6 observed the steppe eagle Aquila nipalensis to morph its wings into a distinct M-shape during the pitch-up phase of its perching sequence. 10 is likely not the alula tip vortex but rather the separated boundary layer of the alula.Ī second proposed function of the alula is that it promotes LEV formation over the swept-back hand-wing of birds in flight scenarios when the arm-wing is completely stalled 4, 6. These observations led Sander to remark that the streamwise vortex measured by Lee et al. Sander notes the breakdown of the alula vortices at higher angles of attack, α ≥ 25 deg, due to their interaction with a separated wing boundary layer. did not measure nor mention the LEV on the hand-wing and Sander only observed an LEV when simulating flapping motion. Sander 18 also observed an alula tip vortex as well as an alula leading-edge vortex in their computations on a simplified bird-alula model. 10 measured a streamwise vortex aft of the alula, calling this the alula tip vortex, and a stall-delaying effect outboard of the alula.
This function has been partially corroborated by several recent experimental and computational works. First, that the alula generates a small vortex which separates the attached-flow system on the inner, thick-profiled, arm-wing section and the separated leading-edge vortex (LEV) on the outer, thin-profiled, hand-wing section. These observations have prompted a revaluation of the aerodynamics of the alula for which two updated interpretations of its function have been proposed 4, 6. Subsequent research depicting separated flow over real and model swift wings in steady flight 15, 16 and Passerines in slow-flapping flight 17 suggests that the alula likely prevents wing stall through the maintenance of separated-edge flows rather than preventing flow separation from occurring in the first place. These devices prevent wing stall by ensuring the flow remains smoothly attached to the wing. 1) has led to early comparisons of it to flow control devices on aircraft such as leading-edge slots/slats 7, 8, 9. The gap formed between the deflected alula and the top surface of the wing (see Fig. These findings shed new light on avian wing anatomy and the role of unconventional aerodynamics in shaping it.įull size image Aerodynamics of the alulaĭespite consensus among researchers regarding the importance of the alula in avian flight, the aerodynamic mechanisms underlying its function remain debated. We found the position of the alula on non-aquatic birds selected for alula optimization to be located at or near the lift-maximizing position predicted by wind tunnel experiments. To test this, we perform experiments on model wings in a wind tunnel to approximate this distance and compare our results to positional measurements of the alula on spread-wing specimens. Specifically, we test the hypothesis that the relative distance of the alula from the wing tip is that which maximizes LEV-lift when the wing is spread and operated in a deep-stall flight condition. Here, we explore scaling trends of the alula’s spanwise position and its connection to this function. New research into the aerodynamics of this structure suggests that its primary function is to induce leading-edge vortex (LEV) flow over bird’s outer hand-wing to enhance wing lift when manuevering at slow speeds. An aerodynamic structure ubiquitous in Aves is the alula a small collection of feathers muscularized near the wrist joint.
0 kommentar(er)
