Active control of noise generated from axial-flow turbomachinery

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Our experiments are divided into two parts. One is concerned with the fundamental features of pressure fluctuations in the stage which is important in handling the noise. The other is the active control of the noise by adding sound pressure with loud speakers.The main results of the former experiment are summarized as follows :1. The magnitude of the pressure fluctuation, the intensity of the cross-spectrum and the sound pressure level vary similarly for the change of flow rates. This means that the fluctuation is closely related to the generated noise.2. The pressures on a rotor blade fluctuate in phase and have almost the same magnitude over a blade. Such a remarkable feature cannot be explained by the existing theories.3. As a reason for this, it is assumed that compressibility plays a part due to high unsteadiness.The latter experiment was done to know the usefulness of active control and also to obtain the fundamental data. The result shows that our approach to the active control is much effective. The main results are as follows :1. The reduction of overall noise was obtained by about 11 dB and that corresponding to the blade passing frequency by about 23 dB.2. The phase and the amplitude of sound pressure added by loud speakers coincide with those predicted by the theory.