Basic research of the generation mechanism of aerodynamic noise

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In this project, the generation and propagation mechanisms of aerodynamic noises generated by the motions of vortices and shock waves are investigated. We developed a highly-accurate finite difference numerical scheme (6-th order accurate in space and 4-th order in time) to solve the unsteady compressible Navier-Stokes equations. By applying the scheme to shock-vortex interaction problems, we found that noises are generated in response to the generation of reflected shock waves caused by the interaction, and that the generated reflected shock waves interact with vortices again and produce noises successively. This is an important finding on the mechanism of sound generation in supersonic turbulent flows. Next, we applied the scheme to the problems of aeolian tones generated by an obstacle in a uniform flow. The results showed that the generation frequency of the sound is equal to the fluctuation frequency of lift force and also to the shedding frequency of Karman vortices. We also found that the propagation of sound is affected by the Doppler effect and the propagation angle is determined by the Mach number. We also compared our numerical results with the prediction by Curle's acoustic analogy and pointed out that the Curle's analogy in its original form is incomplete and proposed an improved method which gives a close approximation to the correct solutions. We also proposed a new method which is effective to control actively the generation and propagation of aeolian tones generated by a circular cylinder in a uniform flow.