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Bistability in the rotational motion of rigid and flexible flyers

Published online by Cambridge University Press:  26 June 2018

Yangyang Huang
Affiliation:
Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
Leif Ristroph
Affiliation:
Courant Institute of Mathematical Sciences, New York University, New York, NY 10012, USA
Mitul Luhar
Affiliation:
Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
Eva Kanso*
Affiliation:
Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
*
Email address for correspondence: [email protected]

Abstract

We explore the rotational stability of hovering flight in an idealized two-dimensional model. Our model is motivated by an experimental pyramid-shaped object (Weathers et al., J. Fluid Mech, vol. 650, 2010, pp. 415–425; Liu et al., Phys. Rev. Lett., vol. 108, 2012, 068103) and a computational $\wedge$-shaped analogue (Huang et al., Phys. Fluids, vol. 27 (6), 2015, 061706; Huang et al., J. Fluid Mech., vol. 804, 2016, pp. 531–549) hovering passively in oscillating airflows; both systems have been shown to maintain rotational balance during free flight. Here, we attach the $\wedge$-shaped flyer at its apex in oscillating flow, allowing it to rotate freely akin to a pendulum. We use computational vortex sheet methods and we develop a quasi-steady point-force model to analyse the rotational dynamics of the flyer. We find that the flyer exhibits stable concave-down ($\wedge$) and concave-up ($\vee$) behaviour. Importantly, the down and up configurations are bistable and co-exist for a range of background flow properties. We explain the aerodynamic origin of this bistability and compare it to the inertia-induced stability of an inverted pendulum oscillating at its base. We then allow the flyer to flap passively by introducing a rotational spring at its apex. For stiff springs, flexibility diminishes upward stability but as stiffness decreases, a new transition to upward stability is induced by flapping. We conclude by commenting on the implications of these findings for biological and man-made aircraft.

Type
JFM Papers
Copyright
© 2018 Cambridge University Press 

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