Vortex induced vibrations of long distributed structures (risers and mooring cables) is an inherently complicated phenomenon in which due to the riser multi-mode excitations, various combinations of traveling and standing wave patterns along the length is observed. These observations are made based on a series of model scale experiments conducted on a riser for both uniform and linearly sheared flow cases. In these model scale experiments, strain and acceleration measurements are conducted at selected points along the riser. The contour plots of amplitudes of oscillations in these experiments show a mainly traveling wave behavior for linearly sheared flow cases and a mainly standing wave one for the uniform flow cases. In order to model the vortex induced vibrations of the riser used in these experiments, a wake oscillator model is used. In this model, the riser is assumed to be a tensioned string and the wake dynamics is represented by a Van der Pol oscillator whose driving force is in parallel with the riser acceleration. Randomness in the current, added mass and lift coefficients is taken into account by considering random parameters for the wake oscillator model. By using the proper parameters in this wake oscillator model, its results can be compared with the experimental ones. The comparison is made in terms of dynamical behavior (traveling waves versus standing waves, amplitudes and frequencies of oscillations) as well as the fatigue life calculations. The statistics of fatigue life calculations based on the experimental reconstruction compares well with those of the model results showing that the theoretical model can predict fatigue damage of the riser fairly well.
A single van der pol wake oscillator model for coupled cross-flow and in-line vortex-induced vibrations
