二维变分不等式问题的自适应有限元分析

有限元后处理超收敛计算的EEP(单元能量投影)法以及基于该法的自适应有限元分析已在一维变分不等式问题的求解中取得显著成功。以此为基础,该文对二维变分不等式问题成功地实现了自适应有限元分析。该文提出二维区域二分法和二维C 检验技术,有效地提升了松弛迭代的收敛速率,进而应用EEP 超收敛公式计算超收敛解答,用其检验误差并指导网格细分。该文给出的典型数值算例表明该文算法高效、稳定、精确,解答可逐点以最大模度量满足精度要求,堪称为数值精确解。     

基于p型超收敛计算的一维有限元自适应分析

基于提高单元阶次的p型超收敛算法,可以在有限元解答基础上求得超收敛解。用该超收敛解代替精确解可以对有限元解答进行可靠的误差估计。对Zienkiewicz网格划分策略进行一定的改进,得到一种更有效的网格划分策略。基于可靠的误差估计和高效的网格划分,可以进行有限元自适应求解。数值试验表明,该文的自适应求解方案能够得到较优的网格和满足误差限的解答。     

变截面变曲率梁振型的有限元超收敛拼片恢复解和网格自适应分析

该文提出变截面变曲率梁振型的有限元后处理超收敛拼片恢复方法,建立各阶振型的超收敛解,并基于振型超收敛解进行变截面曲梁面内和面外自由振动的自适应分析。在位移型有限元后处理阶段,引入超收敛拼片恢复方法和高阶形函数插值技术,得到振型(位移)的超收敛解。利用振型超收敛解估计当前网格下振型有限元解的能量模形式下的误差,并指导网格进行自适应细分加密分析,获得优化的网格和满足预设误差限的高精度解答。数值算例表明该算法适于求解不同曲线形态、多类边界条件、变截面、变曲率形式的曲梁面内和面外自由振动连续阶频率和振型,解答精确、分析过程高效可靠。     

平面曲梁面外自由振动有限元分析的p型超收敛算法

该文用p型超收敛算法对平面曲梁面外自由振动问题进行求解。该法基于频率和振型结点位移在有限元解答中的超收敛特性,在单元上建立振型近似满足的线性常微分方程边值问题,用更高次元对该线性边值问题进行有限元求解获得各单元上振型的超收敛解,将振型的超收敛解代入Rayleigh商,得到频率的超收敛解。该法作为后处理法,修复计算分别在各个单元上单独进行,故通过少量计算即能显著提高频率和振型的精度和收敛阶。数值算例显示该法稳定、高效,值得进一步研究和推广。     

二阶非线性常微分方程边值问题有限元p型超收敛计算

该文提出二阶非线性常微分方程边值问题有限元求解的p型超收敛算法。该法基于有限元解答中结点解的超收敛特性,以单元端部的有限元解作为单元边界条件,通过泰勒展开技术在单个单元上建立了单元解近似满足的线性常微分方程边值问题,对该局部线性边值问题采用单个高次元进行有限元求解获得该单元上的超收敛解,对每个单元实施上述过程可获得全域的超收敛解。该法为后处理法,且后处理计算仅在单个单元上进行,通过很少量的计算即能显著提高解答的精度和收敛阶。数值结果显示,该法高效、可靠,是一个颇具潜力的方法。     

平面曲梁面内自由振动有限元分析的p型超收敛算法

该文提出一种求解平面曲梁面内自由振动问题的p型超收敛算法。该法基于有限元解答中频率和振型结点位移的固有超收敛特性,在单个单元上建立了振型近似满足的线性常微分方程边值问题,对该局部线性边值问题采用单个高次元进行有限元求解获得该单元上振型的超收敛解,逐单元计算完毕后,将振型的超收敛解代入Rayleigh商,获得频率的超收敛解。该法为后处理法,且后处理计算仅在单个单元上进行,通过少量计算即能显著提高频率和振型的精度和收敛阶。数值算例表明,该法可靠、高效,值得进一步研究和推广。     

