A circular cylinder subjected to forced oscillations at angle α with respect to the free stream shows a number of admissible modes of vortex formation synchronized with the body motion. These modes can be categorized into two basic groups: symmetrical vortex formation; and antisymmetrical vortex formation. Whereas there is a single symmetrical mode, there are four basic antisymmetrical modes. Three of these antisymmetrical modes show period doubling relative to the classical Kármán mode. This doubling arises from the symmetrical perturbation component induced by the cylinder motion at α ± 90°. Synchronization, i.e. phase-locking of the vortex shedding with the cylinder motion, is possible for all of these modes. It occurs even when streamwise (α = 0°) motion induces an antisymmetrical mode.

When synchronization does not occur, there is competition between the symmetrical and antisymmetrical modes; the near-wake structure successively locks-on to each mode over a defined number of cycles, abruptly switching between modes. The number of occurrences of each mode is a well-defined function of excitation frequency and angle α.

If, in contrast to steady-state motion of the cylinder, there is an initial transient motion, the transition between symmetrical and antisymmetrical modes has a markedly different character, emphasizing the importance of initial conditions. Abrupt onset of sinusoidal motion produces an initially synchronized symmetrical mode, which gradually decays to an antisymmetrical mode. The number of excitation cycles to onset of decay to antisymmetrical mode is highly repeatable. Moreover, the mechanism of decay of the near wake from the symmetrical to antisymmetrical mode can occur deterministically over a defined number of cycles.