  Shear-induced vertical mixing in a stratified flow is a key ingredient of
thermohaline circulation. We experimentally determine the vertical flux of
momentum and density of a forced gravity current using high-resolution velocity
and density measurements. A constant eddy viscosity model provides a poor
description of the physics of mixing, but a Prandtl mixing length model
relating momentum and density fluxes to mean velocity and density gradients
works well. For $<Ri_g > \approx 0.08$ and $Re_\lambda \approx 100$, the mixing
lengths are fairly constant, about the same magnitude, comparable to the
turbulent shear length.

Fluid Dynamics;Atmospheric and Oceanic Physics