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Bibliography on: Reynolds Number

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ESP: PubMed Auto Bibliography 21 Oct 2019 at 01:31 Created: 

Reynolds Number

It is well known that relative size greatly affects how organisms interact with the world. Less well known, at least among biologists, is that at sufficiently small sizes, mechanical interaction with the environment becomes difficult and then virtually impossible. In fluid dynamics, an important dimensionless parameter is the Reynolds Number (abbreviated Re), which is the ratio of inertial to viscous forces affecting the movement of objects in a fluid medium (or the movement of a fluid in a pipe). Since Re is determined mainly by the size of the object (pipe) and the properties (density and viscosity) of the fluid, organisms of different sizes exhibit significantly different Re values when moving through air or water. A fish, swimming at a high ratio of inertial to viscous forces, gives a flick of its tail and then glides for several body lengths. A bacterium, "swimming" in an environment dominated by viscosity, possesses virtually no inertia. When the bacterium stops moving its flagellum, the bacterium "coasts" for about a half of a microsecond, coming to a stop in a distance less than a tenth the diameter of a hydrogen atom. Similarly, the movement of molecules (nutrients toward, wastes away) in the vicinity of a bacterium is dominated by diffusion. Effective stirring — the generation of bulk flow through mechanical means — is impossible at very low Re. An understanding of the constraints imposed by life at low Reynolds numbers is essentially for understanding the prokaryotic biosphere.

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Citations The Papers (from PubMed®)

RevDate: 2019-10-18

Hayat T, Waqar Ahmad M, Ijaz Khan M, et al (2019)

Entropy optimization in CNTs based nanomaterial flow induced by rotating disks: A study on the accuracy of statistical declaration and probable error.

Computer methods and programs in biomedicine, 184:105105 pii:S0169-2607(19)31590-1 [Epub ahead of print].

BACKGROUND: CNTs (Carbon nanotubes) being allotropes of carbon, made of graphene and diameters of single and multi-walls carbon nanotubes are typically 0.8 to 2 nm and 5 to 20 mn, although diameter of MWCNTs can exceed 100 nm. Carbon nanotubes lengths range from less than 100 nm to 0.5 m. Their impressive structural, electronic and mechanical attributes subject to their small size and mass, their high electrical and thermal conductivities, and their strong mechanical potency. CNTs based materials are successfully applied in medicine and pharmacy subject to their huge surface area that is proficient of conjugating or adsorbing with a wide variety of genes, drugs, antibodies, vaccines and biosensors etc. Therefore, we have presented a theoretical study about mathematical modeling of CNTs based viscous material flow between two rotating disks. Both types of nanotubes i.e., SWCNTs and MWCNTs are considered. Xue model is used for the mathematical modeling. Fluid flow is due to rotating disks. Main focus here is given to probable error and statistical declaration. Entropy is calculated for both single and multi-walls nanotubes.

METHOD: Nonlinear PDEs are first converted into ODEs and then computed for homotopy convergent solutions.

RESULTS AND CONCLUSION: Statistical declaration and probable error for skin friction and Nusselt number are numerically computed and discussed through Tables. From obtained outcomes it is concluded that magnitude of skin friction increases at both disks surface for higher values of Reynolds number, lower stretching parameter and porosity parameter while it decays for both of disks versus larger rotation parameter. Nusselt number or heat transfer rate also enhances at both disks in the presence of radiation and Reynolds number while it decays against Eckert number.

RevDate: 2019-10-17

Bukowicki M, ML Ekiel-Jeżewska (2019)

Sedimenting pairs of elastic microfilaments.

Soft matter [Epub ahead of print].

The dynamics of two identical elastic filaments settling under gravity in a viscous fluid in the low Reynolds number regime is investigated numerically. A large family of initial configurations symmetric with respect to a vertical plane is considered, as well as their non-symmetric perturbations. The behaviour of the filaments is primarily governed by the elasto-gravitational number, which depends on the filament's length and flexibility, and the strength of the external force. Flexible filaments usually converge toward horizontal and parallel orientation. We explain this phenomenon and show that it occurs also for curved rigid particles of similar shapes. Once aligned, the two fibres either converge toward a stationary, flexibility-dependent distance, or tend to collide or continuously repel each other. Rigid and straight rods perform periodic motions while settling down. Apart from very stiff particles, the dynamics is robust to non-symmetric perturbations.

RevDate: 2019-10-15

Holdenried-Chernoff D, Chen L, A Jackson (2019)

A trio of simple optimized axisymmetric kinematic dynamos in a sphere.

Proceedings. Mathematical, physical, and engineering sciences, 475(2229):20190308.

Planetary magnetic fields are generated by the motion of conductive fluid in the planet's interior. Complex flows are not required for dynamo action; simple flows have been shown to act as efficient kinematic dynamos, whose physical characteristics are more straightforward to study. Recently, Chen et al. (2018, J. Fluid Mech.839, 1-32. (doi:10.1017/jfm.2017.924)) found the optimal, unconstrained kinematic dynamo in a sphere, which, despite being of theoretical importance, is of limited practical use. We extend their work by restricting the optimization to three simple two-mode axisymmetric flows based on the kinematic dynamos of Dudley & James (1989, Proc. R. Soc. Lond. A425, 407-429. (doi:10.1098/rspa.1989.0112)). Using a Lagrangian optimization, we find the smallest critical magnetic Reynolds number for each flow type, measured using an enstrophy-based norm. A Galerkin method is used, in which the spectral coefficients of the fluid flow and magnetic field are updated in order to maximize the final magnetic energy. We consider the t01s01, t01s02 and t02s02 flows and find enstrophy-based critical magnetic Reynolds numbers of 107.7, 142.4 and 125.5 (13.7, 19.6 and 16.4, respectively, with the energy-based definition). These are up to four times smaller than the original flows. These simple and efficient flows may be used as benchmarks in future studies.

RevDate: 2019-10-11

Browne CA, Shih A, SS Datta (2019)

Pore-Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media.

Small (Weinheim an der Bergstrasse, Germany) [Epub ahead of print].

Polymer solutions are frequently used in enhanced oil recovery and groundwater remediation to improve the recovery of trapped nonaqueous fluids. However, applications are limited by an incomplete understanding of the flow in porous media. The tortuous pore structure imposes both shear and extension, which elongates polymers; moreover, the flow is often at large Weissenberg numbers, Wi, at which polymer elasticity in turn strongly alters the flow. This dynamic elongation can even produce flow instabilities with strong spatial and temporal fluctuations despite the low Reynolds number, Re. Unfortunately, macroscopic approaches are limited in their ability to characterize the pore-scale flow. Thus, understanding how polymer conformations, flow dynamics, and pore geometry together determine these nontrivial flow patterns and impact macroscopic transport remains an outstanding challenge. This review describes how microfluidic tools can shed light on the physics underlying the flow of polymer solutions in porous media at high Wi and low Re. Specifically, microfluidic studies elucidate how steady and unsteady flow behavior depends on pore geometry and solution properties, and how polymer-induced effects impact nonaqueous fluid recovery. This work thus provides new insights for polymer dynamics, non-Newtonian fluid mechanics, and applications such as enhanced oil recovery and groundwater remediation.

RevDate: 2019-10-09

Houba T, Dasgupta A, Gopalakrishnan S, et al (2019)

Supersonic turbulent flow simulation using a scalable parallel modal discontinuous Galerkin numerical method.

Scientific reports, 9(1):14442 pii:10.1038/s41598-019-50546-w.

The scalability and efficiency of numerical methods on parallel computer architectures is of prime importance as we march towards exascale computing. Classical methods like finite difference schemes and finite volume methods have inherent roadblocks in their mathematical construction to achieve good scalability. These methods are popularly used to solve the Navier-Stokes equations for fluid flow simulations. The discontinuous Galerkin family of methods for solving continuum partial differential equations has shown promise in realizing parallel efficiency and scalability when approaching petascale computations. In this paper an explicit modal discontinuous Galerkin (DG) method utilizing Implicit Large Eddy Simulation (ILES) is proposed for unsteady turbulent flow simulations involving the three-dimensional Navier-Stokes equations. A study of the method was performed for the Taylor-Green vortex case at a Reynolds number ranging from 100 to 1600. The polynomial order P = 2 (third order accurate) was found to closely match the Direct Navier-Stokes (DNS) results for all Reynolds numbers tested outside of Re = 1600, which had a normalized RMS error of 3.43 × 10-4 in the dissipation rate for a 603 element mesh. The scalability and performance study of the method was then conducted for a Reynolds number of 1600 for polynomials orders from P = 2 to P = 6. The highest order polynomial that was tested (P = 6) was found to have the most efficient scalability using both the MPI and OpenMP implementations.

RevDate: 2019-10-08

Porteous R, Moreau DJ, CJ Doolan (2019)

The effect of the incoming boundary layer thickness on the aeroacoustics of finite wall-mounted square cylinders.

The Journal of the Acoustical Society of America, 146(3):1808.

This paper is concerned with the influence of the incoming wall boundary layer thickness on the noise produced by a square finite wall-mounted cylinder in cross-flow. Acoustic and near wake velocity measurements have been taken in an anechoic wind tunnel for a cylinder in two different near-zero-pressure gradient turbulent boundary layers with thicknesses of 130% and 370% of the cylinder width, W. The cylinders have an aspect ratio of 0.29≤L/W≤22.9 (where L is the cylinder span) and were examined at a Reynolds number, based on width, of ReW = 1.4 × 104. The results presented in this paper demonstrate that increasing the height of the boundary layer delays the production of acoustic tones to higher aspect ratios. The height of the boundary layer changes the balance between upwash and downwash across the cylinder span, resulting in a delayed onset of the shedding regimes and correspondingly, the production of acoustic tones.

RevDate: 2019-10-08

Kawaguchi M, Fukui T, Funamoto K, et al (2019)

Viscosity Estimation of a Suspension with Rigid Spheres in Circular Microchannels Using Particle Tracking Velocimetry.

Micromachines, 10(10): pii:mi10100675.

Suspension flows are ubiquitous in industry and nature. Therefore, it is important to understand the rheological properties of a suspension. The key to understanding the mechanism of suspension rheology is considering changes in its microstructure. It is difficult to evaluate the influence of change in the microstructure on the rheological properties affected by the macroscopic flow field for non-colloidal particles. In this study, we propose a new method to evaluate the changes in both the microstructure and rheological properties of a suspension using particle tracking velocimetry (PTV) and a power-law fluid model. Dilute suspension (0.38%) flows with fluorescent particles in a microchannel with a circular cross section were measured under low Reynolds number conditions (Re ≈ 10-4). Furthermore, the distribution of suspended particles in the radial direction was obtained from the measured images. Based on the power-law index and dependence of relative viscosity on the shear rate, we observed that the non-Newtonian properties of the suspension showed shear-thinning. This method will be useful in revealing the relationship between microstructural changes in a suspension and its rheology.

RevDate: 2019-10-02
CmpDate: 2019-10-02

Singh G, R Lakkaraju (2019)

Wall-mounted flexible plates in a two-dimensional channel trigger early flow instabilities.

Physical review. E, 100(2-1):023109.

A high level of mixing by passive means is a desirable feature in microchannels for various applications, and use of flexible obstacles (or plates) is one of the prime choices in that regard. To gain further insight, we carry out two-dimensional numerical simulations for flow past one or two flexible plates anchored to a channel's opposite walls using a fluid-structure interaction framework. For the inlet flow Reynolds number vs the Strouhal number plane, we observe a sudden flow change from a laminar to a time-periodic vortex shedding state when flexible plates are present in the channel. We found the critical Reynolds number to be Re_{cr}≈370 when a single plate is anchored on the channel wall and Re_{cr}≈290 or even lower when two plates are anchored. With an increase in the inlet flow Reynolds number (up to 3200), we found that vortices detach regularly at the plates' tips, which causes the flow to meander in the channel. In a two-plate anchored configuration, primary vortices generated at the first plate are constrained by the second plate and result in an energetic secondary vortex generation in the downstream side. The overall flow features and the energy dissipation in the channel are mainly controlled by the separation gap between the plates. At high-inlet-flow Reynolds numbers (≥1600), the probability density function (F) of the kinetic energy dissipation in a flexible plate configuration shows a stretched exponential shape in the form F(Z)∼1/sqrt[Z]e^{-pZ^{q}}, where Z is the normalized kinetic energy dissipation and the constants p=0.89 and q=0.86. The observed increase in energy dissipation comes at the cost of an increase in pressure loss in the channel, and we found that the loss is inversely related to the plates' separation gap. From our simulations, we found that if high mixing levels are desired, then two flexible plates anchored to the channel walls is a better choice than a channel flow without obstacles or flow past a single plate. The two-plate configuration with zero separation gap between the plates is best suited to achieve a high mixing level.

RevDate: 2019-10-02
CmpDate: 2019-10-02

Feng Y, Boivin P, Jacob J, et al (2019)

Hybrid recursive regularized lattice Boltzmann simulation of humid air with application to meteorological flows.

Physical review. E, 100(2-1):023304.

An extended version of the hybrid recursive regularized lattice-Boltzmann model which incorporates external force is developed to simulate humid air flows with phase change mechanisms under the Boussinesq approximation. Mass and momentum conservation equations are solved by a regularized lattice Boltzmann approach well suited for high Reynolds number flows, whereas the energy and humidity related equations are solved by a finite volume approach. Two options are investigated to account for cloud formation in atmospheric flow simulations. The first option considers a single conservation equation for total water and an appropriate invariant variable of temperature. In the other approach, liquid and vapor are considered via two separated equations, and phase transition is accounted for via a relaxation procedure. The obtained models are then systematically validated on four well-established benchmark problems including a double diffusive Rayleigh Bénard convection of humid air, two- and three-dimensional thermal moist rising bubble under convective atmospheric environment, as well as a shallow cumulus convection in the framework of large-eddy simulation.

RevDate: 2019-10-02
CmpDate: 2019-10-02

Topayev S, Nouar C, Bernardin D, et al (2019)

Taylor-vortex flow in shear-thinning fluids.

Physical review. E, 100(2-1):023117.

This paper deals with the Taylor-Couette flow of shear-thinning fluids. It focuses on the first principles understanding of the influence of the viscosity stratification and the nonlinear variation of the effective viscosity μ with the shear rate γ[over ̇] on the flow structure in the Taylor-vortex flow regime. A wide gap configuration (η=0.4) is mainly considered. A weakly nonlinear analysis, using the amplitude expansion method at high order, is adopted as a first approach to study nonlinear effects. For the numerical computation, the shear-thinning behavior is described by the Carreau model. The rheological parameters are varied in a wide range. The results indicate that the flow field undergoes a significant change as shear-thinning effects increase. First, vortices are squeezed against the inner wall and the center of the patterns is shifted axially toward the radial outflow boundaries (z=0,z/λ_{z}=1). This axial shift leads to increasing concentration of vorticity at these positions. The outflow becomes stronger than the inflow and the extent of the inflow zone where the vorticity is low increases acoordingly. Nevertheless, the strength of the vortices relative to the velocity of the inner cylinder is weaker. Second, the pseudo-Nusselt number, ratio of the torque to that obtained in the laminar flow, decreases. Third, higher harmonics become more relevant and grow faster with Reynolds number. Finally, the modification of the viscosity field is described.

RevDate: 2019-10-02
CmpDate: 2019-10-02

Krämer A, Wilde D, Küllmer K, et al (2019)

Pseudoentropic derivation of the regularized lattice Boltzmann method.

Physical review. E, 100(2-1):023302.

The lattice Boltzmann method (LBM) facilitates efficient simulations of fluid turbulence based on advection and collision of local particle distribution functions. To ensure stable simulations on underresolved grids, the collision operator must prevent drastic deviations from local equilibrium. This can be achieved by various methods, such as the multirelaxation time, entropic, quasiequilibrium, regularized, and cumulant schemes. Complementing a part of a unified theoretical framework of these schemes, the present work presents a derivation of the regularized lattice Boltzmann method (RLBM), which follows a recently introduced entropic multirelaxation time LBM by Karlin, Bösch, and Chikatamarla (KBC). It is shown that both methods can be derived by locally maximizing a quadratic Taylor expansion of the entropy function. While KBC expands around the local equilibrium distribution, the RLBM is recovered by expanding entropy around a global equilibrium. Numerical tests were performed to elucidate the role of pseudoentropy maximization in these models. Simulations of a two-dimensional shear layer show that the RLBM successfully reproduces the largest eddies even on a 16×16 grid, while the conventional LBM becomes unstable for grid resolutions of 128×128 and lower. The RLBM suppresses spurious vortices more effectively than KBC. In contrast, simulations of the three-dimensional Taylor-Green and Kida vortices show that KBC performs better in resolving small scale vortices, outperforming the RLBM by a factor of 1.8 in terms of the effective Reynolds number.

RevDate: 2019-10-02

Morris RG, M Rao (2019)

Active morphogenesis of epithelial monolayers.

Physical review. E, 100(2-1):022413.

During typical early-stage embryo development, single-cell-thick tissues of tightly bound epithelial cells autonomously generate profound changes in their shape, forming the basis of organism anatomy. We report on a (covariant) active-hydrodynamic theory of such monolayer morphogenesis that is closed under its shape-changing dynamics-i.e., the degrees of freedom that encode monolayer geometry appear properly as broken-symmetry variables. In our theory, the salient physics of tissue-scale deformations emerges from a balance between the displacement and/or shear of a low-Reynolds-number embedding fluid (the "yolk") and cell-autonomous stresses, themselves a result of combining apical contractile stresses with an elastic-like mechanical response under the constraint of constant cell volume. The leading-order hydrodynamic instabilities include both passive constrained-buckling and active deformation, which can be further categorized by cell shape changes that are either "squamous to columnar" or "regular-prism to truncated-pyramid." The deformations resulting from the latter qualitatively reproduce in vivo observations of the onset of both mesoderm and posterior midgut invaginations, which take place during gastrulation in the model organism Drosophila melanogaster.

