Coherent Structures in Dense Granular Flows

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We present a theoretical derivation of a rheology for dense granular flow, based on the process of inelastic collapse of neighboring particles. This collapse creates regions of correlated motion, which control the viscous behavior of the flow on a large scale. The result is a rheology that obeys the scaling form observed experimentally by Pouliquen and by the "G.D.R. Midi" group author. We identify the nature of the constraints imposed by inelastic collapse on the grain-scale motions in the flow; finally, we show using an energy cascade argument that the inelastic collapse need not proceed to the final endpoint in order for the correlations we have identified to build up.