应用多重网格技术进行烟雾与障碍物交互模拟

烟雾在高精度网格中与大量障碍物交互十分耗时.为改善障碍物周围的局部视觉细节并进一步提高模拟的实时性,引入多重网格技术与结点分组方法相结合.使用结点分组方法处理障碍物边界条件;在每个障碍物周围,包围一张局部网格,在每个时间步,为每张网格分别求解N-S方程组并在局部和全局网格间传递信息.实验表明,采用文中方法,在障碍物局部模拟精度和单独使用结点分组方法相近的前提下,模拟速度提高了近3倍,达到了更高的实时性.     

利用流SIMD扩展加速3D曲线网格的流线计算

流线是一种基本的流场可视化技术,计算流线要耗费大量时间,Intel处理器（Pentium Ⅲ,Pentium4）提供流SIMD扩展（SSE）,支持指令 级SIMD操作。3D曲线网格上的流线计算包含速度插值、数值积分、点定位等主要子过程,具有很高的内在SIMD并行性。通过将数据按SSE数据类型组织以及对主要子过程进行SIMD并行化,设计了线流计算的SSE算法。采用向量类库、嵌入汇编两种SSE编码方式分别实现SSE算法,并依据处理器的体系结构优化代码。测试结果表明：SSE大大加速了3D曲线网格的流线计算,向量类库方式比传统计算提高55％左右的性能,嵌入汇编提高75％左右。     

Application of Computer Algebra Systems for Stability Analysis of Difference Schemes on Curvilinear Grids

The paper deals with problems arising in the application of the computer algebra systems for the symbolic–numeric stability analysis of difference schemes and schemes of the finite-volume method approximating the two-dimensional Euler equations for compressible fluid flows on curvilinear spatial grids. We carry out a detailed comparison of the REDUCE 3.6 and Mathematica(Versions 2.2 and 3.0) from the point of view of their applicability to the solution of the above problems. We draw a conclusion that a preference should be given for Mathematica from the viewpoint of the execution of symbolic–numeric computations. We also describe in detail our new symbolic–numeric algorithm for stability investigation, which was implemented with the aid of Mathematica. The proposed method enables us to reduce the needed computer storage at the symbolic stages by a factor of about 20 as compared with the previous algorithms. A feature of the numerical stages is the use of the arithmetic of rational numbers, which enables us to avoid the accumulation of the roundoff errors. We present the examples of the application of the proposed symbolic–numeric method for stability analysis of very complex schemes of the finite-volume method on curvilinear grids, which are widely used in computational fluid dynamics.     

基于粒子系统的烟雾模拟

在计算机图形学领域,最热门的一个研究课题就是如何模拟自然景物,它的应用非常之广,但是,很多自然景物的外观是非常不规则的,而且又是随机变化的,所以模拟起来会有很大的难度。到目前为止,粒子系统技术一直被大家认为是模拟像自然景物这类不规则物体的最成功最有效的一个图形生成算法．在深入研究国内各种烟雾模拟方法的基础上,采用粒子系统和纹理贴图技术并结合烟雾运动的基本物理公式实现对烟雾的有效模拟。通过汽车尾气随风飘散的效果,来满足视觉上对烟雾浓度渐变的要求,能够真实地模拟出不断运动变化的烟团。最后．对研究工作进行总结．并对下一步的研究工作进行展望。     

基于物理模型的烟雾模拟

在分析研究流体物理特性算法基础上,提出一种新的烟雾模拟实现方法。该方法基于物理模型的求解简化方程模拟烟雾的动态变化过程。模型中采用了非粘性欧拉方程,通常它比其他用粘性Navier-Stoke方程建模的更适合用来对气体进行建模并且减少计算量。实验验证该模型还可以正确处理烟雾与移动的物体之间的相互作用。     

可控的烟雾变形模拟

针对烟雾在自然无规则状态下对其变形控制困难的问题,提出一种可控的烟雾变形模拟方法.为实现烟雾的变形,在N-S (Navier-Stokes)方程中引入控制形变的吸引力和驱动力;利用MacCormack方法,有效减少了N-S方程求解过程中的数值耗散,使烟雾更好的保持细节;为加快烟雾的变形速度,在预处理阶段采用体素化的方法把目标三维模型数据离散到局部网格上,并计算出模型外网格点处的吸引力系数;为避免变形烟雾与自由运动烟雾交互时吸引力系数的重新计算,在全局网格中动态跟踪包围目标模型的局部网格.实验证明,采用文中方法,能够有效控制烟雾的变形,并实现变形烟雾与自由运动烟雾的快速交互.     

