Mixing across a density interface produced by grid turbulence

The mixing produced by dropping a horizontal grid through a sharp density interface is examined experimentally. The fraction of the available kinetic energy used to mix the fluid, the flux Richardson number Rf, is measured as a function of the overall Richardson number Ri0. It is found that Rf increases from zero as Ri0 does, reaches a maximum, and then decreases with further increase in Ri0. The final interface thickness is found to be a decreasing function of Ri0, and the equivalent vertical diffusion coefficient is calculated. Qualitative observations of the flow show that the final stages of the decay of motion in the interface are characterized by pancake-shaped modes with large horizontal and small vertical scales.