Effects of Unsteady Actuation on Resonance-Dominated Impinging Jets

The flowfield of an impinging jet has many practical applications in engineering. One such application, the Short Take-Off and Vertical Landing (STOVL) aircraft, has motivated the current study. Many features characteristic of this flowfield are adverse in practice; including noise levels dangerous to nearby personnel, sonic fatigue of the aircraft and nearby structures, hot gas and/or debris ingestion, and loss of lift due to entrainment. One control scheme which has proven to be effective in minimizing some of these effects in previous studies is microjet injection at the nozzle lip. The control had been improved further by pulsing the injection in the range of several hundred hertz (the supersonic impinging jet typically has dominant tones in the range of kHz). In an effort to improve microjet injection one step further, an experimental study has been carried out in evaluating a micro-fluidic actuator's performance in controlling the flow. The actuator, which can generate pulsed supersonic microjets in the range of 1-60kHz, reduced nearfield noise levels by as much as as 5 dB in certain cases, and unsteady pressure levels in the ground and lift-plate by nearly 6 dB. Examination of the nearfield spectra showed that as the actuator began to pulse, a new impinging tone would appear at a higher frequency. In cases where the new and baseline impinging tones were seen, spectrograms showed that each would occur at separate times. Examination of the frequency of these new tones in the view of Powell's model correspond to higher modes of oscillation.