网格融合技术在列车过隧道运动网格数值模拟中的应用

在高速列车过隧道问题的数值模拟中,为提高模拟准确性而考虑转向架、受电弓导流罩、车厢连接处等细部结构后,几何模型变得复杂。为了得到质量高、适用性强的计算网格,在列车附近生成非结构化四面体网格,运动网格及计算区域其余部分划分块结构化六面体网格。在融合面上,利用网格融合技术处理四面体网格的三角形面网格和六面体网格的四边形面网格的联结问题,通过控制节点位置的变化满足拓扑一致,实现无缝连接。通过三维数值模拟计算结果与一维实验结果的对比发现,在同等精度要求下,采用网格融合技术及分区思想生成的网格整体上数量更少,生成速率更高,该方法可推广应用于更复杂几何模型的网格划分中。     

人体面骨三维有限元模型重构及碰撞分析

本文实现了螺旋CT图像构建面颅骨三维有限元模型过程,用CT断层图像输入计算机,采用CT图像三维再现软件和CAD软件构建轮廓线,用非规则形体、有限元软件Ansys划分网格.此模型包括上颌骨、鼻骨、泪骨、颧骨.六面体与四面体的网格细化到平均尺寸6mm,四面体的网格细化到平均4mm.通过与前人实验的对比,可知该模型有一定的价值,并且对比了两种网格对于结果的影响.     

人体面骨三维有限元模型重构及碰撞分析

本文实现了螺旋CT图像构建面颅骨三维有限元模型过程，用CT断层图像输入计算机，采用CT图像三维再现软件和CAD软件构建轮廓线，用非规则形体、有限元软件Ansys划分网格。此模型包括上颌骨、鼻骨、泪骨、颧骨。六面体与四面体的网格细化到平均尺寸6mm，四面体的网格细化到平均4mm。通过与前人实验的对比，可知该模型有一定的价值，并且对比     

汽车变速器箱体精准性优化仿真研究

针对由汽车变速器箱体壁厚不均、结构复杂导致有限元网格精准划分困难,计算误差大的问题,提出了一种基于低阶四面体模型,进行复杂零件最佳网格划分方法研究,在对不同单元尺寸和单元类型变速器箱体有限元模型分析研究,获得单元尺寸和单元类型对有限元分析计算精度和经济性的影响规律。用Hypermesh、Ansys为分析平台,根据局部误差与全局误差理论,进行了网格局部加密有限元网格划分方法。通过对汽车变速器箱体分析计算表明,分析方法与实际情况相吻合。     

大学生方程式赛车车架侧翼碰撞分析与优化

中国大学生方程式汽车大赛(简称"FSC")。本文据《中国大学生方程式汽车大赛规则》要求,在考虑到车架前部结构相对侧面更稳定、安全,且具备缓冲吸能模块而车手不易因车辆正面碰撞受伤,而侧面防护仅存单壳体车架,其在高速侧翼碰撞时易发生形变而致使车手受伤。因而本文通过使用非线性计算功能强大的工程模拟的有限元软件ABAQUS,针对车架侧翼碰撞进行分析,并优化车架侧翼结构;考虑到车架交叉焊接连接位置,网格划分难度大且需采用四面体网格划分,无法保证结果准确度(四面体单元易产生应力集中)。因而本文将通过使用六面体网格和参考点自由度进行耦合的方法,避免了在车架管材交叉焊接处必须使用易产生应力集中的四面体网格的缺陷,同时也降低了网格的划分难度和网格数量,提高了前处理速度和求解速度,对单管部分使用六面体网格划分,而对交叉焊接点以刚性连接参考点的方式简化,以连接部分的单元应力数值作为判断焊点断开的依据,对车架侧翼结构进行快速优化。     

一种适用于任意复杂结构的曲六面体网格生成算法

曲六面体网格自动生成是时域谱元法发展的瓶颈问题。本文采用基于体基函数的协变投影算法,将一个10节点曲四面体单元分解为四个20节点曲六面体单元,从而形成一种适用于任意复杂结构的曲六面体网格生成算法。使用该算法能够将曲四面体单元均匀分割,且确保新形成的曲六面体单元的翘曲最小。典型算例表明,使用该算法建立的曲六面体离散模型,完全满足时域谱元法的计算精度要求。     

