Experimental studies on travelling bubble cavitatin noise

Abstract

An effectively controlled uniform travelling bubble cavitation was made possible at almost all freestream conditions by seeding the flow with artificial nuclei generated by electrolysis within the boundary layer of three test bodies. The size of the nuclei thus generated was determined by laser light scattering method. An improved method of calibration was utilised to obtain the relationship between the size of the nuclei and the electrical signal from the scattered light. The cavitation inception and noise data were obtained on three axisymmetric test bodies having different pressure distribution and viscous characteristics. It was found that the cavitation number at inception with artificial seeding of nuclei was close to the negative value of the minimum pressure coefficient for the corresponding body and was independent of velocity in all cases. Thus, the scale effects due to velocity on cavitation inception normally observed were successfully eliminated. The cavitation noise data were normalised using the parameters of single bubble dynamics and the collapse of data was found to be quite good at least in the region of lower frequencies. The spectral density of cavitation noise at higher frequencies was found to roll off roughly as the reciprocal of four-thirds power of the frequency. Also, the mean square sound pressure was found to vary nearly as the third to fourth power of freestream velocity. These results are in close agreement with the predictions based on theoretical results of Hickling and Plesset which include compressible effects. The spectral density shows a complex behaviour with cavitation number; it first increases with reduction in cavitation number, reaches a peak and then starts to decrease contrary to the predictions from single bubble dynamics theories. There is strong evidence that this phenomenon is due to crowding of cavitation bubbles at lower cavitation numbers resulting in interference effect of neighbouring bubbles. This was established from the observations conducted with varying density of cavitation events.