Intrinsic nucleation dynamics of cloud cavitation

Nucleation phenomena associated with cloud cavitation about a three-dimensional (3-D) NACA $\,$ 16-029 hydrofoil are explored experimentally in a cavitation tunnel where susceptible free stream nuclei are absent. Microbubble nuclei are found to be intrinsically generated by cavity collapse and become sequestered in the low-momentum separated region ahead of the cavity leading edge. Nuclei dynamics upstream of a shedding sheet cavity was investigated using high-speed photography. Measurements were performed at zero incidence for cavitation numbers in the range of $0.55 \gt \sigma \gt 0.45$ , and chord-based Reynolds numbers of $ \textit{Re} = 0.75\times 10^6$ and $ \textit{Re} = 1.5\times 10^6$ . Nuclei are generated each shedding cycle due to cavity breakup from condensation shock-wave phenomena. These nuclei may undergo immediate activation or transport due to pressure gradients, local re-circulation and jetting. Some nuclei remain upstream of the cavity leading edge over multiple cycles. Several phenomena influence this behaviour, including cyclical variation of the boundary layer properties with each shedding cycle. A major conclusion of the work is that these nuclei are produced in a self-sustaining manner from near surface, small scale, interfacial or viscous phenomena rather than from surface or free stream nuclei. Additionally, these experiments reveal the low-momentum region upstream of the cavity to be above vapour pressure, despite the meta-stable tension developed in the boundary layer further upstream of the cavity.