Experimental and Numerical Study of the Flow Around Rigid and Flexible Hydrofoils for Wetted and Cavitating Flow Conditions

The hydroelastic response of a flexible NACA 0015 hydrofoil is investigated for both wetted and cavitating flow conditions. Computational Fluid Dynamics (CFD) analysis are performed using a fully implicit coupling between the ISIS-CFD solver (developed by the METHRIC team at Ecole Centrale de Nantes) and a modal approach for the structure. The RANS (Reynolds Averaged Navier-Stokes) solver is first validated for wetted and cavitating flow conditions around a similar rigid hydrofoil, with experimental results carried out at the hydrodynamic tunnel of the French Naval Academy, including lift and drag measurements and high speed camera images. Then the numerical predictions for the flexible hydrofoil response are compared with experimental bending shapes and vibrations amplitudes, with a focus on cavitating flow conditions. For wetted flow conditions, numerical results show a good agreement with the experiments, for both rigid and flexible hydrofoils. For cavitating flow conditions, the hydroelastic response is dominated by vibrations at the hydrofoil modal frequencies and the re-entrant jet instability frequency. For the lowest values of the cavitation parameter, a large amplitude peak is experimentally observed in the frequency response spectra, due to lock-in between the first modal frequency and the re-entrant jet frequency. Strong harmonics of this dominant peak also appear in the spectra, revealing a non-linear response of the hydrofoil. While the amplitudes of vibrations are well predicted by the computations, the frequency lock-in observed in the experiments is not captured by the numerical model.