Measurement of cavitation bubble cluster downstream of sheet cavity

在水翼船等物体上产生的片状空腔在达到一定尺寸时开始周期性振荡，向下游脱落有害的空腔。云腔坍缩是无数小气泡团簇的集体坍缩，产生脉冲压力波动，产生噪声、高频振动和对物料的侵蚀。在本研究中，我们测量了薄片空腔下游云腔中气泡的数量和空间分布。采用激光全息法测量了箔片上云腔中的气泡。最初，对半径大于10 μm的气泡进行计数大约需要3个月的时间。因此，我们主要测量半径大于35 μm的气泡。结果，我们成功地测量了3个空化数和3个箔弦站的9个云腔中的气泡。结果表明：a)云内气泡存在于靠近云中心的u型涡腔周围，特别是云的下游。较大的气泡存在于离涡腔更近的地方。这表明，当涡旋腔坍塌时，它们分裂成小气泡，为周围的气泡提供能量。还可以看到，在靠近箔表面的地方有较大的气泡。这被认为是由于较低的压力接近箔表面。b)当气泡半径大于35 μm时，气泡数密度函数与气泡半径成反比为5.1。得到了气泡数密度函数与气泡半径关系的经验方程。我们还测量了片腔表面的蒸发速率，这决定了初始云腔的总蒸汽体积。还测量了再入射流的特性，它撕裂了片状腔，导致腔脱落。

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.