数值流形方法研究及应用进展

基于有限覆盖技术的数值流形方法是一种新的广义的数值方法.该方法的场函数近似原理和有限元、无网格、单位分解等方法相似,但在网格划分、覆盖形式、近似函数等方面有其自身的特点和优势.对该方法近年来在理论研究和应用方面取得的重要进展进行了综述.在理论研究方面, 目前已对不同形式物理覆盖流形单元的性能进行了研究,结果表明流形单元的精度较有限单元高,且提高覆盖函数的阶次能提高单元的精度;同时理论研究已由二维低阶流形方法推广到三维高阶流形方法,由线性流形方法推广到非线性流形方法,由基于能量原理的流形方法推广到基于加权余量的流形方法,非协调流形方法、无网格流形方法等也已开展了研究; 此外,覆盖系统的自动生成、覆盖函数的形式以及边界条件的处理方法等流形方法相关理论的研究也取得了进展.在应用方面,开展了有关岩石破坏和裂纹扩展等非连续变形分析更深入的研究,并已逐步推广到金属塑性变形分析、多孔介质变形分析以及温度场的数值分析等多个领域.针对目前流形方法的研究和应用现状,该文展望了流形方法理论及实现方法的研究方向、及其在计算流体力学、金属成形等大变形问题、多物理场分析等领域的应用前景.      

质点和刚体的惯性运动

<正> 质点惯性运动的概念基于惯性定律.惯性运动的本质在于物体具有惯性.惯性运动的本意是要说明它是不受力作用的,按物体之惯性而进行的一种运动.遵循这一思想,要把惯性运动的概念推广到刚体,应根据动力学定律由动力学基本特征量来下定义.有关文章提出惯性运动应由运动学基本特征来下定义,值得商榷.     

也谈惯性运动

惯性运动在运动规律中是最简单的。它有明确的运动学含义和动力学含义。把惯性运动的概念由质点扩大到刚体,是基于它们有同一形式的运动微分方程和同一形式的最简单的运动积分。离开寻求最简单的运动积分这一目标,把惯性运动的概念扩大到非自由质点系并无实际意义。有关文章新提出的"广义惯性 ...     

sine—Gordon方程的多重脉动跳跃轨道

本文利用ｓｉｎｅ－Ｇｏｒｄｏｎ方程的广义渐近惯性流形上的常微分方程，证明了在一定参数条件下存在多重脉动跳跃轨道，本工作与先前工作（１－４）证实可用ｓｉｎｅ－Ｇｏｒｏｄｎ方程的有限维形式来讨论其的动力学行为。     

关于近似惯性流形及其数值方法的研究

本文简述了近年来在无穷维动力系统研究中一些数学理论的进展状况，主要目的是结合二维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程讨论近似惯性流形及其数值方法的构造和意义．     

流形Trefftz直接法

基于数值流形方法,结合广义节点的概念,构造了一类节点位移用任意阶多项式展开的流形Trefftz直接法。这种方法融合了数值流形方法和Trefftz直接法的优点。计算结果表明,流形Trefftz直接法与Trefftz直接法以及其他计算方法相比有较高的精度。     

sine—Gordon方程的动力学行为讨论

本文以ｓｉｎｅ－Ｃｏｒｄｏｎ方程为全了把无穷维动力系统理论和非一动力学方法结合起来讨论无穷维动力学系统的动力学行为的一种方法，首先证明了有限维广义渐惯性流形的存在性。然后给出了该流形上常微分方程组，且讨论了解的不变子空间结构。最后用摄动和数值计算讨论了动力学行为，所得结果与（６）所得的一致。这种方法也许为时军蝗分析提供了一条有效途径     

基于有限变形理论的数值流形方法研究

原有数值流形方法通过积累每一时步的小变形而得到结构最终的大变形,然而,当结构发生大变形、大转动时往往产生较大计算误差. 针对该问题,从动量守恒方程以及应力边界条件的积分弱形式出发,引入流形方法的插值函数,建立了基于有限变形理论的数值流形方法. 通过对比改进前后流形方法的计算迭代格式,指出了原有流形方法计算大变形问题时的误差来源. 最后,通过大变形悬臂梁和旋转块体算例对有限变形流形方法进行了验证. 数值结果表明,改进后的流形方法能够很好地处理大变形大转动问题,消除了转动所带来的计算误差,其计算结果与解析解及ABAQUS 软件求得的数值解相吻合.      

