Influence of nucleation on cavitation inception in tip leakage flows

Cavitation in a tip leakage flow is experimentally investigated in a cavitation tunnel using a stationary hydrofoil analogy. The experiments were performed for different tip clearances (tau = gap height=maximum profile thickness) and hydrofoil incidences (alpha). The chord-based Reynolds number remained fixed at Re = 3 x 10(6). The influence of nucleation on both inception and developed cavitation is evaluated by performing tests with two populations of freestream nuclei: a low concentration with strong critical tensions for activation and a high concentration with weak critical tensions. These populations represent the extremes that would be expected in practical tip leakage flows. Cavitation was characterized using high-speed imaging and acoustic measurements. Following a survey of developed cavitation topology for a range s and a values, alpha = 6 degrees was selected for further investigation of cavitation inception as it demonstrated a rich variety of physical processes. From the acoustic measurements, the worst performance in terms of cavitation inception was observed at an intermediate gap height of around tau = 0.6-0.8 for the "strong water" case. Broadly, cavitation and inception is intermittent when nuclei are sparse, becoming continuous as additional nuclei are introduced. While a continuous cavity in the seeded flow resulted in a higher baseline acoustic signature, sparse populations allow the leakage vortex to sustain tension, which can result in extremely loud incipient events. Optimization of gap height will, therefore, depend on the expected nuclei population during operation.