Stability of vortex shedding modes in the wake of a ring at low Reynolds numbers

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Abstract
The vortex street behind a ring of circular cross section and large aspect ratio is investigated experimentally. Different modes of annular and helical vortex shedding are identified by phase and frequency measurements. Their stability domains overlap in a large interval in Reynolds number where mode selection depends on initial conditions only. A new instability of the vortex shedding process involving characteristic mode transitions has been observed. This instability can be explained in the context of a Ginzburg-Landau model by a mechanism resembling formally the Eckhaus instability of spatially periodic patterns.