物理模拟龙卷风的风速和气压降分布特征

与季风、台风相比,龙卷风具有空间尺度最小、风速最大的特点,常规气象观测仪器无法记录龙卷风的风场信息;到目前为止,极少量的龙卷风风场信息来自于多普勒雷达探测得到的龙卷风风速、气压特征信息。因此,基于龙卷风实测数据尚不能充分研究龙卷风的风场特征。在实验室中,利用缩尺的物理模拟器生成龙卷风风场,研究其风场特点,对于充分认识龙卷风特性具有重要意义。利用北京交通大学龙卷风模拟器生成了5种涡流比的龙卷风风场,总结分析了龙卷风的切向风速、径向风速、竖向风速以及气压降的空间分布规律,与龙卷风实测结果和龙卷风理论模型进行了对比分析。结果表明,利用北京交通大学龙卷风模拟器生成的龙卷风风场与真实龙卷风的形状相似,风速、气压降的分布特点与真实龙卷风实测值和修正兰金涡理论值吻合或基本吻合,验证了龙卷风模拟器的有效性,为进一步研究建筑、桥梁、输电线等基础设施的龙卷风风效应提供了试验平台。     

基于失效概率偏导数的局部与全局混合灵敏度分析

基于失效概率偏导数的局部灵敏度与矩独立的全局灵敏度定义了一种新的混合灵敏度指标,该灵敏度指标不仅继承了传统的矩独立灵敏度的优点,而且反映了矩独立灵敏度和基于方差的灵敏度之间的内在联系。针对该矩独立的混合灵敏度指标计算量大的问题,该文首先将其转化为基于方差的混合灵敏度指标,然后利用能够高效计算条件矩的态相关参数(SDP)法进行求解。为了进一步提高计算效率,该文建立了基于重要抽样和截断重要抽样的SDP方法。算例结果验证了该文所提指标的合理性及所提方法的准确性。方法方法     

Management Selection, Operations Research and Relevance: A CRITIQUE OF “STATISTICAL CONTAMINATION AND THE SELECTION OF MANAGEMENT”

Editor's Note: The reviewers of the preceding paper by Martin and Horowitz expressed diametrically opposing viewpoints concerning their recommendations for publication. Both reviewers felt the paper was well written and technically sound; one reviewer endorsed the work and recommended publication as a valuable contribution to the literature. The other reviewer felt just as strongly that the authors' work is a misapplication of mathematical modeling. We made the decision to publish the paper and accompany it with a statement of rebuttal from the opposing reviewer, as it appears here.     

一种水泥混凝土用聚合物共混物的微观形态及其变化机理研究

一种水混凝土用聚合物共混物的微观形态及其变化机理研究表明,聚合物总浓度不超过20％时,等量酯比的共混物微观形态随聚合物总浓度的增大而逐渐由致密体结构变成网络状结构,聚合物总浓度为20％时,不同配比的共混物微观形态比等量配比的共混物的网络状诚结构均匀为特征,B聚合物在共混体系中兼起消泡剂作用,同时探讨了各共混物微观形态与两聚合物分子链在共混体系中的物理化学行为之间的关系。     

A COMPARISON OF PROCEDURAL AND DECLARATIVE PROGRAMMING LANGUAGES FOR THE COMPUTATION OF PARETO OPTIMAL SOLUTIONS

This paper investigates the relative merits of declarative programming and procedural programming languages for optimization problems through an example taken from the field of multicriteria decision making. The results obtained from this work are presented and discussed.     

THE THEOREMS OF STRUCTURAL AND GEOMETRIC VARIATION: A REVIEW

This paper reviews the application of the Theorems of Structural and Geometric Variation to optimization, design and non-linear analysis.     

