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 Ri 0 . It is found that Rf increases from zero as Ri 0 does, reaches a maximum, and then decreases with further increase in Ri 0 . The final interface thickness is found to be a decreasing function of Ri 0 , 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.