Hysteresis at low Reynolds number: onset of two-dimensional vortex shedding

Hysteresis has been observed in a study of the transition between laminar flow and vortex shedding in a quasi-two-dimensional system. The system is a vertical, rapidly flowing soap film which is penetrated by a rod oriented perpendicular to the film plane. Our experiments show that the transition from laminar flow to a periodic von Karman vortex street can be hysteretic, i.e., vortices can survive at velocities lower than the velocity needed to generate them.