RevDate: 2019-09-23

Phuong NL, Van Quang T, Khoa ND, et al (2019)

CFD analysis of the flow structure in a monkey upper airway validated by PIV experiments.

Respiratory physiology & neurobiology pii:S1569-9048(19)30218-6 [Epub ahead of print].

Inhalation exposure to airborne contaminants has adverse effects on humans; however, related research is typically conducted using in vivo/in vitro tests on animals. Extrapolating the test results is complicated by anatomical and physiological differences between animals and humans and a lack of understanding of the transport mechanism inside their respective respiratory tracts. This study determined the detailed air-flow structure in the upper airway of a monkey. A steady computational fluid dynamics simulation, which was validated by previous particle image velocimetry measurements, was adopted for flow rates of 4 L/min and 10 L/min to analyze the flow structure from the nasal/oral cavities to the trachea region in a monkey airway model. The low Reynolds number type k-ε model provided a reasonably accurate prediction of the airflow in a monkey upper airway. Furthermore, it was confirmed that large velocity gradients were generated in the nasal vestibule and larynx regions, as well as increased turbulent air kinetic energy and wall sheer stress.

RevDate: 2019-09-22

Shi L, Wu J, Krenn HW, et al (2019)

Temporal model of fluid-feeding mechanisms in a long proboscid orchid bee compared to the short proboscid honey bee.

Journal of theoretical biology pii:S0022-5193(19)30387-X [Epub ahead of print].

Bees (Apidae) are flower-visiting insects that possess highly efficient mouthparts for the ingestion of nectar and other sucrose fluids. Their mouthparts are composed of mandibles and a tube-like proboscis. The proboscis forms a food canal, which encompasses a protrusible and hairy tongue to load and imbibe nectar, representing a fluid-feeding technique with a low Reynolds number. The western honey bee, Apis mellifera ligustica, can rhythmically erect the tongue microtrichia to regulate the glossal shape, achieving a tradeoff between nectar intake rate and viscous drag. Neotropical orchid bees (Euglossa imperialis) possess a proboscis longer than the body and combines this lapping-sucking mode of fluid-feeding with suction feeding. This additional technique of nectar uptake may have different biophysics. In order to reveal the effect of special structures of mouthparts in terms of feeding efficiency, we build a temporal model for orchid bees considering fluid transport in multi-states including active suction, tongue protraction and viscous dipping. Our model indicates that the dipping technique employed by honey bees can contribute to more than seven times the volumetric and energetic intake rate at a certain nectar concentration compared with the combined mode used by orchid bees. The high capability of the honey bee's proboscis to ingest nectar may inspire micropumps for transporting viscous liquid with higher efficiency.

RevDate: 2019-09-20

Onyiriuka EJ, Ighodaro OO, Adelaja AO, et al (2019)

A numerical investigation of the heat transfer characteristics of water-based mango bark nanofluid flowing in a double-pipe heat exchanger.

Heliyon, 5(9):e02416 pii:e02416.

In this study, the heat transfer characteristics of a new class of nanofluids made from mango bark was numerically simulated and studied during turbulent flow through a double pipe heat exchanger. A range of volume fractions was considered for a particle size of 100 nm. A two-phase flow was considered using the mixture model. The mixture model governing equations of continuity, momentum, energy and volume fraction were solved using the finite-volume method. The results showed an increase of the Nusselt number by 68% for a Reynolds number of 5,000 and 45% for a Reynolds number of 13 000, and the heat transfer coefficient of the nanofluid was about twice that of the base fluid. In addition, the Nusselt number decreased by an average value of 0.76 with an increase of volume fraction by 1%. It was also found that there was a range of Reynolds numbers in which the trend of the average heat transfer coefficient of the nanofluid was completely reversed, and several plots showing zones of higher heat transfer which if taken advantage of in design will lead to higher heat transfer while avoiding other zones that have low heat transfer. It is hoped that these results will influence the thermal design of new heat exchangers.

RevDate: 2019-09-19

Thurman D, Culley D, Poinsatte P, et al (2016)


Journal of turbomachinery, 138(9):.

Surface infrared thermography, hotwire anemometry, and thermocouple surveys were performed on two new film cooling hole geometries: spiral/rifled holes and fluidic sweeping holes. The spiral holes attempt to induce large-scale vorticity to the film cooling jet as it exits the hole to prevent the formation of the kidney shaped vortices commonly associated with film cooling jets. The fluidic sweeping hole uses a passive in-hole geometry to induce jet sweeping at frequencies that scale with blowing ratios. The spiral hole performance is compared to that of round holes with and without compound angles. The fluidic hole is of the diffusion class of holes and is therefore compared to a 777 hole and Square holes. A patent-pending spiral hole design showed the highest potential of the non-diffusion type hole configurations. Velocity contours and flow temperature were acquired at discreet cross-sections of the downstream flow field. The passive fluidic sweeping hole shows the most uniform cooling distribution but suffers from low span-averaged effectiveness levels due to enhanced mixing. The data was taken at a Reynolds number of 11,000 based on hole diameter and freestream velocity. Infrared thermography was taken for blowing ratios of 1.0, 1.5, 2.0, and 2.5 at a density ratio of 1.05. The flow inside the fluidic sweeping hole was studied using 3D unsteady RANS.

RevDate: 2019-09-19

Blumenthal BT, Elmiligui AA, Geiselhart KA, et al (2019)

Computational Investigation of a Boundary-Layer Ingesting Propulsion System for the Common Research Model.

Journal of aircraft, 55(3):1141-1153.

The present paper examines potential propulsive and aerodynamic benefits of integrating a Boundary-Layer Ingestion (BLI) propulsion system into the Common Research Model (CRM) geometry and the NASA Tetrahedral Unstructured Software System (TetrUSS). The Numerical Propulsion System Simulation (NPSS) environment is used to generate engine conditions for Computational Fluid Dynamics (CFD) analyses. Improvements to the BLI geometry are made using the Constrained Direct Iterative Surface Curvature (CDISC) design method. Potential benefits of the BLI system relating to cruise propulsive power are quantified using a power balance method, and a comparison to the baseline case is made. Iterations of the BLI geometric design are shown, and improvements between subsequent BLI designs are presented. Simulations are conducted for a cruise flight condition of Mach 0.85 at an altitude of 38,500 feet, with Reynolds number of 40 million based on mean aerodynamic chord and an angle of attack of 2° for all geometries. Results indicate an 8% reduction in engine power requirements at cruise for the BLI configuration compared to the baseline geometry. Small geometric alterations of the aft portion of the fuselage using CDISC has been shown to marginally increase the benefit from boundary-layer ingestion further, resulting in an 8.7% reduction in power requirements for cruise, as well as a drag reduction of approximately twelve counts over the baseline geometry.

RevDate: 2019-09-19

Wang R, Wang J, W Yuan (2019)

Analysis and Optimization of a Microchannel Heat Sink with V-Ribs Using Nanofluids for Micro Solar Cells.

Micromachines, 10(9): pii:mi10090620.

It is crucial to control the temperature of solar cells for enhancing efficiency with the increasing power intensity of multiple photovoltaic systems. In order to improve the heat transfer efficiency, a microchannel heat sink (MCHS) with V-ribs using a water-based nanofluid as a coolant for micro solar cells was designed. Numerical simulations were carried out to investigate the flows and heat transfers in the MCHS when the Reynolds number ranges from 200 to 1000. The numerical results showed that the periodically arranged V-ribs can interrupt the thermal boundary, induce chaotic convection, increase heat transfer area, and subsequently improve the heat transfer performance of a MCHS. In addition, the preferential values of the geometric parameters of V-ribs and the physical parameters of the nanofluid were obtained on the basis of the Nusselt numbers at identical pump power. For MCHS with V-ribs on both the top and bottom wall, preferential values of V-rib are rib width d / W = 1 , flare angle α = 75 ° , rib height h r / H = 0.3 , and ratio of two slant sides b / a = 0.75 , respectively. This can provide sound foundations for the design of a MCHS in micro solar cells.

RevDate: 2019-09-19

Tang L, Pan X, Feng J, et al (2019)

Experimental Investigation on the Relationship Between COD Degradation and Hydrodynamic Conditions in Urban Rivers.

International journal of environmental research and public health, 16(18): pii:ijerph16183447.

Due to extensive pollution and the relatively weak flow replacement in urban rivers, determining how to fully utilize the self-purification abilities of water bodies for water quality protection has been a complex and popular topic of research and social concern. Organic pollution is an important type of urban river pollution, and COD (chemical oxygen demand) is one of the key pollution factors. Currently, there is a lack of research on the relationship between COD degradation and the flow characteristics of urban rivers. In this paper, COD degradation experiments were conducted in an annular flume with Jinjiang River water at controlled flow velocities and the COD degradation coefficients under different hydraulic conditions were analyzed. A good correlation was observed between the degradation coefficient and hydraulic conditions. According to dimensional analysis, the relationship between the COD degradation coefficient and hydraulic conditions such as the flow velocity, water depth, Reynolds number (Re), and Froude number (Fr) was established as K COD = 86400 u h F r 0.8415 R e - 1.2719 + 0.258 . The COD degradation coefficients of the Chishui River in Guizhou Province ranged from 0.175-0.373 1/d based on this formula, and the field-measured values varied from 0.234-0.463 1/d. The error in the formula ranged from 5.4-25.3%. This study provides a scientific basis for the prediction of the COD degradation coefficients of urban rivers.

RevDate: 2019-09-18

Wang C, Ren F, H Tang (2019)

Enhancing propulsion performance of a flexible heaving foil through dynamically adjusting its flexibility.

Bioinspiration & biomimetics [Epub ahead of print].

This study investigates how dynamically adjusting the bending stiffness of a heaving foil affects its propulsion performance in a flow of Reynolds number 200. The foil is forced to oscillate sinusoidally at the leading edge, and its bending stiffness is tuned in a square-wave manner. Such a fluid-structure interaction (FSI) problem is explored using an immersed boundary lattice Boltzmann method (IBLBM) based numerical framework. The results reveal that when the lower and upper bounds of the foil's time-dependent bending stiffness are moderate, the net thrust can be evidently enhanced compared to those in the corresponding constant-bending-stiffness cases, while the propulsion efficiency is not apparently ameliorated. The most significant enhancement is observed when the bending stiffness has lower and upper bounds of the same duration (i.e., a duty cycle of 1/2) and also it remains at the lower bound during stroke reversals (corresponding to an actuation phase angle of π/2). When the two bounds simultaneously increase or decrease, however, dynamically adjusting the bending stiffness fails to improve the net thrust. Through this study, competitions among various forces/moments, including the inertial force, tension force, bending moment and fluid loading, acting on the foil and their influences on the foil's dynamics are also unveiled.

RevDate: 2019-09-12

Acosta-Avalos D, E Rodrigues (2019)

On the motion of magnetotactic bacteria: theoretical predictions and experimental observations.

European biophysics journal : EBJ pii:10.1007/s00249-019-01394-z [Epub ahead of print].

The movement of magnetotactic bacteria is done in a viscous media in the low Reynolds number regime. In the present research, the simple model for magnetotactic bacteria motion, proposed by Nogueira and Lins de Barros (Eur Biophys J 24:13-21, 1995), was used to numerically simulate their trajectory. The model was done considering a spherical bacterium with a single flagellum and a magnetic moment positioned in the sphere center and parallel to the flagella. The numerical solution shows that the trajectory is a cylindrical helix and that the body Euler angles have linear dependencies on time. Using that information, analytical expressions were obtained for the first time for the center-of-mass coordinates, showing that the trajectories are helixes oriented to the magnetic field direction. They also show that the magnetic moment does not align to the magnetic field, but it precesses around it, being fully oriented only for very high magnetic fields. The analytical solution obtained permits to relate for the first time the flagellar force to the axial velocity and helical radius. Trajectories of uncultivated magnetotactic bacteria were registered in video and the coordinates were obtained for several bacteria in different magnetic fields. The trajectories showed to be a complex mixture of two oscillating functions: one with frequency lower than 5 Hz and the other one with frequency higher than 10 Hz. The simple model of Nogueira and Lins de Barros shows to be incomplete, because is unable to explain the trajectories composed of two oscillating functions observed in uncultivated magnetotactic bacteria.

RevDate: 2019-09-11
CmpDate: 2019-09-11

Falsaperla P, Giacobbe A, G Mulone (2019)

Nonlinear stability results for plane Couette and Poiseuille flows.

Physical review. E, 100(1-1):013113.

We prove that the plane Couette and Poiseuille flows are nonlinearly stable if the Reynolds number is less than Re_{Orr}(2π/(λsinθ))/sinθ when a perturbation is a tilted perturbation in the direction x^{'} which forms an angle θ∈(0,π/2] with the direction i of the basic motion and does not depend on x^{'}. Re_{Orr} is the critical Orr-Reynolds number for spanwise perturbations which is computed for wave number 2π/(λsinθ), with λ being any positive wavelength. By taking the minimum with respect to λ, we obtain the critical energy Reynolds number for a fixed inclination angle and any wavelength: for plane Couette flow, it is Re_{Orr}=44.3/sinθ, and for plane Poiseuille flow, it is Re_{Orr}=87.6/sinθ (in particular, for θ=π/2 we have the classical values Re_{Orr}=44.3 for plane Couette flow and Re_{Orr}=87.6 for plane Poiseuille flow). Here the nondimensional interval between the planes bounding the channel is [-1,1]. In particular, these results improve those obtained by Joseph, who found for streamwise perturbations a critical nonlinear value of 20.65 in the plane Couette case, and those obtained by Joseph and Carmi who found the value 49.55 for plane Poiseuille flow for streamwise perturbations. If we fix some wavelengths from the experimental data and the numerical simulations, the critical Reynolds numbers that we obtain are in a very good agreement both with the the experiments and the numerical simulation. These results partially solve the Couette-Sommerfeld paradox.

RevDate: 2019-09-09

Shah F, Khan MI, Hayat T, et al (2019)

Theoretical and mathematical analysis of entropy generation in fluid flow subject to aluminum and ethylene glycol nanoparticles.

Computer methods and programs in biomedicine, 182:105057 pii:S0169-2607(19)31393-8 [Epub ahead of print].

BACKGROUND: Here we have conducted a magnetohydrodynamic (MHD) flow of viscous material with alumina water and ethylene glycol over a stretched surface. The flow is discussed with and without effective Prandtl number. MHD liquid is considered. Electric field is absent. Effect of uniform magnetic field is taken in the vertical direction to the surface. Influence of thermal radiation as well as Joule heating are taken into account for both aluminum oxide-water and aluminum oxide-Ethylene glycol nanofluids. Velocity slip and melting heat effects are considered.

METHODS: The nonlinear flow expressions are numerically solved via ND-solve technique (built-in-Shooting).

RESULTS: The physical impacts of flow variables like mixed convection parameter, magnetic parameter, Reynold number, Eckert number, melting parameter and heat source/sink parameter are graphically discussed. Moreover, entropy generation (irreversibility) and Bejan number are discussed graphically through various flow variables. Physical quantities like skin friction coefficient and Sherwood and Nusselt numbers are numerically calculated and discussed through Tables.

CONCLUSIONS: Impact of magnetic and slip parameters on the velocity field show decreasing behavior for both effective and without effective Prandtl number. Temperature field increases for both effective and without effective Prandtl number for higher values of magnetic and radiative parameters. Entropy number is an increasing function of Reynolds number while Bejan number shows opposite impact against Reynolds number. Moreover, heat transfer rate upsurges versus larger melting and radiative parameter.

RevDate: 2019-09-04

DeJonckere P, J Lebacq (2019)

Intraglottal Aerodynamics at Vocal Fold Vibration Onset.

Journal of voice : official journal of the Voice Foundation pii:S0892-1997(19)30237-1 [Epub ahead of print].

The most frequently observed type of voice onset in spontaneous speech in normal subjects is the soft onset, and it may be considered as the "physiological" onset. It starts from an immobile narrow glottal slit crossed by a continuous airflow, and then a few oscillations (even a single one in some cases) precede the first glottal closure. It is a transient event, during which the acting forces, lung pressure, intraglottal pressure, myoelastic tension of the vocal fold (VF) oscillator and inertance of the supraglottal vocal tract, interact to progressively reach the steady state of a sustained oscillation. Combined measurements of flow, area, and pressure provide a detailed qualitative and quantitative analysis of the intraglottal mechanical events at the precise moment of starting oscillation in a physiological (soft or soft/breathy) onset. Our in vivo measurements of airflow and glottal area show that the very first oscillation occurs exactly at the time when turbulence appears at the level of the glottal narrowing, ie, when the Reynolds number reaches its critical value. The turbulence may be assumed to trigger an oscillator consisting in the ensemble of the VFs and the air of the vocal tract, which is known to be weakly damped. Turbulence can act here as an aspecific flick, triggering the oscillator, the frequency of oscillation being determined by its mechanical properties. Furthermore, the first noticeable glottal oscillations are sinusoidal: the VFs are neither steeply sucked together by a negative Bernoulli pressure, nor burst apart by the lung pressure. Our measurements show that, at the critical time, the rising positive lung pressure is balanced by the rising negative Bernoulli pressure generated by the transglottal flow.

RevDate: 2019-09-04

Tsai CD, XY Lin (2019)

Experimental Study on Microfluidic Mixing with Different Zigzag Angles.

