the chaotic attractors of the two dimensional (2-d) Navier-Stokes (N-S) equations is addressed in this paper. First, the Fourier Galerkin method based on a reduced-order modelling approach developed by Chen and Price is applied to the 2-d N-S equations to construct a fifth-order system of nonlinear ordinary differential equations (ODEs). The dynamics of the fifth-order system was studied by analyzing the system's attractor for different values of Reynolds number, Re. Then, control laws are proposed to drive the states of the ODE system to a desired attractor. Finally, an adaptive controller is designed to synchronize two reduced order ODE models having different Reynolds numbers and starting from different initial conditions. Simulation results indicate that the proposed control schemes work well.}, } @article {pmid28342532, year = {2017}, author = {Gülan, U and Binter, C and Kozerke, S and Holzner, M}, title = {Shear-scaling-based approach for irreversible energy loss estimation in stenotic aortic flow - An in vitro study.}, journal = {Journal of biomechanics}, volume = {56}, number = {}, pages = {89-96}, doi = {10.1016/j.jbiomech.2017.03.006}, pmid = {28342532}, issn = {1873-2380}, abstract = {Today, the functional and risk assessment of stenosed arteries is mostly based on ultrasound Doppler blood flow velocity measurements or catheter pressure measurements, which rely on several assumptions. Alternatively, blood velocity including turbulent kinetic energy (TKE) may be measured using MRI. The aim of the present study is to validate a TKE-based approach that relies on the fact that turbulence production is dominated by the flow's shear to determine the total irreversible energy loss from MRI scans. Three-dimensional particle tracking velocimetry (3D-PTV) and phase-contrast magnetic resonance imaging (PC-MRI) simulations were performed in an anatomically accurate, compliant, silicon aortic phantom. We found that measuring only the laminar viscous losses does not reflect the true losses of stenotic flows since the contribution of the turbulent losses to the total loss become more dominant for more severe stenosis types (for example, the laminar loss is 0.0094±0.0015W and the turbulent loss is 0.0361±0.0015W for the Remax=13,800 case, where Remax is the Reynolds number based on the velocity in the vena-contracta). We show that the commonly used simplified and modified Bernoulli's approaches overestimate the total loss, while the new TKE-based method proposed here, referred to as "shear scaling" approach, results in a good agreement between 3D-PTV and simulated PC-MRI (mean error is around 10%). In addition, we validated the shear scaling approach on a geometry with post-stenotic dilatation using numerical data by Casas et al. (2016). The shear scaling-based method may hence be an interesting alternative for irreversible energy loss estimation to replace traditional approaches for clinical use. We expect that our results will evoke further research, in particular patient studies for clinical implementation of the new method.}, } @article {pmid28337415, year = {2017}, author = {Nijp, JJ and Metselaar, K and Limpens, J and Gooren, HP and van der Zee, SE}, title = {A modification of the constant-head permeameter to measure saturated hydraulic conductivity of highly permeable media.}, journal = {MethodsX}, volume = {4}, number = {}, pages = {134-142}, doi = {10.1016/j.mex.2017.02.002}, pmid = {28337415}, issn = {2215-0161}, abstract = {The saturated hydraulic conductivity (Ks) is a key characteristic of porous media, describing the rate of water flow through saturated porous media. It is an indispensable parameter in a broad range of simulation models that quantify saturated and/or unsaturated water flow. The constant-head permeameter test is a common laboratory method to determine Ks on undisturbed soil samples collected from the field. In this paper we show that the application of this conventional method may result in a biased Ks in the case of highly permeable media, such as the top layer of Sphagnum peat and gravel. Tubes in the conventional permeameter, that collect water under the sample, introduce a hydraulic head-dependent resistance for highly permeable media and result in an underestimation of Ks . We present a simple and low-budget alternative of the constant-head permeameter test that overcomes the disadvantages of conventional permeameters. The new method was successfully tested on intact highly permeable peatmoss collected from a northern peatland. •Conventional constant-head permeameters underestimate Ks of highly permeable media due to flow resistance in tubing systems•We developed the low-resistance permeameter to overcome this disadvantage.•Testing of the low-resistance permeameter demonstrated no systematic bias and successful application for highly permeable media.}, } @article {pmid28325440, year = {2017}, author = {Hayat, T and Farooq, S and Alsaedi, A}, title = {Mixed convection peristaltic motion of copper-water nanomaterial with velocity slip effects in a curved channel.}, journal = {Computer methods and programs in biomedicine}, volume = {142}, number = {}, pages = {117-128}, doi = {10.1016/j.cmpb.2017.02.006}, pmid = {28325440}, issn = {1872-7565}, mesh = {Computer Simulation ; Convection ; Copper/*chemistry ; Drug Delivery Systems ; Hot Temperature ; Metal Nanoparticles ; Models, Theoretical ; Motion ; Nanostructures/*chemistry ; *Peristalsis ; Pressure ; Rheology ; Software ; Viscosity ; Water/*chemistry ; }, abstract = {BACKGROUND AND OBJECTIVE: The primary objective of present analysis is to model the peristalsis of copper-water based nanoliquid in the presence of first order velocity and thermal slip conditions in a curved channel. Mixed convection, viscous dissipation and heat generation/absorption are also accounted.

METHOD: Mathematical formulation is simplified under the assumption of small Reynolds number and large wavelength. Regular perturbation technique is employed to find the solution of the resulting equations in terms of series for small Brinkman number. The final expression for pressure gradient, pressure rise, stream function, velocity and temperature are obtained and discussed through graphs. Mathematica software is utilized to compute the solution of the system of equations and to plot the graphical results.

RESULTS: Results indicates that insertion of 30% copper nanoparticles in the basefluid (water) velocity and temperature reduces by almost 3% and 40% respecively. Moreover it is seen that size of the trapped bolus also reduces almost 20% with the insertion of 20% nanoparticles (copper) in the basefluid (water).

CONCLUSION: It is noted that velocity and temperature are decreasing functions of nanoparticle volume fraction. Moreover the temperature rises when heat generation parameter and Brinkman number are enhanced.}, } @article {pmid28297984, year = {2017}, author = {Hejranfar, K and Saadat, MH and Taheri, S}, title = {High-order weighted essentially nonoscillatory finite-difference formulation of the lattice Boltzmann method in generalized curvilinear coordinates.}, journal = {Physical review. E}, volume = {95}, number = {2-1}, pages = {023314}, doi = {10.1103/PhysRevE.95.023314}, pmid = {28297984}, issn = {2470-0053}, abstract = {In this work, a high-order weighted essentially nonoscillatory (WENO) finite-difference lattice Boltzmann method (WENOLBM) is developed and assessed for an accurate simulation of incompressible flows. To handle curved geometries with nonuniform grids, the incompressible form of the discrete Boltzmann equation with the Bhatnagar-Gross-Krook (BGK) approximation is transformed into the generalized curvilinear coordinates and the spatial derivatives of the resulting lattice Boltzmann equation in the computational plane are solved using the fifth-order WENO scheme. The first-order implicit-explicit Runge-Kutta scheme and also the fourth-order Runge-Kutta explicit time integrating scheme are adopted for the discretization of the temporal term. To examine the accuracy and performance of the present solution procedure based on the WENOLBM developed, different benchmark test cases are simulated as follows: unsteady Taylor-Green vortex, unsteady doubly periodic shear layer flow, steady flow in a two-dimensional (2D) cavity, steady cylindrical Couette flow, steady flow over a 2D circular cylinder, and steady and unsteady flows over a NACA0012 hydrofoil at different flow conditions. Results of the present solution are compared with the existing numerical and experimental results which show good agreement. To show the efficiency and accuracy of the solution methodology, the results are also compared with the developed second-order central-difference finite-volume lattice Boltzmann method and the compact finite-difference lattice Boltzmann method. It is shown that the present numerical scheme is robust, efficient, and accurate for solving steady and unsteady incompressible flows even at high Reynolds number flows.}, } @article {pmid28297968, year = {2017}, author = {de Graaf, J and Stenhammar, J}, title = {Lattice-Boltzmann simulations of microswimmer-tracer interactions.}, journal = {Physical review. E}, volume = {95}, number = {2-1}, pages = {023302}, doi = {10.1103/PhysRevE.95.023302}, pmid = {28297968}, issn = {2470-0053}, abstract = {Hydrodynamic interactions in systems composed of self-propelled particles, such as swimming microorganisms and passive tracers, have a significant impact on the tracer dynamics compared to the equivalent "dry" sample. However, such interactions are often difficult to take into account in simulations due to their computational cost. Here, we perform a systematic investigation of swimmer-tracer interaction using an efficient force-counterforce-based lattice-Boltzmann (LB) algorithm [De Graaf et al., J. Chem. Phys. 144, 134106 (2016)JCPSA60021-960610.1063/1.4944962] in order to validate its ability to capture the relevant low-Reynolds-number physics. We show that the LB algorithm reproduces far-field theoretical results well, both in a system with periodic boundary conditions and in a spherical cavity with no-slip walls, for which we derive expressions here. The force-lattice coupling of the LB algorithm leads to a "smearing out" of the flow field, which strongly perturbs the tracer trajectories at close swimmer-tracer separations, and we analyze how this effect can be accurately captured using a simple renormalized hydrodynamic theory. Finally, we show that care must be taken when using LB algorithms to simulate systems of self-propelled particles, since its finite momentum transport time can lead to significant deviations from theoretical predictions based on Stokes flow. These insights should prove relevant to the future study of large-scale microswimmer suspensions using these methods.}, } @article {pmid28297886, year = {2017}, author = {Iyer, KP and Sreenivasan, KR and Yeung, PK}, title = {Reynolds number scaling of velocity increments in isotropic turbulence.}, journal = {Physical review. E}, volume = {95}, number = {2-1}, pages = {021101}, doi = {10.1103/PhysRevE.95.021101}, pmid = {28297886}, issn = {2470-0053}, abstract = {Using the largest database of isotropic turbulence available to date, generated by the direct numerical simulation (DNS) of the Navier-Stokes equations on an 8192^{3} periodic box, we show that the longitudinal and transverse velocity increments scale identically in the inertial range. By examining the DNS data at several Reynolds numbers, we infer that the contradictory results of the past on the inertial-range universality are artifacts of low Reynolds number and residual anisotropy. We further show that both longitudinal and transverse velocity increments scale on locally averaged dissipation rate, just as postulated by Kolmogorov's refined similarity hypothesis, and that, in isotropic turbulence, a single independent scaling adequately describes fluid turbulence in the inertial range.}, } @article {pmid28289562, year = {2017}, author = {Cheng, X and Sun, M}, title = {Aerodynamic forces and flows of the full and partial clap-fling motions in insects.}, journal = {PeerJ}, volume = {5}, number = {}, pages = {e3002}, doi = {10.7717/peerj.3002}, pmid = {28289562}, issn = {2167-8359}, abstract = {Most of the previous studies on Weis-Fogh clap-fling mechanism have focused on the vortex structures and velocity fields. Detailed pressure distribution results are provided for the first time in this study to reveal the differences between the full and the partial clap-fling motions. The two motions are studied by numerically solving the Navier-Stokes equations in moving overset grids. The Reynolds number is set to 20, relevant to the tiny flying insects. The following has been shown: (1) During the clap phase, the wings clap together and create a high pressure region in the closing gap between wings, greatly increasing the positive pressure on the lower surface of wing, while pressure on the upper surface is almost unchanged by the interaction; during the fling phase, the wings fling apart and create a low pressure region in the opening gap between wings, greatly increasing the suction pressure on the upper surface of wing, while pressure on the lower surface is almost unchanged by the interaction; (2) The interference effect between wings is most severe at the end of clap phase and the start of the fling phase: two sharp force peaks (8-9 times larger than that of the one-winged case) are generated. But the total force peaks are manifested mostly as drag and barely as lift of the wing, owing to the vertical orientation of the wing section; (3) The wing-wing interaction effect in the partial clap-fling case is much weaker than that in the full clap-fling case, avoiding the generation of huge drag. Compared with a single wing flapping with the same motion, mean lift in the partial case is enhanced by 12% without suffering any efficiency degradation, indicating that partial clap-fling is a more practical choice for tiny insects to employ.}, } @article {pmid28270051, year = {2017}, author = {Zeinoddini, M and Bakhtiari, A and Schoefs, F and Zandi, AP}, title = {Towards an understanding of marine fouling effects on the vortex-induced vibrations of circular cylinders: partial coverage issue.}, journal = {Biofouling}, volume = {33}, number = {3}, pages = {268-280}, doi = {10.1080/08927014.2017.1291803}, pmid = {28270051}, issn = {1029-2454}, mesh = {Animals ; Biofilms/*growth & development ; Biofouling/*prevention & control ; *Hydrodynamics ; *Models, Theoretical ; Thoracica/*physiology ; *Vibration ; }, abstract = {The results of in-water vortex-induced vibration (VIV) experiments on circular cylinders artificially covered with barnacles are reported. The paper focusses on the effects of the partial coverage and the shape of the fouling elements. An artificial barnacle typical of marine fouling was synthesised using 3-D printing. Coverage ratios of 80, 50 and 30% were examined and the results compared with those from a smooth cylinder. The Reynolds number ranged from 5.8 × 103 to 6.6 × 104. The experimental results show that the fouling reduced the peak VIV amplitude, narrowed the synchronisation region and lowered the hydrodynamic force coefficients such as the coefficients of lift force RMS, the mean drag force and the fluctuating drag force RMS. The shape of the artificial barnacles had little effect on the maximum oscillation amplitude. The coverage ratio appeared to have a lower impact on the lift force than those on the amplitude and the frequency responses.}, } @article {pmid28267999, year = {2017}, author = {Gritti, F}, title = {Extension of Golay's plate height equation from laminar to turbulent flow I - Theory.}, journal = {Journal of chromatography. A}, volume = {1492}, number = {}, pages = {129-135}, doi = {10.1016/j.chroma.2017.02.044}, pmid = {28267999}, issn = {1873-3778}, mesh = {Algorithms ; Chromatography, Supercritical Fluid ; *Models, Theoretical ; Temperature ; }, abstract = {The reduced plate height (RPH) equation of Golay derived in 1958 for open tubular columns (OTC) is extended from laminar to turbulent-like flow. The mass balance equation is solved under near-equilibrium conditions in the mobile phase for changing shapes of the velocity profile across the OTC diameter. The final expression of the general RPH equation is: [Formula: see text] where ν is the reduced linear velocity, k is the retention factor, Dm is the bulk diffusion coefficient in the mobile phase, Da¯ is the average axial dispersion coefficient, Dr¯ is the average radial dispersion coefficient, Ds is the diffusion coefficient of the analyte in the stationary film of thickness df, D is the OTC inner diameter, and n≥2 is a positive number controlling the shape of the flow profile (polynomial of degree n). The correctness of the derived RPH equation is verified for Poiseuille (n=2), turburlent-like (n=10), and uniformly flat (n→∞) flow profiles. The derived RPH equation is applied to predict the gain in speed-resolution of a 180μm i.d.×20m OTC (df=2μm) from laminar to turbulent flow in supercritical fluid chromatography. Using pure carbon dioxide as the mobile phase at 297K, k=1, and increasing the Reynolds number from 2000 (laminar) to 4000 (turbulent), the OTC efficiency is expected to increase from 125 to 670 (×5.4) while the hold-up time decreases from 19 to 9s (×0.5). Despite the stronger resistance to mass transfer in the stationary phase, the projected improvement of the column performance in turbulent flow is explained by the quasi-elimination of the resistance to mass transfer in the mobile phase while axial dispersion remains negligible.}, } @article {pmid28267159, year = {2017}, author = {Peng, Z and Elfring, GJ and Pak, OS}, title = {Maximizing propulsive thrust of a driven filament at low Reynolds number via variable flexibility.}, journal = {Soft matter}, volume = {13}, number = {12}, pages = {2339-2347}, doi = {10.1039/c6sm02880b}, pmid = {28267159}, issn = {1744-6848}, abstract = {At low Reynolds numbers the locomotive capability of a body can be dramatically hindered by the absence of inertia. In this work, we show how propulsive performance in this regime can be significantly enhanced by employing spatially varying flexibility. As a prototypical example, we consider the propulsive thrust generated by a filament periodically driven at one end. The rigid case leads to zero propulsion, as so constrained by Purcell's scallop theorem, while for uniform filaments there exists a bending stiffness maximizing the propulsive force at a given frequency; here we demonstrate explicitly how considerable further improvement can be achieved by simply varying the stiffness along the filament. The optimal flexibility distribution is strongly configuration-dependent: while increasing the flexibility towards the tail-end enhances the propulsion of a clamped filament, for a hinged filament decreasing the flexibility towards the tail-end is instead favorable. The results reveal new design principles for maximizing propulsion at low Reynolds numbers, potentially useful for developing synthetic micro-swimmers requiring large propulsive force for various biomedical applications.}, } @article {pmid28253673, year = {2017}, author = {Van Blitterswyk, J and Rocha, J}, title = {An experimental study of the wall-pressure fluctuations beneath low Reynolds number turbulent boundary layers.}, journal = {The Journal of the Acoustical Society of America}, volume = {141}, number = {2}, pages = {1257}, doi = {10.1121/1.4976341}, pmid = {28253673}, issn = {1520-8524}, abstract = {A more complete understanding of the physical relationships, between wall-pressure and turbulence, is required for modeling flow-induced noise and developing noise reduction strategies. In this study, the wall-pressure fluctuations, induced by low Reynolds number turbulent boundary layers, are experimentally studied using a high-resolution microphone array. Statistical characteristics obtained using traditional cross-correlation and cross-spectra analyses are complimented with wall-pressure-velocity cross-spectra and wavelet cross-correlations. Wall-pressure-velocity correlations revealed that turbulent activity in the buffer layer contributes at least 40% of the energy to the wall-pressure spectrum at all measured frequencies. As Reynolds number increases, the low-frequency energy shifts from the buffer layer to the logarithmic layer, as expected for regions of uniform streamwise momentum formed by hairpin packets. Conditional cross-spectra suggests that the majority of broadband wall-pressure energy is concentrated within the packets, with the pressure signatures of individual hairpin vortices estimated to decay on average within traveling ten displacement thicknesses, and the packet signature is retained for up to seven boundary layer thicknesses on average.}, } @article {pmid28234274, year = {2017}, author = {Qin, Y and Wu, J and Hu, Q and Ghista, DN and Wong, KK}, title = {Computational evaluation of smoothed particle hydrodynamics for implementing blood flow modelling through CT reconstructed arteries.}, journal = {Journal of X-ray science and technology}, volume = {25}, number = {2}, pages = {213-232}, doi = {10.3233/XST-17255}, pmid = {28234274}, issn = {1095-9114}, mesh = {Arterial Occlusive Diseases/diagnostic imaging/pathology/physiopathology ; Blood Flow Velocity/*physiology ; *Computer Simulation ; Humans ; Hydrodynamics ; Image Processing, Computer-Assisted/*methods ; *Models, Cardiovascular ; Tomography, X-Ray Computed/*methods ; }, abstract = {Simulation of blood flow in a stenosed artery using Smoothed Particle Hydrodynamics (SPH) is a new research field, which is a particle-based method and different from the traditional continuum modelling technique such as Computational Fluid Dynamics (CFD). Both techniques harness parallel computing to process hemodynamics of cardiovascular structures. The objective of this study is to develop and test a new robust method for comparison of arterial flow velocity contours by SPH with the well-established CFD technique, and the implementation of SPH in computed tomography (CT) reconstructed arteries. The new method was developed based on three-dimensional (3D) straight and curved arterial models of millimeter range with a 25% stenosis in the middle section. In this study, we employed 1,000 to 13,000 particles to study how the number of particles influences SPH versus CFD deviation for blood-flow velocity distribution. Because further increasing the particle density has a diminishing effect on this deviation, we have determined a critical particle density of 1.45 particles/mm2 based on Reynolds number (Re = 200) at the inlet for an arterial flow simulation. Using this critical value of particle density can avoid unnecessarily big computational expenses that have no further effect on simulation accuracy. We have particularly shown that the SPH method has a big potential to be used in the virtual surgery system, such as to simulate the interaction between blood flow and the CT reconstructed vessels, especially those with stenosis or plaque when encountering vasculopathy, and for employing the simulation results output in clinical surgical procedures.}, } @article {pmid28231298, year = {2017}, author = {Hayat, T and Aziz, A and Muhammad, T and Alsaedi, A}, title = {A revised model for Jeffrey nanofluid subject to convective condition and heat generation/absorption.}, journal = {PloS one}, volume = {12}, number = {2}, pages = {e0172518}, doi = {10.1371/journal.pone.0172518}, pmid = {28231298}, issn = {1932-6203}, mesh = {Algorithms ; Computer Simulation ; *Convection ; *Hot Temperature ; *Hydrodynamics ; Magnetic Fields ; Models, Chemical ; Motion ; Nanoparticles/*chemistry ; Nanotechnology ; Surface Properties ; }, abstract = {Here magnetohydrodynamic (MHD) boundary layer flow of Jeffrey nanofluid by a nonlinear stretching surface is addressed. Heat generation/absorption and convective surface condition effects are considered. Novel features of Brownian motion and thermophoresis are present. A non-uniform applied magnetic field is employed. Boundary layer and small magnetic Reynolds number assumptions are employed in the formulation. A newly developed condition with zero nanoparticles mass flux is imposed. The resulting nonlinear systems are solved. Convergence domains are explicitly identified. Graphs are analyzed for the outcome of sundry variables. Further local Nusselt number is computed and discussed. It is observed that the effects of Hartman number on the temperature and concentration distributions are qualitatively similar. Both temperature and concentration distributions are enhanced for larger Hartman number.}, } @article {pmid28222160, year = {2017}, author = {Hayat, T and Zahir, H and Tanveer, A and Alsaedi, A}, title = {Soret and Dufour effects on MHD peristaltic transport of Jeffrey fluid in a curved channel with convective boundary conditions.}, journal = {PloS one}, volume = {12}, number = {2}, pages = {e0164854}, doi = {10.1371/journal.pone.0164854}, pmid = {28222160}, issn = {1932-6203}, mesh = {Convection ; Diffusion ; Electric Conductivity ; Equipment Design ; Magnetic Fields ; Osmolar Concentration ; Peristalsis ; *Rheology ; Solutions ; *Temperature ; }, abstract = {The purpose of present article is to examine the peristaltic flow of Jeffrey fluid in a curved channel. An electrically conducting fluid in the presence of radial applied magnetic field is considered. Analysis of heat and mass transfer is carried out. More generalized realistic constraints namely the convective conditions are utilized. Soret and Dufour effects are retained. Problems formulation is given for long wavelength and low Reynolds number assumptions. The expressions of velocity, temperature, heat transfer coefficient, concentration and stream function are computed. Effects of emerging parameters arising in solutions are analyzed in detail. It is found that velocity is not symmetric about centreline for curvature parameter. Also maximum velocity decreases with an increase in the strength of magnetic field. Further it is noticed that Soret and Dufour numbers have opposite behavior for temperature and concentration.}, } @article {pmid28208335, year = {2017}, author = {Walchli, B and Thornber, B}, title = {Reynolds number effects on the single-mode Richtmyer-Meshkov instability.}, journal = {Physical review. E}, volume = {95}, number = {1-1}, pages = {013104}, doi = {10.1103/PhysRevE.95.013104}, pmid = {28208335}, issn = {2470-0053}, abstract = {The Reynolds number effects on the nonlinear growth rates of the Richtmyer-Meshkov instability are investigated using two-dimensional numerical simulations. A decrease in Reynolds number gives an increased time to reach nonlinear saturation, with Reynolds number effects only significant in the range Re<256. Within this range there is a sharp change in instability properties. The bubble and spike amplitudes move towards equal size at lower Reynolds numbers and the bubble velocities decay faster than predicted by Sohn's model [S.-I. Sohn, Phys. Rev. E 80, 055302 (2009)PLEEE81539-375510.1103/PhysRevE.80.055302]. Predicted amplitudes show reasonable agreement with the existing theory of Carles and Popinet [P. Carles and S. Popinet, Phys. Fluids Lett. 13, 1833 (2001)10.1063/1.1377863; Eur. J. Mech. B 21, 511 (2002)EJBFEV0997-754610.1016/S0997-7546(02)01199-8] and Mikaelian [K. O. Mikaelian, Phys. Rev. E 47, 375 (1993)1063-651X10.1103/PhysRevE.47.375; K. O. Mikaelian, Phys. Rev. E 87, 031003 (2013)PLEEE81539-375510.1103/PhysRevE.87.031003], with the former being the closest match to the current computations.}, } @article {pmid28208331, year = {2017}, author = {Linkmann, M and Berera, A and Goldstraw, EE}, title = {Reynolds-number dependence of the dimensionless dissipation rate in homogeneous magnetohydrodynamic turbulence.}, journal = {Physical review. E}, volume = {95}, number = {1-1}, pages = {013102}, doi = {10.1103/PhysRevE.95.013102}, pmid = {28208331}, issn = {2470-0053}, abstract = {This paper examines the behavior of the dimensionless dissipation rate C_{ɛ} for stationary and nonstationary magnetohydrodynamic (MHD) turbulence in the presence of external forces. By combining with previous studies for freely decaying MHD turbulence, we obtain here both the most general model equation for C_{ɛ} applicable to homogeneous MHD turbulence and a comprehensive numerical study of the Reynolds number dependence of the dimensionless total energy dissipation rate at unity magnetic Prandtl number. We carry out a series of medium to high resolution direct numerical simulations of mechanically forced stationary MHD turbulence in order to verify the predictions of the model equation for the stationary case. Furthermore, questions of nonuniversality are discussed in terms of the effect of external forces as well as the level of cross- and magnetic helicity. The measured values of the asymptote C_{ɛ,∞} lie between 0.193≤C_{ɛ,∞} ≤0.268 for free decay, where the value depends on the initial level of cross- and magnetic helicities. In the stationary case we measure C_{ɛ,∞} =0.223.}, } @article {pmid28191582, year = {2017}, author = {Yasuda, S and Hayakawa, M and Onoe, H and Takinoue, M}, title = {Twisting microfluidics in a planetary centrifuge.}, journal = {Soft matter}, volume = {13}, number = {11}, pages = {2141-2147}, doi = {10.1039/c6sm02695h}, pmid = {28191582}, issn = {1744-6848}, abstract = {This paper reports a twisting microfluidic method utilising a centrifuge-based fluid extruding system in a planetary centrifuge which simultaneously generates an orbital rotation and an axial spin. In this method, fluid extrusion from a micro-scale capillary to an 'open-space' solution or air enables release of the fluid from the capillary-based microchannel, which physically means that there is a release of fluids from a confined low-Reynolds-number environment to an open non-low-Reynolds-number environment. As a result, the extruded fluids are separated from the axial spin of the capillary, and the difference in the angular rates of the axial spin between the capillary and the extruded fluids produces the 'twisting' of the fluid. In this study, we achieve control of the twist of highly viscous fluids, and we construct a simple physical model for the fluid twist. In addition, we demonstrate the formation of twisted hydrogel microstructures (stripe-patterned microbeads and multi-helical microfibres) with control over the stripe pattern and the helical pitch length. We believe that this method will enable the generation of more sophisticated microstructures which cannot easily be formed by usual channel-based microfluidic devices. This method can also provide advanced control of microfluids, as in the case of rapid mixing of highly viscous fluids. This method can contribute to a wide range of applications in materials science, biophysics, biomedical science, and microengineering in the future.}, } @article {pmid28187884, year = {2017}, author = {Shahzadi, I and Sadaf, H and Nadeem, S and Saleem, A}, title = {Bio-mathematical analysis for the peristaltic flow of single wall carbon nanotubes under the impact of variable viscosity and wall properties.}, journal = {Computer methods and programs in biomedicine}, volume = {139}, number = {}, pages = {137-147}, doi = {10.1016/j.cmpb.2016.10.016}, pmid = {28187884}, issn = {1872-7565}, mesh = {*Nanotubes, Carbon ; *Peristalsis ; *Viscosity ; }, abstract = {OBJECTIVE: The main objective of this paper is to study the Bio-mathematical analysis for the peristaltic flow of single wall carbon nanotubes under the impact of variable viscosity and wall properties.

DESIGN/APPROACH: The right and the left walls of the curved channel possess sinusoidal wave that is travelling along the outer boundary. The features of the peristaltic motion are determined by using long wavelength and low Reynolds number approximation. Exact solutions are determined for the axial velocity and for the temperature profile.

FINDINGS: Graphical results have been presented for velocity profile, temperature and stream function for various physical parameters of interest. Symmetry of the curved channel is disturbed for smaller values of the curvature parameter. It is found that the altitude of the velocity profile increases for larger values of variable viscosity parameter for both the cases (pure blood as well as single wall carbon nanotubes). It is detected that velocity profile increases with increasing values of rigidity parameter. It is due to the fact that an increase in rigidity parameter decreases tension in the walls of the blood vessels which speeds up the blood flow for pure blood as well as single wall carbon nanotubes. Increase in Grashof number decreases the fluid velocity. This is due to the reason that viscous forces play a prominent role that's why increase in Grashof number decreases the velocity profile. It is also found that temperature drops for increasing values of nanoparticle volume fraction. Basically, higher thermal conductivity of the nanoparticles plays a key role for quick heat dissipation, and this justifies the use of the single wall carbon nanotubes in different situations as a coolant.

CONCLUSIONS: Exact solutions are calculated for the temperature and the velocity profile. Symmetry of the curved channel is destroyed due to the curvedness for velocity, temperature and contour plots. Addition of single wall carbon nanotubes shows a decrease in fluid temperature. Trapping phenomena show that the size of the trapped bolus is smaller for pure blood case as compared to the single wall carbon nanotubes.}, } @article {pmid28176897, year = {2017}, author = {Guiffant, G and Flaud, P and Royon, L and Burnet, E and Merckx, J}, title = {Mechanical characteristics of plastic base Ports and impact on flushing efficacy.}, journal = {Medical devices (Auckland, N.Z.)}, volume = {10}, number = {}, pages = {11-15}, doi = {10.2147/MDER.S125605}, pmid = {28176897}, issn = {1179-1470}, abstract = {BACKGROUND: Three types of totally implantable venous access devices, Ports, are currently in use: titanium, plastic (polyoxymethylene, POM), and mixed (titanium base with a POM shell). Physics theory suggests that the interaction between a non-coring needle (NCN, made of stainless steel) and a plastic base would lead to the stronger material (steel) altering the more malleable material (plastic).

OBJECTIVES: To investigate whether needle impacts can alter a plastic base's surface, thus potentially reducing flushing efficacy.

STUDY DESIGN AND METHODS: A Port made of POM was punctured 200 times with a 19-gauge NCN. Following the existing guidelines, the needle tip pricked the base with each puncture. The Port's base was then examined using a two-dimensional optical instrument, and a bi-dimensional numerical simulation using COMSOL® was performed to investigate potential surface irregularities and their impact on fluid flow.

RESULTS: Each needle impact created a hole (mean depth, 0.12 mm) with a small bump beside it (mean height, 0.02 mm) the Reynolds number Rek≈10. A numerical simulation of the one hole/bump set showed that the flushing efficacy was 60% that of flushing along a flat surface.

DISCUSSION: In clinical practice, the number of times a Port is punctured depends on patient and treatment characteristics, but each needle impact on the plastic base may increase the risk of decreased flushing effectiveness. Therefore, the more a plastic Port is accessed, the greater the risk of microorganisms, blood products, and medication accumulation.

CONCLUSIONS: Multiple needle impacts created an irregular surface on the Port's base, which decreased flushing efficacy. Clinical investigation is needed to determine whether plastic base Ports are associated with an increased risk of Port infection and occlusion compared to titanium base Ports.}, } @article {pmid28167586, year = {2017}, author = {Örlü, R and Fiorini, T and Segalini, A and Bellani, G and Talamelli, A and Alfredsson, PH}, title = {Reynolds stress scaling in pipe flow turbulence-first results from CICLoPE.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0187}, pmid = {28167586}, issn = {1364-503X}, abstract = {This paper reports the first turbulence measurements performed in the Long Pipe Facility at the Center for International Cooperation in Long Pipe Experiments (CICLoPE). In particular, the Reynolds stress components obtained from a number of straight and boundary-layer-type single-wire and X-wire probes up to a friction Reynolds number of 3.8×104 are reported. In agreement with turbulent boundary-layer experiments as well as with results from the Superpipe, the present measurements show a clear logarithmic region in the streamwise variance profile, with a Townsend-Perry constant of A2≈1.26. The wall-normal variance profile exhibits a Reynolds-number-independent plateau, while the spanwise component was found to obey a logarithmic scaling over a much wider wall-normal distance than the other two components, with a slope that is nearly half of that of the Townsend-Perry constant, i.e. A2,w≈A2/2. The present results therefore provide strong support for the scaling of the Reynolds stress tensor based on the attached-eddy hypothesis. Intriguingly, the wall-normal and spanwise components exhibit higher amplitudes than in previous studies, and therefore call for follow-up studies in CICLoPE, as well as other large-scale facilities.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167585, year = {2017}, author = {Klewicki, JC and Chini, GP and Gibson, JF}, title = {Prospectus: towards the development of high-fidelity models of wall turbulence at large Reynolds number.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0092}, pmid = {28167585}, issn = {1364-503X}, abstract = {Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier-Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167584, year = {2017}, author = {Dogan, E and Hearst, RJ and Ganapathisubramani, B}, title = {Modelling high Reynolds number wall-turbulence interactions in laboratory experiments using large-scale free-stream turbulence.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0091}, pmid = {28167584}, issn = {1364-503X}, abstract = {A turbulent boundary layer subjected to free-stream turbulence is investigated in order to ascertain the scale interactions that dominate the near-wall region. The results are discussed in relation to a canonical high Reynolds number turbulent boundary layer because previous studies have reported considerable similarities between these two flows. Measurements were acquired simultaneously from four hot wires mounted to a rake which was traversed through the boundary layer. Particular focus is given to two main features of both canonical high Reynolds number boundary layers and boundary layers subjected to free-stream turbulence: (i) the footprint of the large scales in the logarithmic region on the near-wall small scales, specifically the modulating interaction between these scales, and (ii) the phase difference in amplitude modulation. The potential for a turbulent boundary layer subjected to free-stream turbulence to 'simulate' high Reynolds number wall-turbulence interactions is discussed. The results of this study have encouraging implications for future investigations of the fundamental scale interactions that take place in high Reynolds number flows as it demonstrates that these can be achieved at typical laboratory scales.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167583, year = {2017}, author = {Chini, GP and Montemuro, B and White, CM and Klewicki, J}, title = {A self-sustaining process model of inertial layer dynamics in high Reynolds number turbulent wall flows.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0090}, pmid = {28167583}, issn = {1364-503X}, abstract = {Field observations and laboratory experiments suggest that at high Reynolds numbers Re the outer region of turbulent boundary layers self-organizes into quasi-uniform momentum zones (UMZs) separated by internal shear layers termed 'vortical fissures' (VFs). Motivated by this emergent structure, a conceptual model is proposed with dynamical components that collectively have the potential to generate a self-sustaining interaction between a single VF and adjacent UMZs. A large-Re asymptotic analysis of the governing incompressible Navier-Stokes equation is performed to derive reduced equation sets for the streamwise-averaged and streamwise-fluctuating flow within the VF and UMZs. The simplified equations reveal the dominant physics within-and isolate possible coupling mechanisms among-these different regions of the flow.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167582, year = {2017}, author = {Sharma, AS and Moarref, R and McKeon, BJ}, title = {Scaling and interaction of self-similar modes in models of high Reynolds number wall turbulence.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0089}, pmid = {28167582}, issn = {1364-503X}, abstract = {Previous work has established the usefulness of the resolvent operator that maps the terms nonlinear in the turbulent fluctuations to the fluctuations themselves. Further work has described the self-similarity of the resolvent arising from that of the mean velocity profile. The orthogonal modes provided by the resolvent analysis describe the wall-normal coherence of the motions and inherit that self-similarity. In this contribution, we present the implications of this similarity for the nonlinear interaction between modes with different scales and wall-normal locations. By considering the nonlinear interactions between modes, it is shown that much of the turbulence scaling behaviour in the logarithmic region can be determined from a single arbitrarily chosen reference plane. Thus, the geometric scaling of the modes is impressed upon the nonlinear interaction between modes. Implications of these observations on the self-sustaining mechanisms of wall turbulence, modelling and simulation are outlined.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167576, year = {2017}, author = {Duvvuri, S and McKeon, B}, title = {Phase relations in a forced turbulent boundary layer: implications for modelling of high Reynolds number wall turbulence.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0080}, pmid = {28167576}, issn = {1364-503X}, abstract = {Phase relations between specific scales in a turbulent boundary layer are studied here by highlighting the associated nonlinear scale interactions in the flow. This is achieved through an experimental technique that allows for targeted forcing of the flow through the use of a dynamic wall perturbation. Two distinct large-scale modes with well-defined spatial and temporal wavenumbers were simultaneously forced in the boundary layer, and the resulting nonlinear response from their direct interactions was isolated from the turbulence signal for the study. This approach advances the traditional studies of large- and small-scale interactions in wall turbulence by focusing on the direct interactions between scales with triadic wavenumber consistency. The results are discussed in the context of modelling high Reynolds number wall turbulence.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167575, year = {2017}, author = {Brauckmann, HJ and Eckhardt, B and Schumacher, J}, title = {Heat transport in Rayleigh-Bénard convection and angular momentum transport in Taylor-Couette flow: a comparative study.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0079}, pmid = {28167575}, issn = {1364-503X}, abstract = {Rayleigh-Bénard convection and Taylor-Couette flow are two canonical flows that have many properties in common. We here compare the two flows in detail for parameter values where the Nusselt numbers, i.e. the thermal transport and the angular momentum transport normalized by the corresponding laminar values, coincide. We study turbulent Rayleigh-Bénard convection in air at Rayleigh number Ra=107 and Taylor-Couette flow at shear Reynolds number ReS=2×104 for two different mean rotation rates but the same Nusselt numbers. For individual pairwise related fields and convective currents, we compare the probability density functions normalized by the corresponding root mean square values and taken at different distances from the wall. We find one rotation number for which there is very good agreement between the mean profiles of the two corresponding quantities temperature and angular momentum. Similarly, there is good agreement between the fluctuations in temperature and velocity components. For the heat and angular momentum currents, there are differences in the fluctuations outside the boundary layers that increase with overall rotation and can be related to differences in the flow structures in the boundary layer and in the bulk. The study extends the similarities between the two flows from global quantities to local quantities and reveals the effects of rotation on the transport.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167574, year = {2017}, author = {Deguchi, K and Hall, P}, title = {The relationship between free-stream coherent structures and near-wall streaks at high Reynolds numbers.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0078}, pmid = {28167574}, issn = {1364-503X}, abstract = {The present work is based on our recent discovery of a new class of exact coherent structures generated near the edge of quite general boundary layer flows. The structures are referred to as free-stream coherent structures and were found using a large Reynolds number asymptotic approach to describe equilibrium solutions of the Navier-Stokes equations. In this paper, first we present results for a new family of free-stream coherent structures existing at relatively large wavenumbers. The new results are consistent with our earlier theoretical result that such structures can generate larger amplitude wall streaks if and only if the local spanwise wavenumber is sufficiently small. In a Blasius boundary layer, the local wavenumber increases in the streamwise direction so the wall streaks can typically exist only over a finite interval. However, here it is shown that they can interact with wall curvature to produce exponentially growing Görtler vortices through the receptivity process by a novel nonparallel mechanism. The theoretical predictions found are confirmed by a hybrid numerical approach. In contrast with previous receptivity investigations, it is shown that the amplitude of the induced vortex is larger than the structures in the free-stream which generate it.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28167573, year = {2017}, author = {Baars, WJ and Hutchins, N and Marusic, I}, title = {Reynolds number trend of hierarchies and scale interactions in turbulent boundary layers.}, journal = {Philosophical transactions. Series A, Mathematical, physical, and engineering sciences}, volume = {375}, number = {2089}, pages = {}, doi = {10.1098/rsta.2016.0077}, pmid = {28167573}, issn = {1364-503X}, abstract = {Small-scale velocity fluctuations in turbulent boundary layers are often coupled with the larger-scale motions. Studying the nature and extent of this scale interaction allows for a statistically representative description of the small scales over a time scale of the larger, coherent scales. In this study, we consider temporal data from hot-wire anemometry at Reynolds numbers ranging from Reτ≈2800 to 22 800, in order to reveal how the scale interaction varies with Reynolds number. Large-scale conditional views of the representative amplitude and frequency of the small-scale turbulence, relative to the large-scale features, complement the existing consensus on large-scale modulation of the small-scale dynamics in the near-wall region. Modulation is a type of scale interaction, where the amplitude of the small-scale fluctuations is continuously proportional to the near-wall footprint of the large-scale velocity fluctuations. Aside from this amplitude modulation phenomenon, we reveal the influence of the large-scale motions on the characteristic frequency of the small scales, known as frequency modulation. From the wall-normal trends in the conditional averages of the small-scale properties, it is revealed how the near-wall modulation transitions to an intermittent-type scale arrangement in the log-region. On average, the amplitude of the small-scale velocity fluctuations only deviates from its mean value in a confined temporal domain, the duration of which is fixed in terms of the local Taylor time scale. These concentrated temporal regions are centred on the internal shear layers of the large-scale uniform momentum zones, which exhibit regions of positive and negative streamwise velocity fluctuations. With an increasing scale separation at high Reynolds numbers, this interaction pattern encompasses the features found in studies on internal shear layers and concentrated vorticity fluctuations in high-Reynolds-number wall turbulence.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.}, } @article {pmid28163876, year = {2017}, author = {Phillips, N and Knowles, K and Bomphrey, RJ}, title = {Petiolate wings: effects on the leading-edge vortex in flapping flight.}, journal = {Interface focus}, volume = {7}, number = {1}, pages = {20160084}, doi = {10.1098/rsfs.2016.0084}, pmid = {28163876}, issn = {2042-8898}, abstract = {The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolation P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 1-3. The wings were driven using a mechanical device, the 'Flapperatus', to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitude Λ* (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span for P = 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.}, } @article {pmid28163871, year = {2017}, author = {Widmann, A and Tropea, C}, title = {Reynolds number influence on the formation of vortical structures on a pitching flat plate.}, journal = {Interface focus}, volume = {7}, number = {1}, pages = {20160079}, doi = {10.1098/rsfs.2016.0079}, pmid = {28163871}, issn = {2042-8898}, abstract = {The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex-wall interaction.}, } @article {pmid28163870, year = {2017}, author = {Wagner, H and Weger, M and Klaas, M and Schröder, W}, title = {Features of owl wings that promote silent flight.}, journal = {Interface focus}, volume = {7}, number = {1}, pages = {20160078}, doi = {10.1098/rsfs.2016.0078}, pmid = {28163870}, issn = {2042-8898}, abstract = {Owls are an order of birds of prey that are known for the development of a silent flight. We review here the morphological adaptations of owls leading to silent flight and discuss also aerodynamic properties of owl wings. We start with early observations (until 2005), and then turn to recent advances. The large wings of these birds, resulting in low wing loading and a low aspect ratio, contribute to noise reduction by allowing slow flight. The serrations on the leading edge of the wing and the velvet-like surface have an effect on noise reduction and also lead to an improvement of aerodynamic performance. The fringes at the inner feather vanes reduce noise by gliding into the grooves at the lower wing surface that are formed by barb shafts. The fringed trailing edge of the wing has been shown to reduce trailing edge noise. These adaptations to silent flight have been an inspiration for biologists and engineers for the development of devices with reduced noise production. Today several biomimetic applications such as a serrated pantograph or a fringed ventilator are available. Finally, we discuss unresolved questions and possible future directions.}, } @article {pmid28163869, year = {2017}, author = {Tank, J and Smith, L and Spedding, GR}, title = {On the possibility (or lack thereof) of agreement between experiment and computation of flows over wings at moderate Reynolds number.}, journal = {Interface focus}, volume = {7}, number = {1}, pages = {20160076}, doi = {10.1098/rsfs.2016.0076}, pmid = {28163869}, issn = {2042-8898}, abstract = {The flight of many birds and bats, and their robotic counterparts, occurs over a range of chord-based Reynolds numbers from 1 × 104 to 1.5 × 105. It is precisely over this range where the aerodynamics of simple, rigid, fixed wings becomes extraordinarily sensitive to small changes in geometry and the environment, with two sets of consequences. The first is that practical lifting devices at this scale will likely not be simple, rigid, fixed wings. The second is that it becomes non-trivial to make baseline comparisons for experiment and computation, when either one can be wrong. Here we examine one ostensibly simple case of the NACA 0012 aerofoil and make careful comparison between the technical literature, and new experiments and computations. The agreement (or lack thereof) will establish one or more baseline results and some sensitivities around them. The idea is that the diagnostic procedures will help to guide comparisons and predictions in subsequent more complex cases.}, } @article {pmid28163357, year = {2017}, author = {Wang, CY and Mercer, E and Kamranvand, F and Williams, L and Kolios, A and Parker, A and Tyrrel, S and Cartmell, E and McAdam, EJ}, title = {Tube-side mass transfer for hollow fibre membrane contactors operated in the low Graetz range.}, journal = {Journal of membrane science}, volume = {523}, number = {}, pages = {235-246}, doi = {10.1016/j.memsci.2016.09.049}, pmid = {28163357}, issn = {0376-7388}, } @article {pmid28161594, year = {2017}, author = {Tanveer, A and Hayat, T and Alsaadi, F and Alsaedi, A}, title = {Mixed convection peristaltic flow of Eyring-Powell nanofluid in a curved channel with compliant walls.}, journal = {Computers in biology and medicine}, volume = {82}, number = {}, pages = {71-79}, doi = {10.1016/j.compbiomed.2017.01.015}, pmid = {28161594}, issn = {1879-0534}, mesh = {Animals ; Computer Simulation ; Elastic Modulus/*physiology ; Humans ; *Models, Biological ; Peristalsis/*physiology ; Rheology/*methods ; Temperature ; }, abstract = {The novel features of nanofluids made them potentially significant in heat transfer mechanism occurring in medical and industrial processes like microelectronics, pharmaceutical processes, hybrid engines, thermal management of vehicles, refrigerator, chiller, gas temperature reduction and so forth. These processes bear tendency to enhance thermal conductivity and the convective heat transfer more efficiently than base fluid. This unique aspect made nanofluids the topic of interest in recent time via different fluid flow models. The problem in hand is one such application of nanofluids in peristaltic flow through curved channel. Thus peristalsis of Eyring-Powell nanofluid followed through conservation principles of mass, momentum, energy and concentration has been modeled. The whole system is made coupled via viscous dissipation, mixed convection, thermophoresis and Brownian motion. The complexity of system has been executed through a numerical approach after utilizing small Reynolds number and large wavelength concepts. A striking feature of this study is the activation of velocity and temperature with larger Brownian diffusion, whereas reduction is noticed with advancement in thermophoresis. Moreover the numerically obtained results for compliant walls are compatible with those obtained through other techniques.}, } @article {pmid28152457, year = {2017}, author = {He, L and Hang, J and Wang, X and Lin, B and Li, X and Lan, G}, title = {Numerical investigations of flow and passive pollutant exposure in high-rise deep street canyons with various street aspect ratios and viaduct settings.}, journal = {The Science of the total environment}, volume = {584-585}, number = {}, pages = {189-206}, doi = {10.1016/j.scitotenv.2017.01.138}, pmid = {28152457}, issn = {1879-1026}, mesh = {Air Pollutants/*analysis ; *Cities ; Environmental Exposure/*analysis ; Humans ; Models, Theoretical ; Noise ; Vehicle Emissions/*analysis ; Wind ; }, abstract = {Vehicular pollutant exposure of residents and pedestrians in high-rise deep street canyons with viaducts and noise barriers requires special concerns because the ventilation capacity is weak and the literature reported inconsistent findings on flow patterns as aspect ratios (building height/street width, H/W) are larger than 2. By conducting computational fluid dynamics (CFD) simulations coupled with the intake fraction iF and the daily pollutant exposure Et, this paper investigates the impact of street aspect ratios, viaducts and noise barriers on the flow and vehicular passive pollutant exposure in full-scale street canyons (H/W=1-6, W=24m). iF represents the fraction of total emissions inhaled by a population (1ppm=10-6), while Et means the extent of human beings' contact with pollutants within one day. CFD methodologies of passive pollutant dispersion modeling are successfully validated by wind tunnel data in Meroney et al. (1996). As a novelty, the two-main-vortex pattern start appearing in full-scale street canyons as H/W changes from 4 to 5, however previous studies using wind-tunnel-scale models (H=6cm) reported two to five vortexes as H/W=2-5. This finding is validated by both smoke visualization in scale-model outdoor field experiments (H=1.2m, W=0.6m) and CFD simulations of Reynolds number independence. Cases with two main vortexes (H/W=5-6) experience much larger daily pollutant exposure (~103-104mg/m3/day) than those with single main vortex as H/W=1-4 (~101-102mg/m3/day). Moreover leeward-side pollutant exposures are much larger than windward-side as H/W=1-4 while oppositely as H/W=5-6. Assuming a general population density, the total iF is 485-803ppm as H/W=1, 2020-12051ppm as H/W=2-4, and 51112-794026ppm as H/W=5-6. With a single elevated pollutant source, cases with viaducts experience significantly smaller pollutant exposures than cases without viaducts. Road barriers slightly increase pollutant exposure in near-road buildings with H/W=1 while reduce a little as H/W=3 and 5. Two-source cases can experience 2.60-5.52 times pollutant exposure as great as single-source cases.}, } @article {pmid28151968, year = {2017}, author = {Tanveer, A and Hayat, T and Alsaedi, A and Ahmad, B}, title = {Numerical simulation for peristalsis of Carreau-Yasuda nanofluid in curved channel with mixed convection and porous space.}, journal = {PloS one}, volume = {12}, number = {2}, pages = {e0170029}, doi = {10.1371/journal.pone.0170029}, pmid = {28151968}, issn = {1932-6203}, mesh = {Computer Simulation ; Convection ; Hot Temperature ; Humans ; *Hydrodynamics ; *Models, Theoretical ; Nanoparticles ; *Peristalsis/physiology ; Porosity ; Rheology ; Thermal Conductivity ; }, abstract = {Main theme of present investigation is to model and analyze the peristaltic activity of Carraeu-Yasuda nanofluid saturating porous space in a curved channel. Unlike the traditional approach, the porous medium effects are characterized by employing modified Darcy's law for Carreau-Yasuda fluid. To our knowledge this is first attempt in this direction for Carreau-Yasuda fluid. Heat and mass transfer are further considered. Simultaneous effects of heat and mass transfer are examined in presence of mixed convection, viscous dissipation and thermal radiation. The compliant characteristics for channel walls are taken into account. The resulting complex mathematical system has been discussed for small Reynolds number and large wavelength concepts. Numerical approximation to solutions are thus plotted in graphs and the physical description is presented. It is concluded that larger porosity in a medium cause an enhancement in fluid velocity and reduction in concentration.}, } @article {pmid28141804, year = {2017}, author = {Wheeler, RJ}, title = {Use of chiral cell shape to ensure highly directional swimming in trypanosomes.}, journal = {PLoS computational biology}, volume = {13}, number = {1}, pages = {e1005353}, doi = {10.1371/journal.pcbi.1005353}, pmid = {28141804}, issn = {1553-7358}, mesh = {Adaptation, Physiological/*physiology ; Cell Movement/*physiology ; *Cell Size ; Cell Tracking ; Computer Simulation ; Friction ; Hydrodynamics ; Microscopy, Video ; *Models, Biological ; Swimming/physiology ; Trypanosoma/*cytology/*physiology ; Viscosity ; }, abstract = {Swimming cells typically move along a helical path or undergo longitudinal rotation as they swim, arising from chiral asymmetry in hydrodynamic drag or propulsion bending the swimming path into a helix. Helical paths are beneficial for some forms of chemotaxis, but why asymmetric shape is so prevalent when a symmetric shape would also allow highly directional swimming is unclear. Here, I analyse the swimming of the insect life cycle stages of two human parasites; Trypanosoma brucei and Leishmania mexicana. This showed quantitatively how chirality in T. brucei cell shape confers highly directional swimming. High speed videomicrographs showed that T. brucei, L. mexicana and a T. brucei RNAi morphology mutant have a range of shape asymmetries, from wild-type T. brucei (highly chiral) to L. mexicana (near-axial symmetry). The chiral cells underwent longitudinal rotation while swimming, with more rapid longitudinal rotation correlating with swimming path directionality. Simulation indicated hydrodynamic drag on the chiral cell shape caused rotation, and the predicted geometry of the resulting swimming path matched the directionality of the observed swimming paths. This simulation of swimming path geometry showed that highly chiral cell shape is a robust mechanism through which microscale swimmers can achieve highly directional swimming at low Reynolds number. It is insensitive to random variation in shape or propulsion (biological noise). Highly symmetric cell shape can give highly directional swimming but is at risk of giving futile circular swimming paths in the presence of biological noise. This suggests the chiral T. brucei cell shape (associated with the lateral attachment of the flagellum) may be an adaptation associated with the bloodstream-inhabiting lifestyle of this parasite for robust highly directional swimming. It also provides a plausible general explanation for why swimming cells tend to have strong asymmetries in cell shape or propulsion.}, } @article {pmid28117769, year = {2017}, author = {Kulkarni, AA and Patel, RK and Friedman, C and Leftwich, MC}, title = {A Robotic Platform to Study the Foreflipper of the California Sea Lion.}, journal = {Journal of visualized experiments : JoVE}, volume = {}, number = {119}, pages = {}, doi = {10.3791/54909}, pmid = {28117769}, issn = {1940-087X}, mesh = {Animals ; Biomechanical Phenomena ; Extremities ; Female ; *Robotics ; Sea Lions/*anatomy & histology ; *Swimming ; }, abstract = {The California sea lion (Zalophus californianus), is an agile and powerful swimmer. Unlike many successful swimmers (dolphins, tuna), they generate most of their thrust with their large foreflippers. This protocol describes a robotic platform designed to study the hydrodynamic performance of the swimming California sea lion (Zalophus californianus). The robot is a model of the animal's foreflipper that is actuated by motors to replicate the motion of its propulsive stroke (the 'clap'). The kinematics of the sea lion's propulsive stroke are extracted from video data of unmarked, non-research sea lions at the Smithsonian Zoological Park (SNZ). Those data form the basis of the actuation motion of the robotic flipper presented here. The geometry of the robotic flipper is based a on high-resolution laser scan of a foreflipper of an adult female sea lion, scaled to about 60% of the full-scale flipper. The articulated model has three joints, mimicking the elbow, wrist and knuckle joint of the sea lion foreflipper. The robotic platform matches dynamics properties-Reynolds number and tip speed-of the animal when accelerating from rest. The robotic flipper can be used to determine the performance (forces and torques) and resulting flowfields.}, } @article {pmid28085475, year = {2016}, author = {Mo, CJ and Qin, LZ and Zhao, F and Yang, LJ}, title = {Application of the dissipative particle dynamics method to the instability problem of a liquid thread.}, journal = {Physical review. E}, volume = {94}, number = {6-1}, pages = {063113}, doi = {10.1103/PhysRevE.94.063113}, pmid = {28085475}, issn = {2470-0053}, abstract = {We investigate the application of the dissipative particle dynamics method to the instability problem of a long liquid thread surrounded by another fluid. The dispersion curves obtained from simulations are compared with classic theoretical predictions. The results from standard dissipative particle dynamics (DPD) simulations at first have a tendency of gradually approaching to Tomotika's Stokes flow prediction when the Reynolds number is decreased. But they then abnormally deviate again when the viscosity is very large. The same phenomenon is also confirmed in droplet retraction simulations when also compared with theoretical Stokes flow results. On the other hand, when a hard-core DPD model is used, with the decrease of the Reynolds number the simulation results did finally approach Tomotika's predictions when Re≈0.1. A combined presentation of the hard-core DPD results and the standard DPD results, excluding the abnormal ones, demonstrates that they are approximately on a continuum when labeled with Reynolds number. These results suggest that the standard DPD method is a suitable method for investigation of the instability problem of immersed liquid thread in the inertioviscous regime (0.1

METHODS: We used a computational fluid dynamics (CFDs) approach to evaluate the shear stress distribution and minimize its effects under various conditions including changes in the anastomosis angle. A three-dimensional computational domain was designed for arteriovenous end-to-side anastomosis based on anastomosis angles of 45°, 90° and including 135° angle of an obtuse anastomosis using three-dimensional design software. COMSOL Multiphysics® simulation software was used to identify the hemodynamic factors influencing wall shear stress at the anastomosis site using a low Reynolds number k-ε turbulence model that included non-Newtonian blood flow characteristics, the complete cardiac pulse cycle, and distention of blood vessels. In preliminary clinical study, all 201 patients who received a radiocephalic wrist AVF from January 2009 to February 2014 were divided into classic and obtuse angle groups.

