Boundary effects and self-organization in dense granular flows

Boundary effects in gravity-driven, dense granular flows down inclined planes are studied using large-scale molecular dynamics simulations. We find that the flow behavior and structure of the flowing pile changes dramatically as we vary the roughness of the supporting base. For a rough, bumpy base, there are three principal flow regimes that depend on the inclination angle θ: at small angles θ<θ_r, where θ_r is the angle of repose, the system does not flow; for large angles θ>θ_max, where θ_max is the maximum angle for which stable, steady state flow exists, the flow is unstable; and for θ_r<θ<θ_max, the energy input from gravity is balanced by that dissipated through friction and the system reaches a stable, steady state flow. In the stable regime, we find no slip boundary conditions with a bulk density that is independent of the height above the base. For a chute base that is ordered, the steady state regime splits into a further three distinct flow regimes: at lower angles, the flowing system self-organizes into a state of low-dissipation flow consisting of in-plane ordering in the bulk; at higher angles, a high-dissipation regime similar to that for a rough base but with considerable slip at the bottom is observed; and between these two sub-regions we observe a transitional flow regime characterized by large oscillations in the bulk averaged kinetic energy due to the spontaneous ordering and disordering of the system as a function of time.