Comparative examination of tip vortex and surface cavitation noise

Abstract

Ship propeller noise is a problem of current interest. One of the major sound sources is due to tip vortex cavitation. This phenomenon is currently under investigation utilizing elliptically loaded hydrofoils placed in a 190 × 190‐mm high‐speed water tunnel. Results from two different hydrofoils were compared: one with a symmetrical (NACA 66‐012) cross section and another with a cambered (NACA 66‐415) cross section. As predicted, surface cavitation would occur without tip vortex cavitation on the former and tip vortex cavitation could be obtained without surface cavitation on the latter. This permits a quantitative comparison of surface and tip vortex cavitation noise under controlled conditions. The test environment is highly reverberant. Because of this, the acoustic signature of an idealized cavitation noise source in the water tunnel was modeled on the computer. After some experimentation with positioning the sound source in the computer model, the actual time traces of individual bubble implosions in a reverberant field could be closely reproduced. Utilizing an auto‐regressive smoothing routine, the spectrum of the computer generated signal with reverberation was identical to the measured data except for a proportionality constant. This important finding lends support to acoustic research in water tunnels. [Work supported by ONR, SRO program in Hydroacoustics.]