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Unsteady dynamics of rapid perching manoeuvres

Published online by Cambridge University Press:  13 February 2015

Delyle T. Polet ,
David E. Rival  and
Gabriel D. Weymouth
Delyle T. Polet
Affiliation:
Department of Mechanical Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada
David E. Rival
Affiliation:
Department of Mechanical Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada
Gabriel D. Weymouth*
Affiliation:
Southampton Marine and Maritime Institute, University of Southampton, Southampton SO17 1BJ, UK
*
†Email address for correspondence: G.D.Weymouth@soton.ac.uk
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Abstract

A perching bird is able to rapidly decelerate while maintaining lift and control, but the underlying aerodynamic mechanism is poorly understood. In this work we perform a study on a simultaneously decelerating and pitching aerofoil section to increase our understanding of the unsteady aerodynamics of perching. We first explore the problem analytically, developing expressions for the added-mass and circulatory forces arising from boundary-layer separation on a flat-plate aerofoil. Next, we study the model problem through a detailed series of experiments at $\mathit{Re}=22\,000$ and two-dimensional simulations at $\mathit{Re}=2000$. Simulated vorticity fields agree with particle image velocimetry measurements, showing the same wake features and vorticity magnitudes. Peak lift and drag forces during rapid perching are measured to be more than 10 times the quasi-steady values. The majority of these forces can be attributed to added-mass energy transfer between the fluid and aerofoil, and to energy lost to the fluid by flow separation at the leading and trailing edges. Thus, despite the large angles of attack and decreasing flow velocity, this simple pitch-up manoeuvre provides a means through which a perching bird can maintain high lift and drag simultaneously while slowing to a controlled stop.

JFM classification

Biological Fluid Dynamics: Biological Fluid Dynamics Biological Fluid Dynamics: Swimming/flying Vortex Flows: Vortex shedding
Type
Papers
Information
Journal of Fluid Mechanics , Volume 767 , 25 March 2015 , pp. 323 - 341
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Copyright
© 2015 Cambridge University Press 

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