基于水平集-离散元方法的球谐函数颗粒材料缓冲性能分析

在自然环境与工业领域中,颗粒材料是一种常见的缓冲材料,其中大量形态各异的非球形颗粒表现出复杂的力学特性并应用于不同工程领域。本文采用球谐函数构造不同球面度和表面凹凸特性的非规则颗粒,通过水平集方法计算球谐函数颗粒间的接触点和碰撞力,并对冲击过程中球形和凹形颗粒的缓冲性能进行离散元分析。数值结果表明,颗粒床厚度、冲击速度和颗粒形状显著影响球谐函数颗粒材料的缓冲性能。颗粒床底部的冲击力峰值随着颗粒床厚度和表面凹凸性的增加而降低,同时冲击力峰值随着冲击速度和颗粒球面度的增加而增加。与球形颗粒相比,球谐函数颗粒具有凹凸表面和多接触点特性,这有利于冲击荷载向四周扩展并提高凹形颗粒的缓冲效果。     

Supercritical Spatially Homogeneous Branching in Admits no Equilibria

At the beginning of investigations in spatially homogeneous branching processes in Euclidean space (Liemant [1]) it seemed to be obvious that the existence of equilibria implies criticality of branching. This prejudice was disproved by the example [2] of a subcritical homogeneous branching equilibrium in dimension one. We prove that supercritical homogeneous branching processes in Euclidean space and, more general, in a broad class of topological groups have no (non – void, homogeneous) equilibria.     

Numerical analysis of breaking waves using the moving particle semi-implicit method

The numerical method used in this study is the moving particle semi-implicit (MPS) method, which is based on particles and their interactions. The particle number density is implicitly required to be constant to satisfy incompressibility. A semi-implicit algorithm is used for two-dimensional incompressible non-viscous flow analysis. The particles whose particle number densities are below a set point are considered as on the free surface. Grids are not necessary in any calculation steps. It is estimated that most of computation time is used in generation of the list of neighboring particles in a large problem. An algorithm to enhance the computation speed is proposed. The MPS method is applied to numerical simulation of breaking waves on slopes. Two types of breaking waves, plunging and spilling breakers, are observed in the calculation results. The breaker types are classified by using the minimum angular momentum at the wave front. The surf similarity parameter which separates the types agrees well with references. Breaking waves are also calculated with a passively moving float which is modelled by particles. Artificial friction due to the disturbed motion of particles causes errors in the flow velocity distribution which is shown in comparison with the theoretical solution of a cnoidal wave. © 1998 John Wiley & Sons, Ltd.     

Polyepichlorohydrine stabilized core shell microspheres by dispersion polymerization

Polystyrene microspheres ranging in diameter from 2 to 10 μm were prepared by dispersion polymerization in the presence of polyepichlorohydrine as the steric stabilizer precursor in alcoholic media. To clarify the effects of the solvent mixture composition, initiator concentration and steric stabilizer amount on the microsphere characteristics, including the molar mass parameters of the polymeric material, the microsphere size, and the steric stabilizer surface density, a three-factor full factorial design involving two levels of each of the factors and replicate experiments was employed. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 681–688, 1997     

In‐Situ Measurement of Local Particle Flux Densities in a Complex Two‐Phase Flow

An optical measuring technique is presented allowing the exact in‐situ measurement of local particle flux densities in a confined channel flow by counting single particles penetrating an optically well defined measuring volume. This enables a precise flux determination up to the direct vicinity of planar walls. The measurement set‐up and its calibration as well as the whole test facility are described in detail. This measurement technique is used to study the particle transport in electrostatic precipitators. Exemplarily, results of particle flux profiles as well as precipitation, as gained from balances of parts of the precipitator channel, are presented. Furthermore, the possibility to determine particle velocity fluctuations is demonstrated.     

