A 3D BEM-FEM methodology for simulation of high speed train induced vibrations

This work presents an efficient methodology for the analysis of vibrations in a railroad track system, induced by the passage of conventional and high-speed trains. To this end the Boundary Element Method is used to model the soil-tie system within the framework of impulse response techniques. Conventional Finite Element Methods along with Newmark's integration is used for the modeling of the rail system. The two methods are coupled at the tie-rail interface and the solution is obtained following a staggered, time marching scheme in an efficient manner. The methodology accounts for Soil-Structure Interaction and traveling wave effects. In addition, this work identifies the parameters that affect the efficient modeling of railroad track systems as they pertain to the proposed solution methodology.     

Strain and competence contrast estimation from fold shape

A new method to estimate strain and competence contrast from natural fold shapes is developed and verified by analogue and numerical experiments. Strain is estimated relative to the nucleation amplitude, A_N, which is the fold amplitude when the amplification velocities caused by kinematic layer thickening and dynamic folding are identical. A_N is defined as the initial amplitude corresponding to zero strain because folding at amplitudes smaller than A_N is dominantly by kinematic layer thickening. For amplitudes larger than A_N, estimates of strain and competence contrast are contoured in thickness-to-wavelength (H/λ) and amplitude-to-wavelength (A/λ) space. These quantities can be measured for any observed fold shape. Contour maps are constructed using existing linear theories of folding, a new nonlinear theory of folding and numerical simulations, all for single-layer folding. The method represents a significant improvement to the arc length method. The strain estimation method is applied to folds in viscous (Newtonian), power-law (non-Newtonian) and viscoelastic layers. Also, strain partitioning in fold trains is investigated. Strain partitioning refers to the difference in strain accommodated by individual folds in the fold train and by the whole fold train. Fold trains within layers exhibiting viscous and viscoelastic rheology show different characteristic strain partitioning patterns. Strain partitioning patterns of natural fold trains can be used to assess the rheological behaviour during fold initiation.     

井水位对地壳应力-应变响应灵敏度的研究

从丰镇井水位的潮汐效应、地表水体荷载效应与列车荷载效应分别计算出井水位对各类应力 应变的响应灵敏度,其结果分别是6.6×10-9体应变/mm、9.6×10-7体应变/mm与1.0×10-2体应变/mm;在此基础上,推算出该井水位在张北地震前60mm的缓升型异常相应的含水层应变与应力前兆的量级为5.76×10-5体应变与2.3kPa。     

Dynamic interaction of bridge–train system under non‐uniform seismic ground motion

The probability that an earthquake occurs when a train is running over a bridge in earthquake‐prone regions is much higher than before, for high‐speed railway lines are rapidly developed to connect major cities worldwide. This paper presents a finite element method‐based framework for dynamic analysis of coupled bridge–train systems under non‐uniform seismic ground motion, in which rail–wheel interactions and possible separations between wheels and rails are taken into consideration. The governing equations of motion of the coupled bridge–train system are established in an absolute coordinate system. Without considering the decomposition of seismic responses into pseudo‐static and inertia‐dynamic components, the equations of motion of the coupled system are formed in terms of displacement seismic ground motions. The mode superposition method is applied to the bridge structure to make the problem manageable while the Newmark‐β method with an iterative computation scheme is used to find the best solution for the problem concerned. Eight high‐speed trains running over a multi‐span steel truss‐arch bridge subject to earthquakes are taken as a case study. The results from the case study demonstrate that the spatial variation of seismic ground motion affects dynamic responses of the bridge–train system. The ignorance of pseudo‐static component when using acceleration seismic ground motions as input may underestimate seismic responses of the bridge–train system. The probability of separation between wheels and rails becomes higher with increasing train speed. Copyright © 2011 John Wiley & Sons, Ltd.     

Three‐dimensional steady‐state response of a railway system on layered half‐space soil medium subjected to a moving train

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.     

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Variation in Summer Rainfall in North China during the Period 1956-2007 and Links with Atmospheric Circulation

Using gauge precipitation data and NCEP-NCAR reanalysis data, the interdecadal changes in summer precipitation during the period 1956-2007 in North China and the link with atmospheric circulation change over Eurasia are studied. Results show that precipitation amounts decreased by 16.2 mm per decade, which was attributable to a significant reduction in precipitation frequency. Contrary wave trains were found in the subtropical westerly jet (wave guide) over Eurasia for the wet and dry years of North China. ...     

Water transport in planetary ice shells by two-phase flow – a parametric study

We present a two-phase model for the generation of meltwater and its propagation through the outer shells of icy satellites such as Europa, Enceladus or Titan. We exploit the analogy with the process of partial melt generation in the Earth’s interior by adopting the formalism of two-phase flow developed in the mantle-dynamics community, and by means of scaling analysis we derive a reduced system appropriate for our planetary application. The resultant system couples Darcy’s law with the deformation of the viscous ice matrix. We numerically investigate the system in a simplified one-dimensional geometry, corresponding to a laterally uniform ice layer, and analyze the role of various physical parameters. We focus on the leading-order effects, namely (i) the key importance of ice permeability, (ii) the role of complex ice rheology depending on temperature, deformation mechanisms and water content, (iii) the possible contribution of surface tension and (iv) the effects of mechanical coupling between the phases. Our analysis suggests that the global water transport through temperate ice is mainly controlled by ice permeability and can be well approximated by a model in which the complex ice rheology is parameterized in terms of a constant viscosity. While the mechanical coupling between the phases dramatically affects the flow at the local scale, the surface tension appears to be insignificant.     

饱和地基上列车运行引起的地面振动隔振分析

利用推导出的频域内饱和土的以土骨架位移u和孔隙水压力p表示的u-p格式的2.5维有限元方程,分析列车低速和高速运行条件下不同隔振措施的隔振效果,包括空沟、填充沟（混凝土墙）和混凝土板（波阻板）。其中轨道被简化为饱和地基上的Euler梁,通过沿轨道方向的波数变换将三维空间问题降为平面应变问题,对求解的结果进行Fourier反变换,获得三维时域-空间域内的解。研究表明：弹性地基和饱和地基上设置空沟和填充沟隔振效果相差不大,而设置波阻板隔振效果存在较大差异。列车低速运行时,空沟基本无隔振效果,填充沟在一定范围内有隔振效果,波阻板隔振效果最好;列车高速运行时,空沟在较大沟宽和较深沟深条件下才能达到较好隔振效果,填充沟和波阻板只需较小尺寸就能达到理想的隔振效果     

可作为新震源的列车振动及实验研究

利用地震仪器对火车通过大同—秦岭铁路线产生的振动进行了观测. 观测中使用了两套宽频带地震仪和一套短周期地震仪,共获得距铁路0～2.15km的9个观测点的38次列车振动观测数据,全部记录均可分辨出清晰的地震波形. 研究表明：运动列车会在铁路沿线2～3km范围内产生明显的地面振动,接收到的振动频谱在0.05～20Hz范围内平坦,信号频带较宽,观测重复性好. 列车振动为利用地震勘探方法探测地下浅层结构提供了一种重复性很好的新震源.