TOPOLOGY DESIGN OF STRUCTURES SUBJECTED TO PERIODIC LOADING

Although a lot of attention in the topology optimization literature has focused on the optimization of eigenfrequencies in free vibration problems, relatively little work has been done on the optimization of structures subjected to periodic loading. In this paper, we propose two measures, one global and the other local, for the minimization of vibrations of structures subjected to periodic loading. The global measure which we term as the “dynamic compliance” reduces the vibrations in an overall sense, and thus has important implications from the viewpoint of reducing the noise radiated from a structure, while the local measure reduces the vibrations at a user-defined point. Both measures bring about a reduction in the vibration level by moving the natural frequencies which contribute most significantly to the measures, away from the driving frequencies, although, as expected, in different ways. Quite surprisingly, the structure of the dynamic compliance optimization problem turns out to be very similar to the structure of the static compliance optimization problem. The availability of analytical sensitivities results in an efficient algorithm for both measures. We show the effectiveness of the measures by presenting some numerical examples.     

MODEL-ORDER REDUCTION AND PASS-BAND BASED CALCULATIONS FOR DISORDERED PERIODIC STRUCTURES

This paper is concerned with the dynamics of disordered periodic structures. The free vibration problem is considered. A method akin to the Rayleigh method is presented. This method is particularly suitable for the study of periodic structures as it exploits the nominal periodicity leading to an approximation that greatly reduces the order of the model. The method is used to calculate the natural frequencies and mode shapes for a pass-band by treating the unknown phases between the nominally identical bays as the generalized co-ordinates of the problem. An illustrative example of a cyclically coupled beam model is presented. In spite of a very large reduction in the computational effort, the results obtained are very accurate both for frequencies and mode shapes even when strong mode localization is observed. To test the performance of the proposed approximation further, a situation where two pass-bands are brought close to each other is considered (a coupled beam model having inherent bending-torsion coupling). The method presented here is general in its formulation and has the potential of being used for more complex geometries.     

THE CHARACTERISTIC RECEPTANCE METHOD FOR DAMAGE DETECTION IN LARGE MONO-COUPLED PERIODIC STRUCTURES

An approach based on the concept of wave propagation to detect the structural damage in large mono-coupled periodic structures is presented in this paper. The free vibration analysis of a finite mono-coupled periodic structure with a single disorder has been conducted by the characteristic receptance method, and the sensitivity of the natural frequencies to the disorder in flexibility has been discussed. Based on the sensitivity analysis, the locations and magnitude of damage in large mono-coupled periodic systems have been estimated using measured changes in the natural frequencies. The paper also introduces a substructure-based method for improving the computational efficiency and the accuracy of damage detection in large mono-coupled periodic structures. Numerical results from two periodic mass-spring-structures show that the proposed method can provide good predictions of both the locations and magnitude of damage at one or more sites. Furthermore, the proposed method, in which a priori information about the nature such as stiffness of the undamaged structure is not needed, and only measurements of the change in a few of the structure's natural frequencies between the undamaged and damaged states are required, is particularly attractive in practice. However, some issues such as the role of noise in actual measurements, application to multi-coupled periodic structures with complex boundary conditions remain to be resolved before this approach becomes a truly variable method of structural damage assessment.     

多孔复合介质周期结构热传导和质扩散问题的多尺度数值方法

本文针对一类复杂的多孔复合介质的热传导和质扩散问题,给出具体的多尺度渐近展开公式,并在此基础上设计了有限元算法格式,它是宏观和细观相结合的数值方法。理论分析和数值实验均表明：多尺度数值方法对求解多孔复合介质周期结构的热传导和质扩散问题是可行的和有效的。     

PHASE OPERRTORS FOR ATOMIC SYSTEM IN SCHWINGER BOSON REPRESENTATION AND THEIR PROPERTIES

In terms of the Schwinger boson representation of angular momentum We introduce suitable phase operators which exhibit similar properties to those of the phase operators in radiation field into atomic system. With the help of them angular momentum's third component phase uncertainty relation are discussed. Also, the eigenstates of the Hermitian phase operator are derived.     

