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881.
The vibrations of railway tracks on a poroelastic half‐space generated by moving trains are investigated through a vehicle–track–ground coupling model. The theoretical model incorporates a vehicle, a track, and a fully saturated poroelastic half‐space soil medium. The source of vibration excitation is divided into two components: the quasi‐static loads and the dynamic loads. The quasi‐static loads are related to the static component of the axle loads, whereas the dynamic loads are due to the dynamic wheel–rail interaction. A linear Hertizian contact spring is introduced between each wheelset and the rail to consider the dynamic loads. Biot's dynamic theory is used to characterize the poroelastic half‐space soil medium. Using the Fourier transform, the governing equations for the track–ground system are solved and the numerical results are presented for a single axle vehicle model. The different dynamic characteristics of the elastic soil medium and the saturated poroelastic medium are investigated. In addition, the different roles of the moving axle loads and the roughness‐induced dynamic loads are identified. It is concluded that the vibration level of the free field off the track predicted by the poroelastic soil medium is smaller than that predicted by the elastic soil medium for vehicle speed below the Rayleigh wave speed of the poroelastic half‐space, whereas it is larger for vehicle speed above the Rayleigh wave speed. The dynamic loads play an important role in the dynamic responses of the track–ground system. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
882.
Lei Huang Mike Cai Zhi-Qiang Shen Feng Yuan 《Monthly notices of the Royal Astronomical Society》2007,379(3):833-840
The compact dark objects with very large masses residing at the centres of galaxies are believed to be black holes. Due to the gravitational lensing effect, they would cast a shadow larger than their horizon size over the background; the shape and size of this shadow can be calculated. For the supermassive black hole candidate Sgr A*, this shadow spans an angular size of about 50 μas, which is under the resolution attainable with the current astronomical instruments. Such a shadow image of Sgr A* will be observable at about 1 mm wavelength, considering the scatter broadening by the interstellar medium. By simulating the black hole shadow image of Sgr A* with the radiatively inefficient accretion flow model, we demonstrate that analysing the properties of the visibility function can help us determine some parameters of the black hole configuration, which is instructive for the submillimetre Very Long Baseline Interferometry (VLBI) observations of Sgr A* to be made in the near future. 相似文献
883.
The paper presents a three‐dimensional study on the steady‐state response of a track system and layered half‐space soil medium subjected to the load induced by the passages of a moving train with the substructure method. Practically, due to the ground water table being several meters beneath the ground surface, the soil profile can be divided into two layers: the upper layer modeled by an elastic medium and the lower layer by a fully saturated poroelastic medium governed by Biot's theory. In the meanwhile, the rails are regarded as an infinitely long Euler beam, and the sleepers are represented by a continuous mass. The effect of the ballast is accounted for by introducing the Cosserat model for granular medium, and the train is described by a series of moving axle point loads, depending on the geometry of the train. The influences of the thickness, the mass and the rigidity of the elastic layer and the mass of the ballast on rail's displacement responses are carefully investigated. Numerical results show that the influences of these parameters are significant for high train velocity, while vanishes for low velocity. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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886.
Transmission of Elastic P-waves across Single Fractures with a Nonlinear Normal Deformational Behavior 总被引:13,自引:5,他引:13
Summary This paper presents a theoretical study on normally incident elastic P-wave transmission across single dry fractures with
a nonlinear normal deformational behavior. The effects of nonlinear fracture normal behavior on P-wave transmission are examined
without the mixture of fracture shear behavior. The linear displacement discontinuity model for wave propagation across fractures
is extended to a nonlinear model – the hyperbolic elastic model (BB model). Numeric solutions of magnitudes of transmission
(|T
non|) and reflection (|R
non|) coefficients, for normally incident P-wave transmission across the nonlinear deformable fractures, are obtained and related
to the closure behavior of fractures. Parametric studies are conducted to acquire an insight into the effects of the nonlinear
fracture normal deformation on P-wave transmission, in terms of initial normal stiffness and the ratio of current maximum
closure to maximum allowable closure of the fractures, as well as the incident wave amplitude and frequency. Comparisons between
the linear and nonlinear models are presented. It is shown that, |T
lin| and |R
lin| for the linear model are special solutions of |T
non| and |R
non| for the nonlinear model, when the incident wave amplitude is so low that the current maximum closure of fracture incurred
during the wave transmission is much smaller, relative to the maximum allowable closure. In addition, the nonlinear fracture
behavior gives rise to a phenomenon of higher harmonics during the wave transmission across the fracture. The higher harmonics
contribute to the increase of |T
non| from |T
lin|. 相似文献
887.
888.
地缘关系与世界经济地理学科建设 总被引:21,自引:1,他引:21
地缘关系是国际关系的重要表现形式,地缘政治,尤其地缘经济对当代世界经济影响很大,也影响世界经济地理学科的发展。本文着重阐述了地缘经济学与世界经济地理学的关系及其互为影响作用,进而论述了世界经济地理学科建设的任务。 相似文献
889.
本文根据瑞利波测量所得波形的变化特征,分析研究了地基参数对瑞利波弥散曲线的影响规律,提出了利用瑞利波测试剪切波速的方法。在研究过程中,对瑞利波测试技术进行了探讨,并引入了相关分析原理,以消除噪声、系统的非线性以及其它一些人为因素的干扰。最后结合实例,论证该技术用于实际勘探的可行性并指出其存在的问题和有待进一步完善的工作。 相似文献
890.