Coupling effects of top-end dynamic boundary with a planar trajectory of “8” on three-dimensional VIV characteristics

Abstract

The top-end dynamic boundary of riser is usually generated by the motion of the moored floating body in marine environment, inducing the oscillation of equivalent fluid velocity and mechanical features of the riser, and further affecting its vortex-induced vibration (VIV). In this paper, the double-degree-of-freedom (2DOF) top-end dynamic boundary with a trajectory of “8”, which widely exists in fluid-structure interactions of ocean engineering, is studied. A series of tests for a riser model with the aspect ratio of 250 are conducted under the combination of the top-end dynamic boundary and uniform flow, whose three-dimensional (3D) VIV characteristics and behaviors are investigated in detail. The effect of amplitude and frequency of “8”-shape dynamic boundary on VIV is discussed, following which the relationship and difference among four top-end boundaries of “8”-shape, in-line harmonic, cross-flow harmonic and static forms are compared and analyzed. The results indicate that some periodically oscillatory phenomena involving time-varying frequency, mode transition and amplitude modulation are induced by the “8”-shape dynamic boundary. The periodic effect of dynamic boundary on VIV shows orthogonality, which means the cross-flow dynamic boundary is mainly responsible to the oscillatory characteristics in in-line VIV while the in-line dynamic boundary corresponds to cross-flow VIV. At the same time, the in-line dynamic boundary has a certain disturbance on the in-line vibration. The amplitude and frequency responses of in-line VIV could be enhanced by “8”-shape dynamic boundary at low or medium reduced velocity, further growing with the increase of boundary amplitude and frequency.