Boundary Condition for Dense Granular Flows on Smooth Boundary*

When granular material moves relative to a smooth boundary, we often assign a Coulomb-type stress boundary condition with a constant effective wall friction coefficient μ_w. Recent experimental and numercial (DEM) investigations on both steady and transient flows reveal a roubst trend that that μ_w decays monotonically with the distance from a moving boundary such as a free surface, a slip base, or an internal shear band. Individual grain rotation is found to be an internal mechanism to degrade bulk μ_w. We discover that grain anular speed is correlated with granular temperatre which has been reported to correlate with bulk slip velocity or shear rate. We integrate the findings to present a boundary condition for dense granular flows with non-zero velocity relative to its boundary.