Vortex-induced vibration of cylindrical structures

  • Enhao Wang

Student thesis: Doctoral Thesis

Abstract

Vortex-induced vibration (VIV) of cylindrical structures is a classical topic within fluid-structure interaction (FSI). In offshore engineering, it often causes the fatigue of slender structures, such as risers, mooring lines and pipelines. Detailed understanding of this FSI phenomenon and an efficient prediction of such self-excited and self-sustained oscillations are required for the reliable estimation of the fatigue damage and the development of VIV suppression techniques.Over the past few decades, VIV has been extensively studied and the majority of the existing publications in the literature are experiments or semi-empirical modelling. In contrast, FSI simulations by combining high-fidelity computational fluid dynamics (CFD) and computational structural dynamics (CSD) solvers have received less attention. The main objective of this thesis is to investigate VIV of elastically mounted rigid cylinders and flexible cylinders using fully three-dimensional (3D) FSI simulations. Apart from important VIV aspects, such as response amplitude, response frequency and fatigue damage etc., the present research is also focussed on the aspects which have not been fully addressed by previous studies such as correlation lengths and time-dependent 3D flow structures.Two-degree-of-freedom (2DOF) VIV of an elastically mounted circular cylinder with varying in-line (IL) to cross-flow (CF) natural frequency ratios (f* = fnx/fny) is first studied using a 3D CFD approach.Numerical simulation is carried out for a constant mass ratio m* = 2 at a fixed Reynolds number Re = 500. The reduced velocity Vr ranges from 2 to 12. Three natural frequency ratios are considered, i.e., f* = 1, 1.5 and 2. The structural damping is set to zero to maximise the response of the cylinder. The main objective of the first study is to investigate the effect of f* on the 2DOF VIV responses and the 3D characteristics of the flow. It is discovered that there is a significant increase in the vibration amplitude and the peak amplitude shifts to a higher reduced velocity when f* increases from 1 to 2. A single-peak cross-flow response is observed for the identical in-line and cross-flow mass ratios when f* = 2. Dual resonance is found to exist over the range of f* studied.The preferable trajectories of the cylinder in the lock-in range are counterclockwise figure-eight orbits, whereas clockwise orbits primarily occur in the initial branch. The number of clockwise orbits decreases as f* increases from 1 to 2. Oblique figure-eight trajectories appear at Vr = 6, 7 and 8 when f* = 1. The third harmonic component which is observed in the lift fluctuation increases with f*. The correlation decreases in the lock-in range and reaches its minimum value around the transition region between the lock-in and post-lock-in ranges.Three vortex shedding modes (2S, P + S and 2P) appear in the present simulation. A dominant P + S mode is associated with the oblique figure-eight trajectories. Variation of vortex shedding flows along the cylinder is observed leading to the poor correlation of the sectional lift forces.Then, a numerical investigation of VIV of a vertical riser subject to uniform and linearly sheared currents is presented. The model vertical riser tested at the MARINTEK by ExxonMobil is considered. The predicted numerical results are in good agreement with the experimental data. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the fatigue damage indices increase with the flow velocity. Dual resonance is found to occur at most of the locations along the riser. At some locations along the riser, a third harmonic frequency component is observed in the CF response and a frequency component at the CF response frequency is found in the IL response apart from the frequency component at twice the CF response frequency. The maj
Date of Award1 Oct 2015
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorAtilla Incecik (Supervisor) & Qing Xiao (Supervisor)

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