Development Of Viscous Numerical Wave Tank And Its Application In Ocean Engineering

Wave tanks play very significant roles in the ocean engineering field. Investigation of most complicated hydrodynamic problems in ocean engineering can hardly be carried out without the help of wave tanks. In recent years, along with the development of Computational Fluid Dynamics(CFD) techniques, research on Numerical Wave Tanks(NWT) as well as wave-structure interaction has become an important topic in the field of hydrodynamics. Meanwhile, NWT has great significance to ocean engineering applications due to its importance as an essential supplementary means for experimental work in physical wave tank. It can not only be used to predict the hydrodynamic performance for offshore structures, but also provide detailed information of flow field to extend our understanding of the underlying physical mechanics of fluid flow, which is helpful to the design and optimization of offshore structures. The present study aims to develop a computational program which can solve problems taking into consideration both the fluid viscosity and the strong nonlinearity of wave-structure interaction. The program is then employed to construct a three-dimensional viscous NWT and study the wave-structure interaction in ocean engineering.Based on Open FOAM, an open source CFD software package, a three-dimensional viscous NWT is set up where incompressible two-phase flow is governed by Navier-Stokes equations and free surface is captured by the Volume of Fluid(VOF) method. On this basis, a hydrodynamic solver named naoe-FOAM-SJTU is established by integrating the NWT with a six-degree-of-freedom(6DOF) body motion module and a mooring system analysis module, both of which are developed by our CFD team. This solver can efficiently deal with complicated wave-structure interaction problems.A series of different benchmark cases is carried out to validate the reliability of the mathematical model and numerical methods in Open FOAM, and also to investigate the efficiency and accuracy of the solving process of Navier-Stokes equations and the VOF method. The results imply that the complicated fluid flow and strongly nonlinear free surface evolution can be well simulated by using Open FOAM. The validation work provides a basic guarantee for the development of the viscous NWT.A three-dimensional viscous NWT is set up based on a built-in solver in Open FOAM for incompressible viscous two-phase fluid flow. Efficient numerical wave-makers are realized by modeling the movement of physical piston and flap type wave makers or by specifying wave elevation and velocity at the inlet boundary. The wave damping functionality is also implemented to avoid the wave reflection from outlet boundary by adding a source term in the momentum equations. Furthermore, a comprehensive library including many wave theories and wave spectra is established, enabling the generation of different types of waves, including regular waves, irregular waves and even freak waves. Regular waves are first simulated to test the efficiency and accuracy of the wave maker and wave damping function. Influence of computational mesh on generated waves is then analyzed and the basic reference mesh size is also suggested. To further test the capability of the present NWT, several cases are carried out including the simulation of nonlinear evolution of regular waves, solitary waves, transient extreme waves, multidirectional irregular waves and focusing waves. Comparison of numerical results with corresponding wave theories and experimental data demonstrates that the present NWT is equipped with a powerful wave maker which is capable of generating different types of waves. Construction of the present NWT lays a good basis for the further study of wave-structure interaction problems in ocean engineering.Numerical studies of wave run-up phenomena on fixed structures are then carried out using the solver naoe-FOAM-SJTU. Firstly, wave run-up phenomena on a single vertical cylinder under regular waves and solitary waves are simulated separately. Obtained numerical results of wave force and wave run-up height agree well with the corresponding experimental data, showing that the solver naoe-FOAM-SJTU is reliable for dealing with wave-structure interaction problems and can provide reasonable and accurate prediction for both wave force and wave run-up height. Meanwhile, free surface evolution, vortex generation and damping are also studied and analyzed to give better understanding of viscous effects. Furthermore, numerical research of wave run-up and wave forces on multiple cylinders is performed. The influence of the incident wave angle and the gap between cylinders on wave run-up height and wave force are analyzed. The results show that the interference among cylinders increases both wave force and wave run-up height on front cylinders, and also changes the position where the largest wave run-up occurs. The numerical studies of wave run-up phenomena indicate that the solver naoe-FOAM-SJTU is efficient for dealing with wave-structure interaction problems, and can be adopted as a new means of predicting the hydrodynamic performance for offshore structures.Motion response of floating bodies in waves is also studied in present work, which is a hot topic of great interest in ocean engineering. Firstly, a two-dimensional wave tank is set up, in which the body motion responses under regular waves and focusing waves are carried out. The strongly nonlinear phenomenon of green water on deck is well simulated. The numerical results, including body motion responses and free surface evolution, agree well with the experimental data, demonstrating the capability of the solver naoe-FOAM-SJTU for handling the problems involving body motion and green water phenomena. By utilizing the mooring system analysis module, motion responses of a moored full-scale offshore platform under steep regular waves and strongly nonlinear freak waves are simulated. Platform motion responses are analyzed, as well as the strongly nonlinear free surface evolution and the effects of the mooring system on restricting platform responses.In the present work, the efficiency of present NWT is demonstrated through the simulation of different types of waves. The numerical studies of wave run-up, body motion response, and green water phenomena imply the powerful capability of the solver naoe-FOAM-SJTU, which can deal with complicated wave-structure interaction problems involving the influence of fluid viscosity and strong nonlinearity of free surface. Meanwhile, the present numerical wave tank and the solver naoe-FOAM-SJTU feature open source code and modularized functional design, which benefits further program optimization and extension of more functional modules.