Micromachines, 10(9): pii:mi10090583.

This paper presents experimental investigations of passive mixing in a microfluidic channel with different zigzag angles. Zigzag channel is commonly used for microfluidic mixing because it does not need an additional control unit and can be easily implemented in a lab-on-a-chip system. In this work, microfluidic channels with six different zigzag angles, from θ = 0° to θ = 75°, are tested under ten different flow rates corresponding to Reynolds number from 0.309 to 309. Two colored liquids are mixed with the zigzag channels and mixing performance is evaluated based on the color of the pixels on the region of interest from captured images. According to the results, we found that the mixing performance is almost independent of the zigzag angle in the low-speed regime where its Reynolds number is less than 4. The mixing became very much depending on the zigzag angle in the high-speed regime where its Reynolds number is greater than 100. Microfluidic mixing is needed for Lab-on-a-chip applications in both low flow speed, such as medium perfusion for cell culture, and high flow speed, such as high-speed sensing on a point-of-care device. This work is aimed to provide practical information on zigzag mixing for chip design and applications.

RevDate: 2019-09-01

Waheed W, Alazzam A, Al-Khateeb AN, et al (2019)

Dissipative particle dynamics for modeling micro-objects in microfluidics: application to dielectrophoresis.

Biomechanics and modeling in mechanobiology pii:10.1007/s10237-019-01216-3 [Epub ahead of print].

The dissipative particle dynamics (DPD) technique is employed to model the trajectories of micro-objects in a practical microfluidic device. The simulation approach is first developed using an in-house Fortran code to model Stokes flow at Reynolds number of 0.01. The extremely low Reynolds number is achieved by adjusting the DPD parameters, such as force coefficients, thermal energies of the particles, and time steps. After matching the numerical flow profile with the analytical results, the technique is developed further to simulate the deflection of micro-objects under the effect of a deflecting external force in a rectangular microchannel. A mapping algorithm is introduced to establish the scaling relationship for the deflecting force between the physical device and the DPD domain. Dielectrophoresis is studied as a case study for the deflecting force, and the trajectory of a single red blood cell under the influence of the dielectrophoretic force is simulated. The device is fabricated using standard microfabrication techniques, and the experiments involving a dilute sample of red blood cells are performed at two different cases of the actuation voltage. Good agreement between the numerical and experimental results is achieved.

RevDate: 2019-09-01

Chaput R, Majoris JE, Buston PM, et al (2019)

Hydrodynamic and biological constraints on group cohesion in plankton.

Journal of theoretical biology pii:S0022-5193(19)30330-3 [Epub ahead of print].

The dynamics of plankton in the ocean are determined by biophysical interactions. Although physics and biotic behaviors are known to influence the observed patchiness of planktonic populations, it is still unclear how much, and if, group behavior contributes to this biophysical interaction. Here, we demonstrate how simple rules of behavior can enhance or inhibit active group cohesion in plankton in a turbulent environment. In this study, we used coral-reef fish larvae as a model to investigate the interaction between microscale turbulence and planktonic organisms. We synthesized available information on the swimming speeds and sizes of reef fish larvae, and developed a set of equations to investigate the effects of viscosity and turbulence on larvae dispersion. We then calculated the critical dispersion rates for three different swimming strategies - cruise, random-walk, and pause-travel - to determine which strategies could facilitate group cohesion during dispersal. Our results indicate that swimming strategies and migration to low-turbulence regions are the key to maintaining group cohesion, suggesting that many reef fish species have the potential to remain together, from hatching to settlement. In addition, larvae might change their swimming strategies to maintain group cohesion, depending on environmental conditions and/or their ontogenic stage. This study provides a better understanding of the hydrodynamic and biological constraints on group formation and cohesion in planktonic organisms, and reveals a wide range of conditions under which group formation may occur.

RevDate: 2019-09-01

Zhang C, Sanjose M, S Moreau (2019)

Aeolian noise of a cylinder in the critical regime.

The Journal of the Acoustical Society of America, 146(2):1404.

The noise from the flow around a circular cylinder in the critical regime is investigated by combining a compressible wall-resolved large eddy simulation and a Ffowcs Williams and Hawkings analogy on solid and porous surfaces. This simulation is validated by comparing several flow parameters with previous experimental and numerical data in the same flow regime. Significantly reduced drag and increased vortex shedding Strouhal number (0.33) are observed. Two slightly asymmetric laminar separation bubbles (LSBs) on the cylinder surface at about 100° are shown to trigger turbulence through Kelvin-Helmholtz (KH) shear-layer instability. The latter contributes to a narrowband hump in the wall-pressure fluctuations with a tone at a Strouhal number of 27, which can be as intense as the dominant vortex shedding tone. The ratio of the corresponding Strouhal numbers is consistent with the proposed variation with the Reynolds number by Prasad and Williamson [(1997). J. Fluid Mech. 333, 375-402]. The dominant far-field noise source is still the vortex shedding dipolar tone radiating mostly at 90°. Yet, two additional broadband noise sources are evidenced in the wake, one at low frequencies caused by the wake oscillation and another one at high frequencies caused by the KH instability mostly directly toward the LSB locations.

RevDate: 2019-09-01

Gholampour S, Bahmani M, A Shariati (2019)

Comparing the Efficiency of Two Treatment Methods of Hydrocephalus: Shunt Implantation and Endoscopic Third Ventriculostomy.

Basic and clinical neuroscience, 10(3):185-198.

Introduction: Hydrocephalus is one of the most common diseases in children, and its treatment requires brain operation. However, the pathophysiology of the disease is very complicated and still unknown.

Methods: Endoscopic Third Ventriculostomy (ETV) and Ventriculoperitoneal Shunt (VPS) implantation are among the common treatments of hydrocephalus. In this study, Cerebrospinal Fluid (CSF) hydrodynamic parameters and efficiency of the treatment methods were compared with numerical simulation and clinical follow-up of the treated patients.

Results: Studies have shown that in patients under 19 years of age suffering from hydrocephalus related to a Posterior Fossa Brain Tumor (PFBT), the cumulative failure rate was 21% and 29% in ETV and VPS operation, respectively. At first, the ETV survival curve shows a sharp decrease and after two months it gets fixed while VPS curve makes a gradual decrease and reaches to a level lower than ETV curve after 5.7 months. Post-operative complications in ETV and VPS methods are 17% and 31%, respectively. In infants younger than 12 months with hydrocephalus due to congenital Aqueduct Stenosis (AS), and also in the elderly patients suffering from Normal Pressure Hydrocephalus (NPH), ETV is a better treatment option. Computer simulations show that the maximum CSF pressure is the most reliable hydrodynamic index for the evaluation of the treatment efficacy in these patients. After treatment by ETV and shunt methods, CSF pressure decreases about 9 and 5.3 times, respectively and 2.5 years after shunt implantation, this number returns to normal range.

Conclusion: In infants with hydrocephalus, initial treatment by ETV was more reasonable than implanting the shunt. In adult with hydrocephalus, the initial failure in ETV occurred sooner compared to shunt therapy; however, ETV was more efficient.

RevDate: 2019-08-25

Foo YY, Pant S, Tay HS, et al (2019)

4D modelling of fluid mechanics in the zebrafish embryonic heart.

Biomechanics and modeling in mechanobiology pii:10.1007/s10237-019-01205-6 [Epub ahead of print].

Abnormal blood flow mechanics can result in pathological heart malformation, underlining the importance of understanding embryonic cardiac fluid mechanics. In the current study, we performed image-based computational fluid dynamics simulation of the zebrafish embryonic heart ventricles and characterized flow mechanics, organ dynamics, and energy dynamics in detail. 4D scans of 5 days post-fertilization embryonic hearts with GFP-labelled myocardium were acquired using line-scan focal modulation microscopy. This revealed that the zebrafish hearts exhibited a wave-like contractile/relaxation motion from the inlet to the outlet during both systole and diastole, which we showed to be an energy efficient configuration. No impedance pumping effects of pressure and velocity waves were observed. Due to its tube-like configuration, inflow velocities were higher near the inlet and smaller at the outlet and vice versa for outflow velocities. This resulted in an interesting spatial wall shear stress (WSS) pattern where WSS waveforms near the inlet and those near the outlet were out of phase. There was large spatial variability in WSS magnitudes. Peak WSS was in the range of 47.5-130 dyne/cm2 at the inflow and outflow tracts, but were much smaller, in the range of 4-11 dyne/cm2, in the mid-ventricular segment. Due to very low Reynolds number and the highly viscous environment, intraventricular pressure gradients were high, suggesting substantial energy losses of flow through the heart.

RevDate: 2019-08-23

Rackus DG, Riedel-Kruse IH, N Pamme (2019)

"Learning on a chip:" Microfluidics for formal and informal science education.

Biomicrofluidics, 13(4):041501 pii:017903BMF.

Microfluidics is a technique for the handling of small volumes of liquids on the order of picoliters to nanoliters and has impact for miniaturized biomedical science and fundamental research. Because of its multi- and interdisciplinary nature (i.e., combining the fields of biology, chemistry, physics, and engineering), microfluidics offers much potential for educational applications, both at the university level as well as primary and secondary education. Microfluidics is also an ideal "tool" to enthuse and educate members of the general public about the interdisciplinary aspects of modern sciences, including concepts of science, technology, engineering, and mathematics subjects such as (bio)engineering, chemistry, and biomedical sciences. Here, we provide an overview of approaches that have been taken to make microfluidics accessible for formal and informal learning. We also point out future avenues and desired developments. At the extreme ends, we can distinguish between projects that teach how to build microfluidic devices vs projects that make various microscopic phenomena (e.g., low Reynolds number hydrodynamics, microbiology) accessible to learners and the general public. Microfluidics also enables educators to make experiments low-cost and scalable, and thereby widely accessible. Our goal for this review is to assist academic researchers working in the field of microfluidics and lab-on-a-chip technologies as well as educators with translating research from the laboratory into the lecture hall, teaching laboratory, or public sphere.

RevDate: 2019-08-30

Hayat T, Aslam N, Ijaz Khan M, et al (2019)

MHD peristaltic motion of Johnson-Segalman fluid in an inclined channel subject to radiative flux and convective boundary conditions.

Computer methods and programs in biomedicine, 180:104999 pii:S0169-2607(19)31148-4 [Epub ahead of print].

BACKGROUND: In abundant of a digestive tract like smooth muscle tissue, human gastrointestinal tract contracts in sequence to generate a peristaltic wave, which pushes a food along the tract. The peristaltic motion contains circular relaxation smooth muscles, then their shrinkage (contraction) behind the chewed material to keep it from moving backward, then longitudinal contraction to shove it ahead. Therefore, we have conducted a theoretical investigation on peristaltic transport in flow of Johnson-Segalman liquid subject to inclined magnetic field. The energy equation is developed with extra heat transport assumptions like thermal radiative flux and dissipation. The channel walls are heated convectively.

METHODS: Dimensionless problems subject to small Reynolds number and long wavelength are tackled. Perturbation technique is implemented for small Weissenberg number.

RESULTS: The physical importance of involved parameters that directly affect the heat transfer rate temperature and velocity. The pertinent variables are amplitude ratio, wave number, Reynolds number, Hartman number, Prandtl number, Weissenberg number, thermal radiative heat flux, Biot number, elasticity variables and Froude number are graphically discussed. The obtained outcome shows that the velocity field increases against higher values of elasticity variables but velocity the material decays through higher fluid parameter. Temperature field declines through higher Hartman number. Furthermore, it is also examined that the heat transfer rate decays against rising Hartman number.

CONCLUSIONS: The impact of complaint walls on radiative peristaltic transport of Johnson-Segalman liquid in symmetric channel subject to inclined angle. The influence of Johnson-Segalman variable on the velocity field shows decreasing behavior. Velocity also declines against larger Hartman number. Temperature and heat transfer rate boosts through rising values of E1 E2 while decays versus larger E3. Furthermore, reduction in heat transfer coefficient is observed when the values of α and Br are increased.

RevDate: 2019-08-30

Tanveer A, Khan M, Salahuddin T, et al (2019)

Numerical simulation of electroosmosis regulated peristaltic transport of Bingham nanofluid.

Computer methods and programs in biomedicine, 180:105005 pii:S0169-2607(19)30436-5 [Epub ahead of print].

The effects of slip condition and Joule heating on the peristaltic flow of Bingham nanofluid are investigated. The flow is taken in a porous channel with elastic walls. Mathematical formulation is presented under the assumption of long wavelength and small Reynolds number. The transformed equations for the flow are solved to seek values for the nanoparticles velocity, concentration and temperature along the channel length. Graphs are plotted to evaluate the behavior of various physical parameters on flow quantities in both slip and no-slip cases. The main features of the physical parameters are highlighted on the inclined non uniform channel. The results show an increment in velocity with rise in inclination and porosity while it reduces with magnetic field. Moreover, nanofluid favors the heat transfer and decline the concentration.

RevDate: 2019-08-18

Lambert WB, Stanek MJ, Gurka R, et al (2019)

Leading-edge vortices over swept-back wings with varying sweep geometries.

Royal Society open science, 6(7):190514 pii:rsos190514.

Micro air vehicles are used in a myriad of applications, such as transportation and surveying. Their performance can be improved through the study of wing designs and lift generation techniques including leading-edge vortices (LEVs). Observation of natural fliers, e.g. birds and bats, has shown that LEVs are a major contributor to lift during flapping flight, and the common swift (Apus apus) has been observed to generate LEVs during gliding flight. We hypothesize that nonlinear swept-back wings generate a vortex in the leading-edge region, which can augment the lift in a similar manner to linear swept-back wings (i.e. delta wing) during gliding flight. Particle image velocimetry experiments were performed in a water flume to compare flow over two wing geometries: one with a nonlinear sweep (swift-like wing) and one with a linear sweep (delta wing). Experiments were performed at three spanwise planes and three angles of attack at a chord-based Reynolds number of 26 000. Streamlines, vorticity, swirling strength, and Q-criterion were used to identify LEVs. The results show similar LEV characteristics for delta and swift-like wing geometries. These similarities suggest that sweep geometries other than a linear sweep (i.e. delta wing) are capable of creating LEVs during gliding flight.

RevDate: 2019-08-15

Brown AI, DA Sivak (2019)

Theory of Nonequilibrium Free Energy Transduction by Molecular Machines.

Chemical reviews [Epub ahead of print].

Biomolecular machines are protein complexes that convert between different forms of free energy. They are utilized in nature to accomplish many cellular tasks. As isothermal nonequilibrium stochastic objects at low Reynolds number, they face a distinct set of challenges compared with more familiar human-engineered macroscopic machines. Here we review central questions in their performance as free energy transducers, outline theoretical and modeling approaches to understand these questions, identify both physical limits on their operational characteristics and design principles for improving performance, and discuss emerging areas of research.

RevDate: 2019-08-13

Berg O, Singh K, Hall MR, et al (2019)

Thermodynamics of the bladderwort feeding strike-suction power from elastic energy storage.

Integrative and comparative biology pii:5548213 [Epub ahead of print].

The carnivorous plant bladderwort exemplifies the use of accumulated elastic energy to power motion: respiration-driven pumps slowly load the walls of its suction traps with elastic energy (~1 h). During a feeding strike, this energy is released suddenly to accelerate water (~ 1 ms). However, due to the traps' small size and concomitant low Reynolds number, a significant fraction of the stored energy may be dissipated as viscous friction. Such losses and the mechanical reversibility of Stokes flow are thought to degrade the feeding success of other suction feeders in this size range, such as larval fish. In contrast, triggered bladderwort traps are generally successful. By mapping the energy budget of a bladderwort feeding strike, we illustrate how this smallest of suction feeders can perform like an adult fish. The elastic energy stored in loaded bladders-pressure-volume work performed during the loading process-is in the range of 1 µJ, as measured via the volume evacuated during loading, and literature values of internal pressure. We determined the kinetic energy present in the fluid during suction events from flow fields obtained by Particle Image Velocimetry. Such observations are confounded by the difficult-to-resolve timescale and internal flows, so we obtained independent estimates from mathematical and mechanical models. At the beginning of a feeding strike, we find that 0.5 mW of power are delivered by the elastic recoil mechanism, and the same amount appears as kinetic energy in the flow field. A power deficit would represent viscous dissipation heating the fluid by friction. Approximate solution of the Navier-Stokes equations for an idealized bladderwort strike suggests that less than 20% of energy is lost to friction on the timescale relevant to prey capture. This discrepancy would indeed be difficult to detect experimentally. However, even this upper limit is small in comparison with the 60% losses calculated for fish larvae of similar size, the suction of which is assumed to be muscle powered (and for which elastic energy accumulation has not been demonstrated). Our estimates of elastic energy storage and frictional losses during suction events support the hypothesis that small suction feeders convert a large proportion of the elastic energy stored in the trap walls into kinetic energy of the inspired water, with little energy thermalized due to friction.

RevDate: 2019-08-09

Alazzam A, Al-Khaleel M, Riahi MK, et al (2019)

Dielectrophoresis Multipath Focusing of Microparticles through Perforated Electrodes in Microfluidic Channels.

Biosensors, 9(3): pii:bios9030099.

