Multi-phase flow modeling of submarine landslides: Transformation from hyperconcentrated flows into turbidity currents

Sedimentary density flows are major agents of sediment transport on oceans. Occurring after submarine landslides, the sedimentary density flows experience a series of dilution processes. Such flows, then, transform from the hyperconcentrated flows into the concentrated ones, and finally turn into the turbidity currents. This study applies a sophisticated multi-phase flow model to simulate sedimentary density flows caused by submarine granular landslides over a simplified topography with an emphasis on their dilution mechanisms. This model takes account of the particle-particle interactions, the particle-fluid interactions, and the turbulent motions of fluid and particles in the momentum transports. The two effects on turbulence modulation are considered: one is due to the stratification and the other is caused by the correlation between the fluid and sediment turbulent motions. The numerical results suggest that the dilution mechanisms are highly related to Kelvin-Helmholtz vortices. In addition to erosion of the hyperconcentrated region by vortices, this study finds that strong stir in the concentrated region by the vortices is another important dilution mechanism. Compared with the outer regions of the vortices (with zero vorticity), the vortex cores (with non-zero vorticity) have less efficient dilution processes due to rigid-body rotations there. The vortex cores can trap the small particles for a while. The involved flow regimes are also presented. The numerical results reveal that the turbidity currents can be either turbulent or laminar when the Bagnold limit of concentration is adopted to define the boundary of turbidity currents.