A pressure-sensitive rheological origin of high friction angles of granular matter observed in NASA–MGM project

An abnormally high peak friction angle of Ottawa sand was observed in(National Aeronautics and Space Administration) NASA–(Mechanics of Granular Materials) MGM tests in microgravity conditions on the space shuttle. Previous investigations have been unsuccessful in providing a constitutive insight into this behavior of granular materials under extremely low effective stress conditions. Here, a recently proposed unified constitutive model for transient rheological behavior of sand and other granular materials is adopted for the analytical assessment of high peak friction angles. For the first time, this long-eluded behavior of sand is attributed to a hidden rheological transition mechanism, that is not only rate-sensitive, but also pressure-sensitive. The NASA–MGM microgravity conditions show that shear-tests of sand can be performed under abnormally low confining stress conditions. The pressure-sensitive behavior of granular shearing that is previously ignored is studied based on the μ(I) rheology and its variations. Comparisons between the model and the NASA microgravity tests demonstrate a high degree of agreement. The research is highly valid for pressure-sensitive and rate-dependent problems that occur during earthquakes, landslides, and space exploration.

Behaviour of granular matter under gravity-induced stress gradient:A two-dimensional numerical investigation

Gravity is the most important load source in mining and geotechnical engineering,which causes both the stress level and stress gradient inside geomaterials.Different from the stress level,the influence of gravity-induced stress gradient on the behaviour of the material is still unknown.An in-deep study on it will help to promote the understanding of material behaviour,especially for those cases related to unconventional gravity such as terrestrial ng physical modelling and extraterrestrial resource exploitation(g is the terrestrial gravitational acceleration).In this study,a high-order homogenization for granular materials is proposed at first,in which the stress gradient is drawn into the constitutive representation by adopting a representative volume element(RVE).The consolidation and shear strength behaviour of RVE are then investigated by performing numerical biaxial tests.The results show that all the compressibility,shear strength,shear stiffness,volumetric deformation,and critical state behaviour show a stress gradient dependence.A coupling between stress gradient,stress level,and material properties is also observed.These observations suggest that,besides the stress level,extra attention needs to be paid to material responses related to stress gradient during engineering practices.

Buffer capacity of granular matter to impact of spherical projectile based on discrete element method

Granular matter possesses impact-absorbing property due to its energy dissipation character.To investigate the impact-absorbing capacity of granular matter,the discrete element method(DEM)is adopted to simulate the impact of a spherical projectile on to a granular bed.The dynamic responses of the projectile are obtained for both thin and thick granular bed.The penetration depth of the projectile and the first impact peak are investigated with different bed thicknesses and impact velocities.Determining a suitable bed thickness is crucial to the buffering effect of granular matter.The first impact peak is independent of bed thickness when the thickness is larger than the critical thickness.

Automatic extraction method of force chain information and its application in the flow photoelastic experiment of granular matter

The force chain is the core of the multi-scale analysis of granular matter.Accurately extracting the force chain information among particles is of great significance to the study of particle mechanics and geological hazards caused by particle flow.However,in the photoelastic experiment,the precise identification of the branching points of force chains has not been effectively realized.Therefore,this study proposes an automatic extraction method of force chain key information.First,based on the Hough transform and the Euclidean distance,a particle geometric information identification model is established and geometric information such as particle circle center coordinates,radius,contact point location,and contact angle is extracted.Then,a particle contact force information identification model is established following the color gradient mean square method.The model realizes the rapid calibration and extraction of a large number of particle media contact force information.Next,combined with the force chain composition criterion and its quasilinear feature,an automatic extraction method of force chain information is established,which solves the problem of the accurate identification of the force chain branch points.Finally,in the photoelastic experiment of ore drawing from a single drawpoint,the automatic extraction method of force chain information is verified.The results show that the macroscopic distribution of force chains during ore drawing from a single drawpoint is left–right symmetrical.Strong force chains are mostly located on the two sides of the model but in small numbers and they mainly develop vertically.Additionally,the ends are mostly in a combination of Y and inverted Y shapes,while the middle is mostly quasilinear.Weak force chains are abundant and mostly distributed in the middle of the model,and develop in different directions.The proposed extraction method accurately extracts the force chain network from the photoelastic experiment images and dynamically characterizes the force chains of granular matter,which has significant advantages in particle geometry information extraction,force chain branch point discrimination,force chain retrieval,and force chain distribution and its azimuthal characterization.The results provide a scientific basis for studying the macroscopic and microscopic mechanical parameters of granular matter.

