Measurement of cavitation bubble cluster downstream of sheet cavity

在水翼等物体上产生的片状空穴在达到一定尺寸后开始周期性振荡，从而在下游脱落有害的空穴。云泡的崩塌是由众多小气泡组成的团簇集体崩塌，产生脉冲式的压力波动，从而导致噪声、高频振动和材料的侵蚀。在这项研究中，我们测量了薄片空穴下游云腔中气泡的数量和空间分布，并应用激光全息方法测量了薄片截面上云腔中的气泡。最初，对半径大于10μm的气泡进行计数需要大约3个月的时间，因此，我们主要测量半径大于35μm的气泡。结果，我们成功地测量了3个空化数和3个翼弦站的9个云腔中的气泡。结果表明：a)云中心附近的U型涡腔周围存在气泡，尤其是在云下游。更大的气泡存在于离涡腔更近的地方。这表明，当涡旋空穴破裂时，它们分裂成小气泡，为它们提供周围的气泡。还可以看到，靠近铝箔表面有更大的气泡。B)当气泡半径大于35μm时，气泡数密度函数与气泡半径成反比，得到了气泡数密度函数与气泡半径之间关系的经验公式，并测量了片状空腔表面的蒸发速率，确定了初始云空穴的总蒸汽量。还测量了折入射流的特性，它撕裂了片状空腔，导致空腔脱落。

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.