Bammann-Chiesa-Johnson粘塑性本构模型材料参数的一种识别方法

粘塑性本构模型能否成功模拟金属高应变率大应变变形过程依赖于材料参数识别结果的好坏。由于BCJ模型考虑了应变率、温度与材料硬化之间的耦合效应以及应变率、温度历史效应,同时模型中包含了多个材料参数,因此很难通过试验直接识别模型的材料参数。本文针对BCJ模型中的耦合效应和历史效应,基于对模型中材料参数物理涵义的界定,给出了一种对材料参数解耦、分离并进行估计的方法,获得了模型材料参数估计公式,估计了材料参数的取值范围。在此基础上,编制了BCJ模型应力积分径向返回算法和粒子群优化算法的计算程序,应用重新设计了BCJ模型耦合效应和历史效应的反分析方法,在参数取值范围内对材料参数进行了优化识别。以OFHC Cu为例,应用提出的识别方法对BCJ模型的材料参数进行了识别,计算结果和试验结果符合较好。     

Space potential particles to enhance the stability of projection-based particle methods

Particle methods have been seldom verified by a Karman vortex simulation, which is commonly performed as a typical benchmark in computational fluid dynamics. This is mainly due to a difficulty in suppression of occurrence of unphysical voids manifested usually in a strong vortex on account of definition of free surface by the Lagrangian tracking framework with inconsistency in volume conservation. This paper presents a simple and effective scheme as a free-surface boundary condition of projection-based particle methods, namely the MPS (moving particle semi-implicit) and Incompressible SPH (ISPH) methods to handle the free surface with consistency in volume conservation. The new scheme is introduced into the Poisson pressure equation (PPE) with consideration of a potential in void space as space potential particle (SPP), to reproduce physical motions of particles around free surface through a particle–void interaction. The enhancing effect of the newly proposed SPP scheme is shown by simulating a few numerical tests, including a whirling water flow, a two-phase surfacing flow, and a set of Karman vortex simulations.     

Differentially weighted direct simulation Monte Carlo method for particle collision in gas–solid flows

In gas–solid flows, particle–particle interaction (typical, particle collision) is highly significant, despite the small particles fractional volume. Widely distributed polydisperse particle population is a typical characteristic during dynamic evolution of particles (e.g., agglomeration and fragmentation) in spite of their initial monodisperse particle distribution. The conventional direct simulation Monte Carlo (DSMC) method for particle collision tracks equally weighted simulation particles, which results in high statistical noise for particle fields if there are insufficient simulation particles in less-populated regions. In this study, a new differentially weighted DSMC (DW-DSMC) method for collisions of particles with different number weight is proposed within the framework of the general Eulerian–Lagrangian models for hydrodynamics. Three schemes (mass, momentum and energy conservation) were developed to restore the numbers of simulation particle while keeping total mass, momentum or energy of the whole system unchanged respectively. A limiting case of high-inertia particle flow was numerically simulated to validate the DW-DSMC method in terms of computational precision and efficiency. The momentum conservation scheme which leads to little fluctuation around the mass and energy of the whole system performed best. Improved resolution in particle fields and dynamic behavior could be attained simultaneously using DW-DSMC, compared with the equally weighted DSMC. Meanwhile, computational cost can be largely reduced in contrast with direct numerical simulation.     

Mixing state of individual carbonaceous particles during a severe haze episode in January 2013, Nanjing,China

