Prosenjit Bagchi

Effect of turbulence on the drag and lift of a particle

A direct numerical simulation (DNS) is used to study the effect of a freestream isotropic turbulent flow on the drag and lift forces on a spherical particle. The particle diameter is about 1.5–10 times the Kolmogorov scale, the particle Reynolds number is about 60–600, and the freestream turbulence intensity is about 10%–25%. The isotropic turbulent field considered here is stationary, i.e., frozen in time. It is shown that the freestream turbulence does not have a substantial and systematic effect on the time-averaged mean drag. The standard drag correlation based on the instantaneous or mean relative velocity results in a reasonably accurate prediction of the mean drag obtained from the DNS. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. In...

Computational Fluid Dynamic Simulation of Aggregation of Deformable Cells in a Shear Flow

We present computational fluid dynamic (CFD) simulation of aggregation of two deformable cells in a shearflow. This work is motivated by an attempt to develop computational models of aggregation of red blood cells (RBCs). Aggregation of RBCs is a major determinant of blood viscosity in microcirculation under physiological and pathological conditions. Deformability of the RBCs plays a major role in determining their aggregability. Deformability depends on the viscosity of the cytoplasmic fluid and on the rigidity of the cell membrane, in a macroscopic sense. This paper presents a computational study of RBC aggregation that takes into account the rheology of the cells as well as cell-cell adhesion kinetics. The simulation technique considered here is two dimensional and based on the front tracking/immersed boundary method for multiple fluids. Results presented here are on the dynamic events of aggregate formation between two cells, and its subsequent motion, rolling, deformation, and breakage. We show that the rheological properties of the cells have significant effects on the dynamics of the aggregate. A stable aggregate is formed at higher cytoplasmic viscosity and membrane rigidity. We also show that the bonds formed between the cells change in a cyclic manner as the aggregate rolls in a shearflow. The cyclic behavior is related to the rolling orientation of the aggregate. The frequency and amplitude of oscillation in the number of bonds also depend on the rheological properties.

Effect of free rotation on the motion of a solid sphere in linear shear flow at moderate Re

The effect of free rotation on the drag and lift forces on a solid sphere in unbounded linear shear flow is investigated. The sphere Reynolds number, Re=|ur|d/ν, is in the range 0.5–200, where ur is the slip velocity. Direct numerical simulations of three-dimensional flow past an isolated sphere are performed using spectral methods. The sphere is allowed to rotate and translate freely in the shear flow in response to the hydrodynamic forces and torque acting on it. The effect of free rotation is studied in a systematic way by considering three sets of simulations. In the first set of simulations, we study how fast a pure rotational or translational motion of the sphere approaches steady state. The “history” effect of rotational and translational motions are compared. Results at high Re are found to be significantly different from the analytical prediction based on low Re theory. In steady simulations, the sphere is allowed to rotate in a torque-free condition. The torque-free rotation rate and the drag an...

Response of the wake of an isolated particle to an isotropic turbulent flow

The interaction of an isolated spherical particle with an isotropic turbulent flow is considered using direct numerical simulations (DNS). The particle Reynolds number is varied from about 50 to 600 and the particle diameter is varied from about 1.5 to 10 times the Kolmogorov scale. The Reynolds number based on the Taylor microscale of the free-stream turbulent field considered here is 164. The DNS technique employed here is the first of its kind to address particle–turbulence interaction and it resolves the smallest scales in the free-stream turbulent flow and the complex vortical structures in the particle wake. The primary objective of this paper is to present new results on the effect of the free-stream turbulence on the particle wake and vortex shedding, and the modulation of free-stream turbulence in the particle wake. The parameters of the present simulations are comparable to those of the experimental study by Wu & Faeth (1994a, b), and agreement between the present computational results and the experimental measurement is demonstrated. The effect of free-stream turbulence on the mean and instantaneous wake structure is studied. The time-averaged mean wake in a turbulent ambient flow shows a lower velocity deficit and a flatter profile. However, in agreement with the experimental results of Wu & Faeth the mean wake in a turbulent flow behaves like a self-preserving laminar wake. At Reynolds numbers below about 210 the effect of free-stream turbulence is to introduce wake oscillations. For Reynolds numbers in the range 210 to 280, free-stream turbulence is observed to promote early onset of vortex shedding. The nature of the shed vortices is somewhat different from that in a uniform flow. Increasing the free-stream turbulence intensity suppresses the process of vortex shedding, and only marginally increases the wake oscillation. The modulation of freestream turbulence in the wake is studied in terms of the distribution of kinetic energy and RMS velocity fluctuation. The free-stream energy lost in the wake is recovered faster in a turbulent ambient flow than in a uniform ambient flow. The energy of the velocity fluctuation is enhanced in the wake at low free-stream intensities, and is damped or marginally increased at higher intensities. The fluctuation energy is not equi-partitioned among the streamwise and cross-stream components. The RMS streamwise fluctuation is always enhanced, whereas the RMS cross-stream fluctuation is enhanced only at low free-stream intensities, and damped at higher intensities.

