Monodisperse dry granular flows on inclined planes: Role of roughness

Recent studies have pointed out the importance of the basal friction on the dynamics of granular flows. We present experimental results on the influence of the roughness of the inclined plane on the dynamics of a monodisperse dry granular flow. We found experimentally that there exists a maximum of the friction for a given relative roughness. This maximum is shown to be independent of the slope angle. This behavior is observed for four planes with different bump sizes (given by the size of the beads glued on the plane) from 200 m to 2 mm. The relative roughness corresponding to the maximum of the friction can be predicted with a geometrical model of stability of one single bead on the plane. The main parameters are the size of the bumps and the size of the flowing beads. In order to obtain a higher precision, the model also takes into account the spacing between the bumps of the rough plane. Experimental results and model are in good agreement for all the planes we studied. Other parameters, like the sphericity of the beads, or irregularities in the thickness of the layer of glued particles, are shown to be of influence on the friction.     

圆截面直流道中微粒黏弹性聚焦机理研究

微粒黏弹性聚焦技术近年来受到了广泛的研究重视, 但影响粒子聚焦特性的关键参数调控机理仍不清楚. 基于此目的, 本文量化研究了圆截面直流道中非牛顿流体诱导微粒黏弹性聚焦的行为, 给出了流速和流道长度对粒子聚焦特性的调控机理. 具体而言: 首先, 对比分析不同黏度牛顿流体(水和22 wt%甘油水溶液)和非牛顿流体(8 wt%聚乙烯吡咯烷酮水溶液)中粒子横向迁移行为, 发现非牛顿流体中粒子将在弹性力主导下聚焦至流道中心区域, 而牛顿流体中粒子则在惯性升力主导下迁移形成Segré-Silberberg圆环. 其次, 量化分析粒子尺寸和驱动流速对黏弹性聚焦效果的影响, 发现随着流速的增加, 粒子聚焦效果逐渐变好并最终趋于稳定, 且大粒子较小粒子具有更好的聚焦效果. 最后, 研究粒子沿流道长度的动态聚焦过程, 推导并验证了粒子聚焦所需安全流道长度的数学模型, 发现大粒子聚焦所需安全流道长度显著短于小粒子. 上述研究结果对于提升粒子黏弹性聚焦机理和过程的理解, 实现微粒聚焦特性的灵活控制具有非常重要的意义.     

Density inversion in rapid granular flows: the supported regime

This paper presents numerical findings on rapid 2D and 3D granular flows on a bumpy base. In the supported regime studied here, a strongly sheared, dilute and agitated layer spontaneously appears at the base of the flow and supports a compact packing of grains moving as a whole. In this regime, the flow behaves like a sliding block on the bumpy base. In particular, for flows on a horizontal base, the average velocity decreases linearly in time and the average kinetic energy decreases linearly with the travelled distance, those features being characteristic of solid-like friction. This allows us to define and measure an effective friction coefficient, which is independent of the mass and velocity of the flow. This coefficient only loosely depends on the value of the micromechanical friction coefficient whereas the infuence of the bumpiness of the base is strong. We give evidence that this dilute and agitated layer does not result in significantly less friction. Finally, we show that a steady regime of supported flows can exist on inclines whose angle is carefully chosen.     

Improving mixture of grains by using bi-dimensional Galton boards

This paper presents an experimental and numerical study that deals with the problem of mixing grains falling down through a bi-dimensional Galton board (BGB). The special issue addressed here is the influence of the presence of lateral walls in the BGB. Disks of equal diameters but different species are launched from the top of the device. During the fall, disks collide with obstacles (arranged to form a triangular lattice) and with the lateral walls. The exit distribution of particles at the bottom of the board is determined and the incidence of the presence of walls in the mixing quality is studied as a function of W the relative separation between lateral walls. Two types of indexes are evaluated to characterize the efficiency in the obtained mixture. The presence of walls has proven to be crucial to enhance the quality of the mixture of particles.     

