Numerical Investigation of the Influence ofudSpan-wise Force Variation in Circular CylindersudUndergoing Vortex Induced Vibrations at HighudReynolds Number

摘要

The focus of this research is on the developmentudof a new approach for simulating vortex induced vibrations onudmarine risers at high Reynolds numbers. This method considersudthe span-wise variation of the lift and drag forces, and determinesudthe moment acting on the cylinder. The predicted motion thenudconsists of a rotational component to accompany the traditionaludcross-stream and stream-wise translations normally associatedudwith vortex induced vibrations. This was accomplished by describingudthe motion of the cylinder using a set of springs anduddampers. A moment acting on the cylinder causes the springs onudone end to compress, and stretch on the other, thus rotating theudcylinder.udA Large Eddy Simulation (LES) computational fluid dynamicsudcode running on 16 3Ghz processors was used to calculate theudunsteady flow and at each time step the hydrodynamic forcesudacting on the cylinder were calculated in a separate routine basedudon the pressure distribution around the cylinder. This informationudwas then used to solve two second-order ordinary differentialudequations, which gave the velocity and displacement of theudcylinder in cross-flow and rotational planes. This information wasudtransferred back to the code where the cylinder was displacedudand another cycle of calculations was started.udThe simulated results showed that the correlation length wasudhigher for a cylinder subject to pure translation compared to audcylinder free to translate and rotate in the cross-stream direction.udThis has implications for current numerical and experimentaludtechniques since it has been traditionally assumed that theudflow around a circular cylinder becomes two-dimensional duringudvortex induced vibrations. Consequently, empirical,numericaludand experimental models have generally only considered crossudstream and/or stream-wise translation. The extent to which theudexperimental apparatus or harmonic model may have influencedudthe behavior of the riser by eliminating span-wise amplitudeudvariation is important information that should be considered forudfuture riser designs.