太赫兹自由电子激光放大器研究

太赫兹自由电子激光(FEL)是获得高功率太赫兹辐射的重要途径,目前运行的太赫兹FEL装置基本上都采用振荡器结构,若采用FEL放大器模式,则可产生具有更高峰值功率的太赫兹辐射。本文以北京大学超导电子直线加速器的束流参数为基础,通过模拟分析确定了太赫兹FEL放大器对太赫兹种子源、电子束流及波荡器等的要求。模拟结果显示,太赫兹种子的峰值功率在10 W以上即可实现太赫兹FEL放大;在较易实现的参数条件下,可获得峰值功率数兆瓦的太赫兹辐射。     

Stability of fiber trajectories for winding toroidal pressure vessels

In this paper the fiber trajectory stability of filament wound toroidal pressure vessels is evaluated for two most frequently used patterns: single helical winding, helical and hoop winding. The basic equations of equilibrium for fibers on a torus are given based on netting analysis. The governing equations that determine helical winding angles along the meridional direction are derived for the two winding patterns. The slippage coefficients of the obtained fiber trajectories are calculated using the non-geodesic law and differential geometry. The condition between the hoop-to-helical thickness ratio and the relative bend radius of the toroid is also formulated to prevent fiber bridging on the concave surface. The fiber slippage and bridging tendencies are outlined and compared, corresponding to various relative bend radii and hoop-to-helical thickness ratios. The results show that the single helical winding process provides better stability of fiber trajectories, in terms of both fiber slippage and bridging, than the helical and hoop winding. The toroidal vessel with larger relative bend radius requires lower coefficient of friction between the fiber bundle and the supporting surface. The present analysis for fiber trajectory stability affords a useful reference tool for designing filament-wound toroidal pressure vessels.     

机织物纱线滑移规律及影响因素分析

探讨机织物纱线滑移规律及影响因素。通过分析纱线滑移的形成原因,从织物组织设计、织物结构、微观受力等方面总结了纱线滑移的规律,进而从组织设计、织造参变数、织造方式三个方面分析了影响纱线滑移的主要因素,总结出解决纱线滑移问题的途径和方法。认为:通过在以上三个方面采取措施,可以有效解决纱线滑移问题,改善布面风格;同时也可利用纱线滑移规律实现特殊的织物风格。     

低渗透气藏水平井气水两相产能研究

低渗透含水气藏开发过程中,水平井底见水不仅增加了地层流体渗流的复杂性,而且降低了水平井产量。以气水两相渗流理论为基础,建立了气水两相运动方程,定义气水两相启动压力梯度以及气水两相广义拟压力,考虑应力敏感、滑脱效应、紊流效应和表皮效应对气水同产水平井产能的影响,利用保角变换方法推导出低渗透气藏中水平井气水两相三项式产能新公式。通过实例计算,利用新公式计算的无阻流量结果与实际产能测试结果相对误差较小,仅为5.13%,说明新公式精确可靠,且敏感性分析表明,水平气井产水量随着滑脱因子以及水气质量比的增大而增大,而随着启动压力梯度以及应力敏感指数的增大而减小,但启动压力梯度对产水量影响较小,若生产压差较大,启动压力梯度可以忽略不计。     

汽车碰撞过程中计及安全带约束的人体动力学响应分析

采用基于Euler绳索模型的理论 ,提出了一种新的带有滑移特性的安全带模型。采用多体动力学理论Kane Huston方法 ,进行了计及安全带约束的人体动力学建模及响应的分析 ,并与实验结果进行了比较 ,得到了较一致的结果 ,验证了模型的正确性 ,从而为进行汽车碰撞过程分析提供了有用的工具     

滑移转向机器人不确定度分析及路面识别方法

传感器的不确定度是移动机器人定位中的关键问题。文章对Pineer3-AT滑移转向机器人在转弯时运动学状态进行分析,发现了滑动偏差受地面与轮的摩擦系数及左右两轮的速度影响,滑动偏差的大小即为里程计的不确定度,通过Adams与Matlab/Simulink联合仿真实验得到了在不同地面不同轮速的滑动偏差的大小多组数据,并对结果拟合,建立了滑动偏差模型,并通过实验进行了验证对比。对结果分析,可以得出,摩擦系数越大,在相同速度下滑动偏差越小,根据这一特性,提取小车在不同轮速下的滑动偏差作为地面分类的原始数据。通过k-近邻(KNN)方法,对地面进行分类,识别率达到70%以上。     

Influence of Substrate Properties on the Dewetting Dynamics of Viscoelastic Polymer Films

We studied the dewetting process of thin polystyrene (PS) films on silicon substrates, coated with a thin, irreversibly adsorbed polydimethylsiloxane (PDMS) layer, by optical microscopy and atomic force microscopy. Besides demonstrating the exceptional potential of dewetting for a sensitive characterization of rheological properties of PS thin films, characterized by a stress-relaxation time, τ₁, we focused on the influence of the frictional behaviour (energy dissipation mechanism) at the interface between the PDMS-coated silicon wafer and the PS film on the dewetting process. Our results show that the initial stages of dewetting depend sensitively on the thickness and the way the PDMS layer was adsorbed. The maximum width of the dewetting rim at τ₁ increased with increasing PDMS layer thickness, which can be interpreted as an increase of the effective, velocity-dependent slippage length. Interestingly, τ₁ was found to be almost independent of the substrate properties. Our results demonstrate that dewetting is a really powerful approach for rheological and frictional studies of thin polymer films.     

Evolution of Rim Instabilities in the Dewetting of Slipping Thin Polymer Films

Using optical microscopy, we investigated the amplification of instabilities of the moving rim which formed during dewetting of slipping polymer films. At the onset, the wavelength of the rim instability grew in time and proportional to the width of the rim. At later stages, these instabilities led to finger and subsequent droplet formation. Droplet size was found to be proportional to the width of the rim at break-off of droplets, which, in turn, was proportional to the initial film thickness. Our experiments suggest that the decrease of the dewetting velocity with increasing width of the rim is the key mechanism responsible for this instability. Droplet formation provided a possibility for self adjustment of the dewetting front resulting in a constant mean self-regulated dewetting velocity. This mean velocity was significantly higher than the velocity for the corresponding stable rim.     

Splines in the parameter domain of surfaces and their application in filament winding

For non-axisymmetric filament winding in the composite industry, it is difficult to automatically design fiber paths with uniform distribution, full coverage, and non-slippage. Geodesics and semi-geodesics are commonly used as non-slippage trajectories in the fiber path design. However, the uniqueness of these curves from the initial conditions severely restricts the freedom of design, which plays an important role in producing high performance parts. This paper presents a new class of trajectories with more freedom by generalizing splines, and gives the conditions to make these splines on cylinders and cones stable. The proposed trajectory has been applied to filament winding on elbows. Practical results show that the fiber distribution is excellent, and the finished product has extremely uniform thickness.     

The Slippage Configuration Is Always the Least Favorable Configuration for Two Alternatives

Abstract

Performance guarantees for multinomial selection procedures are usually derived by finding the least favorable configuration (LFC)—the one for which the probability of correct selection is minimum outside the indifference zone—and then evaluating the procedure on that configuration. The slippage configuration has been proved to be the LFC for several procedures and has been conjectured to be the worst for some other procedures. The principal result of this article unifies and extends all previous results for two alternatives: the slippage configuration is the worst for all procedures that have a finite expected number of trials and always select the alternative with more successes. A generalization of the key inequality in the proof to an arbitrary number of alternatives is conjectured.