Dynamics of flow over a sphere at moderate Re in a highly stratified fluid

Direct numerical simulations (DNS) are performed to investigate the flow past a sphere at $Re = 3700$ and $Fr \in [0.025, 1]$. Unlike previous experimental and numerical studies of flow over a sphere at low $Re$ and low $Fr$, it is found that the fluctuations tend to regenerate at $Fr$ lower than a critical value for moderate $Re = 3700$. High stratification suppress vertical motion and, for a three-dimensional body, the fluid flows horizontally around the sides leading to a new regime of unsteady vortex shedding. Vertically thin layers of shear interspersed between quasi-two dimensional motions undergo secondary Kelvin-Helmholtz (KH) instabilities if the buoyancy Reynolds number, $Re_b \geq O(1)$. The combined effect of unsteady vortex shedding, enhanced horizontal shear, and secondary KH instabilities results in the regeneration of turbulence at low $Fr$. There is an $increase$ in the coefficient of drag $C_d$ at high stratification (low $Fr$), for $Re = 3700$. This result is contrary to previous experiments on flow over sphere at low $Re$ where $C_d$ was found to $decrease$ with increase in stratification in the low-$Fr$ regime.