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.