大规模并行粒子模拟中基于主从架构的负载平衡策略研究与实现

粒子方法作为一种直观而且普适的模拟方法,在化工、材料、生物等领域得到了广泛的应用。本文采用主从(Master／Slave)并行模型,基于正交递归对分法(recursive coordinate bisection,RCB)和规则网格,设计出一种适用于具有多种复杂粒子的粒子系统并行模拟的负载平衡方法。应用该方法,在集群系统上采用离散元方法模拟了某钢铁公司滚筒系统。模拟采用4个节点,根据是否利用负载平衡模块分别进行测试,计算结果分析表明采用本文的负载平衡策略,计算效率提高了约14％。本文的算法具有较好的通用性,可以方便地应用于其它粒子方法的大规模并行计算问题。     

Issues in computing contact forces for non-penetrating rigid bodies

In rigid-body simulation it is necessary to compute the forces that arise between contacting bodies to prevent interpenetration. This paper studies the problem of rigid-body simulation when the bodies being simulated are restricted to contact at only finitely many points. Some theoretical and practical issues in computing contact forces for systems with large numbers of contact points are considered. Both systems of rigid bodies with and without Coulomb friction are studied. Complexity results are derived for certain classes of configurations and numerical methods for computing contact forces are discussed.A preliminary version of Sections 2.2, 2.3, 7, and 8.2, and the proof in Section 6 has appeared in conference form [3], [4].     

一种非线性非高斯随机系统的故障诊断方法

针对非线性非高斯随机系统在线故障诊断的问题,运用粒子滤波器提出了一种基于方差自适应粒子滤波器的非线性非高斯随机系统的故障诊断方法,可以用来解决系统的参数偏差型故障诊断问题。通过对连续搅拌釜式反应器(CSTR)的仿真研究,可以看出,该算法实现简单,易于对系统进行在线估计,对于发生缓变和突变的参数偏差型故障的检测与估计均较为有效。     

From ‘simulation optimization’ to ‘simulation configuration’ of systems

Simulation optimization aims at determining the best values of input parameters, given an output criterion. We address here the much more complex case where design options have to be chosen, which can themselves necessitate choices about other sub-options, as well as numerical parameters. This so-called configuration problem is addressed using simulation associated with a distributed evolutionary algorithm, and is illustrated through an example. This approach allows a large variety of new types of problems to be solved, such as design of complex dynamic systems. In the light of these results, the limits in the optimized configuration of systems are shown to be now determined by the modeling and simulation tools and concepts. In this respect, a possible research direction, based on so-called functional simulation, is discussed and some research perspectives are introduced.     

Improving performance of rigid body dynamics simulation by removing inaccessible regions from geometric models

Rigid body simulations require collision detection for determining contact points between simulated bodies. Collision detection performance can become dramatically slow, if geometric models of rigid bodies have intricate inaccessible regions close to their boundaries, particularly when bodies are in close proximity. As a result, frame rates of rigid body simulations reduce significantly in the states in which bodies come into close proximity. Thus, removing inaccessible regions from models can significantly improve rigid body simulation performance without influencing the simulation accuracy because inaccessible regions do not come in contact during collisions. This paper presents an automated pair-wise contact preserving model simplification approach based upon detection and removing of inaccessible regions of a given model with respect to another colliding model. We introduce a pose independent data-structure called part section signature to perform accessibility queries on 3D models based on a conservative approximation scheme. The developed approximation scheme is conservative and does not oversimplify but may undersimplify models, which ensures that the contact points determined using simplified and unsimplified models are exactly identical. Also, we present a greedy algorithm to reduce the number of simplified models that are needed to be stored for satisfying memory constraints in case of a simulation scene with more than two models. This paper presents test results of the developed simplification algorithm on a variety of part models. We also report results of collision detection performance tests in rigid body simulations using simplified models, which are generated using developed algorithms, and their comparison with the identical performance tests on respective unsimplified models.     

