Finite element methods on piecewise equidistant meshes for interior turning point problems

We consider linear second order singularly perturbed two-point boundary value problems with interior turning points. Piecewise linear Galerkin finite element methods are constructed on various piecewise equidistant meshes designed for such problems. These methods are proved to be convergent, uniformly in the singular perturbation parameter, in a weighted energy norm and the usualL ² norm. Supporting numerical results are presented.     

The Dual Singular Function Method for 2D Boundary Integral Equations

The accuracy of standard boundary element methods for elliptic boundary value problems deteriorates if the boundary of the domain contains corners or if the boundary conditions change along the boundary. Here we first investigate the convergence behaviour of standard spline Galerkin approximation on quasi-uniform meshes for boundary integral equations on polygonal domains. It turns out, that the order of convergence depends on some constant describing the singular behaviour of solutions near corner points of the boundary. In order to recover the full order of convergence for the Galerkin approximation we propose the dual singular function method which is often used for improving the accuracy of finite element methods. The theoretical convergence results are confirmed and illustrated by a numerical example.     

Singularity cancellation method for time-domain boundary element formulation of elastodynamics: A direct approach

In the implementation of time-domain boundary element method for elasto-dynamic problems, there are two types of singularities: the wave front singularity arising when the product of wave velocity and time is equal to the distance between the source point and the field point, and the spatial singularity arising when the source point coincides with the field point. In this paper, the singularity of the first type in the integrand is eliminated by an analytical integration over time, Cauchy principal value and Hadamard finite part integral. Four types of singularities with different orders appear in the integrand after analytical time integration. In order to accurately calculate the integral, in which the integrand is piecewise continuous, the integral domain is subdivided into several patches based on the relation between the product of wave velocity and time and the distance. In singular patches, the integrands are separated into a regular part and a singular part. The regular part can be computed by traditional numerical integration method such as Gaussian integration, while the singular part can be analytically integrated. Using the proposed method, the spatial singular integrals can be calculated directly. Numerical examples using various kinds of elements are presented to verify the proposed method.     

一类四阶奇异非线性椭圆方程的加权模误差估计

利用有限元方法研究了一类四阶奇异非线性椭圆方程,利用有限元的逆性质,给出了考虑数值积分影响时的加权H2模误差估计.     

Uniformly stable rectangular elements for fourth order elliptic singular perturbation problems

In this article, we consider rectangular finite element methods for fourth order elliptic singular perturbation problems. We show that the non‐ C⁰ rectangular Morley element is uniformly convergent in the energy norm with respect to the perturbation parameter. We also propose a C⁰ extended high order rectangular Morley element and prove the uniform convergence. Finally, we do some numerical experiments to confirm the theoretical results. © 2012 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2013     

Wave induced vibrations of gravity platforms: a stochastic theory

This paper presents a consistent stochastic method to evaluate wave induced vibrations of offshore platforms with special emphasis on gravity type structures. The wavy sea surface is idealized as a stationary and homogeneous stochastic Gaussian (or semi-Gaussian) field. The wave loading processes are treated by applying the concepts of probabilistic potential theory and the stochastic linearization method. The structure is modelled according to the finite element method using boundary spring-dashpot elements to idealize the soil-foundation system. The linearized equations of motion are solved by the frequency response method accounting properly for the temporal and spatial structure of waves as well as for time dependent system function. Numerical results for a 170m high concrete platform are presented including a parametric study of effects due to the shape of the wave spectral density.     

Analysis of split weighted least-squares procedures for pseudo-hyperbolic equations

In this paper, we introduce two novel split weighted least-squares finite element procedures for pseudo-hyperbolic equations arising in the modelling of nerve conduction process. By selecting the weighted least-squares functional properly, each procedure can be split into two independent symmetric positive definite sub-procedures. One of sub-procedures is for the primitive unknown variable, which is the same as the standard Galerkin finite element procedure and the other is for the introduced flux variable. Optimal order error estimates are developed and the numerical example is given to show the efficiency of the introduced schemes.     

