辽宁区域地震短波通信传播特性和频率选择探讨     

牛峰  刘民 《东北地震研究》1997,13(2):40-50

辽宁区域地震短波通信网对电磁波在无线通信中的传播特性及频率选择等问题，没有进行深入的理论研究和探讨。笔者将通信工作的实践经验升至理论的高度，研究探讨辽宁区域地震短波通信传播特性和频率选择办法，达到在短波数据通信中，选择最佳频率，增加沟通率的目的  相似文献   

稳定电流场的反射、透射、衍射——波动性          下载免费PDF全文

刘方文  马冠群 《地球物理学报》1992,35(3):396-401

稳定电流场是否有反射、透射现象,它们服从什么规律,以及稳定电流场是否具有波动性等问题,本文将予以探讨。 一、稳定电流场的反射与透射 在用“电像法”解点电流源平界面问题时,一般用反射、透射的概念,并用虚源等效地代替界面,指的是全界面的、源的“反射”与“透射”,要求界面为无限平面,其结果没有普遍性。因此,尚不能当作一般的物理规律来看待。从弹性波与稳定电流场的类比中,有人曾  相似文献   

Polysomatism and modules of gehlenite composition and structure in the Hanic phase (Ca5Al6MgSiO17)     

I. Dódony  P. R. Buseck 《Physics and Chemistry of Minerals》2001,28(6):428-434

 Discrete, well-ordered, single-phase crystals (the Hanic phase) form when powders the bulk composition of Ca₅Al₆MgSiO₁₇ are allowed to react in the absence of a melt. If, on the other hand, either the powders or crystals of Ca₅Al₆MgSiO₁₇ are molten and then cooled, complex intergrowths form. These intergrowths consist of modules having the composition and structure of Ca₃Al₄MgO₁₀ and gehlenite (Ca₂Al₂SiO₇), forming a polysomatic series in which the modules occur in varied proportions. High-resolution transmission electron microscope (HRTEM) images reveal the details of these intergrowths. Mass balance is achieved by the formation of this polysomatic series together with Mg-substituted mayenite (Ca_12−xMg_xAl₁₄O₃₃). The tetrahedral sheets in gehlenite have close structural similarities to those of Ca₅Al₆O₁₄. Based on our HRTEM images, we hypothesize chemical substitutions between gehlenite and Ca₅Al₆O₁₄, resulting in gehlenite-like modules that are depleted in Si and correspondingly enriched in Al and Ca. Received: 2 August 2000 / Accepted: 18 January 2001  相似文献   

天然气——21世纪的能源   总被引：2，自引：1，他引：1  

史斗 《地球科学进展》1998,13(3):251-260

天然气是一种生态型能源或理想的城市能源。综述了世纪之交的全球天然气工业发展状况,包括储量的增长、输气管道的延长、液化天然气厂的发展和天然气利用技术的进步。介绍了有关国家的天然气利用法规和政策。预测了21世纪全世界天然气供需趋势。论述了为什么说天然气是21世纪的能源。指出了当前和未来全球天然气工业面临的挑战。  相似文献   

Transmission of obliquely incident waves at low-crested breakwaters: Theoretical interpretations of experimental observations     

Baoxing Wang  Ashwini K. Otta  Andrew J. Chadwick 《Coastal Engineering》2007

The paper presents a series of analytical and numerical investigations of oblique wave transmission at low-crested breakwaters. For a smooth breakwater, two important features of wave height and direction are analyzed to establish the generic nature of the wave transmission process at oblique incidence. The proposed framework of research is validated against laboratory data from the EU-sponsored project, DELOS. The numerical simulations exhibit a significant decrease of the transmission coefficient with increasingly oblique incidence at a smooth breakwater. The roles of wave-breaking, nonlinearity, wave-induced currents and set-up in determining the characteristics of oblique wave transmission are demonstrated in the paper. It is found that both the amplitude-dependent phase velocity and the decrease of mean wave period contribute to the change of mean wave direction on the transmission side. An attempt has also been made to qualitatively explain the different behaviour of oblique wave transmission at a rubble-mound breakwater.  相似文献   

