Hirofumi Niiya, Akinori Awazu, Hiraku Nishimori
It has been known that the granular flow of polystyrene particles down a slope forms a wavy pattern with many heads at the moving front of the resulting avalanche. In experiments of granular flow using low-density particles, the instability of the moving front and the subsequent head formation are driven by gravity and air drag. To elucidate the instability mechanism of granular avalanches, we propose a particle method considering gravity as the driving force for the avalanche, the contact interaction between granular particles, and the long-range interaction between granular particles through the ambient fluid as a type of air drag. Using this model, we simulate the head formation at the moving front of the avalanche, and we investigate the particle flow caused by the air drag. It is found that the air drag destabilizes the shape of the avalanche that deforms into a wavy pattern, leading to the generation of a pair of granular vortex convection currents inside the head. Further, the relationship between the particle radius and head size is found to satisfy the positive linear scaling law. Moreover, the hydrodynamic interaction between the particles causes their aggregation at the moving front of the avalanche, and this aggregation effect generates the head-tail structure. These numerical results are qualitatively consistent with the results of previous experiments.
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http://arxiv.org/abs/1302.0572
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