Dynamics of cavitation inception in high Reynolds number shear flow

Cavitation inception in high Reynolds number shear flow is an inherently complex phenomenon due to the stochastic character of both the underlying flow and the nuclei population naturally occurring in the water. Within the present study, the dynamic behaviour of cavitation inception in the shear layer formed in the wake of a nominally two-dimensional backward-facing step is investigated in a water tunnel for step height based Reynolds numbers of 0.8 × 10⁶ ≤ Re ≤ 1.4 × 10⁶ using high-speed imaging and acoustic measurements. Discrete incipient cavities were observed to form as either ‘spherical’ or stretched bubbly structures across a wide range of sizes. The collapse of an incipient cavity resulted in a remnant micro-bubble cloud dispersed into the shear layer and the recirculation zone in the wake of the step. These micro-bubbles, generally larger in size than those naturally occurring in the water, act as preferential sites for re-nucleation, triggering almost invariably the formation of developed shear layer cavitation. Due to the effect of re-nucleation, a hysteresis between the incipient and desinent cavitation number is observed. The effect of hysteresis is found to increase with a decrease in Re.