离散颗粒层被横向推移过程中的力学行为研究

研究发现,颗粒物质层被匀速推移挤压过程中,所需推移力先以线性规律增加,在某一确定点后,则会以指数规律增加.而颗粒物质是由众多离散颗粒组成的软凝聚态物质,其宏观上反映的是离散颗粒的个体性质和凝聚态物质的集体效应.颗粒与颗粒之间以及颗粒与边界之间的细观尺度接触力链的构成以及演变规律将会直接影响各种宏观受力情况,其摩擦力与挤压力便是力链的主要构成形式.围绕着定量描述细观力链特征,从而揭示力的变化规律这一目标,采用计算机模拟的方法,依照球形颗粒Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论,对重力作用下不同数目的三维等径球体颗粒层的推移情况进行了离散元仿真模拟,量化分析了推移力变化规律、各摩擦力变化规律以及力链分布规律,发现摩擦力与挤压力在颗粒堆积的不同阶段对力链的构成起到了不同的主次作用,使得力链发生强弱演变,从而发现了推移颗粒物质层时推移力的变化规律及原因.这些结果有利于从力链角度揭示颗粒内部和颗粒与各边界之间的受力情况.     

静态堆积颗粒中的力链分布

颗粒物质是由众多离散颗粒组成的软凝聚态物质,涉及多个物理层次结构和机制,是多尺度问题. 首先阐述了颗粒物质多尺度力学的研究框架,指出颗粒间接触力链构成的细观尺度是核心,颗粒物质显示出的独特静态堆积特性和动态流变特性都与细观尺度力链的复杂演变规律直接相关. 围绕着定量描述力链特征这一目标,采用严格的球形颗粒Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论,对重力作用下12000个球心共面的二维等径颗粒静态堆积进行了离散动力学模拟,对力链分布特征、接触力规律等做了量化分析,考察了颗粒 关键词： 颗粒物质 力链 离散模型 多尺度力学     

颗粒物质内自旋小球运动行为的数值模拟研究

针对大耳沙蜥在沙子中的运动行为,以球形物体为研究对象,利用3维离散元数值模拟程序LIGGGHTS模拟了旋转运动模式对颗粒物质中球形物体平动和上升/下降行为的影响,定量分析了旋转速度以及颗粒间摩擦系数等因素的具体影响.研究结果表明:球形物体与颗粒物质基底颗粒间的摩擦系数以及球形物体的自转角速度对球形物体的运动有明显影响,摩擦系数越大物体运动越明显,自转角速度越大物体运动越明显.该结果比较好地解释了沙漠生物外表具有鳞片的原因.     

Injection of granular material

Some civil engineering projects require the injection of granular matter such as sand into the ground. However, granular matter resists pumping through tubes and thus it is difficult to inject such matter into the ground. With the help of several methods forces and movements of grains were visualized. Force chains and arches in a two dimensional granular matter model were visualized with pola rized light and photo-elastic material. The movement of sand grains was visualized on a glass plate in a half-space ground model. With th e p article im age velocimetry (PIV) method the vector field of the movement and the field of the resulting shear rate were assessed. In this way a new method for the injection of granular material, such as, sand into the ground was devised. The method appears to be applicable to lift a building to counter subsidence.     

Unclustering transition in freely cooling wet granular matter

The free cooling of one-dimensional wet granular matter is presented in the framework of the minimal capillary model. We demonstrate analytically and by extensive simulations that above a critical density, the clustering of wet granular matter is not monotonic in time, but undergoes a sharp unclustering transition. This precipitation of granular droplets out of the gas takes place when the granular temperature comes close to the energy scale set by the capillary interaction.     

Direct observation of medium-range crystalline order in granular liquids near the glass transition

Collective behavior of driven granular matter is often strikingly analogous to that of thermal systems. Here we use a vibrated quasi-two-dimensional granular matter as a model system and investigate the mechanism of the liquid-glass transition. We demonstrate by direct observation the existence of long-lived medium-range crystalline order, which is found to be closely related to both dynamic heterogeneity and slow dynamics. Our findings are remarkably similar to recent numerical results on model thermal liquids and thus open an intriguing possibility of understanding the dynamic arrest in both thermal and athermal systems in a unified manner.     

