On the interaction between bubbles and the free surface with high density ratio 3D lattice Boltzmann method

The bubbles rise up and burst at the free surface is a complex two-phase process.A free energy lattice Boltzmann method(LBM)model is adopted in this paper to study this phenomenon.The interface capturing technique[Zheng et al.,2006]is used to deal with the high density ratio problem.The Laplace law and the air-water interface capturing ability are validated for the multiphase model.The interaction between the single bubble or multiple bubbles and the free surface are studied by the multiphase model.The force acting on the bubble and the evolution of the free surface is studied.Meanwhile,effect of the initial distance between two adjacent bubbles on interaction effects of multiple bubbles is investigated as well.

Free-surface Simulations of Newtonian and Non-Newtonian Fluids with the Lattice Boltzmann Method

This paper describes the application of a three-dimensional lattice Boltzmann method （LBM） to Newtonian and non-Newtonian （Bingham fluid in this work） flows with free surfaces. A mass tracking algorithm was incorporated to capture the free surface, whereas Papanastasiou＇s modified model was used for Bingham fluids. The lattice Boltzmann method was first validated using two benchmarks： Newtonian flow through a square cross-section tube and Bingham flow through a circular cross-section tube. Afterward, the dam-break problem for the Newtonian fluid and the slump test for Bingham fluid were simulated to validate the free-surface-capturing algorithm. The numerical results were in good agreement with analytical results, as well as other simulations, thereby proving the validity and correctness of the current method. The proposed method is a promising substitute for time-consuming and costly physical experiments to solve problems encountered in geotechnical and geological engineering, such as the surge and debris flow induced by a landslide or earthquake.

Coexisting lattice contractions and expansions with decreasing thicknesses of Cu(100)nano-films

Lattice parameters are a basic quantity in material characterization,and a slight alteration in lattice parameters directly affects the properties of materials.However,there are still considerable controversies as to whether lattice expansion or contraction occurs in metallic nanomaterials with size reduction.Here,the size dependences of the lattice parameter and surface free energy of clean Cu(100)films are investigated via simulations.Lattice parameters of the exposed surfaces contract,whereas lattice expansion occurs along the direction perpendicular to the surfaces with decreasing film thicknesses.This is striking since the metallic bonds usually lack strong directionality,and it is always regarded that the lattice variations in all directions are consistent.The contraction parallel to the surface is more severe than the expansion perpendicular to the surface in films.The lattices change from cubic to tetragonal with decreasing film thickness.Consequently,common contractions and occasional expansions of the lattice parameters of Cu nanoparticles have been observed in previous experiments.Increasing free energy and surface free energy with decreasing thicknesses is the thermodynamic origin of the lattice variations.Our study therefore provides a comprehensive physical basis for the surface effects on the lattice variations.

Modified surface tension model for free surface flow with single-phase lattice Boltzmann method

A single-phase lattice Boltzmann model with modified surface tension is developed in this paper to solve the problem of high-density-ratio free surface flow.The computational efficiency and accuracy are both enhanced.The restriction to the relaxation factor （which needs to be smaller than 1） is circumvented by the new surface tension algebra,due to its rational physical nature compared with the treatment of Xing,Buther and Yang in their paper （Comp.Mater.Sci.,2007,39（2）：282-290）.The proposed stable surface tension scheme is applied to simulate the free deformation of a square droplet with surface tension effect and the process of a droplet impinging on a liquid film.The numerical solution for free deformation of a droplet agrees well with thermodynamic principles,and also achieves high accuracy in comparison with Xing,et al.＇s model.Three typical impinging modes are successfully obtained with the new scheme,and another particular mode found by Wang and Chen is also successfully simulated.The evolutions of liquid crown agree well with the power law related to time.

Impact Sensitivity in Respect of the Crystal Lattice Free Volume and the Characteristics of Plasticity of Some Nitramine Explosives

The relationship between the crystal lattice free volume,ΔV,and impact sensitivity,E_(dr),of ten nitramines has been analyzed. It was found that this relationship is not uniquely determined,i. e. it is not given only by their own ΔV values but fundamentally by the type and intensity of the intermolecular forces in the nitramine crystals. Also,relationships between the E_(dr)values,on the one hand,and bulk modulus,K,and shear modulus,G,on the other,have been the subject of discussion not only for pure cyclic nitramines but also for their PBXs,bonded by a poly-fluoro binder. The closest linear correlation exists between the E_(dr)values and dimensionless K·G^(-1) ratio which indicates the plasticity range. A similar relationship is valid also for the ΔV values. Relationships of the E_(dr) and / or ΔV values with the shear modulus or to the K·G^(-1) ratio reflect an unusual behavior of ε-HNIW to which the published morphological instability of this particular HNIW version might also be related.

