Flow transport enhancement of alula maintaining high lift on fixed wing

The role of alula in enabling birds to achieve high lift at large angles of attack (AoAs) is well-established. However, the mechanisms remain controversial especially for biomimetic high-lift wings which do not generate obvious leading-edge vortex at high incidences. This paper experimentally investigates alula effects on lift force variations, flow structures, and high-lift sustaining mechanisms on fixed high-lift wings, using direct force measurements and stereo particle image velocimetry. Lift measurements reveal four distinct lift stages for alula wings: pre-stall, high lift maintenance plateau (HLMP), lift descent, and deep stall. During pre-stall and deep stall stages, alula has a negligible effect on lift augmentation. However, during HLMP stage, alula wing maintains a stable high lift within a specific AoA range. As AoA increases to lift descent stage, the effectiveness of alula gradually diminishes. The maintenance of high lift during HLMP stage is attributed to alula-enhanced flow transport over the wing. In the streamwise direction, high-speed attached flow through alula path (path between alula and wing) is formed and accelerated due to the combined effects of path contraction and induction of alula trailing-edge shear layer. Additionally, Spanwise flow gradients, driven by alula-induced outward flow and wingtip vortices, further stabilize attachment along the wingspan. These findings elucidate alula-enabled high-lift mechanisms, informing aerodynamic design, and flight control for unmanned aerial vehicles.