Chemical composition, hourly counts, and sizes of atmospheric carbonaceous particles were measured to investigate their mixing state on clear and hazy days. 623,122 carbonaceous particles with sizes 0.2–2.0 μm was analyzed using a single-particle aerosol mass spectrometer from 1st to 17th January 2013. Particle types included biomass/biofuel burning particles (biomass), element carbon (EC-dominant) particles that were also mixed with biomass/biofuel burning species (EC-biomass) or secondary species (EC-secondary), organic carbon (OC), internally mixed OC and EC (OCEC), ammonium-containing (ammonium) and sodium-containing (sodium) particles. On clear days the top ranked carbonaceous particle types were biomass (48.2%), EC-biomass (15.7%), OCEC (11.1%), and sodium (9.6%), while on hazy days they were biomass (37.3%), EC-biomass (17.6%), EC-secondary (16.6%), and sodium (12.7%). The fractions of EC-secondary, ammonium (10%), and sodium particle types were elevated on hazy days. Numbers of EC-secondary particles were more than four times those on clear days (4.1%). Thus, carbonaceous particles mixed with ammonium, nitrate and sulfate during aging and transport, enhancing their light extinction effects and hygroscopic growth under high relative humidity on hazy days, further reducing visibility. Our real-time single-particle data showed that changes to mixing state had a significant impact on light extinction during haze events in Nanjing.     

费托合成Co/SiO2催化剂的粒子尺寸效应:TPD和DRIFTS的研究

以不同配比的乙二醇(EG)/水溶液作溶剂, 用初湿含浸法制备了Co粒子尺寸分别为6、8、12、19 nm 的Co/SiO₂模型催化剂. 采用N₂物理吸附、X射线粉末衍射(XRD)、透射电子显微镜(TEM)、程序升温脱附(TPD)、漫反射红外光谱(DRIFTS)、程序升温表面反应(TPSR)等表征和微分固定床反应器费托(F-T)反应评价实验, 考察了Co粒子尺寸对F-T反应性能的影响. 评价结果显示随着Co粒子尺寸的增大, CO转化率降低, 表观TOF在Co粒子尺寸为8 nm时出现高点. TPD和DRIFTS研究结果表明: 较小的Co粒子对CO的吸附和解离能力较强, 其表面碳物种会覆盖部分表面活性位, 使有效反应位点减少; 较大Co粒子对CO的吸附能力较弱, 表面C*较易脱附及CO*/Co_s比例较高, 导致催化剂加氢能力减弱, CO₂的选择性增高. 当Co粒子尺寸为8 nm左右时, CO_ads和C*在Co粒子表面吸附强度适中并且比例恰当, 使得催化剂表现出较高的F-T活性和产物选择性.     

运用微粒观开发复分解反应教学价值的实践与研究

在酸、碱、盐教学中以复分解反应条件教学为例, 应用“微粒观”统领酸、碱、盐教学。基于“微粒观”构建了本节课的知识结构层级图, 剖析了学生的认识发展过程, 通过教学实践总结、归纳了若干教学策略, 从而突破学生学习难点, 提高学生的迁移和应用能力, 并为高中进一步学习离子反应做好铺垫。     

Synthesis of Submicrometer-sized Sn-MCM-41 Particles and Their Catalytic Performance in Baeyer-Villiger Oxidation

Submicrometer-sized tin-containing MCM-41 particles with a size of several hundred nanometers(Sn-MCM-41/SMPs) were rapidly prepared with tin chloride as tin source and tetraethyl orthosilicate as silicon source via a dilute solution route in sodium hydroxide medium at room temperature. The characterization results show the highly ordered hexagonal mesopores and tetrahedral Sn species in Sn-MCM-41/SMPs. The material proved to be active and selective for Baeyer-Villiger oxidation of adamantanone with aqueous H₂O₂. Notably, Sn-MCM-41/ SMPs displayed a higher initial reaction rate and turnover number(TON) than common micrometer-sized Sn-MCM-41 large particles(Sn-MCM-41/LPs), mainly attributed to the accelerated diffusion of the reactants and enhanced accessibility to the catalytic Sn species via shorter mesopore channels in Sn-MCM-41/SMPs. Furthermore, Sn-MCM-41/SMPs could be reused without the loss of activity after five runs, indicating that Sn active sites in the submicrometer-sized particles are remarkably stable. The study shows that decreasing particle size of Sn-MCM-41 in submicrometer scale is an effective way to achieve catalysts for Baeyer-Villiger oxidations with improved catalytic performance.     

