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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 at post-stall angles of attack. 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 free-stream velocity). The former feature induces LEV roll-up while the latter 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 birds leverage LEVs to augment slow flight.
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Scaling the vorticity dynamics in the leading-edge vortices of revolving wings with two directional length scales
In revolving or flapping wings, radial planetary vorticity tilting (PVTr) is a mechanism that contributes to the removal of radial (spanwise) vorticity within the leading-edge vortex (LEV), while vorticity advection increases its strength. Dimensional analysis predicts that the PVTr and advection should scale with the wing aspect-ratio (AR) in identical fashion, assuming a uniform characteristic length is used. However, the authors’ previous work suggests that the vorticity advection decreases more rapidly than the PVTr as AR increases, indicating that separate normalizations should be applied. Here, we aim to develop a comprehensive scaling for the PVTr and vorticity advection based on simulation results using computational fluid dynamics. Two sets of simulations of revolving rectangular wings at an angle of attack of 45° were performed, the first set with the wing-tip velocity maintained constant, so that the Reynolds number (Re) defined at the radius of gyration equals 110, and the second set with the wing angular velocity maintained constant, so that Re defined at one chord length equals 63.5. We proposed two independent length scales based on LEV geometry, i.e., wing-span for the radial and tangential directions and wing chord for the vertical direction. The LEV size in the radial and tangential directions was limited by the wing-span, while the vertical depth remained invariant. The use of two length scales successfully predicted not only the scaling for the PVTr and the vorticity advection but also the relative magnitude of advection in three directions, i.e., tangential advection was strongest, followed by the vertical (downwash) and then the radial that was negligible.
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- Award ID(s):
- 1903312
- PAR ID:
- 10229602
- Date Published:
- Journal Name:
- Physics of fluids
- Volume:
- 32
- Issue:
- 12
- ISSN:
- 2163-5013
- Page Range / eLocation ID:
- 121903
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi/10.1063/5.0024213
