title: Heavy flags undergo spontaneous oscillations in flowing water.
abstract: By immersing a compliant yet self-supporting sheet into flowing water, we study a heavy, streamlined, and elastic body interacting with a fluid. We find that above a critical flow velocity a sheet aligned with the flow begins to flap with a Strouhal frequency consistent with animal locomotion. This transition is subcritical. Our results agree qualitatively with a simple fluid dynamical model that predicts linear instability at a critical flow speed. Both experiment and theory emphasize the importance of body inertia in overcoming the stabilizing effects of finite rigidity and fluid drag.
authors.affiliation: Applied Mathematics Laboratory, Courant Institute of Mathematical Sciences, New York University, 251 Mercer Street, New York, New York 10012, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Shelley
authors.forename: Nicolas
authors.initials: N
authors.lastname: Vandenberghe
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang