STUDY ON THEREDUCTION OF AERODYNAMIC NOISE BY VORTEX CONTROL

本研究的目的是为降低气动噪声的涡流控制技术提供基础材料。结果总结如下：(1)通过流动显示实验和相位平均测量，研究了叶片（涡发生器）对射流噪声的影响。研究结果揭示了涡环与顺流涡相互作用抑制声音产生的机理。特别是在工程应用中，弄清缠绕在涡环上的流向涡的切断-重连机制，确认涡对的抑制作用是非常重要的。(2)通过测量匀速流动中方形圆柱体的噪声，研究了桩织物对空气动力噪声的降噪效果。结果表明，在筒体上设置桩织物能有效抑制分离流的涡演化和配对，并能降低涡运动引起的噪声。(3)通过速度波动和压力波动的测量，讨论了栅栏分离混合层的声源。结果表明，在涡旋演化区和涡对区声源强度较高。与流动显示同时检测的传播声也表明声源与旋涡运动密切相关。(4)在本研究和相关研究的基础上，提出了降低气动噪声的涡控技术，并对今后的工作提出了建议。

The objective of the present study is to obtain the basic materials for the vortex control techniques to reduce the aerodynamic noise. The results are summarized as follows.(1)The effect of tabs (vortex generators) on jet noise was investigated by flow visualization experiments and phase-average measurements. The results reveal the interaction mechanism of vortex rings and streamwise vortices to lead to the suppression of sound generation. In particular, it is very important in engineering applications to make clear the cut-and-reconnection mechanism of the streamwise vortices wound around the vortex rings and to confirm the suppression of vortex pairing.(2)The effect of pile-fabrics on reducing aerodynamic noise was investigated by measuring the noise from a square cylinder in uniform flow. The results show that the pile-fabrics on the cylinder are effective to suppress the vortex evolution and pairing of separated flow and to reduce the noise related to the vortex motions.(3)The sound source of the separated mixing layer from a fence was discussed by the measurements of velocity and pressure fluctuations. The results reveal that the sound source intensity is high in the regions of vortex evolution and pairing. The propagated sound detected simultaneously with flow visualization also suggests that the sound source is closely related to the vortex motions.(4)Based on the results of the present study and the related studies, the vortex control techniques to reduce aerodynamic noise are proposed, and the future works are suggested.