基于混合常应力−光滑应变单元的极限分析方法

极限分析是岩土工程稳定性评价的重要方法之一。传统的有限元极限分析方法,采用低阶三角形单元时需要引入速度间断面并采用特殊网格布局,或者采用高阶三角形单元等措施来克服体积锁定问题和提高数值精度。在光滑有限元法（smoothed finite element method,简称SFEM）的基础上,提出了一种基于新型混合常应力−光滑应变单元的极限分析方法（mixed constant stress-smoothed strain element limit analysis,简称MCSE-LA方法）。在服从关联流动法则和Mohr-Coulomb屈服准则的基础上,MCSE-LA方法最终将数值极限分析转化为以应力和极限荷载乘子为基本未知量的二阶锥规划（second order cone programming,简称SOCP）问题。MCSE-LA方法具有形式简单、优化变量相对较少和无需显式的写出塑性内能耗散函数的优点,并且根据凸锥优化的对偶理论,可以从对偶问题中获得速度场和塑性乘子等信息。此外,还采用基于最大塑性剪应变率的网格自适应加密算法,该算法在塑性区细化网格,显著提高了新数值极限分析方法的计算效率和精度。最后通过边坡稳定分析的结果对比,验证了MCSE-LA方法的计算精度和效率均高于传统的有限元极限分析方法。     

复杂地形三角网格局部渐进传输方法

在数据高速共享的信息时代,数据传输是共享的前提和基础,而三维数据本身的复杂结构带来的海量数据对其传输、使用造成极大限制,因此,实现三维数据的高效传输十分重要。为解决三维空间数据高效传输的问题,本文提出了针对复杂地形三角网格的局部渐进传输方案。具体过程包括:首先在服务器端通过基于边折叠的三角网格化简和基于遗传算法的化简优化方法对网格模型进行预处理,并在服务器端构造折叠树存储网格简化信息;然后根据客户端的连接请求,向客户端传递化简后的初始网格,并根据用户的局部放大操作,在服务器端实时展开相应位置的网格结点并向客户端传输;最后,客户端将接收到的少量网格结点信息进行重组,实现局部放大效果,从而避免不必要的结点展开和传输。通过对遗传算法的效率和局部渐进传输时间的测试,验证了这一局部渐进传输方案的可行性。     

Multi-view dense matching supported by triangular meshes

We present a new procedure to compute dense 3D point clouds from a sequential set of images. This procedure is considered as a second step of a three-step algorithm for 3D reconstruction from image sequences, whose first step consists of image orientation and the last step is shape reconstruction. We assume that the camera matrices as well as a sparse set of 3D points are available and we strive for obtaining a dense and reliable 3D point cloud. Three novel ideas are presented: (1) for sparse tracking and triangulation, the search space for correspondences is reduced to a line segment by means of known camera matrices and disparity ranges are provided by triangular meshes from the already available points; (2) triangular meshes from extended sets of points are used for dense matching, because these meshes help to reconstruct points in weakly textured areas and present a natural way to obtain subpixel accuracy; (3) two non-local optimization methods, namely, 1D dynamic programming along horizontal lines and semi-global optimization were employed for refinement of local results obtained from an arbitrary number of images. All methods were extensively tested on a benchmark data set and an infrared video sequence. Both visual and quantitative results demonstrate the effectiveness of our algorithm.     

基于边扩张算法和熵编码的3D网格模型的拓扑信息压缩

本文总结了三类不经三角剖分直接编码多边形网格模型拓扑信息的单分辨率压缩法,对其中基于边区域扩张的Face Fixer算法进行了研究,并分别应用同阶自适应区间编码法和算术编码法对三角形网格模型和多边形网格模型进行了压缩.实验结果表明:随着模型数据量的增大,区间编码的压缩率和压缩速度反而高于算术编码,因而对于大数据量的网格...     

A robust well-balanced finite volume model for shallow water flows with wetting and drying over irregular terrain

An unstructured Godunov-type finite volume model is developed for the numerical simulation of geometrically challenging two-dimensional shallow water flows with wetting and drying over convoluted topography. In the framework of sloping bottom model, a modified formulation of shallow water equations is used to preserve mass conservation during flooding and recession. The key ingredient of the model is the use of this combination of the sloping bottom model and the modified shallow water equations to provide a robust technique for wet/dry fronts tracking and, together with centered discretization of the bed slope source term, to exactly preserve the static flow on irregular topographies. The variable reconstruction technique ensures nonnegative reconstructed water depth and reasonable reconstructed velocity, and the friction terms are solved by semi-implicit scheme that does not invert the direction of velocity components. The robustness and accuracy of the proposed model are assessed by comparing numerical and reference results of extensive test cases. Moreover, the results of a dam-break flooding over real topography are presented to show the capability of the model on field-scale application.     

