Development of a New Method for Alleviation of Impulsive Aerodynamic - Noise by Means of Water - Sheet

1. 当压缩波沿着管道传播并到达其开口端时，脉冲波从管道出口向外发射，并向周围区域发射，从而产生脉冲噪声。本文研究了压缩波传播过程中波形的衰减和变化，出口脉冲波的产生过程，以及抑制脉冲噪声的实用方法。压缩波的强度随距离呈指数递减。采用非定常摩擦代替定常摩擦进行了数值计算。非定常摩擦使压缩波形畸变较大，计算波形与实测值吻合较好。在开口端产生的脉冲波的大小几乎与到达出口的压缩波的最大压力变化率成正比。在考虑开端校正的情况下，出口震级的实验结果与气动声学分析结果吻合较好。本文提出了一种利用水片来减轻脉冲噪声强度的方法，并证实了这种方法是非常有用的，尽管控制噪声场的指向性很强。在目前的研究中，还提出了其他一些降低脉冲噪声强度的方法，如主动噪声控制系统的应用。主动控制系统可能是其中最有用的方法。

1. When a compression wave propagates along a tube and reaches its open end, an impulsive wave is emitted outward from its exit toward the surrounding area, which causes an impulsive noise. In the present study, the attenuation and change of waveform of a propagating compression wave, the generation process of the impulsive wave at the exit, and some practical means for alleviation of the impulsive noise have been investigated.2. The strength of a compression wave decreases exponentially with the distance. Numerical calculation has been carried out taking unsteady friction into account instead of steady one. Unsteady friction causes more distortion of the compression waveform and the calculated waveform is in better agreement with the measured one.3. The magnitude of an impulsive wave generated at the open end is nearly proportional to the maximum rate of pressure change of the compression wave coming to the exit. The experimental results of the magnitude at the exit agree well with the results by aeroacoustic analysis where an open end correction is considered.4. A method for alleviation of the magnitude of impulsive noises by means of water-sheet has been developed, and it is confirmed that the present method is quite useful, although the directivity of the controlled noise field is rather strong. Some other methods for reducing the magnitude of impulsive noises such as an application of active noise control system have been developed in the present investigation. The active control system may be the most useful method in these ones.