线弹性结构非平稳随机振动分析的有限元方法

当前结构分析的有效方法是有限单元法,对于结构动力学问题,将变位、应力等物理量通过Fou-rier变换进行谱分解,在谱分解的形式下推求动力刚度矩阵,这样所得的矩阵和有关方程不能用结构的随机振动问题常用的振型分解法求解.本文提出了一个普遍化的求解方法.文中考虑如地震、风震等外载是如下非平稳随机过程:P(t)={P_i(t)},P_i(t)=α_i(t)P_i0(t),α_i(t)是巳知的时间函数,P_i0(t)是平稳随机过程.本文将有限单元法所得的离散化方程进行Fourier变换,利用随机过程谱分解的正交增量性质推导了激励谱和反应谱之间关系的公式.用这些公式可以寻求反应的互功率谱密度矩阵,再根据反应的统计量进行结构的安全度分析.在本文提出的计算方法中,当α_i(t)=1(i=1.,2,…,n)时方法可以简化为求解平稳过程的特殊情况.在实际应用中可以根据地震、风震记录所得的功率谱密度矩阵,按本文方法用计算机对高层、高耸、大跨度等结构问题进行分析,为了说明计算方法的特点,文中首先考虑单自由度情况,其次考虑多自由度情况,列出几个重要统计量的计算公式,并对数值计算方法和安全度分析作了讨论.     

Correlation of design parameters with performance for electrostatic Precipitator. Part I. 3D model development and validation

As the first part of a two-paper series, this paper develops a three-dimensional model that describes corona discharge, turbulent flow, particle charging and tracking in electrostatic precipitators (ESP). To capture the shielding effects between discharge wires in a multi-wire ESP, the corona-discharge-induced space charge density at an arbitrary point between the discharge wire and grounded plates is specified as the sum of two components: (i) a uniform value on the wire surface which is resolved individually for each wire; and (ii) a space-dependent variation relative to the uniform part. The present model is solved with the finite element method and validated with experimental and numerical results from literature. Good agreement is obtained and illustrated in terms of distributions of electric potential, current density, electrohydrodynamic flow pattern, and particle trajectories, as well as corona current and particle collection efficiency. This validated model will be applied in Part II and integrated with the design-of-experiment approach to analyze both individual and interactive effects of design parameters on ESP performance.     

三维非定常等熵流中的Chaplygin方程——Dirac-Pauli表象的复变函数理论及其在流体力学中的应用(Ⅲ)

本文是文[1]的继续。在本文中,我们将等熵气体动力学方程组分成两类问题来处理:其一为三维非定常无旋流(因而也是等熵流),其二为三维非定常等熵无散流(即不可压缩等熵流)。我们应用Dirac-Pauli表象的复变函数理论并采用Legendre变换,将此两类问题的方程组变换到速度空间,从而得到了两种推广的Chaplygin方程。推广的Chaplygin方程是一个线性偏微分方程,它的通解至多由超几何函数表示。由此,我们求得了气体动力学三维非定常等熵流的一般问题的通解。     

Boundary stabilization of vibration of nonlocal micropolar elastic media

In this paper, we study the stabilization problem of vibration of linearized three-dimensional nonlocal micropolar elasticity. For this purpose, we need to demonstrate the well-posedness of the system of equations governing the vibration of three-dimensional nonlocal micropolar media for both forced (i.e. with boundary feedback) and unforced cases. We assume the non-homogeneous system of equations for the unforced (uncontrolled) case to establish the well-posedness. It should be pointed out that the well-posedness of the evolution equations in micropolar case has been studied by many authors; but, the well-posedness in the nonlocal micropolar is an open problem. Our tools in well-posedness analysis are the semigroup techniques. Afterwards, we pursue the stabilization problem and show that the vibration of the nonlocal micropolar elastic media will be eventually dissipated under boundary feedback actions consisting of stress and couple stress feedback laws. These control laws are simple, linear and can be easily implemented in practical applications. The stabilization proof is accomplished using Lyapunov stability and LaSalle’s invariant set theorems.     

