The flow rate of granular materials through an orifice

The flow rate of grains through large orifices is known to be dependent on its diameter to a 5/2 power law. This relationship has been checked for big outlet sizes, whereas an empirical fitting parameter is needed to reproduce the behavior for small openings. In this work, we provide experimental data and numerical simulations covering a wide span of outlet sizes, both in three- and two-dimensions. This allows us to show that the laws that are usually employed are satisfactory only if a small range of openings is considered. We propose a new law for the mass flow rate of grains that correctly reproduces the data for all the orifice sizes, including the behaviors for very large and very small outlet sizes.     

A note on the velocity of granular flow down a bumpy inclined plane

The velocity distribution of granular flow down a bumpy inclined plane is theoretically studied. The characteristic length scale of local transient cluster plays an important role in determining the flow rheology. After discussing the factors influencing the cluster size, we reproduce all observed velocity distributions successfully.This research was supported by the National Key Basic Research and Development Foundation of the Ministry of Science and Technology of China No. G2000048702.     

Cross-sectional and axial flow characteristics of dry granular material in rotating drums

Cross-sectional and axial flow behaviors of dry granular material in rotating drums are closely related to the dynamic characteristics and velocity distributions between the surface layer and bed material. In this study, both 2D and 3D dry granular flow patterns in horizontal rotating drums are experimentally investigated with flow imaging analysis. A dimensionless flow parameter combining the effects of Froude number, relative particle size and volume filling is proposed in this study, which controls the flow characteristics in a rational drum such as dynamic angle of repose, thickness of the flowing layer, relative free surface velocity, and the shear rates in the flowing layer. The dimensionless granular temperature exhibits linear distribution in the flowing layer, being maximum at the free surface and being negligible at the interface in the rolling regime. The measured shear rate of the plug flow departs from drum angular velocity under the wall slip conditions when the drum surface is smooth. Due to the existence of axial convection and lateral surface profile, the mass flux in the flowing layer is always less than that of the plug flow in the 3D granular flows based on sidewall particle images. One the other hand, the mass flux in the flowing layer is always equal or greater than that of the plug flow in the 2D granular flows. 2D granular flows exhibit higher angles of repose and surface velocities than those of the 3D granular flows at the same volume fillings.     

基于磁流体独有特性的各种潜在传感器

详细分析磁流体所具有的磁通门原理、粘度智能性、液体流动性、可浸泡性、磁光效应等各种独有特性,探讨基于上述特性的潜在传感机理及应用方向,将上述特性单独或组合使用,将可以开发出各类新型磁流体传感器,具有用于倾斜、速度、加速度、体积、流量、非磁性体或磁流体密度、磁场以及磁流体磁化强度等传感的潜在可能性,对磁流体传感器研究具有指导意义.     

The dynamics of granular flow in an hourglass

We present experimental investigations of flow in an hourglass with a slowly narrowing elongated stem. The primary concern is the interaction between grains and air. For large grains the flow is steady. For smaller grains we find a relaxation oscillation (ticking) due to the counterflow of air, as previously reported by Wu et al. [Phys. Rev. Lett. 71, 1363 (1993)]. In addition, we find that the air/grain interface in the stem is either stationary or propagating depending on the average grain diameter. In particular, a propagating interface results in power-law relaxation, as opposed to exponential relaxation for a stationary interface. We present a simple model to explain this effect. We also investigate the long-time properties of the relaxation flow and find, contrary to expectations, that the relaxation time scale is remarkably constant. Finally, we subject the system to transverse vibrations of maximum acceleration Γ. Contrary to results for non-ticking flows, the average flow rate increases with Γ. Also, the relaxation period becomes shorter, probably due to the larger effective permeability induced by the vibrations. Received: 11 February 2000     

Steady-state granular flow in a 3D cylindrical hopper with flat bottom: macroscopic analysis

The information of a hopper flow at a particle scale, obtained from discrete particle simulation, is used to investigate the macroscopic dynamic behaviour of granular flow in a cylindrical hopper with flat bottom by means of an averaging technique. The macroscopic properties including velocity, mass density, stress and couple stress are quantified under the cylindrical coordinate framework, and an effort is made to link these variables to the microscopic variables considered. The velocity and density distributions are first illustrated to match qualitatively the experimental and numerical results, confirming the validity of the proposed averaging method. Four components of stress, T_zz, T_rr, T_rz and T_zr, and two dominant components of couple stress, M_{r θ} and M_{z θ}, are then investigated in detail. It is shown that large vertical normal stress is mainly observed in the region close to the bottom corner, large radial normal stress is observed within the particle bed as well as the bottom corner, and large shear stresses in the region adjacent to the vertical wall. The four stresses are relatively small in a region close to the orifice. Their magnitudes are mainly contributed by the interaction forces between particles and between particles and walls. However, the transport of particles also plays a significant role at the orifice, especially, in the vertical normal stress. The couple stress can be ignored except for the regions close to the vertical and bottom walls, where the most dominant components are M_{r θ} adjacent to the vertical wall and M_{z θ} close to the bottom wall. The magnitudes of these macroscopic variables depend on the geometric and physical parameters of the hopper and particles such as the orifice size and wall roughness of the hopper, and the friction and damping coefficients between particles although their spatial distributions are similar.     

