Self organization of granular flow by basal friction variation: Natural jump, moving bore, and flying avalanche

Abstract

The article discusses the response of rapid granular downslope flow to an abrupt change in basal friction. Although granular discharge from silo and hoppers has received considerable attention in the past, the flow behavior for an abrupt change in basal friction is hitherto unexplored. In the present study, the channel floor comprises of a smooth surface (bed friction angle—δS) and a rough surface (bed friction angle—δR) such that the angle of repose of the granular material (θr) lies between δS and δR and the flow features are observed as the channel inclination (θ) is varied from ≈δS to >δR. Experiments are performed in two channels with different extent of rough surface for two grain sizes with the same angle of repose and different inlet depth of granular flow. The visualization studies reveal a rich variety of flow features namely moving bore, rapid granular flow, flying avalanche and granular jump. Natural granular jump formed by mere frictional dissipation without any external forcing has not been reported earlier. The variety of flow features primarily results from the interplay of downslope motion of variable depth, collision‐driven piling of grains, and slope shaving avalanches. The observed flow phenomena have been satisfactorily analyzed by the well‐known depth‐averaged avalanche flow equations under conditions of incompressible granular flow.