栅格法三维六面体网格局部加密算法

有限元数值分析的精度和效率与网格单元的划分质量以及疏密程度密切相关,针对三维六面体网格单元之间疏密过渡必须平缓和协调的要求,提出了一套基于8分法的六面体网格加密模板,并给出了相应的数据结构和模板应用方式.为使所有加密单元都有相对应的加密模板,建立了加密信息场调整规则;对需要进行加密的区域首先补充加密单元,按照节点加密属性调整加密信息场,然后根据单元加密属性对加密单元进行分类,按照全加密单元、面加密单元、边加密单元以及过渡加密单元的顺序依次采用相应的模板进行加密,从而实现三维六面体网格的局部协调加密.实例结果表明,采用该套加密模板的六面体网格局部加密算法能够保障密集网格向稀疏网格的平缓和协调过渡,所生成的网格可满足有限元数值计算的要求.     

子分元素法:一种体网格压缩算法

提出了一个包含六面体,四面体,金字塔以及三棱柱单元的混合体网格的压缩与解压算法。首先对非四面体单元按照最小节点标号策略进行子分,然后利用修改的增长缝合算法压缩子分后的四面体网格,解压阶段再通过面删除操作来恢复原始网格。压缩后每个四面体约需10bits的存储,初步试验表明,对于通常的六面体网格,能将数据模型压缩至原先的1／4。     

三维有限差分计算中大规模网格生成及显示技术

针对爆炸与冲击问题并行仿真计算软件PMMIC-3D（Parallel Multi-Material in Cell 3D）的计算网格为正交六面体网格的特点,开发与PMMIC-3D接口统一的可对任意复杂三维实体模型进行大规模有限差分网格生成的三维前处理软件MESH-3D.MESH-3D采用CSG和STL模型两种建模方式进行复杂实体建模,并采用基于边的整体切片算法,借鉴计算机图形学中的扫描线填充算法完成三维有限差分网格划分.在绘制网格时,删除网格单元的公共面,大大缩短计算时间和减少存储空间,实现对网格的快速消隐显示.MESH-3D可实现百亿量级网格单元的生成和显示.三维前处理软件MESH-3D的开发有力地支持爆炸与冲击问题的仿真计算.     

扫掠法有限元网格生成方法

为了提高有限元网格的生成质量,扫掠法生成六面体网格过程中内部节点定位成为关键一步,在研究复杂扫掠体六面体有限元网格生成算法过程中,提出了一种基于扫掠法的六面体网格生成算法,算法利用源曲面已经划分好的网格和连接曲面的结构化网格,用仿射映射逐层投影,生成目标曲面,提出基于Roca算法的内部节点定位的新算法,运用由外向内推进的波前法思想,生成全部的六面体网格。通过实例表明,该算法快速,稳定,可靠,可处理大量复杂2.5维实体六面体网格生成问题。     

Transformation of Hexaedral Finite Element Meshes into Tetrahedral Meshes According to Quality Criteria

The paper is concerned with algorithms for transforming hexahedral finite element meshes into tetrahedral meshes without introducing new nodes. Known algorithms use only the topological structure of the hexahedral mesh but no geometry information. The paper provides another algorithm which is then extented such that quality criteria for the splitting of faces are respected.     

A strategy of automatic hexahedral mesh generation by using an improved whisker-weaving method with a surface mesh modification procedure

One of the demands for three dimensional (3D) finite element analyses is the development of an automatic hexahedral mesh generator. For this problem, several methods have been proposed by many researchers. However, reliable automatic hexahedral mesh generation has not been developed at present. In this paper, a new strategy of fully automatic hexahedral mesh generation is proposed. In this strategy, the prerequisite for generating a hexahedral mesh is a quadrilateral surface mesh. From the given surface mesh, combinatorial dual cycles (sheet loops for the whisker-weaving algorithm) are generated to produce a hexahedral mesh. Since generating a good quality hexahedral mesh does not depend only on the quality of quadrilaterals of the surface mesh but also on the quality of the sheet loops generated from it, a surface mesh modification method to remove self-intersections from sheet loops is developed. Next, an automatic hexahedral mesh generator by the improved whisker-weaving algorithm is developed in this paper. By creating elements and nodes on 3D real space during the weaving process, it becomes possible to generate a hexahedral mesh with fewer bad-quality elements. Several examples will be presented to show the validity of the proposed mesh generation strategy.     