自由射流高空模拟试车台系统建模与仿真研究

介绍了面向对象的系统建模仿真关键技术,分析了其在解决大型复杂系统动态仿真方面的优势。针对自由射流高空台系统,给出了自由射流供气喷管、气动阀门等一些关键仿真元件的建模思路与模型验证情况。在此基础上搭建了简化的自由射流空气原理系统,并利用简化后的模型开展了对马赫数、马赫数/高度过渡态模拟能力的仿真。结果表明,该套系统原理可行,若要实现工程应用,尚有许多控制上的问题需要克服。最后,给出了另外一种工程应用的思路。系统仿真技术对于大型复杂系统动态问题模拟来说是良好的解决方案,该技术需要大量的早期投入和设备试验数据支持。     

多机器人体间动态碰撞检测方法仿真研究

当前多结合包围求碰撞检测法、Average-Case法、K-DOPs法等实现多机器人体间动态碰撞的检测,均存在寻优性能较差、检测效率较低的问题。为此提出一种基于动态粒子群的多机器人体间动态碰撞检测方法。采用OBB层次包围盒方法,缩小多机器人之间需要动态碰撞检测的区域,同时把动态碰撞检测问题转换为物体特征对间距离机制的非线性优化问题,进而构建层次拓扑框架进行局部碰撞检测,将机器人体引入到粒子群算法中建立混合进化算法,找到动态碰撞检测的最优解,实现多机器人体间动态碰撞检测。仿真结果证明,所提方法的检测效率高达96%,且具有较高的寻优性能。     

Computation and visualization of discrete particle systems on gLite-based grid

Three-dimensional simulation of discrete particle systems is performed by the discrete element method (DEM) software on the gLite-based BalticGrid infrastructure. The performance of a parallel algorithm for particles exchanging processors is investigated by using a number of benchmarks. Polydispersed particle systems are visualized by a novel grid e-service VizLitG designed for convenient access and interactive visualization of remote data files located on the grid. Partial dataset transfer from the storage element is implemented in the visualization e-service. The efficiency tests of VizLitG are performed on the datasets of different sizes. Two granular problems associated with triaxial compaction and hopper discharge are solved.     

Nonlinear contractivity of a class of semi-implicit multistep methods

The stability and contractivity of generalized linear multistep methods are studied for a large class of nonlinear stiff initial value problems. These methods are characterized by the fact that the coefficients of the integration formulas are matrices depending on the Jacobian or on an approximation to the Jacobian. Conditions for the parameters of such a multistep method are given which ensure that the method gives contractive numerical solutions over a large class of nonlinear dissipative systems for sufficiently small stepsizesh, where the restriction onh is not due to the stiffness of the problem. Stability and contractive properties of special methods of this class are reported.     

Transmission Link Dimensioning and Service Assignment for Differentiated Protection

We consider the problem of deciding on which transmission systems to carry a given number of services or sessions to minimize link cost and to guarantee a required degree of reliability. The sessions come in a variety of types, each with different requirements and the transmission systems can meet some of these requirements at some cost. We define a new optimization model for the Link Dimensioning and Service Assignment (LDSA) problem, an extension of current work on link dimensioning problems where a common assumption is that there is a single transmission system type on each link. We find that under realistic assumptions about the number of session types and transmission classes, it is possible to solve the LDSA problem to optimality in reasonable time using a combinatorial approach based on the maximal configuration concept. We also propose an efficient stopping criterion for large problems that gives near-optimal solutions within a few seconds. This efficient solution procedure can then be used to solve different network management problems where capacity assignment and survivability issues are involved.     

Surface Smoothing Procedures in Computational Contact Mechanics

This work addresses the problems arising in the finite element simulation of contact problems undergoing large deformation. The frictional contact problem is formulated in the continuum framework, introducing the interface laws for the normal and tangential stress components in the contact area. The variational formulation is presented, considering different methods to enforce the contact constraints. The spatial discretization within the finite element method is applied, as well as the temporal discretization required to solve the three sources of nonlinearities: geometric, material and frictional contact. The discretization of contact surfaces is discussed in detail, including different surface smoothing procedures. This numerical strategy allows to solve the difficulties associated with the discontinuities in the contact surface geometry introduced by finite element discretization, which leads to nonphysical oscillations of the contact force for large sliding problems. The geometrical accuracy of different interpolation methods is evaluated, paying particular attention to the Nagata patch interpolation recently proposed. In this framework, the Node-to-Nagata contact elements are developed using the augmented Lagrangian method to regularize the variational frictional contact problem. The techniques used to search for contact in case of large deformations are discussed, including self-contact phenomena. Several numerical examples are presented, comprising both the contact between deformable and rigid obstacles and the contact between deformable bodies. The results show that the accuracy and robustness of the numerical simulations is improved when the contact surface is smoothed with Nagata patches.     

