A Comparison of the Turbulence Evolution in a Stratified Fluid With Vertical or Horizontal Shear

The evolution of homogeneous turbulence in a stratified shear flow is studied using direct numerical simulations. Vertically stably stratified flows with uniform vertical shear and with uniform horizontal shear are considered and compared. The Richardson number is varied to include unstratified and strongly stratified cases. For a given value of the Richardson number, the turbulent velocity and density fluctuations were found to be significantly larger in the horizontal shear case due to an increased turbulence production rate that is not directly influenced by buoyancy. Eddy viscosity and eddy diffusivity coefficients are about an order of magnitude larger in the horizontal shear case. Volume visualization shows internal wave development in the strongly stratified cases with both vertical or horizontal shear. A vanishing buoyancy flux, however, is observed in the vertical shear case only. In decaying vertical shear cases, the Ozmidov scale restricts the size of vertical overturns. However, such a restriction was not observed in horizontal shear cases