Measurement of cavitation bubble cluster downstream of sheet cavity

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A sheet cavity generated on a body such as hydrofoils begins to oscillate periodically when it attains a certain size, shedding harmful could cavities downstream. The collapse of cloud cavity is a collective collapse of cluster of numerous small bubbles, resulting in impulsive pressure fluctuations which leads to noise, high frequency vibration and erosion of the materials. In this study, we measured the number and spatial distribution of bubbles in cloud cavities downstream of sheet cavity.A laser holography method was applied to measure the bubbles in cloud cavities on a foil section. In the beginning, it took about 3 months to count the bubbles whose radii are more than 10 μm. Therefore, we measured the bubbles with radii of more than 35 μm mainly. As a result we succeeded in measuring bubbles in 9 cloud cavities for 3 cavitation numbers and 3 foil chord stations. The results can be summarized as follows :a) Bubbles in cloud exist around a U-shape vortex cavity near the center of cloud particularly downstream of it. Larger bubbles exist more closer to the vortex cavities. This suggests that the vortex cavities split into small bubbles, supplying bubbles around them, when they collapse. It is also seen that there are larger bubbles closer to the foil surface. It is considers that this is attributed to lower pressure close to the foil surface.b) The bubble number density function is inversely proportional to the bubble radius to power 5.1 when the bubble radius is larger than 35 μm. We obtained an empirical equation of the relation between the bubble number density function and bubble radius.We also measured evaporation rate at sheet cavity surface, which determines the total vapor volumes of the initial cloud cavity. Also measured are the characteristics of re-entrant jet, which tears off the sheet cavity resulting in could cavity shedding.