Abstract

Due to the effect of fluid viscosity exerted on the motions for a cylindrical structure, a series of complex changes will occur in the motion variations, especially for the motion of single degree of freedom. However, the computation accuracy of traditional computing methods has been limited, with no thought for the fluid viscosity. In this paper, a three-dimensional numerical wave tank is established based on the theory of viscous fluid mechanics. Firstly, a series of convergence analysis of different grid resolutions are carried out, and the parameters of numerical models are optimized. After that, taking the fluid viscosity into consideration, the heave motions of different cylinders with various sizes are investigated via the optimized models. Meanwhile, the equation of wave-cylinder coupling movement is established based on the potential flow theory and linear gravity water wave theory. And the analytical solution is obtained. Finally, through comparing the analytical solution and the numerical results, the basic moving laws of float has been validated and the variations of heave motion of cylindrical structures is analyzed systematically.

INTRODUCTION

Ocean energy is a kind of clean and renewable energy, and wave energy is one of the common ways to use ocean energy. As a new type of energy, which is environmentally friendly, renewable, easy to use and abundant in reserves, wave energy has gradually attracted people's attention. The exploitation and utilization of wave energy is mainly completed by wave energy power generation devices, and the oscillating float wave energy power generation device, as one of the most advantageous wave energy devices at present, has attracted more and more attention. The research on the motion characteristics and hydrodynamic characteristics of cylindrical float in waves is of great significance for the research of oscillating float wave energy generation device.

Since then, a large number of scholars have carried out studies on the oscillation of floats in waves. Based on the potential flow theory and considering the nonlinear free surface condition, Kent (2007) numerically studied the vertical forced oscillation of a submerged horizontal cylinder. Wu (2010) used the potential flow semi analytical method based on the eigenfunction expansion method to derive the hydrodynamic characteristics of floats with complex configurations, and then studied the wave energy capture characteristics of floats with different sizes. The traditional calculation of hydrodynamic force or hydrodynamic coefficient is mostly based on the potential flow theory. In recent research progress, the influence of viscosity is also taken into account. Yeung and Jiang (2011) used The Free-Surface Random- Vortex Method (FSRVM) to simulate the viscous motion by allowing the generation of vorticity, studied the influence of draft ratio and shape on the viscous damping and heave motion of the cylinder, and clarified the source of viscous damping. Yuan and Mao (2020) used the finite volume method (FVM) to study the influence of fluid viscosity on the force of horizontal cylinder under forced oscillation, and explained the reason for the difference between the results of viscous flow and potential flow under different fluid viscosity.

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