Numerical investigation of the influence of span-wise force variation in circular cylinders undergoing vortex induced vibrations at high Reynolds number

The focus of this research is on the development of a new approach for simulating vortex induced vibrations on marine risers at high Reynolds numbers. This method considers the span-wise variation of the lift and drag forces, and determines the moment acting on the cylinder. The predicted motion then consists of a rotational component to accompany the traditional cross-stream and stream-wise translations normally associated with vortex induced vibrations. This was accomplished by describing the motion of the cylinder using a set of springs and dampers. A moment acting on the cylinder causes the springs on one end to compress, and stretch on the other, thus rotating the cylinder. A Large Eddy Simulation (LES) computational fluid dynamics code running on 16 3Ghz processors was used to calculate the unsteady flow and at each time step the hydrodynamic forces acting on the cylinder were calculated in a separate routine based on the pressure distribution around the cylinder. This information was then used to solve two second-order ordinary differential equations, which gave the velocity and displacement of the cylinder in cross-flow and rotational planes. This information was transferred back to the code where the cylinder was displaced and another cycle of calculations was started. The simulated results showed that the correlation length was higher for a cylinder subject to pure translation compared to a cylinder free to translate and rotate in the cross-stream direction. This has implications for current numerical and experimental techniques since it has been traditionally assumed that the flow around a circular cylinder becomes two-dimensional during vortex induced vibrations. Consequently, empirical,numerical and experimental models have generally only considered cross stream and/or stream-wise translation. The extent to which the experimental apparatus or harmonic model may have influenced the behavior of the riser by eliminating span-wise amplitude variation is important information that should be considered for future riser designs.