Study on self-exciting structure of cloud-cavitation flow

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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