Large eddy simulations of forced and stably stratified turbulence: Evolution, spectra, scaling, structures and shear

摘要

Large eddy simulations (LES) of forced stratified turbulence in a triply-periodic box at multiple Froude numbers (Fr-0) are carried out to study the effect of stratification on energy evolution, spectra, scaling, vortical structures and shear for flows at Re = 25000. The dynamic Smagorinsky model is used as the sub-grid scale (SGS) closure with a grid density of 96(3). A three-dimensional (3D) continuous forcing scheme adapted from Rosales et al. (2005), where a constant factor is multiplied to the velocity scales, is applied to LES for the first time in stably-stratified turbulence. Calculations for three different Froude numbers, Fr-0 = 0.1, 0.2 and 0.3, are presented here. The behavior of the energy spectra and flux and evolution of local quantities such as buoyancy Reynolds number, Froude number, Richardson number and shear distribution at a fully developed state are analysed in detail. In addition, the 3D isosurfaces and two-dimensional (2D) isocontours of vorticity are presented, which clearly highlight the suppression of turbulence in the vertical direction. Increase in the intensity of stratification, results in a delay in the attainment of a fully-developed state. Moreover, the dissipation rates and enstrophy at the fully developed state are lower for lower Froude numbers. A kappa(-3) scaling is observed for the vertical kinetic energy spectra and a kappa(-5/3) scaling for the potential energy spectra. For moderate stratification, i.e. at Fr-0 = 0.3, the scaling results based on the Bolgiano and Obukhov theory are verified here as well.