光栅选频CO_2激光器内管的设计

<正> 一般激光器采用的内管都是笔直、光滑的。当用光栅做波长选择器时,未被选取的波长将以近90°的入射角射到内管内壁上,在某种特定的条件下,将会构成谐振腔而产生振荡。这样就不易得到单一的频率和模式,也不易得到稳定的功率输出。

Datas comemorativas e outras datas significativas

A publicação inaugura uma série comemorativa aos 190 anos da Independência do Brasil. Apresenta um levantamento das datas comemorativas e dos feriados nacionais que vigoram no território brasileiro. Estão elencadas principalmente aquelas instituídas por norma jurídica ou documentos oficiais. Inclui também, as datas comemoradas de modo costumeiro ou popular, apesar de haver divergências quanto à precisão das informações obtidas. Traz datas comemorativas instituídas na esfera internacional que buscam encorajar a cooperação entre governos e promover ações mundiais conjuntas voltadas para o bem-estar dos povos e do planeta. Estão incluídas ainda, as semanas comemorativas e as datas variáveis, que são aquelas comemoradas em dias não fixos. A pesquisa contemplou as datas comemorativas de maior significação e importância.

分层介质中快磁声波的传播

<正> 分层介质中阿尔文波的传播特征已有研究,我们讨论分层介质中的快磁声波,对于一维问题,假设基态为在直角坐标系中列出磁流体力学方程组为

关于磁等离子体的运动位形

<正> 在太阳活动区以及许多其它天体物理问题中,需要讨论重力、压力梯度、电磁力与运动之间的相互关系。对于轴对称问题,在柱坐标下可由磁场无源条件定义标量磁势ψ,它满足

运动气体的冻结型和电阻型无力场

本文是文[1]的继续,除了核实文[1]中的结论外,新的结果有: (1)若电场E的势场-φ按满足规范,则a为常数的必要充分条件是:式中A、B和V分别为磁矢势、磁场和气体速度。 (2)得到控制a变化的基本方程,电阻是a从非常数演化到常数的因素。

应用广义变分原理计算扁壳的固有频率

<正> 由于数学上的复杂性,要在满足全部方程和边界条件的情况下来决定扁壳振动的固有频率是有很大困难的。因此一般采用了法等各种近似解法。但在应用这些近似解法时,也还需要满足一定的条件,因而往往也会出现不少困难。本文给出了一个计算扁壳固有频率更一般的近似解法,并对建筑部门常见的对边簡支对边固定和三边簡支一边固定这两组边界条件的矩形底球面扁壳的基本固有频率进行了计算。计算过程表

Elimination of loading reverberation in the split hopkinson torsional bar

The loading reverberation is a multiple wave effect on the specimen in the split Hopkinson torsional bar (SHTB). Its existence intensively destroys the microstructure pattern in the tested material and therefore, interferes with the study correlating the deformed microstructure to the macroscopic stress-strain response. This paper discusses the problem of the loading reverberation and its effects on the post-mortem observations in the SHTB experiment. The cause of the loading reverberation is illustrated by a stress wave analysis. The modification of the standard SHTB is introduced, which involves attaching two unloading bars at the two ends of the original main bar system and adopting a new loading head and a couple of specially designed clutches. The clutches are placed between the main bar system and the unloading bars in order to lead the secondary loading wave out of the main bar system and to cut off the connection in a timely manner. The loading head of the standard torsional bar was redesigned by using a tube-type loading device associated with a ratchet system to ensure the exclusion of the reflected wave. Thus, the secondary loading waves were wholly trapped in the two unloading bars. The wave recording results and the contrasting experiments for examining the post-mortem microstructure during shear banding both before and after the modification highly support the effectiveness of the modified version. The modified SHTB realizes a single wave pulse loading process and will become a useful tool for investigating the relation between the deformed microstructure and the macroscopic stress-strain response. It will play an important role especially in the study of the evolution of the microstructure during the shear banding process. (C) 1995 American Institute of Physics.

多煤层开采时岩层垮落过程的数值模拟

运用自行开发的岩层破断过程分析 (SFPA2D)系统模拟了多煤层开采时岩层的垮落过程。数值模拟结果表明 ,虽然上部煤柱的存在改变了上覆岩层结构 ,但岩层的垮落仍具有明显的周期性 ,地表的水平移动和垂直下沉仍具有明显的对称性 ;当垮落带高度达到上部煤柱后 ,可能出现大面积的垮落 ,产生较大的次生来压现象 ,从而影响下部长壁工作面的安全。通过和相似材料模拟以及理论分析结果的对比 ,本次数值模拟基本上反映了多煤层开采时采动岩体的动态发展过程 ,SFPA2D系统可以作为模拟采动覆岩破坏过程的一种新方法

非线性动力学系统的辨识:时频滤波与骨架曲线

利用非平稳信号的时频分析方法研究了一类非线性系统的频率特性和阻尼特性随运动形态的变化规律 ,得到了能简洁、直观地反映系统基本非线性动力学特性的广义骨架线性系统 (简称GSLS)和骨架曲线·　在此基础上 ,利用时频滤波方法根据系统非平稳响应信号对非线性系统进行辨识·　该项工作为非线性系统反问题的研究提供了一条新的途径·