  Active flow control of canonical laminar separation bubbles by steady and
harmonic vortex generator jets (VGJs) was investigated using direct numerical
simulations. Both control strategies were found to be effective in controlling
the laminar boundary-layer separation. However, the present results indicate
that using the same blowing amplitude, harmonic VGJs were more effective and
efficient in reducing the separated region than the steady VGJs considering the
fact that the harmonic VGJs use less momentum than the steady case. For steady
VGJs, longitudinal structures formed immediately downstream of injection
location led to formation of hairpin-type vortices causing an earlier
transition to turbulence. Symmetric hairpin vortices were shown to develop
downstream of the forcing location for the harmonic VGJs as well. However, the
increased control effectiveness for harmonic VGJs flow control strategy is
attributed to the fact that shear-layer instability mechanism was exploited. As
a result, disturbances introduced by VGJs were strongly amplified leading to
development of large-scale coherent structures, which are very effective in
increasing the momentum exchange, thus, limiting the separated region.