莫桑比克非鲫卵壳膜形成的电镜观察   总被引：3，自引：0，他引：3  

王爱民 《海洋与湖沼》1994,25(4):385-389

于1986年9－12月对取自湖北省水产研究所实验鱼网箱中的莫桑比克非鲫，运用透射电镜观察卵子发生中印壳膜的形成过程。结果表明，随着卵母细胞的生长，在卵母细胞和颗粒细胞之间先后出现3层结构，即Z1，Z2和Z3；初步确定：Z1可能由颗粒细胞分泌形成，z2和z3由印母细胞分泌形成，最终z1，z2和Z3融为一体成为卵壳膜。对印亮膜的结构特征及其与特殊的生殖习性——衔卵孵化的关系的分析认为，衔卵孵化导致卵壳膜的厚度降低。  相似文献   

Damping long waves by a multi-channel structure     

Sheng-Wen Twu  Cheng-Chi Liou 《Ocean Engineering》2003,30(3):351-367

In this paper, a multi-channel structure was developed to attenuate waves with various wave periods. By ignoring energy losses and the factor of channel width, a simple and straightforward method was used to tackle this problem. The theoretical solution showed that a single reflected channel structure could attenuate waves of a certain wave period, while a multi-channel structure could attenuate waves with various wave periods. If an interval of 0.05<relative water depth<0.15 is concerned, a structure consisting of four reflected channels could provide a transmission coefficient of less than 0.2. Experimental tests were conducted to verify the theoretical solutions. Both theoretical solution and experimental data indicated that waves with various wave periods would be effectively attenuated after passing through the multi-channel structure. Although some factors are neglected in treating this problem, the method is applicable. An allowed transmission coefficient can be provided by a multi-channel structure only if the computed peak value is not larger than that value.  相似文献   

热弹性波在流体—固体界面上的反射问题及其基本特征^*     

张丽娟  武晔  张艳芳  张新超  李瑞东  胡顺田  熊仲华  赵永安 《地震》2016,36(2):68-75

本文建立了热弹性流体介质中的波方程,指出了该介质中可能传播的两类波,并建立了热弹性P波的概念和基本特征。此外,本文还研究了热弹性波在液体—固体界面上的反射问题,给出了反射系数、透射系数的表达式以及反射热弹性P波、透射热弹性P波和透射SV型S波的基本特征。  相似文献   

Scattering of internal waves in a two-layer fluid flowing through a channel with small undulations     

Smrutiranjan Mohapatra  Swaroop Nandan Bora 《Ocean Dynamics》2009,59(4):615-625

Using two-dimensional linear water wave theory, we consider the problem of normal water wave (internal wave) propagation over small undulations in a channel flow consisting of a two-layer fluid in which the upper layer is bounded by a fixed wall, an approximation to the free surface, and the lower one is bounded by a bottom surface that has small undulations. The effects of surface tension at the surface of separation is neglected. Assuming irrotational motion, a perturbation analysis is employed to calculate the first-order corrections to the velocity potentials in the two-layer fluid by using Green’s integral theorem in a suitable manner and the reflection and transmission coefficients in terms of integrals involving the shape function c(x) representing the bottom undulation. Two special forms of the shape function are considered for which explicit expressions for reflection and transmission coefficients are evaluated. For the specific case of a patch of sinusoidal ripples having the same wave number throughout, the reflection coefficient up to the first order is an oscillatory function in the quotient of twice the interface wave number and the ripple wave number. When this quotient approaches one, the theory predicts a resonant interaction between the bed and the interface, and the reflection coefficient becomes a multiple of the number of ripples. High reflection of the incident wave energy occurs if this number is large. Again, when a patch of sinusoidal ripples having two different wave numbers for two consecutive stretches is considered, the interaction between the bed and the interface near resonance attains in the neighborhood of two (singular) points along the x-axis (when the ripple wave number of the bottom undulation become approximately twice as large as the interface wave number). The theoretical observations are presented in graphical form.  相似文献   

Scattering of water wave by a submerged horizontal plate and a submerged permeable breakwater     

Hu-Hsiao Hsu  Yung-Chao Wu 《Ocean Engineering》1998,26(4):325-341

Based on a two-dimensional linear water wave theory, this study develops the boundary element method (BEM) to examine normally incident wave scattering by a fixed, submerged, horizontal, impermeable plate and a submerged permeable breakwater in water of finite depth. Numerical results for the transmission coefficients are also presented. In addition, the numerical technique's accuracy is demonstrated by comparing the numerical results with previously published numerical and experimental ones. According to that comparison, the transmission coefficient relies not only on the submergence of the horizontal impermeable plate and the height of the permeable breakwater, but also on the distance between horizontal plate and permeable breakwater. Results presented herein confirm that the transmission coefficient is minimum for the distance approximately equal to four times the water depth.  相似文献