A method for analyzing three-dimensional dynamic contact problems in visco-elastic media with kinetic and static friction

A solution method is presented for analyzing three-dimensional dynamic contact problems in visco-elastic media with kinetic and static friction considered. The approach is an extension to three dimensions of our previous work where a two-dimensional case was considered. The time integration scheme adopted in the algorithm is explicit and has second-order accuracy. No iteration is performed when dynamic contact is determined and dynamic contact conditions are exactly satisfied. Determining friction in a three-dimensional problem is very different from that in a two-dimensional case. Formulations integrating static and kinetic friction are given. In addition a sub-procedure for calculating the direction of kinetic friction is employed. Finally several numerical examples including one benchmark test and four general problems are given to illustrate the precision and effectiveness of the algorithm.     

Modelling of a rolling contact problem with the viscoelastic material response included

This paper gives an approach to the study of a rolling contact problem where bodies in contact exhibit a viscoelastic behaviour. Taking the viscoelastic-elastic analogy into account, the numerical implementation to a certain class of viscoelastic solutions is presented first, followed by a simulation of the rolling contact problem. The simulation is based on the superposition of two special boundary problems, the former concerning a viscoelastic half-space loaded by a concentrated force, F(t), perpendicular to its surface, and the latter concerning a sphere loaded by two opposite concentrated forces.     

An Algorithm with Logarithmic Time Complexity for Interactive Simulation of Hierarchically Articulated Bodies Using a Distributed-memory Architecture

The dynamic simulation of large systems of hierarchically articulated bodies is very time-consuming and cannot be done in real time using current one-processor workstations. This paper discusses the parallelization of dynamic simulation algorithms for such systems. First, the methods used in computer animation for solving initial-value problems are compared. Using test suites based on articulated bodies, the Gear method for non-stiff problems is identified as the solver with the least right-hand-side evaluations of the differential equations for the executed tests. On the basis of this method and the articulated-body method, a ‘simulation engine’ for distributed-memory architectures is presented. By reimplementing the Gear method and rearranging its parts, logarithmic time complexity is achieved for hierarchically articulated bodies. By using the simulation engine, interactive, physically based animation of large articulated figures is possible.     

A benchmarking system for MBS simulation software: Problem standardization and performance measurement

Despite the importance given to the computational efficiency of multibody system (MBS) simulation tools, there is a lack of standard benchmarks to measure the performance of these kinds of software applications. Benchmarking is done on an individual basis: different sets of problems are used, and the procedures and conditions considered to measure computational efficiency are also different. In this scenario, it becomes almost impossible to compare the performance of the different available simulation methods in an objective and quantitative way.This work proposes a benchmarking system for MBS simulation tools. The structure of the benchmark problem collection is defined, and a group of five problems involving rigid bodies is proposed. For these problems, documentation and validated reference solutions in standard formats have been generated, and a procedure to measure the computational efficiency of a given simulation software is described.Finally, the benchmarking system has been applied to evaluate the performance of two different simulation tools: ADAMS/Solver, a popular general-purpose commercial MBS simulation tool, and a custom Fortran code implementation of an Index-3 augmented Lagrangian formulation with projections combined with the implicit single-step trapezoidal rule as integration scheme. Results show that the proposed problems are able to reach the limits of the tested simulation methods, and therefore they can be considered good benchmark problems.     

Analysis of Flexible Multibody Systems with Intermittent Contacts

This paper is concerned with the dynamic analysis of flexible,nonlinear multibody systems undergoing intermittent contacts. Contact isassumed to be of finite duration, and the forces acting between thecontacting bodies which can be either rigid or deformable are explicitlycomputed during simulation. The modeling of contact consists of threeparts: a number of holonomic constraints that define the candidatecontact points on the bodies, a unilateral contact condition which istransformed into a holonomic constraint by the addition of a slackvariable, and a contact model which describes the relationship betweenthe contact force and the local deformation of the contacting bodies.This work is developed within the framework of energy preserving anddecaying time integration schemes that provide unconditional stabilityfor nonlinear, flexible multibody systems undergoing intermittentcontacts.     