冲击荷载下颗粒物质缓冲性能的试验研究

颗粒物质是一种复杂的能量耗散体系. 颗粒间的摩擦和黏滞作用可使冲击荷载引起的能量有效衰减, 颗粒间的力链结构又可将瞬时局部冲击荷载进行空间扩展和时间延长, 达到良好的缓冲效果. 为研究颗粒物质对冲击荷载的缓冲性能, 本文采用重力作用下球体冲击筒内颗粒物质的试验系统, 研究了筒体底部作用力在颗粒材料、颗粒厚度等因素影响下的变化规律. 试验结果表明: 非规则颗粒具有更加良好的缓冲性能, 粗颗粒的缓冲性能略高于细颗粒. 颗粒厚度H是影响缓冲性能的重要因素, 并存在一个临界厚度H_c. 当H<H_c时, 缓冲性能随H的增加而增强; 当H>H_c时, H对缓冲效果的影响不再显著. 以上研究是在同一冲击能量下进行的, 而对于不同冲击能量下的H_c还需要深入开展. 通过颗粒物质对冲击荷载缓冲性能的试验研究, 可揭示颗粒材料的基本物理力学行为, 为其在缓冲减振领域中的应用提供依据.     

不同形状的棒在颗粒物质中受到的静摩擦力

通过实验对横截面周长相同、形状不同的棒在颗粒物质中受到的最大静摩擦力进行了研究.实验结果表明:相同条件下,圆棒、方棒和扁棒受到的最大静摩擦力各不相同.用水平面上颗粒分层、对称分布模型对实验结果进行了定量分析.     

Ultrasound-assisted adsorption of copper(II) ions on hazelnut shell activated carbon

The present study was aimed to removal of Cu(II) ions from aqueous solution by ultrasound-assisted adsorption onto the granular activated carbon obtained from hazelnut shells. The attention was focused on modeling the equilibrium and kinetics of Cu(II) adsorption onto the granular activated carbon. The granular activated carbon was prepared from ground dried hazelnut shells by simultaneous carbonization and activation by water steam at 950 °C for 2 h. Adsorption isotherm data were better fitted by the Langmuir model than the Freundlich model in both the absence and the presence of ultrasound. The maximum adsorption capacity of the adsorbent for Cu(II), calculated from the Langmuir isotherms, in the presence of ultrasound (3.77 mmol/g) is greater than that in the absence of ultrasound (3.14 mmol/g). The adsorption process in the absence and the presence of ultrasound obeyed to the pseudo second-order kinetics. The removal of Cu(II) ions was higher in the presence of ultrasound than in its absence, but ultrasound reduced the rate constant. The intraparticular diffusion model indicated that adsorption of Cu(II) ions on the granular activated carbon was diffusion controlled as well as that ultrasound promoted intraparticular diffusion.     

Dilatancy Transition in a Granular Model

We introduce a model of granular matter and use a volume/strain ensemble to analyze infinitesimal shearing. Monte Carlo simulation suggests the model exhibits a second order phase transition associated with the onset of dilatancy.     

Dense granular flow around a penetrating object: experiment and hydrodynamic model

We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter, together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number.     

Effective elasticity coefficients of native rocks and consolidated granular matter

The elastic coefficients of binary heterogeneous materials, such as several native rock materials or consolidated granular matter will be determined in terms of a perturbation expansion. Furthermore, in order to check the validity of the obtained results, these are compared with numerical investigations using Boole's model of randomly distributed spheres. Finally, we apply the results on several classes of native rocks and consolidated granular materials.     

颗粒物质(上)

颗粒物质是地球上存在最多、最为人们所熟悉的物质类型之一．大量颗粒组成的离散态物质体系具有特别的性质和运动规律．颗粒物质表现出许多不同于固、液、气物质的奇特现象和独特的运动规律．由于对颗粒物质运动规律的理解具有重要科学意义和应用背景，近十年来颗粒物质研究逐步成为物理学研究中的一个活跃领域．文章综述了颗粒物质的一些主要特性，如颗粒物质的静态性质、振动行为、流动特性等，其中也包括文章作者的一些新近研究结果．限于篇幅，文章分为上、下两部分发表．第一部分在对颗粒物质作一般介绍后讨论颗粒物质的静态特性，第二部分则主要讨论颗粒物质振动时的行为和流动性质．     

