The beaming of external Compton emission

We consider a relativistically moving blob consisting of an isotropic electron distribution that Compton-scatters photons from an external isotropic radiation field. We compute the resulting beaming pattern, i.e. the distribution of the scattered photons, in the blob frame as well as in the observer’s frame by using the full Klein–Nishina cross section and the exact incident photon distribution. In the Thomson regime the comparison of our approach with Dermer (1995) results in concurrent characteristics but different absolute number of the scattered photons by a factor of f_corr = 3.09. Additionally, our calculation yields a slightly lower boost factor which varies the more from the corresponding value in Dermer (1995) the higher the spectral index p of the electron distribution gets.     

可拓模式识别算法中经典域的确定方法

可拓模式识别算法是根据各个关联度的对比来识别最终的模式,其中经典域区间的确定有着重要的作用。传统的经典域确定方法是基于数理统计学的,不适用于小样本数据,数据不完全以及数据分布未知等复杂情况。引入Bootstrap算法,并和传统的方法对比,更精确地确定了经典域区间。     

Geometric error analysis of an over-constrained parallel tracking mechanism using the screw theory

This paper deals with geometric error modeling and sensitivity analysis of an overconstrained parallel tracking mechanism. The main contribution is the consideration of overconstrained features that are usually ignored in previous research. The reciprocal property between a motion and a force is applied to tackle this problem in the framework of the screw theory. First of all, a nominal kinematic model of the parallel tracking mechanism is formulated. On this basis, the actual twist of the moving platform is computed through the superposition of the joint twist and geometric errors. The actuation and constrained wrenches of each limb are applied to exclude the joint displacement. After eliminating repeated errors brought by the multiplication of wrenches, a geometric error model of the parallel tracking mechanism is built. Furthermore,two sensitivity indices are defined to select essential geometric errors for future kinematic calibration. Finally, the geometric error model with minimum geometric errors is verified by simulation with SolidWorks software. Two typical poses of the parallel tracking mechanism are selected, and the differences between simulation and calculation results are very small. The results confirm the correctness and accuracy of the geometric error modeling method for over-constrained parallel mechanisms.     

图形背景理论模式下英语强调结构的教学认知

在英语中,说话人要重点突出的部分叫做强调.强调结构一直是学生学习英语的重点和难点。传统的教学方法存在诸多弊端,学生不会综合运用,不懂融会贯通。从新的视角对英语强调结构的教学进行认知势在必行。认知语言学中图形-背景理论为重新感知和理解强调结构提供了理据,进行形象具体化教学,有效提高学习效果。     

基于响应面法的齿轮容差设计方法研究

随着附件传动系统不断向高速、重载方向发展,齿面接触可靠性问题越来越受关注.为提高齿轮可靠性,分析了齿轮传动系统常见误差类型及影响,开展了齿轮容差设计初步探讨.根据齿轮制造、加工及系统动态误差的产生原理和作用方式,基于齿轮接触应力仿真结果,确定了影响齿轮接触应力随机性的5种主要误差形式及其特征量分布规律,提出了基于响应面法的齿轮容差设计方法,给出了齿轮系统公差设计的改进方向:在进行齿轮公差设计时,重点控制H、V方向的轴向平行度公差.     

基于层次评估理论的弹道导弹末段突防制导律研究

针对弹道导弹末段突防问题,应用层次性能评估体系,对弹道导弹末段典型突防制导律进行对比分析.并通过对弹道导弹制导精度、弹道导弹最大过载、弹道飞行时间和反导导弹脱靶量以及它们的均值、均方差的重要性和需求性分析,合理设置评估体系的指标及其权值,应用随机仿真分析方法,对比仿真经典比例导引、修正比例导引和变结构比例导引三种突防制导律的性能.仿真结果证明,层次评估理论可以用于弹道导弹末段突防制导律的评估及优选.     

Analyses toward factors influencing sealing clearance of a metal rubber seal and derivation of a calculation formula

Sealing clearance is a key factor for a metal rubber seal’s sealability. The expansion coef-ficient and expansion deformation in the radial direction of metal rubber have been obtained through a thermal expansion experiment of metal rubber. The influence of the elastic modulus to the sealing clearance has been analyzed theoretically. By combining the temperature and elasticity factors of metal rubber with the elastic mechanics theory, the calculation formula of the sealing clearance has been derived, and the values of the sealing clearance and the leakage rate in certain working conditions have been calculated. Experimental results are consistent with calculation results in a high degree. The calculation formula of the sealing clearance can explain the influences of the temperature and elastic modulus factors of metal rubber on the sealing clearance. It can pro-vide guidance for the study of sealing mechanism of metal rubber seals.     

基于格子Boltzmann方法表面形貌对微通道对流换热的影响

为推动微通道冷却技术在航空发动机中的应用,进一步提高航空发动机性能,本文通过扫描电子显微镜、粗糙度扫描仪等多方式扫描得到实际加工物体表面形貌,结合分形理论进行处理,得到真实、光滑、分形插值、康托尔集四类圆形截面管模型,管径100μm~400μm;采用格子Boltzmann方法进行数值模拟,首先通过与文献中实验结果的对比,验证了此方法的正确性,然后对空气在这四类圆形截面微通道中的对流换热特性进行了数值模拟,计算雷诺数Re范围80~640;结果发现：表面形貌是微通道中对流换热不可忽略的因素,相对粗糙度越大越有利于换热,真实扫描管的换热性能要比相同条件下光滑管高2.22%;分形理论可用于微通道表面形貌的构建,在相同迭代次数下,分形插值模型与真实扫描形貌换热性能相差0.03%,而康托尔集模型与真实扫描形貌换热性能相差1.88%,分形插值模型较康托尔集模型更能真实地反映实际形貌;微通道中,随着雷诺数的增加Nu将不再是常数,而是有增加的趋势。     

突加不平衡下GTF发动机转子系统瞬态响应分析

针对GTF发动机低压转子系统突加不平衡瞬态响应问题,基于有限元法建立了考虑啮合单元、行星架单元、膜盘联轴器单元的多体接触、多转子耦合动力学模型,介绍了考虑非线性因素下突加不平衡瞬态响应计算方法,求解了低压转子系统的突加不平衡瞬态响应,并分析了关键部件刚度对突加不平衡的影响规律。结果表明：当风扇转子发生突加不平衡时,各部件均表现为振动幅值突增,后迅速趋于稳定,低压涡轮转子发生明显拍振,突加不平衡载荷主要由行星架结构和第1.5支点刚性支承承担;行星架刚度主要影响风扇转子突加不平衡响应、第1.5支点和行星架处的外传力;膜盘联轴器刚度主要影响增压级突加不平衡响应、转子在突加不平衡之后稳定运转情况。     

液体火箭发动机健康监控技术研究现状

液体火箭发动机健康监控技术是改进和提高运载火箭、航天器可靠性与安全性的核心技术之一,对其进行研究具有重要的学术价值和工程应用价值。液体火箭发动机健康监控技术的研究主要包括液体火箭发动机故障检测与诊断理论方法、液体火箭发动机健康监控系统两方面。该文介绍了基于模型驱动的方法、基于数据驱动的方法和基于人工智能的方法,阐明了液体火箭发动机故障检测与诊断理论方法的研究现状,通过对美国液体火箭发动机典型健康监控系统的介绍,阐明了液体火箭发动机健康监控系统研究的若干进展及现状,并对液体火箭推进系统健康监控技术的演变趋势作了简要评述。