Dynamics and scaling laws of underwater granular collapse with varying aspect ratios

We perform coupled fluid-particle simulations to understand the granular collapse in an ambient fluid (in particular, water) with a wide range of initial aspect ratios. We observe both similar and distinct features in underwater collapses compared to their dry counterparts. As aspect ratio a increases, the normalized runout distance follows a piecewise power-law growth, transitioning at a = 2.5. We associate this transition with the different growth rates of kinetic energy (with a) in vertical and horizontal directions. The ability of utilizing available energy for horizontal motion becomes limited when a > 2.5. Moreover, the front propagation during underwater collapses can be well scaled by using the initial column height as length scale and considering a reduced gravity (due to buoyancy) in timescale. Under the reduced gravity, the initial fall of tall columns is found to be ballistic, consistent with dry collapses. On the other hand, underwater collapses (especially for large a) exhibit unique dynamics due to the presence of water. The eddies generated in water, which may carry considerable fluid inertia, tend to erode the surface of the granular layer, thus modifying the deposit morphology. The energy conversion is also affected by the ambient fluid. While water obviously consumes energy from the granular phase through fluid-particle interactions, it actually increases the efficiency of energy conversion from vertical to horizontal directions. The latter effect compensates the difference of runout distance between underwater and dry collapses.