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First published online December 28, 2007
Journal of Experimental Biology 211, 267-273 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.006155

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Research Article, General Fluid Dynamic Approaches

Vortex-wake interactions of a flapping foil that models animal swimming and flight

David Lentink^1,*, Florian T. Muijres^1,2, Frits J. Donker-Duyvis² and Johan L. van Leeuwen¹

¹ Department of Experimental Zoology, Wageningen University, 6709 PG Wageningen, The Netherlands
² Faculty of Aerospace Engineering, Delft University of Technology, 2600 GB Delft, The Netherlands

^* Author for correspondence (e-mail: david.lentink{at}wur.nl)

Accepted 15 October 2007

Summary

The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (^*=4–24) at a constant dimensionless flapping amplitude (A^*=1.5) and geometric angle of attack amplitude (A_,geo=15°). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight.

We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (^*<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von Kármán-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occured when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically.

Key words: flapping foil, vortex dynamics, vortex-wake interaction, swimming, flight, soap tunnel

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BIOMECHANICS OF SWIMMING AND FLIGHT

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JEB 2008 211: i. [Full Text]  

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