海洋工程小尺度物体水动力数值计算方法研究

发布时间：2019-05-26 20:57

【摘要】：孤立桩柱、导管架平台、海底管线等是海洋工程中普遍存在的结构物,当其被海浪包围部分的横向尺寸D与波长L之比小于0.2一般称为小尺度结构物。水流和波浪是海洋工程中最主要的两种外在荷载,关于水流和波浪与海洋工程小尺度构筑物相互作用问题一直是人们研究的重点,也是海洋工程中尚未很好解决的主要问题之一。高性能计算机将一种非常重要的和新的研究方法—计算流体力学方法引入到流体力学中,从而产生了研究流体运动规律的“第三种方法”—计算流体力学。利用高性能电子计算机,能够克服理论分析和实验研究的缺点,深化对于流体运动规律的认识并提高解决工程实际问题的能力。计算流体力学数值模拟能帮助理解流体力学问题,为实验提供指导,为设计提供参考,从而节省人力、物力和时间,给出详细和完整的资料,很容易模拟特殊尺寸、高温、有毒、易燃等真实条件和实验中只能接近而无法达到的理想条件。本文在考虑流体的粘性、湍流和自由液面流动等前提条件下,选取海洋工程中的小尺度物体相关水动力问题作为研究内容,选择了浸入边界法和流体体积法相结合的数值计算模型进行数值计算,分别给出了浸入边界法和流体体积法两种方法的数值表达、求解步骤和方法验证。提出了一种浸入边界法中通过直接求解外加力源项的数值计算方法。给出了浸入边界法中连续力法和离散力法两种不同外加力源项处理方式的优缺点和计算步骤,离散力法采用直接在物面附近的流场节点上求解作用力源项,而不通过插值、外推的方式来求解力源项,本文采用离散力法类浸入边界法进行数值计算。详细给出了建立浸入边界法数值模型的实现过程,采用有限差分法对控制方程进行离散和半隐式的两步式投影法对N-S方程进行求解,并给出了外加力源项fin+1的数值计算方法,最后给出了浸入边界法的数值模型的数值实现过程。通过数值计算层流状况下的固定圆柱绕流和旋转圆柱绕流经典算例对于本文建立的数值模型进行验证,将数值计算结果与其他人的试验和数值结果进行对比,结果对比良好,从而证明建立的数值计算模型正确可行。详细论述了VOF的界面重构方法和用于界面捕捉的基本思想,建立VOF方程。数值计算中选取的时间步长必须满足一定的要求,为保持数值计算的稳定性本章给出了利用VOF方法求解时的三个限定条件。为检验数值模型的可靠性,文中以复杂自由表面湍流动问题为例对其进行数值验证,主要对二维矩形液舱受迫运动时的流体晃荡问题和溃坝流动的层流问题验证。建立了一种基于浸入边界法(IBM)和流体体积法(VOF)的数值计算模型。将建立的浸入边界法(IBM)和流体体积法(VOF)相结合的数值计算模型进行了数值验证,主要通过线性周期波通过梯形物体和孤立波通过矩形物体两个算例进行验证,将本文的数值计算结果与已有结果对比从而证明本文建立的数值模型正确可靠,可以很好的求解波浪与结构物的相互作用。然后利用建立的数值模型对波浪作用下近壁圆柱进行了数值计算,该模型可较好的模拟波浪作用下的近壁面圆柱绕流过程,相比于目前仅可模拟稳定流与结构物作用的模型,该模型解决了近壁圆柱波浪作用过程的数值模拟问题,为进一步研究近壁面圆柱在波浪作用下的水动力特性提供了基础。波浪作用下的近壁面圆柱绕流的涡脱落模式是“P+S”,与已有实验结果相符。基于松弛造波法开发了一个适用于求解波浪作用问题的求解器waveFoam,建立了一种三维数值波浪水槽模型。该模型利用开源数值计算软件OpenFoam,通过利用OpenFoam中已有的求解器interFoam开发了新的求解器,该模型可有效的实现数值波浪水槽造波、传播和消波等过程,利用该数值水槽可成功的解决了直立圆柱波浪爬升模拟问题。选用两种不同的波浪参数,探讨了波陡参数对柱体周围的波浪爬升效应及其所受载荷的影响。与势流模型相比,本文采用的粘性流模型可以较好的捕捉实验中出现的二次波峰现象。计算表明,本文建立的数值水槽可以较好用于直立圆柱波浪爬升问题的计算。
[Abstract]:The isolated pile, the jacket platform, the submarine pipeline and the like are the ubiquitous structures in the ocean engineering, and the ratio of the transverse dimension D to the wavelength L when it is surrounded by the sea wave is less than 0.2, which is generally referred to as a small-scale structure. Water flow and wave are the main two kinds of external load in ocean engineering, and the interaction of water flow and wave and small-scale structures of ocean engineering has been the focus of people's research, and is one of the main problems that have not been solved well in the ocean engineering. A very important and new method of computational fluid dynamics is introduced into the fluid mechanics by a high-performance computer, which results in the "the third method" of fluid dynamics and the computational fluid dynamics. With high-performance electronic computer, it is possible to overcome the shortcomings of theoretical analysis and experimental research, to deepen the understanding of the law of fluid movement and to improve the ability to solve the practical problems of the project. The computational fluid dynamics numerical simulation can help to understand the fluid mechanics problem, provide guidance for the experiment and provide reference for the design, thus saving manpower, material resources and time, giving detailed and complete data, Flammable and other real conditions and the ideal conditions that can only be achieved in the experiment. In this paper, the hydrodynamic problem of small-scale objects in ocean engineering is selected as the research content under the precondition of considering the viscosity, turbulence and free liquid level flow of the fluid, and the numerical calculation model of the combination of the immersion boundary method and the fluid volume method is selected for numerical calculation. The numerical expression, the solution and the method validation of the two methods of the immersion boundary method and the fluid volume method are respectively given. In this paper, a method for calculating the numerical value of an applied force source term is proposed in the immersion boundary method. In this paper, the advantages and disadvantages of the method of continuous force method and discrete force method in the immersion boundary method are given, and the calculation step and the discrete force method are used to solve the force source term directly on the flow