Application of a cellular automaton to simulations of granular flow in silos

A cellular automaton based on a gas model of hydrodynamics was used to calculate the kinematics of non-cohesive granular materials during confined flow in a mass flow and funnel flow model silo. In the model, collisions of particles were taken into account during granular flow. In addition, a simplified automaton was used wherein granular flow was assumed as an upward propagation of holes through a lattice composed of cells representing single particles. The advantages and disadvantages of both cellular automata were outlined.     

Regimes of segregation and mixing in combined size and density granular systems: an experimental study

Granular segregation in a rotating tumbler occurs due to differences in either particle size or density, which are often varied individually while the other is held constant. Both cases present theoretical challenges; even more challenging, however, is the case where density and size segregation may compete or reinforce each other. The number of studies addressing this situation is small. Here we present an experimental study of how the combination of size and density of the granular material affects mixing and segregation. Digital images are obtained of experiments performed in a half-filled quasi-2D circular tumbler using a bi-disperse mixture of equal volumes of different sizes of steel and glass beads. For particle size and density combinations where percolation and buoyancy both contribute to segregation, either radial streaks or a “classical” core can occur, depending on the particle size ratio. For particle combinations where percolation and buoyancy oppose one another, there is a transition between a core composed of denser beads to a core composed of smaller beads. Mixing can be achieved instead of segregation if the denser beads are also bigger and if the ratio of particle size is greater than the ratio of particle density. Temporal evolution of these segregated patterns is quantified in terms of a “segregation index” (based on the area of the segregated pattern) and a “shape index” (based on the area and perimeter of the segregated pattern).     

Velocity depth profile of granular matter in a horizontal rotating drum

Using MRI velocimetry, we verify that the velocity depth profile of the flowing layer near the axial center of a half-filled 3D drum has the form V _m[1 − (r/r ₀)²]-Ω r, where r is the depth measured from the cylinder center, except very close to the free surface where it lies below the quadratic form. We confirm that this deviation is due in part to particles reaching the surface with large components of their velocity in the azimuthal direction. We used a 3D cylinder with a radial “paddle” placed at approximately the dynamic angle of repose, covering the top third of the flow, so as to null any azimuthal velocity. It was found that the deviation from the quadratic form was reduced by the presence of the paddle when the comparison is made at the same solid body rotation rate, at the same free surface velocity, and with the paddle placed at different positions, so long as it makes good contact with the surface. Thus, we conclude that a quadratic velocity depth profile may be a fundamental property of granular shear flows in this geometry, when the sole effect of the cylinder rotation is to transport the particles from the end of the flow to the beginning without imparting velocity perpendicular to the flow.     

Cellular automata model of gravity-driven granular flows

A cellular automata model is used to simulate a variety of granular chute flows. The model is tested against several case studies: flow down a chute, flow past an obstacle, chute flow in which complex, counter-rotating vortices result in streamwise surface stripes and flow near a boundary. The model successfully reproduces experimental observations in all of these cases. These results lead us to propose that simple, rule-based, models such as this can improve our detailed understanding of dynamics and flow within an opaque granular bed.     

Discrete-particle simulations of cohesive granular flow using a square-well potential

In the present study, rapid granular flows with attractive inter-particle forces are investigated. In particular, cohesive forces are incorporated into hard-sphere (molecular dynamics) simulations via a square-well potential. The square-well potential treats cohesive forces as both binary and instantaneous. For simple shear flows, an investigation of the input parameter space indicates that two distinct flow regimes are present. For relatively large cohesive forces, the formation of a large, single agglomerate is observed. For moderate cohesive forces, the sheared system is composed of mostly 2-particle, dynamic agglomerates that are fairly evenly distributed throughout the domain. Furthermore, the results for this latter regime indicate that cohesion attenuates the magnitude of the stress components at higher solids fractions (in the collisional regime) as compared to the non-cohesive case. At lower solids fractions (kinetic regime), however the presence of cohesive forces has little impact on the observed stress.The authors would like to thank the U.S. Department of Education GAANN Program in Microparticle and Nanoparticle Technology (Grant No. P2004980454) and the U.S. Department of Energy National Energy Technology Laboratory (via subcontract from Ames National Laboratory) for funding support. The Ames Laboratory is operated for the Department of Energy by Iowa State University under Contract No. W-7405-ENG82.     

Effects of end wall friction in rotating cylinder granular flow experiments

Dry granular flows in half-filled horizontal rotating cylinders were studied by NMR to ascertain end wall effects. We measured the velocity and density profiles in planes perpendicular to the axis of the drums for three cases: (1) Near the end wall of a long ``3D' cylinder, (2) Near the middle of the long cylinder and, (3) For the whole of a short ``2D' cylinder. The velocity near the free surface is fastest for the short cylinder and slowest near the end wall of the long cylinder. We believe that these differences are due to friction between the beads and the end walls, speeding up the flow for the 2D drum and reducing the near-wall speed for the 3D drum.     

