Cavitation in Kármán Vortices and Flow Induced Vibrations

The shedding process of the Kármán vortices at the trailing edge of a 2D hydrofoil at high Reynolds numbers is investigated. The focus is put on the effect of the cavitation on the vortex street morphology. A direct insight is also provided on the role of the structural vibration on the cavitation inception. The shedding frequency, derived from the measurement of flow induced vibration, is found to follow the Strouhal law as far as none of resonance frequencies of the hydrofoil is excited. For lock-off condition, PIV measurements in cavitation free regime and high speed visualizations for developed cavitation reveal strong spanwise 3D instabilities. The comparison of instantaneous velocity fields in cavitation free regime and images of cavitating vortices does not show notable influence of the cavitation on the vortex street morphology. It is also observed that the cavitation inception index increases linearly with the square root of the Reynolds number. For Reynolds numbers ranging from 35’000 to 40’000, the torsion mode of the hydrofoil is excited with a substantial increase of the vibration level. In this case, the spatial coherence of the Kármán vortices is enhanced with a quasi 2D shape and the shedding frequency is locked onto the vibration frequency. The cavitation inception index is found to be significantly increased compared to lock-off conditions. It is thought that the vortex roll-up is amplified by the phase locked vibration of the trailing edge. Former model, successful in describing the cavitation inception for fixed bluff bodies, is extended for taking into account the hydrofoil trailing edge vibration velocity. In addition, we have found that the vortices strength increases for lock-in condition and is directly related to the vibration velocity of the trailing edge normalized by the upstream velocity.