On the viscous scale effects in cavitation inception of marine propellers

A systematic experimental investigation has been carried out to further explore the effects of scale in inception of cavitation of marine propellers. Cavitation tunnel tests were performed with a relatively large rectangular shape NACA66 foil and Meridian type model propeller systematically to explore the effects of free-stream turbulence and artificial roughness on the inception of tip vortex and sheet cavitation. The free-stream turbulence at varying levels was generated by wire mesh screens and measured using a Laser Doppler Anemometry (LDA) facility while the roughness was simulated by varying grades of carborundum applied at the leading edge of the test bodies. The results of the inception measurements were presented systematically and the observed trends were discussed in terms of the turbulence, roughness and air content levels. The analyses of the results indicate that it may be possible to establish correspondence between the levels of the free-stream turbulence and roughness in terms of inducing the similar cavitation inception numbers. The investigation also includes a semi-empirical approach to predict the inception of the tip vortex cavitation based on the Lighthills leading edge correction, which has been modified to take into account of the effect of the free-stream turbulence through a simplified manner. The comparison of the predictions with the measurements presents encouraging correlations as well as forming a basis to develop an extrapolator for the inception of cavitation.