Abstract: This paper discusses the dynamic behavior of the Kelvin-Stuart cat’s eye flow under periodic perturbations. By means of the Melnikov method the conditions to have bifurcations to subharmonics of even order for the oscillating orbits and to have bifurcations to subharmonics of any order for the rotating orbits are given, and further, the coexistence phenomena of the chaotic motions and periodic solutions are presented.

Key words: chaos, bifurcation, transverse, heteroclinic cycle, homoclinic orbit, cat’s eye flow, vortex