CiteULike: Tag grain

The effects of an impact velocity dependent coefficient of restitution on stresses developed by sheared granular materials

C Lun — 2008-03-11T20:36:32-00:00

Acta Mechanica, Vol. 63, No. 1. (29 November 1986), pp. 15-44.

Summary Following the granular flow kinetic theory of Lun, Savage, Jeffrey and Chepurniy, a moment method is used to obtain the approximate form for the single particle velocity distribution function for the case of smooth, slightly inelastic, uniform spherical particles in which the coefficient of restitutione depends upon the particle impact velocity. Constitutive equations for stress are derived and the theory is applied to the case of a simple shear flow. Theoretical predictions of stresses are compared with experimental results. The effect of the impact velocity dependente is to cause the stresses to vary with the shear rate raised to a power less than two; this is consistent with the experimental observations. On the basis of the present theory and comparisons with experimental data it is concluded that theoretical models which include both surface friction and an impact velocity dependente will lead to improved agreement between the theoretical predictions and the measurements.

Solids Movement in Rotary Kilns in the Slumping Regime: Model Using a Control Plane Parallel to the Steepest Descent

Luis Gonzalez — 2008-03-11T21:41:54-00:00

Particle & Particle Systems Characterization, Vol. 22, No. 2. (2005), pp. 119-132.

A new approach is proposed to model the bulk movement of solids in rotary drums operating at low rotation speeds, in slumping and rolling regimes. The model yields an equation similar to Saeman's equation, but which is valid also for the slumping regime and active area in the rolling regime.The model was developed for constant depth using a control surface containing the steepest descent direction, so that any contribution from sliding particles to flow rate can be neglected. By considering an appropriate virtual kiln, the model is extended to the more general, variable depth situation.

A new DPIV proceeding algorithm and its application in particle motion study in a rotary drum

Zhixiao Zhang — 2008-03-11T21:39:40-00:00

Journal of Thermal Science, Vol. 11, No. 2. (5 May 2002), pp. 186-192.

Abstract A new digital particle image velocimetry (DPIV) proceeding method, namely, consecutive motion vector estimation algorithm, which fully employs the global information of sequential gray images and the physics property of flow field to get the precise velocity vector at each pixel, is proposed. Therefore it enlarges the application range of DPIV, especially for high-speed flow, large velocity gradient flow and rotational flow. The new method is applied to study the particle motion qualitatively in a partially filled rotary drum in this work. The results indicate that the newly developed algorithm describes the real flow field of granular motion in the transverse plane of rotary drum more accurately than the conventional cross-correlation method, which is adopted here as the comparative one. It can be concluded from DPIV analysis that the burden bed in rotary drum consists of two distinct regions, the active layer and the plug flow region. Moreover, the boundary of these two regions is an arc and the rotational speed plays an important role in the particles flow field.

Kinetic theories for granular flow: inelastic particles in Couette flow and slightly inelastic particles in a general flow field

CKK Lun — 2008-03-11T20:08:05-00:00

Journal of Fluid Mechanics, Vol. 140 (1984), pp. 223-256.

The flow of an idealized granular material consisting of uniform smooth, but nelastic, spherical particles is studied using statistical methods analogous to those used in the kinetic theory of gases. Two theories are developed: one for the Couette flow of particles having arbitrary coefficients of restitution (inelastic particles) and a second for the general flow of particles with coefficients of restitution near 1 (slightly inelastic particles). The study of inelastic particles in Couette flow follows the method of Savage & Jeffrey (1981) and uses an ad hoc distribution function to describe the collisions between particles. The results of this first analysis are compared with other theories of granular flow, with the Chapman-Enskog dense-gas theory, and with experiments. The theory agrees moderately well with experimental data and it is found that the asymptotic analysis of Jenkins & Savage (1983), which was developed for slightly inelastic particles, surprisingly gives results similar to the first theory even for highly inelastic particles. Therefore the ‘nearly elastic’ approximation is pursued as a second theory using an approach that is closer to the established methods of Chapman-Enskog gas theory. The new approach which determines the collisional distribution functions by a rational approximation scheme, is applicable to general flowfields, not just simple shear. It incorporates kinetic as well as collisional contributions to the constitutive equations for stress and energy flux and is thus appropriate for dilute as well as dense concentrations of solids. When the collisional contributions are dominant, it predicts stresses similar to the first analysis for the simple shear case.

Residence time distribution and material flow studies in a rotary kiln

P Sai — 2008-03-11T21:38:49-00:00

Metallurgical and Materials Transactions B, Vol. 21, No. 6. (14 December 1990), pp. 1005-1011.

