Particle entrainment in unsteady-uniform granular avalanches
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We investigate the mechanism of particle entrainment for dense, dry, collisional granular flows driven by gravity through a set of uniform-unsteady experiments. The flow evolves in time from a rest to a fully developed condition, but without any variability in the longitudinal direction. Using a high speed camera and particle tracking algorithms, we measure the time evolution of the flow depth, the normal-to-bed velocity profile and the granular concentration. We compare these with the predictions of a simple theory, obtained by solving for the time evolution of the momentum balance in the flow direction. In doing this, the velocity profile is assumed to maintain in time the same shape it has in a fully developed flow. This is obtained from the approximate analytical integration of an extended kinetic theory for dry, dense collisional flows of dissipative spheres. The proposed model relies only on measured physical properties of the particles, without any ad hoc calibrated parameter.