  The spatio-temporal dynamics of separation bubbles induced to form in a
fully-developed turbulent boundary layer (with Reynolds number based on
momentum thickness of the boundary layer of 490) over a flat plate are studied
via direct numerical simulations. Two different separation bubbles 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 separation bubble more representative of those
occurring over airfoils and in diffusers. The suction-only separation bubble
exhibits a range of clearly distinguishable modes including a high-frequency
mode and a low-frequency "breathing" mode that has been observed in some
previous experiments. The high-frequency mode is well characterized by
classical frequency scalings for a plane mixing layer and is associated with
the formation and shedding of spanwise oriented vortex rollers. The topology
associated with the low-frequency motion is revealed by applying dynamic mode
decomposition to the data from the simulations and is shown to be dominated by
highly elongated structures in the streamwise direction. The possibility of
G\"{o}rtler instability induced by the streamwise curvature on the upstream end
of the separation bubble as the underlying mechanism for these structures and
the associated low frequency is explored.