Abstract Experiments were conducted in a rotary kiln containing ilmenite particles to study the residence time distribution (RTD) of low-density particles, holdup, and bed depth profile. The variables include feed rate of solids, slope and rotational speed of the kiln, type and size of the tracer, and dam height. Correlations are presented for mean residence time, dispersion number, holdup, and steady-state throughput of solids in terms of the process variables. A simple method is proposed to estimate the dam height that gives rise to a flat profile of solids bed along the length of the kiln.

A cellular automaton for grains in a rotating drum

Dmitri Ktitarev — 2008-03-07T03:11:50-00:00

Computer Physics Communications, Vol. 121-122 ( 1999), pp. 303-305.

We propose a generalization of the cellular automaton model introduced recently by Lai, Jia and Chan [Phys. Rev. Lett. 79 (1997) 4994] for simulating friction induced segregation in a rotating drum. We use our model to investigate geometrical properties of mixing and segregation. Simulations of the model reproduce recent experimental results on mixing efficiency and spontaneous stratification. We also make two predictions - for the dependence of the mixing rate on the speed of rotation and for the stratification of granular system in the drum which is less than half filled.

Granular flow behaviour in the transverse plane of a partially filled rotating cylinder

AA Boateng — 2008-03-11T19:58:23-00:00

Journal of Fluid Mechanics, Vol. 330 (January 1997), pp. 233-249.

Material flow in partially filled rotating cylinders (rotary kilns) is encountered in many practical applications of material processing, for example incineration, calcination, grain drying, etc. The flow behaviour in the cross-section is important to other transport mechanisms such as mixing and energy distribution within the bed material. The paper describes an experimental study which was carried out with the objective of understanding and improving our predictive capabilities of the rheological behaviour of granular materials in rotary cylinders. Measurement techniques similar to that used in chute flows have been employed to measure flow characteristics, e.g. particle velocities, granular temperature, and solid concentration (in the shear layer developed between the free surface and the bulk of the bed) for different materials having a wide range of coefficients of restitution. The results of the experiments provide the necessary assumptions, constraints, and data for granular flows in partially filled rotating cylinders.

Flow of materials in rotary kilns used for sponge iron manufacture: Part III. Effect of ring formation within the kiln

Amit Chatterjee — 2008-03-11T21:20:25-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 393-399.

Abstract The formation of accretions or ’rings’ in rotary kilns used for the manufacture of directly reduced iron (sponge iron) affects the residence time of the charge, kiln hold-up, and the kiln output to a great extent. In this part of the work, the effect of ring formation was simulated at room temperature in a scaled-down model of a rotary kiln by inserting conical dams in the shape of frustrums of cones through the feed end of the kiln. The influence of such conical dams on various operating parameters was evaluated.

Dynamics of a grain on a sandpile model

L Quartier — 2008-03-06T06:39:55-00:00

Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics, Vol. 62, No. 6 Pt B. (December 2000), pp. 8299-8307.

The dynamics of a macroscopic grain rolling on an inclined plane composed of fixed identical grains is investigated both experimentally and theoretically. As real sand, the system exhibits an hysteretic transition between static and dynamical states: for angles smaller than straight phi(d), the roller always stops, for angles larger than straight phi(s), it spontaneously starts rolling down. But for angles between straight phi(d) and straight phi(s), it can be either at rest or in motion with a constant velocity. It is shown that the limit velocity is given by the equilibrium between gravity driving and dissipation by the shocks. Moreover, the rough plane acts as a periodic potential trap whose width and depth decrease when the angle is increased: the static angle straight phi(s) corresponds to the angle for which the trap disappears; the dynamical angle straight phi(d) to that for which the limit velocity is sufficient to escape from the trap. Finally, a continuous description of the force globally acting on the grain is proposed, which preserves this hysteretic behavior.

Flow of materials in rotary kilns used for sponge iron manufacture: Part I. Effect of some operational variables

Amit Chatterjee — 2008-03-11T21:16:15-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 375-381.

Abstract Looking forward to the need of developing coal-based sponge iron technology in India, a country having no significant resources of either coking coal or natural gas, the Research and Development Division of the Tata Iron and Steel Company Limited (TISCO) set up a rotary kiln based direct reduction pilot plant in 1975. In this pilot plant, a totally indigenous technology for production of sponge iron has been developed in which non-coking coal is essentially used as the reductant. For easy scaling up of the TISCO Direct Reduction (TDR) process to units of 300 to 400 tpd capacity, it was thought necessary to explore some of the fundamental aspects of material flow in a rotary kiln. This was carried out by studying the flow of materials in room temperature models. The work was divided into two parts: first, a study of the influence of several operating parameters,viz., rotational speed, inclination of the kiln, effect of circular dams at the feed and the exit ends of the kiln,etc., and second, an investigation of the extent of segregation of a mixture of solids in a rotating kiln. The highlights of the experimental results dealing mainly with the effect of various kiln operating variables on the filling degree profile of the charge in the kiln are presented.

