STUDY ON THEREDUCTION OF AERODYNAMIC NOISE BY VORTEX CONTROL

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The objective of the present study is to obtain the basic materials for the vortex control techniques to reduce the aerodynamic noise. The results are summarized as follows.(1)The effect of tabs (vortex generators) on jet noise was investigated by flow visualization experiments and phase-average measurements. The results reveal the interaction mechanism of vortex rings and streamwise vortices to lead to the suppression of sound generation. In particular, it is very important in engineering applications to make clear the cut-and-reconnection mechanism of the streamwise vortices wound around the vortex rings and to confirm the suppression of vortex pairing.(2)The effect of pile-fabrics on reducing aerodynamic noise was investigated by measuring the noise from a square cylinder in uniform flow. The results show that the pile-fabrics on the cylinder are effective to suppress the vortex evolution and pairing of separated flow and to reduce the noise related to the vortex motions.(3)The sound source of the separated mixing layer from a fence was discussed by the measurements of velocity and pressure fluctuations. The results reveal that the sound source intensity is high in the regions of vortex evolution and pairing. The propagated sound detected simultaneously with flow visualization also suggests that the sound source is closely related to the vortex motions.(4)Based on the results of the present study and the related studies, the vortex control techniques to reduce aerodynamic noise are proposed, and the future works are suggested.