Research on the effect of a movable gurney flap on energy extraction of oscillating hydrofoil

Abstract

A movable Gurney flap was used to enhance the energy extraction of the oscillating hydrofoil. According to the motion of oscillating hydrofoil, the position of Gurney flap was adjusted so that it was always on the pressure side of hydrofoil. The effect of Gurney flap with different length cg and different motion period R was studied under the condition of fixed Reynolds number with variations of kinematic parameters such as reduced frequency f ∗ and pitch amplitude θ. Numerical results show that the application of Gurney flaps significantly improves the average power and efficiency compared with no flap NACA0018 airfoil. By changing the flow structure and pressure distribution around the trailing edge of the airfoil, the Gurney flap facilitates the generation of lifting force, resulting in a higher power coefficient. The average power coefficienC¯P increases with the Gurney flap height cg between 1%c and 7%c, and further increases cg will not further increase energy extraction. When the Gurney flap movement period R is in the range of 2–6, the power coefficient increases as the value of R increases, however C¯P tends to be constant after R > 6. In addition, the increased cg results stronger trailing edge vortices and higher drag.