水波的界带有限元

研究了水波计算的位移法.采用物质坐标,以位移为基本未知量,考虑小变形条件,引入流函数满足不可压缩条件.于是,分析力学的变分原理可以运用了,界带有限元、正则变换、保辛积分等有效手段可使数值求解方便得多.     

基于改进位移模式的二维有限元线法超收敛算法

提出了基于改进位移模式的二维有限元线法超收敛算法．利用单元内部需满足平衡方程的条件,推导了超收敛计算的解析公式的显式,即将高阶有限元线法解的位移模式用常规有限元线法解的位移模式表示．用常规有限元线法解的位移模式与高阶有限元线法解的位移模式之和构造新的位移模式,基于线性形函数,采用变分形式推导了有限元线法求解的修正的常微分方程组．该算法在前和后处理同时使用超收敛计算公式,在原有试函数的基础上,增加了高阶试函数．使得单元内平衡方程的残差减少,从而达到提高精度的目标．对于二维Poisson方程问题,给出了有代表性的算例,结点和单元内的位移、导数的收敛精度得到了极大的提高．     

一个关于流动能量耗散率的minimax变分原理

流动耗散率是湍流理论的核心概念之一．Doering-Constantin变分原理刻画了流动耗散率的上确界(最大值)．在该文的研究中,首先基于优化理论的视角,Doering-Constantin的变分原理被改写为一个不可压缩剪切流耗散率的minimax型的变分原理．其次,博弈论中的Kakutani minimax定理给出该变分原理中minimizing和maximizing计算过程可交换的一个充分条件．这个结果不仅从一个新的角度揭示了谱约束的内涵,也为Doering-Constantin变分原理和Howard-Busse统计理论的等价性从博弈论的角度提供了理论基础．     

只含两个独立变量的扁壳大挠度问题修正的海林格—赖斯内变分泛函

本文首先用海林格-赖斯内变分原理建立任意形状扁壳大挠度问题的泛函，然后用修正的变分原理导出适合于有限单元法的变分泛函表达式．泛函中只包含应力函数F和挠度W两个独立交量．其中也导出了在边界上用上述两个变量表示的中面位移的表达式．推导中考虑了边界的曲率，所以适用于任意形状的边界．     

损伤粘弹性力学的广义变分原理及应用

从粘弹性材料的Boltzmann迭加原理和带空洞材料的线弹性本构关系出发,提出了一种损伤粘弹性材料具有广义力场的本构模型．应用变积方法得到了以卷积形式表示的泛函,并建立了损伤粘弹性固体的广义变分原理和广义势能原理．把它们应用于带损伤的粘弹性Timoshenko梁,得到了Timoshenko梁的统一的运动微分方程、初始条件和边界条件． 这些广义变分原理为近似求解带损伤的粘弹性问题提供了一条途径．     

一类高阶常微分方程的共振周期解的存在唯一性

利用min-max原理的非变分形式给出了一系列有关高阶常微分方程共振周期解的存在唯一性结论．     

广义重调和算子及其在薄板弯曲中的应用

本文用δ－函数具体构造出广义重调和算子，建立相应的二次泛函表达式，并将其应用于弹性薄板的弯曲问题．结果表明．当自变量函数为广义函数时，变分泛函中的自变量函数自然就允许某种程度的不连续性，用Ｌａｇｒａｎｇｅ乘子法所得的修正变分原理实际上是文中给出的变分原理的特殊形式．     

变质量高阶非Четаев型非完整约束系统动力学

本文给出了高阶非型约束加在广义虚位移上的限制条件，建立了变质量高阶非型非线性非完整系统的Ｒｏｕｔｈ方程、方程、Ｎｉｅｌｓｅｎ方程和Ａｐｐｅｌｌ方程；给出了高阶非型约束系统“ｄ”与“δ”之间的交换关系，建立了其积分变分原理；并得到了变质量高阶非型约束系统的广义Ｎｏｅｔｈｅｒ守恒律．     

逆拟变分不等式问题的相关研究

本文研究了Hilbert空间中逆拟变分不等式问题.利用不动点原理得到逆拟变分不等式问题解的存在性和唯一性.利用投影技巧,Wiener-Hopf方程和辅助原理技术分别给出求解逆拟变分不等式的迭代算法,并在一定条件下证明了算法的收敛性.最后通过间隙函数得到误差界.本文改进和推广了最近文献的一些相关结果.     

单位球和多圆柱上带正实部的全纯函数Schwarz-Pick估计

本文给出了Bn和D^n上带正实部的全纯函数高阶导数Schwarz—Pick估计,从而推广了早期C中单位圆盘上带正实部的全纯函数高阶导数的Schwarz—Pick估计的结论．     

A semi-analytical direct optimal control solution for strongly excited and dissipative Hamiltonian systems

A semi-analytical direct optimal control solution for strongly excited and dissipative Hamiltonian systems is proposed based on the extended Hamiltonian principle, the Hamilton-Jacobi-Bellman (HJB) equation and its variational integral equation, and the finite time element approximation. The differential extended Hamiltonian equations for structural vibration systems are replaced by the variational integral equation, which can preserve intrinsic system structure. The optimal control law dependent on the value function is determined by the HJB equation so as to satisfy the overall optimality principle. The partial differential equation for the value function is converted into the integral equation with variational weighting. Then the successive solution of optimal control with system state is designed. The two variational integral equations are applied to sequential time elements and transformed into the algebraic equations by using the finite time element approximation. The direct optimal control on each time element is obtained respectively by solving the algebraic equations, which is unconstrained by the system state observed. The proposed control algorithm is applicable to linear and nonlinear systems with the quadratic performance index, and takes into account the effects of external excitations measured on control. Numerical examples are given to illustrate the optimal control effectiveness.     

