(547b) Impact of Friction On Instabilities In the Homogenous Cooling of Granular Materials

Experiments and molecular dynamics (MD) simulations have demonstrated the existence of vortex and cluster instabilities in the homogeneous cooling of solid, inelastic particles. These instabilities stem from the dissipative nature of (inelastic) collisions, and thus are not present in their molecular counterparts. Recent work has also shown strong quantitative agreement between the predictions from stability analyses of continuum theory and MD simulations for the length scale at which vortex instabilities onset (Mitrano, Dahl, and Hrenya, Physics of Fluids, submitted). However, the effect of friction (another source of dissipation) on such instabilities has not been assessed. In this work, hard-sphere, event-driven MD simulations of the homogenous cooling system are used to study vortex and cluster instabilities in granular systems of frictional particles. A Fourier analysis of the momentum and density fields allows for the detection of vortex and cluster instabilities, respectfully. The specific aims are to determine the critical system length scale at which instabilities appear, to identify the type of instability present at onset, and its evolution over time. The results demonstrate the importance of friction relative to inelasticity on the formation of flow instabilities.