Modelling of pile wave barriers by effective trenches and their screening effectiveness

The three-dimensional problem of isolation of vibration by a row of piles is studied numerically on the basis of a model replacing the row of piles by an effective trench in order to reduce the modelling complexity. The analysis is accomplished with the aid of an advanced frequency domain boundary element method, which is used for both the infilled trench and the soil medium in conjunction with a coupling procedure based on enforcement of equilibrium and compatibility at the trench–soil interface. Linear elastic or viscoelastic material behaviour is assumed for both the piles and the soil. The piles can be tubular or solid and have circular or square cross-section. The vibration source is a vertical force, harmonically varying with time, and the row of piles acts as a passive wave barrier. The effective trench model is constructed by invoking well known homogenization techniques used in the mechanics of fibre-reinforced composite materials, and its accuracy is compared against a rigorous boundary element analysis modelling each pile separately in full contact with the soil medium. On the basis of the effective trench model, the screening effectiveness of a row of piles is studied through parametric studies.     

用单多卜勒雷达确定中尺度气旋环流中心及最大风速半径

首先给出具有 Ｒａｎｋｉｎｅ 模式的中尺度气旋的模拟多卜勒径向速度图,然后利用多卜勒雷达观测的径向速度与实际气流间的几何关系,提出一种确定近似轴对称的中尺度气旋中心及最大风速半径的方法。     

The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north-west Connecticut,USA

Serial sectioning and imaging with a flatbed scanner yielded the three-dimensional size and spatial distribution of garnet porphyroblasts in two garnet schists and one staurolite-bearing schist from the Everett Formation, north-west Connecticut. The dominant garnet-producing reaction in all samples was chlorite+quartz=garnet+H₂O. The appearance of staurolite, and additional garnet growth in the staurolite-bearing sample, was due to the reaction chloritoid=garnet+staurolite+chlorite. Statistical measures of garnet spatial distributions, using the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm. Such clustered nucleation may reflect minor bulk compositional variations. Covariance measures between garnet size and nearest-neighbour distance, using the mark covariance function (MCF), suggest a very weak correlation between crystal size and nearest-neighbour distance for length scales of 2 mm or less. These statistical data suggest that if diffusional gradients were present around growing garnet crystals, they did not influence nucleation and growth patterns at length scales greater than c. 2 mm. Compositional maps, through the garnet centres, show that the smaller crystals have lower Mn core compositions relative to larger crystals, consistent with progressive nucleation during pro-grade metamorphism. Radius-rate plots calculated from compositional X-ray maps show similar growth rates for garnet crystals of different size, consistent with an interface-controlled growth model for garnet. The presence of minor diffusional gradients around growing garnet cannot be entirely dismissed, but the lack of observable reaction rims, the clustered spatial distribution and the radius-rate data are most consistent with an interface-controlled garnet growth model.     

桃园矿10煤底板钻孔注浆堵水半径探讨

桃园煤矿10煤底板裂隙发育不均匀,合理确定钻孔注浆堵水有效半径,是10煤安全开采的保障。经理论计算和实验研究,确定注浆有效半径为1015 m。      

蒸发波导对GNSS海面反射信号有效散射区域的影响

蒸发波导在海洋低空的发生概率高达90%,对舰船雷达、通信等电磁系统具有重要影响.为了分析利用GNSS卫星海面反射信号的时延-相关功率波形反演蒸发波导的可行性,本文提出了GNSS卫星海面反射信号的有效散射区域的概念,并将有效散射区域内的GNSS反射信号区分为GNSS标准反射信号和GNSS波导反射信号;然后,利用射线追踪方法,仿真分析了GNSS卫星海面反射信号的有效散射区域大小对蒸发波导的关键参数——波导高度的敏感性,并分析了在时延-相关功率波形上利用反射信号的传播时延将二者分离开的可行性.结果表明,GNSS卫星海面反射信号的有效散射区域对蒸发波导高度非常敏感,对于2～25m高的GNSS反射信号接收天线,当蒸发波导的高度由0m增加至20m时,GNSS反射信号有效散射区域半径的均值可由约14km迅速扩展至约160km;采用高度角足够大的GNSS卫星可以将有效散射区内的GNSS标准反射信号与GNSS波导反射信号在时延-相关功率波形上分离开.     

