Velocity dependence of damage due to spot cavitation

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Various types of cavitation on a hemispherical cylindrical body have been studied by many investigators. When the Reynolds number is smaller than the critical value, the hemispherical body exhibits transition downstream from laminar separation. In this tuation, the sheet cavitation is observed around the body. However, when the Reynolds number increases to and above the critical value or the laminar separation is eliminated by a trip installed on the body, the sheet cavitation is swept away but the attached spot cavitation is observed occasionally. It occurs at fixed places in the vicinity of the minimum mean pressure and grows into a triangular wedge. It is considered that the spot cavitation is due to isolated local roughness on the surface. In the resent investigation, the spot cavitation is artificially generated from a hollow of diameter 0.3 mm downstream from a boundary-layer trip installed on a hemispherical cylindrical body. The behavior of the artificially generated spot cavit … More ation at the cavitation number of 0.91 is observed by instantaneous photographs and high-speed photography. In the closure region of the main cavity, periodic release of inverse U-shaped vortex cavities or cavity cloud separation is observed. The Strouhal number based on the cavity thickness, the flow velocity, and the frequency of the vortex cavitation release or the cavity cloud separation is approximately 0.41 irrespective of the flow velocity. The frequency of damaging blows in the range of the flow velocity of 25 to 50 m/s at the cavitation number of 0.91 is obtained by counting the number of damaged pits on an aluminum specimen. The pitting rate has maximums at two circumferential locations at approximately 1 .3 mm away from each other. The axial position of the maximum pitting rate is at the mean cavity trailing edge or slightly upstream of the mean cavity trailing edge. The pitting rate at the maximum damage zone varies roughly 5^<th> power of the flow velocity, In addition to the collapses of the inverse U-shaped vortex cavities and the cavity clouds downstream of the main cavity, the collapses of cavitation bubbles by the reentrant flow in the cavity closure region are considered to be the causes of the serious damage. Less