RESULTS: The CFD results showed that the largest anastomosis angle (135°) resulted in lower shear stress, which would help reduce AVF failures. This obtuse angle was preferred, as it minimized the development of anastomotic stenosis and tended to favor primary and primary-assisted patency in clinical study.

CONCLUSIONS: An obtuse radiocephalic wrist AVF shows more favorable patency compared to a classic radiocephalic AVF. Surgeons establishing a radiocephalic wrist AVF would be better to consider an AVF with an obtuse anastomosis.}, } @article {pmid27789458, year = {2016}, author = {Miyagawa, T and Imai, Y and Ishida, S and Ishikawa, T}, title = {Relationship between gastric motility and liquid mixing in the stomach.}, journal = {American journal of physiology. Gastrointestinal and liver physiology}, volume = {311}, number = {6}, pages = {G1114-G1121}, doi = {10.1152/ajpgi.00346.2016}, pmid = {27789458}, issn = {1522-1547}, mesh = {*Computer Simulation ; *Gastric Emptying ; *Gastrointestinal Transit ; Humans ; Muscle Contraction ; Pylorus/*physiology ; }, abstract = {The relationship between gastric motility and the mixing of liquid food in the stomach was investigated with a numerical analysis. Three parameters of gastric motility were considered: the propagation velocity, frequency, and terminal acceleration of peristaltic contractions. We simulated gastric flow with an anatomically realistic geometric model of the stomach, considering free surface flow and moving boundaries. When a peristaltic contraction approaches the pylorus, retropulsive flow is generated in the antrum. Flow separation then occurs behind the contraction. The extent of flow separation depends on the Reynolds number (Re), which quantifies the inertial forces due to the peristaltic contractions relative to the viscous forces of the gastric contents; no separation is observed at low Re, while an increase in reattachment length is observed at high Re. While mixing efficiency is nearly constant for low Re, it increases with Re for high Re because of flow separation. Hence, the effect of the propagation velocity, frequency, or terminal acceleration of peristaltic contractions on mixing efficiency increases with Re.}, } @article {pmid27722471, year = {2016}, author = {Zhao, Y and Shen, AQ and Haward, SJ}, title = {Flow of wormlike micellar solutions around confined microfluidic cylinders.}, journal = {Soft matter}, volume = {12}, number = {42}, pages = {8666-8681}, doi = {10.1039/c6sm01597b}, pmid = {27722471}, issn = {1744-6848}, abstract = {Wormlike micellar (WLM) solutions are frequently used in enhanced oil and gas recovery applications in porous rock beds where complex microscopic geometries result in mixed flow kinematics with strong shear and extensional components. Experiments with WLM solutions through model microfluidic porous media have revealed a variety of complex flow phenomena, including the formation of stable gel-like structures known as a Flow-Induced Structured Phase (FISP), which undoubtedly play an important role in applications of WLM fluids, but are still poorly understood. A first step in understanding flows of WLM fluids through porous media can be made by examining the flow around a single micro-scale cylinder aligned on the flow axis. Here we study flow behavior of an aqueous WLM solution consisting of cationic surfactant cetyltrimethylammonium bromide (CTAB) and a stable hydrotropic salt 3-hydroxy naphthalene-2-carboxylate (SHNC) in microfluidic devices with three different cylinder blockage ratios, β. We observe a rich sequence of flow instabilities depending on β as the Weissenberg number (Wi) is increased to large values while the Reynolds number (Re) remains low. Instabilities upstream of the cylinder are associated with high stresses in fluid that accelerates into the narrow gap between the cylinder and the channel wall; vortex growth upstream is reminiscent of that seen in microfluidic contraction geometries. Instability downstream of the cylinder is associated with stresses generated at the trailing stagnation point and the resulting flow modification in the wake, coupled with the onset of time-dependent flow upstream and the asymmetric division of flow around the cylinder.}, } @article {pmid27780156, year = {2016}, author = {Lu, H and Lua, KB and Lee, YJ and Lim, TT and Yeo, KS}, title = {Ground effect on the aerodynamics of three-dimensional hovering wings.}, journal = {Bioinspiration & biomimetics}, volume = {11}, number = {6}, pages = {066003}, doi = {10.1088/1748-3190/11/5/066003}, pmid = {27780156}, issn = {1748-3190}, mesh = {Animals ; Aviation ; Biomechanical Phenomena ; *Biomimetic Materials ; Biomimetics/*methods ; Computer Simulation ; Flight, Animal/*physiology ; Models, Biological ; Wings, Animal/*physiology ; }, abstract = {This paper reports the results of combined experimental and numerical studies on the ground effect on a pair of three-dimensional (3D) hovering wings. Parameters investigated include hovering kinematics, wing shapes, and Reynolds numbers (Re). The results are consistent with the observation by another study (Gao and Lu, 2008 Phys. Fluids, 20 087101) which shows that the cycle-averaged aerodynamic forces generated by two-dimensional (2D) wings in close proximity to the ground can be broadly categorized into three regimes with respect to the ground clearance; force enhancement, force reduction, and force recovery. However, the ground effect on a 3D wing is not as significant as that on a 2D flapping wing reported in (Lu et al 2014 Exp. Fluids, 55 1787); this could be attributed to a weaker wake capture effect on 3D wings. Also, unlike a 2D wing, the leading edge vortex (LEV) remains attached on a 3D wing regardless of ground clearance. For all the wing kinematics considered, the three above-mentioned regimes are closely correlated to a non-monotonic trend in the strength of downwash due to the restriction of root and tip vortex formation, and a positional shift of wake vortices. The root vortices in interaction with the ground induce an up-wash in-between the two wings, causing a strong 'fountain effect' (Maeda and Liu, 2013 J. Biomech. Sci. Eng., 8 344) that may increase the body lift of insects. The present study further shows that changes in wing planform have insignificant influence on the overall trend of ground effect except for a parallel shift in force magnitude, which is caused mainly by the difference in aspect ratio and leading edge pivot point. On the two Reynolds numbers investigated, the results for the low Re case of 100 do not deviate significantly from those of a higher Re = 5000 except for the difference in force magnitudes, since low Reynolds number generates lower downwash, weaker LEV, and lower rotational circulation. Additionally, lower Re leads to a weaker fountain effect.}, } @article {pmid27778306, year = {2016}, author = {Qureshi, MZ and Rubbab, Q and Irshad, S and Ahmad, S and Aqeel, M}, title = {Heat and Mass Transfer Analysis of MHD Nanofluid Flow with Radiative Heat Effects in the Presence of Spherical Au-Metallic Nanoparticles.}, journal = {Nanoscale research letters}, volume = {11}, number = {1}, pages = {472}, doi = {10.1186/s11671-016-1692-2}, pmid = {27778306}, issn = {1931-7573}, abstract = {Energy generation is currently a serious concern in the progress of human civilization. In this regard, solar energy is considered as a significant source of renewable energy. The purpose of the study is to establish a thermal energy model in the presence of spherical Au-metallic nanoparticles. It is numerical work which studies unsteady magnetohydrodynamic (MHD) nanofluid flow through porous disks with heat and mass transfer aspects. Shaped factor of nanoparticles is investigated using small values of the permeable Reynolds number. In order to scrutinize variation of thermal radiation effects, a dimensionless Brinkman number is introduced. The results point out that heat transfer significantly escalates with the increase of Brinkman number. Partial differential equations that govern this study are reduced into nonlinear ordinary differential equations by means of similarity transformations. Then using a shooting technique, a numerical solution of these equations is constructed. Radiative effects on temperature and mass concentration are quite opposite. Heat transfer increases in the presence of spherical Au-metallic nanoparticles.}, } @article {pmid27771858, year = {2016}, author = {Daniels, DR}, title = {Curvature correction to the mobility of fluid membrane inclusions.}, journal = {The European physical journal. E, Soft matter}, volume = {39}, number = {10}, pages = {96}, doi = {10.1140/epje/i2016-16096-3}, pmid = {27771858}, issn = {1292-895X}, abstract = {Using rigorous low-Reynolds-number hydrodynamic theory on curved surfaces, we provide, via a Stokeslet-type approach, a general and concise expression for the leading-order curvature correction to the canonical, planar, Saffman-Delbrück value of the diffusion constant for a small inclusion embedded in an arbitrarily (albeit weakly) curved fluid membrane. In order to demonstrate the efficacy and utility of this general result, we apply our theory to the specific case of calculating the diffusion coefficient of a locally curvature inducing membrane inclusion. By including both the effects of inclusion and membrane elasticity, as well as their respective thermal shape fluctuations, excellent agreement is found with recently published experimental data on the surface tension dependent mobility of membrane bound inclusions.}, } @article {pmid27643464, year = {2016}, author = {Bhatti, MM and Zeeshan, A and Ellahi, R}, title = {Endoscope analysis on peristaltic blood flow of Sisko fluid with Titanium magneto-nanoparticles.}, journal = {Computers in biology and medicine}, volume = {78}, number = {}, pages = {29-41}, doi = {10.1016/j.compbiomed.2016.09.007}, pmid = {27643464}, issn = {1879-0534}, mesh = {Endoscopy/*methods ; Hemorheology/*physiology ; Humans ; Magnetic Fields ; Magnetite Nanoparticles/*chemistry ; *Models, Biological ; Peristalsis/*physiology ; Titanium/chemistry ; }, abstract = {In this article, endoscope analysis on peristaltic blood flow of Sisko fluid having Titanium magneto-nanoparticles through a uniform tube has been analyzed. The governing flow problem consists of continuity, linear momentum and thermal energy equations. The effect of magnetic field is also taken into account with the help of ohm's law. With the help of long wavelength and zero Reynolds number approximation, the governing equations are simplified. The reduced resulting nonlinear coupled equations are solved analytically with the help of Homotopy perturbation method (HPM). The impact of all the emerging parameters is discussed with the help of graphs for pressure rise, friction forces for outer and inner tube, velocity profile, temperature profile and pressure gradient. Moreover, numerical computation has been used to evaluate the expression for pressure rise and friction forces. Trapping phenomena is also presented with the help of streamlines. The present study depicts many interesting results that provide further study on different blood flow problems.}, } @article {pmid27757417, year = {2016}, author = {Saranadhi, D and Chen, D and Kleingartner, JA and Srinivasan, S and Cohen, RE and McKinley, GH}, title = {Sustained drag reduction in a turbulent flow using a low-temperature Leidenfrost surface.}, journal = {Science advances}, volume = {2}, number = {10}, pages = {e1600686}, doi = {10.1126/sciadv.1600686}, pmid = {27757417}, issn = {2375-2548}, abstract = {Skin friction drag contributes a major portion of the total drag for small and large water vehicles at high Reynolds number (Re). One emerging approach to reducing drag is to use superhydrophobic surfaces to promote slip boundary conditions. However, the air layer or "plastron" trapped on submerged superhydrophobic surfaces often diminishes quickly under hydrostatic pressure and/or turbulent pressure fluctuations. We use active heating on a superhydrophobic surface to establish a stable vapor layer or "Leidenfrost" state at a relatively low superheat temperature. The continuous film of water vapor lubricates the interface, and the resulting slip boundary condition leads to skin friction drag reduction on the inner rotor of a custom Taylor-Couette apparatus. We find that skin friction can be reduced by 80 to 90% relative to an unheated superhydrophobic surface for Re in the range 26,100 ≤ Re ≤ 52,000. We derive a boundary layer and slip theory to describe the hydrodynamics in the system and show that the plastron thickness is h = 44 ± 11 μm, in agreement with expectations for a Leidenfrost surface.}, } @article {pmid27749870, year = {2016}, author = {Longhi, S}, title = {PT-symmetric mode-locking.}, journal = {Optics letters}, volume = {41}, number = {19}, pages = {4518-4521}, pmid = {27749870}, issn = {1539-4794}, abstract = {Parity-time (PT) symmetry is one of the most important accomplishments in optics over the past decade. Here the concept of PT mode-locking (ML) of a laser is introduced, in which active phase-locking of cavity axial modes is realized by asymmetric mode coupling in a complex time crystal. PT ML shows a transition from single- to double-pulse emission as the PT symmetry breaking point is crossed. The transition can show a turbulent behavior, depending on a dimensionless modulation parameter that plays the same role as the Reynolds number in hydrodynamic flows.}, } @article {pmid27742130, year = {2016}, author = {Li, P and Weng, L and Niu, H and Robinson, B and King, T and Conmy, R and Lee, K and Liu, L}, title = {Reynolds number scaling to predict droplet size distribution in dispersed and undispersed subsurface oil releases.}, journal = {Marine pollution bulletin}, volume = {113}, number = {1-2}, pages = {332-342}, doi = {10.1016/j.marpolbul.2016.10.005}, pmid = {27742130}, issn = {1879-3363}, mesh = {Alaska ; *Models, Theoretical ; Particle Size ; Petroleum/*analysis ; Petroleum Pollution/*analysis ; Viscosity ; Water Pollutants, Chemical/*analysis/*chemistry ; }, abstract = {This study was aimed at testing the applicability of modified Weber number scaling with Alaska North Slope (ANS) crude oil, and developing a Reynolds number scaling approach for oil droplet size prediction for high viscosity oils. Dispersant to oil ratio and empirical coefficients were also quantified. Finally, a two-step Rosin-Rammler scheme was introduced for the determination of droplet size distribution. This new approach appeared more advantageous in avoiding the inconsistency in interfacial tension measurements, and consequently delivered concise droplet size prediction. Calculated and observed data correlated well based on Reynolds number scaling. The relation indicated that chemical dispersant played an important role in reducing the droplet size of ANS under different seasonal conditions. The proposed Reynolds number scaling and two-step Rosin-Rammler approaches provide a concise, reliable way to predict droplet size distribution, supporting decision making in chemical dispersant application during an offshore oil spill.}, } @article {pmid27731461, year = {2016}, author = {Beauvier, E and Bodea, S and Pocheau, A}, title = {Front propagation in a vortex lattice: dependence on boundary conditions and vortex depth.}, journal = {Soft matter}, volume = {12}, number = {43}, pages = {8935-8941}, doi = {10.1039/c6sm01547f}, pmid = {27731461}, issn = {1744-6848}, abstract = {We experimentally address the propagation of reaction-diffusion fronts in vortex lattices by combining, in a Hele-Shaw cell and at low Reynolds number, forced electroconvective flows and an autocatalytic reaction in solution. We consider both vortex chains and vortex arrays, the former referring to mixed free/rigid boundary conditions for vortices and the latter to free boundary conditions. Varying the depth of the fluid layer, we observe no variation of the mean front velocities for vortex arrays and a noticeable variation for vortex chains. This questions the two-dimensional character of front propagation in low Reynolds number vortex lattices, as well as the mechanisms of this dependence.}, } @article {pmid27725841, year = {2016}, author = {Liu, X and Yan, W and Liu, Y and Choy, YS and Wei, Y}, title = {Numerical Investigation of Flow Characteristics in the Obstructed Realistic Human Upper Airway.}, journal = {Computational and mathematical methods in medicine}, volume = {2016}, number = {}, pages = {3181654}, doi = {10.1155/2016/3181654}, pmid = {27725841}, issn = {1748-6718}, mesh = {Adult ; Airway Obstruction/diagnostic imaging/*physiopathology ; Asian Continental Ancestry Group ; Biomechanical Phenomena ; China ; Computer Simulation ; Humans ; Larynx/physiopathology ; Lung/diagnostic imaging ; Male ; Models, Statistical ; Models, Theoretical ; Mouth/physiopathology ; Nasal Cavity/physiopathology ; Pharynx/physiopathology ; Respiration ; Respiration Disorders/*diagnostic imaging/physiopathology ; Respiratory Mechanics ; Respiratory System/*physiopathology ; Tomography, X-Ray Computed ; Trachea/physiopathology ; }, abstract = {The flow characteristics in the realistic human upper airway (HUA) with obstruction that resulted from pharyngeal collapse were numerically investigated. The 3D anatomically accurate HUA model was reconstructed from CT-scan images of a Chinese male patient (38 years, BMI 25.7). The computational fluid dynamics (CFD) with the large eddy simulation (LES) method was applied to simulate the airflow dynamics within the HUA model in both inspiration and expiration processes. The laser Doppler anemometry (LDA) technique was simultaneously adopted to measure the airflow fields in the HUA model for the purpose of testifying the reliability of LES approach. In the simulations, the representative respiration intensities of 16.8 L/min (slight breathing), 30 L/min (moderate breathing), and 60 L/min (severe breathing) were conducted under continuous inspiration and expiration conditions. The airflow velocity field and static pressure field were obtained and discussed in detail. The results indicated the airflow experiences unsteady transitional/turbulent flow in the HUA model under low Reynolds number. The airflow fields cause occurrence of forceful injection phenomenon due to the narrowing of pharynx caused by the respiratory illness in inspiration and expiration. There also exist strong flow separation and back flow inside obstructed HUA owing to the vigorous jet flow effect in the pharynx. The present results would provide theoretical guidance for the treatment of obstructive respiratory disease.}, } @article {pmid27708761, year = {2016}, author = {Marth, W and Voigt, A}, title = {Collective migration under hydrodynamic interactions: a computational approach.}, journal = {Interface focus}, volume = {6}, number = {5}, pages = {20160037}, doi = {10.1098/rsfs.2016.0037}, pmid = {27708761}, issn = {2042-8898}, abstract = {We consider a generic model for cell motility. Even if a comprehensive understanding of cell motility remains elusive, progress has been achieved in its modelling using a whole-cell physical model. The model takes into account the main mechanisms of cell motility, actin polymerization, actin-myosin dynamics and substrate mediated adhesion (if applicable), and combines them with steric cell-cell and hydrodynamic interactions. The model predicts the onset of collective cell migration, which emerges spontaneously as a result of inelastic collisions of neighbouring cells. Each cell here modelled as an active polar gel is accomplished with two vortices if it moves. Upon collision of two cells, the two vortices which come close to each other annihilate. This leads to a rotation of the cells and together with the deformation and the reorientation of the actin filaments in each cell induces alignment of these cells and leads to persistent translational collective migration. The effect for low Reynolds numbers is as strong as in the non-hydrodynamic model, but it decreases with increasing Reynolds number.}, } @article {pmid27703591, year = {2016}, author = {Lee, LM and Lee, JW and Chase, D and Gebrezgiabhier, D and Liu, AP}, title = {Development of an advanced microfluidic micropipette aspiration device for single cell mechanics studies.}, journal = {Biomicrofluidics}, volume = {10}, number = {5}, pages = {054105}, doi = {10.1063/1.4962968}, pmid = {27703591}, issn = {1932-1058}, support = {DP2 HL117748/HL/NHLBI NIH HHS/United States ; T32 EB005582/EB/NIBIB NIH HHS/United States ; }, abstract = {Various micro-engineered tools or platforms have been developed recently for cell mechanics studies based on acoustic, magnetic, and optical actuations. Compared with other techniques for single cell manipulations, microfluidics has the advantages with simple working principles and device implementations. In this work, we develop a multi-layer microfluidic pipette aspiration device integrated with pneumatically actuated microfluidic control valves. This configuration enables decoupling of cell trapping and aspiration, and hence causes less mechanical perturbation on trapped single cells before aspiration. A high trapping efficiency is achieved by the microfluidic channel design based on fluid resistance model and deterministic microfluidics. Compared to conventional micropipette aspiration, the suction pressure applied on the aspirating cells is highly stable due to the viscous nature of low Reynolds number flow. As a proof-of-concept of this novel microfluidic technology, we built a microfluidic pipette aspiration device with 2 × 13 trapping arrays and used this device to measure the stiffness of a human breast cancer cell line, MDA-MB-231, through the observation of cell deformations during aspiration. As a comparison, we studied the effect of Taxol, a FDA-approved anticancer drug on single cancer cell stiffness. We found that cancer cells treated with Taxol were less deformable with a higher Young's modulus. The multi-layer microfluidic pipette aspiration device is a scalable technology for single cell mechanophenotyping studies and drug discovery applications.}, } @article {pmid27688703, year = {2016}, author = {Dhanapal, C and Kamalakkannan, J and Prakash, J and Kothandapani, M}, title = {Analysis of Peristaltic Motion of a Nanofluid with Wall Shear Stress, Microrotation, and Thermal Radiation Effects.}, journal = {Applied bionics and biomechanics}, volume = {2016}, number = {}, pages = {4123741}, doi = {10.1155/2016/4123741}, pmid = {27688703}, issn = {1176-2322}, abstract = {This paper analyzes the peristaltic flow of an incompressible micropolar nanofluid in a tapered asymmetric channel in the presence of thermal radiation and heat sources parameters. The rotation of the nanoparticles is incorporated in the flow model. The equations governing the nanofluid flow are modeled and exact solutions are managed under long wavelength and flow Reynolds number and long wavelength approximations. Explicit expressions of axial velocity, stream function, microrotation, nanoparticle temperature, and concentration have been derived. The phenomena of shear stress and trapping have also been discussed. Finally, the influences of various parameters of interest on flow variables have been discussed numerically and explained graphically. Besides, the results obtained in this paper will be helpful to those who are working on the development of various realms like fluid mechanics, the rotation, Brownian motion, thermophoresis, coupling number, micropolar parameter, and the nondimensional geometry parameters.}, } @article {pmid27686531, year = {2016}, author = {Salimi-Kenari, H and Imani, M and Nodehi, A and Abedini, H}, title = {An engineering approach to design of dextran microgels size fabricated by water/oil emulsification.}, journal = {Journal of microencapsulation}, volume = {33}, number = {6}, pages = {511-523}, doi = {10.1080/02652048.2016.1216188}, pmid = {27686531}, issn = {1464-5246}, mesh = {Dextrans/*chemistry ; Emulsions ; Gels/chemistry ; *Models, Chemical ; Oils/*chemistry ; Particle Size ; Water/*chemistry ; }, abstract = {A correlation, based on fluid mechanics, has been investigated for the mean particle diameter of crosslinked dextran microgels (CDMs) prepared via a water/oil emulsification methodology conducted in a single-stirred vessel. To this end, non-dimensional correlations were developed to predict the mean particle size of CDMs as a function of Weber number, Reynolds number and viscosity number similar to ones introduced for liquid-liquid dispersions. Moreover, a Rosin-Rammler distribution function has been successfully applied to the microgel particle size distributions. The correlations were validated using experimentally obtained mean particle sizes for CDMs prepared at different stirring conditions. The validated correlation is especially applicable to medical and pharmaceutical applications where strict control on the mean particle size and size distribution of CDMs are extremely essential. [Formula: see text].}, } @article {pmid27684076, year = {2016}, author = {Bachant, P and Wosnik, M and Gunawan, B and Neary, VS}, title = {Experimental Study of a Reference Model Vertical-Axis Cross-Flow Turbine.}, journal = {PloS one}, volume = {11}, number = {9}, pages = {e0163799}, doi = {10.1371/journal.pone.0163799}, pmid = {27684076}, issn = {1932-6203}, abstract = {The mechanical power, total rotor drag, and near-wake velocity of a 1:6 scale model (1.075 m diameter) of the US Department of Energy's Reference Model vertical-axis cross-flow turbine were measured experimentally in a towing tank, to provide a comprehensive open dataset for validating numerical models. Performance was measured for a range of tip speed ratios and at multiple Reynolds numbers by varying the rotor's angular velocity and tow carriage speed, respectively. A peak power coefficient CP = 0.37 and rotor drag coefficient CD = 0.84 were observed at a tip speed ratio λ0 = 3.1. A regime of weak linear Re-dependence of the power coefficient was observed above a turbine diameter Reynolds number ReD ≈ 106. The effects of support strut drag on turbine performance were investigated by covering the rotor's NACA 0021 struts with cylinders. As expected, this modification drastically reduced the rotor power coefficient. Strut drag losses were also measured for the NACA 0021 and cylindrical configurations with the rotor blades removed. For λ = λ0, wake velocity was measured at 1 m (x/D = 0.93) downstream. Mean velocity, turbulence kinetic energy, and mean kinetic energy transport were compared with results from a high solidity turbine acquired with the same test apparatus. Like the high solidity case, mean vertical advection was calculated to be the largest contributor to near-wake recovery. However, overall, lower levels of streamwise wake recovery were calculated for the RM2 case-a consequence of both the relatively low solidity and tapered blades reducing blade tip vortex shedding-responsible for mean vertical advection-and lower levels of turbulence caused by higher operating tip speed ratio and therefore reduced dynamic stall. Datasets, code for processing and visualization, and a CAD model of the turbine have been made publicly available.}, } @article {pmid27662761, year = {2016}, author = {de Camargo, CL and Shiroma, LS and Giordano, GF and Gobbi, AL and Vieira, LC and Lima, RS}, title = {Turbulence in microfluidics: Cleanroom-free, fast, solventless, and bondless fabrication and application in high throughput liquid-liquid extraction.}, journal = {Analytica chimica acta}, volume = {940}, number = {}, pages = {73-83}, doi = {10.1016/j.aca.2016.08.052}, pmid = {27662761}, issn = {1873-4324}, abstract = {This paper addresses an important breakthrough in the deployment of ultra-high adhesion strength microfluidic technologies to provide turbulence at harsh flow rate conditions. This paper is only, to our knowledge, the second reporting on the generation of high flow rate-assisted turbulence in microchannels. This flow solves a crucial bottleneck in microfluidics: the generation of high throughput homogeneous mixings. We focused on the fabrication of bulky polydimethylsiloxane (PDMS) microchips (without any interfaces) rather than the laborious surface modifications that were employed in the first reporting about turbulence-assisted microfluidics. The fabrication is cleanroom-free, simple, low-cost, fast, solventless, and bondless requiring only a laboratory oven. More specifically, our method relies on the shaping of a nylon scaffold, cure of PDMS with embedded nylon, and removal of this scaffold. The scaffold was obtained by manually wrapping nylon threads. The withdrawing out of the scaffold was completed in few seconds using only a plier. Such microchannels endured flow rates of up to 60.0 mL min(-1) with a strikingly low elastic deformation. The importance in producing turbulence into microscale channels was successfully shown in liquid-liquid extractions. The great energy dissipation rate relative to the turbulence created high throughput and efficient extractions in microfluidics for the first time. The residence time was only 0.01 s at 25.0 mL min(-1) (total flow rate of the immiscible phases). In addition, the partition coefficient determined in a single run was similar to that obtained by the conventional batch shake-flask method that was realized in triplicate.}, } @article {pmid27661694, year = {2016}, author = {Vakarelski, IU and Berry, JD and Chan, DY and Thoroddsen, ST}, title = {Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids.}, journal = {Physical review letters}, volume = {117}, number = {11}, pages = {114503}, doi = {10.1103/PhysRevLett.117.114503}, pmid = {27661694}, issn = {1079-7114}, abstract = {The drag coefficient C_{D} of a solid smooth sphere moving in a fluid is known to be only a function of the Reynolds number Re and diminishes rapidly at the drag crisis around Re∼3×10^{5} . A Leidenfrost vapor layer on a hot sphere surface can trigger the onset of the drag crisis at a lower Re. By using a range of high viscosity perfluorocarbon liquids, we show that the drag reduction effect can occur over a wide range of Re, from as low as ∼600 to 10^{5} . The Navier slip model with a viscosity dependent slip length can fit the observed drag reduction and wake shape.}, } @article {pmid27635104, year = {2016}, author = {Chandler, ID and Guymer, I and Pearson, JM and van Egmond, R}, title = {Vertical variation of mixing within porous sediment beds below turbulent flows.}, journal = {Water resources research}, volume = {52}, number = {5}, pages = {3493-3509}, doi = {10.1002/2015WR018274}, pmid = {27635104}, issn = {0043-1397}, abstract = {River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment-water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber-optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment-water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number, PeK between 1000 and 77,000 and the shear Reynolds number, Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in-sediment mixing coefficient are explored. The variation of in-sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.}, } @article {pmid27627416, year = {2016}, author = {Ren, F and Song, B and Sukop, MC and Hu, H}, title = {Improved lattice Boltzmann modeling of binary flow based on the conservative Allen-Cahn equation.}, journal = {Physical review. E}, volume = {94}, number = {2-1}, pages = {023311}, doi = {10.1103/PhysRevE.94.023311}, pmid = {27627416}, issn = {2470-0053}, abstract = {The primary and key task of binary fluid flow modeling is to track the interface with good accuracy, which is usually challenging due to the sharp-interface limit and numerical dispersion. This article concentrates on further development of the conservative Allen-Cahn equation (ACE) [Geier et al., Phys. Rev. E 91, 063309 (2015)10.1103/PhysRevE.91.063309] under the framework of the lattice Boltzmann method (LBM), with incorporation of the incompressible hydrodynamic equations [Liang et al., Phys. Rev. E 89, 053320 (2014)10.1103/PhysRevE.89.053320]. Utilizing a modified equilibrium distribution function and an additional source term, this model is capable of correctly recovering the conservative ACE through the Chapman-Enskog analysis. We also simulate four phase-tracking benchmark cases, including one three-dimensional case; all show good accuracy as well as low numerical dispersion. By coupling the incompressible hydrodynamic equations, we also simulate layered Poiseuille flow and the Rayleigh-Taylor instability, illustrating satisfying performance in dealing with complex flow problems, e.g., high viscosity ratio, high density ratio, and high Reynolds number situations. The present work provides a reliable and efficient solution for binary flow modeling.}, } @article {pmid27627415, year = {2016}, author = {Ba, Y and Liu, H and Li, Q and Kang, Q and Sun, J}, title = {Multiple-relaxation-time color-gradient lattice Boltzmann model for simulating two-phase flows with high density ratio.}, journal = {Physical review. E}, volume = {94}, number = {2-1}, pages = {023310}, doi = {10.1103/PhysRevE.94.023310}, pmid = {27627415}, issn = {2470-0053}, abstract = {In this paper we propose a color-gradient lattice Boltzmann (LB) model for simulating two-phase flows with high density ratio and high Reynolds number. The model applies a multirelaxation-time (MRT) collision operator to enhance the stability of the simulation. A source term, which is derived by the Chapman-Enskog analysis, is added into the MRT LB equation so that the Navier-Stokes equations can be exactly recovered. Also, a form of the equilibrium density distribution function is used to simplify the source term. To validate the proposed model, steady flows of a static droplet and the layered channel flow are first simulated with density ratios up to 1000. Small values of spurious velocities and interfacial tension errors are found in the static droplet test, and improved profiles of velocity are obtained by the present model in simulating channel flows. Then, two cases of unsteady flows, Rayleigh-Taylor instability and droplet splashing on a thin film, are simulated. In the former case, the density ratio of 3 and Reynolds numbers of 256 and 2048 are considered. The interface shapes and spike and bubble positions are in good agreement with the results of previous studies. In the latter case, the droplet spreading radius is found to obey the power law proposed in previous studies for the density ratio of 100 and Reynolds number up to 500.}, } @article {pmid27627229, year = {2016}, author = {Heidenreich, S and Dunkel, J and Klapp, SH and Bär, M}, title = {Hydrodynamic length-scale selection in microswimmer suspensions.}, journal = {Physical review. E}, volume = {94}, number = {2-1}, pages = {020601}, doi = {10.1103/PhysRevE.94.020601}, pmid = {27627229}, issn = {2470-0053}, abstract = {A universal characteristic of mesoscale turbulence in active suspensions is the emergence of a typical vortex length scale, distinctly different from the scale invariance of turbulent high-Reynolds number flows. Collective length-scale selection has been observed in bacterial fluids, endothelial tissue, and active colloids, yet the physical origins of this phenomenon remain elusive. Here, we systematically derive an effective fourth-order field theory from a generic microscopic model that allows us to predict the typical vortex size in microswimmer suspensions. Building on a self-consistent closure condition, the derivation shows that the vortex length scale is determined by the competition between local alignment forces, rotational diffusion, and intermediate-range hydrodynamic interactions. Vortex structures found in simulations of the theory agree with recent measurements in Bacillus subtilis suspensions. Moreover, our approach yields an effective viscosity enhancement (reduction), as reported experimentally for puller (pusher) microorganisms.}, } @article {pmid27610298, year = {2016}, author = {Maqbool, K and Shaheen, S and Mann, AB}, title = {Exact solution of cilia induced flow of a Jeffrey fluid in an inclined tube.}, journal = {SpringerPlus}, volume = {5}, number = {1}, pages = {1379}, doi = {10.1186/s40064-016-3021-8}, pmid = {27610298}, issn = {2193-1801}, abstract = {The present study investigated the cilia induced flow of MHD Jeffrey fluid through an inclined tube. This study is carried out under the assumptions of long wavelength and low Reynolds number approximations. Exact solutions for the velocity profile, pressure rise, pressure gradient, volume flow rate and stream function are obtained. Effects of pertinent physical parameters on the computational results are presented graphically.}, } @article {pmid27608508, year = {2016}, author = {Li, J and Liu, W and Li, T and Rozen, I and Zhao, J and Bahari, B and Kante, B and Wang, J}, title = {Swimming Microrobot Optical Nanoscopy.}, journal = {Nano letters}, volume = {16}, number = {10}, pages = {6604-6609}, doi = {10.1021/acs.nanolett.6b03303}, pmid = {27608508}, issn = {1530-6992}, abstract = {Optical imaging plays a fundamental role in science and technology but is limited by the ability of lenses to resolve small features below the fundamental diffraction limit. A variety of nanophotonic devices, such as metamaterial superlenses and hyperlenses, as well as microsphere lenses, have been proposed recently for subdiffraction imaging. The implementation of these micro/nanostructured lenses as practical and efficient imaging approaches requires locomotive capabilities to probe specific sites and scan large areas. However, directed motion of nanoscale objects in liquids must overcome low Reynolds number viscous flow and Brownian fluctuations, which impede stable and controllable scanning. Here we introduce a new imaging method, named swimming microrobot optical nanoscopy, based on untethered chemically powered microrobots as autonomous probes for subdiffraction optical scanning and imaging. The microrobots are made of high-refractive-index microsphere lenses and powered by local catalytic reactions to swim and scan over the sample surface. Autonomous motion and magnetic guidance of microrobots enable large-area, parallel and nondestructive scanning with subdiffraction resolution, as illustrated using soft biological samples such as neuron axons, protein microtubulin, and DNA nanotubes. Incorporating such imaging capacities in emerging nanorobotics technology represents a major step toward ubiquitous nanoscopy and smart nanorobots for spectroscopy and imaging.}, } @article {pmid27588859, year = {2016}, author = {Ault, JT and Fani, A and Chen, KK and Shin, S and Gallaire, F and Stone, HA}, title = {Vortex-Breakdown-Induced Particle Capture in Branching Junctions.}, journal = {Physical review letters}, volume = {117}, number = {8}, pages = {084501}, doi = {10.1103/PhysRevLett.117.084501}, pmid = {27588859}, issn = {1079-7114}, abstract = {We show experimentally that a flow-induced, Reynolds number-dependent particle-capture mechanism in branching junctions can be enhanced or eliminated by varying the junction angle. In addition, numerical simulations are used to show that the features responsible for this capture have the signatures of classical vortex breakdown, including an approach flow aligned with the vortex axis and a pocket of subcriticality. We show how these recirculation regions originate and evolve and suggest a physical mechanism for their formation. Furthermore, comparing experiments and numerical simulations, the presence of vortex breakdown is found to be an excellent predictor of particle capture. These results inform the design of systems in which suspended particle accumulation can be eliminated or maximized.}, } @article {pmid27586487, year = {2016}, author = {Hayat, T and Ahmed, B and Abbasi, FM and Ahmad, B}, title = {Mixed convective peristaltic flow of carbon nanotubes submerged in water using different thermal conductivity models.}, journal = {Computer methods and programs in biomedicine}, volume = {135}, number = {}, pages = {141-150}, doi = {10.1016/j.cmpb.2016.07.030}, pmid = {27586487}, issn = {1872-7565}, mesh = {*Models, Theoretical ; *Nanotubes, Carbon ; Water/*chemistry ; }, abstract = {BACKGROUND AND OBJECTIVE: Single Walled Carbon Nanotubes (SWCNTs) are the advanced product of nanotechnology having notable mechanical and physical properties. Peristalsis of SWCNTs suspended in water through an asymmetric channel is examined. Such mechanism is studied in the presence of viscous dissipation, velocity slip, mixed convection, temperature jump and heat generation/absorption.

METHODS: Mathematical modeling is carried out under the low Reynolds number and long wavelength approximation. Resulting nonlinear system is solved using the perturbation technique for small Brinkman's number. Physical analysis and comparison of the results in light of three different thermal conductivity models is also provided.

CONCLUSIONS: It is reported that the heat transfer rate at the boundary increases with an increase in the nanotubes volume fraction. The addition of nanotubes affects the pressure gradient during the peristaltic flow. Moreover, the maximum velocity of the fluid decreases due to addition of the nanotubes.}, } @article {pmid27586475, year = {2016}, author = {Ramesh, K}, title = {Effects of slip and convective conditions on the peristaltic flow of couple stress fluid in an asymmetric channel through porous medium.}, journal = {Computer methods and programs in biomedicine}, volume = {135}, number = {}, pages = {1-14}, doi = {10.1016/j.cmpb.2016.07.001}, pmid = {27586475}, issn = {1872-7565}, mesh = {*Hydrodynamics ; Models, Theoretical ; Porosity ; *Stress, Mechanical ; }, abstract = {BACKGROUND: Assessment of the fluid flow pattern in a non-pregnant uterus is important for understanding embryo transport in the uterus. Fertilization occurs in the fallopian tube and the embryo enters the uterine cavity within three days of ovulation. In the uterus, the embryo is conveyed by the uterine fluid for another three to four days to a successful implantation site at the upper part of the uterus. The movements of fluid within the uterus may be induced by several mechanisms, but they seem to be dominated by myometrial contractions. The intrauterine fluid flow due to these myometrial contractions is peristaltic type motion in nature and the myometrial contractions may occur in both symmetric and asymmetric directions.

OBJECTIVE: The aim of the present article is to investigate the peristaltic transport of couple stress fluid in an asymmetric channel. The channel asymmetry is produced by choosing the peristaltic wave train on the walls to have different wave amplitudes and phase differences. The fluid is filled with a homogeneous porous medium. The effects of slip and convective boundary conditions are also taken into consideration.

METHOD: The flow is investigated in the wave frame of reference moving with constant velocity with the wave. Long wavelength and low Reynolds number approximations are utilized in problem formulation. Exact solutions are presented for the stream function, pressure gradient and temperature.

RESULTS: The graphical analysis is carried out to examine the effects of sundry parameters on flow quantities of interest. Comparative study is also made for couple stress fluid with Newtonian fluid.

