粘-弹性介质中渗流的有限元法分析

R.S.Sandhu和E.L.Wilson提出了弹性介质中渗流分析的有限元法[1],可以用来分析较复杂的工程问题,本文将此方法推广到粘-弹性介质情况.当土骨架是粘-弹性介质时,应力与应变的关系随时间变化,增加了问题的复杂性.本文提出了折算弹性张量的概念,即当有限元法计算中的时间间隔Δt预先给定时,在此间隔内应力增量和应变增量可以近似表示为线性关系.这些线性比例常数将称为折算弹性张量.Sandhu和Wilson方法仅能用于弹性介质情况,根据折算张量概念,本文方法适用范围推广到粘-弹性介质情况.     

Convergence of Weak Galerkin Finite Element Method for Second Order Linear Wave Equation in Heterogeneous Media

Weak Galerkin finite element method is introduced for solving wave equation with interface on weak Galerkin finite element space $(\mathcal{P}_k(K), \mathcal{P}_{k−1}(∂K), [\mathcal{P}_{k−1}(K)]^2).$ Optimal order a priori error estimates for both space-discrete scheme and implicit fully discrete scheme are derived in $L^∞(L^2)$ norm. This method uses totally discontinuous functions in approximation space and allows the usage of finite element partitions consisting of general polygonal meshes. Finite element algorithm presented here can contribute to a variety of hyperbolic problems where physical domain consists of heterogeneous media.     

Dynamics of Wave Scintillation in Random Media

This paper concerns the asymptotic structure of the scintillation function in the simplified setting of wave propagation modeled by an Itô–Schrödinger equation. We show that the size of the scintillation function crucially depends on the smoothness of the initial conditions for the wave equation and on the size of the “array of detectors” where the wave fields are measured. In many practical settings, we show that the estimates are optimal and devise an equation for the appropriately rescaled scintillation function. The estimates are based on a careful analysis of Wigner transforms and of linear kinetic equations involving oscillatory integrals.     

Media Selection and Fuzzy Linear Programming

Mathematical Programming models have been suggested as a tool for optimal media selection. In order to accommodate several objective functions goal programming models have been put forward. It seems, however, that these models are not yet operational and efficient enough to be used in practice.Based on these approaches a model is suggested which seems to be suited to map appropriately fuzzy and subjective phenomena and which is flexible enough to incorporate several criteria (objective functions) simultaneously.An interactive use is envisaged which would help to adapt the model easily to different real world conditions.     

Blow up of Solutions of a Nonlinear Viscoelastic Wave Equation

We consider a nonlinear viscoelastic wave equation with nonlinear source term. Under suitable conditions on g, it is proved that any weak solution with negative initial energy blows up in finite time if p>2.     

激发介质波前曲面和组织中心的结构

通过建立波前曲面上的移动坐标系和边界层内的摄动分析,研究激发介质的波前曲面和组织中心的结构,得到波前曲面所满足的特征方程,组织中心所满足的一般方程．这些特征方程表明激发介质的波前曲面具有扭曲型回卷波、Mbius带型结构等变化;组织中心的轨迹具有打结、连结型的环型结构．得到的理论结果不仅解释了BZ化学反应产生的波形图案,而且给出一般激发介质波前曲面和组织中心可能出现的变化．     

Dynamic Wave Propagation in Porous Media Semi-Infinite Domains

The problem of dynamic wave propagation in semi-infinite domains is of great importance, especially, in subjects of applied mechanics and geomechanics, such as the issues of earthquake wave propagation in an infinite half-space and soil-structure interaction under seismic loading. In such problems, the elastic waves are supposed to propagate to infinity, which requires a special treatment of the boundaries in initial boundary-value problems (IBVP). Saturated porous materials, e. g. soil, basically represent volumetrically coupled solid-fluid aggregates. Based on the continuum-mechanical principles and the established macroscopic Theory of Porous Media (TPM) [1, 2], the governing balance equations yield a coupled system of partial differential equations (PDE). Restricting the discussion to the isothermal and geometrically linear case, this system comprises the solid and fluid momentum balances and the overall volume balance, and can be conveniently treated numerically following an implicit monolithic approach [3]. Therefore, the equations are firstly discretised in space using the mixed Finite Element Method (FEM) together with quasi-static Infinite Elements (IE) at the boundaries that represent the extension of the domain to infinity [4], and secondly in time using an appropriate implicit time-integration scheme. Additionally, a stable implementation of the Viscous Damping Boundary (VDB) method [5] for the simulation of transient waves at infinity is presented, which implicitly treats the damping boundary terms in a weakly imposed sense. The proposed algorithm is implemented into the FE tool PANDAS and tested on a two-dimensional IBVP. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wave Propagation in Dissipative or Dispersive Non-linear Media

A singular perturbation method is developed to investigate onedimensional weak nonlinear waves in dissipative or dispersivemedia. Utilizing this method a boundary value problem for asystem of partial differential equations characterizing wavepropagation in homogeneous dissipative or dispersive media isstudied. In order to obtain a first-order uniformly valid solution,the problem is reduced to an initial value problem for scalarnon-linear partial differential equation. Some special casesarising from the structure of coefficient matrices are examinedand the method is extended to these cases. As an applicationof the perturbation method, various problems of wave propagationin a finite linear viscoelastic half-space are studied.     

