Evaluation of several passive flow control methods applied to reversed pitching airfoils of bi-directional turbines

Reversed pitching hydrofoils/airfoils are often used in many engineering applications, such as bi-directional tidal turbines, helicopters, and vertical-axis propulsions, but how to mitigate the adverse consequences caused by them was seldom investigated previously. In this work, several passive flow control methods, including a front micro-cylinder, Gurney flap (GF), slotted airfoil (SA), airfoil with morphed leading edge (MLE), and airfoil with morphed leading and trailing edges (MLE-MTE), are proposed to assess which one shows the best performance enhancement and alleviates the dynamic stall. Two asymmetrical pitching kinematics with different pitching amplitudes and one symmetrical pitching motion are employed. The main results show that regardless of the pitching kinematic, MLE-MTE can generate a high lift coefficient and lift-to-drag ratio compared with the clean airfoil and airfoils with other flow control methods. Then, the effect of reduced frequency kred is also studied, and it is observed that increasing kred can delay the dynamic stall. Simultaneously, the performance predicted by MLE-MTE is improved significantly with an increase in kred. In addition, the mechanism of asymmetrical performance distribution in up-stroke and down-stroke is clarified clearly based on flow structures and blade loadings of the GF, SA, and MLE-MTE. Finally, by tracing the vortex core region, the trajectory of the reversed dynamic stall vortex and second reversed dynamic stall vortex at two kred values is plotted for different flow control methods. It is concluded that a reversed pitching airfoil with MLE-MTE shows the best performance at different pitching kinematics and reduced frequency.