Flow over a Backward-Facing Step Under Controlled-Plasma, Actuator-Induced Perturbations

Abstract

The effectiveness of an airborne laser can be great ly reduced when the laser has to pass through a compressible free shear layer containing vortical structures. Adaptive optics can compensate for aberrations but the time scale assoc iated with the chaotic structures in a free shear layer is too small to allow the adaptive opti cs to be effective. A possible solution is to force the instabilities in the free shear layer to organize in predetermined manner that could be fed forward to the adaptive optics system. A pla sma actuator was proposed for this application in previous work. In this paper we show an improved design using two exposed electrodes at the corner of the step. The actuator was operated at 5kHz AC voltage signal with square wave amplitude modulation to provide a periodic jet. The modulation frequency of the jet was initially set to match the natural f requency of the shear layer instabilities that were determined from power spectral density analysis of hot film data. The actuator was operated at voltages of 10kV, 8kV and 6kV. Particl e Image Velocimetry results showed that the corner actuator was very effective at producing a 45° jet at the corner of the step. This technique for determining the natural instability f requency and forcing oscillations in the shear layer at those frequencies was effective. It could have been because of the technique for finding the oscillation frequency of it may hav e been the way the plasma actuator produced the oscillation.