Transition from discontinuous avalanches to continuous flow
Seminar Room 1, Newton Institute
AbstractWe investigate experimentally in a half-filled rotating drum set-up the transition from the regime of discontinuous avalanches at low rotation velocities to the regime of continuous flow at higher rotation velocities. For different glass bead diameters, we always observe a smooth transition between the two regimes: At a first critical rotation velocity, brief bursts of continuous flow can be observed in the predominant avalanche flow, while at a higher second critical rotation speed, the last rare events of discontinuous avalanches disappear of the predominant continuous flow. Such a smooth transition by intermittency is very different from the abrupt hysteretic transition described by Rajchenbach . With a model equation describing well the dynamics of unsteady avalanches  and extended to non negligible rotation rate of the drum, we can reproduce the two types of transition depending on the noise level added in the deterministic model equation. The predictions of the model can then be tested: The two critical rotation velocities are measured for the different grain diameters and are shown to be related to the measured noise fluctuations for the first one and to the distribution of maximum angle of stability relative to the neutral angle for the second one. The distribution of life time of each regime have also been measured and shown to be Poissonian, with a mean life time that varies from zero to infinity across the transition zone.  J. Rajchenbach, “Flow in Powders: From Discrete Avalanches to Continuous Regime,” Phys. Rev. Lett. 65, 2221 (1990).  R. Fischer, P. Gondret, B. Perrin, and M. Rabaud, “Dynamics of Dry Granular Avalanches,” Phys. Rev. E 78, 021302 (2008)
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