Stress distribution and surface instability of an inclined granular layer

Static granular materials may avalanche suddenly under continuous quasi-static drives. This phenomenon, which is important in many engineering applications, can be explained by analyzing the stability of the elastic solutions. We show this for a granular layer driven by its inclination angle in gravity, where the elastic problem can be solved generally and analytically. It is found that a loss of stability may occur only at the free surface of the layer. This result is considered to be relevant for understanding surface avalanches and the flows observed experimentally.

Effect of size polydispersity on the structural and vibrational characteristics of two-dimensional granular assemblies

Two-dimensional disordered granular assemblies composed of 2048 polydispersed frictionless disks are simulated using the discrete element method. The height of the first peak of the pair correlation function, gl, the local and global bond orientational parameters ψ6^1 and ψ6^g, and the fluctuations of these parameters decrease with increasing polydispersity s, implying the transition from a polycrystalline state to an amorphous state in the system. As s increases, the peak position of the boson peak aJBp shifts towards a lower frequency and the intensity of the boson peak D（ωBP）/ωBp increases, indicating that the position and the strength of the boson peak are controlled by the polydispersity of the system. Moreover, the inverse of the boson peak intensity ωBP/D（ωBP）, the shear modulus G, and the basin curvature SIS all have a similar dependence on s, implying that the s dependence of the vibrational density of states at low frequencies likely originates from the s dependence of the basin curvature.

Stabilizing effect of plasma discharge on bubbling fluidized granular bed

Fluidized beds have been widely used for processing granular materials. In this paper, we study the effect of plasma on the fluidization behavior of a bubbling fluidized bed with an atmospheric pressure plasma discharger. Experiment results show that the bubbling fluidized bed is stabilized with the discharge of plasma. When the discharge current reaches a minimum stabilization current Cms, air bubbles in the bed will disappear and the surface fluctuation is completely suppressed.A simplified model is proposed to consider the effect of electric Coulomb force generated by the plasma. It is found that the Coulomb force will propel the particles to move towards the void area, so that the bubbling fluidized bed is stabilized with a high enough plasma discharge.

Intruder trajectory tracking in a three-dimensional vibration-driven granular system:Unveiling the mechanism of the Brazil nut effect

We employ a Hall-effect magnetic sensor array to accurately track the trajectory of a single magnetic sphere,referred to as the“intruder,”within a three-dimensional vibro-fluidized granular bed to unravel the underlying physical mechanism governing the motion of the intruder.Within the acceleration range of 3.5 g≥Γ≥1.5 g,we find that,regardless of the intruder's initial position,it consistently reaches the same equilibrium depth when the vibration acceleration(Γ)and frequency(ω)are fixed.ForΓ≤2.5 g,the equilibrium position lies on the surface of the granular bed,while forΓ>2.5 g,it shifts below the surface.Additionally,intruders with different densities exhibit varying equilibrium depths,with higher density resulting in a deeper equilibrium position.To understand the mechanism behind the intruder's upward or downward motion,we measure its rising or sinking velocities under different vibration parameters.Our findings demonstrate that the rising velocity of the intruder,under varying vibration accelerations(Γ)and frequencies(ω),can be collapsed using the ratioΓ/ω,while the sinking velocity remains unaffected by the vibration strength.This confirms that the upward motion of the larger sphere,associated with the Brazil nut effect,primarily arises from the void-filling mechanism of the bed particles.Furthermore,our experiments reveal that the presence of convection within the bed particles has minimal impact on the motion of the intruder.

Effect of number density on velocity distributions in a driven quasi-two-dimensional granular gas

The motion of mono-disperse spherical steel particles in a vibration driven quasi-two-dimensional （2D） square cell is studied. The cell is horizontally vibrated to eliminate the effect of gravity compaction. The velocity distributions at different particle number densities are studied and found to obey the form exp[-β（｜Vy｜/σy）α], in which Vy and （σy are velocity and its variance in the transverse direction, and α and β are fitting parameters. The value of α is found to decrease with the number density of particles increasing. To investigate the effect of the bottom plate, the molecular dynamics simulation without considering any bottom friction is performed. The accordance between the simulation result and the experimental result shows that the influence of bottom plate friction force on the high energy tail of the velocity distribution can be neglected.

Properties of surface waves in granular media under gravity

Acoustical waves propagating along the free surface of granular media under gravity are investigated in the framework of elasticity theory. The influence of stress on a surface wave is analyzed. The results have shown that two types of surface waves, namely sagittal and transverse modes exist depending on initial stress states, which may have some influence on the dispersion relations of surface waves, but the influence is not great. Considering that the present experimental accuracy is far from distinguishing this detail, the validity of elasticity theory on the surface waves propagating in granular media can still be maintained.