Mathematical analysis of powder discharge through longitudinal slits in a slowly rotating drum: Objective measurements of powder flowability

Dong — 2008-03-07T03:05:32-00:00

Journal of Food Engineering, Vol. 21, No. 4. (1994), pp. 421-437.

The discharge of fine powder, e.g. coffee, sugar, flour, dried milk, from a slowly rotating slitted drum is analyzed using simplified Navier-Stokes differential equations for continuum fluid flow. The following equation is derived for the rate of powder discharge from the drum: dmdt= - K(pm1/a)0[middle dot]5 where K is a rate constant related to the particle cohesion but independent of powder density, m1 is the mass of powder remaining in the rotating drum, and [alpha] is a function of the poured density of the powder, [varrho]. The equation allows the cohesion and density contributions to powder flowability to be assessed independently and provides a theoretical basis for the flowability test. The predictions made using the above rate equation are consistent with experimental data for a range of food powders.

Flow of materials in rotary kilns used for sponge iron manufacture: Part II. Effect of kiln geometry

Amit Chatterjee — 2008-03-11T21:14:47-00:00

Metallurgical and Materials Transactions B, Vol. 14, No. 3. (1983), pp. 383-392.

Abstract The present work identifies the basic features of burden movement in a rotary kiln. The cold model study was conducted with iron ore as the feed material to determine the influence of length to diameter ratio (L/D) of a rotary kiln on the filling degree, hold-up, and residence time of the charge. An empirical equation correlating different operating variables has been derived on the basis of the experimental results. The influence of individual parameters under different conditions on the residence time and back spillage has also been evaluated.

The Rotary Kiln: An Investigation of Bed Heat Transfer in the Transverse Plane

S Dhanjal — 2004-12-28T16:39:24-00:00

Metallurgical and Materials Transactions B, Vol. 35, No. 6., 1059.

Experimental investigation of granular flow through an orifice

Hojin Ahn — 2008-03-04T20:57:07-00:00

Powder Technology, Vol. In Press, Corrected Proof

The characteristics of continuous, steady granular flow through a flat-plate orifice have been experimentally investigated. In particular, the normal stress exerted on the orifice plate has been measured by the normal stress gauge which consists of a strain gauge attached to a cantilever beam. The cantilever beam supports the orifice plate which is freely hanging, and thus normal stresses on the orifice plate have been measured by strains developed in the beam due to normal forces on the plate by particles. Discharge rates of granular particles through the orifice have, therefore, been studied as a function of the average normal stress on the orifice plate. The results show that granular flows through the orifice are characterized by three regimes. When the flow is not choked, the discharge rate increases with the increasing normal stress (Regime I). With the further increase of the normal stress, the discharge rate reaches a maximum, at which the flow appears to start choking. Once the flow becomes choked, the discharge rate starts decreasing (Regime II) for further increase of the normal stress and then becomes independent of the normal stress on the orifice plate (Regime III). The transitional Regime II where the discharge rate decreases with the increasing normal stress is observed to be unstable. The asymptotic discharge rates at Regime III for various orifice sizes and particle sizes are in good agreement with results available in the literature. The maximum discharge rates, which are observed when choking just starts, exceed the asymptotic discharge rates by approximately 20-30%.

Granular dynamics of inelastic spheres in Couette flow

Cliff Lun — 2008-03-11T20:42:46-00:00

Physics of Fluids, Vol. 8, No. 11. (1996), pp. 2868-2883.

View this record in Web of Science

A simple kinetic theory for granular flow of binary mixtures of smooth, inelastic, spherical particles

M Farrell — 2008-03-11T20:41:37-00:00

Acta Mechanica, Vol. 63, No. 1. (29 November 1986), pp. 45-60.

Summary Following the approach of the kinetic theory for mixtures of dense gases, the general conservation equations for the rapid flow of a binary mixture of smooth, inelastic, spherical granular particles are derived. Explicit constitutive relations for stress and rate of energy dissipation are obtained by making simple approximations for the particle velocity distribution functions. These approximations are appropriate for cases where collisional interactions are the dominant mechanism for momentum and energy exchange in the system. The theory is applied to the case of simple shear flow. In general, the theory predicts that stresses decrease with increasing concentration of the small particles and decreasing diameter ratio of small to large particles. Theoretical predictions of stresses are compared with experimental results and reasonable agreement is found.

