Characteristic phases of the development of unsteady cloud cavitation: Statistical analysis of uncorrelated velocity fields

In the paper, we identify characteristic phases of the auto-oscillation cycle of unsteady cloud cavitation on a two-dimensional symmetric hydrofoil by analyzing random (uncorrelated) instantaneous two-component velocity fields from a representative PIV database. By applying a certain criterion to the amplitude of the fluctuating velocity, we were able to successfully isolate individual realizations from the entire ensemble into characteristic phases of the quasi-periodic process of attached cavity length pulsations accompanied by detachments of cloud cavities. This ultimately allowed us to phase average the selected velocity fields, to perform phase-by-phase tracking of large-scale vapor and vortex structures developing in the unsteady cavitating flow and, finally, to calculate spatial distributions of vapor concentration and turbulence characteristics in each of these phases. All this provided important information on the phase-averaged turbulence structure, typical of unsteady cloud cavitation, and opened up new opportunities for further in-depth analysis of the interactions of vapor structures and flow turbulence. In particular, it was established that the breakup of the sheet cavity interface followed by the shedding of a large-scale cloud occurs on average at a distance of 36–39% of the chord length from the hydrofoil leading edge. The reverse flow was reliably detected downstream of the cavitation sheet closure and between the suction surface and detached cloud cavity, which directly indicates the presence of a large-scale vortex induced by the circulation of liquid due to the coordinated action of the main flow and re-entrant jet at the moment of the attached cavity breakup.