THE EFFECT OF ELEVATED TEMPERATURE AND ENVIRONMENT ON THE FATIGUE CRACK GROWTH CHARACTERISTICS OF Ti–6Al–2Sn–4Zr–2Mo–0·1Si

Abstract Renewed interest in utilizing titanium at elevated temperature has led to the development of creep resistant near α-alloys. Present work is concerned with the fatigue crack growth characteristics of one such alloy, Ti–6Al–2Sn–4Zr–2Mo–0·1Si, as a function of temperature, environment, and processing history. Increasing crack growth rates were observed in the order, vacuum (298K), vacuum (811K), air (298K), and air (811K). An empirical expression based upon crack opening displacement considerations was found to fit the crack growth data in all but the air (811K) test. The pronounced increase in growth rates in air at 811K, particularly at low ΔK, was caused by oxidation, whereas the increase in growth rates at 811K in vacuum was the result of a decrease in modulus. Processing variables and test frequency had little effect on crack growth rates.     

考虑高架桥和风屏障影响的高速列车龙卷风荷载特性研究

针对高速列车在龙卷风等局地强风作用下的运营安全,开展了高架桥上高速列车在模拟龙卷风作用下气动力识别刚体模型测压实验,研究了不同龙卷风中心作用下列车气动力的空间分布特征,评价了风屏障对列车气动力的影响.结果表明:相比混凝土栏杆,风屏障将减小龙卷风作用下列车的整体阻力、升力和倾覆力矩系数的最不利值,但会增加整体俯仰和横摆力...     

CeSi2薄膜的离子束合成及其室温光致发光特性

将能量为４５ｋｅＶ的Ｃｅ离子注入到Ｓｉ单晶中,研究硅化铈摹离子束合成及其室温光致发光特性,透射电观察表明在单 晶硅的表面形成厚１００ｎｍ的Ｃｅ离子注入层,选区电子衍射和Ｘ射线衍射分析表明注入层内形成了ＣｅＳｉ２,ＣｅＳｉ２的结晶程度随注入剂量的增加而逐渐完善,用远紫外光激发得到了室温蓝紫色ＰＬ谱,以红光（６５０－７００ｎｍ）激发,则上转换蓝光和紫光发射的效率较高,其发光特性比较稳定,蓝光和紫光受激     

CRACK CLOSURE AND THE FATIGUE-CRACK PROPAGATION THRESHOLD AS A FUNCTION OF LOAD RATIO

The phenomenon of crack closure, which involves the premature closing of fatigue cracks during the unloading portion of a fatigue cycle resulting in the development of crack-tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many aspects of the behaviour of fatigue cracks in metallic materials; these include effects of load ratio, variable-amplitude loading, crack size, microstructure, environment and the magnitude of the fatigue threshold. Recently, however, the significance of crack closure has been questioned and alternative suggestions made for many of these phenomena, e.g. the effect of the load ratio (i.e. the ratio R of the minimum to maximum loads) on threshold behaviour. In the light of this, the present paper provides evidence to rebut the assertion that crack closure is an insignificant process. Particular attention is given to the effect of crack closure on the threshold level as a function of load ratio.     

考虑一阶和二阶位移梯度的数字图像相关方法在剪切带测量中的比较

当剪切带中存在二阶位移梯度或应变梯度时,研究了两种数字图像相关(DIC)方法在测量位移、应变及应变梯度中的表现。以基于梯度塑性理论的剪切带的位移解作为基础,通过Matlab仿射变换制作了具有不同平均剪切应变和应变梯度的虚拟剪切带。通过对其计算和分析得到了下列结果：当剪切带的平均剪切应变和应变梯度较高时,与只考虑一阶位移梯度的DIC方法相比,考虑二阶位移梯度的DIC方法优势明显,获得的位移、应变及应变梯度结果与理论解比较吻合,由于DIC方法测量的是平均应变,因此,剪切带中心的峰值应变将被低估;当剪切带的平均剪切应变和应变梯度较低时,剪切带中心的峰值应变可能被高估,受标准偏差的影响,考虑二阶位移梯度的DIC方法没有优势。基于上述研究结果,在单向压缩应力控制加载条件下,对砂样从开始加载至宏观裂纹出现之前变形过程中的3种应变场的4种结果进行了分析。由于应变不超过0.25,因而考虑二阶位移梯度的DIC方法的结果并无优势。