非对称弯曲微流道中粒子惯性聚焦动态过程及流速调控机理研究

设计制作了一种具有非对称弯曲微流道结构的微流控芯片, 搭建实验平台定量表征聚苯乙烯粒子和血细胞沿流道的动态惯性聚焦过程, 并系统研究了流体流速和粒子尺寸对粒子聚焦特性的调控机理. 通过分析粒子荧光图谱和对应量化强度曲线, 将粒子沿流道长度的横向迁移过程分为形成聚焦和平衡位置调整两个阶段, 指出在整个聚焦过程中具有小曲率半径的流道结构起主导作用. 根据全流速段内粒子聚焦特性的演变, 重点分析潜在惯性升力和Dean 曳力的竞争机制, 提出了阐述粒子聚焦流速调控过程的三阶段模型. 进一步比较两种尺寸粒子聚焦位置和聚焦率随流速与流道长度的变化规律, 发现大粒子具有更好的聚焦效果和稳定性, 且两种粒子的相对位置可通过流速进行调整. 最后, 通过分析血细胞在非对称弯流道中的横向迁移特性, 验证了粒子惯性聚焦机理在复杂生物粒子操控方面的适用性. 上述结论为深入研究微流体环境下粒子的运动特性以及开发微流式细胞术等临床即时诊断器件提供了重要参考.     

Interrupted shear of granular media

The response of a granular material during a stop-and-go shear experiment is investigated using an annular shear cell and silicagel powders of different particle sizes. The experimental results are examined on the basis of the Dieterich-Rice-Ruina model for solid friction. In addition to making this analogy with solid friction, we describe a new instability that is observed when restarting shear, where the powder bed is found to slip and compact for short hold times but only dilates for long hold times. The minimum hold time to restore a non-slip behaviour has been investigated for different size particles and normal loadings. The observed dependencies show analogies between this behaviour and the sliding rearrangements seen above the stick-slip threshold.     

Various flows of black and grey particles produced in Mg-emulsion collisions at 4.5 A GeV/c

Various flow phenomena of black particles(b-particles)and grey particles(g-particles)produced in magnesium-emulsion(Mg-Em)collisions at 4.5 A GeV/c are reported.These flows are directed and elliptic transverse flows(vl and v2)related by the azimuthal angle((φ)),directed and elliptic reaction plane flows(vR1 and vR2)related by the projected angle(ψ)on the reaction plane,and directed and elliptic polar direction flows(vp1 and vp2)related by the polar angle((υ)).We extract absolute flows as the direct experimental values minus the isotropic theoretical values.The dependence of the various flows on the target particle multiplicity and on the angles((υ,φ,ψ))is investigated.Our results show that the dependence of b-particle flows on the target size is obvious and for heavy targets the dependence on target particle multiplicity is slight.Compared with b-particles,g-particles have a slight dependence on the target size and target particle multiplicity.     

Numerical Simulation of Random Close Packings in Particle Deformation from Spheres to Cubes

Variation of packing density in particle deforming from spheres to cubes is studied. A new model is presented to describe particle deformation between different particle shapes. Deformation is simulated by relative motion of component spheres in the sphere assembly model of a particle. Random close packings of particles in deformation form spheres to cubes are simulated with an improved relaxation algorithm. Packings in both 2D and 3D cases are simulated. With the simulations, we find that the packing density increases while the particle sphericity decreases in the deformation. Spheres and cubes give the minimum (0.6404) and maximum (0.7755) of packing density in the deformation respectively. In each deforming step, packings starting from a random configuration and from the final packing of last deforming step are both simulated. The packing density in the latter case is larger than the former in two dimensions, but is smaller in three dimensions. The deformation model can be applied to other particle shapes as well.     

Dispersive flow of disks through a two-dimensional Galton board

We report here an experimental and numerical study of the flow properties of disks driven by gravity through a hexagonal lattice of obstacles, i.e. a Galton board. During the fall, particles experience dissipative collisions that scatter them in random directions. A driven-diffusion regime can be achieved under certain conditions. A characteristic length of the motion and its dependence on geometrical parameters of the system is analyzed in the steady regime. The influence of collective effects on the dispersion process is investigated by comparison between single- and many-particle flows. The characterization of the dynamics and the diffusive properties of the flow in a system like a Galton board can be expanded to other granular systems, particularly static solid particle mixers and will give some insight in understanding granular mixing.     

