Active separation control: An overview of Reynolds and Mach numbers effects

Separation control, by nominally two-dimensional periodic excitation, was studied experimentally by the authors and co-workers at Reynolds numbers ranging from 3 × 10⁴ to 4 × 10⁷, including compressibility effects. The tests demonstrated that active control using oscillatory flow excitation can effectively delay flow separation from, and reattach separated flow to, aerodynamic surfaces at various flight conditions. At Reynolds number below 105, where transition does not occur naturally and cannot be passively forced, active separation control may be the only effective method for delaying separation and generating useful lift. The essence of active separation control relies on exploiting instabilities that are inherent in the flow, generally requiring relatively small amplitude excitation. Effective excitation frequencies generate one to four vortices over the controlled region at all times, irrespective of Reynolds number, and perturbations should preferably be amplified over the region that is susceptible to separation. Periodic excitation is vastly superior to steady blowing in terms of performance benefits and eliminates abrupt flow responses, which are undesirable from a control point of view. The effects of compressibility in the absence of shocks are weak and undesirable effects accompanying separation, such as vortex-shedding and buffet, can be significantly reduced or completely eliminated. Separation resulting from shock-wave/boundary-layer interaction can be ameliorated, providing that excitation is introduced upstream of separation.