Quadrilateral mesh generation on trimmed NURBS surfaces

An automatic mesh generation scheme with unstructured quadrilateral elements on trimmed NURBS surfaces has been developed. In this paper NURBS surface geometries in the IGES format have been employed to represent geometric models. For unstructured mesh generation with quadrilateral elements, a domain decomposition algorithm employing loop operators has been modified. As for the surface meshing, an indirect 2D approach is proposed in which both quasi-expanded planes and projection planes are employed. Sample meshes for complex models are presented to demonstrate the robustness of the algorithm.     

Forging process analysis with arbitrary shape die in semi-solid material with high solid fraction

Numerical simulations for industrial net shape forming application by the finite element method (FEM) are based on the integral form of the mechanical equations governing the physical process. However, current commercial packages cannot analyze the die filling and solidification for materials in the semi-solid state because they lack material models for liquid and solid materials and do not address segregation of the liquid and solid phases. Thus, this work presents an overview of the development of a new finite element program entitled SEMI-FORM S/W to predict the solid-phase deformation of semi-solid materials, filling defects and liquid segregation. The automatic mesh generation scheme by the modified looping algorithm using quadrilateral elements is proposed in this present study by a new loop splitting method and a local finite element control. An automatic quadrilateral mesh generator with automatic mesh refinement is developed in order to meet special demands found in the thixoforming process analysis with arbitrarily shaped dies by the node renumbering and modifications of the nodal position. Applying the remeshing criterion carries out the remeshing due to B-spline form. The SEMI-FORM S/W developed in this present study would be recognized as efficient and economical tool for designing and optimizing thixoformed products with arbitrarily shaped dies.     

有限元网格划分中的圣维南原理及其应用

网格划分是有限元分析前处理中的一个关键步骤,如何进行有效的网格划分,既保证计算的精度,又能提高计算效率,是一个重要的研究课题。对此提出了有限元网格划分中的圣维南原理。该原理以为,某局部区域的网格疏密对其稍远处节点的计算精度影响很小。基于该原理提出了网格划分的新思路:首先对全局进行一般的网格划分,然后只对关注地方及其周围的关键区域进行局部细分,就可以达到较高的仿真精度。使用实际算例对该原理进行了验证,结果表明该方法具有较高的精度和计算效率。     

B样条曲面在模具型腔描述及3维网格生成中的应用

采用３次Ｂ样条曲面进行模具型腔及锻件动态几何构形描述,提出基于Ｂ样条曲面描述的３维网格样条生成技术及改进的分块样条生成法,探讨了复杂锻件网格生成中的子域部分、疏密网格过渡、子域拼装等技术技巧。     

MEMS设计中的2.5维六面体网格划分

介绍了一种用于MEMS设计的六面体网格划分方法。MEMS工艺决定了大部分MEMS模型都是层状结构,本文针对这种结构提出了一种简便的2.5维划分方法,适用于所有面平行于(x, y)平面或者z轴的层状结构。该方法在(x, y)平面内将模型划分为四边形网格,然后在z方向延伸形成六面体。相比于如今使用最多的Sweep划分算法,它在划分前的预处理更少,由于仅仅需要输入划分大小, 使得整个划分过程更加方便,速度也比Sweep方法快了10~1 000倍,10 000个六面体以内的划分可在10 s内完成。同时,该方法也可以通过输入工艺过程参数按照实际MEMS工艺步骤产生3维MEMS模型, 完成从2维版图到3维网格的直接生成。     

四叉树法非结构网格剖分技术研究

针对有限元前置处理中二维复杂域四边形网格自动剖分问题,对四叉树网格剖分算法进行了研究。描述了四叉树网格的数据结构及其递归生成过程;提出基于计算机图形学的网格黑白性判断算法;给出两种边界网格处理的修正方法,并对这两种修正方法进行了比较;利用四叉树数据结构的特点实现对网格遍历、查找、插入等操作,并根据最近共同祖先法完成四叉树网格邻域的查询。结果表明:采用该方法可以实现有限元网格全自动剖分,网格生成只依赖于二维域的几何特征,对复杂边界的适应性强,生成的网格域内全部为四边形,只在域边界处出现少量三角形网格,具有较高的质量;网格生成、遍历、查找等数据操作效率高、时间短。     

