二维液桥计算模型及湿颗粒材料离散元模拟

本文研究了二维情况两直接相邻颗粒间液体以液桥形式存在时的液桥临界断裂距离及液桥力的计算过程。导出了用颗粒半径、三相接触角和液桥体积表示的二维液桥临界断裂距离拟合公式及两个接触或非接触颗粒间液桥毛细力随液桥体积的变化关系。引入Voronoi胞元计算定义基于参考颗粒介观结构的物质点的平均饱和度。提出了考虑液桥效应的二维湿颗粒离散元模型,并用以模拟不同初始均匀饱和度下湿颗粒集合体中的吸力效应,验证其有效捕捉以应变局部化为特征的破坏模式的能力。     

片状颗粒间液桥力变化规律的计算研究

研究颗粒间液桥力有助于揭示非饱和土持水特性的内在机理. 为探究片状颗粒间液桥力演化规律, 从细观尺度研究非饱和土的水力特性机理, 使用Surface Evolver软件在两平行的片状颗粒间构建出三维液桥模型, 分析了液桥拉伸过程中接触角、液桥体积、分离距离以及固液接触线钉扎效应等对液桥力变化规律的影响. 基于圆弧假定, 计算相应条件下液桥力以及接触半径的大小, 并与上述模拟结果进行对比分析. 结果表明: 片状颗粒间液桥力随液桥体积增大而递增, 随分离距离的增大而递减, 随固液接触角的增大先增后减或一直递减; 液桥体积一定时, 在钉扎状态下, 其液桥力随着分离距离的增大迅速递增达到峰值, 而后逐渐降低; Surface Evolver模拟与液桥界面环形近似的计算结果相对比, 当固液接触角较大时(θ = 60°和θ = 80°), 二者相对误差在6%以内, 而当固液接触角减小到30°及以下时, 相对误差随之增大, 且颗粒间分离距离越大, 相对误差越大.      

不等径颗粒间液桥力学参数及形态的试验研究

作为一种自然界中广泛存在的力, 液桥力的研究对制药、重金属回收、颗粒分离等领域具有十分重要的意义. 利用纳米多功能拉伸试验机测量不等径颗粒间液桥拉伸过程中的液桥力?位移曲线, 同时配合CCD工业相机记录拉伸全过程液桥形态的变化. 首先分析了液桥力?位移曲线形态、最大液桥力、断裂距离随粒径比及液桥体积的变化规律, 其次基于圆环假设及Y-L方程对本文试验结果的合理性进行验算, 最后针对圆环假设在液桥力计算中存在的不足分析了其原因, 并结合重力对液桥形态的影响对液桥拉伸全过程的形态变化进行了具体分析. 结果表明: 最大液桥力受粒径比的影响较大而受液桥体积的影响较小, 与最大液桥力相反, 断裂距离受液桥体积的影响较大而受粒径比的影响较小; 圆环假设可以较好地预测最大液桥力大小但对拉伸过程中的液桥力预测不准, 这是由于当液桥力达到最大值后液桥的外轮廓已不能用圆环表示; 根据重力对液桥形态的影响, 将拉伸过程液桥外轮廓的变化简化为重力影响可以忽略时的圆环形?抛物线形, 重力影响处于过渡阶段或影响较小时的长轴与短轴之比不断增大的椭圆形, 以及重力影响不可忽略时的“冷却塔形”?双曲线形.      

地下水质球向输运问题的近似解

本文找到了溶质在孔隙介质中球向输运方程的近似解.该公式不仅结构简单、物理意义明确,而且与数值解的结果符合的较好.因此,可用于予测(计算)地下水质球向分布和传播范围.     

