STUDY ON THE SIMULTANEOUS MEASUREMENTS OF VORTEX MOTIONS AND SOUND SOURCES

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The present study reveals the sound-generation mechanism in related to vortex motions, and proposes the ideas to reduce flow-induced noises. The results are summarized as follows.(1) Simultaneous measurements of velocity and fluctuating static-pressure are carried out in an air jet issuing from a sharp-edged circular orifice, and the sound-source terms of the vortex sound theory and the dilatation theory are discussed in relation to vortex motions. The jet is excited in the interaction mode so that velocity and fluctuating static-pressure can be determined in space and time domains by using phase-average technique. The sound-source terms of the theories are obtained from the phase-average velocity and fluctuating static-pressure via the differential process in space and time. The result suggests that intense sound sources are caused by the vortex rolling-up/merging and by the acceleration of vortices during vortex pairing.(2) The intense sound-source regions predicted by pressure measurements correspond to those predicted by the vortex sound theory. The result suggests that the intense sound-source regions can be detected by pressure measurements.(3) The noise propagating from the separated mixing layer from a fence is strongly correlated to the sound source related to the vortex motions.(4) In order to reduce the noise from the separated flow from the front pillar and the side mirror of an automobile, the effects of the devices to suppress vortex evolutions are investigated experimentally. The results show that the devices are very effective to suppress the vortex motions related to sound sources.