Pulsed microjet control of supersonic impinging jets via low-frequency excitation

Several fluid flow problems related to propulsion and power generation exhibit strong acoustic resonances. Produced due to interactions of the acoustics with other underlying unsteady mechanisms such as unsteady heat release or shear flow instability, these resonances manifest as large and sustained pressure oscillations. Active control of such resonances has been shown to be highly effective, resulting in a dramatic reduction in the acoustic resonances using a very small fraction of the system energy. In this paper, a specific fluid flow problem is discussed, that of supersonic impinging jets, and the control of the acoustic resonances that are produced by these jets, via detailed experimental investigations. The actuator used for active control consists of a pulsed microjet, by which it is shown that in a scaled supersonic experimental facility acoustic resonances can be reduced, utilizing a fraction of the mass flowrate needed with a steady microjet. Several parameters related to pulsed injection are varied to evaluate their impact on the resonances, and it is found that duty cycle and pulsing frequency have the most dominant effect on the jet noise as well as on the overall flow field. System identification based models of the uncontrolled and controlled jet are derived to explain the results obtained. The effect of low-frequency pulsing on resonance suppression is explained via the introduction of a controller in the closed-loop whose parameters vary non-linearly with the pulsing parameters.