Flow separation control of an airfoil using flexible upper-surface flaps with inclined protrusions

Airfoil flow separation induces reverse flow in the shear layer. Focusing on this phenomenon, surface modification is adopted on flexible upper-surface flaps (USFs) to enhance their performance in flow separation control. Specifically, the surface modification refers to designing inclined protrusions on the tip of flexible USFs, which face the reverse flow. Systematic wind tunnel experiments, You Only Look Once v8 object detection, aerodynamic force measurements, and flow field visualization were employed to investigate the protrusions' effects on USFs' flapping dynamics, aerodynamic performance, and flow control mechanisms. The experimental results demonstrated that inclined protrusions decrease the critical Reynolds number for flexible USFs to enter the unsteady flapping state and slow the evolution of their flapping dynamics with increasing Reynolds number. These changes expand the Reynolds number range where flexible USFs achieve airfoil flow separation control, resulting in a maximum lift improvement of 10.4%, a drag reduction of 7.4%, and a 21.6% increase in the lift-to-drag ratio. Interestingly, millimeter-scale inclined protrusions are sufficient to induce flapping of flexible USFs, indicating that flexible USFs are highly sensitive to surface boundary conditions. It was also found that increasing protrusion height alters the airfoil's aerodynamic shape, enhancing lift even when the modified USFs are attached to the airfoil.