Numerical Simulations of Vortex-Induced Vibrations in Marine Riser Pipes and Circular Cylinders

Abstract

This thesis presents and discusses the results of two distinct investigations. The first is a Direct Numerical Simulation investigation of prescribed transverse oscillations of a two-dimensional circular cylinder in a fluid flow of Reynolds number 100. The second involves the numerical simulation of the Vortex-Induced Vibrations of long riser pipes in the sub-critical Reynolds number regime, using a strip theory code that employed a Large Eddy Simulation model. Before commencing the long riser investigation the code was thoroughly benchmarked against data from appropriate prescribed cross-stream oscillation experiments; the results of that benchmarking work are also presented in this thesis. The principal objectives of the low Reynolds number Direct Numerical Simulations were to use prescribed oscillations to explain phenomena that have been observed in free oscillation experiments, and also to investigate the different levels and types of synchronisation that exist between the cylinder and its wake in a given amplitude-frequency domain. It was found that the contour of zero hydrodynamic excitation closely matches the response envelopes reported from experimental and numerical investigations of the transverse Vortex-Induced Vibrations of lightly damped cylinders. Furthermore, the zero contour inferred that the maximum amplitude of free cross-stream vibration is 0.56 cylinder diameters in Reynolds number 100 flow, and the shape of the contour confirmed the existence of hystereses at low and high reduced velocities in free vibration. The present study also revealed two new coalesced shedding modes, here labelled C∗(2S) and C∗(P+S), that differ in their formation mechanism from the known C(2S) mode. In the benchmarking of the Large Eddy Simulation code at sub-critical Reynolds numbers a clear trend was observed in which the prediction of the flow physics was altered by changing the level of sub-grid scale turbulence dissipation in the code’s Smagorinsky turbulence dissipation model. It was found that by carefully tuning the level of turbulent dissipation the code could deliver very good predictions of the key physical quantities important in Vortex-Induced Vibrations; namely the component of the lift coefficient at the oscillation frequency and the phase angle by which this lift coefficient leads the cylinder displacement. Regarding the simulations of the Vortex-Induced Vibrations of a long model riser, it has been shown that responses in high modes of vibration at harmonics of the displacement-dominant response frequency (at 3 and 5 times the cross-stream displacement dominant frequency in the cross-stream direction and at 2 and 3 times the in-line displacement dominant frequency in the in-line direction) can be important with regard to the curvature variation along the riser, and can therefore contribute very significantly to the overall fatigue damage rate experienced by a riser undergoing VIV. Comparisons with experimental data in terms of maximum and mean displacements and modes and frequencies of vibration, were generally good for both uniform and linearly sheared flow profiles. Furthermore, it was observed that the majority of the responses involved travelling waves, even when the flow profile was uniform