A new type of axisymmetric vortex shedding from a sphere moving vertically in a stratified fluid

A numerical study is described of the flow past a sphere moving vertically at constant speed in a stratified fluid. It is found that the flow under moderate or strong stratification (Fr=W/Na ≤14, W: sphere velocity, N: buoyancy frequency and a: sphere radius) remains axisymmetric even at Re=500, unlike the flow of a homogeneous fluid which becomes asymmetric for Re>200. A striking feature is that this axisymmetric flow gives, under moderate stratification (7≤Fr≤14), an axisymmetric vortex or 'vorticity' shedding, which has never been observed in the wake of an axisymmetric bluff body. The 'vorticity' shedding occurs even when the familiar vortex shedding can not be identified in the streamlines. The phenomenon corresponds to the observation in previous shadowgraph experiments (e.g. Hanazaki 2009), which showed a periodic generation of knots in the jet downstream of a sphere.