基于有限变形理论的数值流形方法研究简

原有数值流形方法通过积累每一时步的小变形而得到结构最终的大变形,然而,当结构发生大变形、大转动时往往产生较大计算误差. 针对该问题,从动量守恒方程以及应力边界条件的积分弱形式出发,引入流形方法的插值函数,建立了基于有限变形理论的数值流形方法. 通过对比改进前后流形方法的计算迭代格式,指出了原有流形方法计算大变形问题时的误差来源. 最后,通过大变形悬臂梁和旋转块体算例对有限变形流形方法进行了验证. 数值结果表明,改进后的流形方法能够很好地处理大变形大转动问题,消除了转动所带来的计算误差,其计算结果与解析解及ABAQUS 软件求得的数值解相吻合.     

二维定常不可压缩粘性流动N-S方程的数值流形方法

将流形方法应用于定常不可压缩粘性流动N-S方程的直接数值求解,建立基于Galerkin加权余量法的N-S方程数值流形格式,有限覆盖系统采用混合覆盖形式,即速度分量取1阶和压力取0阶多项式覆盖函数,非线性流形方程组采用直接线性化交替迭代方法和Nowton-Raphson迭代方法进行求解.将混合覆盖的四节点矩形流形单元用于阶梯流和方腔驱动流动的数值算例,以较少单元获得的数值解与经典数值解十分吻合.数值实验证明,流形方法是求解定常不可压缩粘性流动N-S方程有效的高精度数值方法.     

Some closure results for inertial manifolds

Suppose that the family of evolution equationsdu/dt+Au+f _N (u)=0 possesses inertial manifolds of the same dimension for a sequence of nonlinear termsf _N withf _N f in the C⁰ norm. Conditions are found to ensure that the limiting equationdu/dt+Au+f(u)=0 also possesses an inertial manifold. There are two cases. The first, where the manifolds for the family have a bounded Lipschitz constant, is straightforward and leads to an interesting result on inertial manifolds for Bubnov-Galerkin approximations. When the Lipschitz constant is unbounded, it is still possible to prove the existence of an exponential attractor of finite Hausdorff dimension for the limiting equation. This more general result is applied to a problem in approximate inertial manifold theory discussed by Sell (1993).For Paul Glendinning, with thanks.     

Approximate inertial manifolds for reaction-diffusion equations in high space dimension

The concept of approximate inertial manifolds was introduced by Foiaset al. (1987) in the case of the two-dimensional Navier-Stokes equations. These manifolds are finite dimensional smooth manifolds such that the orbits enter a very thin neighborhood of the manifold after a transient time; this concept replaces the one of inertial manifold when either an inertial manifold does not exist or its existence is not known. Our aim in this paper is to prove that approximate inertial manifolds exist for reaction-diffusion equations in high space dimension by opposition with exact inertial manifolds whose existence has only been proved in low dimension and for which nonexistence results have been obtained in space dimensionn=4.     

Pullback Attracting Inertial Manifolds for Nonautonomous Dynamical Systems

In this paper we present an abstract approach to inertial manifolds for nonautonomous dynamical systems. Our result on the existence of inertial manifolds requires only two geometrical assumptions, called cone invariance and squeezing property, and two additional technical assumptions, called boundedness and coercivity property. Moreover we give conditions which ensure that the global pullback attractor is contained in the inertial manifold. In the second part of the paper we consider special nonautonomous dynamical systems, namely processes (or two-parameter semi-flows). As a first application of our abstract approach and for reason of comparison with known results we verify the assumptions for semilinear nonautonomous evolution equations whose linear part satisfies an exponential dichotomy condition and whose nonlinear part is globally bounded and globally Lipschitz.     

