Spreading of underwater granular collapse on rough beds by CFD-DEM simulation

We use the coupled method of computational fluid dynamics (CFD) and discrete element method (DEM) to simulate the underwater granular column collapse, which is a simplified two-phase flow model of submarine landslide. With a quantitative description of roughness introduced, the dynamic process of underwater granular collapse on rough bed is studied. We find that as the aspect ratio increases, the final runout distance grows exponentially, and the difference in the final deposition height between the column on smooth and rough beds gradually decreases. Curves of dimensionless runout with aspect ratio for collapse on smooth bed parallel with that on the rough bed. The utilization rate of the initial potential energy in the granular system improves, and the vertical movement of particles gradually becomes dominant. As the roughness increases, the duration time and final runout distance of the column exhibit an exponential reduction with a stable value for large roughness, and the flow dead zone first expands significantly and then remains almost unchanged. The mechanism of its influence is that the utilization efficiency of potential energy and the conversion efficiency of energy in the horizontal direction decrease and tend to stabilize as the roughness increases, with the effect of roughness becomes saturated when the value exceeds 0.6 corresponding to a bed surface with a half covered by particles. These findings enhance the knowledge of underwater landslides, and it is expected to provide guidance in underwater disaster prevention design.