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On the maintenance of an attached leading-edge vortex via model bird alula

Published online by Cambridge University Press:  11 June 2020

Thomas Linehan Open the ORCID record for Thomas Linehan [Opens in a new window]  and
Kamran Mohseni Open the ORCID record for Kamran Mohseni [Opens in a new window]
Thomas Linehan
Affiliation:
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL32611, USA
Kamran Mohseni*
Affiliation:
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL32611, USA Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL32611, USA
*
†Email address for correspondence: mohseni@ufl.edu
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Abstract

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.

JFM classification

Biological Fluid Dynamics: Swimming/flying Wakes/Jets: Separated flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 897 , 25 August 2020 , A17
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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