Directed segregation in compartmentalized bi-disperse granular gas

A bi-disperse granular gas in an asymmetrical two-compartment system is studied experimentally. The presence of asymmetry within the range of our experimental parameters results in a directed segregated state and a directed clustering state. This deterministic system does not depend on the initial conditions. A modified flux model based on Lohse＇s flux model for bi-disperse granular gases is derived. The modified flux model explains qualitatively the experimental results.

Resistance law of a rod penetrating a multilayer granular raft

This paper presents an experimental study on the resistance law of a rod vertically penetrating different kinds of multilayer granular rafts with a constant velocity so as to reveal the mechanical properties of the multilayer granular rafts.The resistance was quasi-static under the chosen velocity.Experiments were conducted with different granular thicknesses,rod diameters and combinations of particles and liquids.The study shows that the resistance–displacement relation of the rod has three smooth stages.In the first stage,the resistance rapidly increased.In the second stage,the resistance curve maintained an almost constant slope.In the third stage,the resistance smoothly changed with its slope continuously increasing.Based on the corresponding physical models for each stage,the study reveals the exponential dependence of the load-bearing capacity of the multilayer granular raft on its thickness,and clarifies the capillary effects on the resistance law.The study extends the knowledge of the granular raft from monolayer to multilayer structure.

A new physical model on the capillary phenomenon of granular particles

Similar to the capillary phenomenon of liquid, granular particles can move up to a certain height along a vertically vibrating tube. The certain height, which is called the equilibrium height, is related to some parameters, e.g., the inner diameter of the tube, the amplitude, and the vibration frequency. In this paper, a theoretical model is proposed to explain the physical origin of the capillary phenomenon and the effects of the inner diameter of the tube, the amplitude, and the vibration frequency on the equilibrium height. In this model, the volumes of the inflowing and outflowing particles in a vibration period are calculated, which can significantly broaden our understanding in the flow of particles in the bottom of the tube. In order to prove the assumption of this physical model that the particles in the bottom of the tube move in the form of sine, several experiments are conducted. The granular climbing heights at different granular positions and different time stages are measured. The results show that granules move in the form of sine, which almost coincides with the motion of the tube. Moreover, motivated by the sampling on the asteroid regolith based on this mechanism, the sampling efficiencies for various vibration amplitudes and frequencies are discussed based on the new proposed model. It is found that there is an optimum frequency at which sampling is the most effective.

颗粒活性炭-纳滤对二级出水中条件致病菌和有机物的去除效能

再生水回用是解决水资源短缺的重要途经,被广泛应用于工业生产、城市绿化方面,但其中含有大量的条件致病菌,对环境及人体健康具有潜在威胁。研究拟对污水厂二级出水采用颗粒活性炭(Granular Activated Carbon,GAC)、生物活性炭(Biological Activated Carbon,BAC)两种过滤单元与纳滤(nanofiltration,NF)组合工艺,来达到水中三种条件致病菌(军团菌、铜绿假单胞菌和鸟分枝杆菌)、大肠杆菌和有机物的去除的目的,采用直接NF、GAC-NF和BAC-NF三种处理工艺对去除效果进行分析。结果表明:在GAC过滤的滤速为5 cm/h,NF的过滤压力为0.4 MPa条件下,直接NF及其组合工艺均可实现二级出水中条件致病菌的完全去除,相较于直接NF和GAC-NF,BAC-NF工艺对水中DOC、UV254、微生物代谢副产物类和腐殖酸类有机物的去除率最高,分别为85.2%、74.5%、76.2%和82.0%;水中条件致病菌与大肠杆菌、DOC之间均有显著相关性,可以通过强化水中大肠杆菌和有机物的去除,达到提高不同组合工艺对条件致病菌削减的目的;BAC表面生物膜细菌中的假单胞菌、气单胞菌、硝化螺旋菌和不动杆菌占比最高,上述细菌对条件致病菌的生长繁殖起抑制作用;生物膜真核微生物中的线虫和轮虫占比最高,这两种微生物能通过捕捉的方式实现对水中条件致病菌的削减。

