Dynamics of stratified shear layer with horizontal shear

The evolution of a stratified shear layer with horizontal shear and vertical stratification is numerically investigated to study the structural organization of the vorticity and density fields as well as to quantify the statistical evolution of the flow. Although the Reynolds number of the flow increases with time facilitating the development of horizontal instabilities of the inflectional mean shear and turbulence, the bulk Richardson number signifying the level of stratification also increases. Remarkably rich dynamics is found: turbulence; the emergence of coherent core/braid regions from turbulence; formation of a lattice of dislocated vortex cores connected by thin horizontal layers; density-driven intrusions at the edges of the shear layer; and, generation of internal waves. Thus, stratification introduces significant vertical variability although it inhibits the vertical component of turbulence. The simulation data is used to help explain how buoyancy enables the formation of such thin layers with large vertical shear (horizontal vorticity) and density gradient from the columnar vortex cores of a shear layer. The statistical evolution of the different energetic quantities is studied. The peak values as well as the cross-stream extent of the horizontal component of the turbulent energy is found to increase at late times with increased stratification. The peak value of the vertical component of the kinetic energy is found to decay, although the cross-stream extent of the profiles is larger than the unstratified case. Significant turbulent potential energy is observed for the different cases simulated, with the profiles spreading outside the nominal shear layer. The molecular dissipation of turbulent kinetic energy and of turbulent potential energy are both found to be substantial even in the case with highest stratification (final Richardson number about 45) and primarily concentrated in thin horizontal layers at the dislocation zones between the cores.