An experimental investigation of cloud cavitation about a sphere

Cloud cavitation occurrence about a sphere is investigated in a variable-pressure water tunnel using low- 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 1.5×106 with cavitation numbers varying between 0.36 and 1.0. High-speed photographic recordings were made at 6 kHz for several cavitation numbers providing insight into cavity shedding and nucleation physics. Shedding phenomena and frequency content were investigated by means of pixel intensity time series data using wavelet analysis. Instantaneous cavity leading edge location was investigated using image processing and edge detection. The boundary layer at cavity separation is shown to be laminar for all cavitation numbers, with Kelvin–Helmholtz instability and transition to turbulence in the separated shear layer the main mechanism for cavity breakup 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 K´arm´ an-type vortices similar to those for low-mode shedding in singlephase subcritical flow. This shedding mode, which exists at supercritical Reynolds numbers for single-phase flow, is eliminated at low cavitation numbers with the onset of supercavitation. Complex interactions between the separating laminar boundary layer and the cavity were observed. In all cases the cavity leading edge was structured in laminar cells separated by well-known ‘divots’. The initial laminar length and divot density were modulated by the unsteady cavity shedding process. At cavitation numbers where shedding was most energetic, with large portions of leading edge extinction, re-nucleation was seen to be circumferentially periodic and to consist of stretched streak-like bubbles that subsequently became fleck-like. This process appeared to be associated with laminar–turbulent transition of the attached boundary layer. Nucleation occurred periodically in time at these preferred sites and formed the characteristic cavity leading edge structure after sufficient accumulation of vapour had occurred. These observations suggest that three-dimensional instability of the decelerating boundary layer flow may have significantly influenced the developing structure of the cavity leading edge.