EROSION MECHANISM OF IMPULSIVE VORITEX CAVITATION BUBBLES AND VORTEX STRUCTURE OF GAS-LIQUID FLOW

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Vortex cavitation was investigated because of its possibility to cause high impulsive force compared with other types. It was tried to make clear the appearance of impact generation and the mechanism of impact. Especially, unsteady motions of vortex cavitation were examined as well as the role of gas-liquid two-phase state. The main results are as follows.First, it is found that an impulsive collapse of cavitation bubbles is caused by unsteady motion with the entrainment flow due to a pair of vortex cavitation in the separated shear layer behind a triangular or a circular cylinder. In addition it is experimentally proved that the axial-collapse pattern can cause erosion pits on the wall material. This collapse pattern which has a rotating axis perpendicular to the wall has been known as the strongest one of other patterns of vortex cavitations. It is also found that one attack generated by the collapse of vortex cavitation consists of a cluster of bubbles in gas-liquid two-phase state because the number of pits is two or up. A soft coated test-plate for erosion was used as a new one to catch directly the collapsing behavior of bubbles. Through experiments each erosion pit was investigated to find out the cluster-like behavior of vortex cavities. As the shedding type of vortex cavities make an important role in an impulsive character the cavitation characteristics about an axisymmetrical inside-flow were examined which had not been clear. Thus some results were obtained as follows ; the shedding cavities are also observed in this flow-type and closely connected to high impulsive force, a re-entrant motion occurs in the separation bubble of two-phase state and closely connected to the bubble shedding, and shedding two-phase vortices shows a ring-like appearance. Furthermore existing researching results were summarized under the present works.