General Three-Dimensional Disturbances to Inviscid Couette Flow

The general solution to the linearized equations governing three-dimensional disturbances to inviscid Couette flow has been obtained. This result extends the Orr solution to initial conditions that do not consist of a single Fourier sine component in the cross-stream coordinate and a plane wave in the streamwise/spanwise coordinates. The time evolution of a measure of disturbance energy for some specific pulsed initial conditions is examined, and it is concluded that, while the rapid algebraic growth to large amplitude followed by decay exemplified by the Orr solution can be of importance for individual cross-stream Fourier components, more realistic initial conditions, which in general consist of the sum of an infinite number of components, often display uniform decay to zero amplitude. However, an interesting example is described in which one positive definite measure of disturbance amplitude remains constant, yet the streamwise/spanwise velocity components grow linearly in time if the initial disturbance is three-dimensional.     

NEW DEVELOPMENTS IN FOREST MODELING: CONVERGENCE BETWEEN APPLIED AND THEORETICAL APPROACHES

ABSTRACT. Additionally to simulations, a mathematical analysis of ecological models could give insights into the system behavior. However, mathematical model analysis has only been performed for simple, abstract models, because models based on biological processes are too complex. Linking simple and detailed approaches could help to obtain mathematically tractable models which are based on physiological and ecological processes. This can be achieved either by extending simpler models with components of complex ones, or by a controlled simplification of complex models. Forest models ranging from models describing processes in organs of individual tress in three-dimensional space to models averaging over all trees of an entire region, are reviewed, categorized by hierarchical level and represented in a general mathematical formulation. Possibilities and approaches of transitions between the approaches, namely linking of approaches and upscaling of individual based models are presented and discussed.     

Electrostatic backscattering by insulating obstacles

We introduce and analyze backscattering data for a three-dimensional obstacle problem in electrostatics. In particular, we investigate the asymptotic behavior of these data as (i) the measurement point goes to infinity and (ii) the obstacles shrink to individual points. We also provide numerical simulations of these data.     

Stability analysis of the cell centered finite-volume M<Emphasis Type="SmallCaps">uscl</Emphasis> method on unstructured grids

The goal of this study is to apply the Muscl scheme to the linear advection equation on general unstructured grids and to examine the eigenvalue stability of the resulting linear semi-discrete equation. Although this semi-discrete scheme is in general stable on cartesian grids, numerical calculations of spectra show that this can sometimes fail for generalizations of the Muscl method to unstructured three-dimensional grids. This motivates our investigation of the influence of the slope reconstruction method and stencil on the eigenvalue stability of the Muscl scheme. A theoretical stability analysis of the first order upwind scheme proves that this method is stable on arbitrary grids. In contrast, a general theoretical result is very difficult to obtain for the Muscl scheme. We are able to identify a local property of the slope reconstruction that is strongly related to the appearance of unstable eigenmodes. This property allows to identify the reconstruction methods that are best suited for stable discretizations. The explicit numerical computation of spectra for a large number of two- and three-dimensional test cases confirms and completes the theoretical results.     

Computation of crack propagation in 3-dimensional anisotropic solids

This contribution presents ideas, how crack propagation in three-dimensional solids composed of anisotropic materials can be predicted using the Griffith energy principle. Since the work of Irwin the change of potential energy caused by a straight elongation of a crack in an isotropic two-dimensional homogeneous structure can be expressed in quadratic terms of the stress intensities at the crack tip. This result was generalized in the last decades using methods of asymptotic analysis by many authors [1] to more complicated geometries, to anisotropic and inhomogeneous materials. With the energy release rate at hand, quasi-static scenarios of crack propagation can be simulated for plane problems [2], but this is still a complicated task for three-dimensional problems [3]. We show an idea how the change of energy caused by propagation of a crack surface in a fully three-dimensional solid of nearly arbitrary shape can be computed in anisotropic materials. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Designing Statistically Similar RVEs for 3D Dual Phase Steel Microstructures

