The flow external to a rotating torus

Imparting a sudden rotation to a torus (or other symmetric smooth object) in an otherwise quiescent, viscous fluid serves to generate boundary layers at the object’s surface. These boundary layers are known to exhibit a finite-time singularity at the equator which manifests in a thickening of the boundary layer and subsequent development of an equatorial jet. Here we consider the post-collision flow dynamics, demonstrating that the equatorial jet serves to shed a finite amplitude toroidal vortex pair. The radial jet is also shown to develop an absolute instability at suitably high Reynolds numbers.