各向异性位势平面问题的规则化边界元法

基于转化域方程为边界积分方程的极限定理及一个新颖的基本解分解技术, 建立间接变量规则化边界积分方程, 它有效地避免了奇异积分的直接计算. 与已有方法比,该方法不将问题变换为各向同性的问题去处理, 因而无需反演运算, 也有别于Galerkin方法, 无需计算重积分. 可计算任意边界位势梯度, 而不仅限于法向通量. 针对椭圆边界的边值问题, 提交一种精确单元来描述边界几何. 数值算例表明, 所提算法稳定且效率高, 所得数值结果与精确解吻合较好.      

正交各向异性弹性问题的规则化边界元法

本文致力于平面正交各向异性弹性问题的规则化边界元法研究,提出了新的规则化边界元法的理论和方法。对问题的基本解的特性进行了研究,确立基本解的积分恒等式,提出一种基本解的分解技术,在此基础上,结合转化域积分方程为边界积分方程的极限定理,建立了新颖的规则化边界积分方程。和现有方法比,本文不必将问题变换为各向同性的去处理,从而不含反演运算,也有别于Galerkin方法,无需计算重积分,因此所提方法不仅效率高,而且程序设计简单。特别是,所建方程可计算任何边界位移梯度,进而可计算任意边界应力,而不仅限于面力。数值实施时,采用二次单元和椭圆弧精确单元来描述边界几何,使用不连续插值逼近边界函数。数值算例表明,本文算法稳定、效率高,所取得的边界量数值结果与精确解相当接近。     

三维位势问题的梯度边界积分方程的新解法

三维位势问题的边界元分析中,关于坐标变量的边界位势梯度的计算是一个困难的问题. 已有一些方法着手解决这个问题,然而,这些方法需要复杂的理论推导和大量的数值计算. 本文提出求解一般边界位势梯度边界积分方程的辅助边值问题法. 该方法构造了与原边界值问题具有相同解域的辅助边值问题,该辅助边值问题具有已知解,因此通过求解此辅助边值问题,可获得梯度边界积分方程对应的系统矩阵,然后将此系统矩阵应用于求解原边值问题,求解过程非常简单,只需求解一个线性系统即可获得原边值问题的解. 值得注意的是,在求解原边值问题时,不再需要重新计算系统矩阵,因此辅助边值问题法的效率并不很差. 辅助边值问题法避免了强奇异积分的计算,具有数学理论简单、程序设计容易、计算精度高等优点,为坐标变量梯度边界积分方程的求解提供了一个新的途径. 3个标准的数值算例验证了方法的有效性.      

三维位势问题的梯度边界积分方程的新解法

三维位势问题的边界元分析中,关于坐标变量的边界位势梯度的计算是一个困难的问题.已有一些方法着手解决这个问题,然而,这些方法需要复杂的理论推导和大量的数值计算.本文提出求解一般边界位势梯度边界积分方程的辅助边值问题法.该方法构造了与原边界值问题具有相同解域的辅助边值问题,该辅助边值问题具有已知解,因此通过求解此辅助边值问题,可获得梯度边界积分方程对应的系统矩阵,然后将此系统矩阵应用于求解原边值问题,求解过程非常简单,只需求解一个线性系统即可获得原边值问题的解.值得注意的是,在求解原边值问题时,不再需要重新计算系统矩阵,因此辅助边值问题法的效率并不很差.辅助边值问题法避免了强奇异积分的计算,具有数学理论简单、程序设计容易、计算精度高等优点,为坐标变量梯度边界积分方程的求解提供了一个新的途径. 3个标准的数值算例验证了方法的有效性.     

调和函数边界积分方程的充要条件

本文以调和函数的边值问题为例,探讨了边界积分方程的充要条件.文中首次提出了超定问题的概念,并建立了超定问题有解的一个充要条件,它也就是直接变量边界积分方程的一个充要条件.文中首次阐明了边界积分方程与变分原理的内在的联系,还指出了间接变量与直接变量两类边界积分方程之间存在着一一对应的关系.文中的慨念、思路和论点不难用于其它有变分原理的问题的边界积分方程.     

