Symmetry breaking leads to forward flapping flight
|NICOLAS VANDENBERGHE a1a2, JUN ZHANG a2a3 and STEPHEN CHILDRESS a2|
a1 Center for Studies in Physics and Biology, The Rockefeller University, NY 10021, USA
a2 Applied Math Laboratory, Courant Institute of Mathematical Sciences, New York University, NY 10012, USA
a3 Department of Physics, New York University, NY 10003, USA
Flapping flight is ubiquitous in Nature, yet cilia and flagella, not wings, prevail in the world of micro-organisms. This paper addresses this dichotomy. We investigate experimentally the dynamics of a wing, flapped up and down and free to move horizontally. The wing begins to move forward spontaneously as a critical frequency is exceeded, indicating that ‘flapping flight’ occurs as a symmetry-breaking bifurcation from a pure flapping state with no horizontal motion. A dimensionless parameter, the Reynolds number based on the flapping frequency, characterizes the point of bifurcation. Above this bifurcation, we observe that the forward speed increases linearly with the flapping frequency. Visualization of the flow field around the heaving and plunging foil shows a symmetric pattern below transition. Above threshold, an inverted von Kármán vortex street is observed in the wake of the wing. The results of our model experiment, namely the critical Reynolds number and the behaviour above threshold, are consistent with observations of the flapping-based locomotion of swimming and flying animals.
(Received December 2 2003)
(Revised February 9 2004)