Hydrofoil flow induced vibration mitigation by passive resonant piezoelectric shunt

International audienceMarine immersed lifting surfaces such as hydrofoils or propeller blades sustain high amplitude flow induced vibrations resulting in structural fatigue and acoustic noise. As the vortex shedding frequency in the wake of the structure tunes with a natural frequency of the structure, resonancemay occur and generate a drastic increase of the vibration amplitude. The present analysis aims to introduce a passive vibration mitigation system based on a resonant piezoelectric shunt used to reduce the vibrations. The studied geometry consists of a NACA 66-306 truncated hydrofoil ofchord length 80 mm immersed at zero degrees of incidence in the hydrodynamic tunnel of the French Naval Academy Research Institute. Chord based Reynolds numbers ranging from 2.40 × 10^5 to 4.40 × 10^5 were investigated to evaluate the vibration mitigation provided by the passiveresonant shunt at flow velocities ranging from 3.0 to 5.5 m.s-1.The system, consisting of a ferrite-core inductor connected to a MFC piezoelectric patch, has been designed to reduce the vibration amplitude when resonance of the twisting mode occurs. The mechanical response of the hydrofoilwas measured by laser vibrometry at a single location in vicinity of the trailing edge. The wake generated by the hydrofoil and the impact of the vibration mitigation on the vortex dynamics was characterised by Time Resolved Particle Image Velocimetry (TR-PIV).The natural vibration frequencies of the immersed hydrofoil under zero flow velocity were identified by spectral analysis and were used to tune the electrical resonance of the piezoelectric shunt to the twisting mode natural frequency. A significant vibration amplitude reduction of 16 dBwas observed for this particular set-up. Afterwards, the mechanical response of the hydrofoil was investigated at different flow velocities. It appeared that the vibration reaches a maximum amplitude at 4.4 m.s-1 corresponding to the resonance velocity. The passive shunt hasdemonstrated a high vibration reduction level around this flow velocity, which was further analysed by a characterization of the wake. The variations of the vertical flow velocity component at a single location behind the hydrofoil were investigated. It has been observed that the vibrationreduction by the passive shunt decreases the velocity fluctuations induced by the vortex shedding and slightly modifies the vortex shedding frequency. Vibration mitigation by passive resonant shunt is of particular interest for industrial applications aiming to reduce structural fatigue andacoustic noise. Effect of piezoelectric shunt on the wake will also open new possibilities for flow control applications