一种基于H-R变分的杂交广义元方法

基于Hellinger Reissner变分原理,通过构造合适的应力场函数使其能更方便和更准确地得到节点上的应力值,同时结合广义有限元构造广义位移插值的方法,在不提高单元节点数目的前提下提高位移场函数的阶次,从而提高其求解精度．这种方法能同时灵活地构造应力场和位移场,在同等精度条件下能占用较少内存和求解更少的方程数目,计算结果也显示了这种方法的有效性和很高的计算精度．     

功率型变分原理和功能型拟变分原理及其应用

自从钱伟长建立了功率型变分原理以来,功率型变分原理和功能型变分原理在理论方面和应用方面有什么区别和联系,成为学术界关注的课题.应用变积方法,根据Jourdain原理和d’Alembert原理,建立了不可压缩黏性流体力学的功率型变分原理和功能型拟变分原理,推导了不可压缩黏性流体力学的功率型变分原理的驻值条件和功能型拟变分原理的拟驻值条件.研究了不可压缩黏性流体力学的功率型变分原理在有限元素法中的应用.研究表明,功率型变分原理与Jourdain原理相吻合,功能型变分原理与d’Alembert原理相吻合.功率型变分原理直接在状态空间中研究问题,不仅在建立变分原理的过程中可以省略在时域空间中的一些变换,而且给动力学问题有限元素法的数值建模带来方便.     

Hybrid stress finite element formulation based on additional internal displacements – application of microscopic geometric perturbation and extension to linear dynamics

Based on the variational principle for hybrid stress finite element method, the assumed stress distribution can be determined through a variational process which includes a microscopic geometric perturbation. An interpretation of such geometric perturbation is discussed. For the creation of, what is called, hyper-bioelement and for the purpose of maintaining invariance, tensor notations of the physical quantities are represented by the use of covariant and contravariant basis vectors of the natural coordinate system. This process is illustrated by a simple 4-node plane element. Furthermore, based on the legitimate variational principle for linear dynamics and the presently proposed method to choose the assumed stress distribution, the finite element formulation different from the ordinary formula is advocated.     

弹性力学平面问题的精确元法

本文在阶梯折算法[1]和精确解析法[2]的基础上,提出构造有限元的新方法——精确元法.该方法不用一般变分原理,可适用于任意变系数正定和非正定偏微分方程.利用该方法得到弹性力学平面问题的一个非协调任意四边形单元.它具有八个自由度.由于没有采用雅可比变换,该单元可以蜕化为三角形单元,在工程中使用起来较为方便.文中给出收敛性证明.文末给出算例,位移和应力均给出较好的结果,在单元的节点上有较好的数值精度.     

弹性厚板的分区广义变分原理

本文提出弹性厚板分区广义变分原理,其要点如下:1.各分区可任意定为势能区或余能区.分区势能、分区余能、分区混合变分原理是它的三种特殊形式.2.每个分区中独立变分变量的个数可任意规定.每个分区可定为单类变量区、二类变量区或三类变量区.3.每个交界线上的位移和力的连接条件可以放宽.这个原理为非协调元的厚板有限元法提供理论基础.各种厚板有限元模型可看作这个原理的特殊应用.特别是弹性厚板分区混合变分原理的提出为分区混合有限元法应用于厚板问题打下了基础.     

更一般的杂交广义变分原理及有限元模型

本文根据钱伟长教授提出的更一般的广义变分原理,给出了适用于有限元法中的更广义杂交变分原理,并由此建立了新的广义杂交模理. 进一步以变厚度薄板弯曲单元为例,对基于各种不同的广义变分原理建立的各种杂交元做了比较.     

A variational arbitrary Lagrangian-Eulerian finite element formulation for standard dissipative solids at finite strains

A novel Arbitrary Lagrangian-Eulerian (ALE) finite element formulation for standard dissipative media at finite strains is presented. In contrast to previously published ALE approaches accounting for dissipative phenomena, the proposed scheme is fully variational. Consequently, no error estimates are necessary and thus, linearity of the problem and the corresponding Hilbert-space are not required. Hence, the resulting Variational Arbitrary Lagrangian-Eulerian (VALE) finite element method can be applied to highly nonlinear phenomena as well. In case of standard dissipative solids, so-called variational constitutive updates provide a variational principle. Based on these updates, the deformation mapping follows from minimizing an incrementally defined (pseudo) potential, i.e., energy minimization is the overriding criterion that governs every aspect of the system. Therefore, it is natural to allow the variational principle to drive mesh adaption as well. Thus, in the present paper, the discretizations of the deformed as well as the undeformed configuration are optimized jointly by minimizing the respective incremental energy of the considered mechanical system. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A mixed finite element method close to the equilibrium model

Summary A new variational formulation of the Dirichlet problem for one elliptic partial differential equation of the second order is established and justified, starting from a non-classical decomposition of the differential operator and the Friedrichs transformation. The variational problem has a unique solution which depends continuously on the right hand side of the given equation and enables to construct mixed finite element models. The Galerkin approximations are vector-functions converging to the cogradient of the solution of the original problem, except one component which tends to the solution itself.