Cavitation Scale Effects - Empirically Found Relations and the Correlation of Cavitation Number and Hydrodynamic Coefficients

Abstract

Scale effects on cavitation phenomena are departures from the classical relation due to variations in a) water quality with regard to its cavitation susceptibility (tensile strength, concentration and size of nuclei) b) body size and flow parameters (flow velocity, viscosity of the fluid, turbulence). These scale effects are an important consideration in the prediction of the prototype cavitation behaviour based on model tests. Test results show that, provided effects of water quality are avoided in the experiments, very clear empirical relations can be established for the scale effects on cavitation inception. The data, on which these empirical relations are based, relate to the inception of various types of cavitation as they occur on the surface of rotationally symmetric bodies, two-dimensional non lift producing bodies, or two- and three-dimensional lift producing foils. It can be shown, that the scaling relations initially found for beginning cavitation also are valid for developed cavitation. In addition lift and/or drag forces were measured on all kinds of test bodies and correlated with the cavitation number, and good correlation is obtained between the cavitation parameter and the corresponding measured drag coefficient. The new findings about the correlation of cavitation numbers with drag coefficients are believed to be the clue to why the empirically found scaling relations are so universal, and a key to the physical explanation for these scale effects.