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

Research output: Contribution to journalArticle

Abstract

The dynamics of swimming fish and flapping flags involves a complicated interaction of their deformable shapes with the surrounding fluid flow. Even in the passive case of a flag, the flag exerts forces on the fluid through its own inertia and elastic responses, and is likewise acted on by hydrodynamic pressure and drag. But such couplings are not well understood. Here we study these interactions experimentally, using an analogous system of flexible filaments in flowing soap films. We find that, for a single filament (or 'flag') held at its upstream end and otherwise unconstrained, there are two distinct, stable dynamical states. The first is a stretched-straight state: the filament is immobile and aligned in the flow direction. The existence of this state seems to refute the common belief that a flag is always unstable and will flap. The second is a flapping state: the filament executes a sinuous motion in a manner akin to the flapping of a flag in the wind. We study further the hydrodynamically coupled interaction between two such filaments, and demonstrate the existence of four different dynamlcal states.

Original languageEnglish (US)
Pages (from-to)835-839
Number of pages5
JournalNature
Volume408
Issue number6814
DOIs
StatePublished - Dec 14 2000

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