Unsteady cavitation dynamics and frequency lock-in of a freely vibrating hydrofoil at high Reynolds number

In this paper, we numerically 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. The fluid–structure interaction of this system is of interest in characterizing the cavitation-induced vibration and noise of marine propellers. The coupled cavitation dynamics and the fluid–structure system are solved using a body-fitted variational framework based on homogeneous mixture-based cavitation with a hybrid URANS-LES turbulence modeling. Specifically, we explore the occurrence of large-amplitude vibrations during unsteady partial cavitating conditions that are absent in the non-cavitating flow configuration. We examine a frequency lock-in phenomenon as the main source of sustained large-amplitude vibration whereby the unsteady lift force locks into a sub-harmonic of the hydrofoil’s natural frequency. We determine the origin of this flow unsteadiness in the vicinity of the trailing edge of the hydrofoil through the interplay between the growing cavity and the adverse pressure gradient. We systematically analyze the impact of flow-induced structural vibration on the combined vortex and cavity synchronization.