Study on Cavitation Inception (2 nd Report)

Abstract

The role of cavitation nuclei on sheet cavitation inception was investigated experimentally for three axisymmetric test bodies, by using a nuclei generator.With no added nuclei, the incipient cavitation number σi showed a large scattering, whereas, when nuclei were added, the scattering became fairly small and σi converged to the upper limit of σi with no added nuclei. It also coincided with the desinent cavitation number σd.By using Laser Doppler Velocimeter, the velocity distribution on axisymmetric bodies were measured, and good agreement was obtained with calculated results. The pressure coefficient at separation point cps was also measured by LDV and was compared with σi, which confirmed the result obtained by Casey that σi is equal to, or slightly less than (-cps).A hypothesis for the mechanism of sheet cavity inception was made, taking account of the surface tension of nuclei and the pressure fluctuation at the reattachment zone of a separated flow. It explained, at least qualitatively, the dependence of σi on u∞ and the relation between σi and (-cps).By controlling the amount of added nuclei, the radius of nuclei which contribute to the sheet cavity inception was estimated to be about 20μm.