An Investigation of Cloud Cavitation about a Sphere

Cloud cavitation occurrence about a sphere is investigated in a variable pressure water tunnel using still and high-speed photography. The model sphere, 0.15 m in diameter, was sting mounted within a 0.6 m square test section and tested at a constant Reynolds number of 1.5 x 106 with cavitation numbers varying between 0.36 and 1.0. High-speed photographic recordings were made at 6 kHz for several cavitation numbers. Shedding phenomena and frequency content is investigated by means of pixel intensity time series data using wavelet analysis. The boundary layer at cavity separation is shown to be laminar for all cavitation numbers, with Kelvin-Helmholtz instability the main mechanism for cavity break up and cloud formation at high cavitation numbers. At intermediate cavitation numbers, cavity lengths allow the development of re-entrant jet phenomena providing a mechanism for shedding of large scale Karman-type vortices similar to those for low mode shedding in single-phase subcritical flow. This shedding mode is eliminated at low cavitation numbers with the onset of supercavitation