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First published online February 20, 2004
Journal of Experimental Biology 207, 1063-1072 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00848
Force production and flow structure of the leading edge vortex on flapping wings at high and low Reynolds numbers
,
Department of Integrative Biology, University of California, Berkeley, CA 94720, USA
Author for correspondence (e-mail:
flyman{at}caltech.edu)
Accepted 22 December 2003
The elevated aerodynamic performance of insects has been attributed in part
to the generation and maintenance of a stable region of vorticity known as the
leading edge vortex (LEV). One explanation for the stability of the LEV is
that spiraling axial flow within the vortex core drains energy into the tip
vortex, forming a leading-edge spiral vortex analogous to the flow structure
generated by delta wing aircraft. However, whereas spiral flow is a
conspicuous feature of flapping wings at Reynolds numbers (Re) of
5000, similar experiments at Re=100 failed to identify a comparable
structure. We used a dynamically scaled robot to investigate both the forces
and the flows created by a wing undergoing identical motion at Re of
120 and 1400. In both cases, motion at constant angular velocity and
fixed angle of attack generated a stable LEV with no evidence of shedding. At
Re=1400, flow visualization indicated an intense narrow region of
spanwise flow within the core of the LEV, a feature conspicuously absent at
Re=120. The results suggest that the transport of vorticity from the
leading edge to the wake that permits prolonged vortex attachment takes
different forms at different Re.
Key words: insect flight, Reynolds number, aerodynamics, flow visualization
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