二维泊松方程问题Lagrange型有限元p型超收敛算法

该文针对二维泊松方程问题的Lagrange型有限元法提出了一种p型超收敛算法。该法受有限元线法对二维问题降维思想的启发,基于网格结点位移的天然超收敛性,通过从网格中取出一行对边相邻的单元作一子域,将子域内各单元另一对边解答取为原有限元解答,在子域上建立真解近似满足的局部偏微分方程边值问题,对该局部边值问题,沿对边方向单向提高单元阶次进行有限元求解获得单元对边上的超收敛解。单元另一对边上的超收敛解可通过另一方向的单元行类似获得。在单元边超收敛解的基础上,依次取出各个单元,以单元边位移超收敛解为Dirichlet边界条件,双向提高单元阶次对原泊松方程问题进行有限元求解即可获得全域超收敛解。数值算例表明,通过简单的后处理计算本法可显著提高解答的精度和收敛阶。     

自适应步长时程分析的新进展：从凝聚单元到降阶单元

研究发现，按最大模度量的自适应步长时程单元的成功求解，需要有限元解的结点精度与单元精度之比，以不低于2为佳；亦即■次单元的单元精度为■，则其结点精度宜达至■。作者提出的凝聚单元，符合此精度比条件，自适应求解表现出色。该文研究发现，■次常规单元的解答，包含了■次凝聚单元的解，进而提出了无须凝聚、无须超收敛计算、无须结点修正的简便高效的单元算法——降阶单元。该文对这一研究进展做一简介，并给出初步算例验证了该法的可行性和有效性。     

结构自由振动问题有限元新型超收敛算法研究

该文以杆件轴向自由振动问题为例提出一个结构自由振动问题的新型超收敛计算方法。该法基于有限元解答中频率和振型结点位移的超收敛特性,建立了单元上振型近似满足的线性常微分方程边值问题,对该线性边值问题采用更高次数的多项式进行有限元求解获得各单元上振型的超收敛解,将振型的超收敛解代入Rayleigh商,获得结构频率的超收敛解。该法简单、直接,通过很少量的计算即能显著提高频率和振型的精度和收敛阶。数值算例显示,该法高效、可靠,是一个颇具潜力的新方法。     

基于EEP超收敛解的自适应有限元法特性分析

基于EEP （单元能量投影）超收敛计算的自适应有限元法,已对一系列问题取得成功,但其自适应特性尚缺乏相关研究。该文以二阶常微分方程为模型问题,同时考察基于EEP和SPR （超收敛分片恢复）超收敛解的自适应分析方法,与有限元最优网格进行了比较分析,进而提出反映自适应有限元收敛特性的估计式,并给出了自适应收敛率β的定义。该文给出的数值试验表明:采用m次单元,对于解答光滑的问题,SPR法与EEP法均可有效用于自适应求解,其位移可按最大模获得m+1的自适应收敛率;对于奇异因子为α（<1）的奇异问题,SPR法失效,而基于EEP法的自适应求解,其位移按最大模可获得m+α的自适应收敛率,远高于α的常规有限元收敛率。     

一维EEP自适应技术新进展:从线性到非线性

有限元后处理中超收敛计算的EEP(单元能量投影)法以及基于该法的自适应分析方法对线性ODE(常微分方程)问题的求解已经获得了全面成功,也推动了非线性ODE问题自适应求解的研究。经过研究,已经实现了一维有限元自适应分析技术从线性到非线性的跨越,该文意在对这方面的进展作一简要综述与报道。该文提出一种基于EEP法的一维非线性有限元自适应求解方法,其基本思想是通过线性化,将现有的线性问题自适应求解方法直接引入非线性问题求解,而无需单独建立非线性问题的超收敛计算公式和自适应算法,从而构成一个统一的、通用的非线性问题自适应求解算法。该文给出的数值算例表明所提出的算法高效、稳定、通用、可靠,解答可逐点按最大模度量满足用户给定的误差限,可作为先进高效的非线性ODE求解器的核心理论和算法。     

A feed-back approach to error control in finite element methods: application to linear elasticity

Recently a refined approach to error control in finite element (FE) discretisations has been proposed, Becker and Rannacher (1995b), (1996), which uses weighted a posteriori error estimates derived via duality arguments. The conventional strategies for mesh refinement in FE models of problems from elasticity theory are mostly based on a posteriori error estimates in the energy norm. Such estimates reflect the approximation properties of the finite element ansatz by local interpolation constants while the stability properties of the continuous model enter through a global coercivity constant. However, meshes generated on the basis of such global error estimates are not appropriate in cases where the domain consists of very heterogeneous materials and for the computation of local quantities, e.g., point values or contour integrals. This deficiency is cured by using certain local norms of the dual solution directly as weights multiplying the local residuals of the computed solution. In general, these weights have to be evaluated numerically in the course of the refinement process, yielding almost optimal meshes for various kinds of error measures. This feed-back approach is developed here for primal as well as mixed FE discretisations of the fundamental problem in linear elasticity.     