Numerical Investigation on the Deformation of the Free Interface During Water Entry and Exit of a Circular Cylinder by Using the Immersed Boundary-Multiphase Lattice Boltzmann Flux Solver

In this work,the deformation of free interface during water entry and exit of a circular cylinder is investigated numerically by using the two-dimensional(2D)immersed boundary-multiphase lattice Boltzmann flux solver(IB-MLBFS).The fluid domain is discretized by finite volume discretization,and the flux on the grid interface is evaluated by lattice Boltzmann equations.Both the implicit velocity correction and the surface flux correction are implemented by using the immersed boundary-method to consider the fluid-structure interaction and the contact interface between the multiphase fluids and the structure.First,the water entry of a circular cylinder is simulated and the results are compared with the experiment,which considered the length-diameter ratio of the circular cylinder.The reliability of 2D simulation is verified and the deformation of the free interface is well investigated.Afterward,the water exit of a circular cylinder with constant velocity is simulated,which is less researched.In addition,the results show the advantage of present IB-MLBFS to some extent.Finally,the water exit and re-entry of a circular cylinder are presented,and the results present the complex deformation of the free interface and the dynamic response of the moving structure.Based on the numerical results,the free interface of the multiphase fluids is well captured,and the contact interface on the boundary of the moving structure is accurately presented by the IB-MLBFS.

Free Electron Characteristic Peculiarities Caused by Lattice Vibrations in Metals

It is shown that the traditional explanation of the free electron properties, such as mean free electron path, drift mobility, and the relaxation time, by lattice vibrations, is not valid for real free randomly moving (RM) electrons in materials with degenerate electron gas. It is shown that the effective density of the free RM electrons in elemental metals is completely determined by density-of-states at the Fermi surface and by absolute temperature. The study has shown that the lattice vibrations excite not only the free RM electrons but also produce the same number of weakly screened ions (so-named electronic defects), which cause the scattering of the free RM electrons and related electron kinetic characteristics.

High velocity flow simulation using lattice Boltzmann method with no-free-parameter dissipation scheme

A finite difference lattice Boltzmann method of second-order accuracy in time is developed based on non-oscillatory scheme with no-free-parameter dissipation （NND） difference scheme in this paper. The NND lattice Boltzmann method is used to simulate high-speed flows by constructing a new equilibrium distribution function of the lattice Boltzmann method. Compared with a variation of lattice Boltzmann method developed by Qu, et al., the present method can capture shock waves and handle oscillations of high velocity flows accurately in larger time steps and in shorter computing time. Numerical results indicate the correctness and capability of simulating shock wave interactions of the NND lattice Boltzmann method.

Study on the Bubble Growth and Departure with A Lattice Boltzmann Method

For the growth and departure of bubbles from an orifice, a free energy lattice Boltzmann model is adopted to deal with this complex multiphase flow phenomenon. A virtual layer is set at the boundary of the flow domain to deal with the no-slip boundary condition. Effects of the viscosity, surface tension, gas inertial force and buoyancy on the characteristics of bubbles when they grow and departure from an orifice in quiescent liquid are studied. The releasing period and departure diameter of the bubble are influenced by the residual gas at the orifice, and the interaction between bubbles is taken into consideration. The relations between the releasing period or departure diameter and the gravity acceleration show fair agreements with previous numerical and theoretical results. And the influence of the gas outflow velocity on bubble formation is discussed as well. For the bubbles growing in cross-flow field, effects of the cross-flow speed and the gas outflow velocity on the bubble formation are discussed, which is related to the application in ship resistance reduction. And optimal choice of the ship speed and gas outflow velocity is studied. Cases in this paper also prove that this high density ratio LBM model has its flexibility and effectiveness on multiphase flow simulations.

Nikodým-Type Theorems for Lattice Group-Valued Measures with Respect to Filter Convergence

We present some convergence and boundedness theorems with respect to filter convergence for lattice group-valued measures. We give a direct proof, based on the sliding hump argument. Furthermore we pose some open problems.