field node near the object surface, and the force source term is not solved by means of interpolation and extrapolation. In this paper, the method of discrete force method is used for numerical calculation. In this paper, the process of establishing the numerical model of the immersion boundary method is given in detail. The two-step projection method is used to solve the control equation by means of a two-step projection method. Finally, the numerical solution of the numerical model of the immersion boundary method is given. In this paper, the numerical model is verified by the numerical calculation of the flow around the fixed cylinder and the flow of the rotating cylinder, and the numerical results are compared with the experimental and numerical results of the others, and the results are good. So that the established numerical calculation model is correct and feasible. In this paper, the interface reconstruction method of the VOF and the basic idea for the interface capture are discussed in detail, and the VOF equation is established. The time steps selected in the numerical calculation must meet certain requirements. In order to maintain the stability of the numerical calculation, the three limiting conditions are given in this chapter. In order to test the reliability of the numerical model, the numerical verification of the turbulent dynamic problem of the complex free surface is taken as an example, and the problem of fluid sloshing and the laminar flow of the dam-break flow during the forced motion of the two-dimensional rectangular liquid tank are mainly verified. A numerical calculation model based on the immersion boundary method (IBM) and the fluid volume method (VOF) is established. The numerical calculation model of the established immersion boundary method (IBM) and the fluid volume method (VOF) is used for numerical verification, which is mainly verified by two numerical examples of a rectangular object through a trapezoidal object and an isolated wave through a linear periodic wave, The numerical results of this paper are compared with the existing results to prove that the numerical model established in this paper is correct and reliable, and the interaction between the wave and the structure can be well solved. the numerical calculation of the near-wall cylinder under the action of the wave is then carried out by using the established numerical model, the model can simulate the flow of the near-wall cylinder under the action of the wave, and compared with the model which can only simulate the action of the steady flow and the structure at present, The model solves the numerical simulation problem of the wave action process of the near-wall cylinder, and provides a basis for further studying the hydrodynamic characteristics of the near-wall cylinder under the action of wave. The vortex shedding mode of the flow around the cylindrical wall under the action of wave is the "P+S", which is in line with the experimental results. A three-dimensional numerical wave water tank model is developed based on the relaxation wave-making method, which is suitable for solving the wave action problem. The model uses open source numerical calculation software, OpenFoam, to develop a new solver by using an existing solver, inter, of OpenFoam, which can effectively realize the process of wave-making, propagation and wave-elimination of the numerical wave water tank. And the numerical water tank can successfully solve the problem of vertical cylindrical wave climbing simulation. Two different wave parameters are used to study the effect of wave-steep parameters on the wave-climbing effect around the column and its load. Compared with the potential flow model, the viscous flow model adopted in this paper can capture the secondary peak in the experiment. The calculation shows that the numerical flume established in this paper can be used for the calculation of the climbing problem of the vertical cylindrical wave.
【学位授予单位】：中国海洋大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P75;P731.2

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