Adaptive computational methods for variational inequalities based on mixed formulations

This work describes concepts for a posteriori error estimation and adaptive mesh design for finite element models where the solution is subjected to inequality constraints. These methods are developed here for several model problems. Based on these examples, unified frameworks are proposed, which provide a systematic way of adaptive error control for problems stated in form of variational inequalities. Copyright © 2006 John Wiley & Sons, Ltd.     

Updated Lagrangian formulation of contact problems using variational inequalities

This article is devoted to the formulation and solution of general frictional contact problems in elasto-plastic solids undergoing large deformations using variational inequalities. An updated Lagrangian formulation is adopted to develop the incremental variational inequality representing this class of problems over the loading history. The Jaumann objective stress rate is incorporated in the formulation of the elasto-plastic constitutive equations to account for large rotations, while Coulomb's law is used to model the friction forces. The resulting variational inequality is treated using mathematical programming in association with a newly developed successive approximation scheme. This scheme, which is based upon the regularization of the frictional work, is used to impose the active contact constraints identified to calculate the incremental changes in the displacement field. The newly developed approach offers the advantages of reducing the active number of variables which is highly desirable in non-linear elasto-plastic problems. The merits of the formulations are demonstrated by application to an illustrative example and to the analysis of the deep drawing process. © 1997 John Wiley & Sons, Ltd.     

An hp‐adaptive finite element method for scattering problems in computational electromagnetics

An hp‐adaptive finite element (FE) approach is presented for a reliable, efficient and accurate solution of 3D electromagnetic scattering problems. The radiation condition in the far field is satisfied automatically by approximation with infinite elements (IE). Near optimal discretizations that can effectively resolve local rapid variations in the scattered field are sought adaptively by mesh refinements blended with graded polynomial enrichments. The p‐enrichments need not be spatially isotropic. The discretization error can be controlled by a self‐adaptive process, which is driven by implicit or explicit a posteriori error estimates. The error may be estimated in the energy norm or in a quantity of interest. A radar cross section (RCS) related linear functional is used in the latter case. Adaptively constructed solutions are compared to pure uniform p approximations. Numerical, highly accurate, and fairly converged solutions for a number of generic problems are given and compared to previously published results. Copyright © 2004 John Wiley & Sons, Ltd.     

Finite element formulation of smart piezoelectric composite plates coupled with acoustic fluid

In the context of noise and vibration reduction by passive piezoelectric devices, this work presents the theoretical formulation and the finite element (FE) implementation of vibroacoustic problems with piezoelectric composite structures connected to electric shunt circuits. The originalities of this work concern (i) the formulation of the electro-mechanical-acoustic coupled system, (ii) the implementation of an accurate and inexpensive laminated composite plate FE with embedded piezoelectric layers connected to resonant shunt circuits, and (iii) the development of an efficient fluid-structure interface element. Various results are presented in order to validate and illustrate the performance of the proposed fully coupled numerical approach.     

On computational methods for variational inequalities

In this note, we focus on optimised mesh design for the Finite Element (FE) method for variational inequalities using global norm estimates for local error control. The strategies are based on the so called dual-weighted-residual (DWR) approach to a posteriori error control for FE-schemes (see, e.g., Rannacher et al. [19, 6, 2]), where error control for the primal problem is established by solving an auxiliary (dual) problem. In this context we blamed (cf. e.g., Rannacher and Suttmeier [18, 19]) global norm estimates being not that useful in applications. But having a closer look at the DWR-concept, one observes that in fact global (energy) error bounds can be employed to establish local error control. Our ideas and techniques are illustrated at the socalled obstacle problem.It turns out, that reliable and efficient energy error control is one main ingredient to establish useful a posteriori error bounds for local quantities. Therefore, in addition, we derive an unified approach to a posteriori error control in the energy norm for elliptic variational inequalities of first kind. Eventually, this framework is applied to Signorinis problem.