CONCLUSIONS: The results revealed that the trapping fluid can be increased and the central line axial velocity can be raised to a considerable extent by increasing Darcy number. Increasing of slip parameter increases the velocity near the boundary of the walls and Brinkman number increases the temperature of the fluid.}, } @article {pmid27575234, year = {2016}, author = {Pareschi, G and Frapolli, N and Chikatamarla, SS and Karlin, IV}, title = {Conjugate heat transfer with the entropic lattice Boltzmann method.}, journal = {Physical review. E}, volume = {94}, number = {1-1}, pages = {013305}, doi = {10.1103/PhysRevE.94.013305}, pmid = {27575234}, issn = {2470-0053}, abstract = {A conjugate heat-transfer model is presented based on the two-population entropic lattice Boltzmann method. The present approach relies on the extension of Grad's boundary conditions to the two-population model for thermal flows, as well as on the appropriate exact conjugate heat-transfer condition imposed at the fluid-solid interface. The simplicity and efficiency of the lattice Boltzmann method (LBM), and in particular of the entropic multirelaxation LBM, are retained in the present approach, thus enabling simulations of turbulent high Reynolds number flows and complex wall boundaries. The model is validated by means of two-dimensional parametric studies of various setups, including pure solid conduction, conjugate heat transfer with a backward-facing step flow, and conjugate heat transfer with the flow past a circular heated cylinder. Further validations are performed in three dimensions for the case of a turbulent flow around a heated mounted cube.}, } @article {pmid27575221, year = {2016}, author = {Sahu, S and Shankar, V}, title = {Passive manipulation of free-surface instability by deformable solid bilayers.}, journal = {Physical review. E}, volume = {94}, number = {1-1}, pages = {013111}, doi = {10.1103/PhysRevE.94.013111}, pmid = {27575221}, issn = {2470-0053}, abstract = {This study deals with the elastohydrodynamic coupling that occurs in the flow of a liquid layer down an inclined plane lined with a deformable solid bilayer and its consequences on the stability of the free surface of the liquid layer. The fluid is Newtonian and incompressible, while the linear elastic constitutive relation has been considered for the deformable solid bilayer, and the densities of the fluid and the two solids are kept equal. A temporal linear stability analysis is carried out for this coupled solid-fluid system. A long-wave asymptotic analysis is employed to obtain an analytical expression for the complex wavespeed in the low wave-number regime, and a numerical shooting method is used to solve the coupled set of governing differential equations in order to obtain the stability criterion for arbitrary values of the wave number. In a previous work on plane Couette flow past an elastic bilayer, Neelmegam et al. [Phys. Rev. E 90, 043004 (2014)PLEEE81539-375510.1103/PhysRevE.90.043004] showed that the instability of the flow can be significantly influenced by the nature of the solid layer, which is adjacent to the liquid layer. In stark contrast, for free-surface flow past a bilayer, our long-wave asymptotic analysis demonstrates that the stability of the free-surface mode is insensitive to the nature of the solid adjacent to the liquid layer. Instead, it is the effective shear modulus of the bilayer G_{eff} (given by H/G_{eff} =H_{1} /G_{1} +H_{2} /G_{2}, where H=H_{1} +H_{2} is the total thickness of the solid bilayer, H_{1} and H_{2} are the thicknesses of the two solid layers, and G_{1} and G_{2} are the shear moduli of the two solid layers) that determines the stability of the free surface in the long-wave limit. We show that for a given Reynolds number, the free-surface instability is stabilized when G_{eff} decreases below a critical value. At finite wave numbers, our numerical solution indicates that additional instabilities at the free surface and the liquid-solid interface can be induced by wall deformability and inertia in the fluid and solid. Interestingly, the onset of these additional instabilities is sensitive to the relative placements of the two solid layers comprising the bilayer. We show that it is possible to delay the onset of these additional instabilities, while still suppressing the free-surface instability, by manipulating the ratio of the shear moduli and the thicknesses of the two solid layers in the bilayer. At moderate Reynolds number and finite wave number, we demonstrate that an exchange of modes occurs between the gas-liquid and liquid-solid interfacial modes as the solid bilayer becomes more deformable. We demonstrate further that dissipative effects in the individual solid layers have an important bearing on the stability of the system, and they could also be exploited in suppressing the instability. This study thus shows that the ability to passively manipulate and control interfacial instabilities increases substantially with the use of solid bilayers.}, } @article {pmid27575215, year = {2016}, author = {Swaminathan, RV and Ravichandran, S and Perlekar, P and Govindarajan, R}, title = {Dynamics of circular arrangements of vorticity in two dimensions.}, journal = {Physical review. E}, volume = {94}, number = {1-1}, pages = {013105}, doi = {10.1103/PhysRevE.94.013105}, pmid = {27575215}, issn = {2470-0053}, abstract = {The merger of two like-signed vortices is a well-studied problem, but in a turbulent flow, we may often have more than two like-signed vortices interacting. We study the merger of three or more identical corotating vortices initially arranged on the vertices of a regular polygon. At low to moderate Reynolds numbers, we find an additional stage in the merger process, absent in the merger of two vortices, where an annular vortical structure is formed and is long lived. Vortex merger is slowed down significantly due to this. Such annular vortices are known at far higher Reynolds numbers in studies of tropical cyclones, which have been noticed to a break down into individual vortices. In the preannular stage, vortical structures in a viscous flow are found here to tilt and realign in a manner similar to the inviscid case, but the pronounced filaments visible in the latter are practically absent in the former. Five or fewer vortices initially elongate radially, and then reorient their long axis closer to the azimuthal direction so as to form an annulus. With six or more vortices, the initial alignment is already azimuthal. Interestingly at higher Reynolds numbers, the merger of an odd number of vortices is found to proceed very differently from that of an even number. The former process is rapid and chaotic whereas the latter proceeds more slowly via pairing events. The annular vortex takes the form of a generalized Lamb-Oseen vortex (GLO), and diffuses inward until it forms a standard Lamb-Oseen vortex. For lower Reynolds number, the numerical (fully nonlinear) evolution of the GLO vortex follows exactly the analytical evolution until merger. At higher Reynolds numbers, the annulus goes through instabilities whose nonlinear stages show a pronounced difference between even and odd mode disturbances. Here again, the odd mode causes an early collapse of the annulus via decaying turbulence into a single central vortex, whereas the even mode disturbance causes a more orderly progression into a single vortex. Results from linear stability analysis agree with the nonlinear simulations, and predict the frequencies of the most unstable modes better than they predict the growth rates. It is hoped that the present findings, that multiple vortex merger is qualitatively different from the merger of two vortices, will motivate studies on how multiple vortex interactions affect the inverse cascade in two-dimensional turbulence.}, } @article {pmid27567769, year = {2016}, author = {Tabe, R and Ghalichi, F and Hossainpour, S and Ghasemzadeh, K}, title = {Laminar-to-turbulence and relaminarization zones detection by simulation of low Reynolds number turbulent blood flow in large stenosed arteries.}, journal = {Bio-medical materials and engineering}, volume = {27}, number = {2-3}, pages = {119-129}, doi = {10.3233/BME-161574}, pmid = {27567769}, issn = {1878-3619}, mesh = {Arterial Occlusive Diseases/*physiopathology ; Arteries/*physiopathology ; Blood Flow Velocity ; Computer Simulation ; Constriction, Pathologic/*physiopathology ; Humans ; Models, Cardiovascular ; Pulsatile Flow ; Stress, Mechanical ; }, abstract = {Laminar, turbulent, transitional, or combine areas of all three types of viscous flow can occur downstream of a stenosis depending upon the Reynolds number and constriction shape parameter. Neither laminar flow solver nor turbulent models for instance the k-ω (k-omega), k-ε (k-epsilon), RANS or LES are opportune for this type of flow. In the present study attention has been focused vigorously on the effect of the constriction in the flow field with a unique way. It means that the laminar solver was employed from entry up to the beginning of the turbulent shear flow. The turbulent model (k-ω SST Transitional Flows) was utilized from starting of turbulence to relaminarization zone while the laminar model was applied again with onset of the relaminarization district. Stenotic flows, with 50 and 75% cross-sectional area, were simulated at Reynolds numbers range from 500 to 2000 employing FLUENT (v6.3.17). The flow was considered to be steady, axisymmetric, and incompressible. Achieving results were reported as axial velocity, disturbance velocity, wall shear stress and the outcomes were compared with previously experimental and CFD computations. The analogy of axial velocity profiles shows that they are in acceptable compliance with the empirical data. As well as disturbance velocity and wall shear stresses anticipated by this new approach, part by part simulation, are reasonably valid with the acceptable experimental studies.}, } @article {pmid27552860, year = {2016}, author = {Maneshian, B and Javadi, Kh and Rahni, MT and Miller, R}, title = {Droplet dynamics in rotating flows.}, journal = {Advances in colloid and interface science}, volume = {236}, number = {}, pages = {63-82}, doi = {10.1016/j.cis.2016.07.005}, pmid = {27552860}, issn = {1873-3727}, abstract = {This paper deals with investigations of droplet dynamics in rotating flows. In many previous studies droplet dynamics was analyzed in simple unidirectional flows. To fill this gap, the focus of this study is an overview on investigations of droplet dynamics in a complex rotating flow. A Lattice Boltzmann Method with high potential in simulation of two-phase unsteady flows is applied to simulate the physics of the problem in a lid-driven cavity. In spite of its simple geometry, there is a complex rotating flow field containing different vortices and shear regions. The Reynolds number based on the cavity length scale and the upper wall velocity, ReL, is considered to be 1000. We discuss here effects of different parameters such as: density ratios (1, 5, 10, 100, and 1000), droplet sizes (D/L=0.097, 0.114, 0.131 and 0.2), and droplet initial positions (1/8, 2/8, and 3/8 of the cavity length, L, out of center). The results are discussed in terms of global flow physics and its interaction with the droplet, drop deformation during its motion along with the main flow, and droplet trajectories. It is shown that there are strong interactions between the droplet and the main carrying flow. During motion, the droplets pass through different flow regions containing acceleration/deceleration zones. Consequently, the droplets experience different shear forces resulting in stretching, shrinking, rotating and dilatation which all contribute to the dynamics of the droplet.}, } @article {pmid27528780, year = {2016}, author = {Liu, H and Ravi, S and Kolomenskiy, D and Tanaka, H}, title = {Biomechanics and biomimetics in insect-inspired flight systems.}, journal = {Philosophical transactions of the Royal Society of London. Series B, Biological sciences}, volume = {371}, number = {1704}, pages = {}, doi = {10.1098/rstb.2015.0390}, pmid = {27528780}, issn = {1471-2970}, mesh = {*Aircraft ; Animals ; Biomechanical Phenomena ; *Biomimetics ; *Flight, Animal ; Insecta/*physiology ; }, abstract = {Insect- and bird-size drones-micro air vehicles (MAV) that can perform autonomous flight in natural and man-made environments are now an active and well-integrated research area. MAVs normally operate at a low speed in a Reynolds number regime of 10(4)-10(5) or lower, in which most flying animals of insects, birds and bats fly, and encounter unconventional challenges in generating sufficient aerodynamic forces to stay airborne and in controlling flight autonomy to achieve complex manoeuvres. Flying insects that power and control flight by flapping wings are capable of sophisticated aerodynamic force production and precise, agile manoeuvring, through an integrated system consisting of wings to generate aerodynamic force, muscles to move the wings and a control system to modulate power output from the muscles. In this article, we give a selective review on the state of the art of biomechanics in bioinspired flight systems in terms of flapping and flexible wing aerodynamics, flight dynamics and stability, passive and active mechanisms in stabilization and control, as well as flapping flight in unsteady environments. We further highlight recent advances in biomimetics of flapping-wing MAVs with a specific focus on insect-inspired wing design and fabrication, as well as sensing systems.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.}, } @article {pmid27518455, year = {2016}, author = {Li, F and Jian, Y and Chang, L and Zhao, G and Yang, L}, title = {Alternating current electroosmotic flow in polyelectrolyte-grafted nanochannel.}, journal = {Colloids and surfaces. B, Biointerfaces}, volume = {147}, number = {}, pages = {234-241}, doi = {10.1016/j.colsurfb.2016.07.064}, pmid = {27518455}, issn = {1873-4367}, mesh = {Electricity ; *Electrochemistry ; Electroosmosis/*instrumentation ; *Nanotechnology ; Polyelectrolytes/*chemistry ; Static Electricity ; }, abstract = {In this work, we investigate the time periodic electroosmotic flow (EOF) of an electrolyte solution through a slit polyelectrolyte-grafted (PE-grafted) nanochannel under applied alternating current (AC) electrical field. The PE-grafted nanochannel is represented by a rigid surface covered by a polyelectrolyte layer (PEL) in a brush-like configuration. Under Debye-Hückel approximation, we obtain analytical solutions of electrical potential in decoupled regime of PE-grafted nanochannel, where the thickness of PEL is independent of the electrostatic effects triggered by polyelectrolyte charges. Based upon the electrical potential obtained above, we calculate EOF velocities with uniform and non-uniform drag coefficients for PE-grafted nanochannel and compare their results. The effects of pertinent dimensionless parameters on EOF velocity amplitude are discussed in detail. Moreover, the amplitude of EOF velocity in a PE-grafted nanochannel is compared with that in a rigid one. It is shown that larger EOF velocity and volume flow rate are found for a PE-grafted nanochannel. In addition, AC EOF velocity is further investigated. The oscillation of velocity reduces and is restricted within the region near the PEL-electrolyte interface for higher oscillating Reynolds number Re.}, } @article {pmid27507620, year = {2016}, author = {Farutin, A and Piasecki, T and Słowicka, AM and Misbah, C and Wajnryb, E and Ekiel-Jeżewska, ML}, title = {Dynamics of flexible fibers and vesicles in Poiseuille flow at low Reynolds number.}, journal = {Soft matter}, volume = {12}, number = {35}, pages = {7307-7323}, doi = {10.1039/c6sm00819d}, pmid = {27507620}, issn = {1744-6848}, abstract = {The dynamics of flexible fibers and vesicles in unbounded planar Poiseuille flow at low Reynolds number is shown to exhibit similar basic features, when their equilibrium (moderate) aspect ratio is the same and vesicle viscosity contrast is relatively high. Tumbling, lateral migration, accumulation and shape evolution of these two types of flexible objects are analyzed numerically. The linear dependence of the accumulation position on relative bending rigidity, and other universal scalings are derived from the local shear flow approximation.}, } @article {pmid27501748, year = {2016}, author = {Daghooghi, M and Borazjani, I}, title = {Self-propelled swimming simulations of bio-inspired smart structures.}, journal = {Bioinspiration & biomimetics}, volume = {11}, number = {5}, pages = {056001}, doi = {10.1088/1748-3190/11/5/056001}, pmid = {27501748}, issn = {1748-3190}, mesh = {Animals ; Biomechanical Phenomena ; *Biomimetic Materials ; Eels/*physiology ; Locomotion ; Models, Biological ; Perciformes/*physiology ; Swimming/*physiology ; }, abstract = {This paper presents self-propelled swimming simulations of a foldable structure, whose folded configuration is a box. For self-locomotion through water the structure unfolds and undulates. To guide the design of the structure and understand how it should undulate to achieve either highest speed or maximize efficiency during locomotion, several kinematic parameters were systematically varied in the simulations: the wave type (standing wave versus traveling wave), the smoothness of undulations (smooth undulations versus undulations of rigid links), the mode of undulations (carangiform: mackerel-like versus anguilliform: eel-like undulations), and the maximum amplitude of undulations. We show that the swimmers with standing wave are slow and inefficient because they are not able to produce thrust using the added-mass mechanism. Among the tested types of undulation at low Reynolds number (Re) regime of [Formula: see text] (Strouhal number of about 1.0), structures that employ carangiform undulations can swim faster, whereas anguilliform swimmers are more economic, i.e., using less power they can swim a longer distance. Another finding of our simulations is that structures which are made of rigid links are typically less efficient (lower propulsive and power efficiencies and also lower swimming speed) compared with smoothly undulating ones because a higher added-mass force is generated by smooth undulations. The wake of all the swimmers bifurcated at the low Re regime because of the higher lateral relative to the axial velocity (high Strouhal number) that advects the vortices laterally creating a double row of vortices in the wake. In addition, we show that the wake cannot be used to predict the performance of the swimmers because the net force in each cycle is zero for self-propelled bodies and the pressure term is not negligible compared to the other terms.}, } @article {pmid27493565, year = {2016}, author = {Smith, FT and Johnson, ER}, title = {Movement of a finite body in channel flow.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {472}, number = {2191}, pages = {20160164}, doi = {10.1098/rspa.2016.0164}, pmid = {27493565}, issn = {1364-5021}, abstract = {A body of finite size is moving freely inside, and interacting with, a channel flow. The description of this unsteady interaction for a comparatively dense thin body moving slowly relative to flow at medium-to-high Reynolds number shows that an inviscid core problem with vorticity determines much, but not all, of the dominant response. It is found that the lift induced on a body of length comparable to the channel width leads to differences in flow direction upstream and downstream on the body scale which are smoothed out axially over a longer viscous length scale; the latter directly affects the change in flow directions. The change is such that in any symmetric incident flow the ratio of slopes is found to be [Formula: see text], i.e. approximately 0.900969, independently of Reynolds number, wall shear stresses and velocity profile. The two axial scales determine the evolution of the body and the flow, always yielding instability. This unusual evolution and linear or nonlinear instability mechanism arise outside the conventional range of flow instability and are influenced substantially by the lateral positioning, length and axial velocity of the body.}, } @article {pmid27475199, year = {2016}, author = {Padois, T and Laffay, P and Idier, A and Moreau, S}, title = {Tonal noise of a controlled-diffusion airfoil at low angle of attack and Reynolds number.}, journal = {The Journal of the Acoustical Society of America}, volume = {140}, number = {1}, pages = {EL113}, doi = {10.1121/1.4958916}, pmid = {27475199}, issn = {1520-8524}, abstract = {The acoustic signature of a controlled-diffusion airfoil immersed in a flow is experimentally characterized. Acoustic measurements have been carried out in an anechoic open-jet-wind-tunnel for low Reynolds numbers (from 5 × 10(4) to 4.3 × 10(5)) and several angles of attack. As with the NACA0012, the acoustic spectrum is dominated by discrete tones. These tonal behaviors are divided into three different regimes. The first one is characterized by a dominant primary tone which is steady over time, surrounded by secondary peaks. The second consists of two unsteady primary tones associated with secondary peaks and the third consists of a hump dominated by several small peaks. A wavelet study allows one to identify an amplitude modulation of the acoustic signal mainly for the unsteady tonal regime. This amplitude modulation is equal to the frequency interval between two successive tones. Finally, a bispectral analysis explains the presence of tones at higher frequencies.}, } @article {pmid27462481, year = {2016}, author = {Zhang, W and Jiang, Y and Li, L and Chen, G}, title = {Effects of wall suction/blowing on two-dimensional flow past a confined square cylinder.}, journal = {SpringerPlus}, volume = {5}, number = {1}, pages = {985}, doi = {10.1186/s40064-016-2666-7}, pmid = {27462481}, issn = {2193-1801}, abstract = {A numerical simulation is conducted to study the laminar flow past a square cylinder confined in a channel (the ratio of side length of the square to channel width is fixed at 1/4) subjected to a locally uniform blowing/suction speed placed at the top and bottom channel walls. Governing equations with boundary conditions are resolved using a finite volume method in pressure-velocity formulation. The flow patterns relevant to the critical spacing values are investigated. Numerical results show that wall blowing has a stabilizing effect on the flow, and the corresponding critical Reynolds number increases monotonically with increasing blowing velocity. Remarkably, steady asymmetric solutions and hysteretic mode transitions exist in a certain range of parameters (Reynolds number and suction speed) in the case of suction.}, } @article {pmid27447509, year = {2016}, author = {Mathai, V and Calzavarini, E and Brons, J and Sun, C and Lohse, D}, title = {Microbubbles and Microparticles are Not Faithful Tracers of Turbulent Acceleration.}, journal = {Physical review letters}, volume = {117}, number = {2}, pages = {024501}, doi = {10.1103/PhysRevLett.117.024501}, pmid = {27447509}, issn = {1079-7114}, abstract = {We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St≪1 in turbulent flow. At a decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles; i.e., they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift velocity governing the enhancement of acceleration variance and the reductions in correlation time and intermittency. Our predictions are in good agreement with experimental and numerical results. The present findings are relevant to a range of scenarios encompassing tiny bubbles and droplets that drift through the turbulent oceans and the atmosphere. They also question the common usage of microbubbles and microdroplets as tracers in turbulence research.}, } @article {pmid27436965, year = {2016}, author = {Muralidhar, SD and Pier, B and Scott, JF and Govindarajan, R}, title = {Flow around a rotating, semi-infinite cylinder in an axial stream.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {472}, number = {2190}, pages = {20150850}, doi = {10.1098/rspa.2015.0850}, pmid = {27436965}, issn = {1364-5021}, abstract = {This paper concerns steady, high-Reynolds-number flow around a semi-infinite, rotating cylinder placed in an axial stream and uses boundary-layer type of equations which apply even when the boundary-layer thickness is comparable to the cylinder radius, as indeed it is at large enough downstream distances. At large rotation rates, it is found that a wall jet appears over a certain range of downstream locations. This jet strengthens with increasing rotation, but first strengthens then weakens as downstream distance increases, eventually disappearing, so the flow recovers a profile qualitatively similar to a classical boundary layer. The asymptotic solution at large streamwise distances is obtained as an expansion in inverse powers of the logarithm of the distance. It is found that the asymptotic radial and axial velocity components are the same as for a non-rotating cylinder, to all orders in this expansion.}, } @article {pmid27427674, year = {2016}, author = {Kristiawan, B and Kamal, S and Yanuar, }, title = {Thermo-Hydraulic Characteristics of Anatase Titania Nanofluids Flowing Through a Circular Conduit.}, journal = {Journal of nanoscience and nanotechnology}, volume = {16}, number = {6}, pages = {6078-6085}, pmid = {27427674}, issn = {1533-4880}, mesh = {*Hydrodynamics ; *Nanotechnology ; *Rheology ; *Temperature ; Titanium/*chemistry ; }, abstract = {The thermo-hydraulic characteristics of anatase titanium dioxide dispersed into distilled water with particle concentration of 0.1, 0.3, and 0.5 vol.% were investigated experimentally in this work. The influence of rheological behavior on hydrodynamic and convective heat transfer characteristics was evaluated under both laminar and turbulent flow conditions in a plain conduit and with twisted tape insert for twist ratio of 7. The nanofluids exhibited a strong shear-thinning behavior at low shear rate particularly higher particle concentration. The non-Newtonian titania nanofluids have also demonstrated a drag reduction phenomena in turbulent flow. At equal Reynolds number, the values of performance evaluation criterion in a conduit inserted a twisted tape were lower than those of in a plain conduit. It implies the unfavourable energy budget for twisted tape insert. The convective heat transfer coefficient does not gradually enhance with an increase of particle concentration. The flow features due mainly to the rheology of colloidal dispersions might be a reason for this phenomenon.}, } @article {pmid27415315, year = {2016}, author = {Apaza, L and Sandoval, M}, title = {Ballistic behavior and trapping of self-driven particles in a Poiseuille flow.}, journal = {Physical review. E}, volume = {93}, number = {6}, pages = {062602}, doi = {10.1103/PhysRevE.93.062602}, pmid = {27415315}, issn = {2470-0053}, abstract = {We study the two- and three-dimensional dynamics of a Brownian self-driven particle at low Reynolds number in a Poiseuille flow. A deterministic analysis is also performed and we find that under certain conditions the swimmer becomes trapped, thus performing closed orbits as observed in related experiments. Further analysis enables us to provide an analytic expression to achieve this trapping phenomenon. We then turn to Brownian dynamics simulations, where we show the effect of a Poiseuille flow, self-propulsion, and confinement on the diffusion of the swimmer in both two and three dimensions. It is found that for long times the mean-square displacement (MSD) along the flow direction is always quadratic in time, whereas for shorter times (before the particle reaches the walls) its MSD has also a quartic time behavior. It is also found that self-propelled particles will spread less in a Poiseuille flow than passive ones under the same circumstances.}, } @article {pmid27404898, year = {2016}, author = {Cole, BC and Marcus, GG and Parsa, S and Kramel, S and Ni, R and Voth, GA}, title = {Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence.}, journal = {Journal of visualized experiments : JoVE}, volume = {}, number = {112}, pages = {}, doi = {10.3791/53599}, pmid = {27404898}, issn = {1940-087X}, mesh = {Anisotropy ; *Printing, Three-Dimensional ; }, abstract = {Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing technology is used to fabricate particles with slender arms connected at a common center. Shapes explored are crosses (two perpendicular rods), jacks (three perpendicular rods), triads (three rods in triangular planar symmetry), and tetrads (four arms in tetrahedral symmetry). Methods for producing on the order of 10,000 fluorescently dyed particles are described. Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four synchronized videos of their motion in a turbulent flow between oscillating grids with Rλ = 91. In this relatively low-Reynolds number flow, the advected particles are small enough that they approximate ellipsoidal tracer particles. We present results of time-resolved 3D trajectories of position and orientation of the particles as well as measurements of their rotation rates.}, } @article {pmid27390636, year = {2016}, author = {Wang, B and Li, H}, title = {POD analysis of flow over a backward-facing step forced by right-angle-shaped plasma actuator.}, journal = {SpringerPlus}, volume = {5}, number = {1}, pages = {795}, doi = {10.1186/s40064-016-2361-8}, pmid = {27390636}, issn = {2193-1801}, abstract = {PURPOSE: This study aims to present flow control over the backward-facing step with specially designed right-angle-shaped plasma actuator and analyzed the influence of various scales of flow structures on the Reynolds stress through snapshot proper orthogonal decomposition (POD).

METHODS: 2D particle image velocimetry measurements were conducted on region (x/h = 0-2.25) and reattachment zone in the x-y plane over the backward-facing step at a Reynolds number of Re h = 27,766 (based on step height [Formula: see text] and free stream velocity [Formula: see text]. The separated shear layer was excited by specially designed right-angle-shaped plasma actuator under the normalized excitation frequency St h ≈ 0.345 along the 45° direction. The spatial distribution of each Reynolds stress component was reconstructed using an increasing number of POD modes.

RESULTS: The POD analysis indicated that the flow dynamic downstream of the step was dominated by large-scale flow structures, which contributed to streamwise Reynolds stress and Reynolds shear stress. The intense Reynolds stress localized to a narrow strip within the shear layer was mainly affected by small-scale flow structures, which were responsible for the recovery of the Reynolds stress peak. With plasma excitation, a significant increase was obtained in the vertical Reynolds stress peak.

CONCLUSIONS: Under the dimensionless frequencies St h ≈ 0.345 and [Formula: see text] which are based on the step height and momentum thickness, the effectiveness of the flow control forced by the plasma actuator along the 45° direction was ordinary. Only the vertical Reynolds stress was significantly affected.}, } @article {pmid27390635, year = {2016}, author = {Hossain, S and Kim, KY}, title = {Parametric investigation on mixing in a micromixer with two-layer crossing channels.}, journal = {SpringerPlus}, volume = {5}, number = {1}, pages = {794}, doi = {10.1186/s40064-016-2477-x}, pmid = {27390635}, issn = {2193-1801}, abstract = {This work presents a parametric investigation on flow and mixing in a chaotic micromixer consisting of two-layer crossing channels proposed by Xia et al. (Lab Chip 5: 748-755, 2005). The flow and mixing performance were numerically analyzed using commercially available software ANSYS CFX-15.0, which solves the Navier-Stokes and mass conservation equations with a diffusion-convection model in a Reynolds number range from 0.2 to 40. A mixing index based on the variance of the mass fraction of the mixture was employed to evaluate the mixing performance of the micromixer. The flow structure in the channel was also investigated to identify the relationship with mixing performance. The mixing performance and pressure-drop were evaluated with two dimensionless geometric parameters, i.e., ratios of the sub-channel width to the main channel width and the channels depth to the main channel width. The results revealed that the mixing index at the exit of the micromixer increases with increase in the channel depth-to-width ratio, but decreases with increase in the sub-channel width to main channel width ratio. And, it was found that the mixing index could be increased up to 0.90 with variations of the geometric parameters at Re = 0.2, and the pressure drop was very sensitive to the geometric parameters.}, } @article {pmid27378067, year = {2016}, author = {Manica, R and Klaseboer, E and Chan, DYC}, title = {The hydrodynamics of bubble rise and impact with solid surfaces.}, journal = {Advances in colloid and interface science}, volume = {235}, number = {}, pages = {214-232}, doi = {10.1016/j.cis.2016.06.010}, pmid = {27378067}, issn = {1873-3727}, abstract = {A bubble smaller than 1mm in radius rises along a straight path in water and attains a constant speed due to the balance between buoyancy and drag force. Depending on the purity of the system, within the two extreme limits of tangentially immobile or mobile boundary conditions at the air-water interface considerably different terminal speeds are possible. When such a bubble impacts on a horizontal solid surface and bounces, interesting physics can be observed. We study this physical phenomenon in terms of forces, which can be of colloidal, inertial, elastic, surface tension and viscous origins. Recent advances in high-speed photography allow for the observation of phenomena on the millisecond scale. Simultaneous use of such cameras to visualize both rise/deformation and the dynamics of the thin film drainage through interferometry are now possible. These experiments confirm that the drainage process obeys lubrication theory for the spectrum of micrometre to millimetre-sized bubbles that are covered in this review. We aim to bridge the colloidal perspective at low Reynolds numbers where surface forces are important to high Reynolds number fluid dynamics where the effect of the surrounding flow becomes important. A model that combines a force balance with lubrication theory allows for the quantitative comparison with experimental data under different conditions without any fitting parameter.}, } @article {pmid27377152, year = {2016}, author = {Dogra, N and Izadi, H and Vanderlick, TK}, title = {Micro-motors: A motile bacteria based system for liposome cargo transport.}, journal = {Scientific reports}, volume = {6}, number = {}, pages = {29369}, doi = {10.1038/srep29369}, pmid = {27377152}, issn = {2045-2322}, mesh = {Bacteria/metabolism ; *Bacterial Physiological Phenomena ; Biological Transport ; Fluorescence Resonance Energy Transfer ; Lipid Bilayers/*metabolism ; Unilamellar Liposomes/*metabolism ; }, abstract = {Biological micro-motors (microorganisms) have potential applications in energy utilization and nanotechnology. However, harnessing the power generated by such motors to execute desired work is extremely difficult. Here, we employ the power of motile bacteria to transport small, large, and giant unilamellar vesicles (SUVs, LUVs, and GUVs). Furthermore, we demonstrate bacteria-bilayer interactions by probing glycolipids inside the model membrane scaffold. Fluorescence Resonance Energy Transfer (FRET) spectroscopic and microscopic methods were utilized for understanding these interactions. We found that motile bacteria could successfully propel SUVs and LUVs with a velocity of 28 μm s(-1) and 13 μm s(-1), respectively. GUVs, however, displayed Brownian motion and could not be propelled by attached bacteria. Bacterial velocity decreased with the larger loaded cargo, which agrees with our calculations of loaded bacteria swimming at low Reynolds number.}, } @article {pmid27370893, year = {2016}, author = {Takasugi, Y and Futagawa, K and Kazuhara, K and Morishita, S and Okuda, T}, title = {Roles of endotracheal tubes and slip joints in respiratory pressure loss: a laboratory study.}, journal = {Journal of anesthesia}, volume = {30}, number = {5}, pages = {789-795}, doi = {10.1007/s00540-016-2210-5}, pmid = {27370893}, issn = {1438-8359}, mesh = {*Airway Resistance ; Humans ; Intubation, Intratracheal/*instrumentation ; Pressure ; }, abstract = {PURPOSE: The endotracheal tube (ETT) constitutes a significant component of total airway resistance. However, a discrepancy between measured and theoretical values has been reported in airway resistance through ETTs. The causes of the discrepancy were estimated by physical and rheological simulations.

METHODS: The pressure losses through total lengths of ETTs and slip joints under a volumetric flow rate of 30 L/min were measured, and the pressure losses through the tubular parts of ETTs with internal diameters (IDs) of 6.0-, 6.5-, 7.0-, 7.5-, and 8.0 mm were measured. The Reynolds number of each setting was calculated, and the pressure losses through the total length of the ETT, the tubular part, and the slip joint of each size of tube were estimated.

RESULTS: The Reynolds numbers were >5000 in all sizes of ETTs. Measured pressure losses were larger in small sized ETTs than in large sized ETTs-520.9 Pascals (Pa) in 6.0-mm ID and 136.4 Pa in 8.0-mm ID tubes. The measured pressure losses through the tubular part were comparable to the predicted values. The measured pressure losses through the slip joints were larger than the predicted values, and they accounted for approximately 25-40% of total pressure losses of the ETTs.

CONCLUSION: Especially in small sized tubes, the pressure loss through the slip joint accounts for a large percentage of the total pressure loss through the ETT. The pressure loss through the slip joint may play a role in the discrepancy between measured and theoretical pressure losses through ETTs.}, } @article {pmid27319152, year = {2016}, author = {Hahn, T and Klemm, A and Ziesse, P and Harms, K and Wach, W and Rupp, S and Hirth, T and Zibek, S}, title = {Optimization and Scale-up of Inulin Extraction from Taraxacum kok-saghyz roots.}, journal = {Natural product communications}, volume = {11}, number = {5}, pages = {689-692}, pmid = {27319152}, issn = {1934-578X}, mesh = {Chemical Fractionation/*methods ; Inulin/*isolation & purification ; Plant Roots/chemistry ; Taraxacum/*chemistry ; }, abstract = {The optimization and scale-up of inulin extraction from Taraxacum kok-saghyz Rodin was successfully performed. Evaluating solubility investigations, the extraction temperature was fixed at 85 degrees C. The inulin stability regarding degradation or hydrolysis could be confirmed by extraction in the presence of model inulin. Confirming stability at the given conditions the isolation procedure was transferred from a 1 L- to a 1 m3-reactor. The Reynolds number was selected as the relevant dimensionless number that has to remain constant in both scales. The stirrer speed in the large scale was adjusted to 3.25 rpm regarding a 300 rpm stirrer speed in the 1 L-scale and relevant physical and process engineering parameters. Assumptions were confirmed by approximately homologous extraction kinetics in both scales. Since T. kok-saghyz is in the focus of research due to its rubber content side-product isolation from residual biomass it is of great economic interest. Inulin is one of these additional side-products that can be isolated in high quantity (- 35% of dry mass) and with a high average degree of polymerization (15.5) in large scale with a purity of 77%.}, } @article {pmid27307513, year = {2016}, author = {Secchi, E and Rusconi, R and Buzzaccaro, S and Salek, MM and Smriga, S and Piazza, R and Stocker, R}, title = {Intermittent turbulence in flowing bacterial suspensions.}, journal = {Journal of the Royal Society, Interface}, volume = {13}, number = {119}, pages = {}, doi = {10.1098/rsif.2016.0175}, pmid = {27307513}, issn = {1742-5662}, mesh = {Bacillus subtilis/*physiology ; Gastrointestinal Microbiome/physiology ; Humans ; Locomotion/*physiology ; Suspensions ; }, abstract = {Dense suspensions of motile bacteria, possibly including the human gut microbiome, exhibit collective dynamics akin to those observed in classic, high Reynolds number turbulence with important implications for chemical and biological transport, yet this analogy has remained primarily qualitative. Here, we present experiments in which a dense suspension of Bacillus subtilis bacteria was flowed through microchannels and the velocity statistics of the flowing suspension were quantified using a recently developed velocimetry technique coupled with vortex identification methods. Observations revealed a robust intermittency phenomenon, whereby the average velocity profile of the suspension fluctuated between a plug-like flow and a parabolic flow profile. This intermittency is a hallmark of the onset of classic turbulence and Lagrangian tracking revealed that it here originates from the presence of transient vortices in the active, collective motion of the bacteria locally reinforcing the externally imposed flow. These results link together two entirely different manifestations of turbulence and show the potential of the microfluidic approach to mimic the environment characteristic of certain niches of the human microbiome.}, } @article {pmid27300988, year = {2016}, author = {Lagubeau, G and Grosjean, G and Darras, A and Lumay, G and Hubert, M and Vandewalle, N}, title = {Statics and dynamics of magnetocapillary bonds.}, journal = {Physical review. E}, volume = {93}, number = {5}, pages = {053117}, doi = {10.1103/PhysRevE.93.053117}, pmid = {27300988}, issn = {2470-0053}, abstract = {When ferromagnetic particles are suspended at an interface under magnetic fields, dipole-dipole interactions compete with capillary attraction. This combination of forces has recently given promising results towards controllable self-assemblies as well as low-Reynolds-number swimming systems. The elementary unit of these assemblies is a pair of particles. Although equilibrium properties of this interaction are well described, the dynamics remain unclear. In this paper, the properties of magnetocapillary bonds are determined by probing them with magnetic perturbations. Two deformation modes are evidenced and discussed. These modes exhibit resonances whose frequencies can be detuned to generate nonreciprocal motion. A model is proposed that can become the basis for elaborate collective behaviors.}, } @article {pmid27300987, year = {2016}, author = {Verjus, R and Angilella, JR}, title = {Critical Stokes number for the capture of inertial particles by recirculation cells in two-dimensional quasisteady flows.}, journal = {Physical review. E}, volume = {93}, number = {5}, pages = {053116}, doi = {10.1103/PhysRevE.93.053116}, pmid = {27300987}, issn = {2470-0053}, abstract = {Inertial particles are often observed to be trapped, temporarily or permanently, by recirculation cells which are ubiquitous in natural or industrial flows. In the limit of small particle inertia, determining the conditions of trapping is a challenging task, as it requires a large number of numerical simulations or experiments to test various particle sizes or densities. Here, we investigate this phenomenon analytically and numerically in the case of heavy particles (e.g., aerosols) at low Reynolds number, to derive a trapping criterion that can be used both in analytical and numerical velocity fields. The resulting criterion allows one to predict the characteristics of trapped particles as soon as single-phase simulations of the flow are performed. Our analysis is valid for two-dimensional particle-laden flows in the vertical plane, in the limit where the particle inertia, the free-fall terminal velocity, and the flow unsteadiness can be treated as perturbations. The weak unsteadiness of the flow generally induces a chaotic tangle near heteroclinic or homoclinic cycles if any, leading to the apparent diffusion of fluid elements through the boundary of the cell. The critical particle Stokes number St_{c} below which aerosols also enter and exit the cell in a complex manner has been derived analytically, in terms of the flow characteristics. It involves the nondimensional curvature-weighted integral of the squared velocity of the steady fluid flow along the dividing streamline of the recirculation cell. When the flow is unsteady and St>St_{c}, a regular motion takes place due to gravity and centrifugal effects, like in the steady case. Particles driven towards the interior of the cell are trapped permanently. In contrast, when the flow is unsteady and St

METHODS: In the present work, the onset of turbulent transition during pulsatile flow through coronary arteries for varying degree of stenosis (i.e., 0%, 30%, 50% and 70%) is quantitatively analyzed by calculating the turbulent parameters distal to the stenosis. Also, the effect of turbulence transition on hemodynamic parameters such as WSS and oscillatory shear index (OSI) for varying degree of stenosis is quantified. The validated transitional shear stress transport (SST) k-ω model used in the present investigation is the best suited Reynolds averaged Navier-Stokes turbulence model to capture the turbulent transition. The arterial wall is assumed to be rigid and the dynamic curvature effect due to myocardial contraction on the blood flow has been neglected.

RESULTS: Our observations shows that for stenosis 50% and above, the WSSavg, WSSmax and OSI calculated using turbulence model deviates from laminar by more than 10% and the flow disturbances seems to significantly increase only after 70% stenosis. Our model shows reliability and completely validated.

