Experimental and numerical study of cylindrical triaxial test on mono-sized glass beads under quasi-static loading condition

This paper aims at studying the shear behavior of homogeneous granular materials by conventional triaxial test. The work is performed both in laboratory tests and by discrete element method simulations. Conventional triaxial tests are performed on glass beads packing, while a cylindrical rigid wall boundary condition based on lame formula and a series of procedures are proposed to simulate the conventional triaxial test. The experimental results on dry and saturated glass beads samples have been studied to find out the effect of saturation condition on the shear behavior. The comparisons between experimental and numerical results show that the numerical model can reproduce deviatoric curves satisfactorily in experimental conditions as long as experimental sample remains cylindrical. It correctly describes the volumetric strains of a numerical sample up to the peak value. Additionally, a parametric study on the influence of main micromechanical parameters has been carried out, which has been compared to experimental tests with glass beads of different textures. The comparison highlights the significant effect of friction coefficients and rolling resistance coefficients on global behavior of granular materials.     

FE-investigations of micro-polar boundary conditions along interface between soil and structure

The interface between granular bodies and structures is analysed with the finite element method and a micro-polar hypoplastic constitutive model. Quasi-static shearing of an infinitely long and narrow granular strip between two rigid walls of different roughness under conditions of free dilatancy and constant vertical pressure is investigated. The constitutive model can reproduce the essential features of granular bodies during shear localization. To model the different roughness of the interface, micro-polar boundary conditions are proposed taking into account the asperity of the wall roughness and grain diameter. Some emphasis is given to the influence of the wall roughness on the thickness of shear zone and the mobilization of wall friction.     

Interface roughness effect on slow cyclic annular shear of granular materials

We experimentally investigate the mechanical behaviour in cyclic shear of a granular material near a solid wall in a pressure controlled annular shear cell. The use of a model system (glass beads and saw-tooth shaped solid surface) enables the study of the influence of the wall roughness. After an initial shakedown procedure ensuring reproducible results in subsequent tests, wall shear stress S, volumetric variation ΔV, and the displacement field of the sample bottom surface, are recorded as functions of wall displacement. A dimensionless roughness parameter R _n is shown to control the interface response. The local grain-level or mesoscale behaviour is directly correlated to the global one on the scale of the whole sample.     

润滑状态下线接触滑动粗糙界面的动摩擦特性研究

滑动粗糙界面的摩擦润滑特性对界面的润滑设计和润滑状态预测具有重要的理论和实际意义。本文通过建立不同润滑状态下的滑动粗糙界面模型,基于界面的法向载荷由润滑油膜和粗糙体共同承担的载荷分配思想,采用Greenwood-Williamson统计模型描述粗糙表面形貌,考虑界面润滑的时变效应和润滑油的粘-压特性,建立了线接触滑动粗糙界面的油膜厚度方程和粗糙体接触压力方程,获得了整个润滑区的润滑油膜载荷比例因子、油膜厚度和摩擦系数随滑动速度的变化关系,推导了界面由混合润滑过渡为液压润滑的临界速度关系表达式,分析了滑动粗糙界面的润滑承载机理,获得了界面油膜厚度、摩擦系数和临界速度随界面形貌参数、法向载荷、润滑油属性参数的变化规律,为机械结构的界面润滑设计、润滑状态预测和润滑优化提供理论和实验参考。     

FE-studies on rapid flow of bulk solids in silos

This paper contains results of numerical modelling of the onset of silo flow for granular material in a model silo with convergent walls. The calculations were performed with a finite element method based on a polar elasto-plastic constitutive relation by Mühlhaus. It differs from the conventional theory of plasticity by the presence of Cosserat rotations and couple stresses using a mean grain diameter as a characteristic length. The characteristic length causes that numerical results do not depend upon the mesh discretisation. The model tests on rapid silo flow of glass beads performed by Renner in a glass hopper with a large wall inclination from the bottom were numerically simulated. The FE-calculations were performed for plane strain by taking into account inertial forces and linear viscous damping. A satisfactory agreement between numerical and experimental results was obtained. In addition, the FE-calculations were performed for very rough walls. Advantages and limitations of a continuum approach for simulations of rapid silo flow were outlined. Received: 10 February 2001     

A study of gravity flow principles of sublevel caving method in dipping narrow veins

