数值流形方法的变分原理与应用

针对线弹性体静力问题,根据数值流形方法的特点及相应的位移模式,得到了面向物理覆盖的数值流形方法的变分原理,详细推导了基于变分原理的数值流形方法的理论计算公式,建立了数值流形方法的控制方程。作为实际应用,给出了相应的数值算例,结果表明,求解精度和效益令人满意。     

一类具有周期变指数和凹凸非线性项椭圆型方程解的多重性

研究一类具有周期变指数和凹凸非线性项的椭圆边值问题,借Ekeland分原理和Nehari流形等理论和方法得到解的多重性.     

三维数值流形方法的理论研究

在二维数值流形方法的基础上,对三维数值流形进行了理论研究．研究了三维覆盖位移函数,进行了三维数值流形的力学分析,给出了三维流形单元的刚度矩阵,详细推导了三维数值流形的Hammer积分及剖分规则,系统地研究三维数值流形的理论体系与数值实现方法．作为数值算例,给出了相应的悬臂梁的计算结果,计算结果表明算法的精度和计算效益较高．     

Euler型梁-柱结构的非线性稳定性和后屈曲分析

在有限变形的假设下,建立了位于非线性弹性基础上非线性弹性Euler型梁-柱结构的广义Hamilton变分原理,并由此导出了任意变截面Euler型梁-柱结构的3维非线性数学模型,其中考虑了转动惯性、几何非线性、材料非线性等因素的影响．作为模型的应用,分析了弹性基础上一端完全固支另一端部分固支,并受轴力作用的均质等截面线性弹性Euler型梁的非线性稳定性和后屈曲;结合打靶法和Newton法,给出了一种计算平凡解(前屈曲状态)、分叉点(临界载荷)和分叉解(后屈曲状态)的数值方法,对前两个分支点和相应分支解,成功地实现了数值计算,并考虑了基础反力和惯性矩对分支点的影响．     

大变形对称弹性理论的广义变分原理

本文以陈至达提出的变形几何非线性理论￣［１］为基础，应用Ｌａｇｒａｎｇｅ乘子法，对大变形对称弹性力学问题进行了研究，给出了相应的位能广义变分原理、余能广义变分原理，以及动力学问题的广义变分原理；同时，文中还证明了位能广义变分原理和余能广义变分原理的等价性。     

基于改进集的集值Ekeland变分原理

Ekeland变分原理在最优化理论及应用研究中具有十分重要的作用.利用非线性标量化函数及相应的非凸分离定理建立了基于改进集的集值Ekeland变分原理.新的Ekeland变分原理包含了一些经典的Ekeland变分原理作为其特例.     

摩擦问题中的边界混合变分不等式

本文以弹性力学中的摩擦问题为背景,讨论了非线性、不可微的混合变分不等式解的存在唯一性,给出相应的边界变分不等式及其解的存在唯一性。为使用边界元方法数值求解提供了理论依据。     

一类非线性抛物型方程广义差分法的变分原理及H~1模误差估计

和有限元方法类似,广义差分法属于基于变分原理的差分格式,是解偏微分方程的一种有效的数值方法.因此,寻求对应于定解问题的广义差分法的变分原理是很重要的,本文第一部分内容即属此.本文还给出了用此方法解一类非线性抛物型方程的H~1模误差估计.     

固体力学中的连续动态演化系统与Hamilton-Jacobi-Bellman方程

在许多固体力学所研究的前沿性领域中,如损伤力学、细观力学、粘塑性问题、蠕变问题等,其最突出的特点就是高度非线性、本构行为的时间相关性以及动态演化性.本文应用控制理论描述这一动态演化力学系统,并结合经典力学变分原理建立相应的用于固体力学的最优控制变分原理.首先,讨论它的连续形式,即相应的Hamilton-Jacobi-Bellman方程;然后,给出相应的数值解形式和算例.     

分数阶临界薛定谔方程解的存在性和多重性

本文利用Ekeland变分原理和Nehari流形方法,研究了一类带有Hardy位势和Hardy-Sobolev临界指数的分数阶薛定谔方程,证明了解的存在性和多重性.     

A high-order numerical manifold method with continuous stress/strain field

Recent attempts to solve solid mechanical problems using the numerical manifold method (NMM) are very fruitful. In the present work, a high-order numerical manifold method (HONMM) which is able to obtain continuous stress/strain field is proposed. By employing the same discretized model as the traditional NMM (TNMM), the proposed HONMM can yield much better accuracy without increasing the number of degrees of freedom (DOFs), and obtain continuous stress/strain field without recourse any stress smoothing operation in the post-processing stage. In addition, the “linear dependence” (LD) issue does not exist in the HONMM, and traditional equation solvers can be employed to solve the simultaneous algebraic equations. A number of numerical examples including four linear elastic continuous problems and five cracked problems are solved with the proposed method. The results show that the proposed HONMM performs much better than the TNMM.     