CONCLUSIONS: Blood flow through stenosed coronary arteries seems to be turbulent in nature for area stenosis above 70% and the transition to turbulent flow begins from 50% stenosis.}, } @article {pmid27190567, year = {2016}, author = {Zhou, R and Wang, C}, title = {Multiphase ferrofluid flows for micro-particle focusing and separation.}, journal = {Biomicrofluidics}, volume = {10}, number = {3}, pages = {034101}, doi = {10.1063/1.4948656}, pmid = {27190567}, issn = {1932-1058}, abstract = {Ferrofluids have demonstrated great potential for a variety of manipulations of diamagnetic (or non-magnetic) micro-particles/cells in microfluidics, including sorting, focusing, and enriching. By utilizing size dependent magnetophoresis velocity, most of the existing techniques employ single phase ferrofluids to push the particles towards the channel walls. In this work, we demonstrate a novel strategy for focusing and separating diamagnetic micro-particles by using the laminar fluid interface of two co-flowing fluids-a ferrofluid and a non-magnetic fluid. Next to the microfluidic channel, microscale magnets are fabricated to generate strong localized magnetic field gradients and forces. Due to the magnetic force, diamagnetic particles suspended in the ferrofluid phase migrate across the ferrofluid stream at the size-dependent velocities. Because of the low Reynolds number and high Péclet number associated with the flow, the fluid interface is sharp and stable. When the micro-particles migrate to the interface, they are accumulated near the interface, resulting in effective focusing and separation of particles. We investigated several factors that affect the focusing and separation efficiency, including susceptibility of the ferrofluid, distance between the microfluidic channel and microscale magnet, and width of the microfluidic channel. This concept can be extended to multiple fluid interfaces. For example, a complete separation of micro-particles was demonstrated by using a three-stream multiphase flow configuration.}, } @article {pmid27183101, year = {2016}, author = {Wada, Y and Koyama, D and Nakamura, K}, title = {Numerical simulation of compressible fluid flow in an ultrasonic suction pump.}, journal = {Ultrasonics}, volume = {70}, number = {}, pages = {191-198}, doi = {10.1016/j.ultras.2016.05.005}, pmid = {27183101}, issn = {1874-9968}, abstract = {Characteristics of an ultrasonic suction pump that uses a vibrating piston surface and a pipe are numerically simulated and compared with experimental results. Fluid analysis based on the finite-difference time-domain (FDTD) routine is performed, where the nonlinear term and the moving fluid-surface boundary condition are considered. As a result, the suction mechanism of the pump is found to be similar to that of a check valve, where the gap is open during the inflow phase, and it is nearly closed during the outflow phase. The effects of Reynolds number, vibration amplitude and gap thickness on the pump performance are analyzed. The calculated result is in good agreement with the previously measured results.}, } @article {pmid27176524, year = {2016}, author = {Maiden, MD and Lowman, NK and Anderson, DV and Schubert, ME and Hoefer, MA}, title = {Observation of Dispersive Shock Waves, Solitons, and Their Interactions in Viscous Fluid Conduits.}, journal = {Physical review letters}, volume = {116}, number = {17}, pages = {174501}, doi = {10.1103/PhysRevLett.116.174501}, pmid = {27176524}, issn = {1079-7114}, abstract = {Dispersive shock waves and solitons are fundamental nonlinear excitations in dispersive media, but dispersive shock wave studies to date have been severely constrained. Here, we report on a novel dispersive hydrodynamic test bed: the effectively frictionless dynamics of interfacial waves between two high viscosity contrast, miscible, low Reynolds number Stokes fluids. This scenario is realized by injecting from below a lighter, viscous fluid into a column filled with high viscosity fluid. The injected fluid forms a deformable pipe whose diameter is proportional to the injection rate, enabling precise control over the generation of symmetric interfacial waves. Buoyancy drives nonlinear interfacial self-steepening, while normal stresses give rise to the dispersion of interfacial waves. Extremely slow mass diffusion and mass conservation imply that the interfacial waves are effectively dissipationless. This enables high fidelity observations of large amplitude dispersive shock waves in this spatially extended system, found to agree quantitatively with a nonlinear wave averaging theory. Furthermore, several highly coherent phenomena are investigated including dispersive shock wave backflow, the refraction or absorption of solitons by dispersive shock waves, and the multiphase merging of two dispersive shock waves. The complex, coherent, nonlinear mixing of dispersive shock waves and solitons observed here are universal features of dissipationless, dispersive hydrodynamic flows.}, } @article {pmid27176410, year = {2016}, author = {Mandal, S and Bandopadhyay, A and Chakraborty, S}, title = {Effect of surface charge convection and shape deformation on the dielectrophoretic motion of a liquid drop.}, journal = {Physical review. E}, volume = {93}, number = {}, pages = {043127}, doi = {10.1103/PhysRevE.93.043127}, pmid = {27176410}, issn = {2470-0053}, abstract = {The dielectrophoretic motion and shape deformation of a Newtonian liquid drop in an otherwise quiescent Newtonian liquid medium in the presence of an axisymmetric nonuniform dc electric field consisting of uniform and quadrupole components is investigated. The theory put forward by Feng [J. Q. Feng, Phys. Rev. E 54, 4438 (1996)10.1103/PhysRevE.54.4438] is generalized by incorporating the following two nonlinear effects-surface charge convection and shape deformation-towards determining the drop velocity. This two-way coupled moving boundary problem is solved analytically by considering small values of electric Reynolds number (ratio of charge relaxation time scale to the convection time scale) and electric capillary number (ratio of electrical stress to the surface tension) under the framework of the leaky dielectric model. We focus on investigating the effects of charge convection and shape deformation for different drop-medium combinations. A perfectly conducting drop suspended in a leaky (or perfectly) dielectric medium always deforms to a prolate shape and this kind of shape deformation always augments the dielectrophoretic drop velocity. For a perfectly dielectric drop suspended in a perfectly dielectric medium, the shape deformation leads to either increase (for prolate shape) or decrease (for oblate shape) in the dielectrophoretic drop velocity. Both surface charge convection and shape deformation affect the drop motion for leaky dielectric drops. The combined effect of these can significantly increase or decrease the dielectrophoretic drop velocity depending on the electrohydrodynamic properties of both the liquids and the relative strength of the electric Reynolds number and electric capillary number. Finally, comparison with the existing experiments reveals better agreement with the present theory.}, } @article {pmid27176408, year = {2016}, author = {Koens, L and Lauga, E}, title = {Rotation of slender swimmers in isotropic-drag media.}, journal = {Physical review. E}, volume = {93}, number = {}, pages = {043125}, doi = {10.1103/PhysRevE.93.043125}, pmid = {27176408}, issn = {2470-0053}, abstract = {The drag anisotropy of slender filaments is a critical physical property allowing swimming in low-Reynolds number flows, and without it linear translation is impossible. Here we show that, in contrast, net rotation can occur under isotropic drag. We first demonstrate this result formally by considering the consequences of the force- and torque-free conditions on swimming bodies and we then illustrate it with two examples (a simple swimmers made of three rods and a model bacterium with two helical flagellar filaments). Our results highlight the different role of hydrodynamic forces in generating translational versus rotational propulsion.}, } @article {pmid27176402, year = {2016}, author = {Hawkins, C and Angheluta, L and Krotkiewski, M and Jamtveit, B}, title = {Reynolds-number dependence of the longitudinal dispersion in turbulent pipe flow.}, journal = {Physical review. E}, volume = {93}, number = {}, pages = {043119}, doi = {10.1103/PhysRevE.93.043119}, pmid = {27176402}, issn = {2470-0053}, abstract = {In Taylor's theory, the longitudinal dispersion in turbulent pipe flows approaches, on long time scales, a diffusive behavior with a constant diffusivity K_{L}, which depends empirically on the Reynolds number Re. We show that the dependence on Re can be determined from the turbulent energy spectrum. By using the intimate connection between the friction factor and the longitudinal dispersion in wall-bounded turbulence, we predict different asymptotic scaling laws of K_{L} (Re) depending on the different turbulent cascades in two-dimensional turbulence. We also explore numerically the K_{L} (Re) dependence in turbulent channel flows with smooth and rough walls using a lattice Boltzmann method.}, } @article {pmid27176396, year = {2016}, author = {Cappanera, L and Guermond, JL and Léorat, J and Nore, C}, title = {Two spinning ways for precession dynamo.}, journal = {Physical review. E}, volume = {93}, number = {}, pages = {043113}, doi = {10.1103/PhysRevE.93.043113}, pmid = {27176396}, issn = {2470-0053}, abstract = {It is numerically demonstrated by means of a magnetohydrodynamic code that precession can trigger dynamo action in a cylindrical container. Fixing the angle between the spin and the precession axis to be 1/2π, two limit configurations of the spinning axis are explored: either the symmetry axis of the cylinder is parallel to the spin axis (this configuration is henceforth referred to as the axial spin case), or it is perpendicular to the spin axis (this configuration is referred to as the equatorial spin case). In both cases, the centro-symmetry of the flow breaks when the kinetic Reynolds number increases. Equatorial spinning is found to be more efficient in breaking the centro-symmetry of the flow. In both cases, the average flow in the reference frame of the mantle converges to a counter-rotation with respect to the spin axis as the Reynolds number grows. We find a scaling law for the average kinetic energy in term of the Reynolds number in the axial spin case. In the equatorial spin case, the unsteady asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. The magnetic field is mainly dipolar in the equatorial spin case, while it is is mainly quadrupolar in the axial spin case.}, } @article {pmid27176354, year = {2016}, author = {Goldfriend, T and Diamant, H and Witten, TA}, title = {Hydrodynamic interactions between two forced objects of arbitrary shape. II. Relative translation.}, journal = {Physical review. E}, volume = {93}, number = {}, pages = {042609}, doi = {10.1103/PhysRevE.93.042609}, pmid = {27176354}, issn = {2470-0053}, abstract = {We study the relative translation of two arbitrarily shaped objects, caused by their hydrodynamic interaction as they are forced through a viscous fluid in the limit of zero Reynolds number. It is well known that in the case of two rigid spheres in an unbounded fluid, the hydrodynamic interaction does not produce relative translation. More generally, such an effective pair-interaction vanishes in configurations with spatial inversion symmetry; for example, an enantiomorphic pair in mirror image positions has no relative translation. We show that the breaking of inversion symmetry by boundaries of the system accounts for the interactions between two spheres in confined geometries, as observed in experiments. The same general principle also provides new predictions for interactions in other object configurations near obstacles. We examine the time-dependent relative translation of two self-aligning objects, extending the numerical analysis of our preceding publication [Goldfriend, Diamant, and Witten, Phys. Fluids 27, 123303 (2015)]PHFLE61070-663110.1063/1.4936894. The interplay between the orientational interaction and the translational one, in most cases, leads over time to repulsion between the two objects. The repulsion is qualitatively different for self-aligning objects compared to the more symmetric case of uniform prolate spheroids. The separation between the two objects increases with time t as t^{1/3} in the former case, and more strongly, as t, in the latter.}, } @article {pmid27168523, year = {2016}, author = {Cheng, X and Sun, M}, title = {Wing-kinematics measurement and aerodynamics in a small insect in hovering flight.}, journal = {Scientific reports}, volume = {6}, number = {}, pages = {25706}, doi = {10.1038/srep25706}, pmid = {27168523}, issn = {2045-2322}, mesh = {Animals ; Biomechanical Phenomena ; Diptera/*physiology ; Flight, Animal/*physiology ; Time Factors ; Wings, Animal/anatomy & histology/*physiology ; }, abstract = {Wing-motion of hovering small fly Liriomyza sativae was measured using high-speed video and flows of the wings calculated numerically. The fly used high wingbeat frequency (≈265 Hz) and large stroke amplitude (≈182°); therefore, even if its wing-length (R) was small (R ≈ 1.4 mm), the mean velocity of wing reached ≈1.5 m/s, the same as that of an average-size insect (R ≈ 3 mm). But the Reynolds number (Re) of wing was still low (≈40), owing to the small wing-size. In increasing the stroke amplitude, the outer parts of the wings had a "clap and fling" motion. The mean-lift coefficient was high, ≈1.85, several times larger than that of a cruising airplane. The partial "clap and fling" motion increased the lift by ≈7%, compared with the case of no aerodynamic interaction between the wings. The fly mainly used the delayed stall mechanism to generate the high-lift. The lift-to-drag ratio is only 0.7 (for larger insects, Re being about 100 or higher, the ratio is 1-1.2); that is, although the small fly can produce enough lift to support its weight, it needs to overcome a larger drag to do so.}, } @article {pmid27166813, year = {2016}, author = {Monteith, CE and Brunner, ME and Djagaeva, I and Bielecki, AM and Deutsch, JM and Saxton, WM}, title = {A Mechanism for Cytoplasmic Streaming: Kinesin-Driven Alignment of Microtubules and Fast Fluid Flows.}, journal = {Biophysical journal}, volume = {110}, number = {9}, pages = {2053-2065}, doi = {10.1016/j.bpj.2016.03.036}, pmid = {27166813}, issn = {1542-0086}, support = {R01 GM046295/GM/NIGMS NIH HHS/United States ; }, mesh = {Biomechanical Phenomena ; *Cytoplasmic Streaming ; *Hydrodynamics ; Kinesin/*metabolism ; *Mechanical Phenomena ; Microtubules/*metabolism ; *Models, Biological ; Movement ; Oocytes/cytology ; }, abstract = {The transport of cytoplasmic components can be profoundly affected by hydrodynamics. Cytoplasmic streaming in Drosophila oocytes offers a striking example. Forces on fluid from kinesin-1 are initially directed by a disordered meshwork of microtubules, generating minor slow cytoplasmic flows. Subsequently, to mix incoming nurse cell cytoplasm with ooplasm, a subcortical layer of microtubules forms parallel arrays that support long-range, fast flows. To analyze the streaming mechanism, we combined observations of microtubule and organelle motions with detailed mathematical modeling. In the fast state, microtubules tethered to the cortex form a thin subcortical layer and undergo correlated sinusoidal bending. Organelles moving in flows along the arrays show velocities that are slow near the cortex and fast on the inward side of the subcortical microtubule layer. Starting with fundamental physical principles suggested by qualitative hypotheses, and with published values for microtubule stiffness, kinesin velocity, and cytoplasmic viscosity, we developed a quantitative coupled hydrodynamic model for streaming. The fully detailed mathematical model and its simulations identify key variables that can shift the system between disordered (slow) and ordered (fast) states. Measurements of array curvature, wave period, and the effects of diminished kinesin velocity on flow rates, as well as prior observations on f-actin perturbation, support the model. This establishes a concrete mechanistic framework for the ooplasmic streaming process. The self-organizing fast phase is a result of viscous drag on kinesin-driven cargoes that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are tethered to the cortex. Fluid moves toward plus ends and microtubules are forced backward toward their minus ends, resulting in buckling. Under certain conditions, the buckling microtubules self-organize into parallel bending arrays, guiding varying directions for fast plus-end directed fluid flows that facilitate mixing in a low Reynolds number regime.}, } @article {pmid27145450, year = {2016}, author = {Xi, J and Si, XA and Kim, J and Zhang, Y and Jacob, RE and Kabilan, S and Corley, RA}, title = {Anatomical Details of the Rabbit Nasal Passages and Their Implications in Breathing, Air Conditioning, and Olfaction.}, journal = {Anatomical record (Hoboken, N.J. : 2007)}, volume = {299}, number = {7}, pages = {853-868}, doi = {10.1002/ar.23367}, pmid = {27145450}, issn = {1932-8494}, support = {R01 HL073598/HL/NHLBI NIH HHS/United States ; }, mesh = {*Air Conditioning ; Animals ; Computer Simulation ; Female ; Magnetic Resonance Imaging ; Nasal Cavity/*anatomy & histology/*physiology ; Pulmonary Ventilation ; Rabbits ; *Respiration ; Smell/*physiology ; }, abstract = {The rabbit is commonly used as a laboratory animal for inhalation toxicology tests and detail knowledge of the rabbit airway morphometry is needed for outcome analysis or theoretical modeling. The objective of this study is to quantify the morphometric dimension of the nasal airway of a New Zealand white rabbit and to relate the morphology and functions through analytical and computational methods. Images of high-resolution MRI scans of the rabbit were processed to measure the axial distribution of the cross-sectional areas, perimeter, and complexity level. The lateral recess, which has functions other than respiration or olfaction, was isolated from the nasal airway and its dimension was quantified separately. A low Reynolds number turbulence model was implemented to simulate the airflow, heat transfer, vapor transport, and wall shear stress. Results of this study provide detailed morphological information of the rabbit that can be used in the studies of olfaction, inhalation toxicology, drug delivery, and physiology-based pharmacokinetics modeling. For the first time, we reported a spiral nasal vestibule that splits into three paths leading to the dorsal meatus, maxilloturbinate, and ventral meatus, respectively. Both non-dimensional functional analysis and CFD simulations suggested that the airflow in the rabbit nose is laminar and the unsteady effect is only significantly during sniffing. Due to the large surface-to-volume ratio, the maxilloturbinate is highly effective in warming and moistening the inhaled air to body conditions. The unique anatomical structure and respiratory airflow pattern may have important implications for designing new odorant detectors or electronic noses. Anat Rec, 299:853-868, 2016. © 2016 Wiley Periodicals, Inc.}, } @article {pmid27136099, year = {2016}, author = {Li, S and Huai, W}, title = {United Formula for the Friction Factor in the Turbulent Region of Pipe Flow.}, journal = {PloS one}, volume = {11}, number = {5}, pages = {e0154408}, doi = {10.1371/journal.pone.0154408}, pmid = {27136099}, issn = {1932-6203}, mesh = {*Friction ; *Models, Theoretical ; }, abstract = {Friction factor is an important element in both flow simulations and river engineering. In hydraulics, studies on the friction factor in turbulent regions have been based on the concept of three flow regimes, namely, the fully smooth regime, the fully rough regime, and the transitional regime, since the establishment of the Nikuradze's chart. However, this study further demonstrates that combining the friction factor with Reynolds number yields a united formula that can scale the entire turbulent region. This formula is derived by investigating the correlation between friction in turbulent pipe flow and its influencing factors, i.e., Reynolds number and relative roughness. In the present study, the formulae of Blasius and Stricklerare modified to rearrange the implicit model of Tao. In addition, we derive a united explicit formula that can compute the friction factor in the entire turbulent regimes based on the asymptotic behavior of the improved Tao's model. Compared with the reported formulae of Nikuradze, the present formula exhibits higher computational accuracy for the original pipe experiment data of Nikuradze.}, } @article {pmid27131850, year = {2016}, author = {Bocanegra Evans, H and Castillo, L}, title = {Index-matched measurements of the effect of cartilaginous rings on tracheobronchial flow.}, journal = {Journal of biomechanics}, volume = {49}, number = {9}, pages = {1601-1606}, doi = {10.1016/j.jbiomech.2016.03.043}, pmid = {27131850}, issn = {1873-2380}, mesh = {Bronchi/*physiology ; Cartilage/*anatomy & histology ; Humans ; Hydrodynamics ; Rheology ; Trachea/*physiology ; }, abstract = {We present a comparison of the flow characteristics in an idealized smooth trachea model and a second model which has a roughness simulating cartilaginous rings. We use refractive index-matched particle image velocimetry (PIV) to measure the velocity field in a two-generation model of the trachea and main bronchi. The flow rate has a trachea-based Reynolds number Re=2800, which is comparable to a resting state. Our results show considerable differences between both cases, the most important of which is the size and magnitude of recirculation zones at the inlet of both bronchi. The smooth case shows a larger separation bubble at the bronchi entrance, which may retain aerosols and have different effects on particles of different sizes. Furthermore, the smooth case displays a higher vorticity along the bottom walls of the bronchi, while a higher vorticity is seen along the trachea walls in the ׳ringed׳ model. These findings suggest that modeling the trachea and main bronchi as smooth tubes may not be justified, since the flow conditions in lower generations will be affected by these differences.}, } @article {pmid27127498, year = {2016}, author = {Brkić, D and Ćojbašić, Ž}, title = {Intelligent Flow Friction Estimation.}, journal = {Computational intelligence and neuroscience}, volume = {2016}, number = {}, pages = {5242596}, doi = {10.1155/2016/5242596}, pmid = {27127498}, issn = {1687-5273}, mesh = {*Friction ; *Hydrodynamics ; *Neural Networks (Computer) ; }, abstract = {Nowadays, the Colebrook equation is used as a mostly accepted relation for the calculation of fluid flow friction factor. However, the Colebrook equation is implicit with respect to the friction factor (λ). In the present study, a noniterative approach using Artificial Neural Network (ANN) was developed to calculate the friction factor. To configure the ANN model, the input parameters of the Reynolds Number (Re) and the relative roughness of pipe (ε/D) were transformed to logarithmic scales. The 90,000 sets of data were fed to the ANN model involving three layers: input, hidden, and output layers with, 2, 50, and 1 neurons, respectively. This configuration was capable of predicting the values of friction factor in the Colebrook equation for any given values of the Reynolds number (Re) and the relative roughness (ε/D) ranging between 5000 and 10(8) and between 10(-7) and 0.1, respectively. The proposed ANN demonstrates the relative error up to 0.07% which had the high accuracy compared with the vast majority of the precise explicit approximations of the Colebrook equation.}, } @article {pmid27121547, year = {2016}, author = {Lee, YJ and Lua, KB and Lim, TT and Yeo, KS}, title = {A quasi-steady aerodynamic model for flapping flight with improved adaptability.}, journal = {Bioinspiration & biomimetics}, volume = {11}, number = {3}, pages = {036005}, doi = {10.1088/1748-3190/11/3/036005}, pmid = {27121547}, issn = {1748-3190}, mesh = {Air ; Aircraft/*instrumentation ; Animals ; Biomimetics/*instrumentation/methods ; Computer Simulation ; *Computer-Aided Design ; Equipment Design ; Equipment Failure Analysis ; Friction ; Miniaturization ; *Models, Theoretical ; Oscillometry/*instrumentation/methods ; Rheology/*instrumentation/methods ; Shear Strength ; Stress, Mechanical ; }, abstract = {An improved quasi-steady aerodynamic model for flapping wings in hover has been developed. The purpose of this model is to yield rapid predictions of lift generation and efficiency during the design phase of flapping wing micro air vehicles. While most existing models are tailored for a specific flow condition, the present model is applicable over a wider range of Reynolds number and Rossby number. The effects of wing aspect ratio and taper ratio are also considered. The model was validated by comparing against numerical simulations and experimental measurements. Wings with different geometries undergoing distinct kinematics at varying flow conditions were tested during validation. Generally, model predictions of mean force coefficients were within 10% of numerical simulation results, while the deviations in power coefficients could be up to 15%. The deviation is partly due to the model not taking into consideration the initial shedding of the leading-edge vortex and wing-wake interaction which are difficult to account under quasi-steady assumption. The accuracy of this model is comparable to other models in literature, which had to be specifically designed or tuned to a narrow range of operation. In contrast, the present model has the advantage of being applicable over a wider range of flow conditions without prior tuning or calibration, which makes it a useful tool for preliminary performance evaluations.}, } @article {pmid27118897, year = {2016}, author = {Shyy, W and Kang, CK and Chirarattananon, P and Ravi, S and Liu, H}, title = {Aerodynamics, sensing and control of insect-scale flapping-wing flight.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {472}, number = {2186}, pages = {20150712}, doi = {10.1098/rspa.2015.0712}, pmid = {27118897}, issn = {1364-5021}, abstract = {There are nearly a million known species of flying insects and 13 000 species of flying warm-blooded vertebrates, including mammals, birds and bats. While in flight, their wings not only move forward relative to the air, they also flap up and down, plunge and sweep, so that both lift and thrust can be generated and balanced, accommodate uncertain surrounding environment, with superior flight stability and dynamics with highly varied speeds and missions. As the size of a flyer is reduced, the wing-to-body mass ratio tends to decrease as well. Furthermore, these flyers use integrated system consisting of wings to generate aerodynamic forces, muscles to move the wings, and sensing and control systems to guide and manoeuvre. In this article, recent advances in insect-scale flapping-wing aerodynamics, flexible wing structures, unsteady flight environment, sensing, stability and control are reviewed with perspective offered. In particular, the special features of the low Reynolds number flyers associated with small sizes, thin and light structures, slow flight with comparable wind gust speeds, bioinspired fabrication of wing structures, neuron-based sensing and adaptive control are highlighted.}, } @article {pmid27108375, year = {2016}, author = {Sultan, T}, title = {Numerical study of the effects of lamp configuration and reactor wall roughness in an open channel water disinfection UV reactor.}, journal = {Chemosphere}, volume = {155}, number = {}, pages = {170-179}, doi = {10.1016/j.chemosphere.2016.04.050}, pmid = {27108375}, issn = {1879-1298}, mesh = {Disinfection/*methods ; *Ultraviolet Rays ; Water Microbiology ; Water Purification/*instrumentation/methods ; }, abstract = {This article describes the assessment of a numerical procedure used to determine the UV lamp configuration and surface roughness effects on an open channel water disinfection UV reactor. The performance of the open channel water disinfection UV reactor was numerically analyzed on the basis of the performance indictor reduction equivalent dose (RED). The RED values were calculated as a function of the Reynolds number to monitor the performance. The flow through the open channel UV reactor was modelled using a k-ε model with scalable wall function, a discrete ordinate (DO) model for fluence rate calculation, a volume of fluid (VOF) model to locate the unknown free surface, a discrete phase model (DPM) to track the pathogen transport, and a modified law of the wall to incorporate the reactor wall roughness effects. The performance analysis was carried out using commercial CFD software (ANSYS Fluent 15.0). Four case studies were analyzed based on open channel UV reactor type (horizontal and vertical) and lamp configuration (parallel and staggered). The results show that lamp configuration can play an important role in the performance of an open channel water disinfection UV reactor. The effects of the reactor wall roughness were Reynolds number dependent. The proposed methodology is useful for performance optimization of an open channel water disinfection UV reactor.}, } @article {pmid27104354, year = {2016}, author = {Aftab, SM and Mohd Rafie, AS and Razak, NA and Ahmad, KA}, title = {Turbulence Model Selection for Low Reynolds Number Flows.}, journal = {PloS one}, volume = {11}, number = {4}, pages = {e0153755}, doi = {10.1371/journal.pone.0153755}, pmid = {27104354}, issn = {1932-6203}, mesh = {*Models, Theoretical ; Viscosity ; }, abstract = {One of the major flow phenomena associated with low Reynolds number flow is the formation of separation bubbles on an airfoil's surface. NACA4415 airfoil is commonly used in wind turbines and UAV applications. The stall characteristics are gradual compared to thin airfoils. The primary criterion set for this work is the capture of laminar separation bubble. Flow is simulated for a Reynolds number of 120,000. The numerical analysis carried out shows the advantages and disadvantages of a few turbulence models. The turbulence models tested were: one equation Spallart Allmars (S-A), two equation SST K-ω, three equation Intermittency (γ) SST, k-kl-ω and finally, the four equation transition γ-Reθ SST. However, the variation in flow physics differs between these turbulence models. Procedure to establish the accuracy of the simulation, in accord with previous experimental results, has been discussed in detail.}, } @article {pmid27078458, year = {2016}, author = {Johnson, PL and Meneveau, C}, title = {Large-deviation statistics of vorticity stretching in isotropic turbulence.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {033118}, doi = {10.1103/PhysRevE.93.033118}, pmid = {27078458}, issn = {2470-0053}, abstract = {A key feature of three-dimensional fluid turbulence is the stretching and realignment of vorticity by the action of the strain rate. It is shown in this paper, using the cumulant-generating function, that the cumulative vorticity stretching along a Lagrangian path in isotropic turbulence obeys a large deviation principle. As a result, the relevant statistics can be described by the vorticity stretching Cramér function. This function is computed from a direct numerical simulation data set at a Taylor-scale Reynolds number of Re(λ)=433 and compared to those of the finite-time Lyapunov exponents (FTLE) for material deformation. As expected, the mean cumulative vorticity stretching is slightly less than that of the most-stretched material line (largest FTLE), due to the vorticity's preferential alignment with the second-largest eigenvalue of strain rate and the material line's preferential alignment with the largest eigenvalue. However, the vorticity stretching tends to be significantly larger than the second-largest FTLE, and the Cramér functions reveal that the statistics of vorticity stretching fluctuations are more similar to those of the largest FTLE. In an attempt to relate the vorticity stretching statistics to the vorticity magnitude probability density function in statistically stationary conditions, a model Kramers-Moyal equation is constructed using the statistics encoded in the Cramér function. The model predicts a stretched-exponential tail for the vorticity magnitude probability density function, with good agreement for the exponent but significant difference (35%) in the prefactor.}, } @article {pmid27078456, year = {2016}, author = {De Santi, F and Fraternale, F and Tordella, D}, title = {Dispersive-to-nondispersive transition and phase-velocity transient for linear waves in plane wake and channel flows.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {033116}, doi = {10.1103/PhysRevE.93.033116}, pmid = {27078456}, issn = {2470-0053}, abstract = {In this study we analyze the phase and group velocity of three-dimensional linear traveling waves in two sheared flows: the plane channel and the wake flows. This was carried out by varying the wave number over a large interval of values at a given Reynolds number inside the ranges 20-100, 1000-8000, for the wake and channel flow, respectively. Evidence is given about the possible presence of both dispersive and nondispersive effects which are associated with the long and short ranges of wavelength. We solved the Orr-Sommerfeld and Squire eigenvalue problem and observed the least stable mode. It is evident that, at low wave numbers, the least stable eigenmodes in the left branch of the spectrum behave in a dispersive manner. By contrast, if the wave number is above a specific threshold, a sharp dispersive-to-nondispersive transition can be observed. Beyond this transition, the dominant mode belongs to the right branch of the spectrum. The transient behavior of the phase velocity of small three-dimensional traveling waves was also considered. Having chosen the initial conditions, we then show that the shape of the transient highly depends on the transition wavelength threshold value. We show that the phase velocity can oscillate with a frequency which is equal to the frequency width of the eigenvalue spectrum. Furthermore, evidence of intermediate self-similarity is given for the perturbation field.}, } @article {pmid27078455, year = {2016}, author = {Nedić, J and Tavoularis, S}, title = {Energy dissipation scaling in uniformly sheared turbulence.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {033115}, doi = {10.1103/PhysRevE.93.033115}, pmid = {27078455}, issn = {2470-0053}, abstract = {The rate of turbulent kinetic energy dissipation in spatially developing, uniformly sheared turbulence is examined experimentally. In the far-downstream fully developed region of the flow, we confirm that the dissipation parameter C(ɛ) is constant. More importantly, however, we find two upstream regions where this parameter could be scaled with the local turbulent Reynolds number as C(ɛ)=ARe(λ)(α); the exponents in these two regions are, respectively, α=-0.6 and 0.5. The observed changes in scaling laws are explained by consideration of structural changes in the turbulence.}, } @article {pmid27078453, year = {2016}, author = {Liang, H and Li, QX and Shi, BC and Chai, ZH}, title = {Lattice Boltzmann simulation of three-dimensional Rayleigh-Taylor instability.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {033113}, doi = {10.1103/PhysRevE.93.033113}, pmid = {27078453}, issn = {2470-0053}, abstract = {In this paper, the three-dimensional (3D) Rayleigh-Taylor instability (RTI) with low Atwood number (A(t)=0.15) in a long square duct (12W × W × W) is studied by using a multiple-relaxation-time lattice Boltzmann (LB) multiphase model. The effect of the Reynolds number on the interfacial dynamics and bubble and spike amplitudes at late time is investigated in detail. The numerical results show that at sufficiently large Reynolds numbers, a sequence of stages in the 3D immiscible RTI can be observed, which includes the linear growth, terminal velocity growth, reacceleration, and chaotic development stages. At late stage, the RTI induces a very complicated topology structure of the interface, and an abundance of dissociative drops are also observed in the system. The bubble and spike velocities at late stage are unstable and their values have exceeded the predictions of the potential flow theory [V. N. Goncharov, Phys. Rev. Lett. 88, 134502 (2002)]. The acceleration of the bubble front is also measured and it is found that the normalized acceleration at late time fluctuates around a constant value of 0.16. When the Reynolds number is reduced to small values, some later stages cannot be reached sequentially. The interface becomes relatively smoothed and the bubble velocity at late time is approximate to a constant value, which coincides with the results of the extended Layzer model [S.-I. Sohn, Phys. Rev. E 80, 055302(R) (2009)] and the modified potential theory [R. Banerjee, L. Mandal, S. Roy, M. Khan, and M. R. Guptae, Phys. Plasmas 18, 022109 (2011)]. In our simulations, the Graphics Processing Unit (GPU) parallel computing is also used to relieve the massive computational cost.}, } @article {pmid27078416, year = {2016}, author = {Jaju, SJ and Kumaran, V}, title = {Structure-rheology relationship in a sheared lamellar fluid.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {032609}, doi = {10.1103/PhysRevE.93.032609}, pmid = {27078416}, issn = {2470-0053}, abstract = {The structure-rheology relationship in the shear alignment of a lamellar fluid is studied using a mesoscale model which provides access to the lamellar configurations and the rheology. Based on the equations and free energy functional, the complete set of dimensionless groups that characterize the system are the Reynolds number (ργL(2)/μ), the Schmidt number (μ/ρD), the Ericksen number (μγ/B), the interface sharpness parameter r, the ratio of the viscosities of the hydrophilic and hydrophobic parts μ(r), and the ratio of the system size and layer spacing (L/λ). Here, ρ and μ are the fluid density and average viscosity, γ is the applied strain rate, D is the coefficient of diffusion, B is the compression modulus, μ(r) is the maximum difference in the viscosity of the hydrophilic and hydrophobic parts divided by the average viscosity, and L is the system size in the cross-stream direction. The lattice Boltzmann method is used to solve the concentration and momentum equations for a two dimensional system of moderate size (L/λ=32) and for a low Reynolds number, and the other parameters are systematically varied to examine the qualitative features of the structure and viscosity evolution in different regimes. At low Schmidt numbers where mass diffusion is faster than momentum diffusion, there is fast local formation of randomly aligned domains with "grain boundaries," which are rotated by the shear flow to align along the extensional axis as time increases. This configuration offers a high resistance to flow, and the layers do not align in the flow direction even after 1000 strain units, resulting in a viscosity higher than that for an aligned lamellar phase. At high Schmidt numbers where momentum diffusion is fast, the shear flow disrupts layers before they are fully formed by diffusion, and alignment takes place by the breakage and reformation of layers by shear, resulting in defects (edge dislocations) embedded in a background of nearly aligned layers. At high Ericksen number where the viscous forces are large compared to the restoring forces due to layer compression and bending, shear tends to homogenize the concentration field, and the viscosity decreases significantly. At very high Ericksen number, shear even disrupts the layering of the lamellar phase. At low Ericksen number, shear results in the formation of well aligned layers with edge dislocations. However, these edge dislocations take a long time to anneal; the relatively small misalignment due to the defects results in a large increase in viscosity due to high layer stiffness and due to shear localization, because the layers between defects get pinned and move as a plug with no shear. An increase in the viscosity contrast between the hydrophilic and hydrophobic parts does not alter the structural characteristics during alignment. However, there is a significant increase in the viscosity, due to pinning of the layers between defects, which results in a plug flow between defects and a localization of the shear to a part of the domain.}, } @article {pmid27078282, year = {2016}, author = {Haward, SJ and Poole, RJ and Alves, MA and Oliveira, PJ and Goldenfeld, N and Shen, AQ}, title = {Tricritical spiral vortex instability in cross-slot flow.}, journal = {Physical review. E}, volume = {93}, number = {3}, pages = {031101}, doi = {10.1103/PhysRevE.93.031101}, pmid = {27078282}, issn = {2470-0053}, abstract = {We examine fluid flow through cross-slot devices with various depth to width ratios α. At low Reynolds number, Re, flow is symmetric and a sharp boundary exists between the two incoming fluid streams. Above an α-dependent critical value, Re(c)(α), a steady symmetry-breaking bifurcation occurs and a spiral vortex structure develops. Order parameters characterizing the instability grow according to a sixth-order Landau potential, and show a progression from second- to first-order transitions as α increases beyond a tricritical value of α ≈ 0.55. Flow simulations indicate the instability is driven by vortex stretching at the stagnation point.}, } @article {pmid27071538, year = {2016}, author = {Xiang, Y and Xue, Y and Lv, P and Li, D and Duan, H}, title = {Influence of fluid flow on the stability and wetting transition of submerged superhydrophobic surfaces.}, journal = {Soft matter}, volume = {12}, number = {18}, pages = {4241-4246}, doi = {10.1039/c6sm00302h}, pmid = {27071538}, issn = {1744-6848}, abstract = {Superhydrophobic surfaces have attracted great attention for drag reduction application. However, these surfaces are subject to instabilities, especially under fluid flow. In this work, we in situ examine the stability and wetting transition of underwater superhydrophobicity under laminar flow conditions by confocal microscopy. The absolute liquid pressure in the flow channel is regulated to acquire the pinned Cassie-Baxter and depinned metastable states. The subsequent dynamic evolution of the meniscus morphology in the two states under shear flow is monitored. It is revealed that fluid flow does not affect the pressure-mediated equilibrium states but accelerates the air exchange between entrapped air cavities and bulk water. A diffusion-based model with varying effective diffusion lengths is used to interpret the experimental data, which show a good agreement. The Sherwood number representing the convection-enhanced mass transfer coefficient is extracted from the data, and is found to follow a classic 1/3-power-law relation with the Reynolds number as has been discovered in channel flows with diffusive boundary conditions. The current work paves the way for designing durable superhydrophobic surfaces under flow conditions.}, } @article {pmid27063642, year = {2016}, author = {Ishimoto, K}, title = {Hydrodynamic evolution of sperm swimming: Optimal flagella by a genetic algorithm.}, journal = {Journal of theoretical biology}, volume = {399}, number = {}, pages = {166-174}, doi = {10.1016/j.jtbi.2016.03.041}, pmid = {27063642}, issn = {1095-8541}, mesh = {*Algorithms ; Animals ; *Biological Evolution ; Flagella/*physiology ; *Hydrodynamics ; Male ; Models, Biological ; Sperm Motility/*physiology ; Spermatozoa/*physiology ; }, abstract = {Swimming performance of spermatozoa is an important index for the success of fertilization. For many years, numerous studies have reported the optimal swimming of flagellar organisms. Nevertheless, there is still a question as to which is optimal among planar, circular helical and ellipsoidal helical beating. In this paper, we use a genetic algorithm to investigate the beat pattern with the best swimming efficiency based on hydrodynamic dissipation and internal torque exertion. For the parameters considered, our results show that the planar beat is optimal for small heads and the helical flagellum is optimum for a larger heads, while the ellipsoidal beat is never optimal. Also, the genetic optimization reveals that the wavenumber and shape of wave envelope are relevant parameters, whereas the wave shape and head geometry have relatively minor effects on efficiency. The optimal beat with respect to the efficiency based on the internal torque exertion of an active elastic flagellum is characterized by a small-wavenumber and large-amplitude wave in a lower-viscosity medium. The obtained results on the optimal waveform are consistent with observations for planar waveforms, but in many respects, the results suggest the necessity of a detailed flagellar structure-fluid interaction to address whether real spermatozoa exhibit hydrodynamically efficient swimming. The evolutional optimization approach used in this study has distinguished biologically important parameters, and the methodology can potentially be applicable to various swimmers.}, } @article {pmid27055766, year = {2016}, author = {Iasiello, M and Vafai, K and Andreozzi, A and Bianco, N}, title = {Analysis of non-Newtonian effects on Low-Density Lipoprotein accumulation in an artery.}, journal = {Journal of biomechanics}, volume = {49}, number = {9}, pages = {1437-1446}, doi = {10.1016/j.jbiomech.2016.03.017}, pmid = {27055766}, issn = {1873-2380}, mesh = {Arteries/*metabolism/physiology ; Diffusion ; Hemodynamics ; Humans ; Lipoproteins, LDL/*metabolism ; *Models, Cardiovascular ; Porosity ; Rheology ; Stress, Mechanical ; }, abstract = {In this work, non-Newtonian effects on Low-Density Lipoprotein (LDL) transport across an artery are analyzed with a multi-layer model. Four rheological models (Carreau, Carreau-Yasuda, power-law and Newtonian) are used for the blood flow through the lumen. For the non-Newtonian cases, the arterial wall is modeled with a generalized momentum equation. Convection-diffusion equation is used for the LDL transport through the lumen, while Staverman-Kedem-Katchalsky, combined with porous media equations, are used for the LDL transport through the wall. Results are presented in terms of filtration velocity, Wall Shear Stresses (WSS) and concentration profiles. It is shown that non-Newtonian effects on mass transport are negligible for a healthy intramural pressure value. Non-Newtonian effects increase slightly with intramural pressure, but Newtonian assumption can still be considered reliable. Effects of arterial size are also analyzed, showing that Newtonian assumption can be considered valid for both medium and large arteries, in predicting LDL deposition. Finally, non-Newtonian effects are also analyzed for an aorta-common iliac bifurcation, showing that Newtonian assumption is valid for mass transport at low Reynolds numbers. At a high Reynolds number, it has been shown that a non-Newtonian fluid model can have more impact due to the presence of flow recirculation.}, } @article {pmid27042817, year = {2016}, author = {Beier, S and Ormiston, JA and Webster, MW and Cater, JE and Norris, SE and Medrano-Gracia, P and Young, AA and Cowan, BR}, title = {Dynamically scaled phantom phase contrast MRI compared to true-scale computational modeling of coronary artery flow.}, journal = {Journal of magnetic resonance imaging : JMRI}, volume = {44}, number = {4}, pages = {983-992}, doi = {10.1002/jmri.25240}, pmid = {27042817}, issn = {1522-2586}, mesh = {Blood Flow Velocity/*physiology ; Computer Simulation ; Coronary Circulation/*physiology ; Coronary Vessels/*diagnostic imaging/*physiology ; Equipment Design ; Equipment Failure Analysis ; Humans ; Image Interpretation, Computer-Assisted/methods ; Magnetic Resonance Angiography/*instrumentation/methods ; *Models, Cardiovascular ; *Phantoms, Imaging ; Reproducibility of Results ; Sensitivity and Specificity ; }, abstract = {PURPOSE: To examine the feasibility of combining computational fluid dynamics (CFD) and dynamically scaled phantom phase-contrast magnetic resonance imaging (PC-MRI) for coronary flow assessment.

MATERIALS AND METHODS: Left main coronary bifurcations segmented from computed tomography with bifurcation angles of 33°, 68°, and 117° were scaled-up ∼7× and 3D printed. Steady coronary flow was reproduced in these phantoms using the principle of dynamic similarity to preserve the true-scale Reynolds number, using blood analog fluid and a pump circuit in a 3T MRI scanner. After PC-MRI acquisition, the data were segmented and coregistered to CFD simulations of identical, but true-scale geometries. Velocities at the inlet region were extracted from the PC-MRI to define the CFD inlet boundary condition.

RESULTS: The PC-MRI and CFD flow data agreed well, and comparison showed: 1) small velocity magnitude discrepancies (2-8%); 2) with a Spearman's rank correlation ≥0.72; and 3) a velocity vector correlation (including direction) of r(2) ≥ 0.82. The highest agreement was achieved for high velocity regions with discrepancies being located in slow or recirculating zones with low MRI signal-to-noise ratio (SNRv) in tortuous segments and large bifurcating vessels.

CONCLUSION: Characterization of coronary flow using a dynamically scaled PC-MRI phantom flow is feasible and provides higher resolution than current in vivo or true-scale in vitro methods, and may be used to provide boundary conditions for true-scale CFD simulations. J. MAGN. RESON. IMAGING 2016;44:983-992.}, } @article {pmid27037586, year = {2016}, author = {Barsanti, L and Coltelli, P and Evangelista, V and Frassanito, AM and Gualtieri, P}, title = {Swimming patterns of the quadriflagellate Tetraflagellochloris mauritanica (Chlamydomonadales, Chlorophyceae).}, journal = {Journal of phycology}, volume = {52}, number = {2}, pages = {209-218}, doi = {10.1111/jpy.12384}, pmid = {27037586}, issn = {1529-8817}, mesh = {Cell Tracking ; Flagella/*physiology/ultrastructure ; Movement ; Time Factors ; Volvocida/anatomy & histology/*physiology/ultrastructure ; }, abstract = {Chlamydomonadales are elective subjects for the investigation of the problems related to locomotion and transport in biological fluid dynamics, whose resolution could enhance searching efficiency and assist in the avoidance of dangerous environments. In this paper, we elucidate the swimming behavior of Tetraflagellochloris mauritanica, a unicellular-multicellular alga belonging to the order Chlamydomonadales. This quadriflagellate alga has a complex swimming motion consisting of alternating swimming phases connected by in-place random reorientations and resting phases. It is capable of both forward and backward swimming, both being normal modes of swimming. The complex swimming behavior resembles the run-and-tumble motion of peritrichous bacteria, with in-place reorientation taking the place of tumbles. In the forward swimming, T. mauritanica shows a very efficient flagellar beat, with undulatory retrograde waves that run along the flagella to their tip. In the backward swimming, the flagella show a nonstereotypical synchronization mode, with a pattern that does not fit any of the modes present in the other Chlamydomonadales so far investigated.}, } @article {pmid27036817, year = {2016}, author = {Liu, C and Du, L and Zhao, Z}, title = {A directional cylindrical anemometer with four sets of differential pressure sensors.}, journal = {The Review of scientific instruments}, volume = {87}, number = {3}, pages = {035105}, doi = {10.1063/1.4943222}, pmid = {27036817}, issn = {1089-7623}, abstract = {This paper presents a solid-state directional anemometer for simultaneously measuring the speed and direction of a wind in a plane in a speed range 1-40 m/s. This instrument has a cylindrical shape and works by detecting the pressure differences across diameters of the cylinder when exposed to wind. By analyzing our experimental data in a Reynolds number regime 1.7 × 10(3)-7 × 10(4), we figure out the relationship between the pressure difference distribution and the wind velocity. We propose a novel and simple solution based on the relationship and design an anemometer which composes of a circular cylinder with four sets of differential pressure sensors, tubes connecting these sensors with the cylinder's surface, and corresponding circuits. In absence of moving parts, this instrument is small and immune of friction. It has simple internal structures, and the fragile sensing elements are well protected. Prototypes have been fabricated to estimate performance of proposed approach. The power consumption of the prototype is less than 0.5 W, and the sample rate is up to 31 Hz. The test results in a wind tunnel indicate that the maximum relative speed measuring error is 5% and the direction error is no more than 5° in a speed range 2-40 m/s. In theory, it is capable of measuring wind up to 60 m/s. When the air stream goes slower than 2 m/s, the measuring errors of directions are slightly greater, and the performance of speed measuring degrades but remains in an acceptable range of ±0.2 m/s.}, } @article {pmid27036184, year = {2016}, author = {Smith, SA and Warrier, S}, title = {Reynolds number effects on mixing due to topological chaos.}, journal = {Chaos (Woodbury, N.Y.)}, volume = {26}, number = {3}, pages = {033106}, doi = {10.1063/1.4943170}, pmid = {27036184}, issn = {1089-7682}, abstract = {Topological chaos has emerged as a powerful tool to investigate fluid mixing. While this theory can guarantee a lower bound on the stretching rate of certain material lines, it does not indicate what fraction of the fluid actually participates in this minimally mandated mixing. Indeed, the area in which effective mixing takes place depends on physical parameters such as the Reynolds number. To help clarify this dependency, we numerically simulate the effects of a batch stirring device on a 2D incompressible Newtonian fluid in the laminar regime. In particular, we calculate the finite time Lyapunov exponent (FTLE) field for three different stirring protocols, one topologically complex (pseudo-Anosov) and two simple (finite-order), over a range of viscosities. After extracting appropriate measures indicative of both the amount of mixing and the area of effective mixing from the FTLE field, we see a clearly defined Reynolds number range in which the relative efficacy of the pseudo-Anosov protocol over the finite-order protocols justifies the application of topological chaos. More unexpectedly, we see that while the measures of effective mixing area increase with increasing Reynolds number for the finite-order protocols, they actually exhibit non-monotonic behavior for the pseudo-Anosov protocol.}, } @article {pmid27033298, year = {2016}, author = {Winzen, A and Roidl, B and Schröder, W}, title = {Combined particle-image velocimetry and force analysis of the three-dimensional fluid-structure interaction of a natural owl wing.}, journal = {Bioinspiration & biomimetics}, volume = {11}, number = {2}, pages = {026005}, doi = {10.1088/1748-3190/11/2/026005}, pmid = {27033298}, issn = {1748-3190}, mesh = {Air ; Animals ; Biomimetics/*methods ; Computer Simulation ; Elastic Modulus/physiology ; Feathers/anatomy & histology/*physiology ; Flight, Animal/*physiology ; Friction ; *Models, Biological ; Rheology/methods ; Stress, Mechanical ; Strigiformes/anatomy & histology/*physiology ; Surface Properties ; Viscosity ; Wings, Animal/anatomy & histology/*physiology ; }, abstract = {Low-speed aerodynamics has gained increasing interest due to its relevance for the design process of small flying air vehicles. These small aircraft operate at similar aerodynamic conditions as, e.g. birds which therefore can serve as role models of how to overcome the well-known problems of low Reynolds number flight. The flight of the barn owl is characterized by a very low flight velocity in conjunction with a low noise emission and a high level of maneuverability at stable flight conditions. To investigate the complex three-dimensional flow field and the corresponding local structural deformation in combination with their influence on the resulting aerodynamic forces, time-resolved stereoscopic particle-image velocimetry and force and moment measurements are performed on a prepared natural barn owl wing. Several spanwise positions are measured via PIV in a range of angles of attack [Formula: see text] 6° and Reynolds numbers 40 000 [Formula: see text] 120 000 based on the chord length. Additionally, the resulting forces and moments are recorded for -10° ≤ α ≤ 15° at the same Reynolds numbers. Depending on the spanwise position, the angle of attack, and the Reynolds number, the flow field on the wing's pressure side is characterized by either a region of flow separation, causing large-scale vortical structures which lead to a time-dependent deflection of the flexible wing structure or wing regions showing no instantaneous deflection but a reduction of the time-averaged mean wing curvature. Based on the force measurements the three-dimensional fluid-structure interaction is assumed to considerably impact the aerodynamic forces acting on the wing leading to a strong mechanical loading of the interface between the wing and body. These time-depending loads which result from the flexibility of the wing should be taken into consideration for the design of future small flying air vehicles using flexible wing structures.}, } @article {pmid27030773, year = {2016}, author = {Chin, DD and Lentink, D}, title = {Flapping wing aerodynamics: from insects to vertebrates.}, journal = {The Journal of experimental biology}, volume = {219}, number = {Pt 7}, pages = {920-932}, doi = {10.1242/jeb.042317}, pmid = {27030773}, issn = {1477-9145}, mesh = {Animals ; Biomechanical Phenomena ; Birds/*physiology ; Chiroptera/*physiology ; Flight, Animal/*physiology ; Insecta/*physiology ; Models, Biological ; Wings, Animal/*physiology ; }, abstract = {More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanisms - added mass, clap and fling, rotational circulation and wing-wake interactions - have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters - the Reynolds number, Rossby number and advance ratio - govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understanding of the biomechanics, movement ecology and evolution of animal flight.}, } @article {pmid27009180, year = {2016}, author = {Li, G and Müller, UK and van Leeuwen, JL and Liu, H}, title = {Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves.}, journal = {Journal of the Royal Society, Interface}, volume = {13}, number = {116}, pages = {}, doi = {10.1098/rsif.2016.0068}, pmid = {27009180}, issn = {1742-5662}, mesh = {Animal Fins/*physiology ; Animals ; Fishes/*physiology ; Locomotion/*physiology ; }, abstract = {Larvae of bony fish swim in the intermediate Reynolds number (Re) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left-right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.}, } @article {pmid26986414, year = {2016}, author = {Köhler, J and Friedrich, J and Ostendorf, A and Gurevich, EL}, title = {Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number.}, journal = {Physical review. E}, volume = {93}, number = {2}, pages = {023108}, doi = {10.1103/PhysRevE.93.023108}, pmid = {26986414}, issn = {2470-0053}, abstract = {We theoretically and experimentally investigate the flow field that emerges from a rodlike microrotor rotating about its center in a nonaxisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo et al. [Phys. Rev. Lett. 96, 134502 (2006)]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices.}, } @article {pmid26986411, year = {2016}, author = {Lavrenteva, O and Prakash, J and Nir, A}, title = {Effect of added mass on the interaction of bubbles in a low-Reynolds-number shear flow.}, journal = {Physical review. E}, volume = {93}, number = {2}, pages = {023105}, doi = {10.1103/PhysRevE.93.023105}, pmid = {26986411}, issn = {2470-0053}, mesh = {Hydrodynamics ; *Models, Theoretical ; Nonlinear Dynamics ; *Shear Strength ; }, abstract = {Equal size air bubbles that are entrapped by a Taylor vortex of the secondary flow in a Couette device, thereby defying buoyancy, slowly form a stable ordered ring with equal separation distances between all neighbors. We present two models of the process dynamics based on force balance on a bubble in the presence of other bubbles positioned on the same streamline in a simple shear flow. The forces taken into account are the viscous resistance, the added mass force, and the inertia-induced repulsing force between two bubbles in a low-Reynolds-number shear flow obtained in Prakash et al. [J. Prakash et al., Phys. Rev. E 87, 043002 (2013)]. The first model of the process assumes that each bubble interacts solely with its nearest neighbors. The second model takes into account pairwise interactions among all the bubbles in the ring. The performed dynamic simulations were compared to the experimental results reported in Prakash et al. [J. Prakash et al., Phys. Rev. E 87, 043002 (2013)] and to the results of quasistationary models (ignoring the added mass effect) suggested in that paper. It is demonstrated that taking into account the effect of added mass, the models describe the major effect of the bubbles' ordering, provide good estimation of the relaxation time, and also predict nonmonotonic behavior of the separation distance between the bubbles, which exhibit over- and undershooting of equilibrium separations. The latter effects were observed in experiments, but are not predicted by the quasistationary models.}, } @article {pmid26951951, year = {2016}, author = {Flamini, V and DeAnda, A and Griffith, BE}, title = {Immersed boundary-finite element model of fluid-structure interaction in the aortic root.}, journal = {Theoretical and computational fluid dynamics}, volume = {30}, number = {1}, pages = {139-164}, doi = {10.1007/s00162-015-0374-5}, pmid = {26951951}, issn = {0935-4964}, support = {P50 GM071558/GM/NIGMS NIH HHS/United States ; R01 HL117063/HL/NHLBI NIH HHS/United States ; }, abstract = {It has long been recognized that aortic root elasticity helps to ensure efficient aortic valve closure, but our understanding of the functional importance of the elasticity and geometry of the aortic root continues to e-volve as increasingly detailed in vivo imaging data become available. Herein, we describe a fluid-structure interaction model of the aortic root, including the aortic valve leaflets, the sinsuses of Valsalva, the aortic annulus, and the sinotubular junction, that employs a version of Peskin's immersed boundary (IB) method with a finite element (FE) description of the structural elasticity. As in earlier work, we use a fiber-based model of the valve leaflets, but this study extends earlier IB models of the aortic root by employing an incompressible hyperelastic model of the mechanics of the sinuses and ascending aorta using a constitutive law fit to experimental data from human aortic root tissue. In vivo pressure loading is accounted for by a backward displacement method that determines the unloaded configurations of the root model. Our model yields realistic cardiac output at physiological pressures, with low transvalvular pressure differences during forward flow, minimal regurgitation during valve closure, and realistic pressure loads when the valve is closed during diastole. Further, results from high-resolution computations indicate that although the detailed leaflet and root kinematics show some grid sensitivity, our IB model of the aortic root nonetheless produces essentially grid-converged flow rates and pressures at practical grid spacings for the high-Reynolds number flows of the aortic root. These results thereby clarify minimum grid resolutions required by such models when used as stand-alone models of the aortic valve as well as when used to provide models of the outflow valves in models of left ventricular fluid dynamics.}, } @article {pmid26943538, year = {2016}, author = {Sadek, M and Alexakis, A and Fauve, S}, title = {Optimal Length Scale for a Turbulent Dynamo.}, journal = {Physical review letters}, volume = {116}, number = {7}, pages = {074501}, doi = {10.1103/PhysRevLett.116.074501}, pmid = {26943538}, issn = {1079-7114}, abstract = {We demonstrate that there is an optimal forcing length scale for low Prandtl number dynamo flows that can significantly reduce the required energy injection rate. The investigation is based on simulations of the induction equation in a periodic box of size 2πL. The flows considered are the laminar and turbulent ABC flows forced at different forcing wave numbers k_{f}, where the turbulent case is simulated using a subgrid turbulence model. At the smallest allowed forcing wave number k_{f} =k_{min} =1/L the laminar critical magnetic Reynolds number Rm_{c} ^{lam} is more than an order of magnitude smaller than the turbulent critical magnetic Reynolds number Rm_{c} ^{turb} due to the hindering effect of turbulent fluctuations. We show that this hindering effect is almost suppressed when the forcing wave number k_{f} is increased above an optimum wave number k_{f} L≃4 for which Rm_{c} ^{turb} is minimum. At this optimal wave number, Rm_{c} ^{turb} is smaller by more than a factor of 10 than the case forced in k_{f} =1. This leads to a reduction of the energy injection rate by 3 orders of magnitude when compared to the case where the system is forced at the largest scales and thus provides a new strategy for the design of a fully turbulent experimental dynamo.}, } @article {pmid26925504, year = {2016}, author = {Hervella, P and Parra, E and Needham, D}, title = {Encapsulation and retention of chelated-copper inside hydrophobic nanoparticles: Liquid cored nanoparticles show better retention than a solid core formulation.}, journal = {European journal of pharmaceutics and biopharmaceutics : official journal of Arbeitsgemeinschaft fur Pharmazeutische Verfahrenstechnik e.V}, volume = {102}, number = {}, pages = {64-76}, doi = {10.1016/j.ejpb.2016.02.015}, pmid = {26925504}, issn = {1873-3441}, mesh = {Chelating Agents/*chemistry ; Chemistry, Pharmaceutical/methods ; Copper/*chemistry ; Hydrophobic and Hydrophilic Interactions ; Lipids/chemistry ; Liposomes/chemistry ; Nanoparticles/*chemistry ; Particle Size ; Porphyrins/chemistry ; }, abstract = {MOTIVATION: In the field of imaging, (18)F-fluorodeoxyglucose (FDG) PET imaging allows evaluation of glucose metabolism and is the most widely used imaging agent clinically for metastatic cancer. While it can certainly detect the metastatic disease, in order to provide a more fully "individualized medicine" strategy of detection and pharmaceutical treatment, what is needed are additional imaging nanoparticles that resemble the subsequently-administered nanoparticle drug delivery system itself. Both of these nanoparticles must also be able to take advantage of what may well be a limited EPR effect in human tumors, which in and of itself still needs to be characterized in the clinic. Administration of FDG, followed by a nanoparticle imaging agent, followed by a therapeutic nanoparticle would constitute such an "individualized medicine strategy", especially for anti-metastasis approaches. It is here that our endogenous-inspired nanoparticle strategies for imaging and therapeutics are focused on encapsulating and retaining imaging ions such as copper inside novel hydrophobic nanoparticles. In this paper, we describe a new approach to label the core of hydrophobic nanoparticles composed of Glyceryl Trioleate (Triolein) with copper using the hydrophobic chelator Octaethyl porphyrin (OEP).

RESEARCH PLAN AND METHODS: The research plan for this study was to (1) Formulate nanoparticles and control nanoparticle size using a modification of the solvent injection technique, named fast ethanol injection; (2) Chelate copper into the octaethyl porphyrin; (3) Encapsulate OEP-Cu in nanoparticles: the encapsulation efficiency of copper into liquid nanoparticles (LNP), solid nanoparticles (SNP) and phospholipid liposomes (PL) was evaluated by UV-Vis and atomic absorption spectroscopy; (4) Retain the encapsulated OEP-Cu in the liquid or solid cores of the nanoparticles in the presence of a lipid sink.

RESULTS: (1) The size of the nanoparticles was found to be strongly dependent on the Reynolds number and the initial concentration of components for the fast injection technique. At high Reynolds number (2181), a minimum value for the particle diameter of ∼30nm was measured. (2) Copper was chelated by OEP in a 1:1mol ratio with an association constant of 2.57×10(5)M(-1). (3) The diameter of the nanoparticles was not significantly affected by the presence of OEP or OEP-Cu. The percentage of encapsulation of copper to nanoparticles was >95% at low OEP-Cu concentrations. In the absence of OEP, copper was not detected in nanoparticles demonstrating the role of the hydrophobic chelator OEP in the encapsulation of the otherwise water-soluble copper inside lipid nanoparticles. (4) The in vitro retention upon incubation at 37°C over a 48h period in the presence of a lipid sink showed a slow transfer of OEP-Cu into the lipid sink (t1/2=7.7h) for SNP; for PL there was an almost instantaneous transfer of OEP-Cu into the lipid sink (t1/2=0.5h), while for the LNP, all OEP-Cu was retained in the LNP over the full 48h period.

