# Séminaire du LPTMS: Vicente Garzo

## Instabilities in granular fluids at moderate densities

### Vicente Garzo, Departamento de Fisica, Universidad de Extremadura

It is well established that when granular materials are externally excited (rapid flow conditions)

they behave like a fluid. In this regime, binary collisions prevail and hence, kinetic theory may

be considered as a quite useful tool to describe the dynamics of grains. The main difference with

respect to ordinary or molecular fluids is that granular systems are constituted by macroscopic

grains that collide inelastically so that the energy decreases with time when the system evolves

freely (homogeneous cooling state, HCS). One of the most characteristic features of granular fluids,

as compared with normal fluids, is the spontaneous formation of velocity vortices and density clus-

ters in the HCS. These instabilities can be well described through a linear stability analysis of the

hydrodynamic equations and follow from the presence of a dissipation or sink term in the equation

for the balance of energy. Apart from its fundamental interest, the determination of the critical

length scale Lc for the onset of instabilities in freely cooling flows offers one of the best opportu-

nities to assess the theoretical predictions of granular hydrodynamics (which are based on kinetic

theory calculations) when one compares those predictions with those independently obtained by

molecular dynamics (MD) simulations. In this talk I will give some insight into the above problem

by determining the critical length scale L c for granular fluids at moderate densities. The comparison

between kinetic theory and MD simulations shows in general an excellent agreement for the onset

of velocity vortices, indicating the applicability of hydrodynamics to monodisperse and polydisperse

granular flows even for strong inelasticity and finite density.

**Date/Time : ** 26/05/2015 - *11:00 - 12:00*

**Location : ** LPTMS, salle 201, 2ème étage, Bât 100, Campus d'Orsay

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