一种高精度的裂纹奇异单元

基于广义伽辽金法的多变量有限元算法,增加了连续体力学有限元模型建立的灵活性.本文利用它,通过数值试验的对比建立了一种高精度的含奇异性的裂纹单元,并对多变量奇异元的构成进行了探讨.     

A posteriori error analysis for a continuous space-time finite element method for a hyperbolic integro-differential equation

An integro-differential equation of hyperbolic type, with mixed boundary conditions, is considered. A continuous space-time finite element method of degree one is formulated. A posteriori error representations based on space-time cells is presented such that it can be used for adaptive strategies based on dual weighted residual methods. A posteriori error estimates based on weighted global projections and local projections are also proved.     

Evaluation of singular integrals for anisotropic elastic boundary element analysis

To improve the numerical evaluation of weakly singular integrals appearing in the boundary element method, a logarithmic Gaussian quadrature formula is usually suggested in the literature. In this formula the singular function is expressed in terms of the distance between source point and field point, which is a real variable. When an anisotropic elastic solid is considered, most of the existing fundamental solutions are written in terms of complex variables. When the problems with holes, cracks, inclusions, or interfaces are considered, to suit for the shape of the boundaries usually a mapping function is introduced and then the solutions are expressed in terms of mapped complex variables. To deal with the trouble induced by the complex variables, in this study through proper change of variables we develop a simple way to improve the evaluation of weakly singular integrals, especially for the problems of anisotropic elastic solids containing holes, cracks, inclusions, or interfaces. By simple matrix expansion, the proposed method is extended to the problems with piezoelectric or magneto-electro-elastic solids. By using the dual reciprocity method, the proposed method employed for the elastostatic fundamental solution can also be applied to the elastodynamic analysis.     

非稳态奇异系数微分方程的时间间断时空有限元方法

针对一类二维单轴奇异系数非稳态问题构造了一种时间间断时空有限元格式,利用以Radau点为节点的Lagrange插值多项式的特性,结合有限差分法和有限元法的技巧证明了格式的稳定性和有限元解的时间最大模、空间加权L²（?）-模误差估计.最后列举了一些数值试验结果,验证了理论结果和格式的可行性.     

Augmented high order finite volume element method for elliptic PDEs in non-smooth domains: Convergence study

The accuracy of a finite element numerical approximation of the solution of a partial differential equation can be spoiled significantly by singularities. This phenomenon is especially critical for high order methods. In this paper, we show that, if the PDE is linear and the singular basis functions are homogeneous solutions of the PDE, the augmentation of the trial function space for the Finite Volume Element Method (FVEM) can be done significantly simpler than for the Finite Element Method. When the trial function space is augmented for the FVEM, all the entries in the matrix originating from the singular basis functions in the discrete form of the PDE are zero, and the singular basis functions only appear in the boundary conditions. That is to say, there is no need to integrate the singular basis functions over the elements and the sparsity of the matrix is preserved without special care. FVEM numerical convergence studies on two-dimensional triangular grids are presented using basis functions of arbitrary high order, confirming the same order of convergence for singular solutions as for smooth solutions.     

Hamilton系统的连续有限元法

利用常微分方程的连续有限元法,对非线性Hamilton系统证明了连续一次、二次有限元法分别是2阶和3阶的拟辛格式,且保持能量守恒;连续有限元法是辛算法对线性Hamilton系统,且保持能量守恒．在数值计算上探讨了辛性质和能量守恒性,与已有的辛算法进行对比,结果与理论相吻合．     

