Vortex shedding from a composite hydrofoil: Experimental evidence of a novel "partial lock-in"

Lock-in is of great importance in many engineering applications due to its practical implications for structural safety. The influence of composite bend-twist coupling on the wake dynamics and vortex-induced vibration around a carbon fiber composite hydrofoil is investigated and compared to a similar stainless-steel hydrofoil. Experiments are conducted by varying linearly and slowly the upstream velocity back and forth between 5 and 15m/s, which allows reaching lock-in conditions for both hydrofoils. Due to the blunt truncation of the trailing edge, both hydrofoils produce strong and alternate vortices in their wake, whose effect is visible on vibration spectrograms. The steel hydrofoil produces a classical lock-in onto its first torsion mode, while the composite hydrofoil exhibits two lock-in phenomena onto both torsion and second bending modes. Interestingly, for the second bending mode, the vibration spectrogram reveals the existence of two frequencies: (i) the resonance frequency, which remains almost constant during the lock-in phase, and (ii) the Strouhal frequency, which increases linearly with the upstream velocity. Using flow visualization, we found that this peculiar behavior is the result of the twist-bending coupling, which leads to the co-existence of two different vortex-shedding mechanisms. Close to the hydrofoil tip, the large vibration amplitude dictates the shedding frequency while the shedding follows the Strouhal law elsewhere. This partial lock-in gradually fades away as the velocity is increased. This result provides guidance for the safe design of composite structures.