基于格子Boltzmann方法的自由能密度模型

基于格子Boltzmann方法的多相流模型具有相界面自动演化、无需边界积分等优势,在模拟复杂多相流系统中获得广泛的研究和应用。本文引入自由能密度计算分子间的相互作用力,提出一种满足热力学一致性和伽利略不变性的多相流模型。使用该模型预测气液两相共存密度的结果与理论值吻合得很好,误差值为-0.01~0.01,并且在低温度下优于改进的伪势模型。同时该模型也可以模拟较大密度比的气液系统,液相与气相的密度比可达10^(7)以上。通过两相分离和液滴撞击液膜等一系列数值模拟,验证了该模型符合伽利略不变性。该模型物理清晰、易于实施,能够结合不同状态方程模拟多相流系统,具有较好的实用性和应用前景。

点阵夹芯圆柱壳自由振动分析的波有限元方法

针对点阵夹芯圆柱壳的自由振动分析发展了波有限元法.首先基于自由波的传播规律,建立了点阵夹芯圆柱壳胞元的二维波有限元控制方程,相比于全尺寸有限元模型,显著缩减了控制方程的自由度规模;其次,基于Neumann级数推导了约束动刚度矩阵求逆的显式表达式,不仅可以提高计算效率,而且使得固有频率从控制方程中分离出来,从而将点阵夹芯圆柱壳的固有频率求解转化为单个胞元自由度规模的二次特征值问题;最后,根据结构振动模态与自由波的关系,给出了圆柱壳轴向和周向的波传播参数的表达式,进而求得点阵夹芯圆柱壳的固有频率和模态.数值算例考虑了多种边界条件下的点阵夹芯圆柱壳自由振动问题,验证了该方法的正确性和高效性.

P-Condense and LP-Condensing Operatorson Semilattices,Join-Complete Lattices and Some Inverse Semigroups

In this paper,the concepts of-condensing operator and 69-condensingoperator on a semigroup are introduced.Then we describe the-condensing operatorsand the 69-condensing operators on semilattices,join-complete lattices,and inversesemigroups.As an application of our results,we also show that the unary operations*:Hg→Hg-1 Hg and*:Hg→HG on the coset semigroupK(G)of a group G are-condensing operators.Consequently,a necessary and sufficient condition for theseoperators to be 69-condensing operators is given.Some further results on condensesemigroups considered by Chen et al.(Acta Math.Hung.119:281-305,2008)arediscussed and obtained in this paper.

Improved efficiency and photo-stability of methylamine-free perovskite solar cells via cadmium doping

Although perovskite solar cells containing methylamine cation can show high power conversion efficiency,stability is a concern.Here,methylamine-free perovskite material Csx FA1–x PbI3 was synthesized by a one-step method.In addition,we incorporated smaller cadmium ions into mixed perovskite lattice to partially replace Pb ions to address the excessive internal strain in perovskite structure.We have found that the introduction of Cd can improve the crystallinity and the charge carrier lifetime of perovskite films.Consequently,a power conversion efficiency as high as 20.59%was achieved.More importantly,the devices retained 94%of their initial efficiency under 1200 h of continuous illumination.

增材制造BCCZ点阵夹芯梁结构的自由振动分析

通过理论、有限元和试验方法研究了悬臂边界条件下BCCZ(体心立方点阵边界添加Z向杆件增强构型)点阵夹芯梁结构的振动行为。基于哈密顿原理和“改进折线”法,获得其振动频率理论模型。并使用SLM(选择性激光熔融)技术和钛合金TC4材料制备了BCCZ点阵夹芯梁试件,进行了模态试验,有限元和试验验证了理论模型的准确性。然后研究了胞元直径、面板厚度和芯子高度等结构几何参数和材料性能对BCCZ点阵结构振动性能的影响规律。结果表明:减小胞元直径,减小面板厚度,提高芯子高度均可以提高BCCZ点阵夹芯梁结构的一阶固有频率参数(一阶固有频率与同质量同面板面积的实体结构之比),胞元直径的影响最明显。面板-芯子-面板材料组合为钛-铝-钛时一阶固有频率最高。研究结果对BCCZ点阵夹芯结构的设计及工程应用具有一定的指导意义。

子格筛法的两种优化方法

子格筛法是求解格上最短向量问题(shortest vector problem,SVP)的一个高效算法,通过在(n-d)维投影子格上调用高斯筛法以求解n维格上的SVP问题,其中d是自由维数.子格筛法的降维思想有效降低了筛法的时间复杂度,缩小了与枚举算法运行效率的差距.本文在简要回顾高斯筛法和子格筛法的算法思想及运行机理的基础上,提出子格筛法的两种优化方法.通过调整高斯筛法的算法参数,改变算法输出包含的信息,增大了子格筛法的自由维数d,从而降低了算法的时间复杂度;在子格筛法最重要的子程序高斯筛法中加入过滤器,在更新列表向量的同时可以更快地完成筛选,从而将高斯筛法的二次搜索过程降低为亚二次搜索过程,节约了算法的运行时间;综合应用上述两种方法优化子格筛法,提出Filteredα-子格筛法.实验数据表明两种优化方法都能有效提升子格筛法的运行效率.在70–86维的随机格上,Filteredα-子格筛法的运行效率相比于初始子格筛法最高提升了约58.8%,平均提高了约41.7%.

