  Researchers have hypothesized that the post-stall lift benefit of bird's
alular feathers, or alula, stems from the maintenance of an attached
leading-edge vortex (LEV) over their thin-profiled, outer hand-wing. Here, we
investigate the connection between the alula and LEV attachment via flow
measurements in a wind tunnel. We show that a model alula, whose wetted area is
1% that of the wing, stabilizes a recirculatory aft-tilted LEV on a
steadily-translating unswept wing inclined at post-stall incidences. The
attached vortex is the result of the alula's ability to smoothly merge
otherwise separate leading- and side-edge vortical flows. We identify two key
processes that facilitate this merging: i) the steering of spanwise vorticity
generated at the wing's leading edge back to the wing plane and ii) an
aft-located wall-jet of high-magnitude root-to-tip spanwise flow (>80% that of
the freestream velocity). The latter feature induces LEV roll-up while the
former feature tilts LEV vorticity aft and evacuates this flow toward the wing
tip via an outboard vorticity flux. We identify the alula's streamwise position
(relative to the leading-edge of the thin wing) as important for vortex
steering and the alula's cant angle as important for high-magnitude spanwise
flow generation. These findings advance our understanding of the likely ways
bird's leverage LEVs to augment slow flight.
