The CFD simulations of vortex-induced vibrations of a flexible cylinder in oscillatory flow have been numerically investigated based on the strip theory. The algorithm PIMPLE in OpenFOAM is adopted to compute the flow field while the small displacement Bernoulli-Euler bending beam theory is used to model the cylinder. Two ends of the flexible cylinder are forced to oscillate harmonically. Features such as the hysteresis between the decelerating and the accelerating stages and the build-up-lock-in-die-out cycle are observed.
Deep water structures can experience Vortex-Induced Vibrations (VIV) when exposed to marine currents. The engineering challenges of VIV for marine risers are of great concern as oil exploration moves to increasingly deeper waters. Experimental studies of vortex-induced vibrations of long flexible cylinders (the riser pipes) subjected to uniform, sheared, and stepped current profiles have been undertaken (Chaplin et al., 2005; Huera-Huarte et al., 2006; Huera-Huarte & Bearman, 2009). Complex phenomena such as multi-mode responses or time-sharing properties and traveling waves along the span of cylinders have been observed.
In practice, offshore floating structures subjected to waves, currents or winds may cause the cylinders to move periodically in the water, then to generate relatively oscillatory flows between the cylinders and the water. Such relative oscillatory flows can play an important part in the vibrations of the flexible cylinders. For a steel catenary riser, the heaving of the floating platform can form relatively vertical oscillatory flows between the sag-bend of the riser and the water. The vibrations of cylinders in this type of time-varying currents can be intricate and quite different from those in constant flows. The time-varying current speeds imply that the vortex shedding frequencies keep going up and down (excitation frequencies). When the vortex shedding frequencies meet one of the cylinder's natural frequencies, lock-in or resonance phenomena occur.
The vibrations of short rigid cylinders in elastic supporting conditions experiencing oscillatory flows have been extensively studied (Sumer & Fredse, 1997). However, only a few investigations into flexible cylinders such as Gonzalez (2001) have been conducted. Gonzalez (2001) numerically simulated the vibrations of a steel catenary riser in oscillatory flows using the Finite Element Method. A model test was also undertaken but only the top tensions were measured. Recently, a series of detailed model tests of a flexible cylinder in oscillatory flows have been conducted in Shanghai Jiao Tong University (Wang et al., 2014; Fu et al., 2014).
