Regime transition in subcritical and supercritical currents

Turbidity currents are sediment-laden flows that travel along sloping surfaces, typically the submarine bottom. They are driven by the density difference between the current and the deep layer of quiescent ambient fluid above them, which is the source of streamwise momentum and thus the source of the turbulence that keeps the sediment in suspension. On the other hand, the stable vertical gradient of density from the bottom to the ambient fluid above damps turbulence. The balance between these mechanisms generates two distinct flow regimes, namely super- and subcritical regimes. In this work, we focus on the transition between these two regimes, using a change in the bed slope. For this, we use highly resolved direct numerical simulations (up to 1.8 billion grid points) of spatially developing turbidity currents on a bed of varying slope. We observe that the flow reaches a quasi self-similar regime before the slope-break and far downstream from it, in a different flow regime. Between these two states, a transition zone appears. In particular, the transition from a subcritical to supercritical regime is characterized by very active turbulent mixing at the interface, with flow properties akin to a turbulent wall jet, before the flow readjusts itself and reaches a supercritical self similar state.