The loss and dilution of ore by sublevel caving mining method is influenced by gravity flow principles of caved granular materials (rocks). Gravity flow of broken rock (muck) in dipping narrow veins is complex. By combined use of dimensional analysis and similitude studies in physical modelling coupled with numerical simulation, this paper reports results of a study of gravity flow of muck in dipping narrow veins on the basis of granular material kinematics and dynamics. By similitude material testing, the paper analyzed the forms of granular motion during the extraction process, studies the impacts of parameters including model thickness and wall roughness on the form and size of the extraction zone. The test results reveal that depending on wall inclination (dip), the axis of granular flow shifts, and the extraction zone is intrusively asymmetric. Also, the greater the distance from the hangingwall of the model and the smoother the wall roughness, the easier the granular ore flows and the greater the volume of the extraction zone. By using PFC\(^{2D}\) modelling, the mechanical mechanism of granular ore flow is analyzed. The model results show that under conditions of dipping narrow veins, the formation and development of the extraction zone are mainly controlled by two mechanisms, namely: the disintegration of the stress arch and the erosion of granular particles. Finally, the study established a mechanistic model of granular flow in dipping narrow veins in sublevel caving mining conditions. By applying the Levenberg-Marquardt nonlinear optimization method the paper defined optimal model parameter values under different experimental conditions.     

Molecular dynamics simulation of the effect of surface roughness and pore on linear friction welding between Ni and Al

Atomic model of Ni–Al linear friction welding is established by molecular dynamics simulation. The effect of surface roughness on linear friction welding and the rules of pore closure are investigated. The simulations results show that rough surfaces are gradually flattened during the friction process. The rough surface of the harder material (Ni) may influence the final structure of the weld. Pores near the interface is closed after linear friction welding. Pore closure in Al slab occurs in friction process by atom diffusion, while it takes place in Ni slab in forging stage mainly by deformation of the interface.     

颗粒状粗糙元对可蚀性地表的保护作用

应用粗糙壁面附近湍流边界层的阻力分解规律,研究了可蚀性地表上散布的颗粒状粗糙元的防风蚀效应。认为按一定密度散布在地表的颗粒状粗糙元(例如粗沙和砾石等),对抑制风蚀起到了两个关键作用:其一,增大地表盖度以减少暴露面积;其二,削弱了暴露面积上的风阻力。基于这一认识,应用量纲分析原理,建立了地表风蚀率与边界层外风速、粗糙元分布密度之间关系的理论公式,并依据有关实验数据分析了公式的可靠性。     

粗糙壁面通道内的流动研究

通过对流动中熵产的数值计算,研究了管道和平面通道内流动中粗糙壁面对摩擦的影响.研究表明,在层流流动中壁面粗糙度的影响也是值得重视的,尽管这种效应以往常常被人们所忽视.除了可得到摩擦因子之外,由于可以得知流场中壁面粗糙单元附近的耗散分布,因此可深入理解这一物理现象的本质.为了对粗糙壁面通道内的流动进行简明的描述,需要对壁面位置和粗糙度参数进行合理的选择,并对各种选择方案进行了讨论和评价.     

Particle deposition in indoor environments: analysis of influencing factors

In this paper, several factors influencing particle deposition in indoor environments are analyzed with an analytical model and a three-dimensional drift flux model combined with the particle deposition boundary conditions for wall surfaces. The influences of flow conditions near the wall surfaces, surface roughness and particle concentration distribution on particle deposition indoors are studied. By modeling particle deposition onto surfaces with the analytical model, it is found that larger friction velocity near the wall surfaces and rougher surface may lead to larger particle deposition velocity when the particle size is small, but when particle size is large enough (the range is up to the actual friction velocity and in this study it is about 1-5 microm), the influence of the friction velocity and roughness could be neglected. Furthermore, the three-dimensional numerical simulations indicate that particle concentration distribution may be very different even for the same particle source and air change rate, which cause a different deposited particle flux. As the particle concentration distribution may not be uniform in most cases, especially for the ventilated rooms, it is important to incorporate particle concentration distribution when analyzing particle deposition in indoor environments. Some suggestions or rules for particle deposition controlling are also presented based on the analysis.     

