Rôle of Impurities in Cavitation Threshold Determination

Abstract

Galloway, Strasberg, Barger, and others have observed that vaporous cavitation thresholds for unfiltered water at frequencies below 100 kHz depend strongly upon the air content of the water sample; and Strasberg has offered theoretical arguments that are consistent with these observations. Greenspan, on the other hand, has observed little dependence on air content at 30–60 kHz for carefully filtered water and for liquids that readily wet solids. Strasberg's theory has been re-examined and extended to include the size of solid impurities (i.e., motes) in the liquid sample and the degree to which the liquid wets them. The results appear to explain the above observations, as well as some temperature and frequency effects, and are entirely consistent with Harvey's basic hypothesis that gas cavities trapped in surface crevices of motes are the primary source of cavitation nuclei. It is concluded that, at sufficiently low frequencies (i.e., below 100 kHz for water), the observed vaporous cavitation threshold depends primarily upon the initial growth of a vapor cavity in an imperfectly wetted crevice. [This work is supported by the Office of Naval Research.]