Numerical computation of connecting orbits on a manifold

In this paper we propose a numerical method for approximating connecting orbits on a manifold and its bifurcation parameters. First we extend the standard nondegeneracy condition to the connecting orbits on a manifold. Then we construct a well-posed system such that the nondegenerate connecting orbit pair on a manifold is its regular solution. We use a difference method to discretize the ODE part in this well-posed system and we find that the numerical solutions still remain on the same manifold. We also set up a modified projection boundary condition to truncate connecting orbits on a manifold onto a finite interval. Then we prove the existence of truncated approximate connecting orbit pairs and derive error estimates. Finally, we carry out some numerical experiments to illustrate the theoretical estimates.     

Feedback Integrators

A new method is proposed to numerically integrate a dynamical system on a manifold such that the trajectory stably remains on the manifold and preserves the first integrals of the system. The idea is that given an initial point in the manifold we extend the dynamics from the manifold to its ambient Euclidean space and then modify the dynamics outside the intersection of the manifold and the level sets of the first integrals containing the initial point such that the intersection becomes a unique local attractor of the resultant dynamics. While the modified dynamics theoretically produces the same trajectory as the original dynamics, it yields a numerical trajectory that stably remains on the manifold and preserves the first integrals. The big merit of our method is that the modified dynamics can be integrated with any ordinary numerical integrator such as Euler or Runge–Kutta. We illustrate this method by applying it to three famous problems: the free rigid body, the Kepler problem and a perturbed Kepler problem with rotational symmetry. We also carry out simulation studies to demonstrate the excellence of our method and make comparisons with the standard projection method, a splitting method and Störmer–Verlet schemes.     

Cover-based bounds on the numerical rank of Gaussian kernels

A popular approach for analyzing high-dimensional datasets is to perform dimensionality reduction by applying non-parametric affinity kernels. Usually, it is assumed that the represented affinities are related to an underlying low-dimensional manifold from which the data is sampled. This approach works under the assumption that, due to the low-dimensionality of the underlying manifold, the kernel has a low numerical rank. Essentially, this means that the kernel can be represented by a small set of numerically-significant eigenvalues and their corresponding eigenvectors.We present an upper bound for the numerical rank of Gaussian convolution operators, which are commonly used as kernels by spectral manifold-learning methods. The achieved bound is based on the underlying geometry that is provided by the manifold from which the dataset is assumed to be sampled. The bound can be used to determine the number of significant eigenvalues/eigenvectors that are needed for spectral analysis purposes. Furthermore, the results in this paper provide a relation between the underlying geometry of the manifold (or dataset) and the numerical rank of its Gaussian affinities.The term cover-based bound is used because the computations of this bound are done by using a finite set of small constant-volume boxes that cover the underlying manifold (or the dataset). We present bounds for finite Gaussian kernel matrices as well as for the continuous Gaussian convolution operator. We explore and demonstrate the relations between the bounds that are achieved for finite and continuous cases. The cover-oriented methodology is also used to provide a relation between the geodesic length of a curve and the numerical rank of Gaussian kernel of datasets that are sampled from it.     

The existence of Silnikov''''s orbits in one coupled Duffing equation

The existence of Silnikov's orbits in one coupled Duffing equation is discussed by using the fiber structure of invariant manifold and high-dimensional Melnikov's method. Example and numerical simulation results are also given to demonstrate the theoretical analysis.     

Computation of Smooth Manifolds Via Rigorous Multi-parameter Continuation in Infinite Dimensions

In this paper, we introduce a constructive rigorous numerical method to compute smooth manifolds implicitly defined by infinite-dimensional nonlinear operators. We compute a simplicial triangulation of the manifold using a multi-parameter continuation method on a finite-dimensional projection. The triangulation is then used to construct local charts and an atlas of the manifold in the infinite-dimensional domain of the operator. The idea behind the construction of the smooth charts is to use the radii polynomial approach to verify the hypotheses of the uniform contraction principle over a simplex. The construction of the manifold is globalized by proving smoothness along the edge of adjacent simplices. We apply the method to compute portions of a two-dimensional manifold of equilibria of the Cahn–Hilliard equation.     

A Dynamical Tikhonov Regularization for Solving Ill-posed Linear Algebraic Systems

The Tikhonov method is a famous technique for regularizing ill-posed linear problems, wherein a regularization parameter needs to be determined. This article, based on an invariant-manifold method, presents an adaptive Tikhonov method to solve ill-posed linear algebraic problems. The new method consists in building a numerical minimizing vector sequence that remains on an invariant manifold, and then the Tikhonov parameter can be optimally computed at each iteration by minimizing a proper merit function. In the optimal vector method (OVM) three concepts of optimal vector, slow manifold and Hopf bifurcation are introduced. Numerical illustrations on well known ill-posed linear problems point out the computational efficiency and accuracy of the present OVM as compared with classical ones.     

High-order adaptive method for computing two-dimensional invariant manifolds of three-dimensional maps

An efficient and accurate numerical method is presented for computing invariant manifolds of maps which arise in the study of dynamical systems. A quasi-interpolation method due to Hering-Bertram et al. is used to decrease the number of points needed to compute a portion of the manifold. Bézier triangular patches are used in this construction, together with adaptivity conditions based on properties of these patches. Several numerical tests are performed, which show the method to compare favorably with previous approaches.