不同直径液滴撞击亲水壁面动态特性实验研究

本文通过可视化实验研究了不同直径液滴撞击亲水壁面的动态特性.实验利用光学原理同时记录了液滴撞击壁面过程的正面及底面图像。实验结果表明:液滴最大铺展因数随液滴初始直径近似呈线性增长关系;随液滴直径增大,液滴铺展至最大时需要的绝对时间、滞留时间、回缩时间均增长,稳定时的液固接触面积变大;液滴铺展至最大铺展因数所需要的无量纲时间约为1.68;液滴直径越小,则撞击后液膜回缩更为迅速.     

Synthesis of multi-walled carbon nanotubes using CoMnMgO catalysts through catalytic chemical vapor deposition

The Co Mg O and Co Mn Mg O catalysts are prepared by a co-precipitation method and used as the catalysts for the synthesis of carbon nanotubes（CNTs） through the catalytic chemical vapor deposition（CCVD）. The effects of Mn addition on the carbon yield and structure are investigated. The catalysts are characterized by temperature programmed reduction（TPR） and X-ray diffraction（XRD） techniques, and the synthesized carbon materials are characterized by transmission electron microscopy（TEM） and thermo gravimetric analysis（TG）. TEM measurement indicates that the catalyst Co Mg O enclosed completely in the produced graphite layer results in the deactivation of the catalyst. TG results suggest that the Co Mn Mg O catalyst has a higher selectivity for CNTs than Co Mg O. Meanwhile, different diameters of CNTs are synthesized by Co Mn Mg O catalysts with various amounts of Co content, and the results show that the addition of Mn avoids forming the enclosed catalyst, prevents the formation of amorphous carbon, subsequently promotes the growth of CNTs, and the catalyst with decreased Co content is favorable for the synthesis of CNTs with a narrow diameter distribution.The Co Mn Mg O catalyst with 40% Co content has superior catalytic activity for the growth of carbon nanotubes.     

大气湍流对航空光电侦察图像质量影响因素分析北大核心CSCD

大气湍流效应是严重影响航空光电侦察系统图像质量的重要因素之一。从大气湍流参数描述出发,研究了大气湍流对光学系统成像质量的影响机理,分析了大气湍流影响光学系统调制传递函数(modulation transfer function,MTF)的影响因素,建立了大气湍流影响光学系统成像MTF的理论模型。仿真结果表明,在大气湍流影响下,光学系统光瞳口径与大气相干直径的比值对光学系统成像MTF影响较大。通过光学系统地面外场实验图像测试对比,验证了实际光学系统受到大气湍流影响后的成像MTF理论模型。研究结果可为大气湍流影响下的航空光电侦察系统设计及提升其成像质量提供理论支持。     

关于紧优双环网络的无限族

双环网络是计算机互连网络和通讯系统的一类重要拓扑结构．1993年,李乔等人提出一个系统的构造方法,构造出69类0紧优和33类1紧优双环网络的无限族,并提出研究下述问题:求k(k>1)紧优双环网络的无限族．2003年,徐俊明等人给出一个4紧优双环网络的无限族．本文首先证明从每一个具体的0紧优双环网络出发,都可以构造若干0紧优双环网络无限族;结合同余方程组理论和数论中的素数理论,给出若干求一般k(k≥0)紧优双环网络无限族(包括非单位步长双环网络无限族)的方法．     

铝球弹丸高速正撞击薄铝板穿孔研究

 低地球轨道上的航天器易受到微流星体及空间碎片的超高速撞击，导致其严重的损伤甚至灾难性的失效。撞击损伤特性研究是航天器防护设计的一个重要问题。通过铝球弹丸超高速正撞击薄铝板的实验研究和数值模拟，证明了AUTODYN-2D软件数值模拟预测薄铝板超高速撞击穿孔直径的有效性。通过对弹丸直径、弹丸撞击速度和薄铝板厚度影响薄铝板超高速撞击穿孔直径的数值模拟，以及利用实验结果和数值模拟结果拟合的曲线，得到了铝球弹丸超高速撞击薄铝板的穿孔规律以及影响薄铝板超高速撞击穿孔直径的主要因素。     

亚微米纤芯光纤芯径大小与光学特性的关系

使用Maxell方程的严格解,研究了亚微米直径光纤的单模条件、传播常数、基模分布、传播效率、非线性系数等光学特性,发现亚微米纤芯直径光纤的单模条件与标准光纤相同,非线性系数在直径为700nm左右时最大;当光纤的直径小于1μm时,传播效率骤减.这些结果有助于新型微光通信和光纤传感器件的设计.     

Practical absorption limits of MPP absorber

The construction and properties of microperforated panel (MPP) absorber are discussed. The absorption limit of the absorber had been shown that low values of the perforate constant k = d(f/10)1/2 and the orifice diameter d (in mm) are essential for MPP to have high absorption in wide frequency band. To find the exact limits, take 1 for k as a start, because both specific resistance and high absorption require k around one. And the orifice diameter d is chosen as 0.1 mm, so that the peak absorption coefficient (resonance absorption) is at 1000 Hz, and high sound frequency may be in the absorption region. Is it possible for a single layer of such an MPP to cover the whole absorption region required in practice? The half-absorption limit is not a good criterion, because low absorption comes in also in some cases. The 0.5 absorption coefficient limit is suggested for practical region, as a standard for comparison. Absorption curves were drawn for different load resistances, of absorption coefficients versus frequency. Ordinary MPP absorber absorbs in slightly over two octaves, and the new absorber with r = 1 (specific resistance equal to the characteristic impedance in air)is slightly better than these, 2.5 octaves. The new absorbers with r > 1, are much better than these, and some satisfies high absorption in broad frequency range. Realization of these will mean great progress of MPP absorbers.     

浅谈发光二极管在电学实验中的应用

发光二极管(LED)具有耗电省、灵敏度高的优点，并且有多种颜色．一般直径为Φ5mm的发光二极管工作电流为15mA，但当电流为0．5mA时，已有清楚显示，用于演示实验时，最好选用直径为12mm的高亮度管．采用LED做电学实验，可取得比传统方法好得多的效果．     

任意k紧优、奇异k紧优双环网无限族的构造

双环网 (double loopnetwork)是具有n个结点和出度为2的有向循环图,它是计算机互连网络的一类重要的拓扑结构,已应用于局域网和分布系统的设计中.给定结点数n,如何构造n个结点的具有最小直径的双环网? 这个问题受到广泛的关注. 与此有关的一个久而未决的主要问题是:任意给定k≥0, 是否有所谓k紧优双环网的无限族? 本文证明了: (1) 对于任意给定的k≥0, 可构造其中一个步长为1的k紧优双环网的无限族, 其结点数n(k,e,c)(其中e充分大)是e的2次整系数多项式且系数含有参数c; (2) 对于任意给定的k≥0, 可构造一个奇异k紧优双环网的无限族.