Hydrodynamic Characters Of A Oscillating Near-wall Circular Cylinder In Steady Current

Marine pipelines are widely used for oil and gas transportation. They are oftenlaid directly on the seabed in intermediate to deep water depths for reasons ofeconomic. Although pipelines are designed to be founded, free spans may formeither because the seabed is uneven or because of local scour below pipelines. Whenexposed to flow action such a pipeline span may undergo vortex-induced vibrations(VIV), which is important for the fatigue life of pipelines. The problems about VIVof free span pipeline can be simplified to the concept that an oscillating near-wallcylinder undergoing steady current. The existing researches about hydrodynamics ofpipelines are mainly focused on stationary cylinder, but the hydrodynamics can betotally changed when pipelines undergo VIV as the case in engineering fields, so it’snecessary to understand the hydrodynamics of the oscillating cylinders. Besides,most forced oscillating experiments were based on wall-free cylinders andaccordingly suitable to predict VIV for riser, not pipeline. There were few near-wallcylinder forced oscillation experiments. Therefore, the forced oscillation tests of anear-wall cylinder are needed to be carried out.Based on the demands above, hydrodynamics of a near-wall circular cylinderoscillating in transverse direction in steady current were experimentally investigatedin this thesis. The effects of Reynolds number, gap ratio, oscillating frequency andamplitude on the hydrodynamics of the cylinder were studied. Numerical simulationof flow around a circular cylinder at very high Reynolds number was conducted.The main contents and conclusions are summarized as follows:(1) Summarized the methods and research status of VIV throughout the world, analyzed the mechanism of vortex shedding, explained some important parametersand concepts about VIV and reviewed some effects to the hydrodynamics of flowaround a circular cylinder.(2) Designed and conducted the near-wall circular cylinder forced oscillationexperiment. Experiment results indicate: a) it is reasonable to utilize the vortexinduced forces of the oscillating cylinder under low Reynolds number into theprediction of the vortex induced vibration of risers in real sea state when theReynolds numbers are in the same subcritical regime; b) the existence of near-wallinfluences the energy transfer between the structure and fluid significantly, whichmeans hydrodynamic coefficients based on free-wall cylinders may not suitable forpredicting VIV of pipelines; c) mean drag coefficient, oscillating drag coefficientand oscillating lift coefficient are all gained with the increasing of oscillatingamplitude for forced oscillation experiments about wall-free and near-wall cylinders.d) in near-wall cylinder forced oscillation experiments, the magnitude of high orderforce components are very small but still have large proportions in low frequencyregion at small G/D, which can’t be negligible.(3) Two-dimensional numerical simulations of flow around a smooth cylinderhad been performed by URANS equations with the standard k-ε turbulence model.Grid and time step size convergence study had been performed and evaluated. Theresults were compared with published numerical and experimental results. Theresults indicate that the deviations of the hydrodynamic quantities such as thetime-averaged drag coefficient, root-mean square value of lift coefficient, Strouhalnumber, the coefficient of pressure on the downstream point of the cylinder, theseparation angle, from those corresponding experimental data become smaller asReynolds numbers increases. The results show that the standard k-ε model withenhanced wall treatment appears to be applicable for higher Reynolds numberturbulence flow.