Effect of wing sweep on a perching maneuver

While landing, birds often perform a perching maneuver, which involves pitching their wings upward while decelerating to a complete stop. This study is motivated by the observation that some birds fold their wings to create a wing sweep during such perching. The objective of this study is to find out whether such a wing sweep helps during a perching maneuver. We use two flat plates, one with a sweep and another without any sweep, and consider a deceleration maneuver where both of them decelerate to stop from Reynolds number Re=13000. We consider two cases: one, where the wings undergo only heaving, and two, where the wings perform both heaving and pitching. The latter maneuver was designed to mimic perching. By performing experiments and simulations, we compare the temporal evolution of the instantaneous forces and the vortex dynamics of both these plates. We show that during a major part of the deceleration, the instantaneous lift forces are higher in the case of the plate with sweep compared to the plate with no sweep during both kinematics. Our results indicate that the higher lift in the swept plate case was contributed by a stable leading-edge-vortex (LEV) which remains attached to the plate. This increase in stability was contributed by the spanwise vorticity convection caused by a distinct spanwise flow on the swept plate, as revealed by the numerical simulation. We also show that combined pitching and heaving resulted in higher force peaks, and the forces also decayed at a faster rate in this case compared to the heave-only case. Finally, by using an analytical model for unsteady flows, we prove that the higher lift characteristics of the swept plate were entirely due to higher circulatory forces.