  We investigate the influence of unsteady partial cavitation on the
fluid-structure interaction of a freely vibrating hydrofoil section at high
Reynolds numbers. We consider an elastically-mounted NACA66 hydrofoil section
that is free to vibrate in the transverse flow direction. Cavitating flow
dynamics coupled with the transverse vibration are studied at low angles of
attack. For the numerical study, we employ a recently developed unified
variational fluid-structure interaction framework based on homogeneous
mixture-based cavitation with hybrid URANS-LES turbulence modeling. We first
validate the numerical implementation against experimental data for turbulent
cavitating flow. For freely oscillating hydrofoil, we observe large-amplitude
vibrations during unsteady partial cavitating conditions that are absent in the
non-cavitating flow. We identify a frequency lock-in phenomenon as the main
source of sustained large-amplitude vibration whereby the unsteady lift forces
lock into a sub-harmonic of the hydrofoil natural frequency. During cavity
collapse and shedding, we find a periodic generation of clockwise vorticity,
leading to unsteady lift generation. We determine the origin of this flow
unsteadiness near the trailing edge of the hydrofoil via the interplay between
the growing cavity and adverse pressure gradient. The flow-induced structural
vibration is observed to have a consequent impact on the cavity dynamics. In
the frequency lock-in regime, large coherent cavitating structures are seen
over the hydrofoil suction surface undergoing a full cavity
growth-detachment-collapse cycle. For the post-lock-in regime, cavity length is
shorter and the attached cavity is observed to undergo high frequency spatially
localized oscillations. In this regime, cavity shedding is primarily limited to
the cavity trailing end and frequency of a complete cavity detachment and
shedding event is reduced.
