Entrainment dynamics in self-adjusting gravity currents using simultaneous velocity-density measurements*

Gravity currents can modify their flow characteristic by entraining and mixing with the ambient fluid. The entrainment in such systems may depend on a variety of intrinsic parameters such as, initial density difference, $\Delta\rho$, total height of the fluid, $H$, and slope of the terrain, $\alpha$. Thus, it is imperative to study the entrainment dynamics of a gravity current in order to have a clear understanding of the mixing transitions that govern the flow physics such as the shear layer thickness, $\delta_{u}$, and the mixing layer thickness, $\delta_{\rho}$. Experiments were conducted in a lock-exchange type facility, where a self-adjusting gravity current is formed, for which the only governing parameter is the Reynolds number, Re=$\frac{u_{f}H}{\nu}$, where $u_{f}$=0.4$\sqrt{g^{'}H}$ is the frontal velocity. Simultaneous PIV-PLIF technique is employed to get the velocity and density statistics. A control volume based flux method is used to calculate the flux entrainment coefficient, E$_{f}$, for a Reynolds number range of Re=400-12000 used in our experiments. The results show transition at Re~4x10$^{3}$, where the mixing occurs due to Kelvin-Helmholtz billows that promote small scale local mixing, and cause a spike in the flux entrainment velocity.