Relating Fresh Concrete Viscosity Measurements from Different Rheometers

Concrete rheological properties need to be properly measured and predicted in order to characterize the workability of fresh concrete, including special concretes such as self-consolidating concrete (SCC). It was shown by a round-robin test held in 2000 [1,2] that different rheometer designs gave different values of viscosity for the same concrete. While empirical correlation between different rheometers was possible, for a procedure that is supposed to “scientifically” improve on the empirical slump tests, this situation is unsatisfactory. To remedy this situation, a new interpretation of the data was developed. In this paper, it is shown that all instruments tested could be directly and quantitatively compared in terms of relative plastic viscosity instead of the plastic viscosity alone. This should eventually allow the measurements from various rheometer designs to be directly calibrated against known standards of plastic viscosity, putting concrete rheometry and concrete workability on a sounder materials science basis.     

冲击-接触问题有限元仿真的并行计算

冲击．接触问题广泛存在于汽车碰撞等的模拟计算中。简单介绍了求解该类问题的显式有限元方法，对显式有限元方法的并行性进行了讨论。根据显式有限元和冲击一接触问题的计算特点，设计并实现了接触均衡的分区算法。算例计算结果表明：该并行算法具有较好的加速比和并行效率。     

Simulation of a confined polymer in solution using the dissipative particle dynamics method

The dynamics of a bead-and-spring polymer chain suspended in a sea of solvent particles are examined by dissipative particle dynamics (DPD) simulations. The solvent is treated as a structured medium, comprised of particles subject to both solvent-solvent and solvent-polymer interactions and to stochastic Brownian forces. Thus hydrodynamic interactions among the beads of the polymer evolve naturally from the dynamics of the solvent particles. DPD simulations are about two orders of magnitude faster than comparable molecular dynamics simulations. Here we report the results of an investigation into the effects of confining the dissolved polymer chain between two closely spaced parallel walls. Confinement changes the polymer configuration statistics and produces markedly different relaxation times for chain motion parallel and perpendicular to the surface. This effect may be partly responsible for the gap width-dependent theological properties observed in nanoscale rheometry.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

聚四氟乙烯包覆铝粉烧结的模拟与分析

为了分析烧结过程对聚四氟乙烯(PTFE)包覆Al粉性能的影响,利用分子动力学手段计算了298 K和678 K时PTFE在Al2O3(001)、(010)及(100)晶面6×6晶层的结合能,利用耗散粒子动力学手段模拟了298 K和678 K时Al2O3/PTFE在不同时刻的介观状态。计算及模拟结果表明:298 K时,PTFE在Al2O3(001)、(010)及(100)晶面的6×6晶层的结合能分别为2 782.67、5 582.97及4 634.32 k J/mol;678 K时PTFE与Al2O3(001)、(010)及(100)晶面的结合能分别是2 835.29、5 537.54及4 608.49 k J/mol。低温时,PTFE和Al2O3混溶性差,两种物质发生明显分相;高温时,PTFE和Al2O3混溶性较好,没有发生明显的分相。烧结过程有助于PTFE在Al2O3中的扩散,同时还可以提高聚合物与Al2O3的混溶性,但对PTFE包覆Al粉的强度影响不大。     

三自由度并联分拣机器人的动力学建模与仿真

目的 针对自动化生产线上分拣机器人的动力可控性问题,提出一种2UU-UPU三自由度并联分拣机器人,以提高分拣的精度可控性。方法 分析该机器人的机构自由度,以及各参数之间的关系,基于闭环矢量法建立并联机构的运动学逆解模型;利用拉格朗日动力学方程推导该机器人的动力学表达式,并进行数值计算,采用Matlab Simulink和Adams进行动力学联合仿真,对理论值和仿真值进行误差分析。结果 揭示了该机器人动平台的运动规律,得到了驱动力矩曲线,理论值与仿真值的误差较小,3个驱动力矩的最大误差分别为0.379%、0.283%、0.146%。结论 通过验证可知,该机构具有较好的动力学特性,这为后续电机的选型和精准控制奠定了基础。     

Parallel multipole implementation of the generalized Helmholtz decomposition for solving viscous flow problems

The evaluation of a domain integral is the dominant bottleneck in the numerical solution of viscous flow problems by vorticity methods, which otherwise demonstrate distinct advantages over primitive variable methods. By applying a Barnes–Hut multipole acceleration technique, the operation count for the integration is reduced from O(N²) to O(NlogN), while the memory requirements are reduced from O(N²) to O(N). The algorithmic parameters that are necessary to achieve such scaling are described. The parallelization of the algorithm is crucial if the method is to be applied to realistic problems. A parallelization procedure which achieves almost perfect scaling is shown. Finally, numerical experiments on a driven cavity benchmark problem are performed. The actual increase in performance and reduction in storage requirements match theoretical predictions well, and the scalability of the procedure is very good. Copyright © 2003 John Wiley Sons, Ltd.     