双激光脉冲在相干介质中的传播特性

从密度矩阵形式的Bloch方程和Maxwell传播方程出发,建立了二能级双脉冲传播的三维半经典理论模式,并给出了数值求解结果-考察了双脉冲在介质中的时空演化,发现了三维空间中双脉冲合并的特性,并与单脉冲的传播进行了比较- 关键词：     

永久磁铁周期性聚焦电子束

在余弦式及非余弦式周期磁场下描写电子轨迹的傍轴方程曾用微扰法求解,得到了一些最佳聚焦条件。在推导中考虑了空间电荷效应及阴极被部分屏蔽的情况,当电子束在周期场中波动比较大时这些结果也能应用。另外用变振幅的方法研究了电子束在周期磁场内的稳定性问题,得到了一些包含有空间电荷密度及阴极屏蔽程度在内的稳定性条件。并绘出图线表示禁区位置因各种不同空间电荷密度及阴极屏蔽程度而改变的情况,知道虽然在用全屏蔽的电子枪情况下,正如过去一些著者所指出的,禁区位置不因空间电荷密度的大小而改变,但在阴极被部分屏蔽的情况下,空间电荷     

EFFECTS OF MODE LOCALIZATION ON INPUT-OUTPUT DIRECTIONAL PROPERTIES OF STRUCTURES

The singular-value decomposition (SVD) is employed to study the effects of localization phenomena on input-output relationships, and power and energy transmission ratios of structures. For weakly and strongly coupled systems, existence of strong localization of singular vectors and abrupt veering of singular value loci are shown. Occurrence of strong localization causes abrupt changes in input-output directional properties. In contrast to eigenvalues, singular values do not veer away by introducing a disorder in weakly coupled systems. In particular, the use of singular values and vectors is computationally advantageous in considering multiple load cases. While eigenvalue-based analyses give information about the resonance frequencies and vibration modes of a structure, singular values of the structure are related to the forced response characteristics and give the dynamic behavior in the frequency domain. Representative examples in structural dynamics are presented.     

有扩散不稳定性的四阶反应-扩散系统的空间周期结构

本文利用分叉理论研究一个含参数的有扩散不稳定性的四阶反应-扩散系统的空间周期结构问题,其中扩散项服从Cahn-Hillard广义扩散定律。首先通过稳定性和奇异性的分析得到从空间均匀定态产生空间周期定态的判别准则,然后利用奇异摄动方法获得这个空间周期定态的二阶近似解。本文的理论结果很好地符合文献[3]中的数值结果。 关键词：     

INVESTIGATION ON EFFECTS OF LEFT-HANDED MATERIALS ON FREQUENCY SELECTIVE CHARACTERISTICS OF DIELECTRIC PERIODIC STRUCTURES

The effects of left-handed materials (LHM) on frequency selective reflection and transmission characteristics of dielectric periodic structures are carefully investigated using a method which combines the multimode network theory with the rigorous mode matching method. In addition, the effects of LHM dielectric loss on the selective properties are also given. It has been shown that the selective behavior of periodic structures can be modified by adding LHM layers above and below the periodic layer. Supported by the National Natural Science Foundation of China (No. 60171037) (No. 60371010) and (No. 63531020)     

Stoner集体电子铁磁理论的圖示

在stoner集体电子铁磁理论的三个前提假设的基础上,一系列的能带图示被提供出来。这些图示鲜明地描绘了Stoner理论的物理情况。Stoner的铁磁判据,即在kθ′/ε₀=ξ>2/3时始能发生铁磁性及在ξ≥2^-1/3时磁化始完全,可利用图示简捷地得到。在铁磁状态下由d-带分裂引致的自d-带至s-带的电子边移,可利用图示加以考虑,并对0°K时的镍作了估算,结果约为1％。     

光脉冲在负色散克尔介质中传输的时空耦合特性

本文综合考虑了在(3+1)维负色散介质中Kerr效应、衍射、色散对光脉冲传输的影响,用变分原理和数值计算的方法对带啁啾高斯光束的传输特性进行了研究,讨论了非线性时空耦合参数,初始束宽、脉宽、散聚特性对时空“自聚焦”特性的影响,计算表明利用会聚光束在负色散克尔介质中传输的耦合作用,可以压缩脉宽,这可作为一种新的压缩脉宽的方法。     

