Development of Aeroacoustic Cavity Noise Controlling Devices Utilizing Strong Fluctuation into Turbulent Flows

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The purpose of this study has been to develop an energy efficient active control device that can control the cavity noise. Our strategy was to provide the phase control of the flow separation at the leading edge of the cavity, where the receptivity of the flow becomes the highest. The control devices installed can introduce wavy patterns of the velocity fluctuations side-by-side with 180 degrees phase difference into the flow. The pressure fluctuations and hence the sound waves coming out are also 180 degrees out of phase. As a result, the opposite-signed sound waves eventually cancel each other faraway from the source.In our study, the suppression of the cavity noise has been tried by synthetic jets according to the strategy. First, the synthetic jets were generated by the sets of the loudspeakers and the resonators. Because it was found that a loudspeaker's contribution was not to the motion of the fluid particles but to the pressure fluctuation, the effective location to produce the synthetic jets was determined. Consequently, the cavity noise was successfully reduced by adjusting the sound frequency of the loudspeakers.Furthermore, another control device utilizing a fluidic oscillator was developed. For the laminar separating flow, the peak of the cavity noise was successfully suppressed to the background noise level and the harmonic components also disappeared from the spectrum. Moreover, we were successful in achieving an effective suppression of the cavity noise even when the separating flow was turbulent using two synchronized fluidic oscillators installed side by side along the spanwise direction.