Using vortex strength wake oscillator in modelling of vortex induced vibrations in two degrees of freedom

A new wake oscillator model is established to predict the structural response characteristics of vortex induced vibration (VIV) in two degrees of freedom. Based on the two-dimensional potential flow approach, the streamwise and transverse fluctuating fluid forces acting on structure are simplified and quantified. The work-energy balanced between the fluid and the structure leads to establish the coupled dynamic model by introducing a displacement variable related to the strength of nascent vortex. Analysis and prediction of the amplitudes, frequencies and phase angles of x-y motions of a rigid 2-D circular cylinder, along with a comparison to the existing experimental results, show that the numerical solutions of the present model can qualitatively and quantitatively capture the important features of VIV. Moreover, the x-y trajectory displays a crescent shape at cross-flow approaching peak amplitude. The reduced-order model also used in predicting VIV response of a 3-D top tensioned riser undergoing a stepped current is presented. The simulation results highlight the combination of standing wave and traveling wave occurring in structure vibration. The trajectories exhibit various patterns of figure-of-eight orbital motions and phase angles between the in-line and cross-flow motion change more rapidly in the standing wave region.