Kazuyuki Matsuda

Measurement and computation of the acoustic field in a cavitation tunnel

Sound pressure distribution around a monotone sound source was measured inside a marine propeller cavitation tunnel and compared with the calculated result by a two-dimensional boundary element method. The measured sound pressure distribution showed some peaks due to the reflection effect of the tunnel test section boundary. As the frequency increased, the sound pressure distribution became more complicated, showing more peaks. The tunnel reverberant effect should be taken into account when the noise data measured in the tunnel are converted into full-scale values. In the boundary element method calculation, the boundary condition at the acrylic observation window of the tunnel was examined in detail. The calculated sound pressure distribution pattern in the tunnel transverse section agreed well with the measured distribution when a reasonable boundary condition was adopted. The boundary element method is an effective method for theoretically predicting the acoustic field inside the cavitation tunnel if the precise boundary condition is adopted.

Characteristics of Acoustic Field inside Cavitation Tunnel

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.