Experimental study on dynamic responses of a tensioned flexible vertical cylinder under waves and combined current and waves

The two degree-of-freedom (2DOF) dynamic responses of a flexible vertical cylinder under waves only and combined current and waves in a finite water depth were experimentally studied. The cylinder has multi-frequency response in both in-line (IL) and crossflow (CF) directions under waves. The response frequencies in the IL direction are multiples of wave frequency. The CF response frequencies include the multiples of wave frequency and the natural frequency of the cylinder, indicating that the CF response is controlled by both the wave actions and the vortex shedding. When the natural frequency of the cylinder is a multiple of the wave frequency, the CF vibration has the largest amplitude. A top tension can suppress the CF response more than the IL response amplitude, and the suppression effect increases with the increase of the top tension. With the decrease of the water depth, the response amplitudes in both directions decrease. Under combined current and waves, the IL response amplitude increases with the increase of KC number and the primary frequency of IL response is still the wave frequency. For a current velocity where VIV occurs, the superimposition of waves on current reduces the CF response amplitude because the vortex shedding frequency moves away from the natural frequency. The multi-frequencies of CF response are related to both the natural frequency of the cylinder and the wave frequency. The trajectories of the cylinder are mainly dependent on the ratio of the natural frequency to the wave frequency under waves or combined current and waves. There can be 2–6 cycles in the CF direction for each IL cycle with different wave frequencies and the top tension.