Statistics of particle pair relative velocity in the homogeneous shear flow

Small scale clustering of inertial particles and relative velocity of particle pairs have been fully characterized for statistically steady homogeneous isotropic flows. Depending on the particle Stokes relaxation time, the spatial distribution of the disperse phase results in a multi-scale manifold characterized by local particle concentration and voids and, because of finite inertia, the two nearby particles have high probability to exhibit large relative velocities. Both effects might explain the speed-up of particle collision rate in turbulent flows. Recently it has been shown that the large scale geometry of the flow plays a crucial role in organizing small scale particle clusters. For instance, a mean shear preferentially orients particle patterns. In this case, depending on the Stokes time, anisotropic clustering may occur even in the inertial range of scales where the turbulent fluctuations which drive the particles have already recovered isotropy. Here we consider the statistics of particle pair relative velocity in the homogeneous shear flow, the prototypical flow which manifests anisotropic clustering at small scales. We show that the mean shear, by imprinting anisotropy on the large scale velocity fluctuations, dramatically affects the particle relative velocity distribution even in the range of small scales where the anisotropic mechanisms of turbulent kinetic energy production are sub-dominant with respect to the inertial energy transfer which drives the carrier fluid velocity towards isotropy. We find that the particles’ populations which manifest strong anisotropy in their relative velocities are the same which exhibit small scale clustering. In contrast to any Kolmogorov-like picture of turbulent transport these phenomena may persist even below the smallest dissipative scales where the residual level of anisotropy may eventually blow-up. The observed anisotropy of particle relative velocity and spatial configuration is suggested to influence the directionality of the collision probability, as inferred on the basis of the so-called “ghost collision” model.     

Pore Stabilization in Cohesive Granular Systems

Cohesive powders tend to form porous aggregates which can be compacted by applying an external pressure. This process is modelled using the Contact Dynamics method supplemented with a cohesion law and rolling friction. Starting with ballistic deposits of varying density, we investigate how the porosity of the compacted sample depends on the cohesion strength and the friction coefficients. This allows to explain different pore stabilization mechanisms. The final porosity depends on the cohesion force scaled by the external pressure and on the lateral distance between branches of the ballistic deposit. Even if cohesion is switched off, pores can be stabilized by Coulomb friction alone. This effect is weak for round particles, as long as the friction coefficient is smaller than 1. However, for non-spherical particles the effect is much stronger.     

Effect of real-time information upon traffic flows on crossing roads

The effect of real-time information on the traffic flows of the crossing roads is studied by simulations based on a cellular automaton model. At the intersection, drivers have to enter a road of a shorter trip-time, by making a turn if necessary, as indicated on the information board. Dynamics of the traffic are expressed as a return map in the density-flow space. The traffic flow is classified into six phases, as a function of the car density. It is found that such a behavior of drivers induces too much concentration of cars on one road and, as a result, causes oscillation of the flow and the density of cars on both roads. The oscillation usually results in a reduced total flow, except for the cases of high car density.     

Effect of gravity on the development of homogeneous shear turbulence laden with finite-size particles

Fully resolved simulations of homogeneous shear turbulence (HST) laden with sedimenting spherical particles of finite size have been performed to clarify the effects of gravity on the development of particle-laden turbulent shear flows. We consider turbulence in a horizontal flow subjected to vertical or horizontal shear. Numerical results show that the development of HST laden with finite-size particles are significantly altered by gravity. The effects of gravity lead to a slower increase in the Taylor-microscale Reynolds number, whose value is found to be well correlated with the average particle Reynolds number. The gravity also causes a slower increase in the turbulence kinetic energy (TKE) through the enhancement of energy dissipation. The change in the Reynolds shear stress (RSS) due to particles also significantly contributes to the relative change in TKE. In vertically sheared cases, RSS has high values between counter-rotating trailing vortices behind the particles, which causes a transient relative increase in TKE. In horizontally sheared cases, on the other hand, RSS is reduced in the wakes of particles, which contributes to a significant relative reduction in TKE.     

