Fluid dynamics: The physics of flapping

The straight-stretched (left) and flapping (right) states

Central to the dynamics of fluids is the interaction of fluid flows with deformable boundaries at high Reynolds number (the Reynolds number, Re, represents the ratio of inertia to viscous force, at high Re inertial forces dominate leading to turbulent flow). This class of problems spans the swimming of fish, insect flight and the flapping of flags. Even the simplest of these systems, the passive case of a flag flapping in the wind, has not been fully explained by fluid physicists. Zhang et al. have studied these interactions experimentally, using a flexible filament (the flag) in flowing soap films. Two distinct stable states are identified: the stretched-straight flag, analogous to gliding flight and the flapping flag state, analogous to swimming.

Flexible filaments in a flowing soap film as a model for one-dimensional flags in a two-dimensional wind
JUN ZHANG, STEPHEN CHILDRESS, ALBERT LIBCHABER, MICHAEL SHELLEY
Nature 408, 835-839 (14 December 2000)
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Swimming in Flatsea
GREG HUBER
Some of the simplest questions are the hardest to answer. Why flags flap is a long-standing puzzle that becomes easier to solve in a two-dimensional world.
Nature 408, 777-778 (14 December 2000)
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physics : Silk and soap show why flags flap
Physicists have hoisted a one-dimensional flag in a two-dimensional wind.
(14 December)

14 December 2000 table of contents