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Anomalous Hydrodynamic Drafting of Interacting Flapping Flags

Leif Ristroph and Jun Zhang
Phys. Rev. Lett. 101, 194502 – Published 3 November 2008

Abstract

In aggregates of objects moving through a fluid, bodies downstream of a leader generally experience reduced drag force. This conventional drafting holds for objects of fixed shape, but interactions of deformable bodies in a flow are poorly understood, as in schools of fish. In our experiments on “schooling” flapping flags, we find that it is the leader of a group who enjoys a significant drag reduction (of up to 50%), while the downstream flag suffers a drag increase. This counterintuitive inverted drag relationship is rationalized by dissecting the mutual influence of shape and flow in determining drag. Inverted drafting has never been observed with rigid bodies, apparently due to the inability to deform in response to the altered flow field of neighbors.

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  • Received 3 April 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.194502

©2008 American Physical Society

Authors & Affiliations

Leif Ristroph1 and Jun Zhang2,*

  • 1Department of Physics, Cornell University, Ithaca, New York 14853, USA
  • 2Department of Physics, New York University, New York, New York 10003, USA and Applied Mathematics Laboratory, Courant Institute of Mathematical Sciences, New York University, New York, New York 10012, USA

  • *jun@cims.nyu.edu

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Issue

Vol. 101, Iss. 19 — 7 November 2008

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