Characteristics of Acoustic Field inside Cavitation Tunnel

Abstract

At present it is usual that the propeller cavitation noise characteristics are evaluated from the measured results at the cavitation tunnel through the scaling laws. However, the test section boundary of the cavitation tunnel is made of plural materials such as stainless steel and acrylic observation window. From this fact, it is predicted that the reflection effect of the boundary is significant and should be removed before the scaling-up to the fullscale. As the first step to this goal, the sound pressure distribution inside the cavitation tunnel was measured and compared with the calculated results by the two-dimensional Boundary Element Method (BEM) in order to argue the effectiveness of BEM for predicting the acoustic field inside the cavitation tunnel.The measurement was carried out using monotone sound to evaluate the acoustic field purely. The sound pressure emitted from a B&K 8100 hydrophone was received by a B&K 8103 hydrophone array. The sound pressure distribution in the transverse section was calculated by a two-dimensional BEM to argue the possibility of BEM application to the theoretical prediction of the tunnel acoustic field. Since the distance attenuation of the sound pressure in the experiment is three-dimensional, the comparison of the sound pressure level is not meaningful. However, since the reflection at the side boundary is considered dominant because of the tunnel test section configuration and the interaction of the sound pressure is mainly determined by the distance (phase) difference, it is considered that the calculated pattern of the sound pressure distribution, i. e. positions of the peaks, can be compared with the experiment. In the research, it was particularly examined what kind of boundary conditions should be given at the tunnel walls.The results obtained were as follows : (1) The measured sound pressure distribution in both the transverse and longitudinal sections showed some loops and nodes. This result showed that the reflection effect of test section boundary was very significant. This effect increased with frequency.(2) The comparison with the experiment showed that calculated pattern of the sound pressure distribution agreed well if we adopted the boundary condition where the loops and nodes of the sound pressure amplitude respectively appeared at the boundaries between the water and the stainless steel and between the acrylic window and the air. This result could be elucidated from the acoustic impedance values of water, stainless steel, acrylic resin and air. It is concluded from this result that BEM is an effective procedure to predict the acoustic field inside the cavitation tunnel theoretically if the precise boundary condition is adopted.