In this study, a particle image velocimetry (PIV) system was used in a large circulating water tank to investigate the wake of a horizontal-axis tidal turbine model, focusing on minor blockage effects and scale influence. A wake map of the turbine was constructed based on PIV measurements, using velocity deficit, turbulence intensity (TI), and turbulence kinetic energy (TKE) as key indicators. The results showed that TKE developed later than TI, forming a plateau-like shape. This plateau was considered the decay region, with the transition and far-wake regions located before and after it, respectively. Additionally, the power law exponent of TI decreased from −0.731 in the decay region to −0.765 in the far wake, indicating a steeper decay further downstream. Overall, the wake map of the tidal stream turbine model exhibited similarities to that of a previously reported wind turbine model.

Flapping-foil hydrokinetic turbines (FHTs), unlike rotary turbines, are inspired by nature and have recently been presented in various tandem forms. In this study, a tandem hydrokinetic turbine with four hydrofoils that mimics a quadrupedal underwater animal and its movements is developed, with each hydrofoil moving in phase and out of phase, and the performance in terms of the power and load is compared and analyzed. As a result of optimizing the flapping frequency and separation distance, the out-of-phase condition showed superior characteristics in terms of power, with similar efficiency and lower fluctuation levels compared to the in-phase condition. In terms of the load on the body, the force levels in the out-of-phase movement were kept lower than those of the in-phase condition, which is advantageous for the design of the structure supporting the turbine. Therefore, the FHT proposed in this study can utilize more than three hydrofoils, similar to a typical rotary turbine, and can improve the FHT performance by adjusting the phase between the hydrofoils.

In this study, a previously developed fully passive hydrokinetic turbine with two flapping foils was used to directly drive a reciprocating pump, and the performance of this system was investigated at an offshore site in Republic of Korea. The fully passive operation of the turbine worked effectively due to its coupling mechanism, and pumping was successfully carried out during flood tides when the pumping height was consistently maintained using a water level gauge and winch system. Pumping occurred at a height of approximately 9 m when the flow velocity reached 1.8 m/s, at which point the corresponding Reynolds number exceeded one million. In one case where a high pumping flow rate was achieved during offshore trials conducted over a period of time, the pumping efficiency reached up to 34% when the reduced frequency of the turbine was 0.126, falling within the known optimum range. The pump driven by the flapping-foil hydrokinetic turbine, which can be positioned near the shore or in shallow water, could provide a viable solution for off-grid communities needing to pump seawater or generate hydroelectric power.