Unsteady cavitation around a Plano-convex hydrofoil with various angle-of-attacks was observed by a blow-down type cavitation tunnel. Liquid nitrogen and hot water was employed as working fluids. In cases that the plane side is downward, a reversely-directed vortex on the plane surface occurs because an adverse pressure gradient is formed on the surface. The vortex rotates an attached cavity on the surface, and then tears the almost whole cavity from the surface. In cases that the convex side is downward, a reversely-directed vortex also occurs, however, only the rear part of the cavity is torn from the surface because the incline of the surface against the main flow gradually increases as a flow goes downstream and the inception point is not fixed. In cases of unsteady cavitation, the change of the surrounding temperature due to the thermo-dynamic effect is smaller than the steady because heat removal of the unsteady cavitation occurs intermittently.
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