A Hartman-Grobman theorem for the Cahn-Hilliard and phase-field equations

In this paper, we study the structural stability of the Cahn-Hilliard equation and the phase-field equations. We show that the Cahn-Hilliard equation and the phase-field equations are topologically conjugate to a decoupled system of a linear equation of infinite dimension and an ordinary differential equation which is the reduced equation on the inertial manifold; particularly, the flow nearby hyperbolic stationary solutions is structurally stable.     

Spectral barriers and inertial manifolds for dissipative partial differential equations

In recent years, the theory of inertial manifolds for dissipative partial differential equations has emerged as an active area of research. An inertial manifold is an invariant manifold that is finite dimensional, Lipschitz, and attracts exponentially all trajectories. In this paper, we introduce the notion of a spectral barrier for a nonlinear dissipative partial differential equation. Using this notion, we present a proof of existence of inertial manifolds that requires easily verifiable conditions, namely, the existence of large enough spectral barriers.     

Exponential tracking and approximation of inertial manifolds for dissipative nonlinear equations

In this paper, we study the long-time behavior of a class of nonlinear dissipative partial differential equations. By means of the Lyapunov-Perron method, we show that the equation has an inertial manifold, provided that certain gap condition in the spectrum of the linear part of the equation is satisfied. We verify that the constructed inertial manifold has the property of exponential tracking (i.e., stability with asymptotic phase, or asymptotic completeness), which makes it a faithful representative to the relevant long-time dynamics of the equation. The second feature of this paper is the introduction of a modified Galerkin approximation for analyzing the original PDE. In an illustrative example (which we believe to be typical), we show that this modified Galerkin approximation yields a smaller error than the standard Galerkin approximation.     

Sine-Gordon方程的截断系统的同宿轨道

研究Ｓｉｎｅ Ｇｏｒｄｏｎ方程的广义渐近惯性流形上的常微分方程组，证实了在一定参数条件下存在Ｗｉｇｇｉｎｓ［１］意义下的同宿轨道．计算表明，与Ｂｉｓｈｏｐ［２］用数值计算得到的Ｓｉｎｅ Ｇｏｒｄｏｎ方程产生混沌的参数值尚有差别，考虑到同宿出现参数值往往低于混沌出现参数值，故结果在定性上正确，而且改进了文［１］中的结果．     

Finite-dimensional description of convective Reaction-Diffusion equations

We are concerned with the asymptotic dynamics of a certain type of semilinear parabolic equation, namely,u _t=u _xx+(f(u))_x+g(u)+h(x) on the interval [0,L]. Under the general condition we prove that this equation admits a dissipative dynamical system and it possesses the global attractor. But for largeL > 0, we do not know whether or not an inertial manifold exists. Here we introduce a nonlinear change of variables so that we transform the above equation to a reaction diffusion system which possesses exactly the same asymptotic dynamics. We then prove the existence of an inertial manifold for the transformed equation; thereby we find the ordinary differential equation which describes completely the long-time dynamics of the orginal equation.     

Inertial Manifolds and Forms for Stochastically Perturbed Retarded Semilinear Parabolic Equations

We construct inertial manifolds for a class of random dynamical systems generated by retarded semilinear parabolic equations subjected to additive white noise. These inertial manifolds are finite-dimensional invariant surfaces, which attract exponentially all trajectories. We study the corresponding inertial forms, i.e., the restriction of the stochastic equation to the inertial manifold. These inertial forms are finite-dimensional Ito equations and they completely describe the long-time dynamics of the system under consideration. The existence of inertial manifolds and the properties of inertial forms allow us to show that under mild additional conditions the system has a global (random) attractor in the sense of the theory of random dynamical systems.     

无穷维动力系统中惯性流形和吸引子

本文综述了无穷维动力系统近年来的发展概况,系统地介绍了它的理论基础以及有关结果,最后就无穷维动力系统发展趋势和意义作了一些讨论.