The effects of the slow change of the tidal current speed on the vortex-induced vibration of slender offshore structures with a circular section were experimentally examined by using an actively controlled wind tunnel. The amplitude of the vortex-induced vibration of the circular section in a sinusoidal flow speed fluctuation was also periodically varied with the same period of the fluctuating flow speed. Unsteady hydrodynamic forces in the fluctuating flow took some different aspects from those in the uniform flow. Observations of the fluctuating flow properties behind the circular cylinder showed that the intermittent lock-in of the vortex-shedding frequency to the natural frequency of the model played an important role.
For slender offshore structures, such as offshore oilrigs, piles and risers, etc, a vortex-induced vibration in tidal currents has been an important problem. It is well known that the vortex-induced vibrations of offshore structures are affected by the tidal current turbulence. However, the effect of the slow change of mean speed without any turbulence on the vortex-induced vibrations was not focused on. It was reported that even if the fluctuation speed of wind was extremely slow, the effects of wind speed fluctuation on the response were observed and quasi-steady forces were not applicable to the vortex-induced vibration in the slowly fluctuating flow.
In the paper, the effects of slow change of tidal current speed on the vortex-induced vibration of offshore structures with circular section are experimentally examined. In slow change of tidal current speed, where the fluctuating period of tidal current speed, T', is much longer than the natural period of offshore structures, To, e.g. T'=100*To, the response properties and the formation of unsteady hydrodynamic forces are experimentally investigated by using an actively controlled wind tunnel.
