Entrainment, mixing, and their dependence on each other are discussed in a variety of stratified flows. In general, entrainment rates decline or are constant as the stratification is increased. The precise dependence of entrainment rate on Richardson number seems to have a variety of possible relationships. Recently, a new model has been proposed in an attempt to describe them in a unified pattern. This model introduces a new parameter, the persistence, to explain the observations. It also distinguishes between cases when the vorticity impinges on the interface from when the vorticity is already there. Surprisingly, mixing seems to be have no effect on stratified entrainment for many of these regimes. However, under some conditions the molecular mixing plays a central role in the flow. This is clear in the limit of large Richardson number, where molecular diffusion controls the rate at which mass or heat crosses a stratified interface. An even more dramatic case, similar to combustion, is mixing-induced buoyancy reversal. There even the global motions are controlled by molecular-scale mixing.

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