Mechanism of competitive grain growth in directional solidification of a nickel-base superalloy

YZ Zhou — 2008-06-24T12:33:44-00:00

Acta Materialia, Vol. 56, No. 11. (June 2008), pp. 2631-2637.

The structure evolution of bicrystal (BC) samples during directional solidification (DS) was explored in an attempt to understand the mechanism of competitive grain growth. It was found that in the case of diverging dendrites the favorably oriented grain overgrows the misaligned grain. However, in the case of converging dendrites the result differs from the prediction of the generally accepted model for competitive grain growth. First, the unfavorably oriented dendrites are able to overgrow the favorably oriented dendrites. Second, the misaligned grain overgrows the favorably oriented grain by blocking the dendrites of the favorably oriented grain at the grain boundary. Based on the experimental results, the process by which a favored <0 0 1> texture is developed during DS process of a nickel-base superalloy is illustrated.

Sensitivity of predictive species distribution models to change in grain size

Antoine Guisan — 2007-04-21T23:13:54-00:00

Diversity & Distributions, Vol. 13, No. 3. (May 2007), pp. 332-340.

Predictive species distribution modelling (SDM) has become an essential tool in biodiversity conservation and management. The choice of grain size (resolution) of environmental layers used in modelling is one important factor that may affect predictions. We applied 10 distinct modelling techniques to presence-only data for 50 species in five different regions, to test whether: (1) a 10-fold coarsening of resolution affects predictive performance of SDMs, and (2) any observed effects are dependent on the type of region, modelling technique, or species considered. Results show that a 10 times change in grain size does not severely affect predictions from species distribution models. The overall trend is towards degradation of model performance, but improvement can also be observed. Changing grain size does not equally affect models across regions, techniques, and species types. The strongest effect is on regions and species types, with tree species in the data sets (regions) with highest locational accuracy being most affected. Changing grain size had little influence on the ranking of techniques: boosted regression trees remain best at both resolutions. The number of occurrences used for model training had an important effect, with larger sample sizes resulting in better models, which tended to be more sensitive to grain. Effect of grain change was only noticeable for models reaching sufficient performance and/or with initial data that have an intrinsic error smaller than the coarser grain size.

Static and Dynamic Properties of a n-C100H202 Melt from Molecular Dynamics Simulations

W Paul — 2008-01-01T20:02:08-00:00

Macromolecules, Vol. 30, No. 25. (15 December 1997), pp. 7772-7780.

Abstract: We present in this work results from atomistic molecular dynamics simulations of a n-C100H202 melt. This work represents a first effort to simulate a fully equilibrated ensemble of chains of sufficient length to follow Gaussian chains statistics and hence Rouse-like chain dynamics, employing well-validated, realistic potentials. In order to allow full equilibration the simulations were performed at experimental densities at high temperatures (450 K). Here we report on the static and dynamic properties of the melt obtained from two models, a united atom and an explicit atom model, and compare these results with experiment. These comparisons allow for a quantitative evaluation of the models and provide insight into the modeled system and the influence of the level of atomistic detail considered.

Molecular phylogeography of domesticated barley traces expansion of agriculture in the Old World

Daisuke Saisho — 2007-10-22T13:15:55-00:00

Genetics, Vol. 177 (18 November 2007), pp. 1765-1776.

Barley (Hordeum vulgare ssp. vulgare) was first cultivated 10,500 years ago in the Fertile Crescent and is one of the founder crops of Eurasian agriculture. Phylogeographic analysis of five nuclear loci and morphological assessment of two traits in >250 domesticated barley accessions reveals that landraces found in South and East Asia are genetically distinct from those in Europe and North Africa. A Bayesian population structure assessment method indicates that barley accessions are subdivided into 6 clusters, and that barley landraces from 10 different geographical regions of Eurasia and North Africa show distinct patterns of distribution across these clusters. Using haplotype frequency data, it appears that the Europe/North Africa landraces are most similar to the Near East population (FST = 0.15) as well as wild barley (FST = 0.11) and are strongly differentiated from all other Asian populations (FST = 0.34 to 0.74). A neighbor-joining analysis using these FST estimates also supports a division between Europe, North African and Near East barley types from more easterly Asian accessions. There is also differentiation in the presence of a naked caryopsis and spikelet row number between eastern and western barley accessions. The data support the differential migration of barley from two domestication events that led to the origin of barley one in the Fertile Crescent and another further east, possibly at the eastern edge of the Iranian Plateau - with European and North African barley largely originating from the former while much of Asian barley arising from the latter. This suggests that cultural diffusion or independent innovation are responsible for the expansion of agriculture to areas of South and East Asia during the Neolithic Revolution. 10.1534/genetics.107.079491

Response of an isolated magnetic grain suspended in a liquid to a rotating field

C Caroli — 2007-10-14T20:49:27-00:00

Zeitschrift für Physik B Condensed Matter, Vol. 9, No. 4. (1969), pp. 311-319.

