mixing length theory

Mixing length theory (or mixing length model) is a simplified model suitable for convection or turbulence in fluids under gravity with pressure and temperature gradients. Both those phenomena are chaotic so a model that approximates their overall effect is useful for incorporating in larger models, of atmospheres, oceans, stellar structure, etc.

Mixing length theory takes the conditions that would make a region of the fluid unstable (i.e., any small disturbance receives positive feedback), works out the distance (the mixing length) after which the fluid set in motion will reach a stable location, and incorporating other factors, yields a resulting movement of material and heat energy. Among the simplifications is that it ignores (as mostly insignificant) some effects on the fluid-in-motion by its surroundings.