Large-eddy simulation of decaying stably stratified turbulence

Abstract

A large-eddy simulation (LES) model is developed and employed to study the interactions among turbulent and internal gravity wave motions in a uniformly stratified fluid at oceanic space and time scales. The decay of a random initial energy spectrum is simulated in a triply periodic domain by solving the full nonlinear, three-dimensional Navier-Stokes equations using pseudospectral techniques and a numerical resolution of 643 modes. Three experiments were performed with mean buoyancy frequencies (N) of 1, 3, and 10 cph for a period of 10 buoyancy times (Nt). The temporal evolution of the domain-averaged statistics is used to examine the nature of decaying stratified turbulence. Initially energy levels rapidly decay as the spectral energy distributions evolve toward more isotropic forms. One of the most exciting observations made here is the rapid transition in the character of the buoyancy flux evolution as part of the "turbulent collapse'. The BF changes suddenly from a state of irreversible mixing to an oscillatory, nearly reversible BF when the Ozmidov length scale is the same order as the vertical energy-containing length scale. -from Authors