arXiv subjects

V. Kumaran

Publications and source records attributed to V. Kumaran.

9 recordsLinked to original sources

arXiv2026

Induced-current magnetophoresis

When an electrically conducting non-magnetic particle is subjected to a spatially varying and oscillating applied magnetic field of amplitude $\mathcal{H} + \mathcal{G} \cdot x$ and frequency $\omega$, an oscillating eddy current is induced. The Lorentz force density, the cross product of the current density and the magnetic field, consists of a steady component and a component with frequency $2 \omega$. If there is a spatial variation in the applied field, there is a steady force on a sphere of radius $R$ proportional to $- \mu_0 R^3 \mathcal{G} \cdot \mathcal{H} $, and a steady force on a thin rod of radius $R$ and length $L$ proportional to $- \mu_0 R^2 L (\mathcal{G} \cdot \mathcal{H} - \tfrac{1}{2} (\mathcal{G} \cdot \hat o)(\mathcal{H} \cdot \hat o))$, where $\mu_0$ is the magnetic permeability. There is torque proportional to $\mu_0 R^2 L (\hat o \times \mathcal{H} ) (\hat o \cdot \mathcal{H} )$ on a thin rod which tends to align the rod direction of the magnetic field. The coefficients in the force and torque expressions are functions of the dimensionless ratio of the radius and the penetration depth of the magnetic field, $\beta R = \sqrt{\mu_0 \omega \kappa R^2}$, where $\kappa$ is the electrical conductivity. It is shown that the effect of particle interactions can be expressed as an anisotropic diffusion term in the equation for the particle number density. The diffusion coefficient is negative, and concentration fluctuations are amplified, in the plane perpendicular to the magnetic field.

physics.class-ph↗

arXiv2025

Energy non-equipartition in vibrofluidized particles

The aim of the present work is to investigate the influence of the realistic model parameters for particle interactions, specifically the spring stiffness coefficient for the tangential force between particles on the energy equipartition in a vibrofluidized system. To achieve this, a three-dimensional vertically vibrated granular system consisting of spherical particles is simulated using the discrete element method (DEM) implemented in the open-source software LAMMPS. Interparticle and wall-particle interactions are determined using the linear-spring dashpot model. Simulations are performed for particles ranging from nearly perfectly smooth to nearly perfectly rough. Two different values for the ratio of the tangential to normal spring stiffness coefficient $\kappa$ ($2/7$ and $3/4$) are chosen for most of the simulations. The ratio of the translational to the rotational kinetic energy ($K$) monotonically decreases with an increase in the friction coefficient, $\mu$ for $\kappa=2/7$; however, for $\kappa = 3/4$, after an initial reduction with $\mu$, $K$ increases and plateaus at $\approx 5$, indicating the absence of equipartition of energy between the translational and rotational modes. Further simulations performed for $0.67 \le \kappa < 1$ confirm non-equipartition of energy for particles with a very high friction coefficient.

cond-mat.soft↗

arXiv2023

Dynamics of Particle-laden Turbulent Suspensions: Effect of particle roughness

The Fluctuating Force Fluctuating Torque (F3T) model is developed and evaluated for the dynamics of a turbulent particle-gas suspension of rough spherical particles in a turbulent Couette flow in the limit where the viscous relaxation time of the particles and the time between collisions are much larger than the integral time for the fluid turbulence. The fluid force/torque exerted on the particles comprise a steady part due to the difference in the particle velocity/angular velocity and the fluid mean velocity/rotation rate, and a fluctuating part due to the turbulent velocity/vorticity fluctuations. The fluctuations are modeled as Gaussian white noise whose variance is determined from the fluid velocity and vorticity fluctuations. The smooth and rough inelastic collision models are considered for particle-particle and particle-wall collisions. The results show that inclusion of roughness is important for accurately predicting the particle dynamics; the second moments of the velocity fluctuations for rough particles are higher than those for smooth particles by a factor of 2-10, while the second moments of the angular velocity fluctuations are higher by 1-2 orders of magnitude. The F3T model quantitatively predicts the number density, mean and root mean square velocity and angular velocity profiles, and the distribution functions for the particle velocity and angular velocity, even though a Gaussian model is used for the highly non-Gaussian distributions for the force and torque fluctuations.

physics.flu-dyn↗

arXiv2022

Stability of a high Mach number flow in a channel

Modal instabilities in a flow through a channel at high Reynolds and Mach numbers are studied for three-dimensional perturbations. In addition to the Tollmien-Schlichting modes, there exist higher modes in a channel flow that do not have a counterpart in the incompressible limit. The stability characteristics of these higher modes, obtained through numerical calculations, are compared with boundary layer and Couette flows that have been previously studied. The dominant higher mode instabilities in a channel flow are shown to be viscous in nature, in contrast to compressible boundary layer modes. For general compressible bounded-domain flows, a necessary condition for the existence of neutral modes in the inviscid limit is obtained. This criterion is used to construct a procedure to determine a critical value of Mach number below which the higher modes remain stable. This criterion also delineates a range of angles of inclination of the wave number with respect to the flow direction which could go unstable at a specified Mach number. Asymptotic analysis is carried out for the lower and upper branch of the stability curve in the limit of high Reynolds number. A common set of relations are identified for these exponents for the upper and lower branch for the continuation of the Tollmien-Schlichting modes and the compressible modes. The scalings for the Tollmien-Schlichting modes are identical to those for an incompressible flow. The scalings for the finite wave number modes are different; the wave speed $c$ scales as $\mbox{Re}^{-\frac{1}{3}}$ for the lower branch and $\mbox{Re}^{-\frac{1}{5}}$ for the upper branch, where $\mbox{Re}$ is the Reynolds number. The asymptotic analysis shows that the stability boundaries for three-dimensional perturbations at high Reynolds numbers can be calculated from the strain rate and the temperature of the base flow at the wall.

