焊点模拟方法对疲劳仿真寿命的影响

为改善焊点疲劳仿真寿命分布的合理性,基于盒状样件,讨论焊点的模拟方法对疲劳仿真寿命的影响,并进行疲劳仿真结果与试验结果的对比分析.结果表明,梁单元焊点模型和ACM焊点模型受网格尺寸的影响较大;而nugget焊点模型受网格尺寸影响相对较小,即使在焊点周边的网格尺寸一致性不好的情况下,该模型也能给出相对合理的寿命分布结果.     

低雷诺数湍流模型的网格特征

低雷诺数模型目前主要应用于二维简单流动的数值仿真中,为研究该湍流模型在三维复杂流动计算中的网格特征,选取不同系列的车身面网格尺寸、车身壁面第一层边界层与壁面法向高度以及边界层层数等3组网格参数,利用ANSYS对阶背式MIRA模型外流场进行数值仿真.数值仿真结果与风洞试验的结果对比表明：数值计算得到的车身表面平均y+值随面网格尺寸增加而呈现减小趋势;网格方案对气动力因数和车身表面压力因数分布影响显著,气动阻力因数仿真值与试验值误差的变化区间为0.83%~7.93%,气动升力因数误差变化区间为10%~104%;气动阻力因数和气动升力因数均随着边界层层数的增加而增大,边界层层数为5时可以得到兼顾气动力因数精度和车身表面压力因数精度的较优仿真结果.     

基于不同网格密度的齿轮强度仿真

对齿轮强度的精确仿真和分析是齿轮优化设计和参数反求的前提。采用的参数化建模方法,创建了精确的轮齿几何模型。在对两类网格模型仿真结果对比分析的基础上,论述了消除舍入误差的必要性。在消除模型舍入误差的条件下,分别对不同网格密度等级的齿轮模型的齿面变形、齿根应力及分布进行数值仿真。对照经验公式的计算结果,进行误差分析,并得出了如下结论:网格的密度等级对齿面最大变形的仿真结果有较大的影响;而对齿面平均变形、总变形和齿根应力影响甚微。     

模型参数对射流侵彻半无限靶板的影响研究

研究聚能装药侵彻半无限靶板的数值模拟计算中,靶板有限元模型的轴向尺寸、径向尺寸以及网格边长对于仿真结果至关重要.为了找出合理的靶板模型尺寸和网格边长,应用动力学分析软件AUTODYN-2D,对口径56mm,半锥角30°的药型罩侵彻半无限靶扳进行了数值仿真,得到了侵彻深度,孔径和仿真计算时间与靶板有限元模型极限尺寸和模型网格边长的关系.结果分析发现,射流半径与网格边长的比例为5,靶板厚度与侵深比例为1.4,靶板半径与侵深比例为0.3比较合适.仿真结果可以为数值模拟和试验设计提供参考.     

基于仿真网格的引信虚拟试验研究

利用蒙特卡洛模拟实现对引信解除保险时间的计算机虚拟试验,以减少或部分替代高价值弹药的靶场射击试验.依据机械设计软件进行三维模型随机尺寸偏差的生成研究,建立了能生成具有一定概率分布特性的尺寸公差的系统;利用仿真网格技术构建了引信虚拟试验系统,并对某引信进行了多组尺寸偏差变化下的虚拟试验,验证了公差对引信解除保险时间的影响.     

三轴导弹仿真转台指向误差分析

仿真误差为仿真设备的精度估算评定与误差补偿提供依据.针对射频仿真系统中转台内框轴的空间误差中的指向误差,分析它与系统各部件几何误差和运动误差之间的关系,阐述数学模型的建立方法,并通过特征转换矩阵得到指向误差模型.对原始模型,充分考虑各误差因素影响对模型进行分析处理,以使具有较强的应用性.最后结合某型导弹转台将其应用于某一飞行试验模型来说明方法,计算结果表明:模型的简化对指向误差的分析的影响可以忽略,同时减小了计算量.分析方法也适应于其他飞行试验模式.     

整车电磁仿真模型精度评估

整车模型是车辆电磁性能仿真分析的基本平台,其精度直接决定仿真结果的准确性和可信度。为实现整车电磁仿真模型的量化评估,提出一种整车电磁仿真模型量化精度评估方法。以模型误差反向表征模型精度指标,用整体误差与局部误差的加权求和表征模型总误差,将整体尺寸与曲面面积的加权求和表征整体误差,采用部件偏差的最大值、平均值和变异系数加权求和表征局部误差。根据模型针对的不同分析问题,各加权系数可做适当调整,适用于各种车辆电磁性能分析任务的整车模型精度评价,具有较好的灵活性。仿真结果显示,该方法可准确预估整车模型分析时结果的可信度。     

