Study on self-exciting structure of cloud-cavitation flow

附着空化具有空化云的脱落和塌陷，并且在恒流条件下具有高冲击力。从基础工程的发展和空化冲击在工业上的积极应用的角度来看，对这种现象的认识是非常有用的。尽管它被认为是空化流特有的自激现象之一，但其周期性的机制仍有待解决。该项目的研究人员已经明确，当重入运动到达附着腔的前缘时，存在从附着腔内部到主流的射流。射流作为一种触发涡流空穴形成后，合并并向下游散落形成大的空化云。本研究重点关注空化云塌陷引起的大压力波的发生。首先，在卡门涡旋型空腔从圆柱体塌陷时，通过实验证明了压力波的存在以及气泡向周围塌陷的连锁反应型传播。此外，还考察了空化水射流周期性脱落的聚结机制和压力波的存在性。特别是开发了一种图像处理方法来定量研究气泡的快速破裂和变形。基于这种图像处理技术，表明伴随着空化云的塌陷，压力波可以引起再入运动。由此，指出了以下过程作为关于空化云流中的自激结构的再入运动的机制： (1)附着空化使空化云脱落，(2)脱落空化云塌陷，(3)压力波传播到附着空腔末端，(4)附着空洞内气泡的连锁反应塌陷和折返运动，(5)折返运动到达附着空洞前部，(6)在附着空洞前部卷起至主流，(7)涡空腔形成并向下游运动， (8)涡腔的增长和空化云的形成 较少

Attached cavitation has a shedding and collapsing of cavitation clouds and high impact under constant flow condition. The understanding of this phenomenon is very useful from a viewpoint of the development of basic engineering and the positive application of cavitation impact for the industrial purpose.The mechanism of the periodicity remains to be solved though it is considered as one of self-exciting phenomena peculiar to cavitation flow. The researchers in this project have made clear the existence of jet flow to the main stream from the inside of attached cavity when a re-entrant motion reaches the leading-edge of the attached cavity. After the jet as a kind of trigger vortex cavities form, coalescence, and shed downstream as a large cavitation cloud.In this study the occurrence of large pressure waves is focused on which is caused by the collapse of cavitation cloud. First the existence of pressure waves and the chain-reaction-type propagation of bubble collapses to the surroundin … More g bubbles was experimentally shown at the collapse of Karman-vortex-type cavity shed from a circular cylinder. In addition the coalescence mechanism and the existence of pressure wave are examined about the periodic shedding in cavitating water jet. Especially an image processing method is developed to investigate quantitatively the rapid collapse and deformation of bubbles. Based on this image processing technique, it is shown that a re-entrant motion can be induced by the pressure wave accompanied by the collapse of cavitation clouds.As a result the following process is pointed out as the mechanism of re-entrant motion concerning the self-exciting structure in the flow of cloud cavitation as follows ; (1) shedding of cavitation clouds from attached cavitation, (2) collapse of shedding cavitation clouds, (3) pressure-wave propagation to the end of attached cavity, (4) chain-reaction collapses of bubbles and re-entrant motion within the attached cavity, (5) arrival of the re-entrant motion at the leading part of attached cavity, (6) roll-up to the main stream at the leading part of attached cavity, (7) formation of vortex cavity and movement to downstream, (8) growth of the vortex cavity and the formation of cavitation cloud Less