Hydrodynamic characteristics and flow structures of pitching hydrofoil with special emphasis on the added force effect

The objective of this paper is to investigate the hydrodynamic characteristics and corresponding flow structures of a pitching Clark-Y hydrofoil with special emphasis on the added force effect. The experiments were performed in the looped cavitation tunnel, and the hydrodynamic characteristics are obtained by the dynamic moment measurement system. The average angle of attack and amplitude of pitching hydrofoil are 10° and 5°, respectively. The whole oscillatory motion is divided into two stages, namely the up stage and down stage. The pitching rate is set with the Reynolds number Re = 4.4 × 10⁵. The incompressible URANS equations are solved by using the coupled k-ω SST turbulence model and γ-Re_θ transition model. It can be shown that the numerical results agree well with the experimental measurements. Compared to the static hydrofoil, the lift of the pitching case is higher in the up stage because of the positive added lift. Meanwhile, the center of pressure is closer to the pitching axis. For the down stage, the lift of the pitching case is lower caused by the negative added lift and the pressure center is further away from the pitching axis. The main reason is that the different pitching direction corresponds to specific position of the added lift relative to the pitching axis, causing the pressure center to move in different directions. When the stall happens, the evolution of lift and the pressure center fluctuates due to the shedding vortex structures. Results show that the added force effect on the hydrodynamic force is negligible compared with the contributions from the vorticity within the flow in the stall phase. As the pitching rate increases, the added force effect becomes more significant, thus leading to the higher lift of the up stage and the lower lift of the down stage. Besides, for the stall phase, the dynamic stall angle is delayed for the fast pitching rate, which is due to the generation and development of the counterclockwise trailing edge vortex.