3-CRS-S并联机构的运动学分析及仿真

目的 对3-CRS-S并联机构进行运动学分析及仿真,验证该机构是否具有优良的运动学性能.方法 运用修正的Kutzbach-Grübler公式对机构进行自由度计算,并分别采用D-H法和数值算法中的粒子群优化算法(PSO)对该3-CRS-S并联机构的位置正逆解进行分析,运用Adams软件对3-CRS-S并联机构进行角度和角速度分析.结果 得出该机构的位置逆解和正解,以及运动学仿真后的角度、角速度图像,该图像曲线均呈现为有规律、周期性的变化,且曲线没有出现有任何断点和突变点,运动范围相对稳定,说明该机构在运行过程中运行平稳.结论 该机构在运动过程中运行平稳,具有良好的运动学性能,在自动化包装机械领域具有广阔的应用前景.     

Parallel load‐balanced simulation for short‐range interaction particle methods with hierarchical particle grouping based on orthogonal recursive bisection

We describe an efficient load‐balancing algorithm for parallel simulations of particle‐based discretization methods such as the discrete element method or smoothed particle hydrodynamics. Our approach is based on an orthogonal recursive bisection of the simulation domain that is the basis for recursive particle grouping and assignment of particle groups to the parallel processors. Particle grouping is carried out based on sampled discrete particle distribution functions. For interaction detection and computation, which is the core part of particle simulations, we employ a hierarchical pruning algorithm for an efficient exclusion of non‐interacting particles via the detection of non‐overlapping bounding boxes. Load balancing is based on a hierarchical PI‐controller approach, where the differences of processor per time step waiting times serve as controller input. Copyright © 2007 John Wiley & Sons, Ltd.     

可视化并行程序开发平台的设计与实现

从方便用户进行并行程序设计的角度出发,开发了一个基于图形的可视化并行程序开发平台.该平台由3层构成:在应用层,用户通过图形的方式确定各进程间的通信关系;在进程层,使用平台提供的图标完成相应的程序设计;在代码层,通过代码产生器生成并行程序.图形方式隐藏了并行机制,降低了并行程序设计的难度.     

Improvements in the reliability and element quality of parallel tetrahedral mesh generation

The paper presents a parallel tetrahedral mesh generation approach based on recursive bidivisions using triangular surfaces. Research was conducted for addressing issues concerning mesh generation reliability and element quality. A novel procedure employing local modification techniques is proposed for repairing the intersecting interdomain mesh instead of directly repeating the bidivision procedure, which improves the robustness of the complete meshing procedure significantly. In addition, a new parallel quality improvement scheme is suggested for optimizing the distributed volume meshes. The scheme is free of any communication cost and highly efficient. Finally, mesh experiments of hundreds of millions of elements are performed to demonstrate the reliability, effectiveness and efficiency of the proposed method and its potential applications to large‐scale simulations of complex aerodynamics models. Copyright © 2012 John Wiley & Sons, Ltd.     

Slow Dynamics of Nonequilibrium Density Fluctuations in Concentrated Hard-Sphere Suspensions

The coupled diffusion equations recently proposed for concentrated hard-sphere suspensions of interacting Brownian particles, the nonlinear deterministic diffusion equation with the self-diffusion coefficient D _S((x, t)) for the average local volume fraction (x, t), and the linear stochastic diffusion equation with D _S((x, t)) for the density fluctuations n(x, t) are numerically solved under a spatially inhomogeneous, nonequilibrium initial state. Thus, in a supercooled region where < _g, the slow evolution of the cluster-like glassy domains with (x, t) _g and the slow relaxation of the nonequilibrium density fluctuations are shown to be caused by the dynamic singularity of the self-diffusion coefficient, D _S((x, t)) (1–(x, t)/_g)², where is a particle volume fraction, _g = (4/3)³/(7 ln 3 – 8 ln 2 + 2) is the colloidal glass transition volume fraction, and is the crossover volume fraction.     

杜仲胶/天然橡胶共混物的分子动力学模拟和耗散粒子动力学模拟

应用分子动力学(MD)和耗散粒子动力学(DPD)模拟方法对杜仲胶(TPI)、天然橡胶(NR)的相容性进行了研究。采用MD模拟方法,在COMPASS力场下,对纯物质在不同聚合度下的溶度参数、一系列共混比的TPI/NR共混物内聚能密度、Flory-Huggins作用参数进行了模拟计算,确定了纯物质单链的聚合度,经判断各比例共混物的相容性均较好;采用DPD模拟方法对TPI/NR共混体系的相结构进行了研究,从等密度图可以进一步判断共混体系的相容性;分析比较两种纯物质的径向分布函数,揭示了其相互作用的本质;经过分析比较静态力学性能,发现共混比为1/3的TPI/NR共混物性能最佳,其结论与实验结果一致。     