Chemical sensors based on nano-polymer films

A procedure is described for formation of a nano-structured organic-organic interface with increased conductivity at the interface of two polymer films. It is established that conductivity is determined by the type and degree of external action. It is shown that on the basis of this interface it is possible to create chemical sensors, and in fact sensors for the relative humidity of air, ethanol vapor pressure, and pH readings. Translated from Izmeritel’naya Tekhnika, No. 4, pp. 62–64, April, 2009.     

Dynamics of a gas bubble rising through a thin immersed layer of granular material: an experimental study

 Packings of non-cohesive grains, immersed in a fluid, differ significantly from classical porous media as the grains, subjected to stresses and flows, can move within the sample, changing then the local properties of the material. We study experimentally the conditions for a gas to pass through a layer of immersed granular material. Above a threshold pressure, which depends mainly on the grains size and on the surface free energy of the liquid-gas interface, the gas creates a channel within the whole thickness of the layer. Received: 23 January 2002     

Hydrodynamics of rapid granular flow of inelastic particles into vacuum

The problem of expansion of a dilute granular gas consisting of smooth, inelastic hard spheres into vacuum was investigated by three different methods: both (1) analytical and (2) computational (CFD) treatments of a hydrodynamic model, and (3) by Discrete Element Method (DEM) simulation. Furthermore, the systems were followed for long times, over which the granular kinetic energy decreases by several hundredfold. The hydrodynamic model assumes that the particles are uniformly distributed in space, yet, in the DEM simulations, the particles are free to cluster. Thus the comparison allows us to evaluate the effects of cluster formation on the system. All three methods give quantitatively similar results for the escape momentum, energy evolution, and for the hydrodynamic velocity distribution, even for restitution coefficients as small as e=0.8. The maximum deviation between the escape momentum computed from the hydrodynamic model and from DEM is shown to be no more than 4%. This means that cluster formation exerts only a relatively minor effect on the hydrodynamic quantities, suggesting to us that the hydrodynamic model in combination with the CFD algorithm may provide an efficient tool in other related problems. In addition, it is one of few cases where hydrodynamic theory and DEM agree quantitatively. Received: 17 June 2002     

Effect of energy nonequipartition on the transport properties in a granular mixture

The Boltzmann kinetic theory is used to analyze the effect of energy nonequipartition on the pressure and the shear viscosity of a granular binary mixture under simple shear flow. Theory and Monte Carlo simulations show that both quantities exhibit a non–monotonic behaviour with the mass ratio in contrast to the predictions made from previous theories based on the equipartition assumption. Our results agree qualitatively well with recent molecular dynamics simulations performed by Alam and Luding [Granular Matter 4, 139 (2002)]. The authors acknowledge partial support from the MCYT (Spain) through Grants No. BFM2001-0718 and ESP2003-02859.     

Electrical resistance drift of molybdenum silicide thin film temperature sensors

For a thermometer to be of practical use, its accuracy of temperature indication must be within a tolerable range. In this paper, patterned molybdenum disilicide (MoSi₂) thin film temperature sensors were fabricated to study their thermoresistance, i.e. resistance vs. temperature (R-T) characteristics. The R-T characteristic of MoSi₂ thin films exhibits a positive deviation from linearity (termed “superlinearity”) instead of showing a simple linearity as for most metals. This superlinear behavior was attributed to thermal expansion and the consequent decrease in the Debye characteristic temperature of MoSi₂. For long-term duration at elevated temperatures, the variation in thickness and composition of the sensor film due to oxidation and other factors may produce drift in the electrical resistance. In this study, the electrical resistance drifts of the sensors as a function of time at temperatures of 1200, 1300 and 1350 °C are presented. For the sensor film tested at 1300 °C, the resistance drift due to the thickness change of the sensor layer was well corrected with the help of an analysis of the oxidation rate of the sensor material. On the other hand, the in-depth composition profile analyzed by Auger electron spectroscopy (AES) indicated no significant composition variation, implying that we could neglect the correction factor for the composition variation in the present study. After the thickness factor was corrected for, a minor drift was still observed; this was also found for the same sensor film tested in an Ar ambient. The exact source of the minor drift is not well understood; further investigations are required.     

Atmosphere analyzer based on an array of resistor gas sensors

The possibilities of an array made from 24 sensors are studied in order to determine a selection of vapors from organic reagents: ethyl, methyl, isopropyl alcohols, acetone, benzene, ether, gasoline, etc. It is shown that in spite of the absence of special selection of sensors and their working temperatures it is possible to distinguish reagents with sufficient confidence. __________ Translated from Izmeritel’naya Tekhnika, No. 2, pp. 59–61, February, 2006.     

Health monitoring in continuous glass fibre reinforced thermoplastics: Manufacturing and application of interphase sensors based on carbon nanotubes

In this study, a new approach for health monitoring of GF reinforced composites is presented by incorporating percolated carbon nanotubes (CNTs) into the composites interphase. This is achieved applying CNT-filled coatings to glass fibres (GF). Taking advantage of the electrical properties of CNTs, this allows a highly localized monitoring of the composites interphase.