Steady planar straining flow past a rigid sphere at moderate Reynolds number

This study focuses on the effect of spatial non-uniformity in the ambient flow on the forces acting on a spherical particle at moderate particle Reynolds numbers. A scaling analysis is performed to obtain conditions under which such effects are important. A direct numerical simulation, based on spectral methods, is used to compute the three-dimensional time-dependent flow past a stationary sphere subject to a uniform flow plus a planar straining flow. The particle Reynolds number, Re, in the range 10 to 300 covering different flow regimes, from unseparated flow to unsteady vortex shedding, is considered. A variety of strain magnitudes and orientations are investigated. A systematic comparison with the potential flow results and axisymmetric strain results is given

Direct Numerical Simulation of Flow and Heat Transfer From a Sphere in a Uniform Cross-Flow

Direct numerical solution for flow and heat transfer past a sphere in a uniform flow is obtained using an accurate and efficient Fourier-Chebyshev spectral collocation method for Reynolds numbers up to 500. We investigate the flow and temperature fields over a range of Reynolds numbers, showing steady and axisymmetric flow when the Reynolds number is less than 210, steady and nonaxisymmetric flow without vortex shedding when the Reynolds number is between 210 and 270, and unsteady three-dimensional flow with vortex shedding when the Reynolds number is above 270. Results from three-dimensional simulation are compared with the corresponding axisymmetric simulations for Re > 210 in order to see the effect of unsteadiness and three-dimensionality on heat transfer past a sphere

Shear versus vortex-induced lift force on a rigid sphere at moderate Re

The lift forces on rigid spheres entrained in a vortex and a linear shear flow are computed using a direct numerical simulation. The sphere Reynolds number is in the range 10 to 100. The lift coefficient in a vortex is shown to be nearly two orders of magnitude higher than that in a shear flow. The inviscid mechanism is shown to be inadequate to account for the enhanced lift force. The effect of free rotation of the sphere is also shown to be too small to account for the enhanced lift force. Flow structure around the sphere is studied to explain the generation of the strong lift force in a vortex.

Platelet dynamics in three-dimensional simulation of whole blood.

A high-fidelity computational model using a 3D immersed boundary method is used to study platelet dynamics in whole blood. We focus on the 3D effects of the platelet-red blood cell (RBC) interaction on platelet margination and near-wall dynamics in a shear flow. We find that the RBC distribution in whole blood becomes naturally anisotropic and creates local clusters and cavities. A platelet can enter a cavity and use it as an express lane for a fast margination toward the wall. Once near the wall, the 3D nature of the platelet-RBC interaction results in a significant platelet movement in the transverse (vorticity) direction and leads to anisotropic platelet diffusion within the RBC-depleted zone or cell-free layer (CFL). We find that the anisotropy in platelet motion further leads to the formation of platelet clusters, even in the absence of any platelet-platelet adhesion. The transverse motion, and the size and number of the platelet clusters are observed to increase with decreasing CFL thickness. The 3D nature of the platelet-RBC collision also induces fluctuations in off-shear plane orientation and, hence, a rotational diffusion of the platelets. Although most marginated platelets are observed to tumble just outside the RBC-rich zone, platelets further inside the CFL are observed to flow with an intermittent dynamics that alters between sliding and tumbling, as a result of the off-shear plane rotational diffusion, bringing them even closer to the wall. To our knowledge, these new findings are based on the fundamentally 3D nature of the platelet-RBC interaction, and they underscore the importance of using cellular-scale 3D models of whole blood to understand platelet margination and near-wall platelet dynamics.

Orbital drift of capsules and red blood cells in shear flow

Many numerical studies have considered the dynamics of capsules and red blood cells in shear flow under the condition that the axis of revolution of such bodies remained aligned in the shear plane. In contrast, several experimental studies have shown that the axis of revolution of red blood cells could drift away from the shear plane in a certain range of controlling parameters. In this article, we present three-dimensional numerical simulations on the orientation dynamics of capsules in simple shear flow with different initial undeformed shapes, namely, prolate, oblate, and biconcave disk. It is observed that unlike rigid ellipsoids in Stokes flow, capsules reorient their axis of revolution either towards the vorticity axis while undergoing a precessing motion or towards the shear plane while undergoing a kayaking-type motion. The specific dynamics are observed to depend on initial shape, capillary number, and the ratio of the internal to external fluid viscosity. Near the physiological values of the vis...

Inertial and viscous forces on a rigid sphere in straining flows at moderate Reynolds numbers

The focus of this paper is the effect of spatial non-uniformity in the ambient flow on the forces acting on a rigid sphere when the sphere Reynolds number,