STABILITY OF VORTEX SHEDDING MODES IN THE WAKE OF A RING AT LOW REYNOLDS-NUMBERS

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