Adaptive finite element analysis of mode I fracture in cement‐based materials

Microscopic studies using advanced experimental techniques have provided better insight into the fracture mechanisms in cement‐based materials. A clear understanding of fracture mechanisms is critical for the development of rigorous computational models for analysing fracture. Fracture analysis is usually carried out by finite element method. Accuracy of FE analysis depends upon the choice of mesh and for the predictions to be reliable, discretization errors are to be minimized. In cohesive crack approach, the non‐linearity is limited to the boundary conditions along the geometric discontinuity while the bulk of the material retains its elastic nature. The paper presents a mesh‐adaptive strategy based on ZZ error estimator to model discrete crack propagation in cement‐based materials. Examples of simulations have demonstrated the potential of the mesh‐adaptive technique in modelling the evolution of the localized strain profiles as well as failure of concrete test specimen. Copyright © 2001 John Wiley & Sons, Ltd.     

Analysis for reflector aluminum mesh panels of five-hundred meter aperture spherical telescope

In this paper, an analysis of the aluminum reflector mesh panels for the Five-hundred meter Aperture Spherical Telescope (FAST) is described. The study concentrates on one element of the reflector surface with the shape of a triangle. For the analysis, the panel is modelled as a mesh of cables so that it can be treated as continuous. It is demonstrated that the results so obtained are reasonable. This revised version was published online in July 2006 with corrections to the Cover Date.     

竹片网水泥砂浆加固承重夯土墙体抗震试验研究

为了提高村镇承重夯土墙体的抗震性能,对2片竹片网水泥砂浆加固墙体和1片未加固对比墙进行拟静力试验,研究其破坏形态、水平承载力、滞回性能和耗能等抗震性能。试验结果表明:与未加固夯土墙体相比,加固后墙体的受力性能、变形性能及耗能能力均得到明显改善;同等加固水平下:斜放竹片网对夯土墙承载力的提高显著,相比对比试件峰值荷载和极限荷载分别提高82.58%和76.97%;正放竹片网加固的墙体滞回曲线更加饱满,骨架曲线下降更加平缓,变形能力和耗能提高更加明显,相比对比试件峰值位移和极限位移分别提高63.33%和327.69%,耗能提高了781%,且正放竹片网加固施工方便,可提倡应用。总之,竹片网水泥砂浆加固方法生态经济,可明显减轻墙体的破坏,有效改善墙体抗震性能,研究成果可对村镇夯土建筑实地加固提供参考。     

Road network selection for medium scales using an extended stroke-mesh combination algorithm

The road network is an essential feature class in topographic maps and databases. Road network selection for smaller scales forms a prerequisite for all other generalization operators and is thus a fundamental operation in the overall process of topographic map and database production. The objective of this paper was to develop an algorithm for automated road network selection from a large-scale (1:10,000) to a medium-scale database (1:50,000). The project was pursued in collaboration with swisstopo, the national mapping agency of Switzerland. Three algorithms (a stroke-based, a mesh-based, and a combined stroke-mesh algorithm) were implemented from the literature and analyzed using swisstopo’s large-scale TLM3D spatial database, with requirements set forth by expert cartographers. Initial experiments showed that the combination algorithm performed best, yet still it could not meet all requirements. Therefore, extensions to the basic stroke-mesh algorithm were developed, significantly improving the selection result with real-world, large test databases. Three extensions introduce modifications to the stroke-mesh combination algorithm. Furthermore, two extensions include external feature classes, ensuring accessibility of points of interest and appropriate network density representation in settlement areas, respectively. The results were evaluated by expert cartographers, who concluded that the proposed approach is ready to be deployed in production at swisstopo.     

Selective omission of road features based on mesh density for automatic map generalization

Selection of roads is an intractable generalization operation due to the difficulty in retaining the density difference and connectivity of a road network. This paper proposes a novel approach of selective omission for roads based on mesh density. The density of a road network and its local variations are calculated using meshes as units. Since maps at different scales usually reveal different densities, different density thresholds for road networks are determined on the basis of theoretical analysis and empirical study of mesh densities on maps at different scales. The selection process starts with the identification of the meshes that have a density beyond the threshold. The mesh with the largest density is first treated. Its bounding road segments are ordered according to their relative importance. The least important segment is eliminated. The remaining segments are then merged with the adjacent mesh, thus forming a new mesh. The selection procedure is repeated until none of the meshes has a density beyond the threshold. Such a process of eliminating road segments and merging meshes can ensure the road network connectivity. In this study, the meshes are classified depending on the types of road segment. For the different mesh types, their density thresholds are set to be different, which can be used as an indicator for the preservation of the density difference. This proposed approach considers topological, geometric and semantic properties of the road network. It was applied to two sets of road networks, and the results of selection are convincing. This methodology has now been adopted for the updating of 1:50,000 maps of China.     