Strength and failure of cemented granular matter

Cemented granular materials (CGMs) consist of densely packed solid particles and a pore-filling solid matrix sticking to the particles. We use a sub-particle lattice discretization method to investigate the particle-scale origins of strength and failure properties of CGMs. We show that jamming of the particles leads to highly inhomogeneous stress fields. The stress probability density functions are increasingly wider for a decreasing matrix volume fraction, the stresses being more and more concentrated in the interparticle contact zones with an exponential distribution as in cohesionless granular media. Under uniaxial loading, pronounced asymmetry can occur between tension and compression both in strength and in the initial stiffness as a result of the presence of bare contacts (with no matrix interposed) between the particles. Damage growth is analyzed by considering the evolution of stiffness degradation and the number of broken bonds in the particle phase. A brutal degradation appears in tension as a consequence of brittle fracture in contrast to the more progressive nature of damage growth in compression. We also carry out a detailed parametric study in order to assess the combined influence of the matrix volume fraction and particle-matrix adherence. Three regimes of crack propagation can be distinguished corresponding to no particle damage, particle abrasion and particle fragmentation, respectively. We find that particle damage scales well with the relative toughness of the particle-matrix interface with respect to the particle toughness. This relative toughness is a function of both matrix volume fraction and particle-matrix adherence and it appears therefore to be the unique control parameter governing transition from soft to hard behavior.     

Initiation and evolution of current ripples on a flat sand bed under turbulent water flow

We investigate the formation and dynamics of sand ripples under a turbulent water flow. Our experiments were conducted in an open flume with spherical glass beads between 100 and 500μm in diameter. The flow Reynolds number is of the order of 10 000 and the particle Reynolds number of the order of 1 to 10. We study the development of ripples by measuring their wavelength and amplitude in course of time and investigate the influence of the grain size and the flow properties. In particular, we demonstrate two different regimes according to the grain size. For fine grains, a slow coarsening process (i.e., a logarithmic increase of the wavelength and amplitude) takes place, while for coarser grains, this process occurs at a much faster rate (i.e., with a linear growth) and stops after a finite time. In the later case, a stable pattern is eventually observed. Besides, we carefully analyze the wavelength of ripples in the first stages of the instability as a function of the grain size and the shear velocity of the flow, and compare our results with other available experimental data and with theoretical predictions based on linear stability analyses.     

Segregation induced instabilities of granular fronts

Experimental investigation of granular flows containing particles of several sizes and moving down slopes shows that segregation of coarse-grained, irregularly shaped particles induces a fingering instability at the propagating front. The size-segregation mechanism involves percolation of small particles downward and a corresponding migration of large ones toward the flow surface. Large particles at the flow surface experience velocities that are greater than average so that they migrate forward and begin to collect at the flow front. In the case of dry cohesionless flows, the instability depends upon these large particles at the flow perimeter being more angular and thus more resistant to flow than the smaller rounder ones in the interior. A simple analytical model predicts the fingering instability when friction of the flow front is greater than that of the following flow. The presence of viscous liquid inhibits both size-segregation and the development of the instability. Fluidization of dry flows permits segregation of large particles to flow perimeters, thus increasing permeability and permitting a similar instability that owes its development to the dry frictional perimeter that surrounds a partly fluidized interior. (c) 1999 American Institute of Physics.     

