Numerical Tests of Kinetic Theory for Sheared Granular Flows

The Revised Enskog Theory generalizes the kinetic theory of dense gases to include granular materials by incorporating inelasticity. It still relies, however, on the assumption that grains interact only through binary collisions. We explore the validity and breakdown of kinetic theory for granular materials utilizing Contact Dynamics simulations of two-dimensional sheared granular materials. Using an analytical expression for the contribution of binary collisions to the stress tensor, we directly measure the ``collisional'' stress and the total (Kirkwood) stress for a wide range of densities and restitution coefficients. Our measurements demonstrate that the ``collisional'' stress becomes negligible at high densities and low restitution coefficients. This suggests that the whole structure of kinetic theory breaks down in these regimes and emphasizes that jamming results from variations of the frictional stress, but not from a crossover between a collision-dominated and friction-dominated regime.