A Lagrangian analysis of scale effects on sheet cavitation inception

Sheet cavitation on foils and propellers is often initiated by free-stream bubbles. Consequently, scale effects on cavitation inception occur due to differences in the bubble spectrum (size vs. concentration), propeller diameter and rotation rate. To better understand the sensitivities, a 2D Lagrangian tracking method is used to compute the trajectories and radial dynamics of free-stream bubbles that cause cavitation on a blade section. Systematic series of conditions are computed by varying the initial radius and the initial position of a bubble. It is shown that the range of initial bubble radii that can cause cavitation decreases from 0.03 to at least 600 µm for a full-scale propeller, to 5 – 160 µm for a small propeller model. If sufficient bubbles in this size range are present in a cavitation facility, this does not explain why sheet cavitation inception is often inhibited on model-scale propellers. However, the bubble dynamics computations did show significantly less bubble growth on the small propeller model.