ABSTRACT

This paper focuses on the inline force and bow wave height on a surface-piercing vertical cylinder moving at a constant forward speed in calm water and in waves. A Computational Fluid Dynamics (CFD) model has been developed using Caelus® open source code. A stationary mesh was developed and the wave libraries were modified to generate current of a speed equal to the velocity of the moving cylinder while still prescribing the desired wave profile. The capability of the CFD model has been illustrated by the very good agreement achieved against results from physical experiments performed in a towing tank for various conditions of increasing complexity.

INTRODUCTION

Cylindrical structural elements are commonly used in offshore and coastal engineering structures such as jacket, jackup and tension-leg platforms and marine pipelines. Therefore, extensive studies on such structures have been conducted over recent years for multiple objectives. For instance, Xiong et al. (2015) experimentally studied the inline force acting on a stationary truncated vertical cylinder of a constant diameter subjected to regular waves. They found that the measured force is influenced by the submerged depth of the cylinder, wave steepness and scatter parameter (function of wavelength and cylinder diameter). Li and Ye (1990) investigated lift and inline forces on a vertical cylinder due to random waves and currents using the Morison equation and found relationships between drag, lift inertia coefficients and Keulegan-Carpenter number. Similarity, Koterayama (1984) studied the hydrodynamic forces and coefficients of a submerged circular cylinder moving forward with a constant velocity in regular waves.

With advances in computational resources, Computational Fluid Dynamics (CFD) modelling has drawn the attention of several researchers interested in drag forces and flow behaviour around cylindrical structures. These models upon validation represent a viable and feasible tool to examine various designs and capture detailed physics. For example, Shao et al. (2013) compared the drag coefficient of a surface-piercing cylinder yawed at different angles in a steady flow and concluded that the drag coefficient differs significantly from that on a vertical cylinder, especially for yaw angles larger than 15°. Yan et al. (2015) investigated the forces acting on fixed and moving cylinders subjected to focused waves. In their study, a CFD model using OpenFOAM® open source code was used to mimic the experiments for the case where the cylinder was fixed. To avoid any negative effects a current might have on wave focusing time and/or location when replicating the more complex experiments of the moving cylinder in waves, they replaced the CFD model by a numerical model based on fully nonlinear potential theory. Suh et al. (2011) developed and validated, in very good agreement, a CFD model using CFDShip-Iowa to capture detailed flow field and vortex shedding for an interface piercing circular cylinder subjected to a uniform flow of medium Reynolds and Froude numbers. Using the validated model, Koo et al. (2010) extended this work to investigate the effect of Reynolds and Froude numbers. Kim et al. (2015) presented validated CFD results for hydrodynamic forces and pressure coefficients of flow past a circular cylinder at subcritical Reynolds numbers, then examined the effects of Reynolds number on the statistical characteristics of the cylinder wake and the shear-layer instability.

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