PROPERTIES OF PROPAGATION OF GUIDED ELECTRON WAVES IN COUPLED ASYMMETRIC QUANTUM WELLS

In this article we investigate the properties of the propagation of guided electron waves in the coupled asymmetric quantum wells. By decomposing the eigenfunctions of electron in the coupled double quantum wells in terms of the basis eigenfunctions of the individual wells, the expression of the mode amplitude func-tions for various bare states in wells has been given. By setting up appropriate boundary conditions one can simulate different injecting manners of electron into the system and study how the electron waves trans-fer among various states. The major electron wave transfer happens between the matching bare states in wells. The particular pattern of the transfer depends on the injecting manner and the energy of the inci-dent electron. We also show the influence of the presence of the multimodes on the variatinn of the mode amplitude functions. It leads to producing some oscillations imposed on the original sinusoidal-like func-tions and causing incomplete transfer between a pair of states in channels due to the beating frequency effect or the interference among various modes.     

原子尺度金属纳米线的结构和性质

当物质尺度减少到几层原子时，形成超细的纳米结、纳米线、或者纳米团簇，原有凝聚态物质的结构和物理性质将不再保持，而呈现出许多令人惊奇的奇异特性。本文重点讨论直径大约3nm以下，具有足够长度的、原子结构往往不同于体材料的准一维金属纳米结构，我们称之为原子尺度金属纳米线或超细金属纳米线（也称为金属原子线）。近年来实验上已经制备和表征出在超高真空中悬挂在两个顶针尖端的Au、Pt、Cu等金属纳米线和纳米管，金属线直径达到1nm以下而长度为6nm以上。通过高分辩电子显微镜观察，它们是同轴圆管（或壳）组成的、类似纳米碳管的单壳或多壳结构，管由绕着线轴的螺旋原子绳构成。理论工作围绕这种新奇结构形态的形成机制、奇异的物理性质和可能的应用前景而同时展开。这是一个崭新的纳米世界，无论是对基础的低维物理还是未来分子电子设备的应用，都将产生深远的影响，有许多奇妙的现象正等待人们去发现。本文将对最近几年原子尺度金属纳米线研究工作的主要进展和发展趋势作一个概述，并重点介绍本组有关的具有螺旋结构的纳米线的各类新奇结构和物理性质。     

高功率微波脉冲大气传输的一些规律

 推导了高功率微波（HPM）脉冲大气击穿阈值功率公式，估计了脉冲从地面向上传输的潜行时间。数值模拟了HPM大气传输的物理图象和潜行时间的变化。     

有限磁场中等离子体圆柱波导的传播特性

分析了有耗介质中等离子体圆柱波导在有限外加磁场中的传播特性.重点讨论了波导传播常数随等离子体参数、介质参数和外加磁场的变化.分析结果表明，有限强磁场中的等离子体波导的传播特性比无外磁场或外加磁场为无穷大时具有更强的控制能力. 关键词：     

各向同性纳米结构Fe-Pt薄膜的结构和磁性

采用直流溅射和热处理技术制备了两个各向同性的纳米结构Fe-Pt永磁合金薄膜系列,并研究了它们的结构和磁性.研究表明,在富Fe双相纳米结构Fe-Pt永磁合金薄膜中,仅由硬磁的FePt相与软磁的Fe₃Pt相组成;在同一系列中,随Fe层厚度的增加,饱和磁极化强度和剩磁明显增大.由Kelly-Henkel图研究指出,在上述Fe-Pt纳米结构永磁合金薄膜中,磁相互作用主要由近邻纳米晶粒间的铁磁交换相互作用控制. 关键词： 磁性薄膜 纳米结构 矫顽力     

自由电子激光中预群聚电子束的传播性质

研究了在自由电子激光(FEL)中预群聚电子束的传播性质,并得到如下结论:在预电子束的传播过程中,基波调制电流的振幅随传播距离增加,其相位保持不变;谐波调制电流的振幅及相位均保持不变。