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The purpose of this present study is to address instability flowing characteristics and mechanism of the water-entry cavity created by a semi-closed cylinder. For this purpose, an experimental study and a numerical study of the water-entry of a semi-closed cylinder are carried out. According to the results of the experiments and comparison, the cavitating flows between the semi-closed cylinder water entry and the sealing cylinder water entry, and the fluctuation flow pattern form of the semi-closed cylinder cavitation is found around the body. The flow characteristics of the cavity shape are gained by analyzing the image data. A further insight into the mechanisms of perturbation to the flow structure and the cavity fluctuation by the air in the opening cell are studied based on the law of conservation of energy in water entry. According to the results of simulation and comparison with the cavity visualization of experiment, three instability flow phenomena of cavity are formed during the different stages of water-entry, i.e., flow separation destroyed, local flow transformed near cavity, and unique cavity shedding pattern. A further insight into the characteristics of the flow and the distribution of pressure and velocity during the stage of the cavity unstabilized flow is gained. Finally, the formation mechanism of the cavity unstabilized flow is studied based on the boundary layer theory and Helmhotz vortex theory. The obtained results show that the water poured into the cell of cylinder after the opening end has impacted free surface causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of flow structure occur around the cavity, then the cavity presents an identical periodic wave flow with air piston motion and the flow stability of cavity is destroyed. At the eve of impacting, the opening end approaches the free surface, which leads to the inflow velocity attenuation rapidly and the pressure increasing in the cell, which creates an initial pressure higher than ambient pressure. Because of the high pressure, air efflux from the cell forms a gas jet injected into the cavity for the first air expansion stage, then the detaching flow is destroyed and the cavity extension diameter is enlarged. The flow in the gas-liquid mixing domain of cavity is seen as an approximate boundary layer flow pattern where favorable pressure gradient on the upwind side and adverse pressure gradient on the lee side appear alternately. As this flow pattern, re-entrant flow acting on the trough of wave cavitation results in the fact that the laminar-turbulent transition is weakened in the trough field and the local gas-liquid mixing domain is thickened to be involved in unstabilized structure as cloud cavitation. The wave cavity presents a partial and multiple shedding pattern occurring at the trough positions in sequence. There is no mutual interference between shedding cavity and the main cavity. Following the cavity shedding, vortex shedding is formed. The vorticity concentrates on the inside of shedding cavity, and the pressure and velocity present a coherent structure.

The objective of this present study is to address the cavitating flow patterns and regimes in the water-entry cavity. For this purpose, an experimental study of vertical water-entry cavity of an end-closed cylindrical shell is investigated by using high-speed video cameras and visualization technique. According to the cavitating flows as observed in the experiments, two flow pattern forms of fluctuation cavitation and cloud cavitation are found around the body. A further insight into the characteristics of the cavity shape and the variation in the cavity fluctuations parameters is gained by analyzing the image data. Furthermore, the experiments at different impact velocities are conducted to analyze the effects of impact velocity on the flow patterns and parameters. Finally, the formation mechanisms of cavitation fluctuations and cavitation clouds are studied based on the basic theory of fluid mechanics. The obtained results show that the cavitation flow pattern form of fluctuation cavitation occurs under the impact velocity condition of low speed, and the cloud cavitation occurs under the velocity condition of high speed. As fluctuation cavitation, the maximal extension diameters of cavitation fluctuate periodically along the water depth, and the speeds of extension and shrinkage are both proportional to the extension diameter. The collapses are different for the two flow pattern cavitations, i.e., the fluctuation cavitation, which is of deep closure and closed at the trough of wave cavitation more than once, and the cloud cavitation, which falls off and forms the leading edge of the cylindrical shell. The frequency fluctuation is independent of the impact velocity, the corresponding pinch-off time decreases with increasing the impact velocity, and the pinch-off time decreases in a nearly linear relation with Froude number. The water poured to the cylindrical shell causes the internal air to compress and expand, and as a consequence of these effects, periodic disturbances of pressure distribution and velocity field occur around the leading edge of the cylindrical shell, then the extended intensity of the cross section of the cavity shows variation in this process, which can be defined as fluctuation cavitation pattern. It appears that the re-entrant flow after the pinch-off at the trailing edge of cavity, then the laminar-turbulent transition is waken as a consequence of the re-entrant flow moving upstream, which flow pattern involved in this structure occurs as cloud cavitation.