明渠层流近床面颗粒拖曳力研究

泥沙颗粒受到的拖曳力是泥沙运动的主要驱动力,而当前应用于计算流体力学-离散颗粒法(CFD-DPM)耦合模型进行水沙运动模拟的泥沙颗粒拖曳力公式均没有考虑明渠流底床边壁作用的影响。求解不可压缩Navier-Stokes方程,对明渠层流不同雷诺数条件下床面附近不同高度处颗粒所受拖曳力进行了模拟,根据模拟结果变化规律,提出了综合考虑床面和水流惯性对标准拖曳力影响的修正拖曳力计算公式。与常用的单颗粒标准拖曳力公式和考虑遮蔽效应的多颗粒拖曳力公式相比,采用本文修正公式得到的水沙作用力更接近高精度数值解,应用于CFD-DPM输沙模拟获得的输沙结果与输沙率公式结果一致,应用分析表明输沙模拟应当采用粗糙底床边界。     

复相陶瓷中颗粒桥联与相变协同增韧作用的力学分析

本文对复相陶瓷内延性金属颗粒和相变颗粒的协同增韧作用进行了力学分析，其中颗粒桥联效应还考虑了桥联颗粒与基体间工艺残余应力的影响，相变效应考虑了相变体膨胀和剪切塑性对增韧的共同贡献。数值计算结果与Ａｍａｚｉｇｏ－Ｂｕｄｉａｎｓｋｙ的结果及２Ｙ－ＴＺＰ／２０ＶＯＬ％Ｎｉ的实验结果作了比较。     

球形和方形湿颗粒团的碰撞模拟研究

本研究利用颗粒离散元方法,并结合接触力学理论,对微米级颗粒组成的湿颗粒聚合体碰撞破损的细观力学机理进行了模拟研究。模拟的颗粒聚合体分成球形和方形。将两种形状的颗粒聚合体碰撞结果做比较,颗粒之间的接触采用干、湿接触两种模型并存的方式来模拟湿颗粒团碰撞中的不可恢复变形过程。在碰撞速度保持不变的情况,考察了不同液体黏度对颗粒聚合体碰撞的影响;在液体黏度保持不变的情况,考察了不同碰撞速度对碰撞结果的影响。研究结果表明,湿颗粒团表现出了与干颗粒团完全不同的损伤模式,没有产生类似于弹性颗粒团的裂解损伤。湿颗粒团在碰撞中,它的内部液桥数量会在与平板碰撞接触的初期快速减少,然后慢慢累计,或会超过湿颗粒团在碰撞前的液桥数量。这一现象在此仿真研究中首次被发现。比如,液体黏度为50mPa·s的方形湿颗粒团以2.0m/s撞击平板时,其内部液桥数可由约9400降低至约8700后,逐渐增加至11300左右;由低黏度液体组成的湿颗粒团在碰撞后会发生反弹现象。而随着液体黏度的增加,恢复系数会趋近于0。     

考虑范德华力曲率效应的双壁碳纳米管外压屈曲

针对双壁碳纳米管外压屈曲问题，研究了层间范德华力的曲率效应对临界外压的影响。应用弹性双层圆柱壳模型，考虑层间范德华力不仅与层间距有关而且与挠度曲率的变化有关，导出了外压屈曲临界压力解析公式。计算得出在不同半径、不同长细比下，外压屈曲临界压力的数值结果，并与经典壳的结果和忽略范德华力曲率效应的结果做了比较。结果显示，对于小半径的双壁碳纳米管曲率效应对外压屈曲有效明显的影响。     

旋流场中PP单球颗粒力学行为实验测试分析

研究PP(Polypropylene)球形颗粒在旋流场中的力学行为,需要一个比较理想的、稳定的旋流场,为此采用了特殊尺寸的实验转鼓。根据PP球形颗粒在这种强制旋流场中的实验现象、测试数据和颗粒受力平衡分析可知:PP颗粒在雷诺数为180～230范围内的强制旋流场中达到稳定运动状态时,几乎静止在中心水平线附近的某一点,只有颗粒的自转运动;且转鼓转速越高,颗粒的质心位置越靠近旋转中心。依据切向受力平衡条件,利用现有的曳力公式对实验数据进行处理,发现颗粒不能达到受力平衡,且最大误差超过22%。为此,利用实验数据拟合方法,得到了旋流场中颗粒曳力系数的新经验公式,其受力平衡误差均小于0.5%。依据径向受力平衡条件,以径向压力梯度力、附加质量力、浮力与重力之差在径向上的分力之和为基准,与低雷诺数下的Saffman升力和Magnus升力的理论计算结果之和相比较,发现二者也不能达到力平衡条件,最大误差超过了55%。本文根据球形颗粒在旋流场中的升力特征,通过直观的力学分析和量纲分析,定义了颗粒旋流场升力概念,导出了相应的升力系数公式;并利用实验数据拟合得到了升力系数与颗粒雷诺数的经验公式;利用经验公式处理实验数据,其力平衡误差不超过7%。     

