ABSTRACT:

ANSYS multi-physics software was applied to a Fluid-Structure Interaction (FSI) task, a large-scale riser with vortex-induced vibration (VIV) undergoing shear inflow environment. The purpose of this paper is to evaluate the performance of the FSI software and to provide the model test some supplementary information, such as the hydrodynamic visualizations of vortex wake and some dynamic characters of the pipe. Basic frequency analysis has been done first to make sure that the ANSYS FSI solution strategy is competent to give a reliable simulation to this pipe system, and then the RMS value of instantaneous amplitude and vortex shedding visualization under different input velocities are provided. Visualization of the vortex wake indicates a hybrid mode of vortex shedding along the pipe in those simulation cases. In this paper, an instantaneous amplitude definition is introduced to give a reasonable conception of RMS for instantaneous amplitude of multi-model VIV.

INTRODUCTION

Flow around a fixed or oscillating cylinder has received much attention in the past few decades. With the development of modern ocean engineering, the need to enhance our knowledge about vortex-induced vibration (VIV) for elastic pipe has greatly risen due to deep-water oil extraction and massive use of underwater cables. These flexible risers/pipes are readily subjected to shear and oscillatory flows due to currents and waves with high degree of complexity, and with intensity and direction changes according to water depth. Among those influencing factors, vortex-induced vibration plays a leading role in determining the life span of marine risers. A better comprehension of the vortex dynamics causing vibration and fatigue to risers is necessary. Semi-empirical models such as Wake-oscillator models, Sdof models and Force-decomposition models could give reasonable numerical simulation results in a special Reynolds number range, but can not provide valuable visualization of fluid domain.

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