New Insight into Flag-Flapping

A new French study demystifies some of what some laypersons call “flag flapping” and others call “the flapping of flags.” The researchers cite, among other influences, previous research done by Ig Nobel Prize winner L. Mahadevan (who was honored for exploring the physics of how sheets get wrinkled). Details are in:

Oblique Waves Lift the Flapping Flag,” Jerome Hœpffner [pictured here, with his head in the clouds] and Yoshitsugu Naka, Physical Review Letters, 107, 194502 (2011). The authors, at Institut Jean Le Rond d’Alembert, Paris and Ecole Centrale de Lille, France, report:

“We may invoke yet a third mechanism for lift through a phenomenon analog to the crack of the whip. This is discussed for flags in Ref. [5] as a ‘snapping event.’ When large waves propagating along the cloth reach the leech, the kinetic energy stored up is focused through reflection at the free boundary and induces a fast flip of the tip: the snap, responsible for quick and large tension event, which peaks at about 5 times the average pull. Now through obliquity, this snapping is directed upward at the peak as sketched in Fig. 4(c), and we may speculate that the snapping at a corner would be even more violent through focusing onto a smaller area. We note that the peak is typically where old flags show most wear.”

(Thanks to investigator Josh Kroll for bringing this to our attention.)