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Aspect ratio affects the equilibrium altitude of near-ground swimmers

Published online by Cambridge University Press:  28 April 2021

Qiang Zhong Open the ORCID record for Qiang Zhong [Opens in a new window] ,
Tianjun Han Open the ORCID record for Tianjun Han [Opens in a new window] ,
Keith W. Moored Open the ORCID record for Keith W. Moored [Opens in a new window]  and
Daniel B. Quinn Open the ORCID record for Daniel B. Quinn [Opens in a new window]
Qiang Zhong*
Affiliation:
Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA22904, USA
Tianjun Han
Affiliation:
Department of Mechanical Engineering, Lehigh University, Bethlehem, PA18015, USA
Keith W. Moored
Affiliation:
Department of Mechanical Engineering, Lehigh University, Bethlehem, PA18015, USA
Daniel B. Quinn
Affiliation:
Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA22904, USA Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA22904, USA
*
†Email address for correspondence: qz4te@virginia.edu
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Abstract

Animals and bio-inspired robots can swim/fly faster near solid surfaces, with little to no loss in efficiency. How these benefits change with propulsor aspect ratio is unknown. Here we show that lowering the aspect ratio weakens unsteady ground effect, thrust enhancements become less noticeable, stable equilibrium altitudes shift lower and become weaker and wake asymmetries become less pronounced. Water-channel experiments and potential flow simulations reveal that these effects are consistent with known unsteady aerodynamic scalings. We also discovered a second equilibrium altitude even closer to the wall (${<}0.35$ chord lengths). This second equilibrium is unstable, particularly for high-aspect-ratio foils. Active control may therefore be required for high-aspect-ratio swimmers hoping to get the full benefit of near-ground swimming. The fact that aspect ratio alters near-ground propulsion suggests that it may be a key design parameter for animals and robots that swim/fly near a seafloor or surface of a lake.

JFM classification

Biological Fluid Dynamics: Swimming/flying Biological Fluid Dynamics: Propulsion
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 917 , 25 June 2021 , A36
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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Zhong et al. supplementary movie

Aspect ratio affects wake topology near the ground. Three-dimensional PIV animation of AR = 2 and AR = 1 cases.

Download Zhong et al. supplementary movie(Video)
Video 3.8 MB