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The objective of this paper is to clarify the mechanism for the reduction of cavity tone with blowing jets aligned in the spanwise direction in the upstream boundary layer. Also, the effects of spacing of the jets on the reduction are focused. To achieve these objectives, direct aeroacoustic simulations were conducted along with wind tunnel experiments. The depth-to-length ratio of cavity was D/L = 0.5. The incoming boundary layer was laminar, where the boundary layer thickness was δ/L = 0.055. The predicted flow fields without control show that two-dimensional large-scale vortices are shed and become acoustic sources in the cavity. The effects of spanwise spacing of spanwise-aligned jets on the cavity flow and tone were clarified with computations and experiments with the different pitches of s/L = 0.1 - 1.0 (s/δ = 1.8-18.2). As a result, the largest reduction level was obtained for s/L = 0.5.

Intense aeroacoustic feedback noises may radiate from flow around an airfoil, rearview mirror with small gaps and so on. Reductions of these noises are important issues in the development of industrial application. The intense noise from a bonnet of the automobile is one of the typical problems of acoustic feedback noise. In order to reduce this noise, plasma actuator (PA) was utilized to control flow and acoustic fields. The aim of this investigation is to clarify the effects of flow control by the PA on noise reduction and the noise reduction mechanism. Wind tunnel experiments were conducted with a half scale bonnet model and a low noise wind tunnel. Simultaneous measurements of flow and noise fields were conducted to understand the generation mechanism of the bonnet noise. Coherent output power (COP) of the velocity fluctuations with reference to far-field sound pressure was measured to visualize noise source distribution.