Basic research of the generation mechanism of aerodynamic noise-Development to complex flow fields and control of the noise-

本课题对复杂流场中气动噪声的产生和传播机理进行了研究。Inoue和Hatakeyama发展了一种高精度的有限差分格式(空间精度为8阶，时间精度为4阶)来求解非定常可压缩N-S方程。他们将该方案应用于均匀流动中多个物体产生的噪音问题，发现流场和声场根据物体之间的间距变化很大。对于在均匀流中串联放置的两个物体的情况，他们发现声波的振幅被极大地抑制(高达单一物体情况的1/50)或增强(高达单一物体情况的十倍)，这取决于两个物体的间距。对于并排排列的两个物体，他们发现，根据物体的间距，会出现6种不同的流动和声场模式。在串列和并列两种情况下，都观察到了依赖于初始条件的分叉现象。这些结果有力地表明了噪声控制的可能性。对于三个物体并排排列的情况，观察到了一种节拍的声音。Fukunishi在实验中发现，通过在空腔中放置控制面板可以显著抑制空腔噪声，并提出了用这种方法控制湍流的可能性。Nakamura对超音速喷流噪声问题进行了数值研究，并较详细地阐明了其产生和传播机制。石井开发了一种高精度的新计算机代码，并将其应用于空腔噪声问题。他阐明了跨度长度对空腔噪声产生的重要性。

In this project, the generation and propagation mechanisms of aerodynamic noises generated in complex flow fields have were investigated. Inoue and Hatakeyama developed a highly-accurate finite difference numerical scheme (8-th order accurate in space and 4-th order in time) to solve the unsteady compressible Navier-Stokes equations. They applied the scheme to noise problems generated by multiple bodies placed in a uniform flow and found that flow and sound fields vary drastically depending on the spacing between the bodies. For the case of two bodies placed in tandem in a uniform flow, they found that the amplitude of sound waves is greatly suppressed (up to 1/50 of the single body case) or enhanced (up to ten times as large as the single body case), depending on the spacing of the bodies. For the case of two bodies in a side-by-side arrangement, they found that 6 different patterns of flow and sound fields appear depending on the spacing of the bodies. In both cases of tandem and side-by-side arrangement, bifurcation phenomenon was observed depending on initial conditions. These results strongly suggest a possibility of noise control. For the case of three bodies in a side-by-side arrangement, a beat of sound was observed. Fukunishi found experimentally that cavity noise can be suppressed significantly by placing a control panel in the cavity and suggested the possibility of turbulence control by this method. Nakamura investigated numerically supersonic jet noise problems and clarified the generation and propagation mechanisms in some detail. Ishii developed a highly-accurate new computer code and applied it to cavity noise problems. He clarified the importance of the span-length on the generation of the cavity noise.