Decaying homogeneous turbulence under strong density stratification at low-Prandtl number

The characteristics of decaying homogeneous turbulence under strong density stratification have been studied using direct numerical simulations. While our previous study dealt with rotating stratified turbulence, here we investigate the detailed flow structure of stratified turbulence without rotation especially at low-Prandtl number. By assuming a low-Prandtl-number fluid, e.g. liquid sodium: Pr ≈ 0.01, gallium: Pr ≈ 0.025, internal gravity waves are markedly attenuated due to the large thermal conductivity, and turbulence soon reaches a two-component state, where vertical energy, coupled with potential energy, significantly decays, and becomes negligible as observed experimentally (Praud et al. in J Fluid Mech 522:1–33, 2005). In the horizontal plane, there appear large-scale vortices with vertical vorticity, and those with the same sign of vorticity increase their horizontal length scale by merging with each other. In the vertical plane, highly sheared regions represented by horizontal vorticity also tend to horizontally increase their length scale and become layered structures by the combined effects of vortex coalescence and energy cascade into higher vertical wavenumbers.