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

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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.