CONCLUSIONS: The main conclusion of this study was that a very hydrophilic ion such as Cu(2+) can indeed be solubilized and retained in the core of hydrophobic nanoparticles when a hydrophobic molecule (OEP) is used as a chelator. The fast-injection technique was shown to provide a very convenient method to formulate both liquid and solid nanoparticles labeled with Cu (well chelated by OEP), with diameters as small as 30nm, and encapsulation efficiencies higher than 95% when the concentration of OEP-Cu loaded into the nanoparticles was equal to or below 2.5mol%. This is expected to be sufficient for PET-imaging studies.}, } @article {pmid26889002, year = {2016}, author = {Murphy, DW and Adhikari, D and Webster, DR and Yen, J}, title = {Underwater flight by the planktonic sea butterfly.}, journal = {The Journal of experimental biology}, volume = {219}, number = {Pt 4}, pages = {535-543}, doi = {10.1242/jeb.129205}, pmid = {26889002}, issn = {1477-9145}, mesh = {Animals ; Biomechanical Phenomena ; Gastropoda/anatomy & histology/*physiology ; Hydrodynamics ; Swimming ; Wings, Animal/anatomy & histology/physiology ; Zooplankton ; }, abstract = {In a remarkable example of convergent evolution, we show that the zooplanktonic sea butterfly Limacina helicina 'flies' underwater in the same way that very small insects fly in the air. Both sea butterflies and flying insects stroke their wings in a characteristic figure-of-eight pattern to produce lift, and both generate extra lift by peeling their wings apart at the beginning of the power stroke (the well-known Weis-Fogh 'clap-and-fling' mechanism). It is highly surprising to find a zooplankter 'mimicking' insect flight as almost all zooplankton swim in this intermediate Reynolds number range (Re=10-100) by using their appendages as paddles rather than wings. The sea butterfly is also unique in that it accomplishes its insect-like figure-of-eight wing stroke by extreme rotation of its body (what we call 'hyper-pitching'), a paradigm that has implications for micro aerial vehicle (MAV) design. No other animal, to our knowledge, pitches to this extent under normal locomotion.}, } @article {pmid26887934, year = {2016}, author = {Wang, G and Yang, F and Zhao, W and Chen, CP}, title = {On micro-electrokinetic scalar turbulence in microfluidics at a low Reynolds number.}, journal = {Lab on a chip}, volume = {16}, number = {6}, pages = {1030-1038}, doi = {10.1039/c5lc01541c}, pmid = {26887934}, issn = {1473-0189}, abstract = {We recently demonstrated the direct observation of micro-electrokinetic turbulence in a microchannel at a low Reynolds number (Re) when a pressure-driven flow was forced electrokinetically. Here, we characterize the corresponding scalar turbulence and surprisingly find that the corresponding turbulent mixing has some typical and important features of scalar turbulence, such as the Obukhov-Corrsin (O-C) -5/3 spectrum of concentration fluctuation, which can commonly be realized only at high Re in macroflows. This discovery could provide a new perspective of scalar turbulence and an avenue for control of transport phenomena in lab-on-a-chip platforms. This will deepen our fundamental understanding of transport phenomena in microfluidics.}, } @article {pmid26886919, year = {2016}, author = {Hayat, T and Shafique, M and Tanveer, A and Alsaedi, A}, title = {Radiative Peristaltic Flow of Jeffrey Nanofluid with Slip Conditions and Joule Heating.}, journal = {PloS one}, volume = {11}, number = {2}, pages = {e0148002}, doi = {10.1371/journal.pone.0148002}, pmid = {26886919}, issn = {1932-6203}, mesh = {*Heating ; Models, Theoretical ; Nanoparticles/*chemistry ; *Peristalsis ; *Rheology ; }, abstract = {Mixed convection peristaltic flow of Jeffrey nanofluid in a channel with compliant walls is addressed here. The present investigation includes the viscous dissipation, thermal radiation and Joule heating. Whole analysis is performed for velocity, thermal and concentration slip conditions. Related problems through long wavelength and low Reynolds number are examined for stream function, temperature and concentration. Impacts of thermal radiation, Hartman number, Brownian motion parameter, thermophoresis, Joule heating and slip parameters are explored in detail. Clearly temperature is a decreasing function of Hartman number and radiation parameter.}, } @article {pmid26883291, year = {2016}, author = {Pinto, SI and Campos, JB}, title = {Numerical study of wall shear stress-based descriptors in the human left coronary artery.}, journal = {Computer methods in biomechanics and biomedical engineering}, volume = {19}, number = {13}, pages = {1443-1455}, doi = {10.1080/10255842.2016.1149575}, pmid = {26883291}, issn = {1476-8259}, mesh = {Computer Simulation ; Coronary Vessels/*physiopathology ; Hemorheology ; Humans ; Models, Cardiovascular ; *Numerical Analysis, Computer-Assisted ; Shear Strength ; *Stress, Mechanical ; Time Factors ; }, abstract = {The present work is about the application of wall shear stress descriptors - time averaged wall shear stress (TAWSS), oscillating shear index (OSI) and relative residence time (RRT) - to the study of blood flow in the left coronary artery (LCA). These descriptors aid the prediction of disturbed flow conditions in the vessels and play a significant role in the detection of potential zones of atherosclerosis development. Hemodynamic descriptors data were obtained, numerically, through ANSYS® software, for the LCA of a patient-specific geometry and for a 3D idealized model. Comparing both cases, the results are coherent, in terms of location and magnitude. Low TAWSS, high OSI and high RRT values are observed in the bifurcation - potential zone of atherosclerosis appearance. The dissimilarities observed in the TAWSS values, considering blood as a Newtonian or non-Newtonian fluid, releases the importance of the correct blood rheologic caracterization. Moreover, for a higher Reynolds number, the TAWSS values decrease in the bifurcation and along the LAD branch, increasing the probability of plaques deposition. Furthermore, for a stenotic LCA model, very low TAWSS and high RRT values in front and behind the stenosis are observed, indicating the probable extension, in the flow direction, of the lesion.}, } @article {pmid26871181, year = {2016}, author = {Tsai, CM and Chu, HY}, title = {Evolution of a plasma vortex in air.}, journal = {Physical review. E}, volume = {93}, number = {1}, pages = {013205}, doi = {10.1103/PhysRevE.93.013205}, pmid = {26871181}, issn = {2470-0053}, mesh = {*Air ; Equipment Design ; *Helium ; Models, Theoretical ; *Motion ; *Plasma Gases ; }, abstract = {We report the generation of a vortex-shaped plasma in air by using a capacitively coupled dielectric barrier discharge system. We show that a vortex-shaped plasma can be produced inside a helium gas vortex and is capable of propagating for 3 cm. The fluctuation of the plasma ring shows a scaling relation with the Reynolds number of the vortex. The transient discharge reveals the property of corona discharge, where the conducting channel within the gas vortex and the blur plasma emission are observed at each half voltage cycle.}, } @article {pmid26871163, year = {2016}, author = {Thiesset, F and Maurice, G and Halter, F and Mazellier, N and Chauveau, C and Gökalp, I}, title = {Geometrical properties of turbulent premixed flames and other corrugated interfaces.}, journal = {Physical review. E}, volume = {93}, number = {1}, pages = {013116}, doi = {10.1103/PhysRevE.93.013116}, pmid = {26871163}, issn = {2470-0053}, abstract = {This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar evolving in a turbulent medium. The latter outcome is likely to have strong implications for modeling the corrugation of turbulent interfaces occurring in many physical situations.}, } @article {pmid26871162, year = {2016}, author = {Nie, D and Lin, J and Chen, R}, title = {Grouping behavior of coaxial settling particles in a narrow channel.}, journal = {Physical review. E}, volume = {93}, number = {1}, pages = {013114}, doi = {10.1103/PhysRevE.93.013114}, pmid = {26871162}, issn = {2470-0053}, abstract = {Using numerical simulations, we studied the grouping behaviors of particles settling along their line of centers in narrow channels having a Reynolds number range of 5 ≤ Re ≤ 50. The calculations are based on our previously developed lattice Boltzmann direct-forcing-fictitious-domain method. We report the grouping behavior and investigate the dependence on the number of particles n, the initial interparticle separation h_{0}, and the Reynolds number Re. In particular, the mode of grouping is found to be independent of the number of particles when the Reynolds numbers is small. The two lowermost particles always come together first and form a vertical doublet and then the next two lowest particles form another doublet, and so on. Therefore, we observe n/2 doublets or (n-1)/2 doublets when n is even or odd, respectively. The uppermost particle is always left behind when n is odd. Furthermore, the separation between these doublets remains constant, displaying a power-law dependence decreasing from top to bottom.}, } @article {pmid26871154, year = {2016}, author = {Wang, G and Yang, F and Zhao, W}, title = {Microelectrokinetic turbulence in microfluidics at low Reynolds number.}, journal = {Physical review. E}, volume = {93}, number = {1}, pages = {013106}, doi = {10.1103/PhysRevE.93.013106}, pmid = {26871154}, issn = {2470-0053}, mesh = {Fluorescein ; Fluorescent Dyes ; Kinetics ; Lab-On-A-Chip Devices ; *Microfluidics ; Models, Theoretical ; *Motion ; Pressure ; Solutions ; Static Electricity ; Water ; }, abstract = {There is commonly no turbulence in microfluidics, and the flows are believed to be either laminar or chaotic, since Reynolds number (Re) in microflows is usually on the order of unity or lower. However, we recently demonstrated that it is possible to achieve turbulence with low Re (based on the measured flow velocity and the width of the channel entrance) when a pressure-driven flow is electrokinetically forced in a quasi T-microchannel. To be able to measure high frequency velocity fluctuations in microchannels, a velocimeter with submicrometer spatial resolution and microsecond temporal resolution, called a laser-induced fluorescence photobleaching anemometer, is developed. Here we characterize the microelectrokinetic turbulence and observe some typical and important features of high Re flows, such as Kolmogorov -5/3 spectrum of velocity fluctuation, which usually can be realized only at very high Re in macroturbulent flows.}, } @article {pmid26853995, year = {2016}, author = {Kim, J and Lee, J and Wu, C and Nam, S and Di Carlo, D and Lee, W}, title = {Inertial focusing in non-rectangular cross-section microchannels and manipulation of accessible focusing positions.}, journal = {Lab on a chip}, volume = {16}, number = {6}, pages = {992-1001}, doi = {10.1039/c5lc01100k}, pmid = {26853995}, issn = {1473-0189}, abstract = {Inertial focusing in microfluidic channels has been extensively studied experimentally and theoretically, which has led to various applications including microfluidic separation and enrichment of cells. Inertial lift forces are strongly dependent on the flow velocity profile and the channel cross-sectional shape. However, the channel cross-sections studied have been limited to circles and rectangles. We studied inertial focusing in non-rectangular cross-section channels to manipulate the flow profile and thus the inertial focusing of microparticles. The location and number of focusing positions are analyzed with varying cross-sectional shapes and Reynolds number. We found that the broken symmetry of non-equilateral triangular channels leads to the shifting of focusing positions with varying Reynolds number. Non-rectangular channels have unique mapping of the focusing positions and the corresponding basins of attraction. By connecting channels with different cross-sectional shapes, we were able to manipulate the accessible focusing positions and achieve focusing of microparticles to a single stream with ∼99% purity.}, } @article {pmid26841796, year = {2016}, author = {Mathijssen, AJ and Doostmohammadi, A and Yeomans, JM and Shendruk, TN}, title = {Hotspots of boundary accumulation: dynamics and statistics of micro-swimmers in flowing films.}, journal = {Journal of the Royal Society, Interface}, volume = {13}, number = {115}, pages = {20150936}, doi = {10.1098/rsif.2015.0936}, pmid = {26841796}, issn = {1742-5662}, mesh = {*Bacteria ; *Bacterial Physiological Phenomena ; Flagella/*physiology ; Locomotion/*physiology ; *Models, Biological ; }, abstract = {Biological flows over surfaces and interfaces can result in accumulation hotspots or depleted voids of microorganisms in natural environments. Apprehending the mechanisms that lead to such distributions is essential for understanding biofilm initiation. Using a systematic framework, we resolve the dynamics and statistics of swimming microbes within flowing films, considering the impact of confinement through steric and hydrodynamic interactions, flow and motility, along with Brownian and run-tumble fluctuations. Micro-swimmers can be peeled off the solid wall above a critical flow strength. However, the interplay of flow and fluctuations causes organisms to migrate back towards the wall above a secondary critical value. Hence, faster flows may not always be the most efficacious strategy to discourage biofilm initiation. Moreover, we find run-tumble dynamics commonly used by flagellated microbes to be an intrinsically more successful strategy to escape from boundaries than equivalent levels of enhanced Brownian noise in ciliated organisms.}, } @article {pmid26830757, year = {2016}, author = {Rosenberg, D and Marino, R and Herbert, C and Pouquet, A}, title = {Variations of characteristic time scales in rotating stratified turbulence using a large parametric numerical study.}, journal = {The European physical journal. E, Soft matter}, volume = {39}, number = {1}, pages = {8}, doi = {10.1140/epje/i2016-16008-7}, pmid = {26830757}, issn = {1292-895X}, abstract = {We study rotating stratified turbulence (RST) making use of numerical data stemming from a large parametric study varying the Reynolds, Froude and Rossby numbers, Re, Fr and Ro in a broad range of values. The computations are performed using periodic boundary conditions on grids of 1024(3) points, with no modeling of the small scales, no forcing and with large-scale random initial conditions for the velocity field only, and there are altogether 65 runs analyzed in this paper. The buoyancy Reynolds number defined as R(B) = ReFr2 varies from negligible values to ≈ 10(5), approaching atmospheric or oceanic regimes. This preliminary analysis deals with the variation of characteristic time scales of RST with dimensionless parameters, focusing on the role played by the partition of energy between the kinetic and potential modes, as a key ingredient for modeling the dynamics of such flows. We find that neither rotation nor the ratio of the Brunt-Väisälä frequency to the inertial frequency seem to play a major role in the absence of forcing in the global dynamics of the small-scale kinetic and potential modes. Specifically, in these computations, mostly in regimes of wave turbulence, characteristic times based on the ratio of energy to dissipation of the velocity and temperature fluctuations, T(V) and T(P), vary substantially with parameters. Their ratio γ=T(V)/T(P) follows roughly a bell-shaped curve in terms of Richardson number Ri. It reaches a plateau - on which time scales become comparable, γ≈0.6 - when the turbulence has significantly strengthened, leading to numerous destabilization events together with a tendency towards an isotropization of the flow.}, } @article {pmid26824542, year = {2016}, author = {Shishkina, O and Wagner, S}, title = {Prandtl-Number Dependence of Heat Transport in Laminar Horizontal Convection.}, journal = {Physical review letters}, volume = {116}, number = {2}, pages = {024302}, doi = {10.1103/PhysRevLett.116.024302}, pmid = {26824542}, issn = {1079-7114}, abstract = {We report the Prandtl-number (Pr) and Rayleigh-number (Ra) dependencies of the Reynolds number (Re) and mean convective heat transport, measured by the Nusselt number (Nu), in horizontal convection (HC) systems, where the heat supply and removal are provided exclusively through a lower horizontal surface of a fluid layer. For laminar HC, we find that Re∼Ra^{2/5} Pr^{-4/5}, Nu∼Ra^{1/5} Pr^{1/10} with a transition to Re∼Ra^{1/2} Pr^{-1}, Nu∼Ra^{1/4} Pr^{0} for large Pr. The results are based on direct numerical simulations for Ra from 3×10^{8} to 5×10^{10} and Pr from 0.05 to 50 and are explained by applying the Grossmann-Lohse approach [J. Fluid Mech. 407, 27 (2000)] transferred from the case of Rayleigh-Bénard convection to the case of laminar HC.}, } @article {pmid26821214, year = {2016}, author = {Maier, AM and Weig, C and Oswald, P and Frey, E and Fischer, P and Liedl, T}, title = {Magnetic Propulsion of Microswimmers with DNA-Based Flagellar Bundles.}, journal = {Nano letters}, volume = {16}, number = {2}, pages = {906-910}, doi = {10.1021/acs.nanolett.5b03716}, pmid = {26821214}, issn = {1530-6992}, mesh = {Biocompatible Materials/*chemistry ; DNA/*chemistry ; Magnetic Fields ; Magnetite Nanoparticles/*chemistry ; Robotics/instrumentation ; }, abstract = {We show that DNA-based self-assembly can serve as a general and flexible tool to construct artificial flagella of several micrometers in length and only tens of nanometers in diameter. By attaching the DNA flagella to biocompatible magnetic microparticles, we provide a proof of concept demonstration of hybrid structures that, when rotated in an external magnetic field, propel by means of a flagellar bundle, similar to self-propelling peritrichous bacteria. Our theoretical analysis predicts that flagellar bundles that possess a length-dependent bending stiffness should exhibit a superior swimming speed compared to swimmers with a single appendage. The DNA self-assembly method permits the realization of these improved flagellar bundles in good agreement with our quantitative model. DNA flagella with well-controlled shape could fundamentally increase the functionality of fully biocompatible nanorobots and extend the scope and complexity of active materials.}, } @article {pmid26802269, year = {2016}, author = {Sultan, T and Ahmad, S and Cho, J}, title = {Numerical study of the effects of surface roughness on water disinfection UV reactor.}, journal = {Chemosphere}, volume = {148}, number = {}, pages = {108-117}, doi = {10.1016/j.chemosphere.2016.01.005}, pmid = {26802269}, issn = {1879-1298}, mesh = {*Bioreactors ; *Construction Materials ; Disinfection/*methods ; Hydrodynamics ; *Models, Theoretical ; Surface Properties ; *Ultraviolet Rays ; Water Pollutants/isolation & purification ; Water Purification/*methods ; }, abstract = {UV reactors are an emerging choice as a big barrier against the pathogens present in drinking water. However, the precise role of reactor's wall roughness for cross flow ultraviolet (CF-UV) and axial flow ultraviolet (AF-UV) water disinfection reactors are unknown. In this paper, the influences of reactor's wall roughness were investigated with a view to identify their role on the performance factors namely dose distribution and reduction equivalent dose (RED). Herein, the relative effects of reactor's wall roughness on the performance of CF-UV and AF-UV reactors were also highlighted. This numerical study is a first step towards the comprehensive analysis of the effects of reactor's wall roughness for UV reactor. A numerical analysis was performed using ANSYS Fluent 15 academic version. The reactor's wall roughness has a significant effect on the RED. We found that the increase in RED is Reynolds number dependent (at lower value of turbulent Reynolds number the effects are remarkable). The effects of reactor's roughness were more pronounced for AF-UV reactor. The simulation results suggest that the study of reactor's wall roughness provides valuable insight to fully understand the effects of reactor's wall roughness and its impact on the flow behavior and other features of CF-UV and AF-UV water disinfection reactors.}, } @article {pmid26780177, year = {2016}, author = {Agbesi, MP and Naylor, S and Perkins, E and Borsuk, HS and Sykes, D and Maclaine, JS and Wang, Z and Cox, JP}, title = {Complex flow in the nasal region of guitarfishes.}, journal = {Comparative biochemistry and physiology. Part A, Molecular & integrative physiology}, volume = {193}, number = {}, pages = {52-63}, doi = {10.1016/j.cbpa.2015.12.007}, pmid = {26780177}, issn = {1531-4332}, mesh = {Animals ; Fishes/metabolism/*physiology ; Nasal Cavity/metabolism/*physiology ; Respiration ; Smell/*physiology ; Swimming/physiology ; Water/metabolism ; }, abstract = {Scent detection in an aquatic environment is dependent on the movement of water. We set out to determine the mechanisms for moving water through the olfactory organ of guitarfishes (Rhinobatidae, Chondrichthyes) with open nasal cavities. We found at least two. In the first mechanism, which we identified by observing dye movement in the nasal region of a life-sized physical model of the head of Rhinobatos lentiginosus mounted in a flume, olfactory flow is generated by the guitarfish's motion relative to water, e.g. when it swims. We suggest that the pressure difference responsible for motion-driven olfactory flow is caused by the guitarfish's nasal flaps, which create a region of high pressure at the incurrent nostril, and a region of low pressure in and behind the nasal cavity. Vortical structures in the nasal region associated with motion-driven flow may encourage passage of water through the nasal cavity and its sensory channels, and may also reduce the cost of swimming. The arrangement of vortical structures is reminiscent of aircraft wing vortices. In the second mechanism, which we identified by observing dye movement in the nasal regions of living specimens of Glaucostegus typus, the guitarfish's respiratory pump draws flow through the olfactory organ in a rhythmic (0.5-2 Hz), but continuous, fashion. Consequently, the respiratory pump will maintain olfactory flow whether the guitarfish is swimming or at rest. Based on our results, we propose a model for olfactory flow in guitarfishes with open nasal cavities, and suggest other neoselachians which this model might apply to.}, } @article {pmid26775865, year = {2015}, author = {Wang, Q and Othmer, HG}, title = {The performance of discrete models of low Reynolds number swimmers.}, journal = {Mathematical biosciences and engineering : MBE}, volume = {12}, number = {6}, pages = {1303-1320}, pmid = {26775865}, issn = {1551-0018}, support = {GM29123-36/GM/NIGMS NIH HHS/United States ; }, mesh = {Animals ; Cell Movement/physiology ; Cell Shape/physiology ; Computer Simulation ; Dictyostelium/cytology/physiology ; Humans ; Mathematical Concepts ; Melanoma/pathology/physiopathology ; *Models, Biological ; Movement/*physiology ; }, abstract = {Swimming by shape changes at low Reynolds number is widely used in biology and understanding how the performance of movement depends on the geometric pattern of shape changes is important to understand swimming of microorganisms and in designing low Reynolds number swimming models. The simplest models of shape changes are those that comprise a series of linked spheres that can change their separation and/or their size. Herein we compare the performance of three models in which these modes are used in different ways.}, } @article {pmid26764831, year = {2015}, author = {Ebrahimian, M and Yekehzare, M and Ejtehadi, MR}, title = {Low-Reynolds-number predator.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063035}, doi = {10.1103/PhysRevE.92.063035}, pmid = {26764831}, issn = {1550-2376}, abstract = {To generalize simple bead-linker model of swimmers to higher dimensions and to demonstrate the chemotaxis ability of such swimmers, here we introduce a low-Reynolds predator, using a two-dimensional triangular bead-spring model. Two-state linkers as mechanochemical enzymes expand as a result of interaction with particular activator substances in the environment, causing the whole body to translate and rotate. The concentration of the chemical stimulator controls expansion versus the contraction rate of each arm and so affects the ability of the body for diffusive movements; also the variation of activator substance's concentration in the environment breaks the symmetry of linkers' preferred state, resulting in the drift of the random walker along the gradient of the density of activators. External food or danger sources may attract or repel the body by producing or consuming the chemical activators of the organism's enzymes, inducing chemotaxis behavior. Generalization of the model to three dimensions is straightforward.}, } @article {pmid26764821, year = {2015}, author = {Iyer, KP and Sreenivasan, KR and Yeung, PK}, title = {Refined similarity hypothesis using three-dimensional local averages.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063024}, doi = {10.1103/PhysRevE.92.063024}, pmid = {26764821}, issn = {1550-2376}, abstract = {The refined similarity hypotheses of Kolmogorov, regarded as an important ingredient of intermittent turbulence, has been tested in the past using one-dimensional data and plausible surrogates of energy dissipation. We employ data from direct numerical simulations, at the microscale Reynolds number R(λ)∼650, on a periodic box of 4096(3) grid points to test the hypotheses using three-dimensional averages. In particular, we study the small-scale properties of the stochastic variable V=Δu(r)/(rε(r))(1/3), where Δu(r) is the longitudinal velocity increment and ε(r) is the dissipation rate averaged over a three-dimensional volume of linear size r. We show that V is universal in the inertial subrange. In the dissipation range, the statistics of V are shown to depend solely on a local Reynolds number.}, } @article {pmid26764819, year = {2015}, author = {Rosén, T and Einarsson, J and Nordmark, A and Aidun, CK and Lundell, F and Mehlig, B}, title = {Numerical analysis of the angular motion of a neutrally buoyant spheroid in shear flow at small Reynolds numbers.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063022}, doi = {10.1103/PhysRevE.92.063022}, pmid = {26764819}, issn = {1550-2376}, abstract = {We numerically analyze the rotation of a neutrally buoyant spheroid in a shear flow at small shear Reynolds number. Using direct numerical stability analysis of the coupled nonlinear particle-flow problem, we compute the linear stability of the log-rolling orbit at small shear Reynolds number Re(a). As Re(a)→0 and as the box size of the system tends to infinity, we find good agreement between the numerical results and earlier analytical predictions valid to linear order in Re(a) for the case of an unbounded shear. The numerical stability analysis indicates that there are substantial finite-size corrections to the analytical results obtained for the unbounded system. We also compare the analytical results to results of lattice Boltzmann simulations to analyze the stability of the tumbling orbit at shear Reynolds numbers of order unity. Theory for an unbounded system at infinitesimal shear Reynolds number predicts a bifurcation of the tumbling orbit at aspect ratio λ(c)≈0.137 below which tumbling is stable (as well as log rolling). The simulation results show a bifurcation line in the λ-Re(a) plane that reaches λ≈0.1275 at the smallest shear Reynolds number (Re(a)=1) at which we could simulate with the lattice Boltzmann code, in qualitative agreement with the analytical results.}, } @article {pmid26764810, year = {2015}, author = {Deng, J and Sun, L and Shao, X}, title = {Dynamical features of the wake behind a pitching foil.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063013}, doi = {10.1103/PhysRevE.92.063013}, pmid = {26764810}, issn = {1550-2376}, abstract = {As an extension of the previous study on the three-dimensional transition of the wake behind a pitching foil [Deng and Caulfield, Phys. Rev. E 91, 043017 (2015)], this investigation draws a comprehensive map on the pitching frequency-amplitude phase space. First, by fixing the Reynolds number at Re=1700 and varying the pitching frequency and amplitude, we identify three key dynamical features of the wake: first, the transition from Bénard-von Kármán (BvK) vortex streets to reverse BvK vortex streets, and second, the symmetry breaking of this reverse BvK wake leading to a deflected wake, and a further transition from two-dimensional (2D) wakes to three-dimensional (3D) wakes. The transition boundary between the 2D and 3D wakes lies top right of the wake deflection boundary, implying a correlation between the wake deflection and the 2D to 3D wake transition, confirming that this transition occurs after the wake deflection. This paper supports the previous extensive numerical studies under two-dimensional assumption at low Reynolds number, since it is indeed two dimensional except for the cases at very high pitching frequencies or large amplitudes. Furthermore, by three-dimensional direct numerical simulations (DNSs), we confirm the previous statement about the physical realizability of the short wavelength mode at β=30 (or λ(z)=0.21) for Re=1500. By comparing the three-dimensional vortical structures by DNSs with that from the reconstruction of Floquet modes, we find a good consistency between them, both exhibiting clear streamwise structures in the wake.}, } @article {pmid26764807, year = {2015}, author = {Allouche, MH and Millet, S and Botton, V and Henry, D and Ben Hadid, H and Rousset, F}, title = {Stability of a flow down an incline with respect to two-dimensional and three-dimensional disturbances for Newtonian and non-Newtonian fluids.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063010}, doi = {10.1103/PhysRevE.92.063010}, pmid = {26764807}, issn = {1550-2376}, abstract = {Squire's theorem, which states that the two-dimensional instabilities are more dangerous than the three-dimensional instabilities, is revisited here for a flow down an incline, making use of numerical stability analysis and Squire relationships when available. For flows down inclined planes, one of these Squire relationships involves the slopes of the inclines. This means that the Reynolds number associated with a two-dimensional wave can be shown to be smaller than that for an oblique wave, but this oblique wave being obtained for a larger slope. Physically speaking, this prevents the possibility to directly compare the thresholds at a given slope. The goal of the paper is then to reach a conclusion about the predominance or not of two-dimensional instabilities at a given slope, which is of practical interest for industrial or environmental applications. For a Newtonian fluid, it is shown that, for a given slope, oblique wave instabilities are never the dominant instabilities. Both the Squire relationships and the particular variations of the two-dimensional wave critical curve with regard to the inclination angle are involved in the proof of this result. For a generalized Newtonian fluid, a similar result can only be obtained for a reduced stability problem where some term connected to the perturbation of viscosity is neglected. For the general stability problem, however, no Squire relationships can be derived and the numerical stability results show that the thresholds for oblique waves can be smaller than the thresholds for two-dimensional waves at a given slope, particularly for large obliquity angles and strong shear-thinning behaviors. The conclusion is then completely different in that case: the dominant instability for a generalized Newtonian fluid flowing down an inclined plane with a given slope can be three dimensional.}, } @article {pmid26764803, year = {2015}, author = {Anbarlooei, HR and Cruz, DO and Ramos, F and Silva Freire, AP}, title = {Phenomenological Blasius-type friction equation for turbulent power-law fluid flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {6}, pages = {063006}, doi = {10.1103/PhysRevE.92.063006}, pmid = {26764803}, issn = {1550-2376}, abstract = {We propose a friction formula for turbulent power-law fluid flows, a class of purely viscous non-Newtonian fluids commonly found in applications. Our model is derived through an extension of the friction factor analysis based on Kolmogorov's phenomenology, recently proposed by Gioia and Chakraborty. Tests against classical empirical data show excellent agreement over a significant range of Reynolds number. Limits of the model are also discussed.}, } @article {pmid26739143, year = {2016}, author = {Low, SC and Eshtiaghi, N and Slatter, P and Baudez, JC and Parthasarathy, R}, title = {Mixing characteristics of sludge simulant in a model anaerobic digester.}, journal = {Bioprocess and biosystems engineering}, volume = {39}, number = {3}, pages = {473-483}, doi = {10.1007/s00449-015-1530-4}, pmid = {26739143}, issn = {1615-7605}, mesh = {*Models, Chemical ; Sewage/*chemistry ; }, abstract = {This study aims to investigate the mixing characteristics of a transparent sludge simulant in a mechanically agitated model digester using flow visualisation technique. Video images of the flow patterns were obtained by recording the progress of an acid-base reaction and analysed to determine the active and inactive volumes as a function of time. The doughnut-shaped inactive region formed above and below the impeller in low concentration simulant decreases in size with time and disappears finally. The 'cavern' shaped active mixing region formed around the impeller in simulant solutions with higher concentrations increases with increasing agitation time and reaches a steady state equilibrium size, which is a function of specific power input. These results indicate that the active volume is jointly determined by simulant rheology and specific power input. A mathematical correlation is proposed to estimate the active volume as a function of simulant concentration in terms of yield Reynolds number.}, } @article {pmid26738932, year = {2016}, author = {Jawed, MK and Reis, PM}, title = {Deformation of a soft helical filament in an axial flow at low Reynolds number.}, journal = {Soft matter}, volume = {12}, number = {6}, pages = {1898-1905}, doi = {10.1039/c5sm02625c}, pmid = {26738932}, issn = {1744-6848}, mesh = {Bacterial Physiological Phenomena ; Biomechanical Phenomena ; *Elasticity ; Flagella/*chemistry/physiology ; Models, Theoretical ; Movement ; Rotation ; }, abstract = {We perform a numerical investigation of the deformation of a rotating helical filament subjected to an axial flow, under low Reynolds number conditions, motivated by the propulsion of bacteria using helical flagella. Given its slenderness, the helical rod is intrinsically soft and deforms due to the interplay between elastic forces and hydrodynamic loading. We make use of a previously developed and experimentally validated computational tool framework that models the elasticity of the filament using the discrete elastic rod method and the fluid forces are treated using Lighthill's slender body theory. Under axial flow, and in the absence of rotation, the initially helical rod is extended. Above a critical flow speed its configuration comprises a straight portion connected to a localized helix near the free end. When the rod is also rotated about its helical axis, propulsion is only possible in a finite range of angular velocity, with an upper bound that is limited by buckling of the soft helix arising due to viscous stresses. A systematic exploration of the parameter space allows us to quantify regimes for successful propulsion for a number of specific bacteria.}, } @article {pmid26736424, year = {2015}, author = {Nita, C and Itu, L and Mihalef, V and Sharma, P and Rapaka, S}, title = {GPU-accelerated model for fast, three-dimensional fluid-structure interaction computations.}, journal = {Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference}, volume = {2015}, number = {}, pages = {965-968}, doi = {10.1109/EMBC.2015.7318524}, pmid = {26736424}, issn = {1557-170X}, mesh = {*Computer Simulation ; }, abstract = {In this paper we introduce a methodology for performing one-way Fluid-Structure interaction (FSI), i.e. where the motion of the wall boundaries is imposed. We use a Graphics Processing Unit (GPU) accelerated Lattice-Boltzmann Method (LBM) implementation and present an efficient workflow for embedding the moving geometry, given as a set of polygonal meshes, in the LBM computation. The proposed method is first validated in a synthetic experiment: a vessel which is periodically expanding and contracting. Next, the evaluation focuses on the 3D Peristaltic flow problem: a fluid flows inside a flexible tube, where a periodic wave-like deformation produces a fluid motion along the centerline of the tube. Different geometry configurations are used and results are compared against previously published solutions. The efficient approach leads to an average execution time of approx. one hour per computation, whereas 50% of it is required for the geometry update operations. Finally, we also analyse the effect of changing the Reynolds number on the flow streamlines: the flow regime is significantly affected by the Reynolds number.}, } @article {pmid26730219, year = {2015}, author = {Huang, D and Chernyshenko, S and Goulart, P and Lasagna, D and Tutty, O and Fuentes, F}, title = {Sum-of-squares of polynomials approach to nonlinear stability of fluid flows: an example of application.}, journal = {Proceedings. Mathematical, physical, and engineering sciences}, volume = {471}, number = {2183}, pages = {20150622}, doi = {10.1098/rspa.2015.0622}, pmid = {26730219}, issn = {1364-5021}, abstract = {With the goal of providing the first example of application of a recently proposed method, thus demonstrating its ability to give results in principle, global stability of a version of the rotating Couette flow is examined. The flow depends on the Reynolds number and a parameter characterizing the magnitude of the Coriolis force. By converting the original Navier-Stokes equations to a finite-dimensional uncertain dynamical system using a partial Galerkin expansion, high-degree polynomial Lyapunov functionals were found by sum-of-squares of polynomials optimization. It is demonstrated that the proposed method allows obtaining the exact global stability limit for this flow in a range of values of the parameter characterizing the Coriolis force. Outside this range a lower bound for the global stability limit was obtained, which is still better than the energy stability limit. In the course of the study, several results meaningful in the context of the method used were also obtained. Overall, the results obtained demonstrate the applicability of the recently proposed approach to global stability of the fluid flows. To the best of our knowledge, it is the first case in which global stability of a fluid flow has been proved by a generic method for the value of a Reynolds number greater than that which could be achieved with the energy stability approach.}, } @article {pmid26723345, year = {2015}, author = {Phong, V and Papamoschou, D}, title = {Normal incidence acoustic insertion loss of perforated plates with bias flow.}, journal = {The Journal of the Acoustical Society of America}, volume = {138}, number = {6}, pages = {3907-3921}, doi = {10.1121/1.4937602}, pmid = {26723345}, issn = {1520-8524}, abstract = {The transmission of sound at normal incidence through perforated plates with bias flow is investigated experimentally and theoretically over a large parameter space. A specially designed experimental apparatus enabled the measurement of insertion loss with bias flow Mach number up to 0.25. A theoretical model for insertion loss was constructed based on inviscid, one-dimensional wave propagation with mean flow through a single contraction/expansion chamber. The mass end correction of the contraction is modified for hole interaction effects and mean flow. Hydrodynamic losses are modeled using a vena contracta coefficient dependent on both perforation geometry and Reynolds number. Losses in acoustic energy that occur in the mixing region downstream of the perforations are modeled as fluctuations in entropy. The proposed model was validated experimentally over a range of plate thickness, porosity, and hole size. The experimental results indicate an increase in insertion loss with increasing frequency, followed by saturation and decline as resonant conditions are established in the perforations. The insertion loss at low frequency increases with increasing Mach number through the perforation. The proposed model captures these trends and its predictions are shown to be more accurate than those of past models.}, } @article {pmid26718062, year = {2016}, author = {O Connor, J and Revell, A and Mandal, P and Day, P}, title = {Application of a lattice Boltzmann-immersed boundary method for fluid-filament dynamics and flow sensing.}, journal = {Journal of biomechanics}, volume = {49}, number = {11}, pages = {2143-2151}, doi = {10.1016/j.jbiomech.2015.11.057}, pmid = {26718062}, issn = {1873-2380}, mesh = {Cilia/metabolism ; Computer Simulation ; Cytoskeleton/metabolism ; Humans ; *Hydrodynamics ; Models, Biological ; Shear Strength ; Stress, Mechanical ; }, abstract = {Complex fluid-structure interactions between elastic filaments, or cilia, immersed in viscous flows are commonplace in nature and bear important roles. Some biological systems have evolved to interpret flow-induced motion into signals for the purpose of feedback response. Given the challenges associated with extracting meaningful experimental data at this scale, there has been particular focus on the numerical study of these effects. Porous models have proven useful where cilia arrangements are relatively dense, but for more sparse configurations the dynamic interactions of individual structures play a greater role and direct modelling becomes increasingly necessary. The present study reports efforts towards explicit modelling of regularly spaced wall-mounted cilia using a lattice Boltzmann-immersed boundary method. Both steady and forced unsteady 2D channel flows at different Reynolds numbers are investigated, with and without the presence of a periodic array of elastic inextensible filaments. It is demonstrated that the structure response depends significantly on Reynolds number. For low Reynolds flow, the recirculation vortex aft of successive filaments is small relative to the cilia spacing and does not fully bridge the gap, in which case the structure lags the flow. At higher Reynolds number, when this gap is fully bridged the structure and flow move in phase. The trapping of vortices between cilia is associated with relatively lower wall shear stress. At low to intermediate Reynolds, vortex bridging is incomplete and large deflection is still possible, which is reflected in the tip dynamics and wall shear stress profiles.}, } @article {pmid26705658, year = {2015}, author = {Klotsa, D and Baldwin, KA and Hill, RJ and Bowley, RM and Swift, MR}, title = {Propulsion of a Two-Sphere Swimmer.}, journal = {Physical review letters}, volume = {115}, number = {24}, pages = {248102}, doi = {10.1103/PhysRevLett.115.248102}, pmid = {26705658}, issn = {1079-7114}, mesh = {Biomechanical Phenomena ; Computer Simulation ; *Models, Theoretical ; *Swimming ; }, abstract = {We describe experiments and simulations demonstrating the propulsion of a neutrally buoyant swimmer that consists of a pair of spheres attached by a spring, immersed in a vibrating fluid. The vibration of the fluid induces relative motion of the spheres which, for sufficiently large amplitudes, can lead to motion of the center of mass of the two spheres. We find that the swimming speed obtained from both experiment and simulation agree and collapse onto a single curve if plotted as a function of the streaming Reynolds number, suggesting that the propulsion is related to streaming flows. There appears to be a critical onset value of the streaming Reynolds number for swimming to occur. We observe a change in the streaming flows as the Reynolds number increases, from that generated by two independent oscillating spheres to a collective flow pattern around the swimmer as a whole. The mechanism for swimming is traced to a strengthening of a jet of fluid in the wake of the swimmer.}, } @article {pmid26698964, year = {2015}, author = {Hay, RF and Gibson, GM and Simpson, SH and Padgett, MJ and Phillips, DB}, title = {'Lissajous-like' trajectories in optical tweezers.}, journal = {Optics express}, volume = {23}, number = {25}, pages = {31716-31727}, pmid = {26698964}, issn = {1094-4087}, abstract = {When a microscopic particle moves through a low Reynolds number fluid, it creates a flow-field which exerts hydrodynamic forces on surrounding particles. In this work we study the 'Lissajous-like' trajectories of an optically trapped 'probe' microsphere as it is subjected to time-varying oscillatory hydrodynamic flow-fields created by a nearby moving particle (the 'actuator'). We show a breaking of time-reversal symmetry in the motion of the probe when the driving motion of the actuator is itself time-reversal symmetric. This symmetry breaking results in a fluid-pumping effect, which arises due to the action of both a time-dependent hydrodynamic flow and a position-dependent optical restoring force, which together determine the trajectory of the probe particle. We study this situation experimentally, and show that the form of the trajectories observed is in good agreement with Stokesian dynamics simulations. Our results are related to the techniques of active micro-rheology and flow measurement, and also highlight how the mere presence of an optical trap can perturb the environment it is in place to measure.}, } @article {pmid26684120, year = {2015}, author = {Stepanov, R and Golbraikh, E and Frick, P and Shestakov, A}, title = {Hindered Energy Cascade in Highly Helical Isotropic Turbulence.}, journal = {Physical review letters}, volume = {115}, number = {23}, pages = {234501}, doi = {10.1103/PhysRevLett.115.234501}, pmid = {26684120}, issn = {1079-7114}, abstract = {The conventional approach to the turbulent energy cascade, based on Richardson-Kolmogorov phenomenology, ignores the topology of emerging vortices, which is related to the helicity of the turbulent flow. It is generally believed that helicity can play a significant role in turbulent systems, e.g., supporting the generation of large-scale magnetic fields, but its impact on the energy cascade to small scales has never been observed. We suggest, for the first time, a generalized phenomenology for isotropic turbulence with an arbitrary spectral distribution of the helicity. We discuss various scenarios of direct turbulent cascades with new helicity effect, which can be interpreted as a hindering of the spectral energy transfer. Therefore, the energy is accumulated and redistributed so that the efficiency of nonlinear interactions will be sufficient to provide a constant energy flux. We confirm our phenomenology by high Reynolds number numerical simulations based on a shell model of helical turbulence. The energy in our model is injected at a certain large scale only, whereas the source of helicity is distributed over all scales. In particular, we found that the helical bottleneck effect can appear in the inertial interval of the energy spectrum.}, } @article {pmid26671398, year = {2015}, author = {Zia, RN and Swan, JW and Su, Y}, title = {Pair mobility functions for rigid spheres in concentrated colloidal dispersions: Force, torque, translation, and rotation.}, journal = {The Journal of chemical physics}, volume = {143}, number = {22}, pages = {224901}, doi = {10.1063/1.4936664}, pmid = {26671398}, issn = {1089-7690}, abstract = {The formulation of detailed models for the dynamics of condensed soft matter including colloidal suspensions and other complex fluids requires accurate description of the physical forces between microstructural constituents. In dilute suspensions, pair-level interactions are sufficient to capture hydrodynamic, interparticle, and thermodynamic forces. In dense suspensions, many-body interactions must be considered. Prior analytical approaches to capturing such interactions such as mean-field approaches replace detailed interactions with averaged approximations. However, long-range coupling and effects of concentration on local structure, which may play an important role in, e.g., phase transitions, are smeared out in such approaches. An alternative to such approximations is the detailed modeling of hydrodynamic interactions utilizing precise couplings between moments of the hydrodynamic traction on a suspended particle and the motion of that or other suspended particles. For two isolated spheres, a set of these functions was calculated by Jeffrey and Onishi [J. Fluid Mech. 139, 261-290 (1984)] and Jeffrey [J. Phys. Fluids 4, 16-29 (1992)]. Along with pioneering work by Batchelor, these are the touchstone for low-Reynolds-number hydrodynamic interactions and have been applied directly in the solution of many important problems related to the dynamics of dilute colloidal dispersions [G. K. Batchelor and J. T. Green, J. Fluid Mech. 56, 375-400 (1972) and G. K. Batchelor, J. Fluid Mech. 74, 1-29 (1976)]. Toward extension of these functions to concentrated systems, here we present a new stochastic sampling technique to rapidly calculate an analogous set of mobility functions describing the hydrodynamic interactions between two hard spheres immersed in a suspension of arbitrary concentration, utilizing accelerated Stokesian dynamics simulations. These mobility functions provide precise, radially dependent couplings of hydrodynamic force and torque to particle translation and rotation, for arbitrary colloid volume fraction ϕ. The pair mobilities (describing entrainment of one particle by the disturbance flow created by another) decay slowly with separation distance: as 1/r, for volume fractions 0.05 ≤ ϕ ≤ 0.5. For the relative mobility, we find an initially rapid growth as a pair separates, followed by a slow, 1/r growth. Up to ϕ ≤ 0.4, the relative mobility does not reached the far-field value even beyond separations of many particle sizes. In the case of ϕ = 0.5, the far-field asymptote is reached but only at a separation of eight radii and after a slow 1/r growth. At these higher concentrations, the coefficients also reveal liquid-like structural effects on pair mobility at close separations. These results confirm that long-range many-body hydrodynamic interactions are an essential part of the dynamics of concentrated systems and that care must be taken when applying renormalization schemes.}, } @article {pmid26652667, year = {2015}, author = {Schultz, MP and Walker, JM and Steppe, CN and Flack, KA}, title = {Impact of diatomaceous biofilms on the frictional drag of fouling-release coatings.}, journal = {Biofouling}, volume = {31}, number = {9-10}, pages = {759-773}, doi = {10.1080/08927014.2015.1108407}, pmid = {26652667}, issn = {1029-2454}, mesh = {Biofilms/*growth & development ; Biofouling/*prevention & control ; *Friction ; Models, Theoretical ; *Paint ; *Ships ; Surface Properties ; }, abstract = {Skin-friction results are presented for fouling-release (FR) hull coatings in the unexposed, clean condition and after dynamic exposure to diatomaceous biofilms for 3 and 6 months. The experiments were conducted in a fully developed turbulent channel flow facility spanning a wide Reynolds number range. The results show that the clean FR coatings tested were hydraulically smooth over much of the Reynolds number range. Biofilms, however, resulted in an increase in skin-friction of up to 70%. The roughness functions for the biofilm-covered surfaces did not display universal behavior, but instead varied with the percentage coverage by the biofilm. The effect of the biofilm was observed to scale with its mean thickness and the square root of the percentage coverage. A new effective roughness length scale (keff) for biofilms based on these parameters is proposed. Boundary layer similarity-law scaling is used to predict the impact of these biofilms on the required shaft power for a mid-sized naval surface combatant at cruising speed. The increase in power is estimated to be between 1.5% and 10.1% depending on the biofilm thickness and percentage coverage.}, } @article {pmid26651792, year = {2015}, author = {Minier, JP and Profeta, C}, title = {Kinetic and dynamic probability-density-function descriptions of disperse turbulent two-phase flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {5}, pages = {053020}, doi = {10.1103/PhysRevE.92.053020}, pmid = {26651792}, issn = {1550-2376}, abstract = {This article analyzes the status of two classical one-particle probability density function (PDF) descriptions of the dynamics of discrete particles dispersed in turbulent flows. The first PDF formulation considers only the process made up by particle position and velocity Z(p)=(x(p),U(p)) and is represented by its PDF p(t; y(p),V(p)) which is the solution of a kinetic PDF equation obtained through a flux closure based on the Furutsu-Novikov theorem. The second PDF formulation includes fluid variables into the particle state vector, for example, the fluid velocity seen by particles Z(p)=(x(p),U(p),U(s)), and, consequently, handles an extended PDF p(t; y(p),V(p),V(s)) which is the solution of a dynamic PDF equation. For high-Reynolds-number fluid flows, a typical formulation of the latter category relies on a Langevin model for the trajectories of the fluid seen or, conversely, on a Fokker-Planck equation for the extended PDF. In the present work, a new derivation of the kinetic PDF equation is worked out and new physical expressions of the dispersion tensors entering the kinetic PDF equation are obtained by starting from the extended PDF and integrating over the fluid seen. This demonstrates that, under the same assumption of a Gaussian colored noise and irrespective of the specific stochastic model chosen for the fluid seen, the kinetic PDF description is the marginal of a dynamic PDF one. However, a detailed analysis reveals that kinetic PDF models of particle dynamics in turbulent flows described by statistical correlations constitute incomplete stand-alone PDF descriptions and, moreover, that present kinetic-PDF equations are mathematically ill posed. This is shown to be the consequence of the non-Markovian characteristic of the stochastic process retained to describe the system and the use of an external colored noise. Furthermore, developments bring out that well-posed PDF descriptions are essentially due to a proper choice of the variables selected to describe physical systems and guidelines are formulated to emphasize the key role played by the notion of slow and fast variables.}, } @article {pmid26651790, year = {2015}, author = {Altmeyer, S and Do, Y and Lai, YC}, title = {Ring-bursting behavior en route to turbulence in narrow-gap Taylor-Couette flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {5}, pages = {053018}, doi = {10.1103/PhysRevE.92.053018}, pmid = {26651790}, issn = {1550-2376}, abstract = {We investigate the Taylor-Couette system where the radius ratio is close to unity. Systematically increasing the Reynolds number, we observe a number of previously known transitions, such as one from the classical Taylor vortex flow (TVF) to wavy vortex flow (WVF) and the transition to fully developed turbulence. Prior to the onset of turbulence, we observe intermittent bursting patterns of localized turbulent patches, confirming the experimentally observed pattern of very short wavelength bursts (VSWBs). A striking finding is that, for a Reynolds number larger than that for the onset of VSWBs, a new type of intermittently bursting behavior emerges: patterns of azimuthally closed rings of various orders. We call them ring-bursting patterns, which surround the cylinder completely but remain localized and separated in the axial direction through nonturbulent wavy structures. We employ a number of quantitative measures including the cross-flow energy to characterize the ring-bursting patterns and to distinguish them from the background flow. These patterns are interesting because they do not occur in the wide-gap Taylor-Couette flow systems. The narrow-gap regime is less studied but certainly deserves further attention to gain deeper insights into complex flow dynamics in fluids.}, } @article {pmid26636667, year = {2016}, author = {Dey, KK and Pong, FY and Breffke, J and Pavlick, R and Hatzakis, E and Pacheco, C and Sen, A}, title = {Dynamic Coupling at the Ångström Scale.}, journal = {Angewandte Chemie (International ed. in English)}, volume = {55}, number = {3}, pages = {1113-1117}, doi = {10.1002/anie.201509237}, pmid = {26636667}, issn = {1521-3773}, abstract = {While momentum transfer from active particles to their immediate surroundings has been studied for both synthetic and biological micron-scale systems, a similar phenomenon was presumed unlikely to exist at smaller length scales due to the dominance of viscosity in the ultralow Reynolds number regime. Using diffusion NMR spectroscopy, we studied the motion of two passive tracers--tetramethylsilane and benzene--dissolved in an organic solution of active Grubbs catalyst. Significant enhancements in diffusion were observed for both the tracers and the catalyst as a function of reaction rate. A similar behavior was also observed for the enzyme urease in aqueous solution. Surprisingly, momentum transfer at the molecular scale closely resembles that reported for microscale systems and appears to be independent of swimming mechanism. Our work provides new insight into the role of active particles on advection and mixing at the Ångström scale.}, } @article {pmid26628288, year = {2015}, author = {Wang, S and Ardekani, AM}, title = {Biogenic mixing induced by intermediate Reynolds number swimming in stratified fluids.}, journal = {Scientific reports}, volume = {5}, number = {}, pages = {17448}, doi = {10.1038/srep17448}, pmid = {26628288}, issn = {2045-2322}, mesh = {Animals ; Humans ; *Models, Theoretical ; *Swimming ; }, abstract = {We study fully resolved motion of interacting swimmers in density stratified fluids using an archetypal swimming model called "squirmer". The intermediate Reynolds number regime is particularly important, because the vast majority of organisms in the aphotic ocean (i.e. regions that are 200 m beneath the sea surface) are small (mm-cm) and their motion is governed by the balance of inertial and viscous forces. Our study shows that the mixing efficiency and the diapycnal eddy diffusivity, a measure of vertical mass flux, within a suspension of squirmers increases with Reynolds number. The mixing efficiency is in the range of O(0.0001-0.04) when the swimming Reynolds number is in the range of O(0.1-100). The values of diapycnal eddy diffusivity and Cox number are two orders of magnitude larger for vertically swimming cells compared to horizontally swimming cells. For a suspension of squirmers in a decaying isotropic turbulence, we find that the diapycnal eddy diffusivity enhances due to the strong viscous dissipation generated by squirmers as well as the interaction of squirmers with the background turbulence.}, } @article {pmid26621672, year = {2016}, author = {Avari, H and Savory, E and Rogers, KA}, title = {An In Vitro Hemodynamic Flow System to Study the Effects of Quantified Shear Stresses on Endothelial Cells.}, journal = {Cardiovascular engineering and technology}, volume = {7}, number = {1}, pages = {44-57}, doi = {10.1007/s13239-015-0250-x}, pmid = {26621672}, issn = {1869-4098}, support = {//Canadian Institutes of Health Research/Canada ; }, mesh = {Animals ; Biomechanical Phenomena/physiology ; Cells, Cultured ; Endothelial Cells/*physiology ; Endothelium, Vascular/cytology/*physiology ; Hemodynamics/*physiology ; Laser-Doppler Flowmetry ; *Models, Cardiovascular ; Stress, Mechanical ; Swine ; }, abstract = {Numerous in vitro systems have previously been developed and employed for studying the effects of hemodynamics on endothelial cell (EC) dysfunction. In the majority of that work, accurate flow quantification (e.g., uniformity of the flow over the ECs) remains elusive and wall shear stress (WSS) quantifications are determined using theoretical relationships (without considering the flow channel aspect ratio effects). In addition, those relationships are not applicable to flows other than steady laminar cases. The present work discusses the development of a novel hemodynamic flow system for studying the effects of various well-quantified flow regimes over ECs. The current work presents a novel hemodynamic flow system applying the concept of a parallel plate flow chamber (PPFC) with live microscopy access for studying the effects of quantified WSS on ECs. A range of steady laminar, pulsatile (carotid wave form) and low-Reynolds number turbulent WSSs were quantified through velocity field measurements by a laser Doppler velocimetry (LDV) system, to validate the functionality of the current hemodynamic flow system. Uniformity of the flow across the channel width can be analyzed with the current system (e.g., the flow was uniform across about 65-75% of the channel width for the steady cases). The WSS obtained from the experiments had higher values in almost all of the cases when compared to the most commonly-used theoretical solution (9% < error < 16%), whereas another relationship, which considers the channel dimensions, had better agreement with the experimental results (1% < error < 8%). Additionally, the latter relationship predicted the uniform flow region in the PPFC with an average difference of <5% when compared to the experimental results. The experimental data also showed that the WSS at various locations (D, E and F) at the test section differed by less than 4% for the laminar cases representing a fully developed flow. WSS was also determined for a low-Re (Re = 2750) turbulent flow using (1) the Reynolds shears stress and (2) the time-averaged velocity profile gradient at the wall, with a good agreement (differences <16%) between the two where the first method returned a higher value than the second. Porcine aortic endothelial cell (PAEC) viability in the system and morphological cell response to laminar WSS of about 11 dyne/cm(2), were observed. These results provide performance validation of this novel in vitro system with many improved features compared to previous similar prototypes for investigation of flow effects on ECs. The integration of the LDV technique in the current study and the comparison of the results with those from theory revealed that great care must be taken when using PPFCs since the commonly used theoretical relation for laminar steady flows is unable to predict the flow uniformity (which may introduce significant statistical bias in biological studies) and the predicted WSS was subjected to greater error when compared to a more comprehensive equation presented in the current work. Moreover, application of the LDV technique in the current system is essential for studies of more complex cases, such as disturbed flows, where the WSS cannot be predicted using theoretical or numerical modelling methods.}, } @article {pmid26598001, year = {2015}, author = {Gaddam, A and Agrawal, A and Joshi, SS and Thompson, MC}, title = {Utilization of Cavity Vortex To Delay the Wetting Transition in One-Dimensional Structured Microchannels.}, journal = {Langmuir : the ACS journal of surfaces and colloids}, volume = {31}, number = {49}, pages = {13373-13384}, doi = {10.1021/acs.langmuir.5b03666}, pmid = {26598001}, issn = {1520-5827}, abstract = {Frictional resistance across rough surfaces depends on the existence of slip on the liquid-gas interface; therefore, prolonging the existence of liquid-gas interface becomes relevant. In this work, we explore manipulation of the cavity shape in order to delay the wetting transition. We propose that liquid-driven vortices generated in the air cavity dissipate sufficient energy to delay the Cassie-Wenzel transition. Toward this, we fabricated cavities on the side walls of a polydimethylsiloxane-based microchannel for easy visualization and analysis of the dynamics of the liquid-gas interface. Two distinct flow regimes are identified in the experimental envelope. In the first regime, the liquid-gas interface is found to be protruding into the flow field, thus increasing the pressure drop at low Reynolds number. In the second regime, flow rate and geometry-based wetting transitions are established at moderate to high Reynolds numbers. We then investigate the effect of different cavity shapes (square, trapezoidal, and U-shape) in delaying the wetting transition by manipulating liquid-driven vortices. Out of the shapes considered in this study, trapezoidal cavities perform better than cavities with vertical walls in delaying the wetting transition due to geometrical squeezing of vortices toward the liquid-gas interface. Numerical simulations corroborate the experimental findings in that cavities with inclined walls exert more force on the liquid-gas interface, thus delaying their wetting transition. The proposed method being passive in nature appears more attractive than previous active methods.}, } @article {pmid26593731, year = {2016}, author = {Sekhar, YR and Sharma, KV and Kamal, S}, title = {Nanofluid heat transfer under mixed convection flow in a tube for solar thermal energy applications.}, journal = {Environmental science and pollution research international}, volume = {23}, number = {10}, pages = {9411-9417}, doi = {10.1007/s11356-015-5715-9}, pmid = {26593731}, issn = {1614-7499}, mesh = {*Convection ; *Hot Temperature ; *Nanoparticles ; *Solar Energy ; Water ; }, abstract = {The solar flat plate collector operating under different convective modes has low efficiency for energy conversion. The energy absorbed by the working fluid in the collector system and its heat transfer characteristics vary with solar insolation and mass flow rate. The performance of the system is improved by reducing the losses from the collector. Various passive methods have been devised to aid energy absorption by the working fluid. Also, working fluids are modified using nanoparticles to improve the thermal properties of the fluid. In the present work, simulation and experimental studies are undertaken for pipe flow at constant heat flux boundary condition in the mixed convection mode. The working fluid at low Reynolds number in the mixed laminar flow range is undertaken with water in thermosyphon mode for different inclination angles of the tube. Local and average coefficients are determined experimentally and compared with theoretical values for water-based Al2O3 nanofluids. The results show an enhancement in heat transfer in the experimental range with Rayleigh number at higher inclinations of the collector tube for water and nanofluids.}, } @article {pmid26590151, year = {2015}, author = {Rolland, J}, title = {Stochastic analysis of the time evolution of laminar-turbulent bands of plane Couette flow.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {11}, pages = {121}, doi = {10.1140/epje/i2015-15121-5}, pmid = {26590151}, issn = {1292-895X}, abstract = {This article is concerned with the time evolution of the oblique laminar-turbulent bands of transitional plane Couette flow under the influence of turbulent noise. Our study is focused on the amplitude of modulation of turbulence (the bands). In order to guide the numerical study of the flow, we first perform an analytical and numerical analysis of a Stochastic Ginzburg-Landau (GL) equation for a complex order parameter. The modulus of this order parameter models the amplitude of modulation of turbulence. Firstly, we compute the autocorrelation function of said modulus once the band is established. Secondly, we perform a calculation of average and fluctuations around the exponential growth of the order parameter. This type of analysis is similar to the Stochastic Structural Stability Theory (S3T). We then perform numerical simulations of the Navier-Stokes equations in order to confront these predictions with the actual behaviour of the bands. Computation of the autocorrelation function of the modulation of turbulence shows quantitative agreement with the model: in the established band regime, the amplitude of modulation follows an Ornstein-Uhlenbeck process. In order to test the S3T predictions, we perform quench experiments, sudden decreases of the Reynolds number from uniform turbulence, in which modulation appears. We compute the average evolution of the amplitude of modulation and the fluctuations around it. We find good agreement between numerics and modeling. The average trajectory grows exponentially, at a rate clearly smaller than that of the formation of laminar holes. Meanwhile, the actual time evolution remains in a flaring envelope, centered on the average, and expanding at the same rate. These results provide further validation of the stochastic modeling for the time evolution of the bands for further studies. Besides, they stress on the difference between the oblique band formation and the formation of laminar holes.}, } @article {pmid26584257, year = {2016}, author = {Zhang, J and Yan, S and Yuan, D and Alici, G and Nguyen, NT and Ebrahimi Warkiani, M and Li, W}, title = {Fundamentals and applications of inertial microfluidics: a review.}, journal = {Lab on a chip}, volume = {16}, number = {1}, pages = {10-34}, doi = {10.1039/c5lc01159k}, pmid = {26584257}, issn = {1473-0189}, mesh = {Humans ; *Microfluidic Analytical Techniques ; Particle Size ; Surface Properties ; }, abstract = {In the last decade, inertial microfluidics has attracted significant attention and a wide variety of channel designs that focus, concentrate and separate particles and fluids have been demonstrated. In contrast to conventional microfluidic technologies, where fluid inertia is negligible and flow remains almost within the Stokes flow region with very low Reynolds number (Re ≪ 1), inertial microfluidics works in the intermediate Reynolds number range (~1 < Re < ~100) between Stokes and turbulent regimes. In this intermediate range, both inertia and fluid viscosity are finite and bring about several intriguing effects that form the basis of inertial microfluidics including (i) inertial migration and (ii) secondary flow. Due to the superior features of high-throughput, simplicity, precise manipulation and low cost, inertial microfluidics is a very promising candidate for cellular sample processing, especially for samples with low abundant targets. In this review, we first discuss the fundamental kinematics of particles in microchannels to familiarise readers with the mechanisms and underlying physics in inertial microfluidic systems. We then present a comprehensive review of recent developments and key applications of inertial microfluidic systems according to their microchannel structures. Finally, we discuss the perspective of employing fluid inertia in microfluidics for particle manipulation. Due to the superior benefits of inertial microfluidics, this promising technology will still be an attractive topic in the near future, with more novel designs and further applications in biology, medicine and industry on the horizon.}, } @article {pmid26577358, year = {2015}, author = {Clark, WD and Eslahpazir, BA and Argueta-Morales, IR and Kassab, AJ and Divo, EA and DeCampli, WM}, title = {Comparison Between Bench-Top and Computational Modelling of Cerebral Thromboembolism in Ventricular Assist Device Circulation.}, journal = {Cardiovascular engineering and technology}, volume = {6}, number = {3}, pages = {242-255}, doi = {10.1007/s13239-015-0230-1}, pmid = {26577358}, issn = {1869-4098}, mesh = {Carotid Arteries/*physiopathology ; Cerebral Cortex/*blood supply ; Coronary Artery Bypass/methods ; *Heart-Assist Devices ; Hydrodynamics ; Intracranial Thrombosis/*physiopathology ; *Models, Cardiovascular ; }, abstract = {Despite improvements in ventricular assist devices (VAD) design, VAD-induced stroke rates remain remarkably high at 14-47%. We previously employed computational fluid dynamics (CFD) to propose adjustment of VAD outflow graft (VAD-OG) implantation to reduce stoke. Herein, we present an in-vitro model of cerebral vessel embolization in VAD-assisted circulation, and compare benchtop results to CFD predictions. The benchtop flow-loop consists of a 3D printed aortic bed using Accura 60 polymer driven by a continuous-flow pump. Three hundred spherical particles simulating thrombi of 2, 3.5, and 5 mm diameters were injected at the mock VAD-OG inlet. A water and glycerin mixture (3.8 cP viscosity) synthetically mimicked blood. The flowrate was adjusted to match the CFD Reynolds number. Catch cans were used to capture and count particles reaching cerebral vessels. VAD-OG geometries were evaluated using comparison of means Z-score range of -1.96 ≤ Z ≤ 1.96 to demonstrate overall agreement between computational and in-vitro techniques. Z-scores were: (i) Z = -1.05 for perpendicular (0°), (ii) Z = 0.32 for intermediate (30°), and (iii) Z = -0.52 for shallow (60°) anastomosis and confirmed agreement for all geometries. This study confirmed added benefits of using a left carotid artery bypass-graft with percent embolization reduction: 22.6% for perpendicular, 21.2% for intermediate, and 11.9% for shallow anastomoses. The shallow anastomosis demonstrated lower degrees of aortic arch flow recirculation, consistent with steady-flow computations. Quantitatively and qualitatively, contemporary steady-flow computational models for predicting VAD-induced cerebral embolization can be achieved in-vitro to validate the CFD equivalent.}, } @article {pmid26565499, year = {2015}, author = {Gravish, N and Peters, JM and Combes, SA and Wood, RJ}, title = {Collective Flow Enhancement by Tandem Flapping Wings.}, journal = {Physical review letters}, volume = {115}, number = {18}, pages = {188101}, doi = {10.1103/PhysRevLett.115.188101}, pmid = {26565499}, issn = {1079-7114}, abstract = {We examine the fluid-mechanical interactions that occur between arrays of flapping wings when operating in close proximity at a moderate Reynolds number (Re≈100-1000). Pairs of flapping wings are oscillated sinusoidally at frequency f, amplitude θ_{M}, phase offset ϕ, and wing separation distance D^{*}, and outflow speed v^{*} is measured. At a fixed separation distance, v^{*} is sensitive to both f and ϕ, and we observe both constructive and destructive interference in airspeed. v^{*} is maximized at an optimum phase offset, ϕ_{max}, which varies with wing separation distance, D^{*} . We propose a model of collective flow interactions between flapping wings based on vortex advection, which reproduces our experimental data.}, } @article {pmid26565366, year = {2015}, author = {Bösch, F and Chikatamarla, SS and Karlin, IV}, title = {Entropic multirelaxation lattice Boltzmann models for turbulent flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {043309}, doi = {10.1103/PhysRevE.92.043309}, pmid = {26565366}, issn = {1550-2376}, abstract = {We present three-dimensional realizations of a class of lattice Boltzmann models introduced recently by the authors [I. V. Karlin, F. Bösch, and S. S. Chikatamarla, Phys. Rev. E 90, 031302(R) (2014)] and review the role of the entropic stabilizer. Both coarse- and fine-grid simulations are addressed for the Kida vortex flow benchmark. We show that the outstanding numerical stability and performance is independent of a particular choice of the moment representation for high-Reynolds-number flows. We report accurate results for low-order moments for homogeneous isotropic decaying turbulence and second-order grid convergence for most assessed statistical quantities. It is demonstrated that all the three-dimensional lattice Boltzmann realizations considered herein converge to the familiar lattice Bhatnagar-Gross-Krook model when the resolution is increased. Moreover, thanks to the dynamic nature of the entropic stabilizer, the present model features less compressibility effects and maintains correct energy and enstrophy dissipation. The explicit and efficient nature of the present lattice Boltzmann method renders it a promising candidate for both engineering and scientific purposes for highly turbulent flows.}, } @article {pmid26565347, year = {2015}, author = {Perrin, VE and Jonker, HJ}, title = {Relative velocity distribution of inertial particles in turbulence: A numerical study.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {043022}, doi = {10.1103/PhysRevE.92.043022}, pmid = {26565347}, issn = {1550-2376}, abstract = {The distribution of relative velocities between particles provides invaluable information on the rates and characteristics of particle collisions. We show that the theoretical model of Gustavsson and Mehlig [K. Gustavsson and B. Mehlig, J. Turbul. 15, 34 (2014)], within its anticipated limits of validity, can predict the joint probability density function of relative velocities and separations of identical inertial particles in isotropic turbulent flows with remarkable accuracy. We also quantify the validity range of the model. The model matches two limits (or two types) of relative motion between particles: one where pair diffusion dominates (i.e., large coherence between particle motion) and one where caustics dominate (i.e., large velocity differences between particles at small separations). By using direct numerical simulation combined with Lagrangian particle tracking, we assess the model prediction in homogeneous and isotropic turbulence. We demonstrate that, when sufficient caustics are present at a given separation and the particle response time is significantly smaller than the integral time scales of the flow, the distribution exhibits the same universal power-law form dictated by the correlation dimension as predicted by the model of Gustavsson and Mehlig. In agreement with the model, no strong dependency on the Taylor-based Reynolds number is observed.}, } @article {pmid26565336, year = {2015}, author = {Kim, I and Wu, XL}, title = {Unified Strouhal-Reynolds number relationship for laminar vortex streets generated by different-shaped obstacles.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {043011}, doi = {10.1103/PhysRevE.92.043011}, pmid = {26565336}, issn = {1550-2376}, abstract = {A structure-based Strouhal-Reynolds number relationship, St=1/(A+B/Re), has been recently proposed based on observations of laminar vortex shedding from circular cylinders in a flowing soap film. Since the new St-Re relation was derived from a general physical consideration, it raises the possibility that it may be applicable to vortex shedding from bodies other than circular ones. The work presented herein provides experimental evidence that this is the case. Our measurements also show that, in the asymptotic limit (Re→∞), St(∞)=1/A≃0.21 is constant independent of rod shapes, leaving B the only parameter that is shape dependent.}, } @article {pmid26565330, year = {2015}, author = {Edlund, EM and Ji, H}, title = {Reynolds number scaling of the influence of boundary layers on the global behavior of laboratory quasi-Keplerian flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {043005}, doi = {10.1103/PhysRevE.92.043005}, pmid = {26565330}, issn = {1550-2376}, abstract = {We present fluid velocity measurements in a modified Taylor-Couette device operated in the quasi-Keplerian regime, where it is observed that nearly ideal flows exhibit self-similarity under scaling of the Reynolds number. In contrast, nonideal flows show progressive departure from ideal Couette as the Reynolds number is increased. We present a model that describes the observed departures from ideal Couette rotation as a function of the fluxes of angular momentum across the boundaries, capturing the dependence on Reynolds number and boundary conditions.}, } @article {pmid26565328, year = {2015}, author = {Phillips, WR}, title = {Drift and pseudomomentum in bounded turbulent shear flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {043003}, doi = {10.1103/PhysRevE.92.043003}, pmid = {26565328}, issn = {1550-2376}, abstract = {This paper is concerned with the evaluation of two Lagrangian measures which arise in oscillatory or fluctuating shear flows when the fluctuating field is rotational and the spectrum of wave numbers which comprise it is continuous. The measures are the drift and pseudomomentum. Phillips [J. Fluid Mech. 430, 209 (2001)] has shown that the measures are, in such instances, succinctly expressed in terms of Lagrangian integrals of Eulerian space-time correlations. But they are difficult to interpret, and the present work begins by expressing them in a more insightful form. This is achieved by assuming the space-time correlations are separable as magnitude, determined by one-point velocity correlations, and spatial diminution. The measures then parse into terms comprised of the mean Eulerian velocity, one-point velocity correlations, and a family of integrals of spatial diminution, which in turn define a series of Lagrangian time and velocity scales. The pseudomomentum is seen to be strictly negative and related to the turbulence kinetic energy, while the drift is mixed and strongly influenced by the Reynolds stress. Both are calculated for turbulent channel flow for a range of Reynolds numbers and appear, as the Reynolds number increases, to approach a terminal form. At all Reynolds numbers studied, the pseudomomentum has a sole peak located in wall units in the low teens, while at the highest Reynolds number studied, Re(τ)=5200, the drift is negative in the vicinity of that peak, positive elsewhere, and largest near the rigid boundary. In contrast, the time and velocity scales grow almost logarithmically over much of the layer. Finally, the drift and pseudomomentum are discussed in the context of coherent wall layer structures with which they are intricately linked.}, } @article {pmid26565230, year = {2015}, author = {Clark, AH and Shattuck, MD and Ouellette, NT and O'Hern, CS}, title = {Onset and cessation of motion in hydrodynamically sheared granular beds.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {4}, pages = {042202}, doi = {10.1103/PhysRevE.92.042202}, pmid = {26565230}, issn = {1550-2376}, abstract = {We performed molecular dynamics simulations of granular beds driven by a model hydrodynamic shear flow to elucidate general grain-scale mechanisms that determine the onset and cessation of sediment transport. By varying the Shields number (the nondimensional shear stress at the top of the bed) and particle Reynolds number (the ratio of particle inertia to viscous damping), we explore how variations of the fluid flow rate, particle inertia, and fluid viscosity affect the onset and cessation of bed motion. For low to moderate particle Reynolds numbers, a critical boundary separates mobile and static states. Transition times between these states diverge as this boundary is approached both from above and below. At high particle Reynolds number, inertial effects become dominant, and particle motion can be sustained well below flow rates at which mobilization of a static bed occurs. We also find that the onset of bed motion (for both low and high particle Reynolds numbers) is described by Weibullian weakest-link statistics and thus is crucially dependent on the packing structure of the granular bed, even deep beneath the surface.}, } @article {pmid26563615, year = {2015}, author = {Tai, J and Lim, CP and Lam, YC}, title = {Visualization of polymer relaxation in viscoelastic turbulent micro-channel flow.}, journal = {Scientific reports}, volume = {5}, number = {}, pages = {16633}, doi = {10.1038/srep16633}, pmid = {26563615}, issn = {2045-2322}, abstract = {In micro-channels, the flow of viscous liquids e.g. water, is laminar due to the low Reynolds number in miniaturized dimensions. An aqueous solution becomes viscoelastic with a minute amount of polymer additives; its flow behavior can become drastically different and turbulent. However, the molecules are typically invisible. Here we have developed a novel visualization technique to examine the extension and relaxation of polymer molecules at high flow velocities in a viscoelastic turbulent flow. Using high speed videography to observe the fluorescein labeled molecules, we show that viscoelastic turbulence is caused by the sporadic, non-uniform release of energy by the polymer molecules. This developed technique allows the examination of a viscoelastic liquid at the molecular level, and demonstrates the inhomogeneity of viscoelastic liquids as a result of molecular aggregation. It paves the way for a deeper understanding of viscoelastic turbulence, and could provide some insights on the high Weissenberg number problem. In addition, the technique may serve as a useful tool for the investigations of polymer drag reduction.}, } @article {pmid26553982, year = {2015}, author = {Ni, R and Michalski, MH and Brown, E and Doan, N and Zinter, J and Ouellette, NT and Shepherd, GM}, title = {Optimal directional volatile transport in retronasal olfaction.}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {112}, number = {47}, pages = {14700-14704}, doi = {10.1073/pnas.1511495112}, pmid = {26553982}, issn = {1091-6490}, support = {R01DC 00997701/DC/NIDCD NIH HHS/United States ; }, mesh = {Female ; Humans ; Middle Aged ; Models, Anatomic ; Nose/anatomy & histology/*physiology ; Smell/*physiology ; Time Factors ; Volatilization ; }, abstract = {The ability of humans to distinguish the delicate differences in food flavors depends mostly on retronasal smell, in which food volatiles entrained into the airway at the back of the oral cavity are transported by exhaled air through the nasal cavity to stimulate the olfactory receptor neurons. Little is known whether food volatiles are preferentially carried by retronasal flow toward the nasal cavity rather than by orthonasal flow into the lung. To study the differences between retronasal and orthonasal flow, we obtained computed tomography (CT) images of the orthonasal airway from a healthy human subject, printed an experimental model using a 3D printer, and analyzed the flow field inside the airway. The results show that, during inhalation, the anatomical structure of the oropharynx creates an air curtain outside a virtual cavity connecting the oropharynx and the back of the mouth, which prevents food volatiles from being transported into the main stream toward the lung. In contrast, during exhalation, the flow preferentially sweeps through this virtual cavity and effectively enhances the entrainment of food volatiles into the main retronasal flow. This asymmetrical transport efficiency is also found to have a nonmonotonic Reynolds number dependence: The asymmetry peaks at a range of an intermediate Reynolds number close to 800, because the air curtain effect during inhalation becomes strongest in this range. This study provides the first experimental evidence, to our knowledge, for adaptations of the geometry of the human oropharynx for efficient transport of food volatiles toward the olfactory receptors in the nasal cavity.}, } @article {pmid26550904, year = {2015}, author = {Jawed, MK and Khouri, NK and Da, F and Grinspun, E and Reis, PM}, title = {Propulsion and Instability of a Flexible Helical Rod Rotating in a Viscous Fluid.}, journal = {Physical review letters}, volume = {115}, number = {16}, pages = {168101}, doi = {10.1103/PhysRevLett.115.168101}, pmid = {26550904}, issn = {1079-7114}, mesh = {*Bacterial Physiological Phenomena ; Locomotion ; *Models, Biological ; Swimming ; Viscosity ; }, abstract = {We combine experiments with simulations to investigate the fluid-structure interaction of a flexible helical rod rotating in a viscous fluid, under low Reynolds number conditions. Our analysis takes into account the coupling between the geometrically nonlinear behavior of the elastic rod with a nonlocal hydrodynamic model for the fluid loading. We quantify the resulting propulsive force, as well as the buckling instability of the originally helical filament that occurs above a critical rotation velocity. A scaling analysis is performed to rationalize the onset of this instability. A universal phase diagram is constructed to map out the region of successful propulsion and the corresponding boundary of stability is established. Comparing our results with data for flagellated bacteria suggests that this instability may be exploited in nature for physiological purposes.}, } @article {pmid26542943, year = {2016}, author = {Ishimoto, K and Cosson, J and Gaffney, EA}, title = {A simulation study of sperm motility hydrodynamics near fish eggs and spheres.}, journal = {Journal of theoretical biology}, volume = {389}, number = {}, pages = {187-197}, doi = {10.1016/j.jtbi.2015.10.013}, pmid = {26542943}, issn = {1095-8541}, mesh = {Algorithms ; Animals ; Biophysical Phenomena ; Computer Simulation ; Female ; Flatfishes/*physiology ; Hydrodynamics ; Male ; Models, Theoretical ; Movement ; Oscillometry ; *Sperm Motility ; Sperm-Ovum Interactions/*physiology ; Spermatozoa/*physiology ; }, abstract = {For teleost fish fertilisation, sperm must proceed through a small opening on the egg surface, referred to as the micropyle. In this paper, we have used boundary element simulations to explore whether the hydrodynamic attraction between sperm and a fish egg can be a sperm guidance cue. Hydrodynamical egg-sperm interactions alone do not increase the chances of an egg encounter, nor do they induce surface swimming for virtual turbot fish sperm across smooth spheres with a diameter of 1mm, which is representative of a turbot fish egg. When a repulsive surface force between the virtual turbot sperm and the egg is introduced, as motivated by surface charge and van-der-Waals interactions for instance, we find that extended surface swimming of the virtual sperm across a model turbot egg occurs, but ultimately the sperm escapes from the egg. This is due to the small exit angle of the scattering associated with the initial sperm-egg interaction at the egg surface, leading to a weak drift away from the egg, in combination with a weak hydrodynamical attraction between both gametes, though the latter is not sufficient to prevent eventual escape. The resulting transience is not observed experimentally but is a detailed quantitative difference between theory and observation in that stable surface swimming is predicted for eggs with radii larger than about 1.8mm. Regardless, the extended sperm swimming trajectory across the egg constitutes a two-dimensional search for the micropyle and thus the egg is consistently predicted to provide a guidance cue for sperm once they are sufficiently close. In addition, the observation that the virtual turbot sperm swims stably next to a flat plane given repulsive surface interactions, but does not swim stably adjacent to a turbot-sized egg, which is extremely large by sperm-lengthscales, also highlights that the stability of sperm swimming near a boundary is very sensitive to geometry.}, } @article {pmid26538006, year = {2015}, author = {Grosjean, G and Lagubeau, G and Darras, A and Hubert, M and Lumay, G and Vandewalle, N}, title = {Remote control of self-assembled microswimmers.}, journal = {Scientific reports}, volume = {5}, number = {}, pages = {16035}, doi = {10.1038/srep16035}, pmid = {26538006}, issn = {2045-2322}, abstract = {Physics governing the locomotion of microorganisms and other microsystems is dominated by viscous damping. An effective swimming strategy involves the non-reciprocal and periodic deformations of the considered body. Here, we show that a magnetocapillary-driven self-assembly, composed of three soft ferromagnetic beads, is able to swim along a liquid-air interface when powered by an external magnetic field. More importantly, we demonstrate that trajectories can be fully controlled, opening ways to explore low Reynolds number swimming. This magnetocapillary system spontaneously forms by self-assembly, allowing miniaturization and other possible applications such as cargo transport or solvent flows.}, } @article {pmid26528815, year = {2015}, author = {Ge, M and Fang, L and Tian, D}, title = {Influence of Reynolds Number on Multi-Objective Aerodynamic Design of a Wind Turbine Blade.}, journal = {PloS one}, volume = {10}, number = {11}, pages = {e0141848}, doi = {10.1371/journal.pone.0141848}, pmid = {26528815}, issn = {1932-6203}, mesh = {*Aviation ; *Models, Theoretical ; }, abstract = {At present, the radius of wind turbine rotors ranges from several meters to one hundred meters, or even more, which extends Reynolds number of the airfoil profile from the order of 105 to 107. Taking the blade for 3MW wind turbines as an example, the influence of Reynolds number on the aerodynamic design of a wind turbine blade is studied. To make the study more general, two kinds of multi-objective optimization are involved: one is based on the maximum power coefficient (CPopt) and the ultimate load, and the other is based on the ultimate load and the annual energy production (AEP). It is found that under the same configuration, the optimal design has a larger CPopt or AEP (CPopt//AEP) for the same ultimate load, or a smaller load for the same CPopt//AEP at higher Reynolds number. At a certain tip-speed ratio or ultimate load, the blade operating at higher Reynolds number should have a larger chord length and twist angle for the maximum Cpopt//AEP. If a wind turbine blade is designed by using an airfoil database with a mismatched Reynolds number from the actual one, both the load and Cpopt//AEP will be incorrectly estimated to some extent. In some cases, the assessment error attributed to Reynolds number is quite significant, which may bring unexpected risks to the earnings and safety of a wind power project.}, } @article {pmid26521001, year = {2015}, author = {Kumar, M and Tordjeman, P and Bergez, W and Cavaro, M}, title = {Note: Void effects on eddy current distortion in two-phase liquid metal.}, journal = {The Review of scientific instruments}, volume = {86}, number = {10}, pages = {106104}, doi = {10.1063/1.4932990}, pmid = {26521001}, issn = {1089-7623}, abstract = {A model based on the first order perturbation expansion of magnetic flux in a two-phase liquid metal flow has been developed for low magnetic Reynolds number Rem. This model takes into account the distortion of the induced eddy currents due to the presence of void in the conducting medium. Specific experiments with an eddy current flow meter have been realized for two periodic void distributions. The results have shown, in agreement with the model, that the effects of velocity and void on the emf modulation are decoupled. The magnitude of the void fraction and the void spatial frequency can be determined from the spectral density of the demodulated emf.}, } @article {pmid26519975, year = {2016}, author = {Adams, MC and Barbano, DM}, title = {Effect of ceramic membrane channel diameter on limiting retentate protein concentration during skim milk microfiltration.}, journal = {Journal of dairy science}, volume = {99}, number = {1}, pages = {167-182}, doi = {10.3168/jds.2015-9897}, pmid = {26519975}, issn = {1525-3198}, mesh = {Animals ; Blood Proteins/analysis ; Ceramics/*chemistry ; Computer Simulation ; *Filtration ; Food Handling ; Hydrodynamics ; *Membranes, Artificial ; Milk/*chemistry ; Milk Proteins/analysis ; Pilot Projects ; Viscosity ; }, abstract = {Our objective was to determine the effect of retentate flow channel diameter (4 or 6mm) of nongraded permeability 100-nm pore size ceramic membranes operated in nonuniform transmembrane pressure mode on the limiting retentate protein concentration (LRPC) while microfiltering (MF) skim milk at a temperature of 50°C, a flux of 55 kg · m(-2) · h(-1), and an average cross-flow velocity of 7 m · s(-1). At the above conditions, the retentate true protein concentration was incrementally increased from 7 to 11.5%. When temperature, flux, and average cross-flow velocity were controlled, ceramic membrane retentate flow channel diameter did not affect the LRPC. This indicates that LRPC is not a function of the Reynolds number. Computational fluid dynamics data, which indicated that both membranes had similar radial velocity profiles within their retentate flow channels, supported this finding. Membranes with 6-mm flow channels can be operated at a lower pressure decrease from membrane inlet to membrane outlet (ΔP) or at a higher cross-flow velocity, depending on which is controlled, than membranes with 4-mm flow channels. This implies that 6-mm membranes could achieve a higher LRPC than 4-mm membranes at the same ΔP due to an increase in cross-flow velocity. In theory, the higher LRPC of the 6-mm membranes could facilitate 95% serum protein removal in 2 MF stages with diafiltration between stages if no serum protein were rejected by the membrane. At the same flux, retentate protein concentration, and average cross-flow velocity, 4-mm membranes require 21% more energy to remove a given amount of permeate than 6-mm membranes, despite the lower surface area of the 6-mm membranes. Equations to predict skim milk MF retentate viscosity as a function of protein concentration and temperature are provided. Retentate viscosity, retentate recirculation pump frequency required to maintain a given cross-flow velocity at a given retentate viscosity, and retentate protein determination by mid-infrared spectrophotometry were all useful tools for monitoring the retentate protein concentration to ensure a sustainable MF process. Using 6-mm membranes instead of 4-mm membranes would be advantageous for processors who wish to reduce energy costs or maximize the protein concentration of a MF retentate.}, } @article {pmid26504982, year = {2015}, author = {Colla, L and Marinelli, L and Fedele, L and Bobbo, S and Manca, O}, title = {Characterization and Simulation of the Heat Transfer Behaviour of Water-Based ZnO Nanofluids.}, journal = {Journal of nanoscience and nanotechnology}, volume = {15}, number = {5}, pages = {3599-3609}, pmid = {26504982}, issn = {1533-4899}, abstract = {This paper deals with the characterization and modelling of water-based nanofluids containing zinc oxide (ZnO) nanoparticles in concentrations ranging between 1 and 10 wt%. Low concentrations were chosen to reduce fouling and excessive pressure drops. First of all, the stability was verified by means of an instrument, based on the dynamic light scattering (DLS) technique, measuring mean nanoparticle diameters and Zeta potential. Moreover, nanofluids pH was measured. Then, thermal conductivities and dynamic viscosities were measured, analysing their dependence on temperature and nanoparticle concentration. Thermal conductivity was measured by means of a hot disk apparatus in the temperature range between 10 and 70 degrees C, while viscosity was measured by a magnetic suspension rheometer in the same range of temperatures. Finally, the heat transfer capability of these fluids was studied measuring their heat transfer coefficients in a dedicated apparatus between 18 and 40 degrees C. Heat transfer coefficient was evaluated at different Reynolds number, in turbulent flow regime. Reynolds and Nusselt numbers were deduced by using previously measured thermal conductivity and viscosity values. Moreover, numerical simulations in two-dimensional turbulent and steady state flow were carried out. No increase in heat transfer coefficient in the temperature range between 18 and 40 degrees C was found. Comparison between experimental and numerical simulation data, in terms of wall temperature profiles, showed a good agreement.}, } @article {pmid26490629, year = {2015}, author = {Lenz, PH and Takagi, D and Hartline, DK}, title = {Choreographed swimming of copepod nauplii.}, journal = {Journal of the Royal Society, Interface}, volume = {12}, number = {112}, pages = {}, doi = {10.1098/rsif.2015.0776}, pmid = {26490629}, issn = {1742-5662}, mesh = {Animals ; Biomechanical Phenomena ; Copepoda/*physiology ; *Models, Biological ; Swimming/*physiology ; }, abstract = {Small metazoan paddlers, such as crustacean larvae (nauplii), are abundant, ecologically important and active swimmers, which depend on exploiting viscous forces for locomotion. The physics of micropaddling at low Reynolds number was investigated using a model of swimming based on slender-body theory for Stokes flow. Locomotion of nauplii of the copepod Bestiolina similis was quantified from high-speed video images to obtain precise measurements of appendage movements and the resulting displacement of the body. The kinematic and morphological data served as inputs to the model, which predicted the displacement in good agreement with observations. The results of interest did not depend sensitively on the parameters within the error of measurement. Model tests revealed that the commonly attributed mechanism of 'feathering' appendages during return strokes accounts for only part of the displacement. As important for effective paddling at low Reynolds number is the ability to generate a metachronal sequence of power strokes in combination with synchronous return strokes of appendages. The effect of feathering together with a synchronous return stroke is greater than the sum of each factor individually. The model serves as a foundation for future exploration of micropaddlers swimming at intermediate Reynolds number where both viscous and inertial forces are important.}, } @article {pmid26456297, year = {2015}, author = {Schwarz, US}, title = {Physical constraints for pathogen movement.}, journal = {Seminars in cell & developmental biology}, volume = {46}, number = {}, pages = {82-90}, doi = {10.1016/j.semcdb.2015.09.025}, pmid = {26456297}, issn = {1096-3634}, mesh = {Algorithms ; Amoeba/*physiology ; Animals ; Bacteria/*growth & development ; Host-Pathogen Interactions ; Humans ; Models, Biological ; Movement/physiology ; Plasmodium/*physiology ; Viruses/*growth & development ; }, abstract = {In this pedagogical review, we discuss the physical constraints that pathogens experience when they move in their host environment. Due to their small size, pathogens are living in a low Reynolds number world dominated by viscosity. For swimming pathogens, the so-called scallop theorem determines which kinds of shape changes can lead to productive motility. For crawling or gliding cells, the main resistance to movement comes from protein friction at the cell-environment interface. Viruses and pathogenic bacteria can also exploit intracellular host processes such as actin polymerization and motor-based transport, if they present the appropriate factors on their surfaces. Similar to cancer cells that also tend to cross various barriers, pathogens often combine several of these strategies in order to increase their motility and therefore their chances to replicate and spread.}, } @article {pmid26452005, year = {2015}, author = {Emetere, ME and Akinyemi, ML and Akin-Ojo, O}, title = {Parametric retrieval model for estimating aerosol size distribution via the AERONET, LAGOS station.}, journal = {Environmental pollution (Barking, Essex : 1987)}, volume = {207}, number = {}, pages = {381-390}, doi = {10.1016/j.envpol.2015.09.047}, pmid = {26452005}, issn = {1873-6424}, mesh = {Aerosols ; Air Pollutants/*analysis ; Atmosphere ; Environmental Monitoring/*methods ; *Models, Theoretical ; Nigeria ; Particle Size ; Particulate Matter/*analysis ; Seasons ; Weather ; Wind ; }, abstract = {The size characteristics of atmospheric aerosol over the tropical region of Lagos, Southern Nigeria were investigated using two years of continuous spectral aerosol optical depth measurements via the AERONET station for four major bands i.e. blue, green, red and infrared. Lagos lies within the latitude of 6.465°N and longitude of 3.406°E. Few systems of dispersion model was derived upon specified conditions to solve challenges on aerosols size distribution within the Stokes regime. The dispersion model was adopted to derive an aerosol size distribution (ASD) model which is in perfect agreement with existing model. The parametric nature of the formulated ASD model shows the independence of each band to determine the ASD over an area. The turbulence flow of particulates over the area was analyzed using the unified number (Un). A comparative study via the aid of the Davis automatic weather station was carried out on the Reynolds number, Knudsen number and the Unified number. The Reynolds and Unified number were more accurate to describe the atmospheric fields of the location. The aerosols loading trend in January to March (JFM) and August to October (ASO) shows a yearly 15% retention of aerosols in the atmosphere. The effect of the yearly aerosol retention can be seen to partly influence the aerosol loadings between October and February.}, } @article {pmid26451802, year = {2015}, author = {Phillips, N and Knowles, K and Bomphrey, RJ}, title = {The effect of aspect ratio on the leading-edge vortex over an insect-like flapping wing.}, journal = {Bioinspiration & biomimetics}, volume = {10}, number = {5}, pages = {056020}, doi = {10.1088/1748-3190/10/5/056020}, pmid = {26451802}, issn = {1748-3190}, mesh = {Animals ; Biomimetics/*instrumentation ; Computer Simulation ; Computer-Aided Design ; Equipment Failure Analysis ; Flight, Animal/*physiology ; Insecta/*physiology ; *Models, Biological ; Rheology/methods ; Robotics/*instrumentation ; Shear Strength/physiology ; Stress, Mechanical ; Viscosity ; Wings, Animal/*physiology ; }, abstract = {Insect wing shapes are diverse and a renowned source of inspiration for the new generation of autonomous flapping vehicles, yet the aerodynamic consequences of varying geometry is not well understood. One of the most defining and aerodynamically significant measures of wing shape is the aspect ratio, defined as the ratio of wing length (R) to mean wing chord (c). We investigated the impact of aspect ratio, AR, on the induced flow field around a flapping wing using a robotic device. Rigid rectangular wings ranging from AR = 1.5 to 7.5 were flapped with insect-like kinematics in air with a constant Reynolds number (Re) of 1400, and a dimensionless stroke amplitude of 6.5c (number of chords traversed by the wingtip). Pseudo-volumetric, ensemble-averaged, flow fields around the wings were captured using particle image velocimetry at 11 instances throughout simulated downstrokes. Results confirmed the presence of a high-lift, separated flow field with a leading-edge vortex (LEV), and revealed that the conical, primary LEV grows in size and strength with increasing AR. In each case, the LEV had an arch-shaped axis with its outboard end originating from a focus-sink singularity on the wing surface near the tip. LEV detachment was observed for AR > 1.5 around mid-stroke at ~70% span, and initiated sooner over higher aspect ratio wings. At AR > 3 the larger, stronger vortex persisted under the wing surface well into the next half-stroke leading to a reduction in lift. Circulatory lift attributable to the LEV increased with AR up to AR = 6. Higher aspect ratios generated proportionally less lift distally because of LEV breakdown, and also less lift closer to the wing root due to the previous LEV's continuing presence under the wing. In nature, insect wings go no higher than AR ~ 5, likely in part due to architectural and physiological constraints but also because of the reducing aerodynamic benefits of high AR wings.}, } @article {pmid26451559, year = {2015}, author = {Linkmann, MF and Morozov, A}, title = {Sudden Relaminarization and Lifetimes in Forced Isotropic Turbulence.}, journal = {Physical review letters}, volume = {115}, number = {13}, pages = {134502}, doi = {10.1103/PhysRevLett.115.134502}, pmid = {26451559}, issn = {1079-7114}, abstract = {We demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from a chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase superexponentially with the Reynolds number. Our results suggest that both isotropic turbulence and wall-bounded shear flows qualitatively share the same phase-space dynamics.}, } @article {pmid26447541, year = {2015}, author = {Lucas, KN and Thornycroft, PJ and Gemmell, BJ and Colin, SP and Costello, JH and Lauder, GV}, title = {Effects of non-uniform stiffness on the swimming performance of a passively-flexing, fish-like foil model.}, journal = {Bioinspiration & biomimetics}, volume = {10}, number = {5}, pages = {056019}, doi = {10.1088/1748-3190/10/5/056019}, pmid = {26447541}, issn = {1748-3190}, mesh = {Animal Fins/*physiology ; Animals ; Biomimetics/*methods ; Computer Simulation ; Elastic Modulus/physiology ; Fishes/*physiology ; *Models, Biological ; Rheology/methods ; Robotics/*methods ; Shear Strength/physiology ; Stress, Mechanical ; Swimming/*physiology ; Viscosity ; }, abstract = {Simple mechanical models emulating fish have been used recently to enable targeted study of individual factors contributing to swimming locomotion without the confounding complexity of the whole fish body. Yet, unlike these uniform models, the fish body is notable for its non-uniform material properties. In particular, flexural stiffness decreases along the fish's anterior-posterior axis. To identify the role of non-uniform bending stiffness during fish-like propulsion, we studied four foil model configurations made by adhering layers of plastic sheets to produce discrete regions of high (5.5 × 10(-5) Nm(2)) and low (1.9 × 10(-5) Nm(2)) flexural stiffness of biologically-relevant magnitudes. This resulted in two uniform control foils and two foils with anterior regions of high stiffness and posterior regions of low stiffness. With a mechanical flapping foil controller, we measured forces and torques in three directions and quantified swimming performance under both heaving (no pitch) and constant 0° angle of attack programs. Foils self-propelled at Reynolds number 21 000-115 000 and Strouhal number ∼0.20-0.25, values characteristic of fish locomotion. Although previous models have emphasized uniform distributions and heaving motions, the combination of non-uniform stiffness distributions and 0° angle of attack pitching program was better able to reproduce the kinematics of freely-swimming fish. This combination was likewise crucial in maximizing swimming performance and resulted in high self-propelled speeds at low costs of transport and large thrust coefficients at relatively high efficiency. Because these metrics were not all maximized together, selection of the 'best' stiffness distribution will depend on actuation constraints and performance goals. These improved models enable more detailed, accurate analyses of fish-like swimming.}, } @article {pmid26446009, year = {2016}, author = {Martin, BA and Yiallourou, TI and Pahlavian, SH and Thyagaraj, S and Bunck, AC and Loth, F and Sheffer, DB and Kröger, JR and Stergiopulos, N}, title = {Inter-operator Reliability of Magnetic Resonance Image-Based Computational Fluid Dynamics Prediction of Cerebrospinal Fluid Motion in the Cervical Spine.}, journal = {Annals of biomedical engineering}, volume = {44}, number = {5}, pages = {1524-1537}, doi = {10.1007/s10439-015-1449-6}, pmid = {26446009}, issn = {1573-9686}, support = {R13 NS063446/NS/NINDS NIH HHS/United States ; R15 NS071455/NS/NINDS NIH HHS/United States ; 492 1R15NS071455-01/NS/NINDS NIH HHS/United States ; }, mesh = {*Arnold-Chiari Malformation/diagnostic imaging/physiopathology ; *Cerebrospinal Fluid ; *Cervical Cord/diagnostic imaging/physiopathology ; Female ; Humans ; *Magnetic Resonance Imaging ; Male ; Motion ; Observer Variation ; }, abstract = {For the first time, inter-operator dependence of MRI based computational fluid dynamics (CFD) modeling of cerebrospinal fluid (CSF) in the cervical spinal subarachnoid space (SSS) is evaluated. In vivo MRI flow measurements and anatomy MRI images were obtained at the cervico-medullary junction of a healthy subject and a Chiari I malformation patient. 3D anatomies of the SSS were reconstructed by manual segmentation by four independent operators for both cases. CFD results were compared at nine axial locations along the SSS in terms of hydrodynamic and geometric parameters. Intraclass correlation (ICC) assessed the inter-operator agreement for each parameter over the axial locations and coefficient of variance (CV) compared the percentage of variance for each parameter between the operators. Greater operator dependence was found for the patient (0.19 < ICC < 0.99) near the craniovertebral junction compared to the healthy subject (ICC > 0.78). For the healthy subject, hydraulic diameter and Womersley number had the least variance (CV = ~2%). For the patient, peak diastolic velocity and Reynolds number had the smallest variance (CV = ~3%). These results show a high degree of inter-operator reliability for MRI-based CFD simulations of CSF flow in the cervical spine for healthy subjects and a lower degree of reliability for patients with Type I Chiari malformation.}, } @article {pmid26439225, year = {2015}, author = {Chen, H and Tang, T and Amirfazli, A}, title = {Fast Liquid Transfer between Surfaces: Breakup of Stretched Liquid Bridges.}, journal = {Langmuir : the ACS journal of surfaces and colloids}, volume = {31}, number = {42}, pages = {11470-11476}, doi = {10.1021/acs.langmuir.5b03292}, pmid = {26439225}, issn = {1520-5827}, abstract = {In this work, a systematic experimental study was performed to understand the fast liquid transfer process between two surfaces. According to the value of the Reynolds number (Re), the fast transfer is divided into two different scenarios, one with negligible inertia effects (Re ≪ 1) and the other with significant inertia effects (Re > 1). For Re ≪ 1, the influences of the capillary number (Ca) and the dimensionless minimum separation (H(min)* = H(min)/V(1/3), where H(min) is the minimum separation between two surfaces and V is the volume of liquid) on the transfer ratio (α, the volume of liquid transferred to the acceptor surface over the total liquid volume) are discussed. On the basis of the roles of each physical parameter, an empirical equation is presented to predict the transfer ratio, α = f(Ca). This equation involves two coefficients which are affected only by the surface contact angles and H(min)* but not by the liquid viscosity or surface tension. When Re > 1, it is shown for the first time that the transfer ratio does not converge to 0.5 with the increase in the stretching speed.}, } @article {pmid26429038, year = {2015}, author = {Słowicka, AM and Wajnryb, E and Ekiel-Jeżewska, ML}, title = {Dynamics of flexible fibers in shear flow.}, journal = {The Journal of chemical physics}, volume = {143}, number = {12}, pages = {124904}, doi = {10.1063/1.4931598}, pmid = {26429038}, issn = {1089-7690}, abstract = {Dynamics of flexible non-Brownian fibers in shear flow at low-Reynolds-number are analyzed numerically for a wide range of the ratios A of the fiber bending force to the viscous drag force. Initially, the fibers are aligned with the flow, and later they move in the plane perpendicular to the flow vorticity. A surprisingly rich spectrum of different modes is observed when the value of A is systematically changed, with sharp transitions between coiled and straightening out modes, period-doubling bifurcations from periodic to migrating solutions, irregular dynamics, and chaos.}, } @article {pmid26428671, year = {2016}, author = {Costa, AP and Xu, X and Khan, MA and Burgess, DJ}, title = {Liposome Formation Using a Coaxial Turbulent Jet in Co-Flow.}, journal = {Pharmaceutical research}, volume = {33}, number = {2}, pages = {404-416}, doi = {10.1007/s11095-015-1798-8}, pmid = {26428671}, issn = {1573-904X}, support = {HHSF223201310117C//PHS HHS/United States ; }, mesh = {Dynamic Light Scattering ; Equipment Design ; Ethanol/chemistry ; Lipids/chemistry ; Liposomes/*chemistry/*ultrastructure ; Particle Size ; Technology, Pharmaceutical/*instrumentation ; Water/chemistry ; }, abstract = {PURPOSE: Liposomes are robust drug delivery systems that have been developed into FDA-approved drug products for several pharmaceutical indications. Direct control in producing liposomes of a particular particle size and particle size distribution is extremely important since liposome size may impact cellular uptake and biodistribution.

