Granular streams rheology and mechanics

In the present papa we describe a model of the transition from a quasi stat ic to dynamic regime in a granular stream. The model was developed using th e results of experiments carried out on a rotating drum partially filled wi th sand grains or glass beads; the experiments provide information about rh eology through grain velocity profiles and through the grain velocity covar iance tensor. The model relies on several assumptions: we express the frict ional stress component, due to prolonged contacts between particles, with a Coulomb law, assuming that the friction angle is equal to the true frictio n angle between the particle surfaces at contact plus the angle between the mean contact plane and the shearing plane. The difficulties involved in me asuring the volume concentration of the grains with the necessary precision and the substantial impossibility of checking the results, suggest a closu re based on the contact angle. We assume that the average contact angle in the frictional regime is the same as the average collisional angle in the c ollisional regime. The collisional contribution to the global stress is exp ressed as a function of the mean concentration, the local grain velocity gr adient and the average contact angle between shearing layers (we implicitly assume that collisions between particles are binary and that multiple cont acts between particles in movement generate friction); the kinetic contribu tion is not taken into account because of its minor relevance at high conce ntration. The numerical model gives a satisfactory reproduction of the experimental g rain velocity profiles. (C) 2000 Elsevier Science Ltd. All rights reserved.