各向异性平面含斜裂纹的奇异积分方程方法

本文应用平面弹性复变方法，将无限各向异性平面中的任意斜裂纹问题归结为求解一组解析函数边值问题，通过构造适当的积分变换将边值问题转化为奇异积分方程，进而应用Lobotto-Chebyshev数值求积公式，求出该奇异积分方程的数值解，并得到了应力强度因子的近似表达式，最后，给出了一些实例的数值结果，对特例的数值结果与精确结果进行比较，吻合的很好。     

Liquid bridge force between two unequal-sized spheres or a sphere and a plane

Liquid bridge force acting between wet particles is an important property in particle characterization. This paper deals with liquid bridge force between either two unequal-sized spherical particles or a sphere and a flat plate under conditions where gravitational effect arising from bridge distortion is negligible. In order to calculate the force of the liquid bridge efficiently and accurately, expressions of liquid configuration and liquid bridge force were derived by building a mechanical model, which assumes the liquid bridge to be circular in shape between either two unequal-sized spheres or a sphere and a plane. To assess the accuracy of the numerical results of the calculated liquid bridge forces, they were compared to the published experimental data.     

The capillary force in micro- and nano-indentation with different indenter shapes

The influence of the indenter shapes and various parameters on the magnitude of the capillary force is studied on the basis of models describing the wet adhesion of indenters and substrates joined by liquid bridges. In the former, we consider several shapes, such as conical, spherical and truncated conical one with a spherical end. In the latter, the effects of the contact angle, the radius of the wetting circle, the volume of the liquid bridge, the environmental humidity, the gap between the indenter and the substrate, the conical angle, the radius of the spherical indenter, the opening angle of the spherical end in the truncated conical indenter are included. The meniscus of the bridge is described using a circular approximation, which is reasonable under some conditions. Different dependences of the capillary force on the indenter shapes and the geometric parameters are observed. The results can be applicable to the micro- and nano-indentation experiments. It shows that the measured hardness is underestimated due to the effect of the capillary force.     

A capillary force model for interactions between two spheres

Based on the method of energy principle, an analytical approach for computing the capillary force for sphere/sphere geometry is presented in this paper. In modeling the capillary force, we consider spheres with both equal and non-equal radii, for both symmetric and asymmetric configurations at liquid/solid interfaces. We use numerical analysis to investigate the validity and efficiency of the derived model. The effect of various parameters including humidity, distance between two spheres, radii of spheres and contact angles on the meniscus force are investigated. Finally the results obtained from the model are compared with experimental measurements, and the accuracy and precision of the presented approach is verified.     

Removal of Particle Bridges From a Porous Material by Ultrasonic Irradiation

Particle bridge formation during the flow of a liquid with particles through a porous material is a fouling mechanism that can block the pores and, hence, decrease the permeability of the material. Ultrasonic irradiation of the material is a cleaning method that can restore the permeability. We make a numerical study of this cleaning method using the lattice-Boltzmann method. We start from a pore blocked by two spherical particles attached to the pore wall by colloidal adhesion forces, thus forming a particle bridge. Next we calculate the hydrodynamic force exerted by a high-frequency acoustic wave on the two particles. By comparing the hydrodynamic force and the adhesion force we investigate, whether the particle bridge will be removed by the ultrasonic irradiation. A sensitivity study is carried out to investigate the influence of some relevant parameters, such as the acoustic wave amplitude, the acoustic frequency, the fluid flow velocity and the ratio of particle diameter and pore diameter. An upscaling procedure is applied to translate the microscopic results for the removal of the particles at the pore level to the permeability improvement of the material at the macroscopic level. A comparison is made between numerical results and experimental data. The agreement is reasonable.     