动画烟雾动态模拟算法研究

随着计算机动画技术的发展,烟雾等自然场景的动态模拟在仿真虚拟研究中占有越来越重要的地位.对各种计算机生成动画烟雾的模拟、动态控制等算法技术进行了深入研究,在总结相关算法技术的基础上,利用粒子系统作为基于物理规律的模拟,采用平滑粒子动力学(SPH)模型,将姻雾建模为102～103数量级个数的平滑粒子,并根据粒子密度推导出粒子受到的扩散力,将关于外力的物理参数直接定义在烟的粒子上,提出了动画中烟雾的实时动态模拟算法和模拟步骤.     

基于物理模型的实时烟雾模拟

针对使用直接绘制法渲染烟雾时,在高网格分辨率下实时性差的问题,文中提出了一种新的基于纹理的烟雾渲染算法。该算法采用不可压缩的Navier—Stokes方程作为烟雾模拟的基本物理计算模型,从而保证烟雾物理运动的真实性,同时利用OpenGL中的纹理映射技术渲染烟雾,保证了烟雾渲染效果真实,烟雾模拟细节更能体现出来;另外文中还给出了在烟雾中添加障碍物时边界条件的计算方式。模拟实验结果表明,该算法既能满足烟雾模拟的实时性,又能真实有效地模拟烟雾流动情况。     

Efficient Generation of High-Quality Unstructured Surface and Volume Grids

Procedures are presented for efficient generation of high-quality unstructured surface and volume grids. The overall procedure is based on the well proven Advancing Front/Local-Reconnection (AFLR) method. The AFLR triangular/tetrahedral grid generation procedure is a combination of automatic point creation, advancing type ideal point placement, and connectivity optimization schemes. A valid grid is maintained throughout the grid generation process. This provides a framework for implementing efficient local search operations using a simple data structure. It also provides a means for smoothly distributing the desired point spacing in the field using a point distribution function. This function is propagated through the field by interpolation from the boundary point spacing or by specified growth normal to the boundaries. Points are generated using either advancing- front type point placement for isotropic elements, advancing-point type point placement for isotropic right angle elements, or advancing-normal type point placement for high-aspect-ratio elements. The connectivity for new points is initially obtained by direct subdivision of the elements that contain them. Local-reconnection with a min-max type (minimize the maximum angle) type criterion is then used to optimize the connectivity. The overall procedure is applied repetitively until a complete field grid is obtained. An advancing-normal procedure is coupled with AFLR for anisotropic tetrahedral and pentahedral element grids. Advancing along prescribed normals from solid boundaries generates layers of anisotropic elements. The points are generated such that either pentahedral or tetrahedral elements with an implied connectivity can be directly recovered. The AFLR surface grid procedure uses an approximate physical space grid to define the surface during grid generation. The mapped space coordinates are mapped back to the actual surface at completion. Multiple surface definition patches are grouped into a single surface. A global mapping transformation is generated for the single surface using the grouped surface connectivity. The mapping coordinates are obtained by solving a coupled set of Laplacian equations. The overall procedure has been applied to a wide variety of configurations. Selected results are presented which demonstrate that high-quality unstructured grids can be efficiently and consistently generated for complex configurations.     

Adaptive Task Checkpointing and Replication: Toward Efficient Fault-Tolerant Grids

A grid is a distributed computational and storage environment often composed of heterogeneous autonomously managed subsystems. As a result, varying resource availability becomes commonplace, often resulting in loss and delay of executing jobs. To ensure good grid performance, fault tolerance should be taken into account. Commonly utilized techniques for providing fault tolerance in distributed systems are periodic job checkpointing and replication. While very robust, both techniques can delay job execution if inappropriate checkpointing intervals and replica numbers are chosen. This paper introduces several heuristics that dynamically adapt the above mentioned parameters based on information on grid status to provide high job throughput in the presence of failure while reducing the system overhead. Furthermore, a novel fault-tolerant algorithm combining checkpointing and replication is presented. The proposed methods are evaluated in a newly developed grid simulation environment dynamic scheduling in distributed environments (DSiDE), which allows for easy modeling of dynamic system and job behavior. Simulations are run employing workload and system parameters derived from logs that were collected from several large-scale parallel production systems. Experiments have shown that adaptive approaches can considerably improve system performance, while the preference for one of the solutions depends on particular system characteristics, such as load, job submission patterns, and failure frequency.     

纹理球方法的烟雾模拟

烟雾是普遍存在的自然现象.影视、动画、图形技术日益深入发展,各个领域对于烟雾的真实模拟已不可或缺.模拟需求越来越多、要求也越来越高.传统粒子系统不能满足实际需要;数学物理方法计算复杂,影响其广泛应用;基于纹理的方法越来越受到重视.本文采用基于纹理球的快速模拟方法.分析了真实的烟雾特点、运动变化等因素;设计了关键的控制变量.算法具有灵活的处理方式和开放的可扩展性.最后给出了实际的模拟效果,基本能够达到实时性要求.     