弹性力学平面问题的无奇异边界积分方程

将弹性力学平面问题归化成无奇异边界积分方程,避免了传统的边界元法中的柯西主值（CPV）积分和Hadamard-Finite-Parts(HFP)积分的计算,建立完整的数值求解体系。     

弹性薄板弯曲问题的等价的直接变量边界积分方程

建立平面弹性薄板弯曲问题理论中具有直接变量的等价边界积分方程，传统的直接变量边界积分方程，它们都不是等价的，对此进行了深入的讨论。     

热弹性平面问题的规则化边界积分方程

对于热弹性平面问题,过去广泛集中在直接变量边界元法研究,本文研究间接变量规则化边界元法,建立了间接变量规则化边界积分方程。和直接边界元法相比,间接法具有降低密度函数的连续性要求、位移梯度方程中的热载荷体积分具有较弱奇异性等优点。数值实施中,用精确单元描述边界几何,不连续插值函数逼近边界量。算例表明,本文方法效率高,所得数值结果与精确解相当吻合。     

各向异性势问题充要的随机边界积分方程

本文针对各向异性势问题提出了一类充分必要的随机边界积分方程，数值计算结果表明在退化尺度附近，充要的随机边界积分程较习用的随机边界积分方程有较大的优越性。     

三维有限体平片裂纹的超奇异积分方程与边界元法

利用Ｓｏｍｉｇｌｉａｎａ公式及有限部积分的概念，导出了含任意平片裂纹三维有限体问题的超奇异积分方程组，并联合使用有限部积分与边界元法，建立了数值求解方法．在裂纹前沿附近单元，采用与理论分析一致的平方根位移模型，以提高数值结果的精度．最后计算了若干典型例子的应力强度因子．     

Analytical Treatment of Boundary Integrals in Direct Boundary Element Analysis of Plan Potential and Elasticity Problems

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An efficient three-level explicit time-split scheme for solving two-dimensional unsteady nonlinear coupled Burgers' equations

This work deals with the numerical solutions of two-dimensional viscous coupled Burgers' equations with appropriate initial and boundary conditions using a three-level explicit time-split MacCormack approach. In this technique, the differential operators split the two-dimensional problem into two pieces so that the two-step explicit MacCormack scheme can be easily applied to each subproblem. This reduces the computational cost of the algorithm. For low Reynolds numbers, the proposed method is second-order accurate in time and fourth-order convergent in space, whereas it is second-order convergent in both time and space for high Reynolds numbers problems. This observation shows the utility and efficiency of the considered method compared with a broad range of numerical schemes widely studied in the literature for solving the two-dimensional time-dependent nonlinear coupled Burgers' equations. A large set of numerical examples that confirm the theoretical results are presented and critically discussed.     

Analyzing the 2D fracture problems via the enriched boundary element-free method

In this paper, the enriched boundary element-free method for two-dimensional fracture problems is presented. An improved moving least-squares (IMLS) approximation, in which the orthogonal function system with a weight function is used as the basis function, is used to obtain the shape functions. The IMLS approximation has greater computational efficiency and precision than the existing moving least-squares (MLS) approximation, and does not lead to an ill-conditioned system of equations. Combining the boundary integral equation (BIE) method and the IMLS approximation, a boundary element-free method (BEFM), for two-dimensional fracture problems is obtained. For two-dimensional fracture problems, the enriched basis function is used at the tip of the crack, and then the enriched BEFM is presented. In comparison with other existing meshless boundary integral equation methods, the BEFM is a direct numerical method in which the basic unknown quantity is the real solution of the nodal variables, and the boundary conditions can be implemented easily, which leads to a greater computational precision. When the enriched BEFM is used, the singularity of the stresses at the tip of the crack can be shown better than that in the BEFM. For the purposes of demonstration, some selected numerical examples are solved using the enriched BEFM.     

