QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

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

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JEB Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Lentink, D. Articles by van Leeuwen, J. L. Search for Related Content PubMed PubMed Citation Articles by Lentink, D. Articles by van Leeuwen, J. L.

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

View larger version (4K): [in this window] [in a new window]   Fig. 1. A graphical representation of the non-dimensional parameters of a sinusoidally flapping foil: dimensionless wavelength *, amplitude ratio A*, amplitude-based Strouhal number StA, geometric angle of attack amplitude A,geo, effective angle of attack amplitude A,eff, and stroke-averaged Reynolds number Re.

View larger version (19K): [in this window] [in a new window]   Fig. 2. Flapping mechanism as mounted on the soap-tunnel framework (further illustrated in Fig. 4). The flapping mechanism consists of a crank mechanism that generates a stroke and angle of attack amplitude that are 90° out of phase with respect to each other. The angle of attack amplitude is reduced with a series of pulleys. The stroke amplitude is reduced with the aid of a pantograph. The flapper is driven by a DC motor. We mounted a special dial-plate with one microswitch (four are drawn) on the motor housing. This switch is pressed by a disk with a small knob in a phase we predetermined with the dial; in this way the camera can be triggered in a specific phase of the stroke. The angle of attack and stroke amplitude can be varied independently by changing the distance between the motor and the sled, indicated by `set distance', and the arm length of the crankshaft (`set crank amplitude'), which is hidden under the motor house in its current position. Finally, the angle of the stroke plane of the foil can be set with respect to the free-stream direction by rotating the whole crank mechanism, which is indicated by `set body angle'; in this study it is zero (as drawn).

View larger version (4K): [in this window] [in a new window]   Fig. 3. Stroke kinematics generated with our flapping mechanism. The stroke kinematics deviates with magnitude +/–d from a sine with amplitude A.

View larger version (12K): [in this window] [in a new window]   Fig. 4. The soap-film tunnel is mounted in an inclined frame and driven by gravity. It consists of three sections: a divergent section (i), the constant width (60 mm) test section in which the foil flaps (ii) and a convergent section (iii). The soap reservoir (a) produces a constant head by using an overflow. The soap flows from the reservoir (a) through a tuning valve (b) and an oval nozzle made out of a plastic pipette (c). At the pipette (c) the soap film starts: it runs down, driven by gravity, between two 1 mm thick Nylon wires (d) into tunnel sections (i–iii). The Nylon wires are pulled apart with 0.2 mm Dynema fishing lines (e). Finally the soap is collected in a reservoir (f) and is drained into the main soap reservoir and pumped (P) back again to the top reservoir (a).

View larger version (96K): [in this window] [in a new window]   Fig. 5. Visualization of the evolution of vortex-wake topology and the attachment of the LEV for decreasing dimensionless wavelength *. The wake dynamics evolves from a wavy von Kármán wake (WK) into an aperiodic wake densely packed with large interacting vortices (A–G, left: overview wake, right, zoomed in on LEV). The soap film flows from left to right and all images have been taken mid-stroke during the downstroke. The leading edge vortex is indicated by LEV, a vortex pair by P, a single vortex by S, vortex tearing by t, and vortex merging by m. Note that the naming of the wakes is simplified and should be taken as a guideline: we have neglected a few tiny vortices that are shed at some advanced ratios for simplicity. (Note: A, *=24; B, *=10; C, *=7.9; D, *=6.8; E, *=6.3; F, *=4.5; G, *=4.0.)

View larger version (13K): [in this window] [in a new window]   Fig. 6. Summary of vortex-wake topologies, the attachment of the LEV and the number of shed LEVs and TEVs as a function of dimensionless wavelength *. Note that the effective angle of attack amplitude A strongly increases with decreasing *, which in part explains the increasing vortex size. The more densely packed wake at low * is a direct result of the smaller distances between the shed vortices. Solid lines (1–3) indicate bifurcations found in one movie sequence. Filled and open circles are for easy distinction between modes only.