一维粘弹性波动方程弹性系数的识别方法

本文就一维粘弹性波动方程弹性系数的求解问题，给出了一个新的求解方法．通过对算法进行分析可知，该方法具有较小的计算量，并且具较好的数值稳定性．数值模拟表明了该方法的可行性及有效性．     

粘弹性柱壳的若干动力学性质

本文讨论了轴向压力和一致分布的径向压力作用下粘弹性柱壳的动力稳定性问题.利用Laplace变换,得到了常载作用下粘弹壳的稳定性条件.综合利用动力学的经典方法,得到了由粘弹性壳所定义的动力系统的各种动力学性质及各种参数对结构稳定性的影响.     

基于非局部理论的黏弹性纳米杆轴向振动与波传播研究

根据非局部理论和Kelvin黏弹性理论，针对黏弹性纳米杆自由振动和波传播的轴向动力学问题进行研究.首先，推导了黏弹性纳米杆的轴向动力学微分控制方程.然后，通过无量纲化讨论了3种典型边界纳米杆的前三阶振动特性.最后，研究黏弹性纳米杆波的传播问题，导出了圆频率、波速与波数之间的关系.数值结果表明，非局部效应使第一、二阶固有频率持续减小，第三阶频率先增大再减小，出现结构刚度削弱和增强两种趋势.特别地，对于自由端存在集中质量的情形，第二阶频率随着黏性系数增大出现了多值情况，易导致杆件失稳.数值算例还说明了非局部效应的增强可有效降低黏性材料的阻尼效应，产生逃逸频率，使得纵波能够在高波数段传播.另外，黏性系数在低波数段对阻尼比影响可忽略不计，而在高波数段下，黏性系数越大则阻尼比越大.     

Stress Singularities in Viscoelastic Media 2. Plane-Strain Stress Singularities at Corners

The stress singularities that evolve at the corner of a notchedviscoelastic angular plate subject to mode I deformation isdiscussed when prescribed, but arbitrary, displacements aresymmetrically applied to both radial edges of the sector. Thesolution procedure, based on Laplace and Mellin transforms (withtransform parameters p and s, respectively), leads to an eigenvalueproblem in the complex p-plane, which is dependent on Poisson'sratio and characterizes the singular behaviour of the stressfields. Although simple solutions to the transcendental eigenequationare not available, the real-time evolution of the stress concentrationsis obtained by monitoring a particular branch of the eigenvalueequation as it moves in the complex p-plane. A correspondingpath is traced in an appropriately cut strip in the s-plane,in which solutions to the eigenequation are single-valued. Analyticcontinuation in the p-plane thus allows the Laplace and Mellininversions to be performed and the real-time behaviour of theplane-stress components to be expressed as contour integralswithin the strips in the complex s-plane. Cast in this form,the stress components are evaluated numerically when the viscoelasticmaterial is represented as a standard linear solid. Their dependenceon the angular variation within the plate, the applied load,and the effects of the viscoelastic material properties is exhibitedfor a number of situations, and in each case contrasted withshort- and long-time asymptotic curves based on Tauberian theorems.     

地下水在粘弹性含水层系中不定常渗流

本文研究了地下水在粘弹性含水层系中不定常渗流动态。在前人工作的基础上导出了新的微分-积分方程组。已知的微分方程组是它的特殊情况。新的线性微分-积分方程组描写了弱压缩流体在粘弹性含水层系中流动。用Laplace变换的方法求得了微分-积分方程组的解析解。粘弹性增加了含水层系的非均质性,即具有延迟和补给的性质。数值反演解和解析解符合得较好。它们给出了地下水在这种非均质含水层系中水位变化动态。     

Analysis of Wave Propagation in 1D Inhomogeneous Media

In this paper, we consider the one-dimensional inhomogeneous wave equation with particular focus on its spectral asymptotic properties and its numerical resolution. In the first part of the paper, we analyze the asymptotic nodal point distribution of high-frequency eigenfunctions, which, in turn, gives further information about the asymptotic behavior of eigenvalues and eigenfunctions. We then turn to the behavior of eigenfunctions in the high- and low-frequency limit. In the latter case, we derive a homogenization limit, whereas in the first we show that a sort of self-homogenization occurs at high frequencies. We also remark on the structure of the solution operator and its relation to desired properties of any numerical approximation. We subsequently shift our focus to the latter and present a Galerkin scheme based on a spectral integral representation of the propagator in combination with Gaussian quadrature in the spectral variable with a frequency-dependent measure. The proposed scheme yields accurate resolution of both high- and low-frequency components of the solution and as a result proves to be more accurate than available schemes at large time steps for both smooth and nonsmooth speeds of propagation.     

线性流形的性质

线性流形作为n维数组向量空间的子集合,与线性方程组的解和n维数组向量空间的子空间有着密切的联系.通过讨论线性流形的一些基本性质,研究其与线性方程组的解,线性子空间的关系,从而加深对这一概念的理解.     

星系密度波的线性增长

本文以线性密度波局部渐近解为初值,求解二维非定常流体力学方程组和泊松方程,研究星系密度波的线性增长.数值计算结果表明,线性密度波将在几千万年时间内增长到与基态同量级,形成在中心区域具有棒形结构的螺旋图样.螺旋结构的图样速度及扰动密度的增长率随空间位置及时间变化.讨论了准稳螺旋结构假设的近似性.