Statistical and dynamical questions in stratified turbulence

We examine homogeneous turbulence under stably stratified and neutral conditions, including decaying and randomly forced cases. Our tools include direct numerical simulations (DNSs) and elements of statistical theory. Our DNS at 5123 permit large scales to develop from the dynamics at smaller, energy-containing scales. The preceding resolution permits a Taylor microscale R ~ 150. The size distribution of such large scales is closely related to conservation principles, such as angular momentum, energy, and scalar variance; we relate these principles to our DNS results. Stratified turbulence decays more slowly than isotropic turbulence with the same initial conditions. We offer a simple explanation in terms of the diminution of energy transfer to small scales resulting from phase mixing of gravity waves. Enstrophy structures in stratified flows (scattered pancakes) are distinctly different from those found from isotropic turbulence (vortex tubes). For the forced case, we examine the modification of the inertial range induced by strong stratification (k-5/3 →~ k-2). We note that the development of the vertically sheared horizontal flow (VSHF) mode of Smith and Waleffe (2002) is closely associated with strong gravity waves at large scales.