黄土高原中东部沙尘与非沙尘天气花粉组成及来源范围

通过黄土高原两个不同地区(浑源、洛川)沙尘与非沙尘天气花粉组合的对比来探讨主要花粉类型最大传播距离。结果表明：沙尘天气花粉汇集量明显高于非沙尘天气,特别是蒿属和藜科花粉,沙尘天气平均是非沙尘天气的3倍以上。随风速增大,花粉可能的最大源区范围明显增大。风速低于3.3 m/s时,蒿属和藜科花粉多来自100 km范围内,其他类型花粉来源范围不超过20 km;风速大于5.9 m/s时,蒿属和藜科花粉源区范围增至300 km以上;其他类型花粉不超过100 km。风速达到12.5 m/s时,蒿属及藜科最大花粉源区范围可达1000 km,其他类型花粉不超过300 km。浑源地区不论平均风速还是最大风速均明显高于洛川地区,因此,主要花粉类型可能的最大源区范围达200 km,大于洛川地区(低于100 km)。     

利用卫星轨道资料获取降水云团尺度的新方案

提出一种新的统计方案,以目标面积比为核心度量指标,并参照连续圆形目标定义目标面积比意义上的等效半径,实现对目标宏观尺度大小的量化估计。此方法能有效减轻轨道截断效应所造成的目标尺度低估,可对通过计数类方法得到的目标空间尺度做出有效修正。     

不同倒角半径下方柱绕流的数值模拟及水动力特性研究

为了研究不同倒角半径对方柱绕流特性的影响,采用有限体积法,模拟了雷诺数Re为22 500、倒角半径为0.1D(D为方柱边长的长度)、0.2D和0.3D时方柱的绕流过程。方柱近壁面采用增强壁面函数,模型采用SST k–?湍流模型。根据模拟结果给出了不同倒角半径下方柱的流场涡量图以及阻力系数Cd和升力系数Cl;利用快速傅里叶变换法得到斯托罗哈数St。结果表明,倒角半径的增加改变了方柱的分离点,使得尾流区长度增加,旋涡尺度减小;Cd和Cl的振动幅值呈现先减小后增大的趋势,倒角半径为0.1D和0.2D时方柱受力较小,不存在倒角时方柱受力较大,倒角半径为0.3D时方柱受力最大;随着倒角半径的增加,柱体截面形式越接近圆形,斯托罗哈数逐渐增大,漩涡脱落频率更快。     

基于有效邻域波场近似的起伏地表保幅高斯束偏移

随着我国陆上地震勘探向复杂地表探区的转移,高精度、适应性强的地震成像方法在地震资料的处理、解释及后续属性分析、储层预测中具有重要意义.本文基于有效邻域波场近似理论发展了一种成像精度更高且适用于复杂起伏地表条件的叠前保幅高斯束偏移方法.在传统水平地表高斯束偏移的基础上,本文根据中心射线附近有效邻域内高斯束表征的近似波场,导出了起伏地表条件下具有相对振幅保持的高斯束偏移公式,并给出了一种精度更高的旁轴射线传播角度计算方法.同现有的高斯束偏移方法相比,本文方法不仅考虑了起伏地表对高斯束走时的线性影响,而且首次引入了由地表高程差异和近地表速度变化引起的二次时差校正项和振幅校正项,使得成像结果更加准确可靠.两个典型模型算例验证了本文方法的正确性和有效性.     

Common‐shot Fresnel beam migration based on wave‐field approximation in effective vicinity under complex topographic conditions

Gaussian beam migration is a versatile imaging method for geologically complex land areas, which overcomes the limitation of Kirchhoff migration in imaging multiple arrivals and has no steep‐dip limits of one‐way wave‐equation migration. However, its imaging accuracy depends on the geometry of Gaussian beam that is determined by the initial parameter of dynamic ray tracing. As a result, its applications in exploration areas with strong variations in topography and near‐surface velocity are limited. Combined with the concept of Fresnel zone and the theory of wave‐field approximation in effective vicinity, we present a more robust common‐shot Fresnel beam imaging method for complex topographic land areas in this paper. Compared with the conventional Gaussian beam migration for irregular topography, our method improves the beam geometry by limiting its effective half‐width with Fresnel zone radius. Moreover, through a quadratic travel‐time correction and an amplitude correction that is based on the wave‐field approximation in effective vicinity, it gives an accurate method for plane‐wave decomposition at complex topography, which produces good imaging results in both shallow and deep zones. Trials of two typical models and its application in field data demonstrated the validity and robustness of our method.