Walking in the Wake of a Cylinder

A cylinder in a flow produces a series of alternating vortices known as a von Karman vortex street. Changing the flow speed and rotating the cylinder both allow researchers to tune the frequency of these shed vortices. What happens to an object in the wake?

For a simple hydrofoil tethered to the cylinder, the object wends back and forth along the vortices. But when that hydrofoil sits at the end of a double-pendulum, something very interesting happens. The whole apparatus follows a consistent trajectory similar to a human walking gait. Researchers are using this motion to build a robot that will help physical therapy patients regain a natural walking style. (Image and video credit: A. Carleton et al.)