风环境下烟雾的实时模拟

在复杂环境仿真中,烟雾的模拟具有重要的意义.提出一种基于流体力学方程的烟雾模拟方法,并实现了脉动风与烟雾的交互,使烟雾的形态和运动更加逼真自然.采用简化的Von Karman模型建立风场,并引入特征正交分解技术(POD)和快速傅立叶变换技术(FFT)简化其建立过程.利用非粘性不可压缩的N-S(Navier-Stokes)方程描述烟雾运动,并将风场的作用加入到N-S方程中,利用破开算子法和MacCormack法简化N-S方程的求解过程,减少计算量,并保证求解过程在大的时间步长上的稳定性.同时,引入了"漩涡限制",将数值求解过程中的插值耗散补充回流场,增强烟雾的真实性.实验证明,利用本文的方法能够实时高效的模拟出自然逼真三维烟雾效果.     

基于物理模型的烟雾实时模拟

本文提出了一种基于物理模型的烟雾的实时数值模拟方法.真实感和实时性是计算机图形学追求的两个目标.传统的动画技术生成的物体运动是虚拟的,并不能完全反映物体的真实运动.与传统的动画技术相比,基于物理的动画更能表现运动的真实性.在用非粘性不可压欧拉方程表示烟雾的物理模型的基础上,利用破开算子法将其分解成外力项、对流项和投影项分别进行求解,每一步都稳定,因而整个求解也就稳定.求解过程的稳定性保证了模拟可以用大时间步长,也就保证了模拟的实时性.与传统的方法相比,能同时满足计算机图形学的真实感和实时性要求.     

烟幕防护影响坦克作战效能的仿真分析

对坦克的防护性能进行仿真足坦克作战仿真的重要内容。该文首先对烟幕使用过程进行了分析,并对烟幕性能进行量化描述,在此基础上建立了烟幕作用下的射弹的命中概率模型。根据分析的目的进行作战方案和评价准则的设计。将所建模型应用到已开发的装甲车辆作战仿真系统,对不同作战方案进行仿真。采用适用于分布类型不明确的小样本检验的Wilcoxon秩和检验。对不同方案的作战结果进行对比分析。实现烟幕对坦克作战效能的影响的分析。     

实时稳定的二维烟雾模拟

烟雾模拟是计算机动画模拟的重要组成部分.在模拟过程中保证实时性和稳定性是烟雾动画模拟的一个重要方面.该文的目的是在计算机上模拟出真实感的烟雾.文中采用无粘不可压欧拉方程作为烟雾的基本物理模型,对这种简化的流体方程求解可以加快计算的速度,使模拟的速度加快.数值上,引入了CIP方法的一种改进形式USCIP方法来求解N-S方程的对流项,该方法是无条件稳定的.求解方法的稳定实现了大时间步长模拟,保证了模拟实时性和稳定性,最后模拟得到二维的烟雾效果.实验证明本方法能实时稳定地模拟出具有真实感的二维烟雾效果.     

Efficient LBM Visual Simulation on Face-Centered Cubic Lattices

The Lattice Boltzmann method (LBM) for visual simulation of fluid flow generally employs cubic Cartesian (CC) lattices such as the D3Q13 and D3Q19 lattices for the particle transport. However, the CC lattices lead to suboptimal representation of the simulation space. We introduce the face-centered cubic (FCC) lattice, fD3Q13, for LBM simulations. Compared to the CC lattices, the fD3Q13 lattice creates a more isotropic sampling of the simulation domain and its single lattice speed (i.e., link length) simplifies the computations and data storage. Furthermore, the fD3Q13 lattice can be decomposed into two independent interleaved lattices, one of which can be discarded, which doubles the simulation speed. The resulting LBM simulation can be efficiently mapped to the GPU, further increasing the computational performance. We show the numerical advantages of the FCC lattice on channeled flow in 2D and the flow-past-a-sphere benchmark in 3D. In both cases, the comparison is against the corresponding CC lattices using the analytical solutions for the systems as well as velocity field visualizations. We also demonstrate the performance advantages of the fD3Q13 lattice for interactive simulation and rendering of hot smoke in an urban environment using thermal LBM.     

基于OpenGVS战场火焰烟雾的模拟

战场特效有助于虚拟战场环境气氛的烘托,其模拟质量直接影响着整个战场仿真的质量.对于战场上的烟雾、火焰等不规则物体,不能采用传统的几何方法建模,而只能采用粒子系统等复杂的数学模型进行模拟.但在战场环境模拟时,同一场景中常常会出现多处火焰、烟雾的情况,这时巨大的计算量势必大大影响场景仿真的实时性.文章对基于OpenGVS的特效实现方法进行了研究,提出一种基于OpenGVS开发平台,将粒子系统模型与二维纹理动画模拟相结合的实现方式,明显减少了模拟所需计算量.将此方法运用到某型高炮训练仿真器的场景模拟中,实际应用表明,保证实时性的同时使战场特效得到了高质量的模拟.