Study On Numerical Simulation Methods For Hydrodynamics Of Small Scale Objects In Ocean Engineering

Isolated piles, jacket platforms and subsea pipelines are prevalent in marine engineering structures. When it is partially surrounded by wave transverse, the ration between dimension D and L wavelength of less than 0.2 is generally known as the small-scale structures. Currents and waves are the two most important external loads in ocean engineering. Interaction issue between currents waves and small-scale engineering structures issue has been a focus of research. And one of the main problems is not well solved in ocean engineering. The high speed digital computer has introduced a fundamental important new third approach in fluid dynamics — the approach of computational fluid dynamics. Make use of the new tool, it can overcome the disadvantages of theoretical and experimental research, deepen the understanding of the law for fluid movement and increase the ability to solve practical engineering problems. Computational fluid dynamics can help understand the hydrodynamics problems, provide guidance for the experiment and a reference for the design, save manpower, material and time, give a detailed and complete information, and it is easy to simulate special size, high temperature, toxic and flammable real-world conditions and ideal conditions which are beyond the experiment can be achieved.In this dissertation, considering the fluid viscosity, turbulence and free surface flows, the small scale objects in ocean engineering are selected to study relevant hydrodynamic with numerical model combination of immersed boundary method (IBM) and volume of fluid (VOF). Above two methods for numerical expression, solution procedure and validation are given. Advantages and disadvantages and calculation steps of two different force source term treatments in continuous and discrete force method of immersed boundary method (IBM) are also given. Force source term or boundary conditions in discrete force method are solved directly on the flow field node in the vicinity of the object plane, rather than to be solved by interpolation and extrapolation. The discrete force method of immersed boundary method is applied to numerical simulation. The implementation process of establishing immersed boundary method numerical model is presented in detail. Navier-Stokes equations are solved with the finite difference method and semi-implicit method for two step projection method. It is noted that the imposed terms fin+1 have to be obtained firstly. And the numerical computation cycle for each time step is given. Based on the established numerical model, a stationary cylinder and rotating cylinder under laminar flow are to be simulated. Compared with experimental and other’s numerical results, my numerical results are good results. It is concluded that the numerical scheme based on immersed boundary (IB) method is correct and feasible.In the integrated modeling effort, free surface motions are tracked with the Volume of Fluid (VOF) method including the use of a piecewise linear interface calculation (PLIC) scheme. Time step selected must meet certain requirements in order to maintain the stability of numerical simulation, so chapter4 gives three qualifiers. The numerical model (VOF) has been validated with two cases including two-dimensional rectangular tanks forced movement of sloshing and dam-break water flow problems.A numerical model based on the immersed boundary method (IBM) and the volume of fluid method (VOF) is established. Both the IBM and VOF methods have been validated separately by carrying out various case studies in two dimensions. In addition, the combined two dimensional IB-VOF model has also been rigorously validated by carrying out two case studies, including progressive periodic waves propagating over a submerged trapezoid and the simulations of a solitary wave passing over a shelf. The numerical results compared very well with the experimental data and numerical results reported by other researchers, including the details of free surface evolutions and the velocity field near the object. These case studies exhibit the model’s capability in predicting the nonlinear free surface movement. The well-validated IB-VOF model has been applied to simulate the progressive periodic waves propagating over a circular cylinder sitting on the bed. A vortex pair is formed in each half wave cycle and the vortex shedding mode is’P+S’.The new solver waveFoam for solving the problem of wave problem is developed based on relaxation method by using interFoam solver already existed in OpenFoam. A 3D numerical wave tank of the viscous fluid theory was created based on Navier-Stokes equations and continuity equations, used by VOF method to trace free surface and relaxation wave method to generate, propagate and absorb wave in open source software OpenFOAM. Meanwhile make use of the 3D wave tank to simulate wave run-up on a vertical cylinder. Two different wave parameters was adopted, wave steepness parameters were discussed for the wave effect and run-up around the vertical cylinder. Compared with the potential flow model, viscous flow model used in this paper can better capture the secondary crest phenomenon appeared in the experiments. Calculations show that the numerical tank established in this paper can be better used to solve the problem of wave run-up on cylindrical offshore structures.