Abstract
The S-shaped hydrofoil is often used in the design of reversible machinery due to its centrally symmetrical camber line. The objective of this paper is to study the influence of cloud cavitation on the flow structure and the unsteady characteristics of lift and drag around an S-shaped hydrofoil via experimental tests and numerical simulations. In the experimental component, the tests were carried out in a cavitation tunnel and a high-speed camera was used to record the cavitation details around the S-shaped hydrofoil with different cavitation numbers. The experimental results show that sheet cavitation gradually transforms into cloud cavitation with a decrease in the inlet cavitation number, the maximum cavity length increases faster after the occurrence of cloud cavitation, and the shedding cycle time of cloud cavitation gradually increases with a decrease in the inlet cavitation number. In the numerical component, the numerical results are in good agreement with the experimental data. The numerical results show that the movement of the re-entrant jet is the main factor for the formation of the cloud cavitation around the S-shaped hydrofoil. The shedding cloud cavity induces the U-shaped vortex structure around the S-shaped hydrofoil, and it produces a higher vorticity distribution around the cavity. The periodic motion of cloud cavity causes the unsteady fluctuation of the lift–drag coefficient of the S-shaped hydrofoil, and because of the unique pressure distribution characteristics of the S-shaped hydrofoil, the lift and drag coefficient appeared as two peaks in one typical cycle of cloud cavitation.
Highlights
IntroductionCavitation is a phenomenon that occurs inside a liquid when the pressure is lower than the saturated vapor pressure [1]
Knapp et al [5] discovered the existence of the re-entrant jet in the study of cloud cavitation around an axisymmetric body; Kubota et al [6] selected laser Doppler anemometry (LDA) and a high-speed camera (HSC) to collect the unstable flow of cloud cavitation around a foil
The results show that everything collapses when the position of the cloud cavitation moves to l/C = 0.75, which is close to the experimental observation
Summary
Cavitation is a phenomenon that occurs inside a liquid when the pressure is lower than the saturated vapor pressure [1]. The cloud cavitation process is produced by the instability fracture of sheet cavitation has obvious periodicity, which is easy to induce vibration, noise, erosion and other problems in hydraulic machinery [3,4]. Cloud cavitation is a complex periodic flow that includes phase–change, vortex, and flow separation and it attracts many research studies that use experiments and numerical simulations. The cloud cavitation structure has an obvious three-dimensional stretching effect, and there is an obvious vortex concentration center in its center. The re-entrant jets close to the hydrofoil are the main cause of the formation of cloud cavitation. By placing a lateral obstacle in the hydrofoil, Kawanami et al [7] successfully prevented the re-entrant jets and controlled the shedding of the sheet cavitation
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