This paper presents focusing of microparticles in multiple paths within the direction of the flow using dielectrophoresis. The focusing of microparticles is realized through partially perforated electrodes within the microchannel. A continuous electrode on the top surface of the microchannel is considered, while the bottom side is made of a circular meshed perforated electrode. For the mathematical model of this microfluidic channel, inertia, buoyancy, drag and dielectrophoretic forces are brought up in the motion equation of the microparticles. The dielectrophoretic force is accounted for through a finite element discretization taking into account the perforated 3D geometry within the microchannel. An ordinary differential equation is solved to track the trajectories of the microparticles. For the case of continuous electrodes using the same mathematical model, the numerical simulation shows a very good agreement with the experiments, and this confirms the validation of focusing of microparticles within the proposed perforated electrode microchannel. Microparticles of silicon dioxide and polystyrene are used for this analysis. Their initial positions and radius, the Reynolds number, and the radius of the pore in perforated electrodes mainly conduct microparticles trajectories. Moreover, the radius of the pore of perforated electrode is the dominant factor in the steady state levitation height.

RevDate: 2019-08-07

Samanta T, Tian H, VM Nakariakov (2019)

Evidence for Vortex Shedding in the Sun's Hot Corona.

Physical review letters, 123(3):035102.

Vortex shedding is an oscillating flow that is commonly observed in fluids due to the presence of a blunt body in a flowing medium. Numerical simulations have shown that the phenomenon of vortex shedding could also develop in the magnetohydrodynamic (MHD) domain. The dimensionless Strouhal number, the ratio of the blunt body diameter to the product of the period of vortex shedding and the speed of a flowing medium, is a robust indicator for vortex shedding, and, generally of the order of 0.2 for a wide range of Reynolds number. Using an observation from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we report a wavelike or oscillating plasma flow propagating upward against the Sun's gravitational force. A newly formed shrinking loop in the postflare region possibly generates the oscillation of the upflow in the wake of the hot and dense loop through vortex shedding. The computed Strouhal number is consistent with the prediction from previous MHD simulations. Our observation suggests the possibility of vortex shedding in the solar corona.

RevDate: 2019-08-06

Yu X, Li Y, Liu Y, et al (2019)

Flow Patterns of Viscoelastic Fracture Fluids in Porous Media: Influence of Pore-Throat Structures.

Polymers, 11(8): pii:polym11081291.

Viscoelastic surfactant (VES) fluid and hydrolyzed polyacryamide (HPAM) solution are two of the most common fracturing fluids used in the hydraulic fracturing development of unconventional reservoirs. The filtration of fracturing fluids in porous media is mainly determined by the flow patterns in pore-throat structures. In this paper, three different microdevices analogue of porous media allow access to a large range of Deborah number (De) and concomitantly low Reynolds number (Re). Continuous pore-throat structures were applied to study the feedback effect of downstream structure on upstream flow of VES fluid and HPAM solution with Deborah (De) number from 1.11 to 146.4. In the infinite straight channel, flow patterns between VES fluids and HPAM solution were similar. However, as pore length shortened to 800 μm, flow field of VES fluid exhibited the triangle shape with double-peaks velocity patterns. The flow field of HPAM solution presented stable and centralized streamlines when Re was larger than 4.29 × 10-2. Additionally, when the pore length was further shortened to 400 μm, double-peaks velocity patterns were vanished for VES fluid and the stable convergent flow characteristic of HPAM solution was observed with all flow rates.

RevDate: 2019-08-20

Feng X, Ren Y, Hou L, et al (2019)

Tri-fluid mixing in a microchannel for nanoparticle synthesis.

Lab on a chip, 19(17):2936-2946.

It is becoming more difficult to use bulk mixing and bi-fluid micromixing in multi-step continuous-flow reactions, multicomponent reactions, and nanoparticle synthesis because they typically involve multiple reactants. To date, most micromixing studies, both passive and active, have focused on how to efficiently mix two fluids, while micromixing of three or more fluids together (multi-fluid mixing) has been rarely explored. This study is the first on tri-fluid mixing in microchannels. We investigated tri-fluid mixing in three microchannel models: a straight channel, a classical staggered herringbone mixing (SHM) channel, and a three-dimensional (3D) X-crossing microchannel. Numerical simulations and experiments were jointly conducted. A two-step experimental process was performed to determine the tri-fluid mixing efficiencies of these microchannels. We found that the SHM cannot significantly enhance mixing of three streams especially for a Reynolds number (Re) higher than 10. However, the 3D X-crossing channel based on splitting-and-recombination (SAR) showed effective tri-mixing performance over a wide Re range up to 275 (with a corresponding flow rate of 1972.5 μL min-1), thereby enabling high microchannel throughput. Furthermore, this tri-fluid micromixing process was used to synthesize a kind of Si-based nanoparticle. This achieved a narrower particle size distribution than traditional bulk mixing. Therefore, SAR-based tri-fluid mixing is an alternative for chemical and biochemical reactions where three reactants need to be mixed.

RevDate: 2019-08-03

Henein C, Awwad S, Ibeanu N, et al (2019)

Hydrodynamics of Intravitreal Injections into Liquid Vitreous Substitutes.

Pharmaceutics, 11(8): pii:pharmaceutics11080371.

Intravitreal injections have become the cornerstone of retinal care and one of the most commonly performed procedures across all medical specialties. The impact of hydrodynamic forces of intravitreal solutions when injected into vitreous or vitreous substitutes has not been well described. While computational models do exist, they tend to underestimate the starting surface area of an injected bolus of a drug. Here, we report the dispersion profile of a dye bolus (50 µL) injected into different vitreous substitutes of varying viscosities, surface tensions, and volumetric densities. A novel 3D printed in vitro model of the vitreous cavity of the eye was designed to visualize the dispersion profile of solutions when injected into the following vitreous substitutes-balanced salt solution (BSS), sodium hyaluronate (HA), and silicone oils (SO)-using a 30G needle with a Reynolds number (Re) for injection ranging from approximately 189 to 677. Larger bolus surface areas were associated with faster injection speeds, lower viscosity of vitreous substitutes, and smaller difference in interfacial surface tensions. Boluses exhibited buoyancy when injected into standard S1000. The hydrodynamic properties of liquid vitreous substitutes influence the initial injected bolus dispersion profile and should be taken into account when simulating drug dispersion following intravitreal injection at a preclinical stage of development, to better inform formulations and performance.

RevDate: 2019-08-05

Gvozdić B, Dung OY, van Gils DPM, et al (2019)

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer.

The Review of scientific instruments, 90(7):075117.

A new vertical water tunnel with global temperature control and the possibility for bubble and local heat and mass injection has been designed and constructed. The new facility offers the possibility to accurately study heat and mass transfer in turbulent multiphase flow (gas volume fraction up to 8%) with a Reynolds-number range from 1.5 × 104 to 3 × 105 in the case of water at room temperature. The tunnel is made of high-grade stainless steel permitting the use of salt solutions in excess of 15% mass fraction. The tunnel has a volume of 300 l. The tunnel has three interchangeable measurement sections of 1 m height but with different cross sections (0.3 × 0.04 m2, 0.3 × 0.06 m2, and 0.3 × 0.08 m2). The glass vertical measurement sections allow for optical access to the flow, enabling techniques such as laser Doppler anemometry, particle image velocimetry, particle tracking velocimetry, and laser-induced fluorescent imaging. Local sensors can be introduced from the top and can be traversed using a built-in traverse system, allowing, for example, local temperature, hot-wire, or local phase measurements. Combined with simultaneous velocity measurements, the local heat flux in single phase and two phase turbulent flows can thus be studied quantitatively and precisely.

RevDate: 2019-08-01

O'Neill G, NS Tolley (2019)

The Complexities of Nasal Airflow - Theory and Practice.

Journal of applied physiology (Bethesda, Md. : 1985) [Epub ahead of print].

The objective of this study was to investigate the effects of nasal valve area, valve stiffness and turbinate region cross-sectional area on airflowrate, nasal resistance, flow limitation and inspiratory 'hysteresis' by the use of a mathematical model of nasal airflow. The model of O'Neill and Tolley (1988) describing the effects of valve area and stiffness on the nasal pressure-flow relationship was improved by the incorporation of additional terms involving i) airflow through the turbinate region, ii) the dependency of the flow coefficients for the valve and turbinate region on the Reynolds number and iii) effects of unsteady flow. The model was found to provide a good fit for normal values for nasal resistance and for pressure-flow curves reported in the literature for both congested and decongested states. Also, by showing the relative contribution of the nasal valve and turbinate region to nasal resistance, the model sheds light in explaining the generally poor correlation between nasal resistance measurements and the results from acoustic rhinometry. Furthermore, by proposing different flow conditions for the acceleration and deceleration phases of inspiration, the model produces an inspiratory loop (commonly referred to as 'hysteresis') consistent with those reported in the literature. With simulation of nasal flaring, the magnitude of the loop, the nasal resistance and flow limitation all show a similar change as observed in the experimental results.

RevDate: 2019-07-31

R Ferreira R, Fukui H, Chow R, et al (2019)

The cilium as a force sensor-myth versus reality.

Journal of cell science, 132(14): pii:132/14/jcs213496.

Cells need to sense their mechanical environment during the growth of developing tissues and maintenance of adult tissues. The concept of force-sensing mechanisms that act through cell-cell and cell-matrix adhesions is now well established and accepted. Additionally, it is widely believed that force sensing can be mediated through cilia. Yet, this hypothesis is still debated. By using primary cilia sensing as a paradigm, we describe the physical requirements for cilium-mediated mechanical sensing and discuss the different hypotheses of how this could work. We review the different mechanosensitive channels within the cilium, their potential mode of action and their biological implications. In addition, we describe the biological contexts in which cilia are acting - in particular, the left-right organizer - and discuss the challenges to discriminate between cilium-mediated chemosensitivity and mechanosensitivity. Throughout, we provide perspectives on how quantitative analysis and physics-based arguments might help to better understand the biological mechanisms by which cells use cilia to probe their mechanical environment.

RevDate: 2019-08-03

Jain K (2019)

Transition to turbulence in an oscillatory flow through stenosis.

Biomechanics and modeling in mechanobiology pii:10.1007/s10237-019-01199-1 [Epub ahead of print].

Onset of flow transition in a sinusoidally oscillating flow through a rigid, constant area circular pipe with a smooth sinusoidal obstruction in the center of the pipe is studied by performing direct numerical simulations, with resolutions close to the Kolmogorov microscales. The studied pipe is stenosed in the center with a 75% reduction in area in two distinct configurations-one that is symmetric to the axis of the parent pipe and the other that is offset by 0.05 diameters to introduce an eccentricity, which disturbs the flow thereby triggering the onset of flow transition. The critical Reynolds number at which the flow transitions to turbulence for a zero-mean oscillatory flow through a stenosis is shown to be nearly tripled in comparison with studies of pulsating unidirectional flow through the same stenosis. The onset of transition is further explored with three different flow pulsation frequencies resulting in a total of 90 simulations conducted on a supercomputer. It is found that the critical Reynolds number at which the oscillatory flow transitions is not affected by the pulsation frequencies. The locations of flow breakdown and re-stabilization post-stenosis are, however, respectively shifted closer to the stenosis throat with increasing pulsation frequencies. The results show that oscillatory physiological flows, while more stable, exhibit fluctuations due to geometric complexity and have implications in studies of dispersion and solute transport in the cerebrospinal fluid flow and understanding of pathological conditions.

RevDate: 2019-08-12

Krishnam U, Sharma V, PK Jha (2019)

The Reynolds number modulated low frequency dynamical modes of aqueous medium embedded spherical virus and implications to detecting and killing viruses.

Journal of biomolecular structure & dynamics [Epub ahead of print].

RevDate: 2019-09-02

Liu G, Xue Q, X Zheng (2019)

Phase-difference on seal whisker surface induces hairpin vortices in the wake to suppress force oscillation.

Bioinspiration & biomimetics, 14(6):066001.

Seals are able to use their uniquely shaped whiskers to track hydrodynamic trails generated 30 s ago and detect hydrodynamic velocities as low as 245 [Formula: see text]m s-1. The high sensibility has long thought to be related to the wavy shape of the whiskers. This work revisited the hydrodynamics of a seal whisker model in a uniform flow, and discovered a new mechanism of seal whiskers in reducing self-induced noises, which is different from the long thought-of effect of the wavy shape. It was reported that the major and minor axes of the elliptical cross-sections of seal whisker are out of phase by approximately 180 degrees. Three-dimensional numerical simulations of laminar flow (Reynolds number range: 150-500) around seal-whisker-like cylinders were performed to examine the effect of the phase-difference on hydrodynamic forces and wake structures. It was found that the phase-difference induced hairpin vortices in the wake over a wide range of geometric and flow parameters (wavelength, wavy amplitude and Reynolds number), therefore substantially reducing lift-oscillations and self-induced noises. The formation mechanism of the hairpin vortices was analyzed and is discussed in details. The results provide valuable insights into an innovative vibration reduction and hydrodynamic sensing mechanism.

RevDate: 2019-07-24

Jian X, Zhang W, Deng Q, et al (2019)

Turbulent lithosphere deformation in the Tibetan Plateau.

Physical review. E, 99(6-1):062122.

In this work, we show that the Tibetan Plateau deformation demonstrates turbulence-like statistics, e.g., spatial invariance across continuous scales. A dual-power-law behavior is evident to show the existence of two possible conservation laws for the enstrophy-like cascade in the range 500≲r≲2000km and kinetic-energy-like cascade in the range 50≲r≲500km. The measured second-order structure-function scaling exponents ζ(2) are similar to their counterparts in the Fourier scaling exponents observed in the atmosphere, where in the latter case the earth's rotation is relevant. The turbulent statistics observed here for nearly zero-Reynolds-number flow can be interpreted by the geostrophic turbulence theory. Moreover, the intermittency correction is recognized with an intensity close to that of the hydrodynamic turbulence of high-Reynolds-number turbulent flows, implying a universal scaling feature of very different turbulent flows. Our results not only shed new light on the debate regarding the mechanism of the Tibetan Plateau deformation but also lead to new challenges for the geodynamic modeling using Newton or non-Newtonian models because the observed turbulence-like features have to be taken into account.

RevDate: 2019-07-18

Dölger J, Kiørboe T, A Andersen (2019)

Dense Dwarfs versus Gelatinous Giants: The Trade-Offs and Physiological Limits Determining the Body Plan of Planktonic Filter Feeders.

The American naturalist, 194(2):E30-E40.

Most marine plankton have a high energy (carbon) density, but some are gelatinous with approximately 100 times more watery bodies. How do those distinctly different body plans emerge, and what are the trade-offs? We address this question by modeling the energy budget of planktonic filter feeders across life-forms, from micron-sized unicellular microbes such as choanoflagellates to centimeter-sized gelatinous tunicates such as salps. We find two equally successful strategies, one being small with high energy density (dense dwarf) and the other being large with low energy density (gelatinous giant). The constraint that forces large-but not small-filter feeders to be gelatinous is identified as a lower limit to the size-specific filter area, below which the energy costs lead to starvation. A further limit is found from the maximum size-specific motor force that restricts the access to optimum strategies. The quantified constraints are discussed in the context of other resource-acquisition strategies. We argue that interception feeding strategies can be accessed by large organisms only if they are gelatinous. On the other hand, organisms that use remote prey sensing do not need to be gelatinous, even if they are large.

RevDate: 2019-08-10

Samson JE, Miller LA, Ray D, et al (2019)

A novel mechanism of mixing by pulsing corals.

The Journal of experimental biology, 222(Pt 15): pii:jeb.192518.

The dynamic pulsation of xeniid corals is one of the most fascinating phenomena observed in coral reefs. We quantify for the first time the flow near the tentacles of these soft corals, the active pulsations of which are thought to enhance their symbionts' photosynthetic rates by up to an order of magnitude. These polyps are approximately 1 cm in diameter and pulse at frequencies between approximately 0.5 and 1 Hz. As a result, the frequency-based Reynolds number calculated using the tentacle length and pulse frequency is on the order of 10 and rapidly decays as with distance from the polyp. This introduces the question of how these corals minimize the reversibility of the flow and bring in new volumes of fluid during each pulse. We estimate the Péclet number of the bulk flow generated by the coral as being on the order of 100-1000 whereas the flow between the bristles of the tentacles is on the order of 10. This illustrates the importance of advective transport in removing oxygen waste. Flow measurements using particle image velocimetry reveal that the individual polyps generate a jet of water with positive vertical velocities that do not go below 0.1 cm s-1 and with average volumetric flow rates of approximately 0.71 cm3 s-1 Our results show that there is nearly continual flow in the radial direction towards the polyp with only approximately 3.3% back flow. 3D numerical simulations uncover a region of slow mixing between the tentacles during expansion. We estimate that the average flow that moves through the bristles of the tentacles is approximately 0.03 cm s-1 The combination of nearly continual flow towards the polyp, slow mixing between the bristles, and the subsequent ejection of this fluid volume into an upward jet ensures the polyp continually samples new water with sufficient time for exchange to occur.

RevDate: 2019-09-04

Liao P, Xing L, Zhang S, et al (2019)

Magnetically Driven Undulatory Microswimmers Integrating Multiple Rigid Segments.

Small (Weinheim an der Bergstrasse, Germany), 15(36):e1901197.

Mimicking biological locomotion strategies offers important possibilities and motivations for robot design and control methods. Among bioinspired microrobots, flexible microrobots exhibit remarkable efficiency and agility. These microrobots traditionally rely on soft material components to achieve undulatory propulsion, which may encounter challenges in design and manufacture including the complex fabrication processes and the interfacing of rigid and soft components. Herein, a bioinspired magnetically driven microswimmer that mimics the undulatory propulsive mechanism is proposed. The designed microswimmer consists of four rigid segments, and each segment is connected to the succeeding segment by joints. The microswimmer is fabricated integrally by 3D laser lithography without further assembly, thereby simplifying microrobot fabrication while enhancing structural integrity. Experimental results show that the microswimmer can successfully swim forward along guided directions via undulatory locomotion in the low Reynolds number (Re) regime. This work demonstrates for the first time that the flexible characteristic of microswimmers can be emulated by 3D structures with multiple rigid segments, which broadens possibilities in microrobot design. The proposed magnetically driven microswimmer can potentially be used in biomedical applications, such as medical diagnosis and treatment in precision medicine.

