Dynamics of Attached Cavities on Bodies of Revolution

Abstract

Attached cavitation was generated on two axisymmetric bodies, a Schiebe body and a modified ellipsoidal body (the I. T. T. C. body), both with a 50.8 mm diameter. Tests were conducted for a range of cavitation numbers and for Reynolds numbers in the range of Re = 4.4 × 105 to 4.8 × 105. Partially stable cavities were observed. The steady and dynamic volume fluctuations of the cavities were recorded through measurements of the local fluid impedance near the cavitating surface suing a series of flush mounted electrodes. These data were combined with photographic observations. On the Schiebe body, the cavitation was observed to form a series of incipient spot cavities which developed into a single cavity as the cavitation number was lowered. The incipient cavities were observed to fluctuate at distinct frequencies. Cavities on the I. T. T. C. started as a single patch on the upper surface of the body which grew to envelope the entire circumference of the body as the cavitation number was lowered. These cavities also fluctuated at distinct frequencies associated with oscillations of the cavity closure region. The cavities fluctuated with Strouhal numbers (based on the mean cavity thickness) in the range of St = 0.002 to 0.02, which are approximately one tenth the value of Strouhal numbers associated with Ka´rma´n vortex shedding. The fluctuation of these stabilized partial cavities may be related to periodic break off and filling in the cavity closure region and to periodic entrainment of the cavity vapor. Cavities on both headforms exhibited surface striations in the streamwise direction near the point of cavity formation, and a frothy mixture of vapor and liquid was detected under the turbulent cavity surface. As the cavities became fully developed, the signal generated by the frothy mixture increased in magnitude with frequencies in the range of 0 to 50 Hz.