Liquid Bridges Between Contacting Balls

The problem studied is that of a rotationally symmetric liquid bridge between two contacting balls of equal radius, with the same contact angle with both balls, and in the absence of gravity. The bridge surface must be of constant mean curvature, hence a Delaunay surface. If the contact angle is less than \({\frac{\pi}{2}}\) , existence of a rotationally symmetric bridge is shown for a large range of the relevant parameter, giving unduloidal, catenoidal, and nodoidal bridges. If the contact angle is greater than or equal to \({\frac{\pi}{2}}\) , it is shown that no stable rotationally symmetric bridge which is symmetric across the perpendicular bisector of the line segment between the two centers of the balls exists. Existence therefore depends discontinuously on contact angle.     

Capillary bridges and capillary forces between two axisymmetric power–law particles

Capillary interactions are fundamentally important in many scientific and industrial fields. However, most existing models of the capillary bridges and capillary forces between two solids with a mediated liquid, are based on extremely simple geometrical configurations, such as sphere–plate, sphere–sphere, and plate–plate. The capillary bridge and capillary force between two axisymmetric power–law profile particles with a mediated constant-volume liquid are investigated in this study. A dimensionless method is adopted to calculate the capillary bridge shape between two power–law profile particles based on the Young–Laplace equation. The critical rupture criterion of the liquid bridge is shown in four forms that produce consistent results. It was found that the dimensionless rupture distance changes little when the shape index is larger than 2. The results show that the power–law index has a significant influence on the capillary force between two power–law particles. This is directly attributed to the different shape profiles of power–law particles with different indices. Effects of various other parameters such as ratio of the particle equivalent radii, liquid contact angle, liquid volume, and interparticle distance on the capillary force between two power–law particles are also examined.     

考虑等效曲率的超二次曲面单元非线性接触模型

基于连续函数包络的超二次曲面单元可有效地描述自然界和工业生产中的非球体颗粒形态, 并通过非线性迭代方法精确计算单元间的接触力. 对于具有复杂几何形态的超二次曲面单元, 线性接触模型不能准确地计算不同接触模式下的作用力. 考虑超二次曲面单元相互作用时不同颗粒形状及表面曲率的影响, 本文发展了相应的非线性黏弹性接触模型. 该模型将不同接触模式下的法向刚度和黏滞力统一表述为单元间局部接触点处等效曲率半径的函数; 切向接触作用则借鉴基于Mohr-Coulomb摩擦定律的球体单元非线性接触模型的计算方法. 为检验超二次曲面单元接触模型的可靠性, 对球形颗粒间的法向碰撞、椭球体颗粒间的斜冲击过程、圆柱体的静态堆积和椭球体的动态卸料过程进行离散元模拟, 并与有限元数值结果及试验结果进行对比验证. 计算表明, 考虑接触点处等效曲率半径的超二次曲面非线性接触模型可准确地计算单元间的接触碰撞作用, 并合理地反映非球形颗粒体系的运动规律. 在此基础上进一步分析了不同长宽比和表面尖锐度对卸料过程中颗粒流动特性的影响, 为非球形颗粒材料的流动特性分析提供了一种有效的离散元方法.      

Exact solution for capillary interactions between two particles with fixed liquid volume

The capillary interactions, including the capillary force and capillary suction, between two unequal-sized particles with a fixed liquid volume are investigated. The capillary interaction model is used within the Young-Laplace framework. With the profile of the meridian of the liquid bridge, the capillary suction, and the liquid volume as state variables, the governing equations with two-fixed-point boundary are first derived using a variable substitution technique, in which the gravity effects are neglected. The capillary suction and geometry of the liquid bridge with a fixed volume are solved with a shooting method. In modeling the capillary force, the Gorge method is applied. The effects of various parameters including the distance between two particles, the ratio of particle radii, and the liquid-solid contact angles are discussed.