Coherent structures and enstrophy dynamics in highly stratified flow past a sphere at Re = 3700

Vortex dynamics of flow past a sphere in a linearly stratified environment is investigated. Simulations are carried out for a flow with Reynolds number of 3700 and for several Froude numbers ($Fr$) ranging as low as 0.025. Isosurface of Q criterion is used to identify vortical structures whose cross-section and orientation are found to be affected by buoyancy. At low $Fr = 0.025$, pancake eddies and surfboard-like inclined structures emerge in the near wake and have a regular streamwise spacing that is associated with the frequency of vortex shedding from the sphere. Similar to turbulent kinetic energy, the enstrophy in the near wake decreases with decreasing $Fr$ (increasing stratification) until a minimum at $Fr = 0.5$ but the trend reverses in the low-$Fr$ regime. Vortex stretching by fluctuating and mean strain are both responsible for enhancing vorticity with relatively small contribution from the baroclinic term. Decreasing $Fr$ to $O(1)$ values tends to suppress vortex stretching. Upon further reduction of $Fr$ below 0.25, the vortex stretching term takes large values near the sphere.