The Relevance of Interactions in Nanoparticle Systems – Application to Particulate Thin Films

In the field of Particle Technology, processes cannot yet be designed from basic molecular understanding. Nanotechnology, however, begins to bridge this gap between molecules and particles and may thus open up new ways not only for the production and handling of particulate matter but also for the engineered design of advanced material properties. The visions and applications in nanoparticle technology cover a broad range, for instance quantum dots in information technology, refractory particles for advanced ceramics, highly active substances in pharmacy, catalysts or micro‐ and mesoporous adsorbents, to name only a few applications. Starting from the concept of product engineering, we investigate the basic preconditions for tailoring nanoparticulate properties, i.e. the control of the particle interactions. This concept is then applied to particulate thin film formation as an example of structure formation. The structure of dip‐coated samples, defined as the order of particles within the layer, was found to vary with the pH and the ion concentration adjusted in the sol bath. It was observed that the surface roughness scanned by an atomic force microscope increased with increasing electrolyte concentration. The structural evolution of the particulate network was studied by measuring the viscosity as a function of shear rate and solid concentration. Finally, the influence of coating structures on the transmission properties was determined. Especially in the case of LaCl₃ a strong dependence was observed. The results explicitly confirm a correlation between microscopic structure and macroscopic properties.     

基于修正PSO-UKF的SINS/GPS组合导航滤波算法

针对噪声时变特性引起滤波精度下降的问题,提出了一种基于修正粒子群技术( PSO)的自适应UKF算法.为了克服传统粒子群算法过早收敛,容易陷入局部最优的问题,基于粒子的适应值方差提出了一种惯性权值实时修正算法,有效改善了传统PSO算法.在使用新息序列对观测噪声进行实时跟踪的同时,通过构造合理的适应度函数将修正PSO算法和...     

Unusual ethylene polymerization results with metallocene catalysts supported on silica

With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of supporting methylaluminoxane and metallocene on porous silica has a substantial influence on the progress of polymerization. In particular, fragmentation of catalyst particles during polymerization can be circumvented, maintaining the catalyst activity, if active catalyst sites are being generated on the particle surface only. A method of preparation for such newly designed supported metallocene catalysts is presented, where the active catalyst sites are located exclusively on the particle surface. Furthermore, the kinetics of ethylene polymerization and morphology properties prior to and after polymerization are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 677–682, 1999     

Modified Lagrangian vortex method with improved boundary conditions for water waves past a thin bottom‐standing barrier

Herein, the modified Lagrangian vortex method (LVM), a hybrid analytical‐numerical algorithm per se, is devised to simulate the process of vortex formation and shedding from the sharp edge of a zero‐thickness vertical plate under linear water‐wave attack. Application of the Helmholtz decomposition facilitates a convenient switch between the inviscid‐ and viscous‐flow models, thereby enabling easy incorporation of vorticity effects into the potential‐flow calculations for the viscous‐dominated region. In evaluating the potential‐flow component, making good use of the quickly convergent technique with singular basis functions, correctly capturing the singular behavior in velocity fields near the tip of the plate, leads to a considerable reduction of computational burdens and to 12‐decimal‐place accuracy. The viscous correction is carried out via the meshless LVM with improved boundary conditions. Comparisons with previously published results show good agreement. Simulations of vortex generation and evolution illuminate the ability of the present method, and provide a supplement to pertinent experimental works. The hybrid scheme proposed herein allows flexibility for the former LVM and convenience in the code development. Such a compromise fits particularly well for the high‐resolution modeling of sharp‐edged vortex shedding without heavy numerical developments. Copyright © 2014 John Wiley & Sons, Ltd.     

超强激光等离子体中钻孔效应的二维粒子模拟

 当非均匀超短脉冲强激光照射均匀分布的过稠密等离子体时, 相对论效应导致电子质量增大,相对论电子等离子体频率减小,激光能更深地进入等离子体产生强烈吸收。由于入射激光的径向不均匀性,钻孔效应显著。通过二维粒子模拟结果清晰地观察到了等离子体通道的产生。