METHODS: A device consisting of an injection-port was fabricated to form a coaxial turbulent jet in co-flow that produces liposomes via the ethanol injection method. By altering the injection-port dimensions and flow rates, a fluid flow profile (i.e., flow velocity ratio vs. Reynolds number) was plotted and associated with the polydispersity index of liposomes.

RESULTS: Certain flow conditions produced unilamellar, monodispersed liposomes and the mean particle size was controllable from 25 up to >465 nm. The mean liposome size is highly dependent on the Reynolds number of the mixed ethanol/aqueous phase and independent of the flow velocity ratio.

CONCLUSIONS: The significance of this work is that the Reynolds number is predictive of the liposome particle size, independent of the injection-port dimensions. In addition, a new model describing liposome formation is outlined. The significance of the model is that it relates fluid dynamic properties and lipid-molecule physical properties to the final liposome size.}, } @article {pmid26417381, year = {2015}, author = {Khalafvand, SS and Hung, TK and Ng, EY and Zhong, L}, title = {Kinematic, Dynamic, and Energy Characteristics of Diastolic Flow in the Left Ventricle.}, journal = {Computational and mathematical methods in medicine}, volume = {2015}, number = {}, pages = {701945}, doi = {10.1155/2015/701945}, pmid = {26417381}, issn = {1748-6718}, mesh = {Adult ; Biomechanical Phenomena ; Blood Flow Velocity ; Diastole ; Heart Ventricles/anatomy & histology ; Hemodynamics ; Humans ; Hydrodynamics ; Imaging, Three-Dimensional ; Magnetic Resonance Angiography ; Mitral Valve/physiology ; *Models, Cardiovascular ; *Ventricular Function, Left ; }, abstract = {Blood flow characteristics in the normal left ventricle are studied by using the magnetic resonance imaging, the Navier-Stokes equations, and the work-energy equation. Vortices produced during the mitral valve opening and closing are modeled in a two-dimensional analysis and correlated with temporal variations of the Reynolds number and pressure drop. Low shear stress and net pressures on the mitral valve are obtained for flow acceleration and deceleration. Bernoulli energy flux delivered to blood from ventricular dilation is practically balanced by the energy influx and the rate change of kinetic energy in the ventricle. The rates of work done by shear and energy dissipation are small. The dynamic and energy characteristics of the 2D results are comparable to those of a 3D model.}, } @article {pmid26413565, year = {2015}, author = {Prabu, PM and Padmanaban, KP}, title = {Laminar Wall Jet Flow and Heat Transfer over a Shallow Cavity.}, journal = {TheScientificWorldJournal}, volume = {2015}, number = {}, pages = {926249}, doi = {10.1155/2015/926249}, pmid = {26413565}, issn = {1537-744X}, abstract = {This paper presents the detailed simulation of two-dimensional incompressible laminar wall jet flow over a shallow cavity. The flow characteristics of wall jet with respect to aspect ratio (AR), step length (X u), and Reynolds number (Re) of the shallow cavity are expressed. For higher accuracy, third-order discretization is applied for momentum equation which is solved using QUICK scheme with SIMPLE algorithm for pressure-velocity coupling. Low Reynolds numbers 25, 50, 100, 200, 400, and 600 are assigned for simulation. Results are presented for streamline contour, velocity contour, and vorticity formation at wall and also velocity profiles are reported. The detailed study of vortex formation on shallow cavity region is presented for various AR, X u , and Re conditions which led to key findings as Re increases and vortex formation moves from leading edge to trailing edge of the wall. Distance between vortices increases when the step length (X u) increases. When Re increases, the maximum temperature contour distributions take place in shallow cavity region and highest convection heat transfer is obtained in heated walls. The finite volume code (FLUENT) is used for solving Navier-Stokes equations and GAMBIT for modeling and meshing.}, } @article {pmid26406780, year = {2015}, author = {Shimizu, Y and Ohta, M}, title = {Influence of plaque stiffness on deformation and blood flow patterns in models of stenosis.}, journal = {Biorheology}, volume = {52}, number = {3}, pages = {171-182}, doi = {10.3233/BIR-14016}, pmid = {26406780}, issn = {1878-5034}, mesh = {Arteries/chemistry/physiopathology ; Blood Flow Velocity ; Blood Pressure ; Constriction, Pathologic/*physiopathology ; Humans ; Models, Cardiovascular ; Pulsatile Flow ; Rheology ; *Vascular Stiffness ; }, abstract = {BACKGROUND: Blood flow in stenotic vessels strongly influences the progression of vascular diseases. Plaques in stenotic blood vessels vary in stiffness, which influences plaque behavior and deformation by pressure and flow. Concurrent changes in plaque geometry can, in turn, affect blood flow conditions. Thus, simultaneous studies of blood flow and plaque deformation are needed to fully understand these interactions.

OBJECTIVES: This study aims to identify the change of flow conditions attendant to plaque deformation in a model stenotic vessel.

METHODS: Three plaques of differing stiffness were constructed on a vessel wall using poly (vinyl alcohol) hydrogels (PVA-H) with defined stiffness to facilitate simultaneous observations of blood flow and plaque deformation. Flow patterns were observed using particle image velocimetry (PIV).

RESULTS: Decreases in Reynolds number (Re) with increased plaque deformation suggest that velocity decrease is more critical to establishment of the flow pattern than expansion of the model lumen. Upon exiting the stenosis, the location of the flow reattachment point, shifted further downstream in all models as plaque stiffness decreased and depended on the increase in upstream pressure.

