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Shape of retracting foils that model morphing bodies controls shed energy and wake structure

Published online by Cambridge University Press:  20 September 2016

S. C. Steele ,
J. M. Dahl ,
G. D. Weymouth  and
M. S. Triantafyllou
S. C. Steele*
Affiliation:
Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
J. M. Dahl
Affiliation:
Ocean Engineering, University of Rhode Island, 215 South Ferry Road, Narragansett, RI 02882, USA
G. D. Weymouth
Affiliation:
Southampton Marine and Maritime Institute, University of Southampton, University Rd, Southampton SO17 1BJ, UK
M. S. Triantafyllou
Affiliation:
Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
*
†Email address for correspondence: s_chin@mit.edu
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Abstract

The flow mechanisms of shape-changing moving bodies are investigated through the simple model of a foil that is rapidly retracted over a spanwise distance as it is towed at constant angle of attack. It is shown experimentally and through simulation that by altering the shape of the tip of the retracting foil, different shape-changing conditions may be reproduced, corresponding to: (i) a vanishing body, (ii) a deflating body and (iii) a melting body. A sharp-edge, ‘vanishing-like’ foil manifests strong energy release to the fluid; however, it is accompanied by an additional release of energy, resulting in the formation of a strong ring vortex at the sharp tip edges of the foil during the retracting motion. This additional energy release introduces complex and quickly evolving vortex structures. By contrast, a streamlined, ‘shrinking-like’ foil avoids generating the ring vortex, leaving a structurally simpler wake. The ‘shrinking’ foil also recovers a large part of the initial energy from the fluid, resulting in much weaker wake structures. Finally, a sharp edged but hollow, ‘melting-like’ foil provides an energetic wake while avoiding the generation of a vortex ring. As a result, a melting-like body forms a simple and highly energetic and stable wake, that entrains all of the original added mass fluid energy. The three conditions studied correspond to different modes of flow control employed by aquatic animals and birds, and encountered in disappearing bodies, such as rising bubbles undergoing phase change to fluid.

JFM classification

Biological Fluid Dynamics: Biological Fluid Dynamics Vortex Flows: Vortex shedding Wakes/Jets: Wakes
Type
Papers
Information
Journal of Fluid Mechanics , Volume 805 , 25 October 2016 , pp. 355 - 383
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Copyright
© 2016 Cambridge University Press 

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Steele et al. supplementary movie

Wake visualization using simulation results of a retracting square-tipped foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 3.4 MB

Steele et al. supplementary movie

Wake visualization using simulation results of a retracting square-tipped foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 991.5 KB

Steele et al. supplementary movie

Wake visualization using simulation results of a retracting streamlined-tipped foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 2.9 MB

Steele et al. supplementary movie

Wake visualization using simulation results of a retracting streamlined-tipped foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 669.3 KB

Steele et al. supplementary movie

Wake visualization using simulation results of a retracting hollow foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 3.1 MB

Steele et al. supplementary movie

Wake visualization using simulation results of a retracting hollow foil. Within each panel, left side shows vortex cores colored by the intensity; right side shows non-dimensional vorticity. Flow is from left to right; foil is retracted in the vertical direction.

Download Steele et al. supplementary movie(Video)
Video 708.2 KB