Microbubble generation from condensation and turbulent breakup of sheet cavitation

The formation, size and concentration of microbubbles generated in the wake of a cavitating hydrofoil are investigated experimentally for several Reynolds and cavitation numbers. The present work is restricted to bubble generation from stable sheet cavitation. Interactions between the cavity, the overlying boundary layer and associated interfacial effects are investigated qualitatively and quantitatively using high-resolution and high-speed photography. The physics of cavity breakup and condensation and microbubble formation are also investigated using high-speed photography. Size and concentration of microbubble populations are measured in the far wake using diffused laser shadowgraphy. The optical setup of the shadowgraphy permitted microbubbles in the size range of 5 to 300 µm to be resolved. Coherent spatial and temporal features of the sheet cavitation are shown to be functions of both Reynolds and cavitation numbers. For the range of microbubble sizes measured, concentrations are shown to increase with Reynolds number and reduce with decreasing cavitation number. In contrast, void fraction increases with decreasing cavitation number indicating a greater production of larger diameter bubbles.