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

TL;DR

This study explores how unsteady cavitation influences the fluid-structure interaction of a freely vibrating hydrofoil at high Reynolds numbers, revealing a frequency lock-in phenomenon that causes large amplitude vibrations and affects cavity dynamics.

Contribution

It introduces a unified variational FSI framework with cavitation modeling and identifies the frequency lock-in mechanism causing large vibrations in cavitating hydrofoils.

Findings

Large-amplitude vibrations occur during unsteady partial cavitation.

Frequency lock-in causes the unsteady lift forces to resonate with the hydrofoil's natural frequency.

Cavity dynamics are significantly affected by flow unsteadiness and structural vibrations.

Abstract

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.