In the present paper we describe a model of the transition from a quasi static to dynamic regime in a granular stream. The model was developed using the results of experiments carried out on a rotating drum partially filled with sand grains or glass beads; the experiments provide information about rheology through grain velocity profiles and through the grain velocity covariance tensor. The model relies on several assumptions: we express the frictional stress component, due to prolonged contacts between particles, with a Coulomb law, assuming that the friction angle is equal to the true t?iction angle between the particle surfaces at contact plus the angle between the mean contact plane and the shearing plane. The difficulties involved in measuring the volume concentration of the grains with the necessary precision and the substantial impossibility of checking the results, suggest a closure 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 collisional regime. The collisional contribution to the global stress is expressed as a function of the mean concentration, the local grain velocity gradient and the average contact angle between shearing layers (we implicitly assume that collisions between particles are binary and that multiple contacts between particles in movement generate friction); the kinetic contribution is not taken into account because of its minor relevance at high concentration. The numerical model gives a satisfactory reproduction of the experimental grain velocity profiles.
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