RevDate: 2019-07-18

Hoell C, Löwen H, Menzel AM, et al (2019)

Creeping motion of a solid particle inside a spherical elastic cavity: II. Asymmetric motion.

The European physical journal. E, Soft matter, 42(7):89 pii:10.1140/epje/i2019-11853-4.

An analytical method is proposed for computing the low-Reynolds-number hydrodynamic mobility function of a small colloidal particle asymmetrically moving inside a large spherical elastic cavity, the membrane of which is endowed with resistance toward shear and bending. In conjunction with the results obtained in the first part (A. Daddi-Moussa-Ider, H. Löwen, S. Gekle, Eur. Phys. J. E 41, 104 (2018)), in which the axisymmetric motion normal to the surface of an elastic cavity is investigated, the general motion for an arbitrary force direction can now be addressed. The elastohydrodynamic problem is formulated and solved using the classic method of images through expressing the hydrodynamic flow fields as a multipole expansion involving higher-order derivatives of the free-space Green's function. In the quasi-steady limit, we demonstrate that the particle self-mobility function of a particle moving tangent to the surface of the cavity is larger than that predicted inside a rigid stationary cavity of equal size. This difference is justified by the fact that a stationary rigid cavity introduces additional hindrance to the translational motion of the encapsulated particle, resulting in a reduction of its hydrodynamic mobility. Furthermore, the motion of the cavity is investigated, revealing that the translational pair (composite) mobility, which linearly couples the velocity of the elastic cavity to the force exerted on the solid particle, is solely determined by membrane shear properties. Our analytical predictions are favorably compared with fully-resolved computer simulations based on a completed-double-layer boundary integral method.

RevDate: 2019-08-27

Novelli GL, Ferrari LA, Vargas GG, et al (2019)

A synergistic analysis of drag reduction on binary polymer mixtures containing guar gum.

International journal of biological macromolecules, 137:1121-1129.

Drag reduction by the addition of polymer additives has been widely studied. However, there are only a few studies on binary polymer mixtures, here named blends. In this work, xanthan gum, polyacrylamide and poly(ethylene oxide) were associated with guar gum and drag reduction was used as a parameter to determine the synergistic interaction between polymers. The aim was to verify the relation of the synergy with the rigidity of the polymeric chains, the molecular weights and the magnitude of the molecular interactions between the studied polymers. To that end, several ratios of mixtures were tested at different Reynolds numbers in a rotational rheometer with double-gap concentric cylinders geometry. Finally, experiments were done to verify the behaviour of the blends over time at a fixed Reynolds number. From all these tests, it was documented that blends containing rigid chain polymers show positive synergism in the interaction in at least one of the ratios and that this interaction is more pronounced when the molecular weights are closer and intermolecular forces are stronger. It was also noted that, in general, blends are great substitutes for solutions containing only one type of polymer.

RevDate: 2019-07-24

Mazinani S, Al-Shimmery A, Chew YMJ, et al (2019)

3D Printed Fouling-Resistant Composite Membranes.

ACS applied materials & interfaces, 11(29):26373-26383.

Fouling remains a long-standing unsolved problem that hinders the widespread use of membrane applications in industry. This article reports the use of numerical simulations coupled with extensive material synthesis and characterization to fabricate fouling-resistant 3D printed composite membranes. The membranes consist of a thin polyethersulfone selective layer deposited onto a 3D printed flat and double sinusoidal (wavy) support. Fouling and cleaning of the composite membranes were tested by using bovine serum albumin solution in a cross-flow ultrafiltration setup. The transmembrane pressure was regulated at 1 bar and the cross-flow Reynolds number (Re) varied between 400 and 1000. In comparison to the flat membrane, the wavy membrane showed superior performance in terms of pure water permeance (PWP) (10% higher) and permeance recovery ratio (87% vs 53%) after the first filtration cycle at Re = 1000. Prolong testing showed that the wavy membrane could retain approximately 87% of its initial PWP after 10 complete filtration cycles. This impressive fouling-resistant behavior is attributed to the localized fluid turbulence induced by the 3D printed wavy structure. These results show that not only the lifetime of membrane operations could be favorably extended but also the operational costs and environmental damage of membrane-based processes could also be significantly reduced.

RevDate: 2019-07-09

Chajwa R, Menon N, S Ramaswamy (2019)

Kepler Orbits in Pairs of Disks Settling in a Viscous Fluid.

Physical review letters, 122(22):224501.

We show experimentally that a pair of disks settling at negligible Reynolds number (∼10^{-4}) displays two classes of bound periodic orbits, each with transitions to scattering states. We account for these dynamics, at leading far-field order, through an effective Hamiltonian in which gravitational driving endows orientation with the properties of momentum. This treatment is successfully compared against the measured properties of orbits and critical parameters of transitions between types of orbits. We demonstrate a precise correspondence with the Kepler problem of planetary motion for a wide range of initial conditions, find and account for a family of orbits with no Keplerian analog, and highlight the role of orientation as momentum in the many-disk problem.

RevDate: 2019-08-11

Ren Z, Hu W, Dong X, et al (2019)

Multi-functional soft-bodied jellyfish-like swimming.

Nature communications, 10(1):2703 pii:10.1038/s41467-019-10549-7.

The functionalities of the untethered miniature swimming robots significantly decrease as the robot size becomes smaller, due to limitations of feasible miniaturized on-board components. Here we propose an untethered jellyfish-inspired soft millirobot that could realize multiple functionalities in moderate Reynolds number by producing diverse controlled fluidic flows around its body using its magnetic composite elastomer lappets, which are actuated by an external oscillating magnetic field. We particularly investigate the interaction between the robot's soft body and incurred fluidic flows due to the robot's body motion, and utilize such physical interaction to achieve different predation-inspired object manipulation tasks. The proposed lappet kinematics can inspire other existing jellyfish-like robots to achieve similar functionalities at the same length and time scale. Moreover, the robotic platform could be used to study the impacts of the morphology and kinematics changing in ephyra jellyfish.

RevDate: 2019-08-30

Klusak E, NJ Quinlan (2019)

High-Resolution Measurements of Leakage Flow Inside the Hinge of a Large-scale Bileaflet Mechanical Heart Valve Hinge Model.

Cardiovascular engineering and technology, 10(3):469-481.

PURPOSE: It is believed that non-physiological leakage flow through hinge gaps during diastole contributes to thrombus formation in Bileaflet Mechanical Heart Valves (BMHVs). Because of the small scale and difficulty of experimental access, fluid dynamics inside the hinge cavity has not yet been characterised in detail. The objective is to investigate small-scale structure inside the hinge experimentally, and gain insight into its role in stimulating cellular responses.

METHODS: An optically accessible scaled-up model of a BMHV hinge was designed and built, preserving dynamic similarity to a clinical BMHV. Particle Image Velocimetry (PIV) was used to visualize and quantify the flow fields inside the hinge at physiological Reynolds number and dimensionless pressure drop. The flow was measured at in-plane and out-of-plane spatial resolution of 32 and 86 μm, respectively, and temporal resolution of [Formula: see text] RESULTS: Likely flow separation on the ventricular surface of the cavity has been observed for the first time, and is a source of unsteadiness and perhaps turbulence. The shear stress found in all planes exceeds the threshold of platelet activation, ranging up to 168 Pa.

CONCLUSIONS: The scale-up approach provided new insight into the nature of the hinge flow and enhanced understanding of its complexity. This study revealed flow features that may induce blood element damage.

RevDate: 2019-06-28

Cafiero G, JC Vassilicos (2019)

Non-equilibrium turbulence scalings and self-similarity in turbulent planar jets.

Proceedings. Mathematical, physical, and engineering sciences, 475(2225):20190038.

We study the self-similarity and dissipation scalings of a turbulent planar jet and the theoretically implied mean flow scalings. Unlike turbulent wakes where such studies have already been carried out (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493); Obligado et al. 2016 Phys. Rev. Fluids1, 044409. (doi:10.1103/PhysRevFluids.1.044409)), this is a boundary-free turbulent shear flow where the local Reynolds number increases with distance from inlet. The Townsend-George theory revised by (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493)) is applied to turbulent planar jets. Only a few profiles need to be self-similar in this theory. The self-similarity of mean flow, turbulence dissipation, turbulent kinetic energy and Reynolds stress profiles is supported by our experimental results from 18 to at least 54 nozzle sizes, the furthermost location investigated in this work. Furthermore, the non-equilibrium dissipation scaling found in turbulent wakes, decaying grid-generated turbulence, various instances of periodic turbulence and turbulent boundary layers (Dairay et al. 2015 J. Fluid Mech. 781, 166-198. (doi:10.1017/jfm.2015.493); Vassilicos 2015 Annu. Rev. Fluid Mech. 95, 114. (doi:10.1146/annurev-fluid-010814-014637); Goto & Vassilicos 2015 Phys. Lett. A3790, 1144-1148. (doi:10.1016/j.physleta.2015.02.025); Nedic et al. 2017 Phys. Rev. Fluids2, 032601. (doi:10.1103/PhysRevFluids.2.032601)) is also observed in the present turbulent planar jet and in the turbulent planar jet of (Antonia et al. 1980 Phys. Fluids23, 863055. (doi:10.1063/1.863055)). Given these observations, the theory implies new mean flow and jet width scalings which are found to be consistent with our data and the data of (Antonia et al. 1980 Phys. Fluids23, 863055. (doi:10.1063/1.863055)). In particular, it implies a hitherto unknown entrainment behaviour: the ratio of characteristic cross-stream to centreline streamwise mean flow velocities decays as the -1/3 power of streamwise distance in the region, where the non-equilibrium dissipation scaling holds.

RevDate: 2019-07-12

Garwood RJ, Behnsen J, Haysom HK, et al (2019)

Olfactory flow in the sturgeon is externally driven.

Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 235:211-225.

Fluid dynamics plays an important part in olfaction. Using the complementary techniques of dye visualisation and computational fluid dynamics (CFD), we investigated the hydrodynamics of the nasal region of the sturgeon Huso dauricus. H. dauricus offers several experimental advantages, including a well-developed, well-supported, radial array (rosette) of visible-by-eye olfactory sensory channels. We represented these features in an anatomically accurate rigid model derived from an X-ray scan of the head of a preserved museum specimen. We validated the results from the CFD simulation by comparing them with data from the dye visualisation experiments. We found that flow through both the nasal chamber and, crucially, the sensory channels could be induced by an external flow (caused by swimming in vivo) at a physiologically relevant Reynolds number. Flow through the nasal chamber arises from the anatomical arrangement of the incurrent and excurrent nostrils, and is assisted by the broad, cartilage-supported, inner wall of the incurrent nostril. Flow through the sensory channels arises when relatively high speed flow passing through the incurrent nostril encounters the circular central support of the olfactory rosette, decelerates, and is dispersed amongst the sensory channels. Vortices within the olfactory flow may assist odorant transport to the sensory surfaces. We conclude that swimming alone is sufficient to drive olfactory flow in H. dauricus, and consider the implications of our results for the three other extant genera of sturgeons (Acipenser, Pseudoscaphirhynchus and Scaphirhynchus), and for other fishes with olfactory rosettes.

RevDate: 2019-06-26

Man Y, E Kanso (2019)

Morphological transitions of axially-driven microfilaments.

Soft matter, 15(25):5163-5173.

The interactions of microtubules with motor proteins are ubiquitous in cellular and sub-cellular processes that involve motility and cargo transport. In vitro motility assays have demonstrated that motor-driven microtubules exhibit rich dynamical behaviors from straight to curved configurations. Here, we theoretically investigate the dynamic instabilities of elastic filaments, with free-ends, driven by single follower forces that emulate the action of molecular motors. Using the resistive force theory at low Reynolds number, and a combination of numerical techniques with linear stability analysis, we show the existence of four distinct regimes of filament behavior, including a novel buckled state with locked curvature. These successive instabilities recapitulate the full range of experimentally-observed microtubule behavior, implying that neither structural nor actuation asymmetry are needed to elicit this rich repertoire of motion.

RevDate: 2019-06-19

He G, Wang J, A Rinoshika (2019)

Orthogonal wavelet multiresolution analysis of the turbulent boundary layer measured with two-dimensional time-resolved particle image velocimetry.

Physical review. E, 99(5-1):053105.

The turbulent boundary layer flow measured by two-dimensional time-resolved particle image velocimetry is analyzed using the discrete orthogonal wavelet method. The Reynolds number of the turbulent boundary layer based on the friction velocity is Re_{τ}=235. The flow field is decomposed into a number of wavelet levels which have different characteristic scales. The velocity statistics and coherent structures at different wavelet levels are investigated. It is found that the fluctuation intensities and their peak locations differ for varying scales. The proper orthogonal decomposition (POD) of different wavelet components reveals a cascade of scales of coherent structures, especially the small-scale ones that are usually difficult to be identified in POD modes of the undecomposed flow field. The interactions among the scales are investigated in terms of large-scale amplitude modulations of the small-scale structures. In previous studies the velocity fluctuations are separated into two parts, the large scale and the small scale, divided usually by the boundary layer thickness. In the present study, however, the scales smaller than the boundary layer thickness are further separated. Therefore, the modulation analysis is a refined investigation that differentiates the modulation effects on separated small scales. The results reveal that the modulation effects vary among the small scales.

RevDate: 2019-06-19

Puljiz M, AM Menzel (2019)

Displacement field around a rigid sphere in a compressible elastic environment, corresponding higher-order Faxén relations, as well as higher-order displaceability and rotateability matrices.

Physical review. E, 99(5-1):053002.

An efficient route to the displacement field around a rigid spherical inclusion in an infinitely extended homogeneous elastic medium is presented in a slightly alternative way when compared to some common textbook methods. Moreover, two Faxén relations of next-higher order beyond the stresslet are calculated explicitly for compressible media. They quantify higher-order moments involving the force distribution on a rigid spherical particle in a deformed elastic medium. As a consequence, additional contributions to the distortions of the deformed elastic medium are identified that are absent to lower order. Furthermore, the displaceability and rotateability matrices for an ensemble of rigid spheres are calculated up to (including) sixth order in inverse particle separation distance. These matrices describe the interactions mediated between the rigid embedded particles by the elastic environment. In this way, additional coupling effects are identified that are absent to lower order, particularly when rotations and torques are involved. All methods and results can formally be transferred to the corresponding case of incompressible hydrodynamic low-Reynolds-number Stokes flow by considering the limit of an incompressible environment. The roles of compressibility of the embedding medium and of the here additionally derived higher-order contributions are highlighted by some selected example configurations.

RevDate: 2019-06-19

Nie D, J Lin (2019)

Discontinuity in the sedimentation system with two particles having different densities in a vertical channel.

Physical review. E, 99(5-1):053112.

The two-dimensional lattice Boltzmann method was used to numerically study a sedimentation system with two particles having different densities in a vertical channel for Galileo numbers in the range of 5≤Ga≤15 (resulting in a Reynolds number, based on the settling velocity, approximately ranging between 0.6 and 7). Two types of periodic motion, differing from each other in terms of the size of the limit cycle, the magnitude of the time period, and their changes upon increasing the density difference between particles, are identified depending on whether there is a wake effect. The most prominent features of this system are discontinuous changes in the settling velocity (6.7≤Ga<9.7) and time period of oscillation (10.5≤Ga≤15) at a critical value of the density difference between particles. The first discontinuity results in an abrupt increase in the Reynolds number, associated with a Hopf bifurcation without the presence of vortex shedding. The second discontinuity is accompanied by the disappearance of "abnormal rotation" (referring to the situation in which a particle appears to roll up a wall when settling) of the heavy particle, which directly results from a sharp increase in the amplitude of oscillation induced by the enhanced wake effect at another critical density difference between particles. The wall effects on these discontinuous changes were also examined.

RevDate: 2019-06-19

Wang L, FB Tian (2019)

Numerical simulation of flow over a parallel cantilevered flag in the vicinity of a rigid wall.

Physical review. E, 99(5-1):053111.

Flow over a parallel cantilevered flag in the vicinity of a rigid wall is numerically studied using an immersed boundary-lattice Boltzmann method (IB-LBM) in two-dimensional domain, where the dynamics of the fluid and structure are, respectively, solved by the LBM and a finite-element method (FEM), with a penalty IB to handle the fluid-structure interaction (FSI). Specifically, a benchmark case considering a plate attached to the downstream of a stationary cylinder is first conducted to validate the current solver. Then, the wall effects on the flag are systemically studied, considering the effects of off-wall distance, structure-to-fluid mass ratio, bending rigidity, and Reynolds number. Three flapping modes, including symmetrical flapping, asymmetrical flapping, and chaotic flapping, along with a steady state are observed in the simulations. It is found that the flag is vibrating or stable with a mean angle inclined in the fluid when it is mounted in the vicinity of a rigid wall. The mean inclined angle first increases in the steady state and then decreases in the unsteady state with the off-wall distance. In the unsteady regime, the dependency of the inclined angle on the off-wall distance is similar to that of the gradient of the fluid velocity. In addition, the rigid wall near the flag decreases the lift and drag generation and further stabilizes the flag-fluid system. Contrarily, the flag inertia destabilizes the flag, and large flag inertia induces chaotic vibrating modes.

RevDate: 2019-08-20

Zhang H, Li X, Chuai R, et al (2019)

Chaotic Micromixer Based on 3D Horseshoe Transformation.

