DECAYING STRATIFIED TURBULENCE - COMPARISON BETWEEN A 2-POINT CLOSUREEDQNM MODEL AND DIRECT NUMERICAL SIMULATIONS

A new statistical two-point closure EDQNM (Eddy Damped Quasi Normal Ma rkovian) model for axisymmetric stratified turbulence is presented. Th is model takes into account the detailed anisotropic structure of the flow, including angular dependence, and its closure assumption may inv olve explicit effects of the stratification. In the present paper, a s implified closure assumption is used, in which these explicit effects are not taken into account. The model is tested against three-dimensio nal direct numerical simulations (DNS) of decaying homogeneous stratif ied turbulence. A very good agreement is found between DNS results and EDQNM predictions, when energy transfers between the different modes of motion are considered. However, transfers among wavenumbers are ins ufficiently damped by the EDQNM model, because of the simplified closu re assumption, thus yielding a somewhat less satisfactory agreement. A n interesting prediction of the EDQNM model is the existence of a 'dir ectional' anisotropy at all scales of motion, to the smallest: the lev el of the total (kinetic + potential) energy density spectrum E(k,thet a(k)) varies with the angle theta(k) that the wavenumber vector k make s with the vertical.