Tomographic PIV investigation on coherent vortex structures over shark-skin-inspired drag-reducing riblets

Nature has shown us that the microstructure of the skin of fast-swimming sharks in the ocean can reduce the skin friction drag due to the well-known shark-skin effect.In the present study,the effect of shark-skin-inspired riblets on coherent vortex structures in a turbulent boundary layer(TBL) is investigated.This is done by means of tomographic particle image velocimetry(TPIV) measurements in channel fl ws over an acrylic plate of drag-reducing riblets at a friction Reynolds number of 190.The turbulent fl ws over drag-reducing riblets are verifie by a planar time-resolved particle image velocimetry(TRPIV) system initially,and then the TPIV measurements are performed.Two-dimensional(2D) experimental results with a dragreduction rate of around 4.81% are clearly visible over triangle riblets with a peak-to-peak spacing s+of 14,indicating from the drag-reducing performance that the buffer layer within the TBL has thickened;the logarithmic law region has shifted upward and the Reynolds shear stress decreased.A comparison of the spatial topological distributions of the spanwise vorticity of coherent vortex structures extracted at different wall-normal heights through the improved quadrant splitting method shows that riblets weaken the amplitudesof the spanwise vorticity when ejection(Q2) and sweep(Q4) events occur at the near wall,having the greatest effect on Q4 events in particular.The so-called quadrupole statistical model for coherent structures in the whole TBL is verified Meanwhile,their spatial conditional-averaged topological shapes and the spatial scales of quadrupole coherent vortex structures as a whole in the overlying turbulent fl w over riblets are changed,suggesting that the riblets dampen the momentum and energy exchange between the regions of near-wall and outer portion of the TBL by depressing the bursting events(Q2 and Q4),thereby reducing the skin friction drag.     

颗粒膜的结构及对乳状液稳定性的影响机制

以颗粒膜为基础的乳液因广泛应用于油田、造纸、食品、化妆品、医药等领域而备受关注。本文归纳了在油水界面形成颗粒膜的颗粒密度、粒径及润湿性等特征,并阐述了颗粒在界面吸附扩散的行为。重点从颗粒垂直于界面的分配及方向,颗粒在界面内的排列及进一步形成的空间结构综述颗粒膜结构。总结了颗粒在界面的状态及颗粒膜结构的影响因素,并从能量及力学角度进行分析。颗粒膜对乳状液稳定性影响机制主要体现在颗粒膜的结构及界面黏弹性：颗粒膜的结构阻隔液滴之间的碰撞聚并,这是乳状液稳定的基础;同时,颗粒膜通过改变界面黏弹性使得液滴在运动、碰撞、絮凝时不会轻易崩溃,从而强化了乳状液的稳定性。固体颗粒为乳化剂形成颗粒膜稳定乳状液的机制探究为稳定乳状液制备以及乳状液的破乳提供理论依据,具有现实意义。最后,本文就颗粒膜稳定乳状液的机制研究展望了其未来发展方向及可能的解决途径。     

Block copolymer morphologies confined by square-shaped particle:Hard and soft confinement

The self-assembly of diblock copolymers confined around one square-shaped particle is studied systematically within two-dimensional self-consistent field theory(SCFT).In this model,we assume that the thin block copolymer film is confined in the vicinity of a square-shaped particle by a homopolymer melt,which is equivalent to the poor solvents.Multiple sequences of square-shaped particle-induced copolymer aggregates with different shapes and self-assembled internal morphologies are predicted as functions of the particle size,the structural portion of the copolymer,and the volume fraction of the copolymer.A rich variety of aggregates are found with complex internal self-assembled morphologies including complex structures of the vesicle,with one or several inverted micelle surrounded by the outer monolayer with the particle confined in the core.These results demonstrate that the assemblies of diblock copolymers formed around the square-shaped particle in poor solvents are of immediate interest to the assembly of copolymer and the morphology of biomembrane in the confined environment,as well as to the transitions of vesicles to micelles.