Building a three dimensional sealed geological model to use in numerical stress analysis software: A case study for a dam site

The dam area of the SUOXI hydropower project shows high terrain undulation and complex geological conditions, containing 6 faults and 7 weak inter-beds. A geometric model developed to represent the geology and engineering structures should incorporate the geological realities and should allow suitable mesh generation to perform numerical stress analysis. This is an important precondition to perform rock mass stability analysis of a dam foundation based on a numerical stress analysis software such as FLAC^3D. Using the modeling tools available in FLAC^3D, it is difficult to construct a complex geological model even after performing a large amount of plotting and data analyses. The 3-D geological modeling technique suggested in this paper, named as Sealed Geological Modeling (SGM), is a powerful tool for constructing complex geological models for rock engineering projects that require numerical stress analysis. Applying this technique, first, the geological interfaces are constructed for the dam area of SUOXI hydropower project using various interpolation procedures including geostatistical techniques. Then a unitary wire frame is constructed and the interfaces are connected seamlessly. As the next step, a block tracing technique is used to build a geological model that consists of 130 seamlessly connected blocks. Finally, based on the Advancing Front Technique (AFT), each block is discretized into tetrahedrons and a mesh is generated including 57,661 nodes and 215,471 tetrahedrons which is suitable to perform numerical stress analysis using FLAC^3D.     

A numerical study on local characteristics of predetermined irregular wave trains

In actual sea states, damage to offshore floating structures is usually caused by a few extreme waves or wave groups in an irregular wave train. Accurate simulation of the irregular wave trains can lay a solid foundation for understanding the local flow field and impact loads that would potentially cause such damage. This paper describes how the generation of a single extreme wave was investigated. Determination of the wave-maker motion for generating specified irregular wave trains is the key to this work. First, an experimental irregular wave train was decomposed into a certain number of small-amplitude waves. Fourier series expansion was performed to determine the amplitude and the initial phase angle of each wave component. Then a hydrodynamic transfer function was used to calculate the amplitude of the wave-maker motion associated with each wave component. Superposition was made on all the wave components to get the final wave-maker motion. During the numerical simulation, calculated horizontal velocity profiles of the extreme wave at different moments were analyzed and compared with experimental results, and a satisfactory agreement was obtained. In the simulation, VOF method was employed to capture the free surface, and a dissipation zone was used to deal with wave reflection.     

最小二乘配点法解一维非稳定流问题

最小二乘配点无网格法是一种新型高效的无网格法。该方法除节点外又在研究域内引入辅助点,近似函数仍然只通过节点构造,微分方程在所有节点和辅助点上满足。用最小二乘配点法计算河间承压非稳定流问题,算例表明,最小二乘配点法比有限差分法计算精度高,稳定性好。     

A semi-implicit three-dimensional numerical model for non-hydrostatic pressure free-surface flows on an unstructured,sigma grid

A semi-implicit 3-D numerical formulation for solving non-hydrostatic pressure free-surface flows on an unstructured,sigma grid is proposed.Pressure-splitting and 9 semi-implicit methods are inherited and reformed from Casulli’s z-coordinate formulation.The non-orthogonal sigma-coordinate transformation leads to additional terms. The resulting linear system for the non-hydrostatic correction is diagonally dominant but unsymmetric,and it is solved by the BiCGstab method.In contrast with z-coordinate non-hydrostatic models,the new model fits vertical boundaries much better,which is important for the long-time simulation of sediment transport and riverbed deformation.A lock-exchange density flow is computed to determine whether the new scheme is able to simulate non-hydrostatic free-surface flows.The new model is further verified using the field data of a natural river bend of the lower Yangtze River.Good agreement between simulations and earlier research results,field data is obtained, indicating that the new model is applicable to hydraulic projects in real rivers.     

Flash Code Simulations of Rayleigh-Taylor and Richtmyer-Meshkov Instabilities in Laser-Driven Experiments

We present two- and three-dimensional simulations involving Richtmyer–Meshkov and Rayleigh-Taylor instabilities run with the adaptive mesh refinement code, flash. Variations in the rate of mixing layer growth due to dimensionality, perturbation modes, and simulation resolution are explored. These simulations are designed for detailed comparisons with experiments run on the Omega laser to gain understanding of the mixing processes and to prepare for validation of the Flash code.