Steady chute flow of inelastic discs beyond the maximum angle from μ-I rheology

Studies investigating granular rheology in chute flow configuration suggest that steady flows occur only up to an angle tanθ_max=μ_max where μ_max is the maximum value of the effective friction coefficient in the μ-I rheology description. To the best of our knowledge, studies beyond this maximum value of the inclination angle are not reported and it has been assumed that the flow becomes unsteady and keeps accelerating beyond this angle. We show that this assumption is incorrect and steady, fully developed chute flows well beyond the angle corresponding to maximum effective friction are indeed possible. The flows at such high inclinations show a significant slip at the base along with huge dilation of the layer. Angles as large as 44^o corresponding to the inertial number I~1.7 are observed in our DEM simulations of inelastic, frictional disks. The dip in the effective friction at higher inertial numbers, observed previously in plane-shear flows, is found to be present in chute flow as well. A modified μ-I rheology predicts the properties in the bulk of the layer reasonably well even at these high values of θ, after which transition of the flow to a different regime is observed.