On the relevance of "molecular chaos" for granular flows

With numerical simulations of dissipative, spherical particles the assumption is tested, that the velocities and the positions of colliding particles are uncorrelated ("molecular chaos"). This assumption is a basic ingredient of all theoretical approaches based on a kinetic theory or a pseudo-Liouville operator formalism. The numerical model is an event-driven method for the simulation of rigid spherical particles in two dimensions. In elastic systems, the impact parameter is uniformly distributed or, with other words, the molecular chaos assumption is valid independent of the density. In freely cooling systems, the molecular chaos assumption fails as soon as dissipation becomes strong enough so that convective shear modes are created.