Size effects in underwater granular collapses: Experiments and coupled lattice Boltzmann and discrete element method simulations

Abstract

Immersed granular collapse is a common model case for the study of transient geophysical flows. This paper examines the effects of column size on granular collapses in water, with an emphasis on the granular flow mobility. Laboratory-scale experiments of underwater granular collapses with three different column sizes are carried out, together with their numerical simulations using the coupled lattice Boltzmann and discrete element method. Both experimental and numerical data show that, for an identical aspect ratio, a larger underwater granular collapse results in higher flow mobility and a longer runout distance normalized by the initial column length Li (increased by 18% on average as Li increases from 3 to 10 cm). Simulations show that as the column size increases, there is more potential energy being transferred into the kinetic energies of the fluid and the particles, and there is a positive relationship between the column size and the efficiency of energy conversion of the particle kinetic energies from vertical to horizontal directions, which contributes to a higher underwater granular flow mobility in larger cases. The reason is twofold. First, the fluid inertia scales disproportionately with the column size. A stronger eddy with high inertia is induced in the large case, which penetrates through the flowing layer of the granular phase and pushes the particles forward to reach a longer runout distance. Second, large underwater granular collapses are accompanied with more significant contact lubrication, which promotes basal slip and dissipates less energy during horizontal spreading.