MODELING NONEQUILIBRIUM AND HISTORY EFFECTS OF HOMOGENEOUS TURBULENCEIN STABLY STRATIFIED MEDIUM

Homogeneous turbulence decay in a stably stratified flow has two disti nct characteristics. One is countergradient fluxes that are developed to keep the energy budget in equilibrium. Another is the formation of gravity waves that contribute to the velocity variance with little ver tical mixing. The prediction of these characteristics are investigated using a hierarchy of turbulence models. They include second-order mod els as well as models that solve the transport equations for the turbu lent kinetic energy, its dissipation rate, the temperature variance an d its dissipation rate. In the latter class of models, the vertical he at flux is calculated either from an algebraic equation or from a tran sport equation. The algebraic equation is derived by invoking equilibr ium and nonequilibrium turbulence assumption. Thus, modeling level and the relative importance of nonequilibrium and history effects in the predictions of countergradient fluxes and gravity waves could be asses sed. The investigation reveals that countergradient heat flux can be p redicted even when the equilibrium assumption is invoked. However, the formation of gravity waves can be predicted only when the history eff ects of the vertical heat flux are accounted for properly. The decay r ate of the total energy is very much affected by two model constants i n the temperature variance dissipation rate equation. On the other han d, the calculated frequency and amplitude of the gravity waves are inf luenced by the model constants in the heat flux equation. (C) 1997 by Elsevier Science Inc.