Rheology of Two- and Three-dimensional Granular Mixtures Under Uniform Shear Flow: Enskog Kinetic Theory Versus Molecular Dynamics Simulations

The rheological properties for dilute and moderately dense granular binary mixtures of smooth, inelastic hard disks/spheres under uniform shear flow in steady state conditions are reported. The results are based on the Enskog kinetic theory, numerically solved by a dense gas extension of the Direct Simulation Monte Carlo method for dilute gases. These results are confronted to the ones also obtained by performing molecular dynamics (MD) simulations with good agreement for the lower densities and higher coefficients of restitution. For increasing density and dissipation, the Enskog equation applies qualitatively, but the quantitative differences increase. Possible reasons for deviations of Enskog from MD results are discussed, indicating non-Newtonian flow behavior and anisotropy as the most likely direction in which previous analytical approaches have to be extended.