Hexahedral mesh smoothing via local element regularization and global mesh optimization

The quality of finite element meshes is one of the key factors that affect the accuracy and reliability of finite element analysis results. In order to improve the quality of hexahedral meshes, we present a novel hexahedral mesh smoothing algorithm which combines a local regularization for each hexahedral mesh, using dual element based geometric transformation, with a global optimization operator for all hexahedral meshes. The global optimization operator is composed of three main terms, including the volumetric Laplacian operator of hexahedral meshes and the geometric constraints of surface meshes which keep the volumetric details and the surface details, and another is the transformed node displacements condition which maintains the regularity of all elements. The global optimization operator is formulated as a quadratic optimization problem, which is easily solved by solving a sparse linear system. Several experimental results are presented to demonstrate that our method obtains higher quality results than other state-of-the-art approaches.     

用局部阻尼波形法的自连贯网格生成法

介绍将生成或已有的有限元规则六面体单元改变成形状复杂且光滑的六面体单元的一种新技术——波形法．简单建立了波形法的数学模型并编程进行了网格生成．实验结果表明,只要把复杂外部形状按波的形式传输给指定的规则六面体单元模型,就可以得到具有复杂外部形状的六面体单元网格模型,且网格生成速度快、不出现单元或节点漏洞、欠缺等问题,同时不受单元类型的限制。     

Quasi-automatic 3D finite element model generation for individual single-rooted teeth and periodontal ligament

The paper demonstrates how to generate an individual 3D volume model of a human single-rooted tooth using an automatic workflow. It can be implemented into finite element simulation. In several computational steps, computed tomography data of patients are used to obtain the global coordinates of the tooth's surface. First, the large number of geometric data is processed with several self-developed algorithms for a significant reduction. The most important task is to keep geometrical information of the real tooth. The second main part includes the creation of the volume model for tooth and periodontal ligament (PDL). This is realized with a continuous free form surface of the tooth based on the remaining points. Generating such irregular objects for numerical use in biomechanical research normally requires enormous manual effort and time. The finite element mesh of the tooth, consisting of hexahedral elements, is composed of different materials: dentin, PDL and surrounding alveolar bone. It is capable of simulating tooth movement in a finite element analysis and may give valuable information for a clinical approach without the restrictions of tetrahedral elements. The mesh generator of FE software ANSYS executed the mesh process for hexahedral elements successfully.     

Hexahedral mesh generation constraints

For finite element analyses within highly elastic and plastic structural domains, hexahedral meshes have historically offered some benefits over tetrahedral finite element meshes in terms of reduced error, smaller element counts, and improved reliability. However, hexahedral finite element mesh generation continues to be difficult to perform and automate, with hexahedral mesh generation taking several orders of magnitude longer than current tetrahedral mesh generators to complete. Thus, developing a better understanding of the underlying constraints that make hexahedral meshing difficult could result in dramatic reductions in the amount of time necessary to prepare a hexahedral finite element model for analysis. In this paper, we present a survey of constraints associated with hexahedral meshes (i.e., the conditions that must be satisfied to produce a hexahedral mesh). In presenting our formulation of these constraints, we will utilize the dual of a hexahedral mesh. We also discuss how incorporation of these constraints into existing hexahedral mesh generation algorithms could be utilized to extend the class of geometries to which these algorithms apply. We also describe a list of open problems in hexahedral mesh generation and give some context for future efforts in addressing these problems.     

Meshing complexity: predicting meshing difficulty for single part CAD models

This paper proposes a method for predicting the complexity of meshing computer aided design (CAD) geometries with unstructured, hexahedral, finite elements. Meshing complexity refers to the relative level of effort required to generate a valid finite element mesh on a given CAD geometry. A function is proposed to approximate the meshing complexity for single part CAD models. The function is dependent on a user defined element size as well as on data extracted from the geometry and topology of the CAD part. Several geometry and topology measures are proposed, which both characterize the shape of the CAD part and detect configurations that complicate mesh generation. Based on a test suite of CAD models, the function is demonstrated to be accurate within a certain range of error. The solution proposed here is intended to provide managers and users of meshing software a method of predicting the difficulty in meshing a CAD model. This will enable them to make decisions about model simplification and analysis approaches prior to mesh generation.