Parallel computation of meshless methods for explicit dynamic analysis

A parallel computational implementation of modern meshless methods is presented for explicit dynamic analysis. The procedures are demonstrated by application of the Reproducing Kernel Particle Method (RKPM). Aspects of a coarse grain parallel paradigm are detailed for a Lagrangian formulation using model partitioning. Integration points are uniquely defined on separate processors and particle definitions are duplicated, as necessary, so that all support particles for each point are defined locally on the corresponding processor. Several partitioning schemes are considered and a reduced graph-based procedure is presented. Partitioning issues are discussed and procedures to accommodate essential boundary conditions in parallel are presented. Explicit MPI message passing statements are used for all communications among partitions on different processors. The effectiveness of the procedure is demonstrated by highly deformable inelastic example problems. Copyright © 2000 John Wiley & Sons, Ltd.     

利用耗散颗粒动力学方法模拟优选合成聚表剂的可聚合季铵盐单体

为研究聚合物表面活性剂(聚表剂)分子结构变化对聚表剂和油-水界面性质的影响,优选出最佳性能的聚表剂。以正十六烷基为油相,水为水相,采用耗散颗粒动力学方法(DPD)模拟含有不同可聚合季铵盐表面活性基团的聚表剂在油-水界面的性质,研究了聚表剂摩尔分数及季铵盐基团种类等因素对油-水界面性质的影响。模拟结果表明,模拟体系中聚表剂摩尔分数范围以0.02~0.06为宜;通过模拟优选出最佳季铵盐单体为对甲基苯乙烯基十二烷基二甲基溴化铵(R4),聚表剂AMR4在油水体系中最佳摩尔分数为0.06,此时界面张力值最低、界面效率最佳且聚表剂分子均方根末端距最大。聚表剂分子几乎全都分布在油-水界面层,其界面活性使得油-水界面厚度增大,油-水界面张力降低。     

A parallel and multiscale strategy for the parametric study of transient dynamic problems with friction

The objective of this work is to develop an efficient strategy for the parametric study of dynamic problems involving contacts with friction. Our approach is based on the multiscale LATIN method with domain decomposition. This is a mixed method that deals with the forces and velocities at the interfaces between the different subdomains simultaneously. We propose to take advantage of the capability of the multiscale LATIN method, called the multiparametric strategy, to reuse the solution of a given problem in order to solve similar problems. This strategy has already been applied successfully to a variety of static problems; here, it is extended to dynamics. First, we present the multiscale strategy in dynamics. Then, we show how the multiparametric strategy can be extended to dynamics. We illustrate the capabilities of the method through an academic 3D example and the simulation of a bolted joint. Copyright © 2011 John Wiley & Sons, Ltd.     

基于多体动力学的轮式悬架移动刚-柔并联机械手动力学性能研究

考虑到移动串联机械手不能很好地满足高速、大负载工况下准确作业的要求,提出了轮式悬架柔性移动并联机械手这一新的构型,并对其进行了动力学性能研究.基于多柔性体动力学,在笛卡尔坐标下构建了系统动力学模型,建模方法具有通用性与完备性.在此基础上,分别针对串联、并联、刚性、柔性四类移动机械手的动力学模型进行了数值仿真,并对仿真结果进行了比较、分析,验证了建模方法的正确性并佐证了新构型移动机械手的动力学性能特点,提出了提高柔性并联机械手的工作速度、刚度和工作精度的技术方法,为后续控制算法的构建提供了理论基础.     

Iterative solution of large‐scale acoustic scattering problems with multiple right hand‐sides by a domain decomposition method with Lagrange multipliers

An extension of the FETI‐H method is designed for the solution of acoustic scattering problems with multiple right‐hand sides. A new local pre‐conditioning of this domain decomposition method is also presented. The potential of the resulting iterative solver is demonstrated by numerical experiments for two‐dimensional problems with high wavenumbers, as many as 2.5 million complex degrees of freedom, and a sweep on the angle of the incident wave. Preliminary results for a three‐dimensional submarine problem are also included. The FETI‐H method, whose numerical scalability with respect to the mesh and subdomain sizes was previously established, is shown here to be also numerically scalable with respect to the wavenumber. Copyright © 2001 John Wiley & Sons, Ltd.     

基于正、逆动力学的移动悬架并联机械手动力学建模有效性研究

以多体动力学理论为指导,在笛卡尔坐标系下分别构建了移动悬架并联机械手系统的正、逆运动学模型和动力学模型,并以此为基础,采用数值仿真法对系统逆动力学模型进行求解,然后将部分解作为系统正运动学和正动力学模型的输入和边界条件,并进行数值仿真。最后通过正、逆动力学数值仿真比较,验证了系统动力学模型构建和求解方法的正确性和有效性。