Three-dimensional morphing of similar shapes using a template mesh

Shape morphing is the process of transforming a source shape into a target shape, through a series of intermediate shapes. There are two important problems to be considered in three-dimensional shape morphing: conforming mesh generation and path interpolation. In this paper, a novel approach in which a template mesh is mapped directly to the target mesh is proposed for the efficient treatment of the conforming mesh generation problem. Our mapping technique is based on a shape deformation method using an implicit function and the well-known mesh smoothing scheme, so the implementation of the method is very simple and robust. After mapping the source mesh to the target mesh, i.e., after obtaining a consistent mesh parameterization of the two shapes, the intermediate shapes are obtained by linear interpolation of the modified Laplacian coordinates of the source and target meshes. We demonstrate many examples of morphing between various shapes, including a model of the human head, a head sculpture model, and models of the human body in different poses to show the validity and effectiveness of the proposed method.     

DELAUNAY TRIANGULATION METHOD OF CURVED SURFACES BASED ON RIEMANNIAN METRIC

A method for quality mesh generation of parametric curved surfaces is proposed. It is shown that the main difference between the proposed method and previous ones is that our meshing process is done completely in the parametric domains with the guarantee of mesh quality. To obtain this aim, the Delaunay method is extended to anisotropic context of 2D domains, and a Riemannian metric map is introduced to remedy the mapping distortion from object space to parametric domain. Compared with previous algorithms, the approach is much simpler, more robust and speedy. The algorithm is implemented and examples for several geometries are presented to demonstrate the efficiency and validity of the method.     

基于特征约束的四边形网格划分算法

鉴于有限元分析的特殊需要,提出了一种基于特征约束的四边形网格划分方法。将特征约束分为点约束和线约束,分别采用不同的方法进行处理,然后利用改进的铺砌法进行网格划分,从而生成质量良好的四边形网格。     

薄板冲压成形中基于数控轨迹点的全自动四边形网格生成

基于模具型腔表面的数控轨迹点,提出了一种适用于薄板冲压成 形计算机仿真的全自动四边形网格生成方法。该方法首先利用最大弦长偏差法删除冗余轨迹 点,接着根据相邻特征曲线插值法来重新分布轨迹点,最后生成四边形网格。算法简单、稳 定,能处理任意无孔洞冲压模具型腔表面的数控加工轨迹点的自动四边形网格生成问题。     

基于特征约束的四边形网格划分算法

鉴于有限元分析的特殊需要，提出了一种基于特征约束的四边形网格划分方法。将特征约束分为点约束和线约束，分别采用不同的方法进行处理，然后利用改进的铺砌法进行网格划分。从而生成质量良好的四边形网格。     

基于hypermesh的发动机零部件网格划分

使用hypermesh软件对发动机主要零部件进行网格划分。根据发动机不同零部件自身的结构特点,结合hypermesh软件提供的各种二维和三维的网格划分工具,总结出针对发动机气门、凸轮轴、曲轴、连杆、活塞、气缸盖、机体和油底壳的六面体、四面体及二维四边形网格划分的思路和方法,从而提高发动机主要零部件网格划分的效率和网格的质量。     

回转体非线性边界元网格自动生成的改进连续超限插值法

在有限元法的自动前处理工作基础上，对回转体非线性边界元提出了网格自动生成的改进连续超限插值法。文中详细讨论了改进的连续超限插值函数的构造、网格疏密控制和区域大小的控制方法。针对机械结构中几种典型的回转体给出了边界元网格划分的算例。     

基于样条曲面描述的大变形有限元数值模拟关键技术探索

模具型腔描述、锻件有限元网格划分、界面接触约束是制约三维复杂体成形有限元数值模拟技术应用的3个关键性问题。以3次B样条曲面描述为基础,对三维复杂工具表面离散、变形体三维网格自动划分、变形体边界节点与模具接触状态判断的一体化技术进行了研究。利用B样条曲面的直观性、局部性、通用性、造型灵活性等特点,构造复杂模具型腔曲面及变形体边界,提出三维网格样条生成法、改进的分块样条生成法及接触判断的区域层次搜索技术。     

