縦渦の空力騒音の音源探査と発生機構に関する実験的研究

抄録

Longitudinal vortices generated around automobiles and airplanes are regarded as one of the most dominant aerodynamic noise sources. There have been so many studies to reveal the generation mechanism of aerodynamic noise produced by longitudinal vortex. However, it has not yet been clarified that how longitudinal vortices have been generated and how the vortices produce aerodynamic noise. The paper aims to reproduce longitudinal vortices with a simple delta wing, and to identify noise sources by the beamforming technique, and to clarify aerodynamic sound generation mechanism. The beamforming test was conducted in JAXA (Japan Aerospace Exploration Agency) wind tunnel. The wing was immersed in a uniform flow with an attack angle 15 degrees in the measurement section and the far field noise produced by longitudinal vortex was measured by the far field microphone and the microphone array with 96 microphones installed in the anechoic chamber. The beamforming technique identified two noise sources. One is located at the tip and the other at the rear end of the wing. Regarding noise frequency characteristics, aerodynamic noise consists of two bands, narrow and broad one. It was found that narrow and broad band frequency noise increase in proportion to 4.8th power and 6.5th power of the uniform airflow velocity, respectively. Narrow band frequency noise is generated at the rear end of the wing whereas broad band one is at the tip of the wing. CFD analysis clearly visualized unsteadily fluctuating motions of vortices in the vicinity of the tip of the wing. The tip has the strongest vorticity which becomes weaker and weaker in the rearward direction. It is therefore considered that unsteady motion of vortices with the stronger vorticity at the tip induce the larger level of pressure fluctuations with broad band frequencies. This leads to noise generation of broad band frequency at the tip of the wing.