Study on self-exciting structure of cloud-cavitation flow

在恒流条件下，附着式空化具有空化云脱落、崩塌、冲击力大等特点。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理解这一现象是非常有用的。这种周期性的机理虽然被认为是空化流动特有的自激现象之一，但仍有待于解决。该项目的研究人员明确了当折返运动到达附着腔的前缘时，存在从附着腔内部到主流的喷流。射流作为一种触发旋涡形成空泡后，汇聚并向下游脱落形成大的空化云。本文主要研究空化云坍塌所引起的大压力波的产生。首先，…中压力波的存在和气泡向周围的链式反应传播实验结果表明，从圆柱体上脱落的卡门涡型空腔破裂时，会产生更多的g气泡。此外，还对空化水射流中周期性脱落的聚结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来定量研究气泡的快速坍塌和变形。基于这一图像处理技术，结果表明，伴随空化云崩塌的压力波可以诱导出一种再入运动，从而指出云空化流动中与自激结构有关的再入运动的机理如下；(1)附着空化的空化云脱落；(2)脱落的空化云坍塌；(3)压力波传播到附着空泡的末端；(4)气泡的链式反应坍塌和附着腔内的折返运动；(5)折返运动到达附着空泡的前部；(6)在附着空泡的前部卷起到主流；(7)涡腔的形成和向下游的运动；(8)涡腔的增长和空化云的形成。

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