微小通道冷板流动传热仿真校核方法研究

针对现有冷板流动传热仿真往往基于经验开展、缺乏对仿真模型严谨校核的问题,面向冷板流动传热仿真模型的两个重要方面,即离散网格与流动模型开展校核方法研究,一方面应用Richardson外推法对离散网格进行校核,另一方面通过将层流模型、SST k-ω、Transition k-ω以及RNG k-ε湍流模型与理论或经验关联式对比进行流动模型校核。结果表明,Richardson外推法的可获得量化的离散误差评估,有助于冷板流动传热仿真的网格优化;应用关联式能较简单地评估不同流动模型在不同条件下的适用性,同时发现Transition SST或SST k-ω在总体适用性较好。     

潜艇水平面操纵运动模型误差仿真分析

潜艇水平面运动模型是对潜艇水平面操纵运动进行仿真模拟的基础,运动模型选择的合适与否将直接影响潜艇操纵运动仿真的精确性和实时性。为此,以某型潜艇为研究对象,编制潜艇水平面线性与非线性操纵运动仿真程序,通过仿真并与实艇试验数据比较,分析了两种操纵运动模型的仿真误差,结果表明潜艇水平面操纵运动非线性模型误差远小于线性模型误差,与线性模型相比较,水平面非线性操纵运动模型具有更高的使用价值。     

仿真网格中一种基于知识的动态任务调度算法

仿真网格是以通用网格技术为基础、面向仿真领域的专用网格,目前国际上对仿真网格的研究尚处于起步阶段.现有的分布式仿真HLA(high level architecture)体系结构中的仿真资源和联邦成员是静态绑定的,网格技术的引入使得仿真资源的动态分配成为可能.根据仿真网格任务调度的特点,在仿真网格中建立了一种任务调度模型,并针对该模型,提出了一种新的基于知识的动态任务调度算法KMO,该算法适用于将N个相互独立的计算需求不同的仿真任务调度到M个随时间动态变化的仿真资源上,它能对若干次调度后的结果进行统计并提炼成"知识"反馈给算法预处理部分,使得该算法在动态多变的环境中能获得比较稳定的性能.实验结果表明,在仿真网格环境中,该算法的性能优于网格中传统的任务调度算法.     

An examination of stability in cohesive zone modeling

Stable crack propagation hinges on the driving force and the resistance. In the context of a cohesive approach to fracture, properly resolving the cohesive zone ensures that the resistance is in equilibrium with the driving force. Additional restrictions on the mesh size can stem from crack stability. For high strength, low toughness (brittle) materials, the requirements for stability can exceed those for cohesive zone resolution. Examples in 1-D and 2-D reveal that decrements in the mesh size transition the system from indefinite to positive definite. Moreover, small decreases in the mesh size beyond the transition provide substantial reductions in the condition number. We contrast a physical instability resulting from material properties, geometry, or boundary conditions from a numerical instability resulting from the mesh size and propose that the selected discretization should ensure that the cohesive zone is both resolved and stabilized.     

Automated adaptive 3D forming simulation processes

In this study, an automated adaptive mesh control scheme, based on local mesh modifications, is developed for the finite element simulations of 3D metal-forming processes. Error indicators are used to control the mesh discretization errors, and an h-adaptive procedure is conducted. The mesh size field used in the h-adaptive procedure is processed to control the discretization and geometric approximation errors of the evolving workpiece mesh. Industrial problems are investigated to demonstrate the capabilities of the developed scheme.     

On an h-type mesh-refinement strategy based on minimization of interpolation errors

An adaptive finite element method is proposed which involves an automatic mesh refinement in areas of the mesh where local errors are determined to exceed a pre-assigned limit. The estimation of local errors is based on interpolation error bounds and extraction formulas for highly accurate estimates of second derivatives. Applications to several two-dimensional model problems are discussed. The results indicate that the method can be very effective for both regular problems and problems with strong singularities.     

基于中轴表达的三维模型轮廓提取方法

三维网格模型的轮廓信息在网格检索、网格简化、网格重建中有着广泛应用。现有的轮廓提取方法较为复杂,需要分析和过滤网格模型的几何特征,计算量大且有时无法生成完整的轮廓信息。近年来,三维模型的中轴表达研究趋于成熟,在表达模型几何拓扑关系上有独特的优势。因此,提出了一种基于中轴表达的三维模型轮廓提取方法：首先提取三维模型的中轴表达信息,将中轴角点投影到三维模型表面;然后根据每个区域的拓扑关系选择适合的角点连接关系,将投影点连接形成模型区域轮廓;再针对投影过程中产生的误差进行分析和纠正;最后合并区域轮廓得到三维模型的完整轮廓。通过对多个模型数据库中代表性的三维网格模型进行实验和重建误 差比较,该方法的平均重建质量较现有方法约有 10%的提升,在重建质量和轮廓信息完整度方面优于现有方法。     

