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Flow interactions lead to orderly formations of flapping wings in forward flight

Sophie Ramananarivo1, Fang Fang1,*, Anand Oza1,*, Jun Zhang1,2, and Leif Ristroph1

  • 1Applied Math Laboratory, Courant Institute, New York University, New York, New York, USA
  • 2Department of Physics, New York University, New York, USA and New York University Shanghai, Shanghai, China

  • *F.F. and A.O. contributed equally to this work.
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Phys. Rev. Fluids 1, 071201(R) – Published 15 November, 2016

DOI: https://doi.org/10.1103/PhysRevFluids.1.071201

Abstract

Classic models of fish schools and flying formations of birds are built on the hypothesis that the preferred locations of an individual are determined by the flow left by its upstream neighbor. Lighthill posited that arrangements may in fact emerge passively from hydro- or aerodynamic interactions, drawing an analogy to the formation of crystals by intermolecular forces. Here, we carry out physical experiments aimed at testing the Lighthill conjecture and find that self-propelled flapping wings spontaneously assume one of multiple arrangements due to flow interactions. Wings in a tandem pair select the same forward speed, which tends to be faster than a single wing, while maintaining a separation distance that is an integer multiple of the wavelength traced out by each body. When perturbed, these locomotors robustly return to the same arrangement, and direct hydrodynamic force measurements reveal springlike restoring forces that maintain group cohesion. We also use these data to construct an interaction potential, showing how the observed positions of the follower correspond to stable wells in an energy landscape. Flow visualization and vortex-based theoretical models reveal coherent interactions in which the follower surfs on the periodic wake left by the leader. These results indicate that, for the high-Reynolds-number flows characteristic of schools and flocks, collective locomotion at enhanced speed and in orderly formations can emerge from flow interactions alone. If true for larger groups, then the view of collectives as ordered states of matter may prove to be a useful analogy.

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Outline

Information

Phys. Rev. Fluids 1, 071201(R)– Published 15 November, 2016
  • Received 8 March 2016
  • Revised 27 May 2016
Article part of CHORUS

©2016 American Physical Society

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