On the definition, evolution, and properties of the outer edge of gravity currents: A direct-numerical and large-eddy simulation study

Gravity currents are flows driven by the action of gravity over fluids with different densities. Here, we focus on gravity currents where heavier fluid travels along the bottom of a sloping bed, underneath a large body of stagnant lighter ambient fluid. The thickness of the current increases due to entrainment of ambient fluid into the current. Direct numerical and large eddy simulations of gravity currents and a wall-jet transporting a passive scalar field are performed. We focus on the rate of penetration of mean momentum and mean concentration of the agent responsible for the density difference (temperature, salinity, or sediment volume fraction) into the ambient fluid. The rates of penetration of turbulence-related quantities (i.e., turbulent kinetic energy, Reynolds flux, and stress) into the ambient are analyzed. A robust methodology for defining the upper edge of these quantities and thereby defining the current thickness using these different mean and turbulent quantities is presented. A comparison between downstream evolution of the gravity current with the corresponding behaviors of canonical wall-bounded turbulent flows is presented. The present understanding of turbulent/non-turbulent interface (TNTI) is extended to include subcritical flows where, due to the strong effect of stratification, the TNTI is buried well within the upper edge of the current and confined right above the inner near-bed layer. The present work sheds light on the striking difference between the different definitions of thickness (momentum, concentration, turbulence, etc.) in subcritical gravity currents, where stratification suppresses turbulence in the upper region of the current.