Two-dimensional vortex-induced vibrations of cylindrical structures in oscillatory flows

Abstract

The vortex-induced vibration phenomenon is a critical aspect for the optimal design and reliability analysis of offshore structures subjected to waves and current loadings. Phenomenological models are able to capture some vortex-induced vibration (VIV) features and estimate structural responses within a range of uncertainty. In comparison with steady flow, the subject of VIV in oscillatory flow has not been much explored; hence, there is a necessity to advance numerical and experimental research in this field. This dissertation aims to develop a phenomenological model for a steady flow VIV and perform a sensitivity analysis, apply this model to the case of oscillatory flow VIV and perform parametric investigations, and design and set up a new experimental framework and perform VIV tests in regular waves. A sensitivity analysis, pertaining to the empirical coefficients of the nonlinear wake-cylinder oscillators model, which simulates the two-dimensional coupled in-line and cross-flow VIV of a flexibly-mounted rigid circular cylinder in steady flow, is first performed to relate the input physical parameters to output responses. Wake-oscillator coefficients, cylinder geometric nonlinearity coefficients and hydrodynamic coefficients are chosen as varying input parameters in the implementation of both local and global sensitivity methods. The variability of mean displacements, inline and cross-flow output responses is then obtained for various reduced velocities and mass ratios. The advanced wake-cylinder oscillator model is then applied to the case of two-dimensional VIV of a flexibly-mounted rigid circular cylinder in planar oscillatory flow. The time-domain simulation model is calibrated with CFD results from the literature and shows characteristics of two-dimensional VIV in oscillatory flows. Overall, both in-line and cross-flow responses highly depend on KC, Vr and fr (cylinder-to-flow frequency ratio). The effects of bi-parametric variations of Vr−m*, Vr−KC and Vr−fr are examined and the dependence of hydrodynamic drag, inertia and lift coefficients on the Reynolds number Re is also studied. A new experimental framework based on a spring-pendulum system is developed and experimental VIV tests in regular waves and in combined steady current and regular waves are performed in the wave-current tank of the Hydrodynamic Laboratory at Newcastle University. The experimental results shed light on the features of VIV in oscillatory flows, through response amplitudes, frequencies, and trajectory patterns.