Visualization observations are reported for the time evolving vortex structure at the interface of a counter-current channel flow. The results are primarily qualitative in nature, but show clearly the evolution of a two-dimensional saddle-point flow into a complex, three-dimensional, periodic vortex-pairing process. Generation, pairing and collapse of the flow structure at the free interface is attributed to tilting and stretching of transverse vorticity by the main flow in the longitudinal direction. It is suggested that the present flow configuration may prove of use for investigating mixing in chemically reacting flows due to entraining vortex motions.

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