Spatio-temporal dynamics of pressure-induced turbulent separation bubbles

The spatio-temporal dynamics of separation bubbles (SBs) induced to form in a fully-developed turbulent boundary layer over a flat plate are studied via direct numerical simulations. Two different SBs are examined: one induced by a suction-blowing velocity profile on the top boundary and the other, by a suction-only velocity profile. The latter condition allows reattachment to occur without an externally imposed favourable pressure gradient and leads to a SB more representative of those occurring over airfoils and in diffusers. The suction-only SB exhibits a range of clearly distinguishable modes. The high-frequency modes are well characterized by classical frequency scalings for a plane mixing layer and are associated with the formation and shedding of spanwise oriented vortex rollers. The topology associated with the low-frequency motion is revealed by the application of dynamic mode decomposition (DMD) and is shown to be dominated by highly elongated structures in the streamwise direction. The possibility of G\"{o}rtler instability as the underlying mechanism for these structures is explored.