基于散乱点的全自动四边形网格剖分方法

提出一种基于散乱点的全自动三维非结构化四边形网格剖分方法。对散乱点进行自适应预处理，包括冗余点的自动删除和模型边界提取。使用改进节点计算和铺路面相交搜索处理的铺路算法，自动生成全四边形网格。该方法可有效地控制生成网格单元的尺寸和质量，网格模型满足有限元分析的要求。与常规的基于CAD几何信息的网格生成方法相比，该方法避免了繁琐的模型修补问题，可快速生成高质量的全四边形网格模型。算例证明了该方法的有效性和实用性。     

具有线约束的曲面四边形网格自动生成算法研究

提出了一种具有线约束的曲面四边形网格自动生成算法。该算法分成二个大的步骤:先用约束Delaunay三角化方法生成曲面三角形网格,然后通过定义一些拓扑操作,利用前沿法将三角形网格转化成四边形网格。与传统的算法相比,本文算法能够处理约束边等特殊约束情况。最后给出了例子用于验证本文的算法。     

真实拉延筋参数化建模及其在薄板冲压仿真中的应用

提出一种参数化的拉延筋网格模型建模方法,在自主开发的有限元前处理软件中,建立了具有真实几何尺寸的半圆形、三角形拉延筋和拉深槛的网格模型,并提供了灵活的拉延筋布置手段。提出一种改进的全四边形网格加密方法,对成形过程中将会流过拉延筋区域的板料网格,以及可能与模具上曲率变化大的区域相接触的板料网格进行加密操作,以满足网格的适应性要求。提出一系列的网格拓扑清理模版,对加密后的网格进行拓扑清理操作,有效地提高了板料网格的质量。大量的算例证明所提出方法具有较高的精度和较强的工程实用性。     

含间隙的斜齿轮副扭振分析与试验研究

建立了科齿轮副的间隙型非线性扭振模型，其中考虑了斜齿轮副的啮合综合误差，齿侧间隙和时变啮合刚度。采用三维有限元法计算了斜齿轮副啮合刚度，用三次样条插值拟合得到时变啮合刚度函数。用数值积分方法对系统的非线性动力学微分方程进行了求解，获得了斜齿轮副在外转矩作用下受静态传动误差激励的非线性稳态强迫响应，并对系统的动态响应进行了测试，试验和理论计算结果了一致性证实了本文所提出模型和解法的正确性。     

Quasi Ellipsoid Gear Surface Reconstruction Based on Meshless Local Petrov-Galerkin Method and Transmission Characteristic

Special transmission 3D model simulation must be based on surface discretization and reconstruction,but special transmission usually has complicated tooth shape and movement,so present software can't provide technical support for special transmission 3D model simulation.Currently,theoretical calculation and experimental method are difficult to exactly solve special transmission contact analysis problem.How to reduce calculation and computer memories consume and meet calculation precision is key to resolve special transmission contact analysis problem.According to 3D model simulation and surface reconstruction of quasi ellipsoid gear is difficulty,this paper employes meshless local Petrov-Galerkin(MLPG) method.In order to reduce calculation and computer memories consume,we disperse tooth mesh into finite points-sparseness points cloud or grid mesh,and then we do interpolation reconstruction in some necessary place of the 3D surface model during analysis.Moving least square method(MLSM) is employed for tooth mesh interpolation reconstruction,there are some advantages to do interpolation by means of MLSM,such as high precision,good flexibility and no require of tooth mesh discretization into units.We input the quasi ellipsoid gear reconstruction model into simulation software,we complete tooth meshing simulation.Simulation transmission ratio during meshing period was obtained,compared with theoretical transmission ratio,the result inosculate preferably.The method using curve reconstruction realizes surface reconstruction,reduce simulation calculation enormously,so special gears simulation can be realized by minitype computer.The method provides a novel solution for special transmission 3D model simulation analysis and contact analysis.     

基于交互的B样条曲面重建与误差计算

基于三角网格模型的B样条曲面重建技术得到深入发展,计算与显示重建后的B样条曲面与原始测量三角网格之间的误差对分析曲面重建品质有重要作用。一种较为实用的B样条曲面重建方法是对三角网格模型进行四边界区域划分后进行栅格式采样,再根据采样点进行B样条曲面拟合。针对这种重建方法,研究了一种建立三角网格顶点与四边界区域对应关系的算法,再用离散的方法计算点到对应曲面的距离误差,最后用线性插值方法实现误差彩色．云图的显示。