Solution of the time-dependent,three-dimensional resistive magnetohydrodynamic equations

Resistive magnetohydrodynamics (MHD) is described by a set of eight coupled, nonlinear, three-dimensional, time-dependent, partial differential equations. A computer code, IMP (Implicit MHD Program), has been developed to solve these equations numerically by the method of finite differences on an Eulerian mesh. In this model, the equations are expressed in orthogonal curvilinear coordinates, making the code applicable to a variety of coordinate systems. The Douglas-Gunn algorithm for Alternating-Direction Implicit (ADI) temporal advancement is used to avoid the limitations in timestep size imposed by explicit methods. The equations are solved simultaneously to avoid synchronization errors. While the continuity and magnetic field equations are expressed as conservation laws, the momentum and energy equations are nonconservative. This is to: (1) provide enhanced numerical stability by eliminating errors introduced by the nonvanishing of τ · B on the finite difference mesh; and, (2) allow the simulation of low β plasmas. The resulting finite difference equations are a coupled system of nonlinear algebraic equations which are solved by the Newton-Raphson iteration technique. We apply our model to a number of problems of importance in magnetic fusion research. Ideal and resistive internal kink instabilities are simulated in a Cartesian geometry. Growth rates and nonlinear saturation amplitudes are found to be in agreement with previous analytic and numerical predictions. We also simulate these instabilities in a torus, which demonstrates the versatility of the orthogonal curvilinear coordinate representation.     

A simplified interpolation and conversion method of contour surface model to mesh surface model

Contour line type representation is useful for understanding the surface structure qualitatively to a human. However, for the computer handling, a contour line model may not be suitable and it is required to be converted to the other type model such as a mesh surface model. It may be a problem how to decide mesh data on a mesh line where there are fewer points intersecting with contour lines. In this article, the authors propose a new method to convert a contour line model to a mesh surface model with minimum errors. Mesh lines (equivalent to the planes intersected with contour surface) located on the contour surface and intersected with contour lines are calculated. A mesh line which has maximum number of effective sampling points (that means maximum number of intersections with contour lines, and hereafter referred to as latter) is selected and a sectional shape along this mesh line is decided. The sectional shape is represented by spline curve with parameters, so that the mesh point data on the mesh line can be determined easily, and these decided mesh points are regarded as equivalent as intersection with contour lines. The above processes are repeated until all mesh lines which have intersection points or obtained mesh data have been chosen and calculated. Thus we can convert a contour model to a mesh surface model with minimum loss of contour line informations.     

基于视点的网格模型快速简化算法

在计算机图形学中，经常采用网格模型对物体和场景进行描述，而网格模型的大数据量成为实时绘制的瓶颈。因此，必须对网格模型进行简化，目前的简化算法，主要是以网格模型几何误差的最小化为准则，而忽略了模型的视觉特征。文章提出了一种基于视点的网格模型简化算法，其简化准则是视觉特征的最优化，利用视点相关，建立视点与网格精度的对应关系，对距离视点较近的部分采用较密网格，对距视点较远的部分采用稀疏网格。实验结果表明，该算法能够在保证高度真实感视觉效果的前提下，实现模型较大幅度的简化。     

On fault tolerance of 3-dimensional mesh networks

In this paper, the concept of k-submesh and k-submesh connectivity fault tolerance model is proposed. And the fault tolerance of 3-D mesh networks is studied under a more realistic model in which each network node has an independent failure probability. It is first observed that if the node failure probability is fixed, then the connectivity probability of 3-D mesh networks can be arbitrarily small when the network size is sufficiently large. Thus, it is practically important for multicomputer system manufacturer to determine the upper bound for node failure probability when the probability of network connectivity and the network size are given. A novel technique is developed to formally derive lower bounds on the connectivity probability for 3-D mesh networks. The study shows that 3-D mesh networks of practical size can tolerate a large number of faulty nodes thus are reliable enough for multicomputer systems. A number of advantages of 3-D mesh networks over other popular network topologies are given.     

Adaptive tetrahedral remeshing for modified solid models

During the engineering design process, a designed engineering component is usually repeatedly modified and analyzed to reach final design objectives, and completely mesh regeneration for each design change is very time consuming. An efficient remeshing approach for modified solid models using existing prior existing meshes is proposed in this paper to resolve this issue. It is mainly achieved via three main steps: different face (between the input model and its modification) identification, local destruction zone generation, and local mesh regeneration. In this approach, by carefully selecting a destruction zone to be removed from the original mesh model, a final geometric adaptive mesh model of the modified model is generated, which is very important for downstream analysis accuracy control. Additionally, the complex boundary of the variation region that needs to be remeshed is set up by merging the boundaries of modified region and the destruction zone without using complex intersection operations, which ensure the approach’s robustness. Experimental results on practical engineering parts are also shown to demonstrate the method’s performance.