An Experimental Study On The Scale Effects In Cavitation Noise

Abstract

Cavitation noise studies were made in a water tunnel with particular reference to scale effects. Travelling?bubble cavitation was artificially generated on four axisymmetric bodies of the Schiebe head?forms with varying diameters, having ring electrodes in the stagnation region to seed nuclei by electrolysis. It was found that the cavitation number at inception with the seeding of nuclei was independent of free?stream velocity and body size except for tunnel blockage effects. The cavitation noise data from these bodies were normalised using the theoretically computed parameters of single?bubble dynamics, and the collapse of data was satisfactory. This nondimensional spectrum showed similarities to the classical single?bubble noise spectrum postulated in theories in terms of slopes. The established scaling laws in cavitation noise were examined using the noise data from the geometrically similar bodies. The results showed varying degrees of agreement in different frequency ranges depending on the assumptions made in the maintenance of cavitation similarity. In view of the significance of cavitation event rates in noise scaling, a technique was developed to estimate the number of cavitation bubbles. To achieve this, axisymmetric bodies having point electrodes to generate a train of nuclei and a laser were used. This is a new development in this field of research. Results from these experiments clearly indicate that cavitation noise increases with increase in number density of cavitation events. A statistical study of the time intervals between cavitation events using this technique revealed the absence of any correlating effects predicted in theory on the spectral characteristics of cavitation noise in the range of frequencies used in the present work. The normalised noise spectrum incorporating the cavitation event rates was found to be in agreement with the classical spectrum in terms of magnitude and slopes. This new technique of cavitation event?rate estimation has potential to bring out further details of single?bubble growth and collapse history and also maintenance of cavitation similarity including cavitation event rates. A comparison of the predicted noise spectrum using the existing scaling laws from the smallest?diameter body with the spectrum experimentally obtained for the biggest body showed agreement within reasonable limits. Maintenance of cavitation?similarity criteria with respect to cavitation indices and cavitation event rates showed better results. The significance of bubble?interference effects in cavitation noise scaling was also examined, and it was found that weak or moderate interference does not influence the scaling laws whereas strong interference influences them.