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Wake structure and thrust generation of a flapping foil in two-dimensional flow

Published online by Cambridge University Press:  28 December 2016

A. Andersen ,
T. Bohr ,
T. Schnipper  and
J. H. Walther
A. Andersen*
Affiliation:
Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
T. Bohr
Affiliation:
Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
T. Schnipper
Affiliation:
Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark Department of Mechanical Engineering and Center for Fluid Dynamics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
J. H. Walther
Affiliation:
Department of Mechanical Engineering and Center for Fluid Dynamics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark Swiss Federal Institute of Technology Zürich, Chair of Computational Science, ETH Zentrum, Clausiusstrasse 33, CH-8092 Zürich, Switzerland
*
†Email address for correspondence: aanders@fysik.dtu.dk
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Abstract

We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving, and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically, we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag–thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude, we find that the drag–thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period, in contrast to the common transition scenario in regions with inverted von Kármán wakes.

JFM classification

Biological Fluid Dynamics: Propulsion Vortex Flows: Vortex shedding Wakes/Jets: Wakes/Jets
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 812 , 10 February 2017 , R4
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Copyright
© 2016 Cambridge University Press 

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Footnotes

‡

Present address: Svend Ole Hansen ApS, Sct. Jørgens Allé 5 C, DK-1615 Copenhagen, Denmark.

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Andersen et al. supplementary movie

Pitching foil with StD = 0.08 and AD = 1.14.

Download Andersen et al. supplementary movie(Video)
Video 21.2 MB

Andersen et al. supplementary movie

Heaving foil with StD= 0.10 and AD = 1.80.

Download Andersen et al. supplementary movie(Video)
Video 27.6 MB