Rudimentary Emulation of Covert Feathers on Low-AR Wings for Poststall Lift Enhancement

Abstract

Covert feathers, a group of feathers on the upper surface of bird wings, are one of birds’ features that aid them in flight at high angles of attack, making them more maneuverable. In a previous study, the authors found that a rudimentary emulation of this feature (termed a self-adaptive flap) enhances the poststall lift characteristics of low-aspect-ratio flat-plate wings at . This enhancement, however, was found to vary with the different chordwise locations of the flap and across different planforms. In a continued effort to understand this further, two-dimensional particle image velocimetry investigation was carried out in the midspan plane of the low-aspect-ratio wings. For the varying-span elliptical wings, the results reveal that the flap/shear layer interaction promotes reattachment of the separated shear layer via increased entrainment. This reattachment location varies for the different chordwise location of the flap, resulting in a different extent of the separation bubble and bubble-induced camber, which in turn is responsible for different poststall lift enhancement. On the other hand, for the rectangular wing, the flap promotes large-scale vortex formation. The formation and shedding cycle of large-scale vortices has a significant impact on the mean-body forces because of which the time-averaged poststall lift is enhanced.