粗糙表面固态耦合超声检测研究

固态耦合超声检测时两固体粗糙表面接触界面处的超声波不完全耦合,为提升该界面处超声检测的耦合效果,需深入研究其耦合特性。以粗糙表面的弹簧接触模型为基础,结合粗糙表面接触理论推导出固态耦合超声检测的耦合界面理论模型。根据实际情况以及材料参数分析,得到表征界面耦合效果的平均声反射系数与接触载荷以及接触表面当量粗糙度的关系,并与T模型的计算结果进行对比。在不同当量表面粗糙度和不同接触载荷下分别测得接触界面的平均声反射系数,并对比了不同当量表面粗糙度下界面声耦合效果达到最佳时接触载荷的理论值和实验值,计算得最大相对误差为13.04%,表明实验结果与所提出的理论模型结果基本吻合。所建立的固态耦合超声检测界面理论模型形式简洁,实用性强,并可通过针对性地控制相关参数来改善耦合效果。     

Finite element analysis of magnetic disk with a horizontal subsurface crack

Sliding contact of a rigid rough surface with a semi‐infinite medium including a horizontal subsurface crack was investigated by using linear elastic fracture mechanics and finite element method (FEM). The fractal geometry was used to characterize the rigid rough surface. The propagation of crack was studied with the shear and tensile stress intensity factors. The effect of surface roughness, crack length to depth ratio and friction at the contact and crack interfaces was investigated by using the FEM. It was shown that increasing friction coefficient at the contact interface increases both K_II and K_I.     

Calculations of the coefficient of friction under elastic contact conditions

Expressions for the friction coefficient were derived for the case of elastic contact between rough materials. The computation was based on the assumption that the friction force in the case under consideration is due to hysteresis losses during the deformation of a thin surface layer of one friction pair component by protuberances on the counterpart surface and due to molecular interaction at the solid/solid interface in the contact zones. The minimum value of the friction coefficient was calculated, and it was shown that it depends on the degree of surface roughness, physicomechanical properties of friction pair materials and the magnitude of tangential stresses ₀ at the solid/solid interface.     

颗粒间摩擦对颗粒流润滑影响的力学机制

颗粒流润滑是一种可用于苛刻工况环境的新型润滑方式,颗粒间摩擦对颗粒流润滑影响的规律和特性是阐明颗粒流润滑理论的关键科学问题,也可以为极端工况环境下颗粒流润滑轴承的设计和参数选择提供技术支持。为了分析颗粒间摩擦对颗粒流润滑系统宏、微观特性以及下摩擦副与颗粒润滑介质间摩擦的影响规律,构建了颗粒流润滑离散元数值模型,并对该问题进行了分析和研究。研究结果表明:颗粒间摩擦对颗粒流润滑系统的减摩润滑特性具有显著影响,下摩擦副和颗粒润滑介质之间的平均摩擦系数会随着颗粒间摩擦的增大而增大;颗粒流润滑系统内的微观配位数和滑动率均随着颗粒间摩擦系数的减小而增大;颗粒润滑介质在摩擦副间隙的宏观流动行为具有明显的分层特性,且宏观流动速度随着颗粒间摩擦的增大而减小,进一步的分析结果表明:颗粒润滑介质的波动速度是直接反映其宏观流动速度快慢的关键性参数。     

FE-investigation of shear localization in granular bodies under high shear rate

Shear localization in granular materials under high shear rate is analysed with the finite element method and a micro-polar hypoplastic constitutive model enhanced by viscous terms. We consider plane strain shearing of an infinitely long and narrow granular strip of initially dense sand between two very rough walls under conditions of free dilatancy. The constitutive model can reproduce the essential features of granular materials during shear localization. The calculations are performed under quasi-static and dynamic conditions with different shear rates. In dynamic regime, the viscosity terms are formulated based on a modified Newtonian fluid and according to the formula by Stadler and Buggisch (Proceedings of the conference on Reliable flow of particulate solids, EFCE Pub. Series, vol 49. Chr. Michelsen Institute, Bergen, 1985). Emphasis is given to the influence of inertial and viscous forces on the shear zone thickness and mobilized wall friction angle.     

