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First published online September 5, 2008
Journal of Experimental Biology 211, 2909-2918 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018192

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The near and far wake of Pallas' long tongued bat (Glossophaga soricina)

L. Christoffer Johansson^1,*, Marta Wolf¹, Rhea von Busse², York Winter², Geoffrey R. Spedding^3, and Anders Hedenström¹

¹ Department of Theoretical Ecology, Lund University, Sölvegatan 37, SE-223 62 Lund, Sweden
² Department of Biology, Bielefeld University, D-33501 Bielefeld, Germany
³ Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90098-1191, USA

^* Author for correspondence (e-mail: christoffer.johansson{at}teorekol.lu.se)

Accepted 8 July 2008

The wake structures of a bat in flight have a number of characteristics not associated with any of the bird species studied to this point. Unique features include discrete vortex rings generating negative lift at the end of the upstroke at medium and high speeds, each wing generating its own vortex loop, and a systematic variation in the circulation of the start and stop vortices along the wingspan, with increasing strength towards the wing tips. Here we analyse in further detail some previously published data from quantitative measurements of the wake behind a small bat species flying at speeds ranging from 1.5 to 7 m s^–1 in a wind tunnel. The data are extended to include both near- and far-wake measurements. The near-/far-wake comparisons show that although the measured peak vorticity of the start and stop vortices decreases with increasing downstream distance from the wing, the total circulation remains approximately constant. As the wake evolves, the diffuse stop vortex shed at the inner wing forms a more concentrated vortex in the far wake. Taken together, the results show that studying the far wake, which has been the standard procedure, nevertheless risks missing details of the wake. Although study of the far wake alone can lead to the misinterpretation of the wake topology, the net, overall circulation of the main wake vortices can be preserved so that approximate momentum balance calculations are not unreasonable within the inevitably large experimental uncertainties.

Key words: bats, Glossophaga soricina, flight, aerodynamics, wake, DPIV

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