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Active flow control of canonical laminar separation bubbles by steady andharmonic vortex generator jets (VGJs) was investigated using direct numericalsimulations. Both control strategies were found to be effective in controllingthe laminar boundary-layer separation. However, the present results indicatethat using the same blowing amplitude, harmonic VGJs were more effective andefficient in reducing the separated region than the steady VGJs considering thefact that the harmonic VGJs use less momentum than the steady case. For steadyVGJs, longitudinal structures formed immediately downstream of injectionlocation led to formation of hairpin-type vortices causing an earliertransition to turbulence. Symmetric hairpin vortices were shown to developdownstream of the forcing location for the harmonic VGJs as well. However, theincreased control effectiveness for harmonic VGJs flow control strategy isattributed to the fact that shear-layer instability mechanism was exploited. Asa result, disturbances introduced by VGJs were strongly amplified leading todevelopment of large-scale coherent structures, which are very effective inincreasing the momentum exchange, thus, limiting the separated region.

Control of laminar boundary-layer separation using steady and harmonic vortex generator jets