Micromachines, 10(6): pii:mi10060398.

To improve the efficiency of mixing under laminar flow with a low Reynolds number (Re), a novel three-dimensional Horseshoe Transformation (3D HT) was proposed as the basis for the design of a micromixer. Compared with the classical HT, the Lyapunov exponent of the 3D HT, which was calculated based on a symbolic dynamic system, proved the chaotic enhancement. Based on the 3D HT, a micromixer with a mixing length of 12 mm containing six mixing units was obtained by sequentially applying "squeeze", "stretch", "twice fold", "inverse transformation", and "intersection" operations. Numerical simulation and Peclet Number (Pe) calculations indicated that when the squeeze amplitude 0 < α < 1/2, 0 < β < 1/2, the stretch amplitude γ > 4, and Re ≥ 1, the mass transfer in the mixer was dominated by convective diffusion induced by chaotic flow. When Re = 10, at the outlet of the mixing chamber, the simulated mixing index was 96.4%, which was far less than the value at Re = 0.1 (σ = 0.041). Microscope images of the mixing chamber and the curve trend of pH buffer solutions obtained from a mixing experiment were both consistent with the results of the simulation. When Re = 10, the average mixing index of the pH buffer solutions was 91.75%, which proved the excellent mixing efficiency of the mixer based on the 3D HT.

RevDate: 2019-08-19

Belut E, Sánchez Jiménez A, Meyer-Plath A, et al (2019)

Indoor dispersion of airborne nano and fine particles: Main factors affecting spatial and temporal distribution in the frame of exposure modeling.

Indoor air, 29(5):803-816.

A particle exposure experiment inside a large climate-controlled chamber was conducted. Data on spatial and temporal distribution of nanoscale and fine aerosols in the range of mobility diameters 8-600 nm were collected with high resolution, for sodium chloride, fluorescein sodium, and silica particles. Exposure scenarios studied included constant and intermittent source emissions, different aggregation conditions, high (10 h-1) and low (3.5 h-1) air exchange rates (AERs) corresponding to chamber Reynolds number, respectively, equal to 1 × 105 and 3 × 104 . Results are presented and analyzed to highlight the main determinants of exposure and to determine whether the assumptions underlying two-box models hold under various scenarios. The main determinants of exposure found were the source generation rate and the ventilation rate. The effect of particles nature was indiscernible, and the decrease of airborne total number concentrations attributable to surface deposition was estimated lower than 2% when the source was active. A near-field/far-field structure of aerosol concentration was always observed for the AER = 10 h-1 but for AER = 3.5 h-1 , a single-field structure was found. The particle size distribution was always homogeneous in space but a general shift of particle diameter (-8% to +16%) was observed between scenarios in correlation with the AER and with the source position, presumably largely attributable to aggregation.

RevDate: 2019-06-14

Aghilinejad A, Aghaamoo M, X Chen (2019)

On the transport of particles/cells in high-throughput deterministic lateral displacement devices: Implications for circulating tumor cell separation.

Biomicrofluidics, 13(3):034112 pii:012903BMF.

Deterministic lateral displacement (DLD), which takes advantage of the asymmetric bifurcation of laminar flow around the embedded microposts, has shown promising capabilities in separating cells and particles of different sizes. Growing interest in utilizing high-throughput DLD devices for practical applications, such as circulating tumor cell separation, necessitates employing higher flow rates in these devices, leading to operating in moderate to high Reynolds number (Re) regimes. Despite extensive research on DLD devices in the creeping regime, limited research has focused on the physics of flow, critical size of the device, and deformable cell behavior in DLD devices at moderate to high Re. In this study, the transport behavior of particles/cells is investigated in realistic high-throughput DLD devices with hundreds of microposts by utilizing multiphysics modeling. A practical formula is proposed for the prediction of the device critical size, which could serve as a design guideline for high-throughput DLD devices. Then, the complex hydrodynamic interactions between a deformable cell and DLD post arrays are investigated. A dimensionless index is utilized for comparing different post designs to quantify the cell-post interaction. It is shown that the separation performances in high-throughput devices are highly affected by Re as well as the micropost shapes. These findings can be utilized for the design and optimization of high-throughput DLD microfluidic devices.

RevDate: 2019-08-16

Tshumah-Mutingwende RRMS, F Takahashi (2019)

Physio-chemical effects of freshwaters on the dissolution of elementary mercury.

Environmental pollution (Barking, Essex : 1987), 252(Pt A):627-636.

Elemental mercury (Hg0) is widely used by Artisanal and small-scale gold miners (ASGMs) to extract gold from ore. Due to the unavailability of appropriate waste disposal facilities, Hg0-rich amalgamation tailings are often discharged into nearby aquatic systems where the Hg0 droplets settle in bottom sediment and sediment-water interfaces. Hg0 dissolution and following biogeochemical transformations to methylmercury (MeHg) have been concerned owing to its potential risk to human health and the ecosystem. For reliable estimates of Hg exposure to human bodies using pollutant environmental fate and transport models, knowledge of the Hg0 dissolution rate is important. However, only limited literature is available. Therefore, it was investigated in this study. Dissolution tests in a 'dark chamber' revealed that an increase in medium pH resulted in a decrease in the dissolution rate, whereas, a large Hg0 droplet surface area (SA) and high Reynolds number (Re) resulted in a faster dissolution. A multivariate first order dissolution model of the form:kˆ=-7.9×10-5[pH]+7.0×10-4[logRe]+7.9×10-4[SA]-2.5×10-3 was proposed (adjusted R2 = 0.99). The Breusch-Pagan and White heteroscedasticity tests revealed that the model residuals are homoscedastic (p-value = 0.05) at the 5% significance level. Parameter sensitivity analysis suggests that slow mercury dissolution from the Hg0 droplets to aquatic systems might mask emerging environmental risk of mercury. Even after mercury usage in ASGM is banned, mercury dissolution and following contamination will continue for about 40 years or longer owing to previously discharged Hg0 droplets.

RevDate: 2019-08-29

Almohammadi H, A Amirfazli (2019)

Droplet impact: Viscosity and wettability effects on splashing.

Journal of colloid and interface science, 553:22-30.

HYPOTHESES: The wettability of a surface affects the splashing behavior of a droplet upon impact onto a surface only when surface exhibits either a very high or a very low contact angle. Viscosity affects the splashing threshold in a non-monotony way.

EXPERIMENTS: To examine the roles of drop viscosity and surface wettability on splashing, a wide range of liquid viscosities (1-100 cSt), surface wettabilities (from hydrophilic to hydrophobic), drop velocities (0.5-3.3 m/s), and liquid surface tensions (∼20 and 70 mN/m) were examined. High speed imaging was used.

FINDINGS: Wettability affects the splashing threshold at very extreme limits of the wettability i.e. at very high or very low contact angle values; however, the wettability effect is less prominent on spreading-splashing regime map. For drops of any surface tension impacting surfaces with any wettability, an increase in viscosity (up to ∼5 cSt or Reynolds number of 2000) promotes splashing; whereas using liquids with viscosities larger than 5 cSt, suppress splashing. We explained such behaviors using evolution of the lamella rim, dynamic contact angle, and velocity of the expanding lamella. Finally, to predict the splashing, we developed a general empirical relationship which explains all of ours, and previously reported data.

RevDate: 2019-08-20

Siddiqui AA, M Turkyilmazoglu (2019)

A New Theoretical Approach of Wall Transpiration in the Cavity Flow of the Ferrofluids.

Micromachines, 10(6): pii:mi10060373.

An idea of permeable (suction/injection) chamber is proposed in the current work to control the secondary vortices appearing in the well-known lid-driven cavity flow by means of the water based ferrofluids. The Rosensweig model is conveniently adopted for the mathematical analysis of the physical problem. The governing equation of model is first transformed into the vorticity transport equation. A special finite difference method in association with the successive over-relaxation method (SOR) is then employed to numerically simulate the flow behavior. The effects of intensity of magnetic source (controlled by the Stuart number), aspect ratio of the cavity, rate of permeability (i.e., α p = V 0 U), ratio of speed of suction/injection V 0 to the sliding-speed U of the upper wall of a cavity, and Reynolds number on the ferrofluid in the cavity are fully examined. It is found that the secondary vortices residing on the lower wall of the cavity are dissolved by the implementation of the suction/injection chamber. Their character is dependent on the rate of permeability. The intensity of magnetic source affects the system in such a way to alter the flow and to transport the fluid away from the magnetic source location. It also reduces the loading effects on the walls of the cavity. If the depth of cavity (or the aspect ratio) is increased, the secondary vortices join together to form a single secondary vortex. The number of secondary vortices is shown to increase if the Reynolds number is increased for both the clear fluid as well as the ferrofluids. The suction and injection create resistance in settlement of solid ferroparticles on the bottom. The results obtained are validated with the existing data in the literature and satisfactory agreement is observed. The presented problem may find applications in biomedical, pharmaceutical, and engineering industries.

RevDate: 2019-07-23

Arumuru V, Dash JN, Dora D, et al (2019)

Vortex Shedding Optical Flowmeter based on Photonic Crystal Fiber.

Scientific reports, 9(1):8313 pii:10.1038/s41598-019-40464-2.

In the present work we propose a PCF (photonic crystal fiber) based Modal interferometer detector for sensing low flow velocity by detecting the frequency of vortices shed from a bluff body. The proposed novel design encapsulates the interferometric arm inside a metal casing to protect the sensor from harsh process fluids. The characterization of the developed probe is conducted under no flow conditions using a piezo actuator to evaluate the sensor response over wide frequency range (0-500 Hz). The developed sensors shows a reasonably flat response in the tested frequency range. Experiments are conducted by employing the developed sensor behind a bluff body of a vortex flowmeter to measure the frequency of the shed vortices and hence, the fluid flow rate. The low flow rate sensitivity of the vortex flowmeter is improved many folds by using the present sensor and the minimum Reynolds number detected is Re = 5000. A linear trend is observed between the frequency of the vortices and the flow velocity which is desirable for fluid flow measurement. The PCF based interferometric sensor with metal encapsulation makes the vortex flowmeter, sensitive at low flow rates, robust and economical to be used in industrial application.

RevDate: 2019-07-23

Farouk B, Antao DS, N Hasan (2019)

Acoustically driven oscillatory flow fields in a cylindrical resonator at resonance.

The Journal of the Acoustical Society of America, 145(5):2932.

Generation and development of acoustic waves in an air-filled cylindrical resonator driven by a conical electro-mechanical speaker are studied experimentally and simulated numerically. The driving frequencies of the speaker are chosen such that a standing wave field is produced at each chosen frequency in the resonator. The amplitude of the generated acoustic (pressure) waves is measured along the axis of the resonator by a fast response piezo-resistive pressure transducer, while the radial distribution of the oscillatory axial velocities is measured at the corresponding velocity anti-node locations by a constant temperature hot-film anemometer. For the cases studied, the acoustic Reynolds number ranged between 20.0 and 60.0 and the flow fields were always found to be in the laminar regime. The flow field in the resonator is also simulated by a high-fidelity numerical scheme with low numerical diffusion. Formation of the standing wave and quasi-steady acoustic streaming are numerically simulated by solving the fully compressible form of the Navier-Stokes equations. The effects of the sound field intensity (i.e., input power to the speaker) and driving frequency on the standing wave field and the resultant formation process of the streaming structures are also investigated.

RevDate: 2019-05-31

Kaneda Y, Yamamoto Y, Y Tsuji (2019)

Linear Response Theory for One-Point Statistics in the Inertial Sublayer of Wall-Bounded Turbulence.

Physical review letters, 122(19):194502.

The idea of linear response theory well known in the statistical mechanics for thermal equilibrium systems is applied to one-point statistics in the inertial sublayer of wall-bounded turbulence (WBT). A close analogy between the energy transfer from large to small scales in isotropic turbulence and the momentum transfer in the wall normal direction in WBT plays a key role in the application. The application gives estimates of the influence of the finite Reynolds number on the statistics. The estimates are consistent with data by high-resolution direct numerical simulations of turbulent channel flow.

RevDate: 2019-06-13

Fadel M, Daurelle JV, Fourmond V, et al (2019)

A new electrochemical cell with a uniformly accessible electrode to study fast catalytic reactions.

Physical chemistry chemical physics : PCCP, 21(23):12360-12371.

The electrochemical study of fast catalytic reactions is limited by mass transport when using the conventional electrochemical cell with a rotating disk electrode (RDE). To overcome this issue, it is important to find a new device with improved transport properties that respects electrochemical constraints. We used numerical simulations of computational fluid dynamics to design a new electrochemical cell based on the so-called "jet flow" design for the kinetic studies of catalytic chemical reactions at the surface of an electrode. The new cell is characterized by a high, reliable and uniform mass transport over the electroactive part of its surface. We investigated the effects of the nozzle and the electrode diameters, the nozzle-electrode distance and the Reynolds number on the performance of the jet-electrode in the flow system. Through the optimization of the geometry of this jet electrode cell, we achieved a factor of 3 enhancement in transport compared to the rotating disk electrode. We succeeded in constructing the designed electrode, characterized it with electrochemical techniques, and found an excellent agreement between the transport properties deduced from the numerical simulations and those from the measurements.

RevDate: 2019-06-05

Wang P, Zhang Q, Wang M, et al (2019)

Atomistic insights into cesium chloride solution transport through the ultra-confined calcium-silicate-hydrate channel.

Physical chemistry chemical physics : PCCP, 21(22):11892-11902.

The transport of water and ions in the gel pores of calcium silicate hydrate (C-S-H) determines the durability of cement material. In this study, molecular dynamics was employed to investigate the capillary imbibition process of CsCl solution in the C-S-H channel. The advanced frontier of CsCl solution flow inside the C-S-H capillary shows a concave meniscus shape, which reflects the hydrophilic properties of the C-S-H substrate. Reynolds number calculations show that the transport process is laminar flow and dominated by viscous forces. The invading depth of the CsCl solution deviates from the theoretical prediction of the classic Lucas-Washburn (L-W) equation, but the modified theoretical equation, by incorporating the effect of slip length, dynamic contact angle, and effective viscosity into the L-W equation, can describe the penetration curve of the solution very well. The validity of our developed theoretical equation was confirmed by additional systems with different ion concentrations. In addition, the local structure of ions was analyzed to elucidate the effect of ion concentration on the transport process. The adsorption and accumulation of ions retard the transport process of water. With an increase in the ionic concentration, the effects of immobilization and cluster accumulation became more pronounced, further reducing the transport rate of water. This study provides fundamental insight into the transport behavior of liquid in the gel pores of cement-based material.

RevDate: 2019-07-23

Zhang J, Liu H, Y Ba (2019)

Numerical Study of Droplet Dynamics on a Solid Surface with Insoluble Surfactants.

Langmuir : the ACS journal of surfaces and colloids, 35(24):7858-7870.

Surfactants are widely used in many industrial processes, where the presence of surfactants not only reduces the interfacial tension between fluids but also alters the wetting properties of solid surfaces. To understand how the surfactants influence the droplet motion on a solid surface, a hybrid method for interfacial flows with insoluble surfactants and contact-line dynamics is developed. This method solves immiscible two-phase flows through a lattice Boltzmann color-gradient model and simultaneously solves the convection-diffusion equation for surfactant concentration through a finite difference method. In addition, a dynamic contact angle formulation that describes the dependence of the local contact angle on the surfactant concentration is derived, and the resulting contact angle is enforced by a geometrical wetting condition. Our method is first used to simulate static contact angles for a droplet resting on a solid surface, and the results show that the presence of surfactants can significantly modify surface wettability, especially when the surface is more hydrophilic or more hydrophobic. This is then applied to simulate a surfactant-laden droplet moving on a substrate subject to a linear shear flow for varying effective capillary number (Cae), Reynolds number (Re), and surface wettability, where the results are often compared with those of a clean droplet. For varying Cae, the simulations are conducted by considering a neutral surface. At low values of Cae, the droplet eventually reaches a steady deformation and moves at a constant velocity. In either a clean or surfactant-laden case, the moving velocity of the droplet linearly increases with the moving wall velocity, but the slope is always higher (i.e., the droplet moves faster) in the surfactant-laden case where the droplet exhibits a bigger deformation. When Cae is increased beyond a critical value (Cae,c), the droplet breakup would happen. The presence of surfactants is found to decrease the value of Cae,c, but it shows a non-monotonic effect on the droplet breakup. An increase in Re is able to increase not only droplet deformation but also surfactant dilution. The role of surfactants in the droplet behavior is found to greatly depend upon the surface wettability. For a hydrophilic surface, the presence of surfactants can decrease the wetting length and enables the droplet to reach a steady state faster; while for a hydrophobic surface, it increases the wetting length and delays the departure of the droplet from the solid surface.

RevDate: 2019-07-23

Tian C, Wang X, Liu Y, et al (2019)

In Situ Grafting Hydrophilic Polymeric Layer for Stable Drag Reduction.

Langmuir : the ACS journal of surfaces and colloids, 35(22):7205-7211.

Developing drag reduction techniques has attracted great attention because of their need in practical applications. However, many of the proposed strategies exhibit some inevitable limitations, especially for long period of adhibition. In this work, the dynamic but stable drag reduction effect of superhydrophilic hydrogel-coated iron sphere falling freely in a cylindrical water tank was investigated. The absolute instantaneous velocities and displacements of either the hydrogel-encapsulated or unmodified iron sphere falling freely in water were monitored via a high-speed video. It was revealed that, in the range of Reynolds number from 104 to 106, the optimized hydrogel-coated iron sphere with uniform stability could reduce the resistance by up to 40%, which was mainly due to the boundary slip of water and the delayed boundary separation that resulted from the coated hydrogel. Besides, the deliberate experiments and analysis further indicated that the superhydrophilic hydrogel layer accompanied by the emergence of the drag crisis has largely effected the distribution of flow field at the boundary around the sphere. More importantly, the drag reduction behavior based on the proposed method was thermodynamically stable and resistant to external stimulus, including fluidic oscillator and hydrodynamic pressure. The effective long-term drag reduction performance of the hydrophilic substrate can be expected, correspondingly, and also provides a novel preliminary protocol and avenues for the development of durable drag reduction technologies.

