Study on self-exciting structure of cloud-cavitation flow

附著空化在恒流条件下具有空化云的脱落和塌缩，冲击大的特点。从基础工程的发展和空化效应在工业上的积极应用的角度来看，对这一现象的认识是非常有益的。虽然这种周期性被认为是空化流动特有的自激现象之一，但其机理仍有待解决。本项目研究人员明确了当重入运动到达附着腔前缘时，附着腔内部存在向主流的射流。射流作为一种触发涡后形成空腔，聚并，并向下游散去，形成一大片空化云。本文重点研究了由空化云坍缩引起的大压力波的发生。首先，压力波的存在和气泡崩塌向周围的链式反应式传播，在卡门涡型空腔从圆柱体脱落时，实验显示了更多的气泡。此外，还分析了空化水射流周期性脱落的聚结机理和压力波的存在性。特别是开发了一种图像处理方法来定量研究气泡的快速崩塌和变形。基于该图像处理技术，研究表明，伴随空化云坍缩的压力波可以诱发再入运动。结果表明，云空化流动中自激结构的重入运动机理如下：(1)空化云从附著空化中脱落，(2)脱落空化云坍塌，(3)压力波传播至附著空腔末端，(4)附著空腔内气泡链式崩解和再入运动，(5)再入运动到达附著空腔前段，(6)在附著空腔前段向上卷向主流，(7)涡腔形成并向下游运动，(8)涡腔的生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