Modeling and nonlinear dynamics of fluid-conveying risers under hybrid excitations

The nonlinear dynamical responses of a vertical riser concurrently subjected to hybrid excitations, namely, vortex-induced vibrations (VIVs) and base excitations are investigated. The riser conveying fluid is placed in a uniform cross-flow and subjected to direct harmonic excitations. A van der Pol wake oscillator is used to model the fluctuating lift coefficient. The extended Hamilton’s principle and the Galerkin procedure are used to derive a nonlinear distributed-parameter model for a vertical riser under a combination of vibratory base excitations and vortex-induced vibrations. Linear and nonlinear analyses are performed to investigate the effects of internal fluid velocity, cross-flow speed, and base acceleration on the coupled frequency, onset speed of synchronization, and vibration amplitudes of the riser. The results show that when the cross-flow speed becomes in the synchronization region, vibration behaviors of the riser change from aperiodic to periodic motions, with a jumping phenomenon between these two kinds of motions. It is also demonstrated that the amplitude of the riser can be increased or decreased under combined effects of vortex-induced vibrations and base excitations compared to the separate effect of vortex-induced vibrations or base excitations. The results also show that an increase of the base acceleration results in a wider synchronization region and a significant effect associated with the quenching phenomenon.