Cavity resonance suppression using miniature fluidic oscillators

Abstract

We present a novel approach to suppressing jet-cavity interaction tones using miniature fluidic devices. We first characterize miniature fluidic oscillators and then assess their effectiveness for cavity tone suppression. Further, we evaluate mass flow requirements for effective unsteady fluid mass addition. The fluidic devices used had no moving parts and could provide oscillatory flow of prescribed waveforms (sine, square, and saw-toothed) at frequencies up to 3 KHz. Our testbed for a detailed evaluation of the fluidic excitation (square wave) technique was the flow-induced resonance produced by a jet flowing over a cavity with an (length/depth) ratio of 6. In addition to schlieren photography and acoustic measurements we used photoluminescent Pressure Sensitive Paint (PSP) to map pressures on the cavity's floor for the unperturbed and fluidically excited cases. When located at the upstream end of the cavity floor, the miniature fluidic device was successful in suppressing cavity tones by as much as 10 dB with mass injection rates of the order of only '0.12% of the main jet flow. Similar mass flow rates 'of oscillatory flow near the downstream end of the cavity floor had no effect on the resonant cavity,tones. Additionally, steady upstream mass flow,addition at the same levels as those for fluidic excitation affected cavity tones only marginally (1dB reduction). Furthermore, acoustic excitation at the same frequency as that produced by the fluidic device or its harmonic at comparable amplitudes did not affect the cavity resonance. Our results provide not only an example of the effectiveness of fluidic excitation but offer grounds for believing that vast possibilities exist for its use in aeroacoustic control.