CONCLUSIONS: These results suggest that in addition to luminal area, plaque stiffness should be considered as a measure of the severity of the pathology.}, } @article {pmid26406014, year = {2015}, author = {Wang, C and Si, X and Shen, Y and Zheng, L and Lin, P}, title = {The exterior unsteady viscous flow and heat transfer due to a porous expanding or contracting cylinder.}, journal = {Bio-medical materials and engineering}, volume = {26 Suppl 1}, number = {}, pages = {S279-85}, doi = {10.3233/BME-151315}, pmid = {26406014}, issn = {1878-3619}, mesh = {Body Fluids/*physiology ; Body Temperature/*physiology ; Computer Simulation ; Elastic Modulus/physiology ; Energy Transfer/*physiology ; Humans ; *Models, Biological ; Peristalsis/*physiology ; Porosity ; Pulsatile Flow/*physiology ; Rheology/methods ; Temperature ; Thermal Conductivity ; Viscosity ; }, abstract = {Since the vessels in the biological tissues are characterized by low seepage Reynolds numbers and contracting or expanding walls, more attention is paid on the viscous flow outside the porous pipe with small expansion or contraction. This paper presents a numerical solution of the flow and heat transfer outside an expanding or contracting porous cylinder. The coupled nonlinear similarity equations are solved by Bvp4c, which is a collocation method with MATLAB. The effects of the different physical parameters, namely the permeability Reynolds number,the expansion ratio and the Prandtl number, on the velocity and temperature distribution are obtained and the results are shown graphically.}, } @article {pmid26399987, year = {2016}, author = {Sharp, MK and Diem, AK and Weller, RO and Carare, RO}, title = {Peristalsis with Oscillating Flow Resistance: A Mechanism for Periarterial Clearance of Amyloid Beta from the Brain.}, journal = {Annals of biomedical engineering}, volume = {44}, number = {5}, pages = {1553-1565}, doi = {10.1007/s10439-015-1457-6}, pmid = {26399987}, issn = {1573-9686}, mesh = {*Alzheimer Disease/blood/pathology/physiopathology ; Amyloid beta-Peptides/*blood ; *Cerebral Arteries/metabolism/pathology/physiopathology ; *Cerebrovascular Circulation ; Humans ; *Models, Cardiovascular ; *Pulsatile Flow ; }, abstract = {Alzheimer's disease is characterized by accumulation of amyloid-β (Aβ) in the brain and in the walls of cerebral arteries. The focus of this work is on clearance of Aβ along artery walls, the failure of which may explain the accumulation of Aβ in Alzheimer's disease. Periarterial basement membranes form continuous channels from cerebral capillaries to major arteries on the surface of the brain. Arterial pressure pulses drive peristaltic flow in the basement membranes in the same direction as blood flow. Here we forward the hypothesis that flexible structures within the basement membrane, if oriented such they present greater resistance to forward than retrograde flow, may cause net reverse flow, advecting Aβ along with it. A solution was obtained for peristaltic flow with low Reynolds number, long wavelength compared to channel height and small channel height compared to vessel radius in a Darcy-Brinkman medium representing a square array of cylinders. Results show that retrograde flow is promoted by high cylinder volume fraction and low peristaltic amplitude. A decrease in cylinder concentration and/or an increase in amplitude, both of which may occur during ageing, can reduce retrograde flow or even cause a transition from retrograde to forward flow. Such changes may explain the accumulation of Aβ in the brain and in artery walls in Alzheimer's disease.}, } @article {pmid26396254, year = {2015}, author = {Ober, TJ and Foresti, D and Lewis, JA}, title = {Active mixing of complex fluids at the microscale.}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {112}, number = {40}, pages = {12293-12298}, doi = {10.1073/pnas.1509224112}, pmid = {26396254}, issn = {1091-6490}, abstract = {Mixing of complex fluids at low Reynolds number is fundamental for a broad range of applications, including materials assembly, microfluidics, and biomedical devices. Of these materials, yield stress fluids (and gels) pose the most significant challenges, especially when they must be mixed in low volumes over short timescales. New scaling relationships between mixer dimensions and operating conditions are derived and experimentally verified to create a framework for designing active microfluidic mixers that can efficiently homogenize a wide range of complex fluids. Active mixing printheads are then designed and implemented for multimaterial 3D printing of viscoelastic inks with programmable control of local composition.}, } @article {pmid26395149, year = {2015}, author = {Sousa, PC and Pinho, FT and Oliveira, MS and Alves, MA}, title = {Purely elastic flow instabilities in microscale cross-slot devices.}, journal = {Soft matter}, volume = {11}, number = {45}, pages = {8856-8862}, doi = {10.1039/c5sm01298h}, pmid = {26395149}, issn = {1744-6848}, abstract = {We present an experimental investigation of viscoelastic fluid flow in a cross-slot microgeometry under low Reynolds number flow conditions. By using several viscoelastic fluids, we investigate the effects of the microchannel bounding walls and the polymer solution concentration on the flow patterns. We demonstrate that for concentrated polymer solutions, the flow undergoes a bifurcation above a critical Weissenberg number (Wi) at which the flow becomes asymmetric but remains steady. The appearance of this elastic instability depends on the channel aspect ratio, defined as the ratio between the depth and the width of the channels. At high aspect ratios, when bounding wall effects are reduced, two types of elastic instabilities were observed, one in which the flow becomes asymmetric and steady, followed by a second instability at higher Wi, in which the flow becomes time-dependent. When the aspect ratio decreases, the bounding walls have a stabilizing effect, preventing the occurrence of steady asymmetric flow and postponing the transition to unsteady flow to higher Wi. For less concentrated solutions, the first elastic instability to steady asymmetric flow is absent and only the time-dependent flow instability is observed.}, } @article {pmid26383225, year = {2015}, author = {Vach, PJ and Fratzl, P and Klumpp, S and Faivre, D}, title = {Fast Magnetic Micropropellers with Random Shapes.}, journal = {Nano letters}, volume = {15}, number = {10}, pages = {7064-7070}, doi = {10.1021/acs.nanolett.5b03131}, pmid = {26383225}, issn = {1530-6992}, abstract = {Studying propulsion mechanisms in low Reynolds number fluid has implications for many fields, ranging from the biology of motile microorganisms and the physics of active matter to micromixing in catalysis and micro- and nanorobotics. The propulsion of magnetic micropropellers can be characterized by a dimensionless speed, which solely depends on the propeller geometry for a given axis of rotation. However, this dependence has so far been only investigated for helical propeller shapes, which were assumed to be optimal. In order to explore a larger variety of shapes, we experimentally studied the propulsion properties of randomly shaped magnetic micropropellers. Surprisingly, we found that their dimensionless speeds are high on average, comparable to previously reported nanofabricated helical micropropellers. The highest dimensionless speed we observed is higher than that of any previously reported propeller moving in a low Reynolds number fluid, proving that physical random shape generation can be a viable optimization strategy.}, } @article {pmid26382522, year = {2015}, author = {Gruca, M and Bukowicki, M and Ekiel-Jeżewska, ML}, title = {Periodic and quasiperiodic motions of many particles falling in a viscous fluid.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023026}, doi = {10.1103/PhysRevE.92.023026}, pmid = {26382522}, issn = {1550-2376}, abstract = {The dynamics of regular clusters of many nontouching particles falling under gravity in a viscous fluid at low Reynolds number are analyzed within the point-particle model. The evolution of two families of particle configurations is determined: two or four regular horizontal polygons (called "rings") centered above or below each other. Two rings fall together and periodically oscillate. Four rings usually separate from each other with chaotic scattering. For hundreds of thousands of initial configurations, a map of the cluster lifetime is evaluated in which the long-lasting clusters are centered around periodic solutions for the relative motions, and they are surrounded by regions of chaotic scattering in a similar way to what was observed by Janosi et al. [Phys. Rev. E. 56, 2858 (1997)] for three particles only. These findings suggest that we should consider the existence of periodic orbits as a possible physical mechanism of the existence of unstable clusters of particles falling under gravity in a viscous fluid.}, } @article {pmid26382516, year = {2015}, author = {Takagi, D}, title = {Swimming with stiff legs at low Reynolds number.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023020}, doi = {10.1103/PhysRevE.92.023020}, pmid = {26382516}, issn = {1550-2376}, mesh = {Animals ; Crustacea/physiology ; Extremities/*physiology ; *Models, Biological ; Periodicity ; Swimming/*physiology ; Viscosity ; }, abstract = {Locomotion at low Reynolds number is not possible with cycles of reciprocal motion, an example being the oscillation of a single pair of rigid paddles or legs. Here, I demonstrate the possibility of swimming with two or more pairs of legs. They are assumed to oscillate collectively in a metachronal wave pattern in a minimal model based on slender-body theory for Stokes flow. The model predicts locomotion in the direction of the traveling wave, as commonly observed along the body of free-swimming crustaceans. The displacement of the body and the swimming efficiency depend on the number of legs, the amplitude, and the phase of oscillations. This study shows that paddling legs with distinct orientations and phases offers a simple mechanism for driving flow.}, } @article {pmid26382506, year = {2015}, author = {Schober, J and Schleicher, DR and Federrath, C and Bovino, S and Klessen, RS}, title = {Saturation of the turbulent dynamo.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023010}, doi = {10.1103/PhysRevE.92.023010}, pmid = {26382506}, issn = {1550-2376}, abstract = {The origin of strong magnetic fields in the Universe can be explained by amplifying weak seed fields via turbulent motions on small spatial scales and subsequently transporting the magnetic energy to larger scales. This process is known as the turbulent dynamo and depends on the properties of turbulence, i.e., on the hydrodynamical Reynolds number and the compressibility of the gas, and on the magnetic diffusivity. While we know the growth rate of the magnetic energy in the linear regime, the saturation level, i.e., the ratio of magnetic energy to turbulent kinetic energy that can be reached, is not known from analytical calculations. In this paper we present a scale-dependent saturation model based on an effective turbulent resistivity which is determined by the turnover time scale of turbulent eddies and the magnetic energy density. The magnetic resistivity increases compared to the Spitzer value and the effective scale on which the magnetic energy spectrum is at its maximum moves to larger spatial scales. This process ends when the peak reaches a characteristic wave number k☆ which is determined by the critical magnetic Reynolds number. The saturation level of the dynamo also depends on the type of turbulence and differs for the limits of large and small magnetic Prandtl numbers Pm. With our model we find saturation levels between 43.8% and 1.3% for Pm≫1 and between 2.43% and 0.135% for Pm≪1, where the higher values refer to incompressible turbulence and the lower ones to highly compressible turbulence.}, } @article {pmid26382503, year = {2015}, author = {Roisman, IV and Criscione, A and Tropea, C and Mandal, DK and Amirfazli, A}, title = {Dislodging a sessile drop by a high-Reynolds-number shear flow at subfreezing temperatures.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023007}, doi = {10.1103/PhysRevE.92.023007}, pmid = {26382503}, issn = {1550-2376}, mesh = {Air Movements ; *Models, Theoretical ; *Temperature ; Viscosity ; Wettability ; }, abstract = {The drop, exposed to an air flow parallel to the substrate, starts to dislodge when the air velocity reaches some threshold value, which depends on the substrate wetting properties and drop volume. In this study the critical air velocity is measured for different drop volumes, on substrates of various wettabilities. The substrate initial temperatures varied between the normal room temperature (24.5∘C) and subfreezing temperatures (-5∘C and -1∘C). The physics of the drop did not change at the subfreezing temperatures of the substrates, which clearly indicates that the drop does not freeze and remains liquid for a relatively long time. During this time solidification is not initiated, neither by the air flow nor by mechanical disturbances. An approximate theoretical model is proposed that allows estimation of the aerodynamic forces acting on the sessile drop. The model is valid for the case when the drop height is of the same order as the thickness of the viscous boundary in the airflow, but the inertial effects are still dominant. Such a situation, relevant to many practical applications, was never modeled before. The theoretical predictions for the critical velocity of drop dislodging agree well with the experimental data for both room temperature and lower temperatures of the substrates.}, } @article {pmid26382501, year = {2015}, author = {Nath, SK and Mukhopadhyay, B}, title = {Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023005}, doi = {10.1103/PhysRevE.92.023005}, pmid = {26382501}, issn = {1550-2376}, abstract = {We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as magnetorotational instability (MRI), is responsible for transporting matter in the presence of a weak magnetic field. However, there are some shortcomings, which question the effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g., transient growth (TG), can play an important role in bringing nonlinearity into the system, even at weak magnetic fields. In addition, it should be determined whether MRI or TG is primarily responsible for revealing nonlinearity in order to make the flow turbulent. Our results prove explicitly that the flows with a high Reynolds number (Re), which is the case for realistic astrophysical accretion disks, exhibit nonlinearity via TG of perturbation modes faster than that by modes producing MRI. For a fixed wave vector, MRI dominates over transient effects only at low Re, lower than the value expected to be in astrophysical accretion disks, and low magnetic fields. This calls into serious question the (overall) persuasiveness of MRI in astrophysical accretion disks.}, } @article {pmid26382500, year = {2015}, author = {Man, Y and Lauga, E}, title = {Phase-separation models for swimming enhancement in complex fluids.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023004}, doi = {10.1103/PhysRevE.92.023004}, pmid = {26382500}, issn = {1550-2376}, mesh = {*Models, Biological ; Rheology ; *Swimming ; Viscosity ; }, abstract = {Swimming cells often have to self-propel through fluids displaying non-Newtonian rheology. While past theoretical work seems to indicate that stresses arising from complex fluids should systematically hinder low-Reynolds number locomotion, experimental observations suggest that locomotion enhancement is possible. In this paper we propose a physical mechanism for locomotion enhancement of microscopic swimmers in a complex fluid. It is based on the fact that microstructured fluids will generically phase-separate near surfaces, leading to the presence of low-viscosity layers, which promote slip and decrease viscous friction near the surface of the swimmer. We use two models to address the consequence of this phase separation: a nonzero apparent slip length for the fluid and then an explicit modeling of the change of viscosity in a thin layer near the swimmer. Considering two canonical setups for low-Reynolds number locomotion, namely the waving locomotion of a two-dimensional sheet and that of a three-dimensional filament, we show that phase-separation systematically increases the locomotion speeds, possibly by orders of magnitude. We close by confronting our predictions with recent experimental results.}, } @article {pmid26382498, year = {2015}, author = {Mandal, S and Bandopadhyay, A and Chakraborty, S}, title = {Effect of interfacial slip on the cross-stream migration of a drop in an unbounded Poiseuille flow.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023002}, doi = {10.1103/PhysRevE.92.023002}, pmid = {26382498}, issn = {1550-2376}, abstract = {We analyze the motion and deformation of a buoyant drop suspended in an unbounded fluid which is undergoing a quadratic shearing flow at small Reynolds number in the presence of slip at the interface of the drop. The boundary condition at the interface is accounted for by means of a simple Navier slip condition. Expressions for the velocity and the shape deformation of the drop are derived considering small but finite interface deformation, and results are presented for the specific cases of sedimentation, shear flow, and Poiseuille flow with previously reported results as the limiting cases of our general expressions. The presence of interfacial slip is found to markedly affect axial as well as cross-stream migration velocity of the drop in Poiseuille flow. The effect of slip is more prominent for drops with larger viscosity wherein the drop velocity increases. The presence of significant interface slippage always leads to migration of a deformed drop towards the centerline of the channel for any drop-to-medium viscosity ratio, which is in contrast to the case of no slip at the interface, which allows drop migration towards or away from the centerline depending on the viscosity ratio. We obtain the effect of slip on the cross-stream migration time scale, which quantifies the time required to reach a final steady radial position in the channel. The presence of slip at the drop interface leads to a decrease in the cross-stream migration time scale, which further results in faster motion of the drop in the cross-stream direction. Gravity in the presence of Poiseuille flow is shown to affect not only the axial motion, but also the cross-stream migration velocity of the drop; interfacial slip always increases the drop velocities.}, } @article {pmid26382497, year = {2015}, author = {Burnishev, Y and Steinberg, V}, title = {Turbulence and turbulent drag reduction in swirling flow: Inertial versus viscous forcing.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {023001}, doi = {10.1103/PhysRevE.92.023001}, pmid = {26382497}, issn = {1550-2376}, abstract = {We report unexpected results of a drastic difference in the transition to fully developed turbulent and turbulent drag reduction (TDR) regimes and in their properties in a von Karman swirling flow with counter-rotating disks of water-based polymer solutions for viscous (by smooth disks) as well as inertial (by bladed disks) forcing and by tracking just torque Γ(t) and pressure p(t) . For the viscous forcing, just a single TDR regime is found with the transition values of the Reynolds number (Re) Re turb c =Re TDR c ≃(4.8±0.2)×10(5) independent of ϕ , whereas for the inertial forcing two turbulent regimes are revealed. The first transition is to fully developed turbulence, and the second one is to the TDR regime with both Re turb c and Re TDR c depending on polymer concentration ϕ . Both regimes differ by the values of C f and C p , by the scaling exponents of the fundamental turbulent characteristics, by the nonmonotonic dependencies of skewness and flatness of the pressure PDFs on Re, and by the different frequency power spectra of p with the different dependencies of the main vortex peak frequency in the p power spectra on ϕ and Re. Thus our experimental results show the transition to the TDR regime in a von Karman swirling flow for the viscous and inertial forcings in a sharp contrast to the recent experiments [Phys. Fluids 10, 426 (1998); Phys. Rev. E 47, R28(R) (1993); and J. Phys.: Condens. Matter 17, S1195 (2005)] where the transition to TDR is observed in the same swirling flow with counter-rotating disks only for the viscous forcing. The latter result has led its authors to the wrong conclusion that TDR is a solely boundary effect contrary to the inertial forcing associated with the bulk effect, and this conception is currently rather widely accepted in literature.}, } @article {pmid26382334, year = {2015}, author = {Piñeirua, M and Godoy-Diana, R and Thiria, B}, title = {Resistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {2}, pages = {021001}, doi = {10.1103/PhysRevE.92.021001}, pmid = {26382334}, issn = {1550-2376}, abstract = {In this Rapid Communication, we address a crucial point regarding the description of moderate to high Reynolds numbers aquatic swimmers. For decades, swimming animals have been classified in two different families of propulsive mechanisms based on the Reynolds number: the resistive swimmers, using local friction to produce the necessary thrust force for locomotion at low Reynolds number, and the reactive swimmers, lying in the high Reynolds range, and using added mass acceleration (described by perfect fluid theory). However, inertial swimmers are also systems that dissipate energy, due to their finite size, therefore involving strong resistive contributions, even for high Reynolds numbers. Using a complete model for the hydrodynamic forces, involving both reactive and resistive contributions, we revisit here the physical mechanisms responsible for the thrust production of such swimmers. We show, for instance, that the resistive part of the force balance is as crucial as added mass effects in the modeling of the thrust force, especially for elongated species. The conclusions brought by this work may have significant contributions to the understanding of complex swimming mechanisms, especially for the future design of artificial swimmers.}, } @article {pmid26367403, year = {2015}, author = {Mekni, MA and Achour, W and Ben Hassen, A}, title = {New Robbins device to evaluate antimicrobial activity against bacterial biofilms on central venous catheters.}, journal = {La Tunisie medicale}, volume = {93}, number = {3}, pages = {153-157}, pmid = {26367403}, issn = {0041-4131}, mesh = {Anti-Bacterial Agents/*therapeutic use ; Bacteriological Techniques/*instrumentation ; Biofilms/*growth & development ; Central Venous Catheters/*microbiology ; Daptomycin/*therapeutic use ; Humans ; Staphylococcus epidermidis/*physiology ; }, abstract = {BACKGROUND: Layouts of biomedical devices were tightly related with the emergence of Staphylococcus epidermidis as a major cause of nosocomial infections because of its ability to form biofilm on the biomaterial surfaces. This fact led researchers to develop in-vitro models to simulate what is really happening during biofilm formation process in order to have a better understanding of this phenomena and then to control it and to resolve the associated problems. The aim of this paper was to develop a homemade dynamic device based on instruments used in clinical practice, easy to mount, with low coast and with no sophisticated features.

METHODS: used to evaluate this dispositive were hydrodynamic calculation and enumeration of bacterial cells on petri dishes and with real time polymerase chain reaction during simulation of Staphylococcus epidermidis biofilm eradication with daptomycin.

RESULTS: With hydrodynamic calculation, Reynolds number was estimated to be 26.62 corresponding to a perfect laminar flux giving suitable dynamic growth environment for such experiment. Data recovered from cell enumeration with the two methods showed that bacterial colonization of the tested catheter segment was significantly reduced after 24 and 48h of treatment with daptomycin (P<0.01) reflecting a considerable reliability of this device.

