Experimental study on a flapping-foil hydrokinetic turbine with a fully passive activating mechanism and crank-rocker power transmission*

A rotary turbine is commonly used to extract energy from water flow. However, a new type of hydrokinetic turbine called a flapping-foil hydrokinetic turbine (FHT) has been developed, taking inspiration from underwater creatures. In this study, a passive pitch control method was implemented by synchronizing the movements of the front and rear foils. By linking the flapping movement of one hydrofoil with the pitch movement of the other hydrofoil, a portion of the flapping energy induced by the flow was utilized for pitch control. Through experiments, it was observed that the pitch angle and flapping angle were effectively interlocked by adjusting the gear ratio of the connecting sprocket, allowing the turbine to be self-driven.

The power transmission method used a crank-rocker mechanism. A transmission equation was derived using the rocker angle as the input angle and the crank angle as the output angle. The angular velocity and torque of the crank were obtained from measurements of the rocker's angular velocity and torque during experiments. The transmission power efficiency to the crank was predicted to be over 90% for both foils, and this was validated against the data obtained from the experimentally measured crank information.

The passive pitch control and crank-rocker power transmission proposed in this study considerably improved the performance of the FHT. With further research, it is expected that the FHT could serve as a promising alternative to rotary turbines.