颗粒物质在冲击作用下的堆积分布

沙堆在冲击作用下的行为和静态沙堆的行为有着很大的不同.通过管道产生“高斯冲击”(速度水平分量成高斯分布)来研究沙堆堆积时的分布情况，发现沙堆自身的稳定性和外界冲击是影响沙堆形状的两大重要因素.给出了沙堆在冲击作用下高度随时间的演化方程.通过该方程，成功地得到了沙堆中心点的高度与外界参量之间的关系，并且提出了一种节能高效的颗粒混合方法. 关键词： 颗粒物质 堆积 高斯分布 冲击     

Bianchi cosmologies with anisotropic matter: Locally rotationally symmetric models

The dynamics of cosmological models with isotropic matter sources (perfect fluids) is extensively studied in the literature; in comparison, the dynamics of cosmological models with anisotropic matter sources is not. In this paper we consider spatially homogeneous locally rotationally symmetric solutions of the Einstein equations with a large class of anisotropic matter models including collisionless matter (Vlasov), elastic matter, and magnetic fields. The dynamics of models of Bianchi types I, II, and IX are completely described; the two most striking results are the following. (i) There exist matter models, compatible with the standard energy conditions, such that solutions of Bianchi type IX (closed cosmologies) need not necessarily recollapse; there is an open set of forever expanding solutions. (ii) Generic type IX solutions associated with a matter model like Vlasov matter exhibit oscillatory behavior toward the initial singularity. This behavior differs significantly from that of vacuum/perfect fluid cosmologies; hence “matter matters”. Finally, we indicate that our methods can probably be extended to treat a number of open problems—in particular, the dynamics of Bianchi type VIII and Kantowski-Sachs solutions.     

Open problems in active chaotic flows: Competition between chaos and order in granular materials

There are many systems where interaction among the elementary building blocks-no matter how well understood-does not even give a glimpse of the behavior of the global system itself. Characteristic for these systems is the ability to display structure without any external organizing principle being applied. They self-organize as a consequence of synthesis and collective phenomena and the behavior cannot be understood in terms of the systems' constitutive elements alone. A simple example is flowing granular materials, i.e., systems composed of particles or grains. How the grains interact with each other is reasonably well understood; as to how particles move, the governing law is Newton's second law. There are no surprises at this level. However, when the particles are many and the material is vibrated or tumbled, surprising behavior emerges. Systems self-organize in complex patterns that cannot be deduced from the behavior of the particles alone. Self-organization is often the result of competing effects; flowing granular matter displays both mixing and segregation. Small differences in either size or density lead to flow-induced segregation and order; similar to fluids, noncohesive granular materials can display chaotic mixing and disorder. Competition gives rise to a wealth of experimental outcomes. Equilibrium structures, obtained experimentally in quasi-two-dimensional systems, display organization in the presence of disorder, and are captured by a continuum flow model incorporating collisional diffusion and density-driven segregation. Several open issues remain to be addressed. These include analysis of segregating chaotic systems from a dynamical systems viewpoint, and understanding three-dimensional systems and wet granular systems (slurries). General aspects of the competition between chaos-enhanced mixing and properties-induced de-mixing go beyond granular materials and may offer a paradigm for other kinds of physical systems. (c) 2002 American Institute of Physics.     

Period tripling causes rotating spirals in agitated wet granular layers

Pattern formation of a thin layer of vertically agitated wet granular matter is investigated experimentally. Rotating spirals with three arms, which correspond to the kinks between regions with different colliding phases, are the dominating pattern. This preferred number of arms corresponds to period tripling of the agitated granular layer, unlike predominantly subharmonic Faraday crispations in dry granular matter. The chirality of the spatiotemporal pattern corresponds to the rotation direction of the spirals.     

Particle dynamics in sheared granular matter

The particle dynamics and shear forces of granular matter in a Couette geometry are determined experimentally. The normalized tangential velocity V(y) declines strongly with distance y from the moving wall, independent of the shear rate and of the shear dynamics. Local rms velocity fluctuations deltaV(y) scale with the local velocity gradient to the power 0.4+/-0.05. These results agree with a locally Newtonian, continuum model, where the granular medium is assumed to behave as a liquid with a local temperature [deltaV(y)](2) and density dependent viscosity.