The large scale vortices in the wake of a rigid cylinder undergoing forced oscillations at a constant amplitude are investigated throughout the synchronisation range by using a schlieren method. A precise description is given of the different regimes observed in the frequency range. The vortex shedding is locked to the cylinder oscillation so that the ratio between the two pulsations remains rational and keeps a constant value over the synchronisation range. The phase angle of the vortex shedding versus reduced velocity has been determined for every regime. It exhibits strong variations when the cylinder to vortex shedding frequencies ratio varies from 0.5 to 1.
The hydrodynamic forces applied to a cable normal to a flow result from a fluid-structure coupling between the wake, which generates an oscillating crosswise force, and the cable, which oscillates due to this force. The phenomenon becomes critical when the wake frequency is near a cable vibration mode, causing an increase in the vibration amplitude and sudden changes of phase between the cable movement and the unsteady crosswise force. When the wake frequency locks onto the cable oscillation frequency, there is a large increase in drag. Reviews of the research conducted in last years on the problem of synchronization of an oscillating obstacle with its wake were published by Parkinson (1974), Sarpkaya (1979), Bearman(1984) and Griffin and Hall (1991). These works showed that large phase variations between the unsteady lift Fy and the cylinder position Y appear when a cylinder is subjected to forced oscillation near synchronization. These changes of phase mean that the instant of the oscillation cycle for which the vortex shedding generates maximum lift varies enormously with the reduced speed. Contrary to the above properties, relatively well described. concerning the cylinder movement and applied forces, the near wake structure is still poorly known.