The dynamics of avalanches of granular materials from initiation to runout. Part II. Experiments

Summary This paper describes a model to predict the flow of an initially stationary mass of cohesionsless granular material down a rough curved bed and checks it against laboratory experiments that were conducted with two different kinds of granular materials that are released from rest and travel in a chute consisting of a straight inclined section, a curved segment that is followed by a straight horizontal segment. This work is of interest in connection with the motion of landslides, rockfalls and ice and dense flow snow avalanches. Experiments were performed with two different granular materials, nearly spherical glass beads of 3 mm nominal diameter, Vestolen particles (a light plastic material) of lense type shape and 4 mm nominal diameter and 2,5 mm height. Piles of finite masses of these granular materials with various initial shapes and weight were released from rest in a 100 mm wide chute with the mentioned bent profile. The basal surface consisted of smooth PVC, but was in other experiments also coated with drawing paper and with sandpaper. The granular masses under motion were photographed and partly video filmed and thus the geometry of the avalanche was recorded as a function of position and time. For the two granular materials and for the three bed linings the angle of repose and the bed friction angle were determined. The experimental technique with which the laboratory avalanches were run are described in detail as is the reliability of the generated data. We present and use the depth-averaged field equations of balance of mass and linear momentum as presented by Savage and Hutter [28]. These are partial differential equations for the depth averaged streamwise velocity and the distribution of the avalanche depth and involve two phenomenological parameters, the internal angle of friction, ø, and a bed friction angle, , both as constitutive properties of Coulomb-type behaviour. We present the model but do not derive its equations. The numerical integration scheme for these equations is a Lagrangian finite difference scheme used earlier by Savage and Hutter [27],[28]. We present this scheme for completeness but do not discuss its peculiarities. Comparison of the theoretical results with experiments is commenced by discussing the implementation of the initial conditions. Observations indicate that with the onset of the motion a dilatation is involved that should be accomodated for in the definition of the initial conditions. Early studies of the temporal evolution of the trailing and leading edges of the granular avalanche indicate that their computed counterparts react sensitively to variations in the bed friction angle but not to those of the internal angle of friction. Furthermore, a weak velocity dependence of the bed friction angle, , is also scen to have a small, but negligible influence on these variables. We finally compare the experimental results with computational findings for many combinations of the masses of the granular materials and bed linings. It is found that the experimental results and the theoretical predictions agree satisfactorily. They thus validate the simple model equations that were proposed in Savage and Hutter [28].     

Stick–slip friction of gecko-mimetic flaps on smooth and rough surfaces

The discovery and understanding of gecko ‘frictional-adhesion’ adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick–slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick–slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick–slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems.     

NUMERICAL SIMULATION OF GRANULAR COUETTE FLOWS BETWEEN TWO ROUGH PARALLEL PLATES

This paper presents a numerical method for calculating the granular Couette flows between two parallel plates. A kinetic model which includes the frictional energy loss effects is employed, and the equations of motion are solved using a numerical iterative method. The boundary conditions are satisfied by ensuring the balance of momentum and energy at such boundaries. The mean velocity, the fluctuation kinetic energy and the solid volume fraction profiles are evaluated under a variety of conditions. The mean velocity profiles are compared with the molecular dynamic simulation results, and good agreement is observed. The study shows that the slip velocity may vary considerably depending on the surface roughness, coefficient of restitution and friction coefficient.     

Particle finite element analysis of the granular column collapse problem

The problem of granular column collapse is investigated by means of an axisymmetric version of the particle finite element method (PFEM). The granular medium is represented by a simple rate-independent plasticity model and the frictional contact between the granular flow and its rigid basal surface is accounted for. In the version of the PFEM developed for this study, the governing equations of the boundary value problem are cast in terms of an optimization problem and solved using mathematical programming tools. The agreement between model and experiment is generally satisfactory, quantitatively as well as qualitatively. However, the friction angle of the granular material, as well as the exact interface conditions between the base and granular material, are shown to have a relatively significant influence on the results.     

Experimental validation of a particle-based method for heat transfer incorporating interstitial gas conduction in dense granular flow using a rotary drum

Conductive heat transfer in granular material is important in many industrial processes. For dense systems where the materials have low thermal conductivity, much of the heat transfer will occur through interstitial gases. Industry requires flexible and efficient computational methods to capture these phenomena at scale. In this study, a recently proposed particle-based model that includes the contribution of the interstitial gases was validated using an experiment. This model was originally derived from a multi-scale analysis of static, random packings. To test this approach in dense, dynamic systems, the model results were compared to experimental data for glass beads in an indirectly heated rotating drum. Infrared (IR) thermography was used to track the temperature evolution of the glass beads and the drum wall temperature. Discrete element simulations were performed with the experimental wall temperature used as a transient wall boundary condition. Results from the simulation show good agreement with the experimental data both for the bulk average temperature and for the bed profile, demonstrating the model’s ability to capture the gas contribution in dynamic systems.