Mixing in stratified fluids

Laboratory experiments on mixing in stratified fluids are examined in the light of some simple energy arguments applied to an analysis of mixing at density interfaces. It is shown that as the stability of the flow, as measured by an overall Richardson number Rio , increases, the velocity interface becomes considerably thicker than the density interface. This mismatch in interface thickness provides the extra kinetic energy required for mixing in these strongly stratified flows. The behaviour of the flux Richardson number Rf as a function of Rio is also discussed. It is found that the results from a number of different experiments are quite similar, and that Rf increases from zero as Rio does, reaches a maximum value (0.2 & plusmn; 0.05) and then decreases again with further increase in Rio . Finally, some recent ideas of Posmentier (1977) on the formation of interfaces in a stratified, turbulent flow are compared with observations.