RevDate: 2019-06-18

Li H, Huang B, M Wu (2019)

Experimental and Numerical Investigations on the Flow Characteristics within Hydrodynamic Entrance Regions in Microchannels.

Micromachines, 10(5): pii:mi10050317.

Flow characteristics within entrance regions in microchannels are important due to their effect on heat and mass transfer. However, relevant research is limited and some conclusions are controversial. In order to reveal flow characteristics within entrance regions and to provide empiric correlation estimating hydrodynamic entrance length, experimental and numerical investigations were conducted in microchannels with square cross-sections. The inlet configuration was elaborately designed in a more common pattern for microdevices to diminish errors caused by separation flow near the inlet and fabrication faults so that conclusions which were more applicable to microchannels could be drawn. Three different microchannels with hydraulic diameters of 100 μm, 150 μm, and 200 μm were investigated with Reynolds (Re) number ranging from 0.5 to 50. For the experiment, deionized water was chosen as the working fluid and microscopic particle image velocimetry (micro-PIV) was adopted to record and analyze velocity profiles. For numerical simulation, the test-sections were modeled and incompressible laminar Navier-Stokes equations were solved with commercial software. Strong agreement was achieved between the experimental data and the simulated data. According to the results of both the experiments and the simulations, new correlations were proposed to estimate entrance length. Re numbers ranging from 12.5 to 15 was considered as the transition region where the relationship between entrance length and Re number converted. For the microchannels and the Reynolds number range investigated compared with correlations for conventional channels, noticeable deviation was observed for lower Re numbers (Re < 12.5) and strong agreement was found for higher Re numbers (Re > 15).

RevDate: 2019-06-20

Xin C, Yang L, Li J, et al (2019)

Conical Hollow Microhelices with Superior Swimming Capabilities for Targeted Cargo Delivery.

Advanced materials (Deerfield Beach, Fla.), 31(25):e1808226.

Inspired by flagellate microorganisms in nature, the microhelix is considered as an ideal model for transportation in fluid environment with low Reynolds number. However, how to promote the swimming and loading capabilities of microhelices with controllable geometries remains challenging. In this study, a novel kind of conical hollow microhelices is proposed and a method is developed to rapidly fabricate these microhelices with controllable parameters by femtosecond vortex beams generated from spatial light modulation along helical scanning. Conical hollow microhelices with designable heights (H = 45-75 µm), diameters (D = 6-18 µm), pitch numbers (Pi = 2-4), taper angles (T = 0.1-0.6 rad), and pitch periods (ΔP = 10-30 µm) are efficiently fabricated. In addition, compared with straight microhelices, the forward swimming capability of conical microhelices increases by 50% and the lateral drift of the conical hollow microhelices is reduced by 70%. Finally, the capabilities of these conical hollow microhelices for nanocargo loading and release by the inner hollow core, as well as transportation of neural stem cells by the outer surface are demonstrated. This work provides new insights into faster and simultaneous transportation of multicargoes for hybrid drug delivery, targeted therapy, and noninvasive surgery in vivo.

RevDate: 2019-07-23

Feng Y, Gao Y, Tang K, et al (2019)

Numerical investigation on turbulent oscillatory flow through a jet pump.

The Journal of the Acoustical Society of America, 145(3):1417.

A jet pump with an asymmetrical channel can induce a time-averaged pressure drop in oscillatory flow, which can effectively suppress Gedeon streaming in looped thermoacoustic engines. In this work, the flow characteristics and time-averaged pressure drop caused by a jet pump in turbulent oscillatory flow are investigated through numerical simulation. Through the analysis of the dimensionless governing equations, the emphasis is put on the effects of Womersley number and maximum acoustic Reynolds number on the performance of the jet pump. Meanwhile, the steady flow resistance coefficients are also measured numerically. The results indicate that the oscillatory flow resistance coefficients are relatively insensitive to Womersley number when it is less than 46. Moreover, the oscillatory flow resistance coefficients agree well with the steady state flow results, which validate the quasi-static assumption in turbulent oscillatory flow. However, further increasing Womersley number will lead to a reduction in the time-averaged pressure drop. The simulation method and results, as well as the hydrodynamic mechanism beneath the results, are presented and discussed in detail.

RevDate: 2019-07-23

Ramadan AB, Abd El-Rahman AI, AS Sabry (2019)

Assessment of the transition k-k-ω model application to transitional oscillatory pipe flows.

The Journal of the Acoustical Society of America, 145(3):1195.

The flow transition from laminar to turbulent inside of typical thermoacoustic devices influences the heat-exchange capacities of these devices and dramatically impacts overall performances as well. A few measurements [Eckmann and Grotberg (1991), J. Fluid Mech. 222, 329-350; Hino, Sawamoto, and Takasu (1976). J. Fluid Mech. 75, 193-207] and direct simulations [Feldmann and Wagner (2012). J. Turbul. 13(32), 1-28; Feldmann and Wagner (2016a). New Results in Numerical and Experimental Fluid Mechanics X, pp. 113-122] were reported that describe the transitional oscillatory flows; however, almost no turbulence model has been developed that enables accurate detection and characterization of the reported intermittent turbulent fluctuations. The present work aims to assess the applicability of the k-kL-ω transition model to transitional oscillatory pipe flows. A sinusoidal pressure gradient is introduced into ANSYS finite-volume solver for flow field simulation at different acoustic frequencies, while a friction Reynolds number of 1440 is retained. The stationary turbulent and the laminar oscillatory pipe flows are first considered for validation and model calibration against published LDA measurements [Durst, Kikura, Lekakis, Jovanovic, and Ye (1996). Exp. Fluids 20, 417-428] and DNS results [Feldmann and Wagner (2012). J. Turbul. 13(32), 1-28] in addition to the Sexl's laminar-flow theory [Sexl (1930). Zeitschrift Phys. 61(5), 349-362]. Investigation of the total fluctuation kinetic energy of transitional oscillations reveals the appearance of intermittent fluctuations within the near-wall region at Wo = 13 during deceleration, while fully turbulent oscillations are predicted in the entire pipe domain at Wo = 5. Although the present results are qualitatively in good agreement with reported experimental [Eckmann and Grotberg (1991). J. Fluid Mech. 222, 329-350] and DNS findings [Feldmann and Wagner (2012). J. Turbul. 13(32), 1-28], the velocity profiles show poor agreement with corresponding DNS data during flow acceleration at Wo = 5.

RevDate: 2019-07-23

Zhou T, Sun Y, Fattah R, et al (2019)

An experimental study of trailing edge noise from a pitching airfoil.

The Journal of the Acoustical Society of America, 145(4):2009.

In this study, the far-field noise from a pitching NACA 0012 airfoil was measured at a Reynolds number of 6.6 × 104. The pitching motion was in sinusoidal functions with a mean incident angle of 0°. Cases with the pitching amplitude varying from 7.5° to 15° and frequency from 3 to 8 Hz were tested, corresponding to the reduced frequency from 0.094 to 0.25. A microphone was placed in the far-field and the particle image velocimetry technique was utilized to study the flow structures near the trailing edge. The short-time Fourier transformation results of the noise signals revealed that a high-level narrow-band noise hump occurred at a specific angle of attack in a pitching cycle. At the corresponding moment, a coherent vortex street convecting on the airfoil surface was observed, and the vortex shedding frequency was in good agreement with the central frequency of the noise hump. The occurrence of the noise humps was attributed to the laminar boundary layer separation. In one pitching period, the moment when the narrow-band noise hump occurs is independent from the pitching amplitude and it is delayed as the pitching frequency increases. Larger pitching frequency or amplitude results in lower peak level of the noise humps.

RevDate: 2019-05-03

Gao Y, Yang X, Fu C, et al (2019)

10 kHz simultaneous PIV/PLIF study of the diffusion flame response to periodic acoustic forcing.

Applied optics, 58(10):C112-C120.

Response of a laminar diffusion dimethyl-ether flame forced by an acoustic field is provided. A forcing frequency of 100 Hz, which is chosen based on the typical thermo-acoustic instability frequency in a practical combustor, is applied to the flame at a Reynolds number of 250. The development of the forced vortical structures present in this flame has been investigated utilizing a burst mode laser with a repetition rate of 10 kHz. Flame/vortex interaction is visualized by planar laser-induced fluorescence (PLIF) of formaldehyde, which is used to identify the early-stage fuel decomposition in the flame. The flame structure is also correlated with the velocity field, which is obtained utilizing particle imaging velocimetry (PIV). The resulting phase-resolved and time-averaged velocity and vortex images indicate that the amplitude of excitation has pronounced effects on the flame via modifying the local heat release.

RevDate: 2019-05-03

Wang S, Liu X, Wang G, et al (2019)

High-repetition-rate burst-mode-laser diagnostics of an unconfined lean premixed swirling flame under external acoustic excitation.

Applied optics, 58(10):C68-C78.

Lean premixed swirling flames are important in practical combustors, but a commonly encountered problem of practical swirl combustors is thermo-acoustic instability, which may cause internal structure damage to combustors. In this research, a high-repetition-rate burst-mode laser is used for simultaneous particle image velocimetry and planar laser-induced fluorescence measurement in an unconfined acoustically excited swirl burner. The time-resolved flow field and transient flame response to the acoustic perturbation are visualized at 20 kHz, offering insight into the heat release rate oscillation. The premixed mixture flow rate and acoustic modulation are varied to study the effects of Reynolds number, Strouhal number, and acoustic modulation amplitude on the swirling flame. The results suggest that the Strouhal number has a notable effect on the periodic movements of the inner recirculation zone and swirling flame configuration.

RevDate: 2019-07-23

Mozhi Devan Padmanathan A, Sneha Ravi A, Choudhary H, et al (2019)

Predictive Framework for the Spreading of Liquid Drops and the Formation of Liquid Marbles on Hydrophobic Particle Bed.

Langmuir : the ACS journal of surfaces and colloids, 35(20):6657-6668.

In this work, we have developed a model to describe the behavior of liquid drops upon impaction on hydrophobic particle bed and verified it experimentally. Poly(tetrafluoroethylene) (PTFE) particles were used to coat drops of water, aqueous solutions of glycerol (20, 40, and 60% v/v), and ethanol (5 and 12% v/v). The experiments were conducted for Weber number (We) ranging from 8 to 130 and Reynolds number (Re) ranging from 370 to 4460. The bed porosity was varied from 0.8 to 0.6. The experimental values of βmax (ratio of the diameter at the maximum spreading condition to the initial drop diameter) were estimated from the time-lapsed images captured using a high-speed camera. The theoretical βmax was estimated by making energy balances on the liquid drop. The proposed model accounts for the energy losses due to viscous dissipation and crater formation along with a change in kinetic energy and surface energy. A good agreement was obtained between the experimental βmax and the estimated theoretical βmax. The proposed model yielded a least % absolute average relative deviation (% AARD) of 5.5 ± 4.3 compared to other models available in the literature. Further, it was found that the liquid drops impacting on particle bed are completely coated with PTFE particles with βmax values greater than 2.

RevDate: 2019-07-25

Hatoum H, Mo XM, Crestanello JA, et al (2019)

Modeling of the Instantaneous Transvalvular Pressure Gradient in Aortic Stenosis.

Annals of biomedical engineering, 47(8):1748-1763.

The simplified and modified Bernoulli equations break down in estimating the true pressure gradient across the stenotic aortic valve due to their over simplifying assumptions of steady and inviscid conditions as well as the fundamental nature in which aortic valves are different than idealized orifices. Nevertheless, despite having newer models based on time-dependent momentum balance across an orifice, the simplified and modified Bernoulli continue to be the clinical standard because to date, they remain the only models clinically implementable. The objective of this study is to (1) illustrate the fundamental considerations necessary to accurately model the time-dependent instantaneous pressure gradient across a fixed orifice and (2) propose empirical corrections when applying orifice based models to severely stenotic aortic valves. We introduce a general model to predict the time-dependent instantaneous pressure gradient across an orifice that explicitly model the Reynolds number dependence of both the steady and unsteady terms. The accuracy of this general model is assessed with respect to previous models through pulse duplicator experiments on a round orifice model as well as an explanted stenotic surgical aortic valve both with geometric areas of 0.6 cm2 (less than 1 cm2 which is the threshold for stenosis determination) over cardiac outputs of 3 and 5 L/min and heart rates of 60, 90 and 120 bpm. The model and the raw experimental data corresponding to the orifice showed good agreement over a wide range of cardiac outputs and heart rates (R2 exceeding 0.91). The derived average and peak transvalvular pressure gradients also demonstrated good agreement with no significant differences between the respective peaks (p > 0.09). The new model proposed holds promise with its physical and closed form representation of pressure drop, however accurate modeling of the time-variability of the valve area is necessary for the model to be applied on stenotic valves.

RevDate: 2019-06-12

Heidarinejad G, Roozbahani MH, M Heidarinejad (2019)

Studying airflow structures in periodic cylindrical hills of human tracheal cartilaginous rings.

Respiratory physiology & neurobiology, 266:103-114.

The objective of this study is to assess tracheobronchial flow features with the cartilaginous rings during a light exercising. Tracheobronchial is part of human's body airway system that carries oxygen-rich air to human's lungs as well as takes carbon dioxide out of the human's lungs. Consequently, evaluation of the flow structures in tracheobronchial is important to support diagnosis of tracheal disorders. Computational Fluid Dynamics (CFD) allows evaluating effectiveness of tracheal cartilage rings in human body under different configurations. This study utilizes Large Eddy Simulation (LES) to model an anatomically-based human large conducting airway model with and without cartilaginous rings at the breathing conditions at Reynolds number of 5,176 in trachea region. It is observed that small recirculating areas shaped between rings cavities. While these recirculating areas are decaying, similar to periodic 2D-hills, the cartilaginous rings contribute to the construction of a vortical flow structure in the main flow. The separated vortically-shaped zone creates a wake in the flow and passes inside of the next ring cavity and disturb its boundary layer. At last, the small recirculation flow impinges onto tracheal wall. The outcome of this impinge flow is a latitudinal rotating flow perpendicular to the main flow in a cavity between the two cartilaginous rings crest which appear and disappear within a hundredth of a second. Kelvin-Helmholtz instability is observed in trachea caused by shear flow created behind of interaction between these flow structures near to tracheal wavy wall and main flow. A comparison of the results between a smooth wall model named simplified model and a rough wall model named modified model shows that these structures do not exist in simplified model, which is common in modeling tracheobronchial flow. This study proposes to consider macro surface roughness to account for the separating and rotating instantaneous flow structures. Finally, solving trachea airflow with its cartilages can become one of major issues in measuring the validity and capability of solving flow in developing types of sub-grid scale models as a turbulence studies benchmark.

RevDate: 2019-04-24

Vasilopoulos K, Sarris IE, P Tsoutsanis (2019)

Assessment of air flow distribution and hazardous release dispersion around a single obstacle using Reynolds-averaged Navier-Stokes equations.

Heliyon, 5(4):e01482 pii:e01482.

The flow around a cubical building, with a pollution source at the central point of the top of the cube, is studied. The Reynolds-averaged Navier-Stokes and species concentration equations are solved for Reynolds number, Re = 40,000, is based on the height of the cube. The predictions obtained with the standard, the Kato-Launder, and the low-Reynolds number k-epsilon models are examined with various wall functions for the near wall treatment of the flow. Results are compared against Martinuzzi and Tropea measurements (J. of Fluids Eng., 115, 85-92, 1993) for the flow field and against Li and Meroney (J. of Wind Eng. and Industrial Aerodynamics, 81, 333-345, 1983) experiments and Gaussian models for the concentration distribution. It is found that the present unstructured mesh model performs similarly to the structured mesh models. Results from the Kato-Launder model are closer to the experimental data for the flow patterns and contaminant distribution on the cube's roof. However, the Kato-Launder model has an over-prediction for the recirculation zone and the contaminant distribution windward of the cube. The standard k-epsilon and the low-Reynolds number k-epsilon models predict similar flow patterns and are closer to the experimental data of the cube's windward and side face.

RevDate: 2019-04-24

Martins Afonso M, Mitra D, D Vincenzi (2019)

Kazantsev dynamo in turbulent compressible flows.

Proceedings. Mathematical, physical, and engineering sciences, 475(2223):20180591.

We consider the kinematic fluctuation dynamo problem in a flow that is random, white-in-time, with both solenoidal and potential components. This model is a generalization of the well-studied Kazantsev model. If both the solenoidal and potential parts have the same scaling exponent, then, as the compressibility of the flow increases, the growth rate decreases but remains positive. If the scaling exponents for the solenoidal and potential parts differ, in particular if they correspond to typical Kolmogorov and Burgers values, we again find that an increase in compressibility slows down the growth rate but does not turn it off. The slow down is, however, weaker and the critical magnetic Reynolds number is lower than when both the solenoidal and potential components display the Kolmogorov scaling. Intriguingly, we find that there exist cases, when the potential part is smoother than the solenoidal part, for which an increase in compressibility increases the growth rate. We also find that the critical value of the scaling exponent above which a dynamo is seen is unity irrespective of the compressibility. Finally, we realize that the dimension d = 3 is special, as for all other values of d the critical exponent is higher and depends on the compressibility.