This contribution presents a method for the construction of three-dimensional Statistically Similar Representative Volume Elements (SSRVEs) for dual phase steels (DP steels). From such kind of advanced high strength steels, enhanced material properties are observed, which originate in the interaction of the individual constituents of the material on the microscale. Our aim is to directly incorporate the microstructure in the material modeling, which can be accomplished by applying i. e. the FE² method. A RVE representing the real material is used in the microscopic boundary value problem, which is solved at each macroscopic integration point. Since such RVEs usually exhibit a high complexity due to the underlying real microstructure, high computational costs are a drawback of the approach. We replace this RVE with a SSRVE, which has a lower complexity but which is still able to represent the mechanical behavior of the RVE and thus of the real microstructure. Virtual experiments show the performance of the method. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Three-Dimensional Pallet Chart: An Analysis of the Factors Affecting the Set of Feasible Layouts for a Class of Two-Dimensional Packing Problems

The two-dimensional orthogonal packing problem of packing identical rectangles into a large containing rectangle is important in pallet loading and has recently received much attention in O.R. publications. In this paper, we examine the conditions under which the set of feasible layouts remains unchanged and show that these conditions can be represented by a series of planes in three-dimensional space. We call this representation the three-dimensional pallet chart because it is an extension of the two-dimensional pallet charts presently used in many practical situations. The strength of this result is demonstrated by three examples of its use. Accurate two-dimensional charts are produced from the three-dimensional version with a minimum of calculation, and a complete sensitivity analysis to changes in box and pallet dimensions can be carried out visually by viewing the chart at different angles. Finally, the result is used to generate a new procedure for determining the maximum number of rectangles which can be fitted. This method is shown to be accurate for over 90% of observations in a random sample of 5000—an improvement of 20% over previous methods.     

Nonlinear dynamics and circuit implementation for a new Lorenz-like attractor

In this paper, a new Lorenz-like chaotic system is reported. Nonlinear characteristic and basic dynamic properties of the three-dimensional autonomous system are studied by means of nonlinear dynamics theory. The chaotic system is not only demonstrated by theoretical analysis and numerical simulation, such as equilibria stability analysis, Lyapunov-exponent spectra, Lyapunov dimension, bifurcation diagrams, but also implemented via an electronic circuit.     

Recurrence of Ancestral Lines and Offspring Trees in Time Stationary Branching Populations

For time stationary Galton- Watson-branching populations on a general type space, the structure of the “individually positive recurrent part” of the system is described: its building blocks consist of finitely many “clans” with positive recurrent trunks. Conditions are given when this nubsystem is void, and when it equals the full system. In addition, positive recurrence on the clan level is characterized. Whereas individual positive recurrence turns out to be a symmetric concept with respect to forward and backward time direction (i. e., with respect to anceatral lines and offspring trees), with individual null recurrence this symmetry can fail even in the absence of branching, i.e., for independently migrating particle systems (Example 13.1). For discrete type spaces a classification of types as to the various individual recurrence concepts (positive, null, forward and backward in time) is proposed and illustrated by a couple of results and examples. For finite type spaces conditiom on the branching dynamics and its mean matrix for the existence of nontrivial equilibria are given.     

Change in the Form of Evolution: Transition from Primate to Hominid Forms of Social Organization

In this paper I sketch a model for the transition from biologically to culturally based forms of social organization. The impetus for the transition arises from increased individualization among the non-human primates that can be observed as one moves phylogenetically from the Cercopithecoids and Ceboids (Old and New World monkeys) to the hominoids, especially the African apes. Increased individualization introduced a conflict with coherent and stable social integration that was only resolved among the hominid ancestors to modern Homo sapiens by shifting to a cultural/conceptual, rather than a behavioral/biological, basis for social organization. The shift entailed a change from evolution driven by individual fitness to evolution driven by the structural coherency of a conceptual system for social organization; that is, to selection based on group, rather than individual, level traits. Conceptually the transition depended upon the evolution of mental capacities such as a theory of mind and recursion, both of which are absent or occur only in minimal form among the non-human primates.     