A hybrid optimization approach for efficient calibration of computationally intensive hydrological models

ABSTRACT

The calibration of hydrological models is formulated as a blackbox optimization problem where the only information available is the objective function value. Distributed hydrological models are generally computationally intensive, and their calibration may require several hours or days which can be an issue for many operational contexts. Different optimization algorithms have been developed over the years and exhibit different strengths when applied to the calibration of computationally intensive hydrological models. This paper shows how the dynamically dimensioned search (DDS) and the mesh adaptive direct search (MADS) algorithms can be combined to significantly reduce the computational time of calibrating distributed hydrological models while ensuring robustness and stability regarding the final objective function values. Five transitional features are described to adequately merge both algorithms. The hybrid approach is applied to the distributed and computationally intensive HYDROTEL model on three different river basins located in Québec (Canada).     

Demonstration and application of NMM‐based fractured porous medium flow model

In natural rock masses, the shapes of three‐dimensional (3‐D) blocks cut by arbitrary fracture networks may be very complex. Owing to the geometric complexity and difficulty of mesh discretization of 3‐D blocks and fracture facets, explicit consideration of fracture networks in flow analysis of fractured porous medium (FPM) is very challenging. Using the numerical manifold method based on independent covers (NMMIC), an FPM flow model was proposed that can deal with very complex 3‐D fracture networks. In this paper, the convergence of NMMIC was first demonstrated. The theoretical basis of the arbitrary refinement of computational meshes was proven. Moreover, three peculiarities of NMMIC meshes, that is, arbitrary shape, arbitrary connection, and arbitrary refinement of independent covers, were concluded. Finally, some two‐dimensional (2‐D) tunnel flow examples were analyzed and the numerical results were compared with the analytical results. 3‐D examples with complex fracture distributions were also analyzed. In addition, the computational scale of the developed program was tested by increasing the number of computational elements. The results show that our model can accurately analyze the groundwater flow of rocks surrounding tunnels with complex fracture distributions.     

数值流形法在处理强奇异性问题时的网格无关性

数值流形方法（NMM）的最大优势在于可以统一地处理岩土力学中的连续和非连续变形问题。它在求解断裂力学问题时无需强制裂纹与数学网格保持一致,非常适合应用于岩土工程中由连续到非连续的破坏过程模拟。在裂纹扩展过程中,裂纹与数学网格的相对位置将会是任意的,如裂纹尖端可能落在网格内部、网格节点上或网格边上等。因此,对同一条裂纹,通过旋转和移动数学网格构造了它们之间的这种相对位置关系以及一些可能对计算结果产生影响的极端情况,并以应力强度因子作为衡量标准,研究了NMM在处理线弹性断裂力学问题时的网格依赖性。研究表明,NMM即使在处理强奇异性问题时依然有着很好的网格无关性,进一步证实了它在模拟裂纹扩展问题时的鲁棒性。     

高精度地震时深转换方法研究及应用

地震体的时深转换是利用地震资料进行构造及储层解释的一个非常关键的环节,而常规时深转换方法精度较低,一定程度上影响了勘探井位布置及开发方案的制定。采用时深一体化网格方法,即顶底界面以多井合成记录标定求取的平面时深关系为准,顶底之间利用时间域和深度域网格一一对应关系进行“物理”搬运的方法,从而减小转换前后的累计误差,使地震体及地震反演体时深转换前后的波形与砂体形态保持一致。利用该方法将喇嘛甸油田北部区块反演结果转换到深度域,精细刻画了断层附近砂体的分布特征,指导了L213等3口水平井的部署,砂体预测准确率达到98%以上,年增油17 000 t,展现了新的时深转换方法的精度及可靠性。     

基于地理加权回归的漫湾库区景观破碎化及影响因子分析

应用地理加权回归模型分析漫湾库区景观破碎化指数——有效筛网大小与相关因子之间的空间关系。选取的解释变量分别是距道路的距离、距乡村的距离、距河流的距离、坡度。结果表明：大坝修建后4种解释变量与有效筛网大小呈现较显著的正相关性。与线性回归模型相比,地理加权回归模型的拟合效果显著提高。1974~1988年,有效筛网大小对各影响因子最敏感的区域面积呈现显著的时空变化这为确定水电站建设及其他因素对景观破碎化影响的大小,并进一步改善库区景观破碎化的现状提供了依据。