多相多组分Peng-Robinson流体的正则化格子Boltzmann方法研究及模拟

为了提高石油、天然气等能源的采收率,并最大限度地保护环境.需要对油藏储层内的多相多组分石油混合物的热力学性质和界面行为进行深入研究.Peng-Robinson状态方程被广泛应用于描述和预测烷烃类混合物的热力学性质.文章根据扩散界面理论和Peng-Robinson自由能模型,得到描述多相多组分流体热力学性质和界面行为的流体动力学方程组.该耦合方程组中的交叉扩散项和Peng-Robinson自由能模型的强非线性,给数值求解带来了极大的挑战.文章基于格子Boltzmann方法,针对每个组分的质量守恒方程,构建了带有多分布函数的正则化格子Boltzmann方法.针对势形式的动量守恒方程,构建了能有效降低两相界面虚假速度的正则化格子Boltzmann方法.通过Chapman-Enskog多尺度分析,能够准确恢复至耦合的流体动力学方程组.最后,通过一系列的数值实验,如气-液两相共存线,丙烷和戊烷、甲烷和乙烷共存时的相平衡状态等,验证所发展的多相多组分正则化格子Boltzmann方法满足还原一致性和热力学一致性,同时具有较好的数值稳定性.此外,该正则化格子Boltzmann方法能够有效消除两相界面处的虚假速度,并能准确刻画多相多组分流体的热力学性质和界面行为.

基于格的分层无证书代理签名方案

现有基于经典数论问题假设的无证书代理签名方案无法抵御量子计算机攻击,在应用于有大量用户的系统时会存在单点失效和不易扩展等局限。针对这些问题,提出一种基于格的分层无证书代理签名方案。首先,采用拒绝采样技术和无陷门技术提高密钥生成的计算效率;其次,不同层级的原始签名人和代理签名人通过交换随机选取的矩阵进行互认证,实现代理授权;最后,在随机预言机模型下的小整数解(SIS)困难问题假设下证明了该方案的安全性。相较于现有的代理签名方案,所提方案允许签名人来自不同层级且隶属于不同密钥生成中心(KGC)。性能评价实验结果表明,该方案的公钥尺寸是一个常数,代理签名和验证开销与层级无关,且代理密钥和签名尺寸非层级的线性量。因此,该方案可更好地满足大规模分布式异构网络对均衡负载的需求,是高效可行的。

大跨铝合金自由曲面网壳设计研究

G60拉斐尔科创云廊屋盖的一期工程尺寸为736m×126m,为超长大跨铝合金自由曲面网壳结构。屋盖通过钢结构树状柱支撑于11栋高层建筑屋顶。阐述结构设计过程中关键指标的设计以及荷载取值的原则,通过应变能最小的原则开展屋盖结构找形的研究,介绍屋盖结构中支撑体系设计和网格布置的考虑因素,进一步说明结构分析的关键计算结果。重点介绍本项目的铝合金板式节点性能研究,开展两组板式节点足尺轴压试验,并与有限元模拟对比分析,结果验证了有限元建模的有效性,并为结构设计的安全性和构造的合理性提供数据支撑。同时进行了3组试件抗剪加强节点试验,对比分析3种不同类型板式节点的受力特性和破坏现象。结果表明抗剪键具有良好工作性能。

基于IB-LBM的水下运动分析

近水面水下运动与深水运动不同,当近水物体运动时,自由面表面形状将随时间不断变化,自由面对物体产生的影响是不可忽略的。近些年来,处理近水两相运动问题的数值计算方法有很多种,该文采用隐式直接力格式的浸入边界方法与基于伪势模型的格子玻尔兹曼方法,其具有其他处理N-S方程的数值计算方法的独特优点,可以处理连续或非连续流体问题。并且验证气泡上升模型,模拟近水面水下运动问题,得到水下方块对近水面的影响过程。该方法是对气液两相流问题及近水面处理问题的一次尝试。