Study on self-exciting structure of cloud-cavitation flow

附着空泡在恒定流条件下具有空泡云的脱落、溃灭和较大的冲击力。从基础工程的发展和空化冲击在工业上的积极应用的观点来看，对这一现象的认识是非常有用的。虽然它被认为是空化流特有的自激现象之一，但其机理仍有待解决。本课题的研究工作明确了当再入运动到达附壁凹腔前缘时，附壁凹腔内存在向主流方向的射流。射流作为一种触发器后，涡腔形成、合并，并向下游脱落，形成大的空泡云，本文重点研究空泡云崩溃引起的大压力波的发生。首先，压力波的存在和气泡崩溃到周围环境的链式反应式传播 关于我们 实验表明，在卡门涡型空泡从圆柱脱落的崩溃。此外，还对空化射流中周期性脱落现象的聚并机理和压力波的存在进行了探讨。特别是发展了一种图像处理方法，用于定量研究气泡的快速溃灭和变形。基于这一图像处理技术，研究了空泡云溃灭时伴随的压力波可诱发空泡云的重入运动，指出了云空泡流中自激结构的重入运动机理是：（1）附着空泡的空泡云脱落，（2）脱落空泡云的溃灭，（3）压力波向附着空泡端部的传播，（4）附着腔内气泡的链式反应坍塌和折返运动，（5）折返运动到达附着腔的前部，（6）在附着腔的前部卷起到主流，（7）涡腔的形成和向下游的运动;（8）涡腔的增长和空泡云的形成

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