AVAILABLE POTENTIAL-ENERGY AND MIXING IN DENSITY-STRATIFIED FLUIDS

A conceptual framework for analysing the energetics of density-stratif ied Boussinesq fluid flows is discussed. The concept of gravitational available potential energy is used to formulate an energy budget in wh ich the evolution of the background potential energy, i.e. the minimum potential energy attainable through adiabatic motions, can be explici tly examined. For closed systems, the background potential energy can change only due to diabatic processes. The rate of change of backgroun d potential energy is proportional to the molecular diffusivity. Chang es in the background potential energy provide a direct measure of the potential energy changes due to irreversible diapycnal mixing. For ope n systems, background potential energy can also change due to boundary fluxes, which can be explicitly measured. The analysis is particularl y appropriate for evaluation of diabatic mixing rates in numerical sim ulations of turbulent flows. The energetics of a shear driven mixing l ayer is used to illustrate the analysis.