CONCLUSION: the simple dispositive developed in this work has shown acceptable hydrodynamic proprieties and good reliability making research on biofilm easy to reach.}, } @article {pmid26362281, year = {2016}, author = {Wang, Q and Othmer, HG}, title = {Computational analysis of amoeboid swimming at low Reynolds number.}, journal = {Journal of mathematical biology}, volume = {72}, number = {7}, pages = {1893-1926}, doi = {10.1007/s00285-015-0925-9}, pmid = {26362281}, issn = {1432-1416}, mesh = {Dictyostelium/*physiology ; *Models, Biological ; Swimming ; }, abstract = {Recent experimental work has shown that eukaryotic cells can swim in a fluid as well as crawl on a substrate. We investigate the swimming behavior of Dictyostelium discoideum amoebae who swim by initiating traveling protrusions at the front that propagate rearward. In our model we prescribe the velocity at the surface of the swimming cell, and use techniques of complex analysis to develop 2D models that enable us to study the fluid-cell interaction. Shapes that approximate the protrusions used by Dictyostelium discoideum can be generated via the Schwarz-Christoffel transformation, and the boundary-value problem that results for swimmers in the Stokes flow regime is then reduced to an integral equation on the boundary of the unit disk. We analyze the swimming characteristics of several varieties of swimming Dictyostelium discoideum amoebae, and discuss how the slenderness of the cell body and the shapes of the protrusion effect the swimming of these cells. The results may provide guidance in designing low Reynolds number swimming models.}, } @article {pmid26353555, year = {2015}, author = {Wang, J and Wang, Y and Chen, Z and Chen, K and Li, B}, title = {The Numerical Simulation of Liquid-Vapor Stratified Flow in Horizontal Metal-Foam Tubes.}, journal = {Journal of nanoscience and nanotechnology}, volume = {15}, number = {4}, pages = {3161-3167}, pmid = {26353555}, issn = {1533-4899}, abstract = {In this paper, a boiling stratified flow model in a metal-foam tube is proposed. First, based on Branuer non-equilibrium gas-liquid interface model, a force balance on the gas-liquid interface in metal-foam is calculated. The shape of the interface of upper gas phase and lower liquid phase in metal foam tube is obtained. As for the lower liquid phase, the energy conservation equations of liquid and metal foam are formulated, which account for porosity and fiber diameter of foam on the basis of non-local thermal equilibrium model (NTEM), respectively. Therefore, a profile of temperature difference between liquid and metal foam can be obtained. For the upper gas phase, an empirical correlation developed by other researchers is utilized to obtain temperature difference between gas phase and solid wall. In addition, the variation of the Reynolds number with increasing mass quality along the flow direction is acquired. Ultimately, an average circumference heat transfer coefficient is calculated. The results of circumference heat transfer coefficient agree well with available experimental data, showing the prediction of the proposed stratified flow model is feasible. The reason resulting in discrepancies between the prediction and experiment data is also illustrated.}, } @article {pmid26353536, year = {2015}, author = {Wei, B and Yang, M and Wang, Z and Xu, H and Zhang, Y}, title = {Flow and Thermal Performance of a Water-Cooled Periodic Transversal Elliptical Microchannel Heat Sink for Chip Cooling.}, journal = {Journal of nanoscience and nanotechnology}, volume = {15}, number = {4}, pages = {3061-3066}, pmid = {26353536}, issn = {1533-4899}, abstract = {Flow and thermal performance of transversal elliptical microchannels were investigated as a passive scheme to enhance the heat transfer performance of laminar fluid flow. The periodic transversal elliptical micro-channel is designed and its pressure drop and heat transfer characteristics in laminar flow are numerically investigated. Based on the comparison with a conventional straight micro- channel having rectangular cross section, it is found that periodic transversal elliptical microchannel not only has great potential to reduce pressure drop but also dramatically enhances heat transfer performance. In addition, when the Reynolds number equals to 192, the pressure drop of the transversal elliptical channel is 36.5% lower than that of the straight channel, while the average Nusselt number is 72.8% higher; this indicates that the overall thermal performance of the periodic transversal elliptical microchannel is superior to the conventional straight microchannel. It is suggested that such transversal elliptical microchannel are attractive candidates for cooling future electronic chips effectively with much lower pressure drop.}, } @article {pmid26347566, year = {2015}, author = {Berthé, R and Lehmann, FO}, title = {Body appendages fine-tune posture and moments in freely manoeuvring fruit flies.}, journal = {The Journal of experimental biology}, volume = {218}, number = {Pt 20}, pages = {3295-3307}, doi = {10.1242/jeb.122408}, pmid = {26347566}, issn = {1477-9145}, mesh = {Animals ; Behavior, Animal/physiology ; Biomechanical Phenomena ; Drosophila melanogaster/*physiology ; Extremities/physiology ; *Flight, Animal ; Posture ; Wings, Animal/physiology ; }, abstract = {The precise control of body posture by turning moments is key to elevated locomotor performance in flying animals. Although elevated moments for body stabilization are typically produced by wing aerodynamics, animals also steer using drag on body appendages, shifting their centre of body mass, and changing moments of inertia caused by active alterations in body shape. To estimate the instantaneous contribution of each of these components for posture control in an insect, we three-dimensionally reconstructed body posture and movements of body appendages in freely manoeuvring fruit flies (Drosophila melanogaster) by high-speed video and experimentally scored drag coefficients of legs and body trunk at low Reynolds number. The results show that the sum of leg- and abdomen-induced yaw moments dominates wing-induced moments during 17% of total flight time but is, on average, 7.2-times (roll, 3.4-times) smaller during manoeuvring. Our data reject a previous hypothesis on synergistic moment support, indicating that drag on body appendages and mass-shift inhibit rather than support turning moments produced by the wings. Numerical modelling further shows that hind leg extension alters the moments of inertia around the three main body axes of the animal by not more than 6% during manoeuvring, which is significantly less than previously reported for other insects. In sum, yaw, pitch and roll steering by body appendages probably fine-tune turning behaviour and body posture, without providing a significant advantage for posture stability and moment support. Motion control of appendages might thus be part of the insect's trimming reflexes, which reduce imbalances in moment generation caused by unilateral wing damage and abnormal asymmetries of the flight apparatus.}, } @article {pmid26347563, year = {2015}, author = {van Bokhorst, E and de Kat, R and Elsinga, GE and Lentink, D}, title = {Feather roughness reduces flow separation during low Reynolds number glides of swifts.}, journal = {The Journal of experimental biology}, volume = {218}, number = {Pt 20}, pages = {3179-3191}, doi = {10.1242/jeb.121426}, pmid = {26347563}, issn = {1477-9145}, mesh = {Animals ; Biomechanical Phenomena ; Birds/anatomy & histology/*physiology ; Feathers/*anatomy & histology ; *Flight, Animal ; *Models, Biological ; Wings, Animal/anatomy & histology/*physiology ; }, abstract = {Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar-turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance.}, } @article {pmid26337704, year = {2015}, author = {Lee, JS and Park, SJ and Lee, JH and Weon, BM and Fezzaa, K and Je, JH}, title = {Origin and dynamics of vortex rings in drop splashing.}, journal = {Nature communications}, volume = {6}, number = {}, pages = {8187}, doi = {10.1038/ncomms9187}, pmid = {26337704}, issn = {2041-1723}, abstract = {A vortex is a flow phenomenon that is very commonly observed in nature. More than a century, a vortex ring that forms during drop splashing has caught the attention of many scientists due to its importance in understanding fluid mixing and mass transport processes. However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we show that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along the drop wall, as demonstrated by a phase diagram established here, with different power-law dependencies of the angular velocities on the Reynolds number. These results provide important insight that allows understanding and modelling any type of vortex rings in nature, beyond just vortex rings during drop splashing.}, } @article {pmid26328576, year = {2015}, author = {Huhn, F and van Rees, WM and Gazzola, M and Rossinelli, D and Haller, G and Koumoutsakos, P}, title = {Quantitative flow analysis of swimming dynamics with coherent Lagrangian vortices.}, journal = {Chaos (Woodbury, N.Y.)}, volume = {25}, number = {8}, pages = {087405}, doi = {10.1063/1.4919784}, pmid = {26328576}, issn = {1089-7682}, abstract = {Undulatory swimmers flex their bodies to displace water, and in turn, the flow feeds back into the dynamics of the swimmer. At moderate Reynolds number, the resulting flow structures are characterized by unsteady separation and alternating vortices in the wake. We use the flow field from simulations of a two-dimensional, incompressible viscous flow of an undulatory, self-propelled swimmer and detect the coherent Lagrangian vortices in the wake to dissect the driving momentum transfer mechanisms. The detected material vortex boundary encloses a Lagrangian control volume that serves to track back the vortex fluid and record its circulation and momentum history. We consider two swimming modes: the C-start escape and steady anguilliform swimming. The backward advection of the coherent Lagrangian vortices elucidates the geometry of the vorticity field and allows for monitoring the gain and decay of circulation and momentum transfer in the flow field. For steady swimming, momentum oscillations of the fish can largely be attributed to the momentum exchange with the vortex fluid. For the C-start, an additionally defined jet fluid region turns out to balance the high momentum change of the fish during the rapid start.}, } @article {pmid26315920, year = {2015}, author = {Broderick, SP and Houston, JG and Walsh, MT}, title = {The influence of the instabilities in modelling arteriovenous junction haemodynamics.}, journal = {Journal of biomechanics}, volume = {48}, number = {13}, pages = {3591-3598}, doi = {10.1016/j.jbiomech.2015.07.038}, pmid = {26315920}, issn = {1873-2380}, mesh = {Arteriovenous Fistula/*physiopathology ; Biomechanical Phenomena ; Blood Pressure ; Computer Simulation ; Humans ; Hydrodynamics ; Models, Biological ; Regional Blood Flow ; }, abstract = {The arteriovenous junction is characterised by high flow rates, large pressure difference and typically a palpable thrill or audible bruit, associated with turbulent flow. However, the arteriovenous junction is frequently studied with the assumption of streamline flow. This assumption is based on the Reynolds number calculation, although other factors can contribute to turbulent generation. In this study, the presence of instabilities is examined and the influencing factors discussed. This was performed using a pseudo-realistic geometry with adapted graft angles, vein diameter, outflow split ratio and graft inlet velocity values. Correlation was performed between steady and unsteady averaged simulation cases with correlation performance ranked. Overall the arteriovenous junction is capable of possessing highly disturbed flows, in which strict modelling requirements are necessary to capture such instabilities and avoid erroneous conclusions. Vein diameter and flow split ratio contribute to turbulent generation, thus Reynolds number cannot be used as a sole turbulent criterion in the arteriovenous junction.}, } @article {pmid26314259, year = {2015}, author = {Krieger, MS and Dias, MA and Powers, TR}, title = {Minimal model for transient swimming in a liquid crystal.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {8}, pages = {94}, doi = {10.1140/epje/i2015-15094-3}, pmid = {26314259}, issn = {1292-895X}, mesh = {Elasticity ; *Hydrodynamics ; Liquid Crystals/*chemistry ; *Models, Theoretical ; Rotation ; Viscosity ; }, abstract = {When a microorganism begins swimming from rest in a Newtonian fluid such as water, it rapidly attains its steady-state swimming speed since changes in the velocity field spread quickly when the Reynolds number is small. However, swimming microorganisms are commonly found or studied in complex fluids. Because these fluids have long relaxation times, the time to attain the steady-state swimming speed can also be long. In this article we study the swimming startup problem in the simplest liquid crystalline fluid: a two-dimensional hexatic liquid crystal film. We study the dependence of startup time on anchoring strength and Ericksen number, which is the ratio of viscous to elastic stresses. For strong anchoring, the fluid flow starts up immediately but the liquid crystal field and swimming velocity attain their sinusoidal steady-state values after a time proportional to the relaxation time of the liquid crystal. When the Ericksen number is high, the behavior is the same as in the strong-anchoring case for any anchoring strength. We also find that the startup time increases with the ratio of the rotational viscosity to the shear viscosity, and then ultimately saturates once the rotational viscosity is much greater than the shear viscosity.}, } @article {pmid26314256, year = {2015}, author = {Felderhof, BU}, title = {Efficient swimming of an assembly of rigid spheres at low Reynolds number.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {8}, pages = {90}, doi = {10.1140/epje/i2015-15090-7}, pmid = {26314256}, issn = {1292-895X}, mesh = {*Hydrodynamics ; Microfluidics ; *Models, Theoretical ; Viscosity ; }, abstract = {The swimming of an assembly of rigid spheres immersed in a viscous fluid of infinite extent is studied in low-Reynolds-number hydrodynamics. The instantaneous swimming velocity and rate of dissipation are expressed in terms of the time-dependent displacements of sphere centers about their collective motion. For small-amplitude swimming with periodically oscillating displacements, optimization of the mean swimming speed at given mean power leads to an eigenvalue problem involving a velocity matrix and a power matrix. The corresponding optimal stroke permits generalization to large-amplitude motion in a model of spheres with harmonic interactions and corresponding actuating forces. The method allows straightforward calculation of the swimming performance of structures modeled as assemblies of interacting rigid spheres. A model of three collinear spheres with motion along the common axis is studied as an example.}, } @article {pmid26298765, year = {2015}, author = {Adams, MC and Hurt, EE and Barbano, DM}, title = {Effect of ceramic membrane channel geometry and uniform transmembrane pressure on limiting flux and serum protein removal during skim milk microfiltration.}, journal = {Journal of dairy science}, volume = {98}, number = {11}, pages = {7527-7543}, doi = {10.3168/jds.2015-9753}, pmid = {26298765}, issn = {1525-3198}, mesh = {Animals ; Blood Proteins/analysis ; Caseins/analysis ; Ceramics/*chemistry ; Filtration ; *Food Handling ; Hydrodynamics ; Hydrogen-Ion Concentration ; Membranes, Artificial ; Milk/*chemistry ; Milk Proteins/analysis ; }, abstract = {Our objectives were to determine the effects of a ceramic microfiltration (MF) membrane's retentate flow channel geometry (round or diamond-shaped) and uniform transmembrane pressure (UTP) on limiting flux (LF) and serum protein (SP) removal during skim milk MF at a temperature of 50°C, a retentate protein concentration of 8.5%, and an average cross-flow velocity of 7 m·s(-1). Performance of membranes with round and diamond flow channels was compared in UTP mode. Performance of the membrane with round flow channels was compared with and without UTP. Using UTP with round flow channel MF membranes increased the LF by 5% when compared with not using UTP, but SP removal was not affected by the use of UTP. Using membranes with round channels instead of diamond-shaped channels in UTP mode increased the LF by 24%. This increase was associated with a 25% increase in Reynolds number and can be explained by lower shear at the vertices of the diamond-shaped channel's surface. The SP removal factor of the diamond channel system was higher than the SP removal factor of the round channel system below the LF. However, the diamond channel system passed more casein into the MF permeate than the round channel system. Because only one batch of each membrane was tested in our study, it was not possible to determine if the differences in protein rejection between channel geometries were due to the membrane design or random manufacturing variation. Despite the lower LF of the diamond channel system, the 47% increase in membrane module surface area of the diamond channel system produced a modular permeate removal rate that was at least 19% higher than the round channel system. Consequently, using diamond channel membranes instead of round channel membranes could reduce some of the costs associated with ceramic MF of skim milk if fewer membrane modules could be used to attain the required membrane area.}, } @article {pmid26296866, year = {2015}, author = {Higuchi, M and Terada, K and Sugano, K}, title = {Coning phenomena under laminar flow.}, journal = {European journal of pharmaceutical sciences : official journal of the European Federation for Pharmaceutical Sciences}, volume = {80}, number = {}, pages = {53-55}, doi = {10.1016/j.ejps.2015.08.004}, pmid = {26296866}, issn = {1879-0720}, mesh = {Nuclear Proteins ; *Pharmacokinetics ; RNA-Binding Proteins ; *Rheology ; Rotation ; Technology, Pharmaceutical/*methods ; }, abstract = {The purpose of the present study was to investigate coning phenomena in the paddle dissolution test under laminar flow (Reynolds number <500). The minimum rotation speed at which the coning phenomena disappear (no coning rpm, NCrpm) was measured in viscous media (23 to 147mPa∙s) using various particles. The exponent values of particle size, density, and viscosity parameters in the Zwietering equation were found to be 0.066, 0.38, and 0.22, respectively. NCrpm was appropriately predicted by the Zwietering equation (average error: 8rpm). These values are very different from those for turbulent flow, suggesting that the main physical forces governing the motion of particles can be different between turbulent flow and laminar flow. This point should be taken into account when understanding the dissolution of drug products in viscous fluids representing the fed state.}, } @article {pmid26278133, year = {2015}, author = {Hassanpourfard, M and Nikakhtari, Z and Ghosh, R and Das, S and Thundat, T and Liu, Y and Kumar, A}, title = {Bacterial floc mediated rapid streamer formation in creeping flows.}, journal = {Scientific reports}, volume = {5}, number = {}, pages = {13070}, doi = {10.1038/srep13070}, pmid = {26278133}, issn = {2045-2322}, mesh = {Bacteria/*metabolism ; Bacterial Adhesion/physiology ; Biofilms ; Elasticity ; Green Fluorescent Proteins/genetics/metabolism ; Microfluidic Analytical Techniques/instrumentation/*methods ; Microscopy, Fluorescence ; Microscopy, Video ; Pseudomonas fluorescens/physiology ; }, abstract = {One of the central puzzles concerning the interaction of low Reynolds number fluid transport with bacterial biomass is the formation of filamentous structures called streamers. In this manuscript, we report our discovery of a new kind of low Re bacterial streamers, which appear from pre-formed bacterial flocs. In sharp contrast to the biofilm-mediated streamers, these streamers form over extremely small timescales (less than a second). Our experiments, carried out in a microchannel with micropillars rely on fluorescence microscopy techniques to illustrate that floc-mediated streamers form when a freely-moving floc adheres to the micropillar wall and gets rapidly sheared by the background flow. We also show that at their inception the deformation of the flocs is dominated by recoverable large strains indicating significant elasticity. These strains subsequently increase tremendously to produce filamentous streamers. Interestingly, we find that these fully formed streamers are not static structures and show viscous response at time scales larger than their formation time scales. Finally, we show that such novel streamer formation can lead to rapid clogging of microfluidic devices.}, } @article {pmid26274284, year = {2015}, author = {Wedin, H and Cherubini, S and Bottaro, A}, title = {Effect of plate permeability on nonlinear stability of the asymptotic suction boundary layer.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {1}, pages = {013022}, doi = {10.1103/PhysRevE.92.013022}, pmid = {26274284}, issn = {1550-2376}, abstract = {The nonlinear stability of the asymptotic suction boundary layer is studied numerically, searching for finite-amplitude solutions that bifurcate from the laminar flow state. By changing the boundary conditions for disturbances at the plate from the classical no-slip condition to more physically sound ones, the stability characteristics of the flow may change radically, both for the linearized as well as the nonlinear problem. The wall boundary condition takes into account the permeability K̂ of the plate; for very low permeability, it is acceptable to impose the classical boundary condition (K̂=0). This leads to a Reynolds number of approximately Re(c)=54400 for the onset of linearly unstable waves, and close to Re(g)=3200 for the emergence of nonlinear solutions [F. A. Milinazzo and P. G. Saffman, J. Fluid Mech. 160, 281 (1985); J. H. M. Fransson, Ph.D. thesis, Royal Institute of Technology, KTH, Sweden, 2003]. However, for larger values of the plate's permeability, the lower limit for the existence of linear and nonlinear solutions shifts to significantly lower Reynolds numbers. For the largest permeability studied here, the limit values of the Reynolds numbers reduce down to Re(c)=796 and Re(g)=294. For all cases studied, the solutions bifurcate subcritically toward lower Re, and this leads to the conjecture that they may be involved in the very first stages of a transition scenario similar to the classical route of the Blasius boundary layer initiated by Tollmien-Schlichting (TS) waves. The stability of these nonlinear solutions is also investigated, showing a low-frequency main unstable mode whose growth rate decreases with increasing permeability and with the Reynolds number, following a power law Re(-ρ), where the value of ρ depends on the permeability coefficient K̂. The nonlinear dynamics of the flow in the vicinity of the computed finite-amplitude solutions is finally investigated by direct numerical simulations, providing a viable scenario for subcritical transition due to TS waves.}, } @article {pmid26274272, year = {2015}, author = {Zhou, L and Rauh, C and Delgado, A}, title = {Multifractal-cascade model for inertial and dissipation ranges based on the wavelet reconstruction method.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {1}, pages = {013010}, doi = {10.1103/PhysRevE.92.013010}, pmid = {26274272}, issn = {1550-2376}, abstract = {The discrete wavelet is introduced to construct the turbulent velocity fields. The simple binary cascade model p model is served as the inertial range model for velocity increments. The dissipation model, which follows Foias et al. [Phys. Fluids A 2, 464 (1990)] takes the form of exp(-gk). The length of inertial and dissipation ranges is computed according to the different construction levels. Based on the binary cascade theory and the proposed dissipation model, the Reynolds number regarding to the cascade process can be estimated. The dissipation rate calculated from the proposed model not only agrees with the existing experiment data, but also suggests that the dissipation rate is not an independent variable with respect to the Reynolds number.}, } @article {pmid26274266, year = {2015}, author = {Rorai, C and Mininni, PD and Pouquet, A}, title = {Stably stratified turbulence in the presence of large-scale forcing.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {92}, number = {1}, pages = {013003}, doi = {10.1103/PhysRevE.92.013003}, pmid = {26274266}, issn = {1550-2376}, abstract = {We perform two high-resolution direct numerical simulations of stratified turbulence for Reynolds number equal to Re≈25000 and Froude number, respectively, of Fr≈0.1 and Fr≈0.03. The flows are forced at large scale and discretized on an isotropic grid of 2048(3) points. Stratification makes the flow anisotropic and introduces two extra characteristic scales with respect to homogeneous isotropic turbulence: the buoyancy scale, L(B), and the Ozmidov scale, ℓ(oz). The former is related to the number of layers that the flow develops in the direction of gravity, and the latter is regarded as the scale at which isotropy is recovered. The values of L(B) and ℓ(oz) depend on the Froude number, and their absolute and relative amplitudes affect the repartition of energy among Fourier modes in ways that are not easy to predict. By contrasting the behavior of the two simulated flows we identify some surprising similarities: After an initial transient the two flows evolve towards comparable values of the kinetic and potential enstrophy and energy dissipation rate. This is the result of the Reynolds number being large enough in both flows for the Ozmidov scale to be resolved. When properly dimensionalized, the energy dissipation rate is compatible with atmospheric observations. Further similarities emerge at large scales: The same ratio between potential and total energy (≈0.1) is spontaneously selected by the flows, and slow modes grow monotonically in both regimes, causing a slow increase of the total energy in time. The axisymmetric total energy spectrum shows a wide variety of spectral slopes as a function of the angle between the imposed stratification and the wave vector. One-dimensional energy spectra computed in the direction parallel to gravity are flat from the forcing up to buoyancy scale. At intermediate scales a ∼k(-3) parallel spectrum develops for the Fr≈0.03 run, whereas for weaker stratification, the saturation spectrum does not have enough scales to develop and instead one observes a power law compatible with Kolmogorov scaling. Finally, the spectrum of helicity is flat until L(B), as observed in the nocturnal planetary boundary layer.}, } @article {pmid26269230, year = {2015}, author = {van Leeuwen, JL and Voesenek, CJ and Müller, UK}, title = {How body torque and Strouhal number change with swimming speed and developmental stage in larval zebrafish.}, journal = {Journal of the Royal Society, Interface}, volume = {12}, number = {110}, pages = {0479}, doi = {10.1098/rsif.2015.0479}, pmid = {26269230}, issn = {1742-5662}, mesh = {Animals ; Biomechanical Phenomena ; *Models, Biological ; Swimming/*physiology ; Zebrafish/*physiology ; }, abstract = {Small undulatory swimmers such as larval zebrafish experience both inertial and viscous forces, the relative importance of which is indicated by the Reynolds number (Re). Re is proportional to swimming speed (vswim) and body length; faster swimming reduces the relative effect of viscous forces. Compared with adults, larval fish experience relatively high (mainly viscous) drag during cyclic swimming. To enhance thrust to an equally high level, they must employ a high product of tail-beat frequency and (peak-to-peak) amplitude fAtail, resulting in a relatively high fAtail/vswim ratio (Strouhal number, St), and implying relatively high lateral momentum shedding and low propulsive efficiency. Using kinematic and inverse-dynamics analyses, we studied cyclic swimming of larval zebrafish aged 2-5 days post-fertilization (dpf). Larvae at 4-5 dpf reach higher f (95 Hz) and Atail (2.4 mm) than at 2 dpf (80 Hz, 1.8 mm), increasing swimming speed and Re, indicating increasing muscle powers. As Re increases (60 → 1400), St (2.5 → 0.72) decreases nonlinearly towards values of large swimmers (0.2-0.6), indicating increased propulsive efficiency with vswim and age. Swimming at high St is associated with high-amplitude body torques and rotations. Low propulsive efficiencies and large yawing amplitudes are unavoidable physical constraints for small undulatory swimmers.}, } @article {pmid26267247, year = {2015}, author = {Rashidi, MM and Freidoonimehr, N and Momoniat, E and Rostami, B}, title = {Study of Nonlinear MHD Tribological Squeeze Film at Generalized Magnetic Reynolds Numbers Using DTM.}, journal = {PloS one}, volume = {10}, number = {8}, pages = {e0135004}, doi = {10.1371/journal.pone.0135004}, pmid = {26267247}, issn = {1932-6203}, mesh = {*Models, Theoretical ; }, abstract = {In the current article, a combination of the differential transform method (DTM) and Padé approximation method are implemented to solve a system of nonlinear differential equations modelling the flow of a Newtonian magnetic lubricant squeeze film with magnetic induction effects incorporated. Solutions for the transformed radial and tangential momentum as well as solutions for the radial and tangential induced magnetic field conservation equations are determined. The DTM-Padé combined method is observed to demonstrate excellent convergence, stability and versatility in simulating the magnetic squeeze film problem. The effects of involved parameters, i.e. squeeze Reynolds number (N1), dimensionless axial magnetic force strength parameter (N2), dimensionless tangential magnetic force strength parameter (N3), and magnetic Reynolds number (Rem) are illustrated graphically and discussed in detail. Applications of the study include automotive magneto-rheological shock absorbers, novel aircraft landing gear systems and biological prosthetics.}, } @article {pmid26252657, year = {2015}, author = {Nabawy, MR and Crowthe, WJ}, title = {A Quasi-Steady Lifting Line Theory for Insect-Like Hovering Flight.}, journal = {PloS one}, volume = {10}, number = {8}, pages = {e0134972}, doi = {10.1371/journal.pone.0134972}, pmid = {26252657}, issn = {1932-6203}, mesh = {Animals ; Biomechanical Phenomena ; Computer Simulation ; Flight, Animal/*physiology ; Insecta/*physiology ; *Models, Biological ; Time Factors ; Wings, Animal/anatomy & histology/physiology ; }, abstract = {A novel lifting line formulation is presented for the quasi-steady aerodynamic evaluation of insect-like wings in hovering flight. The approach allows accurate estimation of aerodynamic forces from geometry and kinematic information alone and provides for the first time quantitative information on the relative contribution of induced and profile drag associated with lift production for insect-like wings in hover. The main adaptation to the existing lifting line theory is the use of an equivalent angle of attack, which enables capture of the steady non-linear aerodynamics at high angles of attack. A simple methodology to include non-ideal induced effects due to wake periodicity and effective actuator disc area within the lifting line theory is included in the model. Low Reynolds number effects as well as the edge velocity correction required to account for different wing planform shapes are incorporated through appropriate modification of the wing section lift curve slope. The model has been successfully validated against measurements from revolving wing experiments and high order computational fluid dynamics simulations. Model predicted mean lift to weight ratio results have an average error of 4% compared to values from computational fluid dynamics for eight different insect cases. Application of an unmodified linear lifting line approach leads on average to a 60% overestimation in the mean lift force required for weight support, with most of the discrepancy due to use of linear aerodynamics. It is shown that on average for the eight insects considered, the induced drag contributes 22% of the total drag based on the mean cycle values and 29% of the total drag based on the mid half-stroke values.}, } @article {pmid26252016, year = {2015}, author = {Zhang, C and Hedrick, TL and Mittal, R}, title = {Centripetal Acceleration Reaction: An Effective and Robust Mechanism for Flapping Flight in Insects.}, journal = {PloS one}, volume = {10}, number = {8}, pages = {e0132093}, doi = {10.1371/journal.pone.0132093}, pmid = {26252016}, issn = {1932-6203}, mesh = {*Acceleration ; Animals ; Biomechanical Phenomena ; Computer Simulation ; Flight, Animal/*physiology ; Models, Biological ; Moths/*physiology ; Time Factors ; Wings, Animal/*physiology ; }, abstract = {Despite intense study by physicists and biologists, we do not fully understand the unsteady aerodynamics that relate insect wing morphology and kinematics to lift generation. Here, we formulate a force partitioning method (FPM) and implement it within a computational fluid dynamic model to provide an unambiguous and physically insightful division of aerodynamic force into components associated with wing kinematics, vorticity, and viscosity. Application of the FPM to hawkmoth and fruit fly flight shows that the leading-edge vortex is the dominant mechanism for lift generation for both these insects and contributes between 72-85% of the net lift. However, there is another, previously unidentified mechanism, the centripetal acceleration reaction, which generates up to 17% of the net lift. The centripetal acceleration reaction is similar to the classical inviscid added-mass in that it depends only on the kinematics (i.e. accelerations) of the body, but is different in that it requires the satisfaction of the no-slip condition, and a combination of tangential motion and rotation of the wing surface. Furthermore, the classical added-mass force is identically zero for cyclic motion but this is not true of the centripetal acceleration reaction. Furthermore, unlike the lift due to vorticity, centripetal acceleration reaction lift is insensitive to Reynolds number and to environmental flow perturbations, making it an important contributor to insect flight stability and miniaturization. This force mechanism also has broad implications for flow-induced deformation and vibration, underwater locomotion and flows involving bubbles and droplets.}, } @article {pmid26244665, year = {2015}, author = {Yu, X and Sun, Z and Huang, R and Zhang, Y and Huang, Y}, title = {A Thermal Equilibrium Analysis of Line Contact Hydrodynamic Lubrication Considering the Influences of Reynolds Number, Load and Temperature.}, journal = {PloS one}, volume = {10}, number = {8}, pages = {e0134806}, doi = {10.1371/journal.pone.0134806}, pmid = {26244665}, issn = {1932-6203}, mesh = {Computer Simulation ; Convection ; Friction ; *Hydrodynamics ; *Lubrication ; Models, Chemical ; Temperature ; Weight-Bearing ; }, abstract = {Thermal effects such as conduction, convection and viscous dissipation are important to lubrication performance, and they vary with the friction conditions. These variations have caused some inconsistencies in the conclusions of different researchers regarding the relative contributions of these thermal effects. To reveal the relationship between the contributions of the thermal effects and the friction conditions, a steady-state THD analysis model was presented. The results indicate that the contribution of each thermal effect sharply varies with the Reynolds number and temperature. Convective effect could be dominant under certain conditions. Additionally, the accuracy of some simplified methods of thermo-hydrodynamic analysis is further discussed.}, } @article {pmid26226349, year = {2015}, author = {Maertens, AP and Triantafyllou, MS and Yue, DK}, title = {Efficiency of fish propulsion.}, journal = {Bioinspiration & biomimetics}, volume = {10}, number = {4}, pages = {046013}, doi = {10.1088/1748-3190/10/4/046013}, pmid = {26226349}, issn = {1748-3190}, mesh = {Animals ; Computer Simulation ; Energy Metabolism/*physiology ; Energy Transfer/*physiology ; Fishes/*physiology ; Friction ; *Models, Biological ; Rheology/*methods ; Shear Strength/physiology ; Stress, Mechanical ; Viscosity ; }, abstract = {The system efficiency of a self-propelled flexible body is ill-defined, hence we introduce the concept of quasi-propulsive efficiency, defined as the ratio of the power needed to tow a body in rigid-straight condition over the power it requires for self-propulsion, both measured for the same speed. Through examples we show that the quasi-propulsive efficiency is a rational non-dimensional metric of the propulsive fitness of fish and fish-like mechanisms, consistent with the goal to minimize fuel consumption under size and velocity constraints. We perform two-dimensional viscous simulations and apply the concept of quasi-propulsive efficiency to illustrate and discuss the efficiency of two-dimensional undulating foils employing first carangiform and then anguilliform kinematics. We show that low efficiency may be due to adverse body-propulsor hydrodynamic interactions, which cannot be accounted for by an increase in friction drag, as done previously, since at the Reynolds number Re = 5 000 considered in the simulations, pressure is a major contributor to both thrust and drag.}, } @article {pmid26196807, year = {2015}, author = {Linkmann, MF and Berera, A and McComb, WD and McKay, ME}, title = {Nonuniversality and Finite Dissipation in Decaying Magnetohydrodynamic Turbulence.}, journal = {Physical review letters}, volume = {114}, number = {23}, pages = {235001}, doi = {10.1103/PhysRevLett.114.235001}, pmid = {26196807}, issn = {1079-7114}, abstract = {A model equation for the Reynolds number dependence of the dimensionless dissipation rate in freely decaying homogeneous magnetohydrodynamic turbulence in the absence of a mean magnetic field is derived from the real-space energy balance equation, leading to Cϵ=Cϵ,∞+C/R-+O(1/R-(2)), where R- is a generalized Reynolds number. The constant Cϵ,∞ describes the total energy transfer flux. This flux depends on magnetic and cross helicities, because these affect the nonlinear transfer of energy, suggesting that the value of Cϵ,∞ is not universal. Direct numerical simulations were conducted on up to 2048(3) grid points, showing good agreement between data and the model. The model suggests that the magnitude of cosmological-scale magnetic fields is controlled by the values of the vector field correlations. The ideas introduced here can be used to derive similar model equations for other turbulent systems.}, } @article {pmid26195761, year = {2015}, author = {Wang, J and Li, Q and E, W}, title = {Study of the instability of the Poiseuille flow using a thermodynamic formalism.}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {112}, number = {31}, pages = {9518-9523}, doi = {10.1073/pnas.1501288112}, pmid = {26195761}, issn = {1091-6490}, mesh = {Friction ; Kinetics ; Magnetic Phenomena ; Models, Theoretical ; *Rheology ; Skin Physiological Phenomena ; Solutions ; Thermodynamics ; }, abstract = {The stability of the plane Poiseuille flow is analyzed using a thermodynamic formalism by considering the deterministic Navier-Stokes equation with Gaussian random initial data. A unique critical Reynolds number, Rec ≈ 2,332, at which the probability of observing puffs in the solution changes from 0 to 1, is numerically demonstrated to exist in the thermodynamic limit and is found to be independent of the noise amplitude. Using the puff density as the macrostate variable, the free energy of such a system is computed and analyzed. The puff density approaches zero as the critical Reynolds number is approached from above, signaling a continuous transition despite the fact that the bifurcation is subcritical for a finite-sized system. An action function is found for the probability of observing puffs in a small subregion of the flow, and this action function depends only on the Reynolds number. The strategy used here should be applicable to a wide range of other problems exhibiting subcritical instabilities.}, } @article {pmid26180066, year = {2015}, author = {Gemmell, BJ and Jiang, H and Buskey, EJ}, title = {A tale of the ciliate tail: investigation into the adaptive significance of this sub-cellular structure.}, journal = {Proceedings. Biological sciences}, volume = {282}, number = {1812}, pages = {20150770}, doi = {10.1098/rspb.2015.0770}, pmid = {26180066}, issn = {1471-2954}, mesh = {Biomechanical Phenomena ; Ciliophora/*physiology ; *Escape Reaction ; *Hydrodynamics ; Species Specificity ; Swimming ; Video Recording ; }, abstract = {Ciliates can form an important link between the microbial loop and higher trophic levels primarily through consumption by copepods. This high predation pressure has resulted in a number of ciliate species developing rapid escape swimming behaviour. Several species of these escaping ciliates also possess a long contractile tail for which the functionality remains unresolved. We use high-speed video, specialized optics and novel fluid visualization tools to evaluate the role of this contractile appendage in two free-swimming ciliates, Pseudotontonia sp. and Tontonia sp., and compare the performance to escape swimming behaviour of a non-tailed species, Strobilidium sp. Here, we show that 'tailed' species respond to hydrodynamic disturbances with extremely short response latencies (less than or equal to 0.89 ms) by rapidly contracting the tail which carries the cell body 2-4 cell diameters within a few milliseconds. This provides an advantage over non-tailed species during the critical first 10-30 ms of an escape. Two small, short-lived vortex rings are created during contraction of the tail. The flow imposed by the ciliate jumping can be described as two well-separated impulsive Stokeslets and the overall flow attenuates spatially as r(-3). The high initial velocities and spatio-temporal arrangement of vortices created by tail contractions appear to provide a means for rapid escape as well as hydrodynamic 'camouflage' against fast striking, mechanoreceptive predators such as copepods.}, } @article {pmid26179936, year = {2015}, author = {Murakami, R and Tsai, CH and Kaneko, M and Sakuma, S and Arai, F}, title = {Cell pinball: phenomenon and mechanism of inertia-like cell motion in a microfluidic channel.}, journal = {Lab on a chip}, volume = {15}, number = {16}, pages = {3307-3313}, doi = {10.1039/c5lc00535c}, pmid = {26179936}, issn = {1473-0189}, mesh = {Biomechanical Phenomena ; Cell Movement ; Erythrocytes/*cytology/metabolism ; Humans ; Microfluidic Analytical Techniques/instrumentation/*methods ; Microscopy, Confocal ; Microspheres ; Sodium Chloride/chemistry ; }, abstract = {An unexpected phenomenon of red blood cells bouncing back and forth between the walls inside a microfluidic channel was observed during experiments, and is presented as "Cell Pinball" in this paper. In general, cells in a microfluidic environment are supposed to move along the streamlines parallel to the channel walls when the Reynolds number is small, and the inertia of the cells becomes negligible. However, the cell pinball presented in this paper does not only move along the streamlines but also moves across the channel with the velocity component perpendicular to the streamlines while the Reynolds number is only 0.74. Furthermore, the motion in the direction perpendicular to the streamlines reverses when the cell pinball hits a wall as it "bounces" at the wall. This phenomenon caught our attention and is investigated with both microbead visualization and confocal microscopy. Consistent patterns of rotation with respect to the direction of motion are observed. A kinematic model is proposed to interpret the phenomenon, and it is believed that the phenomenon is caused by the separation of the centroid of the cell and the contact point. The model successfully interprets the features of cell pinball, and the estimated separation between the centroid and the contact point is presented.}, } @article {pmid26172798, year = {2015}, author = {Piedra, S and Ramos, E and Herrera, JR}, title = {Dynamics of two-dimensional bubbles.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {6}, pages = {063013}, doi = {10.1103/PhysRevE.91.063013}, pmid = {26172798}, issn = {1550-2376}, abstract = {The dynamics of two-dimensional bubbles ascending under the influence of buoyant forces is numerically studied with a one-fluid model coupled with the front-tracking technique. The bubble dynamics are described by recording the position, shape, and orientation of the bubbles as functions of time. The qualitative properties of the bubbles and their terminal velocities are described in terms of the Eötvos (ratio of buoyancy to surface tension) and Archimedes numbers (ratio of buoyancy to viscous forces). The terminal Reynolds number result from the balance of buoyancy and drag forces and, consequently, is not an externally fixed parameter. In the cases that yield small Reynolds numbers, the bubbles follow straight paths and the wake is steady. A more interesting behavior is found at high Reynolds numbers where the bubbles follow an approximately periodic zigzag trajectory and an unstable wake with properties similar to the Von Karman vortex street is formed. The dynamical features of the motion of single bubbles are compared to experimental observations of air bubbles ascending in a water-filled Hele-Shaw cell. Although the comparison is not strictly valid in the sense that the effect of the lateral walls is not incorporated in the model, most of the dynamical properties observed are in good qualitative agreement with the numerical calculations. Hele-Shaw cells with different gaps have been used to determine the degree of approximation of the numerical calculation. It is found that for the relation between the terminal Reynolds number and the Archimedes number, the numerical calculations are closer to the observations of bubble dynamics in Hele-Shaw cells of larger gaps.}, } @article {pmid26172790, year = {2015}, author = {Cadot, O and Evrard, A and Pastur, L}, title = {Imperfect supercritical bifurcation in a three-dimensional turbulent wake.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {6}, pages = {063005}, doi = {10.1103/PhysRevE.91.063005}, pmid = {26172790}, issn = {1550-2376}, abstract = {The turbulent wake of a square-back body exhibits a strong bimodal behavior. The wake randomly undergoes symmetry-breaking reversals between two mirror asymmetric steady modes [reflectional symmetry-breaking (RSB) modes]. The characteristic time for reversals is about 2 or 3 orders of magnitude larger than the natural time for vortex shedding. Studying the effects of the proximity of a ground wall together with the Reynolds number, it is shown that the bimodal behavior is the result of an imperfect pitchfork bifurcation. The RSB modes correspond to the two stable bifurcated branches resulting from an instability of the stable symmetric wake. An attempt to stabilize the unstable symmetric wake is investigated using a passive control technique. Although the controlled wake still exhibits strong fluctuations, the bimodal behavior is suppressed and the drag reduced. This promising experiment indicates the possible existence of an unstable solution branch corresponding to a reflectional symmetry preserved (RSP) mode. This work is encouraging to develop a control strategy based on a stabilization of this RSP mode to reduce mean drag and lateral force fluctuations.}, } @article {pmid26163996, year = {2016}, author = {Elcner, J and Lizal, F and Jedelsky, J and Jicha, M and Chovancova, M}, title = {Numerical investigation of inspiratory airflow in a realistic model of the human tracheobronchial airways and a comparison with experimental results.}, journal = {Biomechanics and modeling in mechanobiology}, volume = {15}, number = {2}, pages = {447-469}, doi = {10.1007/s10237-015-0701-1}, pmid = {26163996}, issn = {1617-7940}, mesh = {Biomechanical Phenomena ; Bronchi/*physiology ; Humans ; *Models, Biological ; *Numerical Analysis, Computer-Assisted ; Pulmonary Ventilation/*physiology ; *Respiration ; Time Factors ; Trachea/*physiology ; }, abstract = {In this article, the results of numerical simulations using computational fluid dynamics (CFD) and a comparison with experiments performed with phase Doppler anemometry are presented. The simulations and experiments were conducted in a realistic model of the human airways, which comprised the throat, trachea and tracheobronchial tree up to the fourth generation. A full inspiration/expiration breathing cycle was used with tidal volumes 0.5 and 1 L, which correspond to a sedentary regime and deep breath, respectively. The length of the entire breathing cycle was 4 s, with inspiration and expiration each lasting 2 s. As a boundary condition for the CFD simulations, experimentally obtained flow rate distribution in 10 terminal airways was used with zero pressure resistance at the throat inlet. CCM+ CFD code (Adapco) was used with an SST k-ω low-Reynolds Number RANS model. The total number of polyhedral control volumes was 2.6 million with a time step of 0.001 s. Comparisons were made at several points in eight cross sections selected according to experiments in the trachea and the left and right bronchi. The results agree well with experiments involving the oscillation (temporal relocation) of flow structures in the majority of the cross sections and individual local positions. Velocity field simulation in several cross sections shows a very unstable flow field, which originates in the tracheal laryngeal jet and propagates far downstream with the formation of separation zones in both left and right airways. The RANS simulation agrees with the experiments in almost all the cross sections and shows unstable local flow structures and a quantitatively acceptable solution for the time-averaged flow field.}, } @article {pmid26160306, year = {2015}, author = {Carugo, D and Capretto, L and Roy, B and Carboni, M and Caine, M and Lewis, AL and Hill, M and Chakraborty, S and Zhang, X}, title = {Spatiotemporal dynamics of doxorubicin elution from embolic beads within a microfluidic network.}, journal = {Journal of controlled release : official journal of the Controlled Release Society}, volume = {214}, number = {}, pages = {62-75}, doi = {10.1016/j.jconrel.2015.07.003}, pmid = {26160306}, issn = {1873-4995}, mesh = {Algorithms ; Antibiotics, Antineoplastic/*administration & dosage/*chemistry ; Capillaries/metabolism ; Chemoembolization, Therapeutic/*methods ; Doxorubicin/*administration & dosage/*chemistry ; Drug Delivery Systems ; Drug Design ; Injections, Intravenous ; Kinetics ; Lab-On-A-Chip Devices ; Microfluidics ; Microspheres ; Models, Biological ; Spectrometry, Fluorescence ; }, abstract = {Anticancer treatment using embolic drug-eluting beads (DEBs) has shown multifarious advantages compared to systemic chemotherapy. However, there is a growing need for a better understanding of the physical parameters governing drug-elution from embolic devices under physiologically relevant fluidic conditions. In the present study, we investigated the spatiotemporal dynamics of doxorubicin hydrochloride elution from drug-loaded hydrogel embolic beads within a microfluidic device consisting of a network of interconnected microchannels which replicates the architectural properties of microvascular systems. Drug-elution has been investigated experimentally at a single-bead level, using in-house developed microscopy- and spectrofluorimetry-based methods. Results demonstrated that the kinetics of drug-elution and the amount of eluted drug strongly depended on the location of the embolic event within the embolised channel (e.g. fractional amount of eluted drug after 3h was equal to ~0.2 and ~0.6 for completely-confined and partially-confined bead, respectively). Drug-elution from partially-confined bead showed a counterintuitive dependence on the local Reynolds number (and thus on the mean fluid velocity), as a result of dynamic changes in bead compressibility causing the displacement of the bead from the primary embolic site. Conversely, the kinetics of drug-elution from fully-confined bead was less affected by the local Reynolds number and bead displayed faster elution from the surface area exposed to the systemic flow, which was associated with the formation of fluid eddies nearby the bead post embolisation.}, } @article {pmid26158210, year = {2016}, author = {Ali, N and Javid, K and Sajid, M and Anwar Bég, O}, title = {Numerical simulation of peristaltic flow of a biorheological fluid with shear-dependent viscosity in a curved channel.}, journal = {Computer methods in biomechanics and biomedical engineering}, volume = {19}, number = {6}, pages = {614-627}, doi = {10.1080/10255842.2015.1055257}, pmid = {26158210}, issn = {1476-8259}, mesh = {Body Fluids/physiology ; Humans ; *Models, Theoretical ; Peristalsis/*physiology ; Reproducibility of Results ; Rheology ; Viscosity ; }, abstract = {Peristaltic motion of a non-Newtonian Carreau fluid is analyzed in a curved channel under the long wavelength and low Reynolds number assumptions, as a simulation of digestive transport. The flow regime is shown to be governed by a dimensionless fourth-order, nonlinear, ordinary differential equation subject to no-slip wall boundary conditions. A well-tested finite difference method based on an iterative scheme is employed for the solution of the boundary value problem. The important phenomena of pumping and trapping associated with the peristaltic motion are investigated for various values of rheological parameters of Carreau fluid and curvature of the channel. An increase in Weissenberg number is found to generate a small eddy in the vicinity of the lower wall of the channel, which is enhanced with further increase in Weissenberg number. For shear-thinning bio-fluids (power-law rheological index, n < 1) greater Weissenberg number displaces the maximum velocity toward the upper wall. For shear-thickening bio-fluids, the velocity amplitude is enhanced markedly with increasing Weissenberg number.}, } @article {pmid26154384, year = {2015}, author = {Arrieta, J and Cartwright, JH and Gouillart, E and Piro, N and Piro, O and Tuval, I}, title = {Geometric Mixing, Peristalsis, and the Geometric Phase of the Stomach.}, journal = {PloS one}, volume = {10}, number = {7}, pages = {e0130735}, doi = {10.1371/journal.pone.0130735}, pmid = {26154384}, issn = {1932-6203}, mesh = {Animals ; Computer Simulation ; Gastric Juice/*physiology ; Humans ; Models, Anatomic ; Models, Biological ; Nonlinear Dynamics ; Peristalsis/*physiology ; Stomach/*physiology ; }, abstract = {Mixing fluid in a container at low Reynolds number--in an inertialess environment--is not a trivial task. Reciprocating motions merely lead to cycles of mixing and unmixing, so continuous rotation, as used in many technological applications, would appear to be necessary. However, there is another solution: movement of the walls in a cyclical fashion to introduce a geometric phase. We show using journal-bearing flow as a model that such geometric mixing is a general tool for using deformable boundaries that return to the same position to mix fluid at low Reynolds number. We then simulate a biological example: we show that mixing in the stomach functions because of the "belly phase," peristaltic movement of the walls in a cyclical fashion introduces a geometric phase that avoids unmixing.}, } @article {pmid26135361, year = {2015}, author = {Alexandrov, DV and Galenko, PK}, title = {Thermo-solutal and kinetic regimes of an anisotropic dendrite growing under forced convective flow.}, journal = {Physical chemistry chemical physics : PCCP}, volume = {17}, number = {29}, pages = {19149-19161}, doi = {10.1039/c5cp03018h}, pmid = {26135361}, issn = {1463-9084}, abstract = {A thermo-diffusional problem of a free dendrite growing in a binary mixture is considered analytically. Effects of the anisotropy and convective flow on the stable mode of the dendrite with four-fold crystal symmetry are studied. Special analysis is given for the parabolic dendrite growing at arbitrary Péclet numbers and with small anisotropy of surface energy and atomic kinetics. The stable growth mode is analyzed through the solvability condition giving the stability criterion for the dendrite tip velocity V and dendrite tip diameter ρ as a function of growth Péclet number, Pg, flow Péclet number, Pf, and Reynolds number, Re. Using the obtained criterion of stability, a complete sequence of transitions in growth regimes (namely, from solute diffusion-limited to thermally controlled and further to kinetically-limited regimes) of the anisotropic dendrite is derived and revealed. Limiting cases to known criteria for small and high growth Péclet numbers of the solidifying system with and without convective fluid flow are found. Two-dimensional solidification regimes and scalings obtained are discussed for their extension to three-dimensional dendritic growth.}, } @article {pmid26133874, year = {2015}, author = {Huisman, SG and van der Veen, RC and Bruggert, GW and Lohse, D and Sun, C}, title = {The boiling Twente Taylor-Couette (BTTC) facility: Temperature controlled turbulent flow between independently rotating, coaxial cylinders.}, journal = {The Review of scientific instruments}, volume = {86}, number = {6}, pages = {065108}, doi = {10.1063/1.4923082}, pmid = {26133874}, issn = {1089-7623}, abstract = {A new Taylor-Couette system has been designed and constructed with precise temperature control. Two concentric independently rotating cylinders are able to rotate at maximum rates of f(i) = ± 20 Hz for the inner cylinder and f(o) = ± 10 Hz for the outer cylinder. The inner cylinder has an outside radius of r(i) = 75 mm, and the outer cylinder has an inside radius of r(o) = 105 mm, resulting in a gap of d = 30 mm. The height of the gap is L = 549 mm, giving a volume of V = 9.3 L. The geometric parameters are η = r(i)/r(o) = 0.714 and Γ = L/d = 18.3. With water as working fluid at room temperature, the Reynolds numbers that can be achieved are Re(i) = ω(i)r(i)(r(o) - r(i))/ν = 2.8 × 10(5) and Re(o) = ω(o)r(o)(r(o) - r(i))/ν = 2 × 10(5) or a combined Reynolds number of up to Re = (ω(i)r(i) - ω(o)r(o))(r(o) - r(i))/ν = 4.8 × 10(5). If the working fluid is changed to the fluorinated liquid FC-3284 with kinematic viscosity 0.42 cSt, the combined Reynolds number can reach Re = 1.1 × 10(6). The apparatus features precise temperature control of the outer and inner cylinders separately and is fully optically accessible from the side and top. The new facility offers the possibility to accurately study the process of boiling inside a turbulent flow and its effect on the flow.}, } @article {pmid26116202, year = {2015}, author = {Vasey, G and Lukeman, R and Wyeth, RC}, title = {Additional Navigational Strategies Can Augment Odor-Gated Rheotaxis for Navigation under Conditions of Variable Flow.}, journal = {Integrative and comparative biology}, volume = {55}, number = {3}, pages = {447-460}, doi = {10.1093/icb/icv073}, pmid = {26116202}, issn = {1557-7023}, mesh = {Animals ; *Chemotaxis ; Fishes/*physiology ; Invertebrates/*physiology ; Models, Biological ; *Movement ; *Odorants ; Rheology ; *Spatial Navigation ; }, abstract = {The navigation strategies animals use to find sources of odor depend on the olfactory stimuli, the properties of flowing fluids, and the locomotory capabilities of the animal. In high Reynolds number environments, animals typically use odor-gated rheotaxis to find the source of turbulent odor plumes. This strategy succeeds because, although turbulence creates an intermittent chemical cue, the animal follows the (continuous) directional cue created by the flow that is transporting the chemical. However, in nature, animals may lose all contact with an odor plume as variations in the direction of bulk flow cause the plume to be rotated away before the animal reaches the source of the odor. Our goal was to use a mathematical model to test the hypothesis that strategies that augment odor-gated rheotaxis would be beneficial for finding the source of an odor plume in such variable flow. The model links a stochastic variable-direction odor plume with a turbulence-based intermittent chemical signal and four different movement strategies, including: odor-gated rheotaxis alone (as a control), odor-gated rheotaxis augmented by further rheotaxis in the absence of odor, odor-gated rheotaxis augmented by a random walk, and odor-gated rheotaxis augmented by movement actively guided by the heading of the flow when the odor was still present. We found that any of the three augmented strategies could improve on strict odor-gated rheotaxis. Moreover, variations in performance caused the best strategy to depend on the speed of movement of the animal and the magnitude of the variation in flow, and more subtly on the duration over which the augmented strategy was performed. For most combinations of parameters in the model, either augmenting with a random walk or following the last-known heading were the best-performing strategies. Overall, our results suggest that marine animals that rely on odor cues to navigate in turbulent environments may augment odor-gated rheotaxis with additional movements that will increase the probability of finding the sources of odors. Moreover, we believe our approach to modeling odor plumes in variable flows is a valuable step toward mathematically capturing the key conditions experienced by animals navigating on the basis of odors carried by flows.}, } @article {pmid26100535, year = {2015}, author = {Mirbod, P and Meng, D}, title = {Analysis of bolus formation in micropipette ejection systems.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {6}, pages = {59}, doi = {10.1140/epje/i2015-15059-6}, pmid = {26100535}, issn = {1292-895X}, mesh = {*Hydrodynamics ; Injections/*instrumentation ; Microfluidics ; Models, Theoretical ; }, abstract = {The ejection of drugs from micropipettes is practiced frequently in biomedical research and clinical studies however, little is known about the dynamics of this process. The fundamentals of disperse fluid injection via a capillary into an ambient immiscible fluid have been investigated extensively. Here, we experimentally investigate the bolus formation in micropipette ejection systems, where the injection and ambient fluid are the same. We experimentally measure the temporal evolution of the bolus formation in the same fluid. There are three different bolus formation mechanisms that arise from different Re t regimes: a) a nearly spherical bolus, b) a pear-like bolus, and c) a large distortion or axial jet. We examine the scaled dimensions of the bolus, R b/D t, L b/D t, H/D t, and α, as a function of the dimensionless parameters such as tip Reynolds number, Re t, dimensionless value of g/(D t (.) V t), the dimensionless time, tV t/D t, and the distance between the edge of the micropipette and the free surface, D/D t. The bolus radius for 0.2 < Re t < 30 grows according to t (1/2) in the entire time range, which allows us to estimate the time for complete bolus formation.}, } @article {pmid26098511, year = {2015}, author = {Yadav, V and Duan, W and Butler, PJ and Sen, A}, title = {Anatomy of Nanoscale Propulsion.}, journal = {Annual review of biophysics}, volume = {44}, number = {}, pages = {77-100}, doi = {10.1146/annurev-biophys-060414-034216}, pmid = {26098511}, issn = {1936-1238}, mesh = {Animals ; Bacteria/chemistry/cytology ; Bacterial Physiological Phenomena ; Chemotaxis ; Cilia/physiology/ultrastructure ; Flagella/physiology ; Locomotion ; Molecular Motor Proteins/chemistry ; *Motion ; Nanotechnology/instrumentation/*methods ; }, abstract = {Nature supports multifaceted forms of life. Despite the variety and complexity of these forms, motility remains the epicenter of life. The applicable laws of physics change upon going from macroscales to microscales and nanoscales, which are characterized by low Reynolds number (Re). We discuss motion at low Re in natural and synthetic systems, along with various propulsion mechanisms, including electrophoresis, electrolyte diffusiophoresis, and nonelectrolyte diffusiophoresis. We also describe the newly uncovered phenomena of motility in non-ATP-driven self-powered enzymes and the directional movement of these enzymes in response to substrate gradients. These enzymes can also be immobilized to function as fluid pumps in response to the presence of their substrates. Finally, we review emergent collective behavior arising from interacting motile species, and we discuss the possible biomedical applications of the synthetic nanobots and microbots.}, } @article {pmid26084355, year = {2015}, author = {Pan, R and Geng, J and Cai, J and Tyree, MT}, title = {A comparison of two methods for measuring vessel length in woody plants.}, journal = {Plant, cell & environment}, volume = {38}, number = {12}, pages = {2519-2526}, doi = {10.1111/pce.12566}, pmid = {26084355}, issn = {1365-3040}, mesh = {Acer/*anatomy & histology ; Plant Stems/anatomy & histology ; Populus/*anatomy & histology ; Quercus/*anatomy & histology ; Vitis/*anatomy & histology ; Wood/anatomy & histology ; }, abstract = {Vessel lengths are important to plant hydraulic studies, but are not often reported because of the time required to obtain measurements. This paper compares the fast dynamic method (air injection method) with the slower but traditional static method (rubber injection method). Our hypothesis was that the dynamic method should yield a larger mean vessel length than the static method. Vessel length was measured by both methods in current year stems of Acer, Populus, Vitis and Quercus representing short- to long-vessel species. The hypothesis was verified. The reason for the consistently larger values of vessel length is because the dynamic method measures air flow rates in cut open vessels. The Hagen-Poiseuille law predicts that the air flow rate should depend on the product of number of cut open vessels times the fourth power of vessel diameter. An argument is advanced that the dynamic method is more appropriate because it measures the length of the vessels that contribute most to hydraulic flow. If all vessels had the same vessel length distribution regardless of diameter, then both methods should yield the same average length. This supports the hypothesis that large-diameter vessels might be longer than short-diameter vessels in most species.}, } @article {pmid26083027, year = {2015}, author = {Noreen, S and Qasim, M}, title = {Influence of Hall Current and Viscous Dissipation on Pressure Driven Flow of Pseudoplastic Fluid with Heat Generation: A Mathematical Study.}, journal = {PloS one}, volume = {10}, number = {6}, pages = {e0129588}, doi = {10.1371/journal.pone.0129588}, pmid = {26083027}, issn = {1932-6203}, mesh = {Body Fluids/*physiology ; Body Temperature ; Hot Temperature ; Humans ; Hydrodynamics ; Models, Biological ; Motion ; *Peristalsis ; Pressure ; *Rheology ; Viscosity ; }, abstract = {In this paper, we study the influence of heat sink (or source) on the peristaltic motion of pseudoplastic fluid in the presence of Hall current, where channel walls are non-conducting in nature. Flow analysis has been carried out under the approximations of a low Reynolds number and long wavelength. Coupled equations are solved using shooting method for numerical solution for the axial velocity function, temperature and pressure gradient distributions. We analyze the influence of various interesting parameters on flow quantities. The present study can be considered as a mathematical presentation of the dynamics of physiological organs with stones.}, } @article {pmid26066439, year = {2015}, author = {Bos, WJ and Kadoch, B and Schneider, K}, title = {Angular statistics of Lagrangian trajectories in turbulence.}, journal = {Physical review letters}, volume = {114}, number = {21}, pages = {214502}, doi = {10.1103/PhysRevLett.114.214502}, pmid = {26066439}, issn = {1079-7114}, abstract = {The angle between subsequent particle displacement increments is evaluated as a function of the time lag in isotropic turbulence. It is shown that the evolution of this angle contains two well-defined power laws, reflecting the multiscale dynamics of high-Reynolds number turbulence. The probability density function of the directional change is shown to be self-similar and well approximated by an analytically derived model assuming Gaussianity and independence of the velocity and the Lagrangian acceleration.}, } @article {pmid26066263, year = {2015}, author = {Low, R and Pothérat, A}, title = {Bounds on the attractor dimension for magnetohydrodynamic channel flow with parallel magnetic field at low magnetic Reynolds number.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {5}, pages = {053022}, doi = {10.1103/PhysRevE.91.053022}, pmid = {26066263}, issn = {1550-2376}, abstract = {We investigate aspects of low-magnetic-Reynolds-number flow between two parallel, perfectly insulating walls in the presence of an imposed magnetic field parallel to the bounding walls. We find a functional basis to describe the flow, well adapted to the problem of finding the attractor dimension and which is also used in subsequent direct numerical simulation of these flows. For given Reynolds and Hartmann numbers, we obtain an upper bound for the dimension of the attractor by means of known bounds on the nonlinear inertial term and this functional basis for the flow. Three distinct flow regimes emerge: a quasi-isotropic three-dimensional (3D) flow, a nonisotropic 3D flow, and a 2D flow. We find the transition curves between these regimes in the space parametrized by Hartmann number Ha and attractor dimension d(att). We find how the attractor dimension scales as a function of Reynolds and Hartmann numbers (Re and Ha) in each regime. We also investigate the thickness of the boundary layer along the bounding wall and find that in all regimes this scales as 1/Re, independently of the value of Ha, unlike Hartmann boundary layers found when the field is normal to the channel. The structure of the set of least dissipative modes is indeed quite different between these two cases but the properties of turbulence far from the walls (smallest scales and number of degrees of freedom) are found to be very similar.}, } @article {pmid26066258, year = {2015}, author = {Rosén, T and Do-Quang, M and Aidun, CK and Lundell, F}, title = {Effect of fluid and particle inertia on the rotation of an oblate spheroidal particle suspended in linear shear flow.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {5}, pages = {053017}, doi = {10.1103/PhysRevE.91.053017}, pmid = {26066258}, issn = {1550-2376}, mesh = {*Hydrodynamics ; *Mechanical Phenomena ; *Models, Theoretical ; Rheology ; *Rotation ; }, abstract = {This work describes the inertial effects on the rotational behavior of an oblate spheroidal particle confined between two parallel opposite moving walls, which generate a linear shear flow. Numerical results are obtained using the lattice Boltzmann method with an external boundary force. The rotation of the particle depends on the particle Reynolds number, Re(p)=Gd(2)ν(-1) (G is the shear rate, d is the particle diameter, ν is the kinematic viscosity), and the Stokes number, St=αRe(p) (α is the solid-to-fluid density ratio), which are dimensionless quantities connected to fluid and particle inertia, respectively. The results show that two inertial effects give rise to different stable rotational states. For a neutrally buoyant particle (St=Re(p)) at low Re(p), particle inertia was found to dominate, eventually leading to a rotation about the particle's symmetry axis. The symmetry axis is in this case parallel to the vorticity direction; a rotational state called log-rolling. At high Re(p), fluid inertia will dominate and the particle will remain in a steady state, where the particle symmetry axis is perpendicular to the vorticity direction and has a constant angle ϕ(c) to the flow direction. The sequence of transitions between these dynamical states were found to be dependent on density ratio α, particle aspect ratio r(p), and domain size. More specifically, the present study reveals that an inclined rolling state (particle rotates around its symmetry axis, which is not aligned in the vorticity direction) appears through a pitchfork bifurcation due to the influence of periodic boundary conditions when simulated in a small domain. Furthermore, it is also found that a tumbling motion, where the particle symmetry axis rotates in the flow-gradient plane, can be a stable motion for particles with high r(p) and low α.}, } @article {pmid26066247, year = {2015}, author = {Mishra, PK and Herault, J and Fauve, S and Verma, MK}, title = {Dynamics of reversals and condensates in two-dimensional Kolmogorov flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {5}, pages = {053005}, doi = {10.1103/PhysRevE.91.053005}, pmid = {26066247}, issn = {1550-2376}, abstract = {We present numerical simulations of the different two-dimensional flow regimes generated by a constant spatially periodic forcing balanced by viscous dissipation and large-scale drag with a dimensionless damping rate 1/Rh. The linear response to the forcing is a 6×6 square array of counterrotating vortices, which is stable when the Reynolds number Re or Rh are small. After identifying the sequence of bifurcations that lead to a spatially and temporally chaotic regime of the flow when Re and Rh are increased, we study the transitions between the different turbulent regimes observed for large Re by varying Rh. A large-scale circulation at the box size (the condensate state) is the dominant mode in the limit of vanishing large-scale drag (Rh large). When Rh is decreased, the condensate becomes unstable and a regime with random reversals between two large-scale circulations of opposite signs is generated. It involves a bimodal probability density function of the large-scale velocity that continuously bifurcates to a Gaussian distribution when Rh is decreased further.}, } @article {pmid26066225, year = {2015}, author = {Joglekar, M and Feudel, U and Yorke, JA}, title = {Geometry of the edge of chaos in a low-dimensional turbulent shear flow model.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {5}, pages = {052903}, doi = {10.1103/PhysRevE.91.052903}, pmid = {26066225}, issn = {1550-2376}, abstract = {We investigate the geometry of the edge of chaos for a nine-dimensional sinusoidal shear flow model and show how the shape of the edge of chaos changes with increasing Reynolds number. Furthermore, we numerically compute the scaling of the minimum perturbation required to drive the laminar attracting state into the turbulent region. We find this minimum perturbation to scale with the Reynolds number as Re(-2).}, } @article {pmid26064617, year = {2015}, author = {Montenegro-Johnson, TD and Gadêlha, H and Smith, DJ}, title = {Spermatozoa scattering by a microchannel feature: an elastohydrodynamic model.}, journal = {Royal Society open science}, volume = {2}, number = {3}, pages = {140475}, doi = {10.1098/rsos.140475}, pmid = {26064617}, issn = {2054-5703}, abstract = {Sperm traverse their microenvironment through viscous fluid by propagating flagellar waves; the waveform emerges as a consequence of elastic structure, internal active moments and low Reynolds number fluid dynamics. Engineered microchannels have recently been proposed as a method of sorting and manipulating motile cells; the interaction of cells with these artificial environments therefore warrants investigation. A numerical method is presented for large-amplitude elastohydrodynamic interaction of active swimmers with domain features. This method is employed to examine hydrodynamic scattering by a model microchannel backstep feature. Scattering is shown to depend on backstep height and the relative strength of viscous and elastic forces in the flagellum. In a 'high viscosity' parameter regime corresponding to human sperm in cervical mucus analogue, this hydrodynamic contribution to scattering is comparable in magnitude to recent data on contact effects, being of the order of 5°-10°. Scattering can be positive or negative depending on the relative strength of viscous and elastic effects, emphasizing the importance of viscosity on the interaction of sperm with their microenvironment. The modulation of scattering angle by viscosity is associated with variations in flagellar asymmetry induced by the elastohydrodynamic interaction with the boundary feature.}, } @article {pmid26037711, year = {2015}, author = {Krujatz, F and Illing, R and Krautwer, T and Liao, J and Helbig, K and Goy, K and Opitz, J and Cuniberti, G and Bley, T and Weber, J}, title = {Light-field-characterization in a continuous hydrogen-producing photobioreactor by optical simulation and computational fluid dynamics.}, journal = {Biotechnology and bioengineering}, volume = {112}, number = {12}, pages = {2439-2449}, doi = {10.1002/bit.25667}, pmid = {26037711}, issn = {1097-0290}, mesh = {*Chemical Phenomena ; *Hydrodynamics ; Hydrogen/*metabolism ; *Light ; Photobioreactors/*microbiology ; Rhodobacter sphaeroides/*growth & development/*metabolism ; }, abstract = {Externally illuminated photobioreactors (PBRs) are widely used in studies on the use of phototrophic microorganisms as sources of bioenergy and other photobiotechnology research. In this work, straightforward simulation techniques were used to describe effects of varying fluid flow conditions in a continuous hydrogen-producing PBR on the rate of photofermentative hydrogen production (rH2) by Rhodobacter sphaeroides DSM 158. A ZEMAX optical ray tracing simulation was performed to quantify the illumination intensity reaching the interior of the cylindrical PBR vessel. 24.2% of the emitted energy was lost through optical effects, or did not reach the PBR surface. In a dense culture of continuously producing bacteria during chemostatic cultivation, the illumination intensity became completely attenuated within the first centimeter of the PBR radius as described by an empirical three-parametric model implemented in Mathcad. The bacterial movement in chemostatic steady-state conditions was influenced by varying the fluid Reynolds number. The "Computational Fluid Dynamics" and "Particle Tracing" tools of COMSOL Multiphysics were used to visualize the fluid flow pattern and cellular trajectories through well-illuminated zones near the PBR periphery and dark zones in the center of the PBR. A moderate turbulence (Reynolds number = 12,600) and fluctuating illumination of 1.5 Hz were found to yield the highest continuous rH2 by R. sphaeroides DSM 158 (170.5 mL L(-1) h(-1)) in this study.}, } @article {pmid26030270, year = {2015}, author = {Walker, D and Kübler, M and Morozov, KI and Fischer, P and Leshansky, AM}, title = {Optimal Length of Low Reynolds Number Nanopropellers.}, journal = {Nano letters}, volume = {15}, number = {7}, pages = {4412-4416}, doi = {10.1021/acs.nanolett.5b01925}, pmid = {26030270}, issn = {1530-6992}, abstract = {Locomotion in fluids at the nanoscale is dominated by viscous drag. One efficient propulsion scheme is to use a weak rotating magnetic field that drives a chiral object. From bacterial flagella to artificial drills, the corkscrew is a universally useful chiral shape for propulsion in viscous environments. Externally powered magnetic micro- and nanomotors have been recently developed that allow for precise fuel-free propulsion in complex media. Here, we combine analytical and numerical theory with experiments on nanostructured screw-propellers to show that the optimal length is surprisingly short-only about one helical turn, which is shorter than most of the structures in use to date. The results have important implications for the design of artificial actuated nano- and micropropellers and can dramatically reduce fabrication times, while ensuring optimal performance.}, } @article {pmid26029795, year = {2015}, author = {Jang, B and Gutman, E and Stucki, N and Seitz, BF and Wendel-García, PD and Newton, T and Pokki, J and Ergeneman, O and Pané, S and Or, Y and Nelson, BJ}, title = {Undulatory Locomotion of Magnetic Multilink Nanoswimmers.}, journal = {Nano letters}, volume = {15}, number = {7}, pages = {4829-4833}, doi = {10.1021/acs.nanolett.5b01981}, pmid = {26029795}, issn = {1530-6992}, abstract = {Micro- and nanorobots operating in low Reynolds number fluid environments require specialized swimming strategies for efficient locomotion. Prior research has focused on designs mimicking the rotary corkscrew motion of bacterial flagella or the planar beating motion of eukaryotic flagella. These biologically inspired designs are typically of uniform construction along their flagellar axis. This work demonstrates for the first time planar undulations of composite multilink nanowire-based chains (diameter 200 nm) induced by a planar-oscillating magnetic field. Those chains comprise an elastic eukaryote-like polypyrrole tail and rigid magnetic nickel links connected by flexible polymer bilayer hinges. The multilink design exhibits a high swimming efficiency. Furthermore, the manufacturing process enables tuning the geometrical and material properties to specific applications.}, } @article {pmid26015833, year = {2015}, author = {Kim, H and Cheang, UK and Kim, D and Ali, J and Kim, MJ}, title = {Hydrodynamics of a self-actuated bacterial carpet using microscale particle image velocimetry.}, journal = {Biomicrofluidics}, volume = {9}, number = {2}, pages = {024121}, doi = {10.1063/1.4918978}, pmid = {26015833}, issn = {1932-1058}, abstract = {Microorganisms can effectively generate propulsive force at the microscale where viscous forces overwhelmingly dominate inertia forces; bacteria achieve this task through flagellar motion. When swarming bacteria, cultured on agar plates, are blotted onto the surface of a microfabricated structure, a monolayer of bacteria forms what is termed a "bacterial carpet," which generates strong flows due to the combined motion of their freely rotating flagella. Furthermore, when the bacterial carpet coated microstructure is released into a low Reynolds number fluidic environment, the propulsive force of the bacterial carpet is able to give the microstructure motility. In our previous investigations, we demonstrated motion control of these bacteria powered microbiorobots (MBRs). Without any external stimuli, MBRs display natural rotational and translational movements on their own; this MBR self-actuation is due to the coordination of flagella. Here, we investigate the flow fields generated by bacterial carpets, and compare this flow to the flow fields observed in the bulk fluid at a series of locations above the bacterial carpet. Using microscale particle image velocimetry, we characterize the flow fields generated from the bacterial carpets of MBRs in an effort to understand their propulsive flow, as well as the resulting pattern of flagella driven self-actuated motion. Comparing the velocities between the bacterial carpets on fixed and untethered MBRs, it was found that flow velocities near the surface of the microstructure were strongest, and at distances far above, the surface flow velocities were much smaller.}, } @article {pmid26005774, year = {2015}, author = {Lu, X and Xuan, X}, title = {Continuous Microfluidic Particle Separation via Elasto-Inertial Pinched Flow Fractionation.}, journal = {Analytical chemistry}, volume = {87}, number = {12}, pages = {6389-6396}, doi = {10.1021/acs.analchem.5b01432}, pmid = {26005774}, issn = {1520-6882}, mesh = {Chemical Fractionation ; *Microfluidic Analytical Techniques ; Particle Size ; }, abstract = {Many of the fluids encountered in chemical and biomedical applications exhibit non-Newtonian behavior. However, the majority of current particle separation methods have been demonstrated in Newtonian fluids only. This work presents an experimental study of continuous particle separation in viscoelastic solutions via a combined action of elastic and inertial lift forces, which we term elasto-inertial pinched flow fractionation (eiPFF). The parametric effects on eiPFF are systematically investigated in terms of dimensionless numbers. It is found that eiPFF offers much higher particle throughput and separation resolution than the traditional steric effects-based PFF. Moreover, eiPFF works most efficiently when the Reynolds number, Re, is of order 1 and hence fills perfectly into the gap of our recently proposed inertia-enhanced PFF (iPFF) technique (Anal. Chem. 2015, 87, 4560-4565) that favors Re of the order 10 or more. However, the particle separation via eiPFF does not increase monotonically with the elasticity number at higher polymer concentrations and is strongly affected by the aspect ratio of channel width to height, both of which have not been previously reported. More surprisingly, the elasto-inertial deflection of small particles can be even greater than that of large particles in a high-aspect-ratio channel for Re less than 1.}, } @article {pmid26004808, year = {2015}, author = {Hu, R and Li, F and Lv, J and He, Y and Lu, D and Yamada, T and Ono, N}, title = {Microfluidic analysis of pressure drop and flow behavior in hypertensive micro vessels.}, journal = {Biomedical microdevices}, volume = {17}, number = {3}, pages = {9959}, doi = {10.1007/s10544-015-9959-4}, pmid = {26004808}, issn = {1572-8781}, mesh = {Animals ; *Blood Flow Velocity ; Computer Simulation ; Humans ; Microcirculation ; Microfluidics/*methods ; Microvessels/*physiopathology ; *Models, Cardiovascular ; Retinal Artery/*physiopathology ; Retinal Artery Occlusion/*physiopathology ; }, abstract = {The retinal arterial network is the only source of the highly nutrient-consumptive retina, thus any insult on the arteries can impair the retinal oxygen and nutrient supply and affect its normal function. The aim of this work is to study the influences of vascular structure variation on the flow and pressure characteristics via microfluidic devices. Two sets of micro-channel were designed to mimic the stenosed microvessels and dichotomous branching structure in the retinal arteries. Three working fluids including red blood cell (RBC) suspension were employed to investigate the pressure drop in the stenosed channel. The flow behaviors of RBC suspensions inside the micro channels were observed using high speed camera system. Pressure drop of different working fluids and RBC velocity profiles in the stenosed channel were obtained. Moreover, hematocrit levels of RBC suspensions inside the bifurcated channels were analyzed from the sequential images of RBC flow. The results of the flow in the stenosed channel show that RBCs drift from the center of the channels, and RBC velocity is influenced not only by the inlet flow rate but also the interaction between RBCs. The measured pressure drops in the stenosed channel increase notably with the increase of fluid viscosity. Furthermore, the dimensionless pressure drop due to the stenosis decreases with Reynolds number. On the other hand, the results of flow through the bifurcated channels show that as the ratio of the daughter-branch width to the mother-channel width increases, the ratio of hematocrit in two connected branches (Ht/Hd) decreases, which is in favorable agreement with the available analysis results.}, } @article {pmid25998171, year = {2015}, author = {Dasgupta, R and Tomar, G and Govindarajan, R}, title = {Numerical study of laminar, standing hydraulic jumps in a planar geometry.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {5}, pages = {130}, doi = {10.1140/epje/i2015-15045-0}, pmid = {25998171}, issn = {1292-895X}, abstract = {We solve the two-dimensional, planar Navier-Stokes equations to simulate a laminar, standing hydraulic jump using a Volume-of-Fluid method. The geometry downstream of the jump has been designed to be similar to experimental conditions by including a pit at the edge of the platform over which liquid film flows. We obtain jumps with and without separation. Increasing the inlet Froude number pushes the jump downstream and makes the slope of the jump weaker, consistent with experimental observations of circular jumps, and decreasing the Reynolds number brings the jump upstream while making it steeper. We study the effect of the length of the domain and that of a downstream obstacle on the structure and location of the jump. The transient flow which leads to a final steady jump is described for the first time to our knowledge. In the moderate Reynolds number regime, we obtain steady undular jumps with a separated bubble underneath the first few undulations. Interestingly, surface tension leads to shortening of wavelength of these undulations. We show that the undulations can be explained using the inviscid theory of Benjamin and Lighthill (Proc. R. Soc. London, Ser. A, 1954). We hope this new finding will motivate experimental verification.}, } @article {pmid26001019, year = {2015}, author = {Henríquez Rivera, RG and Sinha, K and Graham, MD}, title = {Margination regimes and drainage transition in confined multicomponent suspensions.}, journal = {Physical review letters}, volume = {114}, number = {18}, pages = {188101}, doi = {10.1103/PhysRevLett.114.188101}, pmid = {26001019}, issn = {1079-7114}, mesh = {Blood ; *Blood Chemical Analysis ; Hydrodynamics ; *Models, Biological ; *Models, Chemical ; Suspensions/*chemistry ; }, abstract = {A mechanistic theory is developed to describe segregation in confined multicomponent suspensions such as blood. It incorporates the two key phenomena arising in these systems at low Reynolds number: hydrodynamic pair collisions and wall-induced migration. In simple shear flow, several regimes of segregation arise, depending on the value of a "margination parameter" M. Most importantly, there is a critical value of M below which a sharp "drainage transition" occurs: one component is completely depleted from the bulk flow to the vicinity of the walls. Direct simulations also exhibit this transition as the size or flexibility ratio of the components changes.}, } @article {pmid25990633, year = {2015}, author = {Montino, A and DeSimone, A}, title = {Three-sphere low-Reynolds-number swimmer with a passive elastic arm.}, journal = {The European physical journal. E, Soft matter}, volume = {38}, number = {5}, pages = {127}, doi = {10.1140/epje/i2015-15042-3}, pmid = {25990633}, issn = {1292-895X}, abstract = {One of the simplest model swimmers at low Reynolds number is the three-sphere swimmer by Najafi and Golestanian. It consists of three spheres connected by two rods which change their lengths periodically in non-reciprocal fashion. Here we investigate a variant of this model in which one rod is periodically actuated while the other is replaced by an elastic spring. We show that the competition between the elastic restoring force and the hydrodynamic drag produces a delay in the response of the passive elastic arm with respect to the active one. This leads to non-reciprocal shape changes and self-propulsion. After formulating the equations of motion, we study their solutions qualitatively and numerically. The leading-order term of the solution is computed analytically. We then address questions of optimization with respect to both actuation frequency and swimmer's geometry. Our results can provide valuable conceptual guidance in the engineering of robotic microswimmers.}, } @article {pmid25974595, year = {2015}, author = {Grafke, T and Frishman, A and Falkovich, G}, title = {Time irreversibility of the statistics of a single particle in compressible turbulence.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043022}, doi = {10.1103/PhysRevE.91.043022}, pmid = {25974595}, issn = {1550-2376}, abstract = {We investigate time irreversibility from the point of view of a single particle in Burgers turbulence. Inspired by the recent work for incompressible flows [Xu et al., Proc. Natl. Acad. Sci. USA 111, 7558 (2014)], we analyze the evolution of the kinetic energy for fluid markers and use the fluctuations of the instantaneous power as a measure of time irreversibility. For short times, starting from a uniform distribution of markers, we find the scaling 〈[E(t)-E(0)](n)〉∝t and 〈p(n)〉∝Re(n-1) for the power as a function of the Reynolds number. Both observations can be explained using the "flight-crash" model, suggested by Xu et al. Furthermore, we use a simple model for shocks that reproduces the moments of the energy difference, including the pre-factor for 〈E(t)-E(0)〉. To complete the single-particle picture for Burgers we compute the moments of the Lagrangian velocity difference and show that they are bifractal. This arises in a similar manner to the bifractality of Eulerian velocity differences. In the above setting, time irreversibility is directly manifest as particles eventually end up in shocks. We additionally investigate time irreversibility in the long-time limit when all particles are located inside shocks and the Lagrangian velocity statistics are stationary. We find the same scalings for the power and energy differences as at short times and argue that this is also a consequence of rare "flight-crash" events related to shock collisions.}, } @article {pmid25974591, year = {2015}, author = {Felderhof, BU}, title = {Stokesian spherical swimmers and active particles.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043018}, doi = {10.1103/PhysRevE.91.043018}, pmid = {25974591}, issn = {1550-2376}, abstract = {The net steady state flow pattern of a distorting sphere is studied in the framework of the bilinear theory of swimming at low Reynolds number. It is argued that the starting point of a theory of interacting active particles should be based on such a calculation, since any arbitrarily chosen steady state flow pattern is not necessarily the result of a swimming motion. Furthermore, it is stressed that as a rule the phase of stroke is relevant in hydrodynamic interactions, so that the net flow pattern must be used with caution.}, } @article {pmid25974590, year = {2015}, author = {Deng, J and Caulfield, CP}, title = {Three-dimensional transition after wake deflection behind a flapping foil.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043017}, doi = {10.1103/PhysRevE.91.043017}, pmid = {25974590}, issn = {1550-2376}, abstract = {We report the inherently three-dimensional linear instabilities of a propulsive wake, produced by a flapping foil, mimicking the caudal fin of a fish or the wing of a flying animal. For the base flow, three sequential wake patterns appear as we increase the flapping amplitude: Bénard-von Kármán (BvK) vortex streets; reverse BvK vortex streets; and deflected wakes. Imposing a three-dimensional spanwise periodic perturbation, we find that the resulting Floquet multiplier |μ| indicates an unstable "short wavelength" mode at wave number β=30, or wavelength λ=0.21 (nondimensionalized by the chord length) at sufficiently high flow Reynolds number Re=Uc/ν≃600, where U is the upstream flow velocity, c is the chord length, and ν is the kinematic viscosity of the fluid. Another, "long wavelength" mode at β=6 (λ=1.05) becomes critical at somewhat higher Reynolds number, although we do not expect that this mode would be observed physically because its growth rate is always less than the short wavelength mode, at least for the parameters we have considered. The long wavelength mode has certain similarities with the so-called mode A in the drag wake of a fixed bluff body, while the short wavelength mode appears to have a period of the order of twice that of the base flow, in that its structure seems to repeat approximately only every second cycle of the base flow. Whether it is appropriate to classify this mode as a truly subharmonic mode or as a quasiperiodic mode is still an open question however, worthy of a detailed parametric study with various flapping amplitudes and frequencies.}, } @article {pmid25974586, year = {2015}, author = {McComb, WD and Berera, A and Yoffe, SR and Linkmann, MF}, title = {Energy transfer and dissipation in forced isotropic turbulence.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043013}, doi = {10.1103/PhysRevE.91.043013}, pmid = {25974586}, issn = {1550-2376}, abstract = {A model for the Reynolds-number dependence of the dimensionless dissipation rate C(ɛ) was derived from the dimensionless Kármán-Howarth equation, resulting in C(ɛ)=C(ɛ,∞)+C/R(L)+O(1/R(L)(2)), where R(L) is the integral scale Reynolds number. The coefficients C and C(ɛ,∞) arise from asymptotic expansions of the dimensionless second- and third-order structure functions. This theoretical work was supplemented by direct numerical simulations (DNSs) of forced isotropic turbulence for integral scale Reynolds numbers up to R(L)=5875 (R(λ)=435), which were used to establish that the decay of dimensionless dissipation with increasing Reynolds number took the form of a power law R(L)(n) with exponent value n=-1.000±0.009 and that this decay of C(ɛ) was actually due to the increase in the Taylor surrogate U(3)/L. The model equation was fitted to data from the DNS, which resulted in the value C=18.9±1.3 and in an asymptotic value for C(ɛ) in the infinite Reynolds-number limit of C(ɛ,∞)=0.468±0.006.}, } @article {pmid25974583, year = {2015}, author = {Beaume, C and Chini, GP and Julien, K and Knobloch, E}, title = {Reduced description of exact coherent states in parallel shear flows.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043010}, doi = {10.1103/PhysRevE.91.043010}, pmid = {25974583}, issn = {1550-2376}, abstract = {A reduced description of exact coherent structures in the transition regime of plane parallel shear flows is developed, based on the Reynolds number scaling of streamwise-averaged (mean) and streamwise-varying (fluctuation) velocities observed in numerical simulations. The resulting system is characterized by an effective unit Reynolds number mean equation coupled to linear equations for the fluctuations, regularized by formally higher-order diffusion. Stationary coherent states are computed by solving the resulting equations simultaneously using a robust numerical algorithm developed for this purpose. The algorithm determines self-consistently the amplitude of the fluctuations for which the associated mean flow is just such that the fluctuations neither grow nor decay. The procedure is used to compute exact coherent states of a flow introduced by Drazin and Reid [Hydrodynamic Stability (Cambridge University Press, Cambridge, UK, 1981)] and studied by Waleffe [Phys. Fluids 9, 883 (1997)]: a linearly stable, plane parallel shear flow confined between stationary stress-free walls and driven by a sinusoidal body force. Numerical continuation of the lower-branch states to lower Reynolds numbers reveals the presence of a saddle node; the saddle node allows access to upper-branch states that are, like the lower-branch states, self-consistently described by the reduced equations. Both lower- and upper-branch states are characterized in detail.}, } @article {pmid25974578, year = {2015}, author = {Chantry, M and Kerswell, RR}, title = {Localization in a spanwise-extended model of plane Couette flow.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043005}, doi = {10.1103/PhysRevE.91.043005}, pmid = {25974578}, issn = {1550-2376}, abstract = {We consider a nine-partial-differential-equation (1-space and 1-time) model of plane Couette flow in which the degrees of freedom are severely restricted in the streamwise and cross-stream directions to study spanwise localization in detail. Of the many steady Eckhaus (spanwise modulational) instabilities identified of global steady states, none lead to a localized state. Spatially localized, time-periodic solutions were found instead, which arise in saddle node bifurcations in the Reynolds number. These solutions appear global (domain filling) in narrow (small spanwise) domains yet can be smoothly continued out to fully spanwise-localized states in very wide domains. This smooth localization behavior, which has also been seen in fully resolved duct flow (S. Okino, Ph.D. thesis, Kyoto University, Kyoto, 2011), indicates that an apparently global flow structure does not have to suffer a modulational instability to localize in wide domains.}, } @article {pmid25974576, year = {2015}, author = {Cerbus, RT and Goldburg, WI}, title = {Predicting two-dimensional turbulence.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043003}, doi = {10.1103/PhysRevE.91.043003}, pmid = {25974576}, issn = {1550-2376}, abstract = {Prediction is a fundamental objective of science. It is more difficult for chaotic and complex systems like turbulence. Here we use information theory to quantify spatial prediction using experimental data from a turbulent soap film. At high Reynolds number, Re, where a cascade exists, turbulence becomes easier to predict as the inertial range broadens. The development of a cascade at low Re is also detected.}, } @article {pmid25974574, year = {2015}, author = {Verma, MK and Kumaran, V}, title = {Stability of the flow in a soft tube deformed due to an applied pressure gradient.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {043001}, doi = {10.1103/PhysRevE.91.043001}, pmid = {25974574}, issn = {1550-2376}, abstract = {A linear stability analysis is carried out for the flow through a tube with a soft wall in order to resolve the discrepancy of a factor of 10 for the transition Reynolds number between theoretical predictions in a cylindrical tube and the experiments of Verma and Kumaran [J. Fluid Mech. 705, 322 (2012)]. Here the effect of tube deformation (due to the applied pressure difference) on the mean velocity profile and pressure gradient is incorporated in the stability analysis. The tube geometry and dimensions are reconstructed from experimental images, where it is found that there is an expansion and then a contraction of the tube in the streamwise direction. The mean velocity profiles at different downstream locations and the pressure gradient, determined using computational fluid dynamics, are found to be substantially modified by the tube deformation. The velocity profiles are then used in a linear stability analysis, where the growth rates of perturbations are calculated for the flow through a tube with the wall modeled as a neo-Hookean elastic solid. The linear stability analysis is carried out for the mean velocity profiles at different downstream locations using the parallel flow approximation. The analysis indicates that the flow first becomes unstable in the downstream converging section of the tube where the flow profile is more pluglike when compared to the parabolic flow in a cylindrical tube. The flow is stable in the upstream diverging section where the deformation is maximum. The prediction for the transition Reynolds number is in good agreement with experiments, indicating that the downstream tube convergence and the consequent modification in the mean velocity profile and pressure gradient could reduce the transition Reynolds number by an order of magnitude.}, } @article {pmid25974532, year = {2015}, author = {Andersen, A and Wadhwa, N and Kiørboe, T}, title = {Quiet swimming at low Reynolds number.}, journal = {Physical review. E, Statistical, nonlinear, and soft matter physics}, volume = {91}, number = {4}, pages = {042712}, doi = {10.1103/PhysRevE.91.042712}, pmid = {25974532}, issn = {1550-2376}, mesh = {Animals ; Biomechanical Phenomena ; Chlamydomonas reinhardtii/physiology ; Ciliophora/physiology ; Copepoda/physiology ; Crustacea/physiology ; Flagella/physiology ; Hydrodynamics ; *Models, Biological ; *Swimming/physiology ; }, abstract = {The stresslet provides a simple model of the flow created by a small, freely swimming and neutrally buoyant aquatic organism and shows that the far field fluid disturbance created by such an organism in general decays as one over distance squared. Here we discuss a quieter swimming mode that eliminates the stresslet component of the flow and leads to a faster spatial decay of the fluid disturbance described by a force quadrupole that decays as one over distance cubed. Motivated by recent experimental results on fluid disturbances due to small aquatic organisms, we demonstrate that a three-Stokeslet model of a swimming organism which uses breast stroke type kinematics is an example of such a quiet swimmer. We show that the fluid disturbance in both the near field and the far field is significantly reduced by appropriately arranging the propulsion apparatus, and we find that the far field power laws are valid surprisingly close to the organism. Finally, we discuss point force models as a general framework for hypothesis generation and experimental exploration of fluid mediated predator-prey interactions in the planktonic world.}, } @article {pmid25967293, year = {2015}, author = {Mishra, GK and Kumar, A and Prakash, O and Biswal, R and Dixit, SK and Nakhe, SV}, title = {Flow and thermal characteristics of high Reynolds number (2800-17,000) dye cell: simulation and experiment.}, journal = {Applied optics}, volume = {54}, number = {11}, pages = {3106-3114}, pmid = {25967293}, issn = {1539-4522}, abstract = {This paper presents computational and experimental studies on wavelength/frequency fluctuation characteristics of a high pulse repetition rate (18 kHz) dye laser pumped by a frequency-doubled Nd:YAG laser (532 nm). The temperature gradient in the dye solution is found to be responsible for wavelength fluctuations of the dye laser at low flow rates (2800