平面COUETTE流动的湍流斑研究

湍流斑是近壁剪切流动中的重要现象,它们的生成和发展与流动转捩和湍流的形成密切相关.采用Navier-Stokes方程直接数值模拟和壁面脉冲模型研究平面Cou-ette流动中湍流斑的生成和演化.高精度高分辨率的三维耦合紧致差分格式被用于数值计算.探讨了平面Couette流动中湍流斑的重要特征,包括高频脉动的形成、雷诺应力的产生、扰动的急剧增长和湍流斑形状的变化,特别是流向涡的复杂演化过程.结果表明层流流动中的湍流斑具有湍流流动中一些基本特征.     

Analysis of the brightness temperature features of the lunar surface using 37 GHz channel data from the Chang'E-2 microwave radiometer

Compared with other remote observations, brightness temperatures (T_B) derived from microwave emission measurements provide a unique means to characterize the physical properties of the lunar surface. Using Chang’E-2 microwave radiometer data, we produced 12 global T_B images of the lunar surface during a diurnal cycle with different local times separated by approximately 2?h. There are two types of remarkable T_B units on the lunar surface, the “hot regions” occurring during the lunar day in the lunar Maria regions and the “microwave cold spots” occurring during the nighttime typically related to young craters. Compared with their surroundings, the hot regions are much warmer during the lunar day and slightly colder at night, while the microwave cold spots are much colder during the lunar night and slightly warmer in the daytime. Moreover, the T_B heating and cooling rates of these two units are larger than others at the same average latitude where they are located during the lunar day, especially after sunrise and before sunset. The hot regions have a good agreement with the mare regions with high TiO₂ abundance. Besides, brightness temperatures in the lunar Maria correlate closely with their TiO₂ abundance. For most microwave cold spots, they agree with the young craters, and their brightness temperature distributions have a significant negative correlation with the lunar surface nighttime temperature and rock abundance.     

Resistance Spot Welding Method for Metal-Based Fiber Bragg Grating Sensors

Aiming at the requirements of installation and work environment of metal-based fiber Bragg grating(FBG)sensor,a resistance spot welding method is presented.Firstly,the effect of welding parameters on welding quality is discussed theoretically,and a suitable resistance spot welding method for the metal-based FBG sensor is proposed for the first time.Then through serial resistance spot welding tests,the feasibility and practicability of the method are verified,and optimal welding parameters for two different tested metals are obtained.Fatigue performance test validates FBG sensors installed by the proposed method with good fatigue properties and long-term stable measurement performance.The research results can provide technical guidance for engineering structure long-term safety monitoring.     

Solitary,explosive and periodic solutions for electron acoustic solitary waves with non-thermal hot ions

A theoretical investigation has been made for electron acoustic waves propagating in a system of unmagnetized collisionless plasma consists of a cold electron fluid and ions with two different temperatures in which the hot ions obey the non-thermal distribution. The reductive perturbation method has been employed to derive the Korteweg–de Vries equation for small but finite amplitude electrostatic waves. It is found that the presence of the energetic population of non-thermal hot ions δ, initial normalized equilibrium density of low temperature ions μ and the ratio of temperatures of low temperature ions to high temperature ions β do not only significantly modify the basic properties of solitary structure, but also change the polarity of the solitary profiles. At the critical hot ions density, the KdV equation is not appropriate for describing the system. Hence, a new set of stretched coordinates is considered to derive the modified KdV equation. In the vicinity of the critical hot ions density, neither KdV nor modified KdV equation is appropriate for describing the electron acoustic waves. Therefore, a further modified KdV equation is derived. An algebraic method with computerized symbolic computation, which greatly exceeds the applicability of the existing tanh, extended tanh methods in obtaining a series of exact solutions of the various KdV-type equations, is used here. Numerical studies have been reveals different solutions e.g., bell-shaped solitary pulses, singular solitary “blowup” solutions, Jacobi elliptic doubly periodic wave, Weierstrass elliptic doubly periodic type solutions, in addition to explosive pulses. The results of the present investigation may be applicable to some plasma environments, such as Earth’s magnetotail region.     

置氢TC4钛合金粉末烧结材料高温流变行为及组织演变

采用热压缩试验研究了置氢量0.42%(质量分数)TC4钛合金粉末烧结材料在温度850～1000℃和应变速率0.001～0.10 s-1范围内的流变行为和组织演变,分析了该合金烧结材料在试验参数范围内变形的应力-应变曲线特征.动力学分析获得置氢TC4钛合金粉末烧结材料高温压缩变形的应力指数和变形激活能分别为3.97和50...     

高动态下的星体目标质心提取方法

高动态情况下,星点像斑在星敏感器探测器上会呈现出拖尾现象,星点质心无法被准确提取。针对上述难点,本文提出了一种高动态条件下基于差异哈希算法的星点质心提取方法。该方法分为三步：第一,建立动态星斑的数学模型;第二,利用差异哈希算法和汉明距离实现星跟踪窗口内星体目标粗定位;第三,在粗定位区域使用阈值分割与连通域法提取星点质心。实验结果表明,该方法能够适应各种长度的曝光时间,并实现星敏感器在3（°）/s条件下的稳定跟踪。在曝光时间50ms,角速度3（°）/s的条件下,星对角距误差为13”,平均提取率为96%,相比于传统方法,分别提高了29.6%和22.9%。     

Manufacturing With Lasers Developments and Opportunities

Developments and opportunities for manufacturing with lasers are considered in three broad themes. Process enhancements, in particular hybrid-laser arc welding and twin spot laser welding are described. Two examples of novel manufacturing processes (laser joining of aluminium to steel and laser direct metal deposition) are considered. Finally, a new laser source (Yb: YAG fibre laser) is introduced as an additional competing processing capability.     

低温红外镜头星点检测数据处理方法研究

在实验室模拟低温真空环境对红外镜头进行星点检测时，不能在像面处直接测得星点光源的像，而是通过在焦面处放置反射镜使系统二次成像的方法测得二次成像的弥散斑大小。二次像斑尺寸的变化将能反映出由温度变化引起的被测镜头焦面处像斑尺寸的变化。通过理论推导得出一次像与二次像弥散斑大小的数学关系，并通过对不同尺寸星点光源、不同像质光学系统的仿真分析，得出星点检测法的检测精度与星点孔直径有关，而与镜头本身成像品质无关。     

电阻点焊的可靠性控制

为了提高了生产点焊工艺中焊点焊接质量可靠性，提出了优化焊点质量的途径和方法，确定了合理的质量保证措施。通过全过程质量控制的方法对焊接质量加以有效的保证，并对焊接过程进行分析和监控，确定外界因素对焊点质量的影响大小，制定改进措施，采用可靠性工程技术进行辅助分析，提高点焊产品的焊接质量。     

SiO2 气凝胶热参数测试及评价

利用非稳态阶跃平面热源法对SiO2气凝胶的热参数进行了高温实验研究,获得了不同温度和压力条件下SiO2气凝胶的热导率、热扩散率以及比热容等.结果表明,SiO2气凝胶800℃的热导率比室温增大约62％.在相同气压且低于600℃时,其热导率受比热容影响,而在高于600℃时,则受热扩散率影响;在相同温度且高于10 kPa时,热导率亦受热扩散率影响.