physics.flu-dyn↗

arXiv2022

The effect of inter-particle hydrodynamic and magnetic interactions in a magnetorheological fluid

A magnetorheological fluid, which consists of magnetic particles suspended in a viscous fluid, flows freely with well-dispersed particles in a the absence of a magnetic field, but particle aggregation results in flow cessation when a field is applied. The mechanism of dynamical arrest is examined by analysing interactions between magnetic particles in a magnetic field subject to a shear flow. An isolated spherical magnetic particle undergoes a transition between a rotating state at low magnetic field and a static orientation at high magnetic field. The effect of interactions for spherical dipolar and polarisable particles with static orientation is examined for a dilute viscous suspension. There are magnetic interactions due to the magnetic field disturbance at one particle caused by the dipole moment of another, hydrodynamic interactions due to the antisymmetric force moment of a non-rotating particle in a shear flow, and a modification of the magnetic field due to the particle magnetic moment density. When there is a concentration variation, the torque balance condition results in a disturbance to the orientation of the particle magnetic moment. The net force and the drift velocity due to these disturbances is calculated, and the collective motion generated is equivalent to an anisotropic diffusion process. When the magnetic field is in the flow plane, the diffusion coefficients in the two directions perpendicular to the field direction are negative, implying that concentration fluctuations are unstable in these directions. This instability could initiate field-induced dynamical arrest in a magnetorheological fluid.

physics.flu-dyn↗

arXiv2018

Turbulence collapses at a threshold particle loading in a dilute particle-gas suspension

Two mechanisms are considered responsible for the turbulence modification due to suspended particles in a turbulent gas-particle suspension. Turbulence augmentation is due to the enhancement of fluctuations by wakes behind particles, whereas turbulence attenuation is considered to result from the increased dissipation due to the particle drag. In order to examine the turbulence attenuation mechanism, Direct Numerical Simulations (DNS) of a particle-gas suspension are carried out at a Reynolds number of about 3333 based on the average gas velocity $\bar{u}$, channel width $h$, and the gas kinematic viscosity. The particle Reynolds number based on the particle diameter $d_p$, gas kinematic viscosity and the flow velocity $\bar{u}$ is about 42 and the Stokes number is in the range $7-450$. The particle volume fraction is in the range $0-2 \times 10^{-3}$, and the particle mass loading is in the range $0-9$. As the volume fraction is increased, a discontinuous decrease in the turbulent velocity fluctuations is observed at a critical volume fraction. from $9 \times 10^{-4}$ to $1 \times 10^{-3}$. There is a reduction, by one order of magnitude, in the mean square fluctuating velocities in all directions and in the Reynolds stress. Though there is a modest increase in the energy dissipation due to particle drag, this increase is smaller than the decrease in the turbulent energy production; moreover, there is a decrease in the total energy dissipation rate when there is turbulence collapse. Thus, turbulence attenuation appears to be due to a disruption of the turbulence production mechanism, and not due to the increased dissipation due to the particles. There is a discontinuous collapse in the turbulence intensities at a critical particle loading, instead of the continuous decrease as the particle loading is increased.

physics.flu-dyn↗

arXiv2008

Fast decay of the velocity autocorrelation function in dense shear flow of inelastic hard spheres

We find in complementary experiments and event driven simulations of sheared inelastic hard spheres that the velocity autocorrelation function $ψ(t)$ decays much faster than $t^{-3/2}$ obtained for a fluid of elastic spheres at equilibrium. Particle displacements are measured in experiments inside a gravity driven flow sheared by a rough wall. The average packing fraction obtained in the experiments is 0.59, and the packing fraction in the simulations is varied between 0.5 and 0.59. The motion is observed to be diffusive over long times except in experiments where there is layering of particles parallel to boundaries, and diffusion is inhibited between layers. Regardless, a rapid decay of $ψ(t)$ is observed, indicating that this is a feature of the sheared dissipative fluid, and is independent of the details of the relative particle arrangements. An important implication of our study is that the non-analytic contribution to the shear stress may not be present in a sheared inelastic fluid, leading to a wider range of applicability of kinetic theory approaches to dense granular matter.

cond-mat.soft↗

arXiv1999

Structure and Rheology of the Defect-gel States of Pure and Particle-dispersed Lyotropic Lamellar Phases

We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of shear-treatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics.

cond-mat.soft↗

arXiv1999

Temperature scaling in a dense vibro-fluidised granular material

The leading order "temperature" of a dense two dimensional granular material fluidised by external vibrations is determined. An asymptotic solution is obtained where the particles are considered to be elastic in the leading approximation. The velocity distribution is a Maxwell-Boltzmann distribution in the leading approximation. The density profile is determined by solving the momentum balance equation in the vertical direction, where the relation between the pressure and density is provided by the virial equation of state. The predictions of the present analysis show good agreement with simulation results at higher densities where theories for a dilute vibrated granular material, with the pressure-density relation provided by the ideal gas law, are in error. The theory also predicts the scaling relations of the total dissipation in the bed reported by McNamara and Luding (PRE v 58, p 813).

cond-mat.soft↗