Mechanism study of flow characteristics on small HAWT blade surfaces based on airfoil concavity under yaw conditions

Aiming to solve the power output reduction caused by the flow separation, this study applied a passive flow control method on blade suction surfaces of a small horizontal axis wind turbine. An airfoil with a semi-elliptical concavity was introduced, and several concave blades were, thus, designed. Among them, the blade with a concavity located at 80% chord and a length of 350 mm was selected for further analysis according to the aerodynamic performance. As a result, it has been found that the concave airfoil had better performance at high wind speeds, low rotational speeds, and small yaw angles, especially the positive yaw conditions. The flow field mechanism could be interpreted with a positive pressure gradient generated by the airfoil concavity. Under the positive yaw angle of 10 degrees, the concavity effect resulted in a greater aerodynamic lift. The azimuth angle of 0 degrees shows an obvious control effect at the blade tip. On the contrary, the concavity has little effect at an azimuth angle of 120 degrees near the leading-edge. At the azimuth angle of 240 degrees, a significant concavity effect at the blade root could be found, while the aerodynamic benefits were not as remarkable as an azimuth angle of 0 degrees in the vicinity of the blade tip. Aside from that, when the flow separation was serious due to the three-dimensional rotational effect, the concavity has no distinct effect on separation control. In essence, the airfoil concavity had a favorable impact on flow separation control and effectively enhanced the power output of the wind turbine. Published under an exclusive license by AIP Publishing.