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Vortex rings are an essential flow phenomenon in the natural and industrial flows. Vortex rings can be formed by the roll-up of a vortex sheet. In the experiments, they are usually generated in starting jets by piston–cylinder equipment [1].

Numerical simulations for the classical and synthetic jets under various Strouhal numbers at different Reynolds numbers (Re=100, 650, 1500) are performed to investigate the effect of interaction of the leading vortex with the second vorticity during the evolution process. The velocity and vorticity fields for the starting jet are obtained by using large eddy simulation (LES) method. It is found that the second vorticity can be generated in the classical jet due to interaction between the shear layer and trailing jet after the leading vortex pitches off (Re≥650).

When Re<650, the flow field for the classical jet will become stable, and only the leading vortex can be formed in the vorticity field. However, when Re≥650, the periodic pitching off of the second vorticity can be generated owing to the small disturbance among the vorticity, shear layer as well as instability of the trailing jet. At Re=1500, the second vortex will be merged into the leading vortex, the interaction between leading and second vortex will become more apparent.

When St>0.11, the leading vortex will begin to merge with the second vortex at Re=100, which is different from the classical jet. The whole process indicates the physical separation from the trailing jet for the main vortex [2]. Owing to the effect of the instability of shear layer and trailing jet, the moment of separation for the leading vortex and merging between the leading vortex and second vortex not only is actually sensitive to the Reynolds number, but it also shows a closely relationship with the Strouhal number in synthetic jets.