弹性力学的一种边界无单元法

首先对移动最小二乘副近法进行了研究，针对其容易形成病态方程的缺点，提出了以带权的正交函数作为基函数的方法－改进的移动最小二乘副近法，改进的移动最小二乘逼近法比原方法计算量小，精度高，且不会形成病态方程组，然后，将弹性力学的边界积分方程方法与改进的移动最小二乘逼近法结合，提出了弹性力学的一种边界无单元法，这种边界无单元法法是边界积分方程的无网格方法，与原有的边界积分方程的无网格方法相比，该方法直接采用节点变量的真实解为基本未知量，是边界积分方程无网格方法的直接解法，更容易引入界条件，且具有更高的精度，最后给出了弹性力学的边界无单元法的数值算例，并与原有的边界积分方程的无网格方法进行了较为详细的比较和讨论。     

A regular boundary element method for fluid flow

The Boundary Element Method is now well established as a valid numerical technique for the solution of field problems, equal to the Finite Element Method in generality and surpassing it in computational efficiency in some cases.¹ In this paper is presented a 'Regular Boundary Element Method' as applied to inviscid laminar fluid flow problems. It involves the formation of a system of regular integral equations obtained by moving the singularity outside the domain of the given problem. It is also shown that non-conforming elements may be used whereby freedoms are not defined at the geometric nodes under the boundary element discretization. A linear element is developed here; higher order variants could easily be defined. Satisfactory numerical results have been obtained using the proposed regular method with both conventional (continuous across the boundary) and non-conforming boundary elements for two-dimensional inviscid laminar fluid flow problems having regular and singular solutions.     

改进的平均源边界节点法模拟平面位势问题

平均源边界节点法ASBNM是一种最近提出的边界型无网格法。该方法仅使用边界节点不涉及任何单元和积分的概念,具有方法简单和程序设计容易等特点。但是,对于依赖于边界积分方程的边界型无网格法,关键问题是如何准确高效地估计影响矩阵的对角元。本文提出直接计算影响矩阵对角元的方法,是已有ASBNM法的改进,将对角元的计算转化为一个纯几何问题,因此适用于任何二维边值问题。数值算例证明了本文方法的有效性和准确性。     

NOVEL REGULARIZED BOUNDARY INTEGRAL EQUATIONS FOR POTENTIAL PLANE PROBLEMS

The universal practices have been centralizing on the research of regulariza-tion to the direct boundary integal equations (DBIEs). The character is elimination of singularities by using the simple solutions. However, up to now the research of regular ization to the first kind integral equations for plane potential problems has never been found in previous literatures. The presentation is mainly devoted to the research on the regularization of the singular boundary integral equations with indirect unknowns. A novel view and idea is presented herein, in which the regularized boundary integral equations with indirect unknowns without including the Cauchy principal value (CPV) and Hadamard-finite-part (HFP) integrals are established for the plane potential problems. With some numerical results, it is shown that the better accuracy and higher efficiency, especially on the boundary, can be achieved by the present system.     

二阶非定常多宗量热传导反问题的正则解

引入Bregman距离函数及其加权函数作为正则项，应用Tikhonov正则 化方法，对二阶非定常多宗量热传导反问题进行求解. 利用测量信息和计算信息构造最小二 乘函数，将多宗量反演识别问题转化为一个优化问题. 空间上采用8节点等参元进行离散， 时域上采用时域精细算法进行离散，建立了二阶非定常多宗量热传导问题的有限元正/反演数 值模型. 该模型不仅考虑了非均质和参数分布的影响，而且也便于正反演问题的敏度分析， 可对导热系数和边界条件等宗量进行有效的单一和组合识别. 给出了相关的数值验证，对信 息测量误差以及不同正则项的计算效率作了探讨. 数值结果表明，该方法能够对二阶非定常 多宗量热传导反问题进行有效的求解，并具有较高的计算精度.