The viscosity-affected regime of decaying stratified turbulence subjected to strong stratification

The decay of turbulence in the viscosity-affected stratified regime is studied through direct numerical simulations (DNS) of a homogeneous turbulent field subjected to strong stratification. The initial velocity fields are specified according to a prescribed energy spectrum at a Taylor scale Reynolds number of Re-lambda = 100. The simulations use a pseudo-spectral code with a 768(3) grid resolution. Very low Froude numbers i.e, Fr-0 = 0.001, 0.01 and 0.1, corresponding to buoyancy Reynolds number of R << 1, are investigated here. Evolution of various parameters such as kinetic energy, potential energy, vertical shear, and length scales - energy spectra and fluxes across vertical and horizontal directions, and - flow structures via the Q-criterion isosurfaces, are analyzed in detail. Q-criteria is utilized for the visualization of flows. It is observed that the laminarization of flow and inverse cascade is the predominant consequence of stable stratification. The behavior of the flow in the R << 1 regime, where viscous shear is the main parameter, is shown to be different from the stratified turbulence regime ( R >> 1). The coherent vortical structure is found to be sensitive to the strength of stratification showing a disk- and elongated strip-like structures within the strongly stratified regime. It is shown that the horizontal energy spectra exhibit a slope, steeper than -5/3 (slope of turbulent energy spectra), varying from -14/5 to -5 in the inertial subrange. Many new features are understood in terms of power law exhibited by bulk quantities like length scales, turbulent kinetic energy, and slope of horizontal spectra in the inertial subrange for the viscosity-affected regime (R << 1). Finally, it is demonstrated that the increase in the negative value of energy flux leads to various slopes in the energy spectra, which are associated with the change of laminarization structures.