Glassy dynamics

We review dynamic processes in supercooled liquids and glasses as studied by dielectric spectroscopy. It is the only experimental technique which allows one to follow the tremendous slow-down of diffusive motion of particles in disordered condensed matter over more than 18 decades in frequency or time. The dielectric techniques used are treated in detail. As an introduction for non-specialists, the time and temperature evolution of the basic spectral features associated with various dynamic relaxation processes are discussed in detail. Among them are the structural relaxation, the occurrence of fast processes and the boson peak. The relevance of these features for glass formation is discussed. The present article may also serve as a review for recent experimental and theoretical studies on glass-forming liquids.     

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在强度静力分析基础上,对S弯盒壁厚、加强筋间距等参数进行了优化设计和校核。在模态分析基础上,对模型进行了频谱分析和动态时程分析,获得地震动下确保结构安全运行的设计参数。     

弹性矩形板非线性振动的多模态解

本文将非线性振动矩形板的振型函数展开为梁函数和Ｂ样条函的乘积形式。由哈密顿原理导出了系统的运动微分方程，得到了以多个线性模态表示的大振幅振动板的位移和非线性频率比。计算结果表明：该法具有很高的计算速度和精度。     

Fast convergence and higher-order mode calculation of optical waveguides: perturbation method with finite difference algorithm

We present an efficient algorithm for determining mode eigenvalues as well as field distributions of optical waveguides with two-dimensional transverse refractive index profile. The algorithm is devised with the analytical perturbation correction method combined with the finite difference approximation of Helmoltz's equation. The technique is simple and does not involve solving any eigenvalue equation or matrix formalism. The algorithm reduces abruptly the computation time required for the field convergence to mode, and can calculate any higher-order modes without the need of any pre-conditioning the field w.r.t. waveguide geometry, or calculation of previous order modes and/orthogonalization. The analysis can yield precisely both scalar and polarized modes. By applying it to waveguide problems whose solutions are otherwise known, the efficacy of the method has been established.     

相对论返波管模式变换器的研究

根据相对论返波管的需求设计计算了一种模式变换器兼反射器。它将工作模式TM11模转换成TE11模的同时将TE11模反射回去,用普通的喇叭形成高斯波束发射输出。在我们的设计方案中,它是旁壁对称正弦波纹的圆波导,变换器的中心频率为10GHZ。数值模拟结果显示,变换器的长度为7个周期时,中心频率点转换的效率已经超过了90％,工作频带能够达到± 0.3GHz。     

软X射线皮秒变象管扫描相机

软X射线皮秒变象管扫描相机技术是一种用变象管扫描相机测量X射线脉冲而能达到皮秒时间分辨率和一维微米级空间分辨率的技术。X射线扫描相机是研究伴有X射线辐射的一切高速瞬变过程的极为重要的测量工具。例如在激光核聚变的研究中,加热压缩期间靶球发生内爆过程中高温高密度等离子体的光谱辐射主要为软X射线,对这些软X射线的研究,可以获得有关等离子体的电子温度、电子能量分布、各种不稳定性的形态及这些参量随时间变化的资料。     

光纤光栅的反射率特性

基于光纤光栅在光通信中的重要应用,根据耦合模理论数值模拟了波长在1.55μm附近时光纤光栅长度与反射率的关系,分析图形得到了在1.50μm到1.55μm之间出现较强的反射率,几乎接近100%,同时分析了折射率变化对反射率的影响,研究结果为光通信中无源光器件的制作提供理论依据。     

Development of fracture facets from a crack loaded in mode I+III: Solution and application of a model 2D problem

It is experimentally well-known that a crack loaded in mode I+III propagates through formation of discrete fracture facets inclined at a certain tilt angle on the original crack plane, depending on the ratio of the mode III to mode I initial stress intensity factors. Pollard et al. (1982) have proposed to calculate this angle by considering the tractions on all possible future infinitesimal facets and assuming shear tractions to be zero on that which will actually develop. In this paper we consider the opposite case of well-developed facets; the stress field near the lateral fronts of such facets becomes independent of the initial crack and essentially 2D in a plane perpendicular to the main direction of crack propagation.To determine this stress field, we solve the model 2D problem of an infinite plate containing an infinite periodic array of cracks inclined at some angle on a straight line, and loaded through uniform stresses at infinity. This is done first analytically, for small values of this angle, by combining Muskhelishvili's (1953) formalism and a first-order perturbation procedure. The formulae found for the 2D stress intensity factors are then extended in an approximate way to larger angles by using another reference solution, and finally assessed through comparison with some finite element results.To finally illustrate the possible future application of these formulae to the prediction of the stationary tilt angle, we introduce the tentative assumption that the 2D mode II stress intensity factor is zero on the lateral fronts of the facets. An approximate formula providing the tilt angle as a function of the ratio of the mode III to mode I stress intensity factors of the initial crack is deduced from there. This formula, which slightly depends on the type of loading imposed, predicts somewhat smaller angles than that of Pollard et al. (1982).     

A theoretical study for one-dimensional modeling for VOC in a catalytic converter

Modeling for catalytic oxidation of a volatile organic compound, acetaldehyde for the purpose of controlling tail-pipe emissions from vehicular exhaust was carried out. The model developed was one-dimensional unsteady state model, using mass and energy balance equations to predict results for acetaldehyde conversion in an adiabatic monolith operating under warm-up conditions. The equations consisted of a set of partial differential equations that are coupled and solved using Backward Implicit Scheme. Analysis of the behavior of the converter during warm up period was carried out and it was observed that the warm up was faster for a fresh catalyst and this warm up period could be reduced by introducing hot incoming exhaust gas at higher temperature.     

Beam Delivery of the Vanderbilt Free Electron Laser with Hollow Wave Guides: Effect on Temporal and Spatial Pulse Propagation

Hollow Wave Guides were evaluated as a beam delivery system for the Free Electron Laser (FEL) at Vanderbilt in preparation of surgical applications. They can transmit the mid-infrared wavelength range (2µm - 9µm) and tolerate the high peak intensity (>10¹⁴ W/m²) in the micropulse of the FEL. Changes in the temporal and spatial beam characteristics induced by the transmission through 1.5 meter Hollow Wave Guides with bore radii of 250 µm and 530 µm were investigated. Temporal broadening of the micro pulses was studied using intensity autocorrelation measurements and beam profile measurements were performed with a pyroelectric camera. Results demonstrate significant pulse broadening and development of higher order modes induced by sub-optimal coupling of the beam into the Hollow Wave Guide. Bending of the Hollow Wave Guide induced additional losses and reduced propagation of higher modes responsible for broadening the pulse. Calculations with a geometrical ray model support the findings on pulse broadening. Optimal coupling conditions are extremely critical for maximal transmission performance of the Hollow Wave Guide. Design consequences for a FEL delivery system are discussed.