水平振动式滚磨光整加工中力链演化及量化分析

滚磨光整加工中磨块颗粒介质内力链网络影响颗粒群的宏观行为,进而影响加工效果。本文建立水平振动式滚磨光整加工离散元模型,分析颗粒介质介观尺度力链网络动态演化机制与宏观运动行为的联系,并对力链网络进行量化表征研究。结果表明:力链网络复杂的时空分布特性决定了颗粒群的运动特征,提高激振参数,颗粒介质出现流态化现象,力链数目减少,强力链长度变短、准直系数减小、强度提高、方向偏向重力方向。研究工作为形成滚磨光整加工多尺度研究策略提供了一定的分析方法。

基于三维离散元模型粉末压制中力链对阻塞行为的影响机制

基于三维离散元建模技术,按照实验级配分布生成三维颗粒堆积,模拟单轴粉末压制过程,研究了压制过程中力链的演化规律及拓扑特性,通过颗粒系统中的速度与配位数分析了阻塞现象,并讨论了力链与阻塞行为间的关联机制。结果表明,在粉末压制过程中,随着上模冲压强从0逐渐增加至60 MPa,力链颗粒数目占比迅速上升至40.0%以上,高应力颗粒数目占比稳定在12.5%以上;当压强大于60 MPa后,力链开始出现弯曲并形成环状结构,从而导致了阻塞现象的产生,因此将60 MPa定为阻塞点。阻塞现象对粉体的致密化产生了影响,在压制过程中,与下层粉体颗粒相比,上层区域的粉体颗粒轴向速度更快,变化幅度更大;当压制结束时,上层区域的粉体颗粒致密化程度比下层区域更高。

基于外积求解颗粒平衡问题

提出一种简洁有效的基于外积的方法,对无摩擦的二维圆盘、二维凸颗粒以及三维球体的力学平衡方程进行求解和分析。理论结果表明,圆盘与球体上输出力的大小与输入力的大小成正比,因此引入了力传递的概念,并且导出力传递系数与接触结构的关系。对于凸颗粒,导出了输出力大小与输入力以及力矩的关系式。在已知圆盘上的平均正压力时,导出了各个接触力大小的公式并给出其几何意义。文中提出的方法以及计算的结果,对于求解其他物理问题中的向量方程有意义,对于进一步研究力学网络上的平衡方程有重要价值。

岩土类颗粒物质宏-细观力学研究进展

岩土类颗粒物质在自然界、工程建设以及日常生活中普遍存在,其运动特性的研究在力学界已经开展了几十年.在近20年开展的一系列小尺寸物理实验中,颗粒物质表现出许多新奇现象,人们从物理角度开展了系统研究,在统计力学中,颗粒固体的流体动力学等理论研究以及实验检测技术等方面都取得突破性进展,深刻地揭示了颗粒材料的物理机制,促使力学界对颗粒物质研究中原有的概念、假设、模型和理论进行重新认识.本文主要介绍颗粒物质的细观结构特点,以Rowe应力剪胀关系为例的宏细观研究思路,非线性、非弹性本构关系的细观推导,宏观强度与颗粒摩擦系数的关系,尺寸效应和相似关系等基础问题,介绍了岩土颗粒力学的新进展,介绍了常用的实验量测手段.基于我们的经验,提出了今后需要加强的几个方面:如分析颗粒离散元理论和计算细节;开展理想实验来验证理论和计算结果;结合迅速发展的高性能计算能力,开展高科学置信度大规模计算.目前,岩土类颗粒物质的宏细观研究尚处于起步阶段,到达实际工程应用还有很长的路要走;相关的工程技术要取得实质进展,还需依赖颗粒物质等基础研究的突破.

颗粒物质与颗粒流

颗粒物质研究是当前物理学研究的热点之一.在我国,对颗粒物质的研究才刚刚起步.本文主要是简单介绍颗粒物质的基本概念以及颗粒物质研究对工农业生产的意义,同时介绍国内外最近几年在颗粒物质方面的研究进展,再报道作者在颗粒物质研究方面的一些初步结果.

刚性块体压入颗粒体系时的受力及力链演变

颗粒体系由大量离散颗粒组成,普遍存在于自然界中,比如砂土地基、泥石流及滑坡体等。外荷载通常沿着准直形的路径在体系内传递,形成区别于单个颗粒和整个体系的细观结构——力链,亦即颗粒物质具有典型的多尺度特征。采用颗粒离散元方法模拟刚性块体压入颗粒体系的过程,计算得到了刚性块体底部所受阻力与压入深度的幂率关系;给出力链的判断准则和搜索强力链程序,构建颗粒体系强力链网络,并分析了压入试验过程中力链的演变及其长度的分布规律。