RevDate: 2019-06-10

Zhang M, Zhang W, Wu Z, et al (2019)

Comparison of Micro-Mixing in Time Pulsed Newtonian Fluid and Viscoelastic Fluid.

Micromachines, 10(4): pii:mi10040262.

Fluid mixing plays an essential role in many microfluidic applications. Here, we compare the mixing in time pulsing flows for both a Newtonian fluid and a viscoelastic fluid at different pulsing frequencies. In general, the mixing degree in the viscoelastic fluid is higher than that in the Newtonian fluid. Particularly, the mixing in Newtonian fluid with time pulsing is decreased when the Reynolds number Re is between 0.002 and 0.01, while it is enhanced when Re is between 0.1 and 0.2 compared with that at a constant flow rate. In the viscoelastic fluid, on the other hand, the time pulsing does not change the mixing degree when the Weissenberg number Wi ≤ 20, while a larger mixing degree is realized at a higher pulsing frequency when Wi = 50.

RevDate: 2019-04-24

Duran-Matute M, van Gorp MD, GJF van Heijst (2019)

Wavelength selection of vortex ripples in an oscillating cylinder: The effect of curvature and background rotation.

Physical review. E, 99(3-1):033105.

We present results of laboratory experiments on the formation, evolution, and wavelength selection of vortex ripples. These ripples formed on a sediment bed at the bottom of a water-filled oscillating cylindrical tank mounted on top of a rotating table. The table is made to oscillate sinusoidally in time, while a constant background rotation was added for some experiments. The changes in bed thickness are measured using a light attenuation technique. It was found that the wavelength normalized with the excursion length depends on both a Reynolds number and the Strouhal number. This differs from straight or annular geometries where the wavelength is proportional to the excursion length. The flow in an oscillating cylinder has the peculiarity that it develops a secondary flow in the radial direction that depends on the excursion length. The effect of this secondary circulation is evident in the radial transport for small values of the Strouhal number or in the orientation of the ripples for strong enough background rotation. Additionally, ripples in an oscillating cylinder present a rich dynamic behavior where the number of ripples can oscillate even with constant forcing parameters.

RevDate: 2019-04-24

Dutta AK, Ramachandran G, S Chaudhuri (2019)

Investigating thermoacoustic instability mitigation dynamics with a Kuramoto model for flamelet oscillators.

Physical review. E, 99(3-1):032215.

In this paper, we present experimental observations and phenomenological modeling of the intermittent dynamics that emerge while mitigating thermoacoustic instability by rotating the otherwise static swirler in a lean premixed, laboratory-scale combustor. Starting with a self-excited thermoacoustically unstable combustor, here we find that a progressive increase in swirler rotation rate does not uniformly decrease amplitudes of coherent, sinusoidal pressure or heat-release-rate oscillations. Instead, these oscillations emerge as high-amplitude bursts separated by low-amplitude noise in the signal. At increased rotational speeds, the high-amplitude coherent oscillations become scarce and their duration in the signal reduces. The velocity field from high-speed particle image velocimetry and simultaneous pressure and chemiluminescence data support these observations. Such an intermittent route to instability mitigation is reminiscent of the opposite transition implemented by changing the Reynolds number from a fully chaotic state to a fully unstable state. To model such dynamics phenomenologically, we discretize the swirling turbulent premixed flame into an ensemble of flamelet oscillators arranged circumferentially around the center body of the swirler. The Kuramoto model is proposed for these flamelet oscillators which is subsequently used to analyze their synchronization dynamics. The order parameter r, which is a measure of the synchronization between the oscillator phases, provides critical insights on the transition from the thermoacoustically unstable to stable states via intermittency. Finally, it is shown that the Kuramoto model for flamelet oscillator can qualitatively reproduce the time-averaged and intermittent dynamics while transitioning from the state of thermoacoustic instability to a state of incoherent noisy oscillations.

RevDate: 2019-04-24

Shaik VA, AM Ardekani (2019)

Swimming sheet near a plane surfactant-laden interface.

Physical review. E, 99(3-1):033101.

In this work we analyze the velocity of a swimming sheet near a plane surfactant-laden interface by assuming the Reynolds number and the sheet's deformation to be small. We observe a nonmonotonic dependence of the sheet's velocity on the Marangoni number (Ma) and the surface Péclet number (Pe_{s}). For a sheet passing only transverse waves, the swimming velocity increases with an increase in Ma for any fixed Pe_{s}. When Pe_{s} is increasing, on the other hand, the swimming velocity of the same sheet either increases (at large Ma) or it initially increases and then decreases (at small Ma). This dependence of the swimming velocity on Ma and Pe_{s} is altered if the sheet is passing longitudinal waves in addition to the transverse waves along its surface.

RevDate: 2019-07-01

Cheng JL, Au JS, MJ MacDonald (2019)

Peripheral artery endothelial function responses to altered shear stress patterns in humans.

Experimental physiology, 104(7):1126-1135.

NEW FINDINGS: What is the central question of this study? What is the effect of altered shear stress pattern, with or without concurrent neurohumoral and metabolic activation, on the acute endothelial function response assessed via brachial artery flow-mediated dilatation? What is the main finding and its importance? Despite generating distinctive shear stress patterns (i.e. increases in anterograde only, anterograde only with neurohumoral and metabolic activation, and both anterograde and retrograde), similar acute improvements were observed in the brachial artery flow-mediated dilatation response in all conditions, indicating that anterograde and/or turbulent shear stress might be the essential element to induce acute increases in endothelial function.

ABSTRACT: Endothelial function is influenced by both the direction and the magnitude of shear stress. Acute improvements in endothelial function have mostly been attributed to increased anterograde shear, whereas results from many interventional models in humans suggest that enhancing shear stress in an oscillatory manner (anterograde and retrograde) might be optimal. Here, we determined the acute brachial artery shear stress (SS) and flow-mediated dilatation (FMD) responses to three shear-altering interventions [passive heat stress (HEAT), mechanical forearm compression (CUFF) and handgrip exercise (HGEX)] and examined the relationship between changes in oscillatory shear index (OSI) and changes in FMD. During separate visits, 10 young healthy men (22 ± 3 years old) underwent 10 min of HEAT, CUFF or HGEX in their left forearm. Anterograde and retrograde SS, Reynolds number, OSI and FMD were assessed at rest and during/after each intervention. Anterograde SS increased during all interventions in a stepwise manner (P < 0.05 between interventions), with the change in HGEX (∆37.7 ± 12.2 dyn cm-2 , P < 0.05) > CUFF (∆25.1 ± 11.9 dyn cm-2 , P < 0.05) > HEAT (∆14.5 ± 7.9 dyn cm-2 , P < 0.05). Retrograde SS increased during CUFF (∆-19.6 ± 4.3 dyn cm-2 , P < 0.05). Anterograde blood flow was turbulent (i.e. Reynolds number ≥ |2000|) during all interventions (P < 0.05). The relative FMD improved after all interventions (P = 0.01), and there was no relationship between ∆OSI and ∆FMD. We elicited changes in SS profiles including increased anterograde SS (HEAT and HGEX) and both increased anterograde and retrograde SS (CUFF); regardless of the SS pattern, FMD improved to the same extent. These findings suggest that the presence of anterograde and/or turbulent SS might be the key to optimizing endothelial function in acute assessment protocols.

RevDate: 2019-04-16

Wei D, Dehnavi PG, Aubin-Tam ME, et al (2019)

Is the Zero Reynolds Number Approximation Valid for Ciliary Flows?.

Physical review letters, 122(12):124502.

Stokes equations are commonly used to model the hydrodynamic flow around cilia on the micron scale. The validity of the zero Reynolds number approximation is investigated experimentally with a flow velocimetry approach based on optical tweezers, which allows the measurement of periodic flows with high spatial and temporal resolution. We find that beating cilia generate a flow, which fundamentally differs from the stokeslet field predicted by Stokes equations. In particular, the flow velocity spatially decays at a faster rate and is gradually phase delayed at increasing distances from the cilia. This indicates that the quasisteady approximation and use of Stokes equations for unsteady ciliary flow are not always justified and the finite timescale for vorticity diffusion cannot be neglected. Our results have significant implications in studies of synchronization and collective dynamics of microswimmers.

RevDate: 2019-04-16

Shekar A, McMullen RM, Wang SN, et al (2019)

Critical-Layer Structures and Mechanisms in Elastoinertial Turbulence.

Physical review letters, 122(12):124503.

Simulations of elastoinertial turbulence (EIT) of a polymer solution at low Reynolds number are shown to display localized polymer stretch fluctuations. These are very similar to structures arising from linear stability (Tollmien-Schlichting modes) and resolvent analyses, i.e., critical-layer structures localized where the mean fluid velocity equals the wave speed. Computations of self-sustained nonlinear Tollmien-Schlichting waves reveal that the critical layer exhibits stagnation points that generate sheets of large polymer stretch. These kinematics may be the genesis of similar structures in EIT.

RevDate: 2019-04-14

M AA, M V (2019)

Demand factor definition-A dimensionless parameter for Vertical Axis Wind Turbines.

MethodsX, 6:567-581 pii:S2215-0161(19)30048-2.

The use of dimensionless numbers like Reynolds Number, Froude Number and Webber Number has historically simplified the process of comparison of phenomena irrespective of their scales and in their classification into different categories. This paper deals with the derivational aspects of a dimensionless parameter named "Demand Factor" for optimization of Vertical Axis Wind Turbine (VAWT). •The input parameters considered in this derivation are power, wind velocity, the aspect ratio of the turbine, density of air and viscosity of air and the output parameters are length of the blade, number of blades, chord length, aerofoil shape, radius of the turbine and angular velocity at rated speed.•Four rounds of variable definition trials are carried out through the arrangement of the input parameters on the numerator and denominator positions. With the filtering out of unsuitable combinations at different stages of elimination, out of 32 combinations the expression that holds the potential to represent demand factor was identified. The process of carrying out single point optimization based on Demand factor expression is discussed along with the steps involved in numerically calculating output parameters.•The expression of Demand factor developed provides a different perspective on the process of design and optimization of VAWTs.

RevDate: 2019-06-10

Wang Y, Wang Y, Z Cheng (2019)

Direct Numerical Simulation of Gas-Liquid Drag-Reducing Cavity Flow by the VOSET Method.

Polymers, 11(4): pii:polym11040596.

Drag reduction by polymer is an important energy-saving technology, which can reduce pumping pressure or promote the flow rate of the pipelines transporting fluid. It has been widely applied to single-phase pipelines, such as oil pipelining, district heating systems, and firefighting. However, the engineering application of the drag reduction technology in two-phase flow systems has not been reported. The reason is an unrevealed complex mechanism of two-phase drag reduction and lack of numerical tools for mechanism study. Therefore, we aim to propose governing equations and numerical methods of direct numerical simulation (DNS) for two-phase gas-liquid drag-reducing flow and try to explain the reason for the two-phase drag reduction. Efficient interface tracking method-coupled volume-of-fluid and level set (VOSET) and typical polymer constitutive model Giesekus are combined in the momentum equation of the two-phase turbulent flow. Interface smoothing for conformation tensor induced by polymer is used to ensure numerical stability of the DNS. Special features and corresponding explanations of the two-phase gas-liquid drag-reducing flow are found based on DNS results. High shear in a high Reynolds number flow depresses the efficiency of the gas-liquid drag reduction, while a high concentration of polymer promotes the efficiency. To guarantee efficient drag reduction, it is better to use a high concentration of polymer drag-reducing agents (DRAs) for high shear flow.

RevDate: 2019-04-10

Stocking JB, Laforsch C, Sigl R, et al (2018)

The role of turbulent hydrodynamics and surface morphology on heat and mass transfer in corals.

Journal of the Royal Society, Interface, 15(149):20180448.

Corals require efficient heat and mass transfer with the overlying water column to support key biological processes, such as nutrient uptake and mitigation of thermal stress. Transfer rates are primarily determined by flow conditions, coral morphology and the physics of the resulting fluid-structure interaction, yet the relationship among these parameters is poorly understood especially for wave-dominated coral habitats. To investigate the interactive effects of these factors on fluxes of heat and mass, we measure hydrodynamic characteristics in situ over three distinct surface morphologies of massive stony corals in a Panamanian reef. Additionally, we implement a numerical model of flow and thermal transport for both current and wave conditions past a natural coral surface, as well as past three simplified coral morphologies with varying ratios of surface roughness spacing-to-height. We find oscillatory flow enhances rates of heat and mass transfer by 1.2-2.0× compared with unidirectional flow. Additionally, increases in Reynolds number and in surface roughness ratio produce up to a 3.3× and a 2.0× enhancement, respectively. However, as waves begin to dominate the flow regime relative to unidirectional currents, the underlying physical mechanisms mediating transfer rates shift from predominantly turbulence-driven to greater control by inertial accelerations, resulting in larger heat and mass transfer for small surface roughness ratios. We show that for rough corals in wave-dominated flows, novel trade-off dynamics for heat and mass transfer exist between broadly spaced roughness that enhances turbulence production versus narrowly spaced roughness that produces greater surface area. These findings have important implications for differential survivorship during heat-induced coral bleaching, particularly as thermal stress events become increasingly common with global climate change.

RevDate: 2019-04-10

Tuttle LJ, Robinson HE, Takagi D, et al (2019)

Going with the flow: hydrodynamic cues trigger directed escapes from a stalking predator.

Journal of the Royal Society, Interface, 16(151):20180776.

In the coevolution of predator and prey, different and less well-understood rules for threat assessment apply to freely suspended organisms than to substrate-dwelling ones. Particularly vulnerable are small prey carried with the bulk movement of a surrounding fluid and thus deprived of sensory information within the bow waves of approaching predators. Some planktonic prey have solved this apparent problem, however. We quantified cues generated by the slow approach of larval clownfish (Amphiprion ocellaris) that triggered a calanoid copepod (Bestiolina similis) to escape before the fish could strike. To estimate water deformation around the copepod immediately preceding its jump, we represented the body of the fish as a rigid sphere in a hydrodynamic model that we parametrized with measurements of fish size, approach speed and distance to the copepod. Copepods of various developmental stages (CII-CVI) were sensitive to the water flow caused by the live predator, at deformation rates as low as 0.04 s-1. This rate is far lower than that predicted from experiments that used artificial predator-mimics. Additionally, copepods localized the source, with 87% of escapes directed away (greater than or equal to 90°) from the predator. Thus, copepods' survival in life-threatening situations relied on their detection of small nonlinear signals within an environment of locally linear deformation.


ESP Quick Facts

ESP Origins

In the early 1990's, Robert Robbins was a faculty member at Johns Hopkins, where he directed the informatics core of GDB — the human gene-mapping database of the international human genome project. To share papers with colleagues around the world, he set up a small paper-sharing section on his personal web page. This small project evolved into The Electronic Scholarly Publishing Project.

ESP Support

In 1995, Robbins became the VP/IT of the Fred Hutchinson Cancer Research Center in Seattle, WA. Soon after arriving in Seattle, Robbins secured funding, through the ELSI component of the US Human Genome Project, to create the original ESP.ORG web site, with the formal goal of providing free, world-wide access to the literature of classical genetics.

ESP Rationale

Although the methods of molecular biology can seem almost magical to the uninitiated, the original techniques of classical genetics are readily appreciated by one and all: cross individuals that differ in some inherited trait, collect all of the progeny, score their attributes, and propose mechanisms to explain the patterns of inheritance observed.

ESP Goal

In reading the early works of classical genetics, one is drawn, almost inexorably, into ever more complex models, until molecular explanations begin to seem both necessary and natural. At that point, the tools for understanding genome research are at hand. Assisting readers reach this point was the original goal of The Electronic Scholarly Publishing Project.

ESP Usage

Usage of the site grew rapidly and has remained high. Faculty began to use the site for their assigned readings. Other on-line publishers, ranging from The New York Times to Nature referenced ESP materials in their own publications. Nobel laureates (e.g., Joshua Lederberg) regularly used the site and even wrote to suggest changes and improvements.

ESP Content

When the site began, no journals were making their early content available in digital format. As a result, ESP was obliged to digitize classic literature before it could be made available. For many important papers — such as Mendel's original paper or the first genetic map — ESP had to produce entirely new typeset versions of the works, if they were to be available in a high-quality format.

ESP Help

Early support from the DOE component of the Human Genome Project was critically important for getting the ESP project on a firm foundation. Since that funding ended (nearly 20 years ago), the project has been operated as a purely volunteer effort. Anyone wishing to assist in these efforts should send an email to Robbins.

ESP Plans

With the development of methods for adding typeset side notes to PDF files, the ESP project now plans to add annotated versions of some classical papers to its holdings. We also plan to add new reference and pedagogical material. We have already started providing regularly updated, comprehensive bibliographies to the ESP.ORG site.

Electronic Scholarly Publishing
961 Red Tail Lane
Bellingham, WA 98226

E-mail: RJR8222 @

Papers in Classical Genetics

The ESP began as an effort to share a handful of key papers from the early days of classical genetics. Now the collection has grown to include hundreds of papers, in full-text format.

Digital Books

Along with papers on classical genetics, ESP offers a collection of full-text digital books, including many works by Darwin (and even a collection of poetry — Chicago Poems by Carl Sandburg).


ESP now offers a much improved and expanded collection of timelines, designed to give the user choice over subject matter and dates.


Biographical information about many key scientists.

Selected Bibliographies

Bibliographies on several topics of potential interest to the ESP community are now being automatically maintained and generated on the ESP site.

ESP Picks from Around the Web (updated 07 JUL 2018 )