The dynamics of a flexible loop in a high-speed flow

We study the behavior of an elastic loop in a fast-flowing soap film. The loop is wetted into the film and is held fixed at a single point against the oncoming flow. We interpret this system as a 2D closed flexible body moving in a quasi-2D flow. The loop is deformed by the flow, and this coupled fluid-structure system shows bi-stability: stationary and oscillatory. In its stationary state, the loop essentially remains motionless and its wake is a von K\'{a}rm\'{a}n vortex street. In its oscillatory state, the loop sheds two vortex dipoles within each oscillation period. The frequency of oscillation of the loop is linearly proportional to the flow velocity.