Entrainment and mixing in a laboratory model of oceanic overflow

Abstract

AbstractWe present experimental measurements of a wall-bounded gravity current, motivated by characterizing natural gravity currents such as oceanic overflows. We use particle image velocimetry and planar laser-induced fluorescence to simultaneously measure the velocity and density fields as they evolve downstream of the initial injection from a turbulent channel flow onto a plane inclined at$10^\circ $with respect to horizontal. The turbulence level of the input flow is controlled by injecting velocity fluctuations upstream of the output nozzle. The initial Reynolds number based on the Taylor microscale of the flow,$R_{\lambda }$, is varied between 40 and 120, and the effects of the initial turbulence level are assessed. The bulk Richardson number$\mathit{Ri}$for the flow is${\sim }$0.3 whereas the gradient Richardson number$\mathit{Ri}_g$varies between 0.04 and 0.25, indicating that shear dominates the stabilizing effect of stratification. Kelvin–Helmholtz instability results in vigorous vertical transport of mass and momentum. We present baseline characterization of standard turbulence quantities and calculate, in several different ways, the fluid entrainment coefficient$E$, a quantity of considerable interest in mixing parameterization for ocean circulation models. We also determine the properties of mixing as represented by the flux Richardson number$\mathit{Ri}_f$as a function of$\mathit{Ri}_g$and diapycnal mixing parameter$K_{\rho }$versus the buoyancy Reynolds number$\mathit{Re}_b$. We find reasonable agreement with results from natural flows.