Ripple formation in weakly turbulent flow

The formation of granular ripples under liquid shear flow in an annular channel is studied experimentally. The erodible granular bed is subject to weakly turbulent flows without a defined sharp boundary layer close to the granular bed. The flow field and the degree of turbulence is characterized quantitatively by using a particle image velocimeter and a laser-Doppler velocimeter, respectively. A new range of particle Reynolds numbers at the lower limit of the Shields diagram were explored. Quantitative measurements of the granular flow on the surface reveal that the threshold for particle motion coincides within the order of one percent with the threshold for ripple formation. In fully developed ripples it was found that on the leeward side of the ripples regions of low-velocity gradients exist where granular motion is scarce, indicating that the coupling between the ripples is mainly caused by the flow field of the liquid.     

Mechanics and Friction at the Nanometer Scale

In this overview, we will give an introduction to experiments in which manipulation is used a means of uncovering the intrinsic response and dynamical behavior of small objects. Experiments done on individual particles reveal new and rich behaviors that are inaccessible to averaging methods. Experiments exploring the stiffness and toughness of carbon nanotubes will be presented showing that nanometer scale engineered materials can far outperform current engineering materials. Through AFM manipulation, imaging and force measurements, the stiffness of this material was found to equal or exceed diamond. Their toughness is also extraordinary. Due to their near crystalline perfection, carbon nanotubes are able to undergo strains exceeding 15% during bending without damage. Through AFM manipulation experiments, these large deformations have been shown to be highly reversible. Experiments in which the lateral force of manipulation of small objects across surfaces is measured show that friction at the nanometer scale occurs without wear processes and is an intrinsic property of the particular interface. Results are also presented showing anisotropic behavior in friction and movement due to commensurate lattice effects. At the nanometer scale, the contacting surfaces can be nearly perfect so that commensurate effects are not partially averaged out by many differently oriented domains. It has been shown that friction can very over an order of magnitude depending on the relative orientation of the contacting surfaces. The relative orientation of object and substrate lattices also can determine the modes of motion. In some cases the particle is confined to move in one direction. In other cases the relative orientation determines whether the particle rolls, rotates in-plane or slides. These effects may have implications on the fundamental mechanisms of friction. They provide a laboratory for testing different geometrical configurations of atoms sliding on atoms. The results may also have implications in the design of nanometer scale electromechanical mechanisms.     

Stress distribution of faceted particles in a silo after its partial discharge

We present experimental and numerical results of the effect that a partial discharge has on the morphological and micro-mechanical properties of non-spherical, convex particles in a silo. The comparison of the particle orientation after filling the silo and its subsequent partial discharge reveals important shear-induced orientation, which affects stress propagation. For elongated particles, the flow induces an increase in the packing disorder which leads to a reduction of the vertical stress propagation developed during the deposit generated prior to the partial discharge. For square particles, the flow favors particle alignment with the lateral walls promoting a behavior opposite to the one of the elongated particles: vertical force transmission, parallel to gravity, is induced. Hence, for elongated particles the flow developed during the partial discharge of the silo leads to force saturation with depth whereas for squares the flow induces hindering of the force saturation observed during the silo filling.     

Numerical studies on velocity, temperature history and heat transfer to the particles injected into the argon plasma

A new approach to study the particle velocity in a thermal plasma in relation to input parameters (power, gas flow rate, injection velocity of the particle and particle size) and nozzle dimensions (nozzle length and diameter) has been made. Injected particle's temperature and thermal history were calculated for particles of three different materials (alumina, tungsten and graphite) in argon plasma. Allowable powder feed rate was calculated for the particles. Heat transfer per particle injected in to the plasma is reported. Liquid fraction of the particle after it reached the melting point is also reported. Particle velocity is found to increase with increase in power, gas flow rate and injection velocity and decrease with increase in particle size, nozzle length and nozzle diameter. Thermal histories of the particles in relation to the plasma temperature and particle diameter are presented. Particle's residence time is found to increase with increase in diameter of the particle. Allowable powder feed rate for complete melting of the particle is higher at higher percentage utilisation of the plasma power. Powder feed rate is seen to decrease with increase in particle size and it is higher for tungsten and lower for graphite particle. Heat transfer rate from plasma to particle is seen to decrease with increase in time and the same is higher for plasmas of higher temperature and smaller sized particle. Received 4 May 2000 and Received in final form 15 March 2001