Vorticity dynamics in separated bluff-body flows

The back-in-time evolution of vorticity in flows over bluff bodies is studied, with particular focus on separation and reattachment. The precursors of vorticity at these locations are decomposed into interior and the boundary contributions. The boundary term was credited by Lighthill for the generation and removal of vorticity, in favorable and adverse pressure gradients, respectively. Separation in two-dimensional flows is then viewed as the result of sustained adverse pressure gradient. An element is missing from this description, which we expose and quantify in our analysis. Furthermore, Lighthill's interpretation does not generalize to three-dimensional separation, for example for the vorticity dynamics near separation on a prolate spheroid at incidence. Our analysis shows that significant cancellations take place between different components of the interior vorticity prior to reaching the separation and reattachment lines. In addition, vorticity tilting enables the transport of vorticity across the separation surface—an effect that is not possible in the two-dimensional case.