Abstract
A statistical analysis of granular avalanches in a half-filled and slowly rotated drum is presented. For large-sized grains the classical coherent oscillation is reproduced, i.e. we observe a quasi-periodic succession of regularly sized avalanches. As particle size is decreased, we see a crossover to a new complex dynamics characterized by long-range time correlations of local avalanches without a typical size, although the size distribution is not scale invariant. In the limit of large values of the time lag avalanches turn gradually to be decorrelated. The trend observed in the system dynamics as particle size is decreased is ascribed to the increase of cohesiveness which promotes bulk disorder. We argue that our experimental findings can be qualitatively predicted by theoretical models with adjustable parameters such as unquenched disorder, random critical slope, fluidization length, inertia and dissipation.