Qualitative analysis of a class of competitive and cooperative systems

Hirsch[1,2] studied the limiting behavior of solutions of competitive or cooperative systems, and showed that ifL is an ω-limit set of a three-dimensional cooperative system, which contains no equilibrium, thenL is a nonattracting closed orbit. Smith^[3] considered a three-dimensional irreducible competitive system and showed that an ω-limit set containing no equilibrium must be a closed orbit which has a simple Floquet multiplier λ<1, and may be attracting. In this paper we carry out the qualitative analysis of a class of competitive and cooperative systems, and a generalization of the result of Levine^[4] is given. The stability problem of closed orbits raised in [5] and [6] is resolved.     

Time-Cost Trade-Off Analysis in Some Constrained Assignment Problems

Several variations of two-dimensional (workers x jobs) and three-dimensional (workers x jobs x machines) time- as well as cost-minimizing assignment problems, which arise owing to (i) precedence relations of some form among the jobs or (ii) capacity restrictions on workers/machines imposed by the requirement that the surplus resources have to be fully employed, have been considered in the literature. In this paper, an algorithm is presented for time-cost trade-off analysis which is applicable to any general pair of such constrained problems. The algorithm is also illustrated by a numerical example.     

X-ray absorption spectra of plasmas

In this paper we present a theoretical method to calculate the absorption spectra of hot dense plasmas. Based on our fully relativistic treatment incorporated with the quantum defect theory to handle the huge number of transition arrays from many configurations with high principal quantum number, we can calculate the absorption spectra for any element or multi-element plasmas with little computational efforts. We calculate the absorption spectra of C10H1605 plasmas, which are in good agreement with the experimental spectra. We can then provide diagnostic analysis for plasmas in relevant inertial confinement fusion (lCF) experiments; namely not only to determine plasmas' temperatures and densities, but also to provide the population densities of various ionic stages. Our theoretical method verified by "benchmark experiments" will be a basic tool to provide "precise" opacity data for the ICF research.``     

Computer aided treatment of 3d-problems in analytic geometry

In this contribution a method for the treatment of analytic geometrical problems is introduced which integrates three-dimensional computer graphics and computer algebra. At this a new computer graphics tool is used which has been developed for the visualization of the corresponding spatial configurations and for the graphical solution of spatial analytic problems. (The virtual sphere device is used for flexible and individual visualization). The tool is suitable both for the demonstration by the teacher and for the interactive work of the students. The computer algebraic treatment is motivated by giving an explanation for the computer graphical solution. Firstly the general algebraic problem is solved and graphically illustrated, after this the general solution is numerically specified. The procedure of analytic solution can be illustrated also computer-graphically.     

RMVT- and PVD-based finite cylindrical layer methods for the three-dimensional buckling analysis of multilayered FGM cylinders under axial compression

The unified formulations of finite cylindrical layer methods (FCLMs) based on the Reissner mixed variational theorem (RMVT) and the principle of virtual displacements (PVD) are developed for the three-dimensional (3D) linear buckling analysis of simply-supported, multilayered functionally graded material (FGM) circular hollow cylinders and laminated composite ones under axial compression. The material properties of the FGM layer are assumed to obey the power-law distributions of the volume fraction of the constituents through the thickness coordinate. In these formulations, the cylinder is divided into a number of finite cylindrical layers, in which the trigonometric functions and Lagrange polynomials are used to interpolate the in- and out-of-surface variations of the primary variables of each individual layer, respectively, as well as the related order of each primary variable can be freely chosen, such as the layerwise linear, quadratic or cubic function distribution through the thickness coordinate. The accuracy and convergence of the RMVT- and PVD-based FCLMs developed in this article are assessed by comparing their solutions with the exact 3D solutions available in the literature.