不可压缩流动的并行数值方法

不可压缩流动的数值模拟是计算流体力学的重要组成部分. 基于有限元离散方法, 本文设计了不可压缩Navier-Stokes (N-S)方程支配流的若干并行数值算法. 这些并行算法可归为两大类: 一类是基于两重网格离散方法, 首先在粗网格上求解非线性的N-S方程, 然后在细网格的子区域上并行求解线性化的残差方程, 以校正粗网格的解; 另一类是基于新型完全重叠型区域分解技巧, 每台处理器用一局部加密的全局多尺度网格计算所负责子区域的局部有限元解. 这些并行算法实现简单, 通信需求少, 具有良好的并行性能, 能获得与标准有限元方法相同收敛阶的有限元解. 理论分析和数值试验验证了并行算法的高效性     

用新的逼近空间求解奇性问题

用新的逼近空间求解奇性问题梁国平，孔林，何江衡（中国科学院数学研究所）ＡＮＥＷＦＩＮＩＴＥＥＬＥＭＥＮＴＳＰＡＣＥＦＯＲＳＩＮＧＵＬＡＲＩＴＹＰＲＯＢＬＥＭＳ￥ＬｉａｎｇＧｕｏ－ｐｉｎｇ；ＫｏｎｇＬｉｎ；ＨｅＪｉａｎｇ－ｈｅｎｇ（Ｉｎｓｔｉｔｕｔｅｏ...     

Semi‐analytical integration of the elastic stiffness matrix of an axisymmetric eight‐noded finite element

The finite element method (FEM) is a numerical method for approximate solution of partial differential equations with appropriate boundary conditions. This work describes a methodology for generating the elastic stiffness matrix of an axisymmetric eight‐noded finite element with the help of Computer Algebra Systems. The approach is described as “semi analytical” because the formulation mimics the steps taken using Gaussian numerical integration techniques. The semianalytical subroutines developed herein run 50[percnt] faster than the conventional Gaussian integration approach. The routines, which are made publically available for download,¹ should help FEM researchers and engineers by providing significant reductions of CPU times when dealing with large finite element models. © 2009 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2010     

Dual finite element analysis of three-dimensional axisymmetric elliptic problems. Part I

This paper consists of two parts in which we propose two types of pure dual finite element (FE) models of a three-dimensional (3D) axisymmetric elliptic problem with mixed boundary conditions. Using cylindrical coordinates and weighted Sobolev spaces, a dual 3D problem is transformed into a 2D problem and finite element spaces of divergence-free vector functions are constructed with the help of a stream function. In part I of the paper a priori and a posteriori error estimates are derived for the first type of the FE model. © 1993 John Wiley & Sons, Inc.     

A uniformly optimal‐order error estimate for a bilinear finite element method for degenerate convection‐diffusion equations

We prove an optimal‐order error estimate in a degenerate‐diffusion weighted energy norm for bilinear Galerkin finite element methods for two‐dimensional time‐dependent convection‐diffusion equations with degenerate diffusion. In the estimate, the generic constants depend only on certain Sobolev norms of the true solution but not the lower bound of the diffusion. This estimate, combined with a known stability estimate of the true solution of the governing partial differential equations, yields an optimal‐order estimate of the Galerkin finite element method, in which the generic constants depend only on the Sobolev norms of the initial and right side data. Preliminary numerical experiments were conducted to verify these estimates numerically. © 2011 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2011     

OPERATOR－SPLITTING METHODS FOR THE SIMULATION OF BINGHAM VISCO－PLASTIC FLOW

IoductlonRom the early seventies to tUs Unlmely death In 2001,Jacqll6sLoms     

A new combined finite element‐upwind finite volume method for convection‐dominated diffusion problems

In this article, we develop a combined finite element‐weighted upwind finite volume method for convection‐dominated diffusion problems in two dimensions, which discretizes the diffusion term with the standard finite element scheme, and the convection and source terms with the weighted upwind finite volume scheme. The developed method leads to a totally new scheme for convection‐dominated problems, which overcomes numerical oscillation, avoids numerical dispersion, and has high‐order accuracy. Stability analyses of the scheme are given for the problems with constant coefficients. Numerical experiments are presented to illustrate the stability and optimal convergence of our proposed method. © 2015 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 32: 799–818, 2016