Ambient fluid entrainment and basal drag in turbidity currents*

Turbidity current are sediment laden flows that move over inclined or horizontal surfaces. These flows are driven by buoyancy resulting from the density difference between the current carrying sediment and the clear ambient fluid above. A strong coupling between turbulence and suspended sediment particles influences the flow dynamics. Here we focus our attention on the process of ambient fluid entrainment happening at the interface between the current and the deep ambient layer. In this work, we study the dependence of the entrainment coefficient with the bulk Richardson number and the settling velocity of the sediment in the flow. We focus our study on the regime where the bulk properties of the flow vary slowly, usually called normal flow conditions. We employ direct numerical simulations (DNS) of temporally evolving, streamwise- spanwise- homogeneous turbidity currents. A new model for entrainment is proposed. Furthermore, the dependence of basal drag on Richardson number and settling velocity is analyzed. In addition, the turbulence structures of the flow and its relation to turbulence production and dissipation is studied for both subcritical and supercritical flows.