Shot peening simulation using discrete and finite element methods

Modeling shot peening process is very complex as it involves the interaction of metallic surfaces with a large number of shots of very small diameter. Conventionally such problems are solved using the finite element software (such as ABAQUS) to predict the stresses and strains. However, the number of shots involved and the number of elements required in a real-life components for a 100% coverage that lasts a considerable duration of peening make such an approach impracticable. Ideally, a method that is suitable for obtaining residual compressive stresses (RCS) and the amount of plastic deformations with the least computational effort seems a dire need.In this paper, an attempt has been made to address this issue by using the discrete element method (DEM) in combination with the finite element method (FEM) to obtain reasonably accurate predictions of the residual stresses and plastic strains. In the proposed approach, the spatial information of force versus time from the DEM simulation is utilized in the FE Model to solve the shot peening problem as a transient problem. The results show that the RCS distribution obtained closely matches with that of the computationally intensive direct FEM simulation. It has also been established, in this paper, that this method works well even in the situations where the robust unit cell approaches are found to be difficult to handle.     

A Parallel Finite Element Method for 3D Two-Phase Moving Contact Line Problems in Complex Domains

Moving contact line problem plays an important role in fluid-fluid interface motion on solid surfaces. The problem can be described by a phase-field model consisting of the coupled Cahn–Hilliard and Navier–Stokes equations with the generalized Navier boundary condition (GNBC). Accurate simulation of the interface and contact line motion requires very fine meshes, and the computation in 3D is even more challenging. Thus, the use of high performance computers and scalable parallel algorithms are indispensable. In this paper, we generalize the GNBC to surfaces with complex geometry and introduce a finite element method on unstructured 3D meshes with a semi-implicit time integration scheme. A highly parallel solution strategy using different solvers for different components of the discretization is presented. More precisely, we apply a restricted additive Schwarz preconditioned GMRES method to solve the systems arising from implicit discretization of the Cahn–Hilliard equation and the velocity equation, and an algebraic multigrid preconditioned CG method to solve the pressure Poisson system. Numerical experiments show that the strategy is efficient and scalable for 3D problems with complex geometry and on a supercomputer with a large number of processors.     

Simulation of dynamics of interacting rigid bodies including friction II: Software system design and implementation

This is the second of two papers that deal with the problem of modeling contact (impact, sliding, rolling) between unconstrained rigid bodies, including friction. In a companion paper [1] we showed that the main underlying problem concerns the ability to do efficient contact mechanics when bodies interact through impact and/or sustained contact. Contact mechanics involves two aspects: detection of contact between bodies and estimation of contact forces. These forces are complicated in character and difficult to estimate because they depend on the material response of the contacting objects, on the duration of contact (very short duration impact, or more sustained contact), frictional interaction at the surfaces, geometry of contact, etc. In [1] we proposed a conceptual model in which linear elastic (springs) and viscous (dampers) elements acting at points of contact between objects generate all contact forces. In this paper we describe how the contact model has been implemented in the software of a working computer simulation system. The major aspects of this process are: formulation of a discrete version of the contact model; calculation of model parameters to reflect material properties; geometric representation of objects (in our system, objects are modeled as convex polyhedra); algorithms to detect and evaluate contacts among objects (a process called contact analysis); and estimation and control of model response for stable numerical integration of equations of motion. A graphical user interface displays a three-dimensional (3-D) perspective animation of the solution using full color shaded surface images. While the simulation may not be accomplished in real time, solutions can be saved in files for real-time visualization.Authors are listed in alphabetical order     

变拓扑柔性多体系统接触碰撞动力学研究

在实际工程领域中存在着大量接触碰撞等非连续动力学问题,现有的解决柔性多体系统连续动力学过程的建模理论与方法,已经无法解决或无法很好解决这些问题.本文基于变拓扑思想,提出了附加接触约束的柔性多体系统碰撞动力学建模理论；通过设计柔性圆柱杆接触碰撞实验,验证了所提出附加约束接触碰撞模型的有效性；针对柔性多体系统全局动力学仿真面临时间和空间的多尺度问题,提出多变量的离散方法,从而提高了柔性多体系统非连续动力学的仿真效率.