Basic research of the generation mechanism of aerodynamic noise

在本项目中，研究了涡流和激波运动产生的气动噪声的产生和传播机制。建立了求解非定常可压缩Navier-Stokes方程的高精度有限差分格式(空间精度为6阶，时间精度为4阶)。将该格式应用于激波-涡间干扰问题，发现干扰产生的反射激波会产生噪声，而产生的反射激波会再次与涡旋相互作用而产生噪声。这是关于超音速湍流发声机理的一个重要发现。接下来，我们将该格式应用于均匀流中障碍物产生的风声问题。结果表明，声的产生频率等于升力的脉动频率，也等于卡门涡的脱落频率。我们还发现，声音的传播受多普勒效应的影响，传播角由马赫数决定。我们还将数值结果与Curle的声学类比预测结果进行了比较，指出Curle的原始形式的类比是不完全的，并提出了一种改进的方法，使其更接近于正确的解。我们还提出了一种新的方法，可以有效地主动控制均匀流动中圆柱体产生的风声的产生和传播。

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.