On the importance of the effect of turbulence in cavitation inception tests of marine propellers

Current methods for predicting the inception of cavitation in marine propellers rely heavily on physical model tests in cavitation tunnels and these tests are subject to complex scale effects. The viscous nature of the flow, particularly the free-stream turbulence, is one of the main factors contributing to the scale effects on the inception of cavitation. Research to explore this factor has been grossly neglected and there is an urgent need for systematic experimental data to establish possible trends in this effect to be included in the procedures used to extrapolate small-scale experimental results to predict the inception of cavitation on the full-scale propeller. This paper presents data and reports on some interesting and important findings on the effects of different turbulence generation techniques from recent research into the scale effects on the inception of cavitation of marine propellers. Systematic measurements were taken with a five-bladed model propeller to explore the viscous scale effects on the tip vortex and sheet-type cavitation using two different methods to generate turbulence. These were a wire mesh screen with variable mesh sizes placed in front of the model propeller and several grades of carborundum applied to the leading edges of the model propeller blades. The results of the measurements displayed distinct but similar trends in the effects of the two methods on the resulting inception values, in particular for the tip vortex cavitation. This finding has an important practical implication on the current cavitation testing procedures, especially for the most widely used 'leading-edge roughness' technique. This technique, which has various practical disadvantages, can now be replaced by a more effective and controlled way of altering the free-stream turbulence. The paper also presents an important set of systematic data on the effect of free-stream turbulence, which is scarce in the literature, and that of leading-edge roughness and it discusses the use of these data for the development of an extrapolator to predict the cavitation inception of a marine propeller at full scale.