An investigation of vortical structures and density overturns in stably stratified homogeneous turbulence by means of direct numerical simulation

Direct numerical simulations (DNS) of stably stratified homogeneous turbulence, with and without a background shear, are performed to investigate two characteristic types of structures in these flows: coherent vortices and density overturns.; Coherent vortices stir the density field by wrapping the isopycnals around their cores. This phenomenon, in the presence of strong stratification, causes high vorticity to concentrate itself in increasingly horizontal structures, the precursors of “pancake” eddies observed in the ocean and laboratory. The associated dynamics are studied in terms of the interaction of vorticity, strain and density gradient.; Density overturns, regions of heavy fluid overlying light in the same fluid column, are considered active sites of stirring and mixing. Field observations of overturns are limited to single time, 1-D vertical profile data and have thereby led to significant controversy over their interpretation. The objective of this study is to develop a more complete description of overturns and provide insight for 1-D measurements. DNS is employed together with a 3-D feature identification and tracking algorithm to investigate the spatial structure, dynamical evolution and energetic significance of overturns. Here, a 3-D overturn patch is defined as a contiguous volume of non-zero Thorpe displacement. Patch characteristics based on the conventional 1-D representation are also determined for comparison. Both individual patches as well as their entire population are studied. The effect of varying degree of stratification is also considered in the analysis. Visualizations show the 3-D spatial structure and demonstrate the effects of background shear. Results show that the patch evolution consists of three distinct phases: an inertially-driven growth phase, buoyancy-dominated collapse phase and final phase of viscous-diffusive buoyant balance. The contribution of the overturns to the overall flow energetics varies for each phase. In general, the majority of diapycnal mixing in the flow is concentrated in the peripheral boundary zone of the overturn patches. Non-overturning motions may be associated with significant available potential energy and buoyancy flux but turbulent overturning is a necessary condition for any significant diapycnal mixing to occur. Finally, the Reynolds number dependence of the results is discussed along with implications for overturn behavior in the ocean.