We have studied the response of an isolated uniaxial magnetic grain suspended in a liquid to an applied fieldh rotating with frequencyω. In the presence of an applied static field (H≫h), at low frequencies (i.e. for fast relaxation), the easy axis followsh, while at high frequencies the behavior is similar to that of a bulk sample. In zero static field, the response of a ferromagnetic grain is more complicated; there exists a critical frequencyωe below which a steady state is reached, with the easy axis followingh. Forω>ωe the mechanical behavior depends crucially on the initial conditions. Finally, a superparamagnetic grain has a (different) critical frequencyωe, below which it reacts similarly to the ferromagnetic particle, while forω>ωe it does not follow steadily the rotating field, but can only oscillate about its initial position.

Grain size effect on the Neel temperature and magnetic properties of nanocrystalline NiFe2O4 spinel

CN Chinnasamy — 2005-09-13T04:46:34-00:00

Journal of Magnetism and Magnetic Materials, Vol. 238, No. 2-3. (January 2002), pp. 281-287.

Nanocrystalline NiFe2O4 spinel ferrites with various grain sizes have been synthesized by ball milling the bulk NiFe2O4. The average grain sizes were estimated from the X-ray line broadening of the (3 1 1) reflection. The Neel temperatures of NiFe2O4 for various grain sizes were determined by magneto thermogravimetric method. The magnetic behaviour has been explained by combining the effects of changes in cation distribution on milling and finite size scaling. The shift in B-H loops has been correlated to the surface spin effects. The high coercivities observed here may be due to high anisotropies of the milled samples. The Hopkinson peak observed just below the Neel temperature has been explained by the mathematical formalism given by the Stoner Wohlfarth model.

Coarse grainings and irreversibility in quantum field theory

C Anastopoulos — 2005-10-25T00:12:42-00:00

Phys. Rev. D, Vol. 56 (July 1997), pp. 1009-1020.

In this paper we are interested in studying coarse graining in field theories using the language of quantum open systems. Motivated by the ideas of Hu and Calzetta on correlation histories we employ the Zwanzig projection technique to obtain evolution equations for relevant observables in self-interacting scalar field theories. Our coarse-graining operation consists in concentrating solely on the evolution of the correlation functions of degree less than n, a treatment which corresponds to the familiar truncation of the BBKGY hierarchy at the nth level. We derive the equations governing the evolution of mean-field and two-point functions thus identifying the terms corresponding to dissipation and noise. We discuss possible applications of our formalism, the emergence of classical behavior, and the connection to the decoherent histories framework.

Effects of High Temperature on Key Enzymes Involved in Starch and Protein Formation in Grains of Two Wheat Cultivars

Zhao — 2008-02-06T12:59:27-00:00

Journal of Agronomy and Crop Science, Vol. 194, No. 1. (February 2008), pp. 47-54.

Impact of Humidity on Temperature-Induced Grain Sterility in Rice (Oryza sativa L)

Weerakoon — 2008-03-12T05:30:42-00:00

Journal of Agronomy and Crop Science, Vol. 194, No. 2. (April 2008), pp. 135-140.

Characterization of atomic motion governing grain boundary migration

Hao Zhang — 2008-04-15T20:22:51-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 74, No. 11. (2006)

Molecular dynamics simulations were employed to study atomic motion within stationary and migrating asymmetric tilt grain boundaries. We employ several measures of the "complexity" of the atomic trajectories, including the van Hove correlation function, the non-Gaussian parameter, and dynamic entropy. There are two key types of dynamical events within the grain boundaries (i) a stringlike cooperative motions parallel to the tilt axis and occurring on a characteristic time scale of 25 ps and (ii) atomic motion across the grain boundary plane occurring on a characteristic time scale of 150 ps. The characteristic times associated with each type of event decreases with increasing driving force for boundary migration. We present evidence as to how the driving force biases these types of events, leading to boundary migration. While the stringlike atomic motion is an intrinsic feature of grain boundary dynamics and is important for grain boundary migration, it is the second type of event that controls grain boundary migration rates.

Refraction of Shear Zones in Granular Materials

Tamás Unger — 2008-03-19T17:28:17-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

We study strain localization in slow shear flow focusing on layered granular materials. A heretofore unknown effect is presented here. We show that shear zones are refracted at material interfaces in analogy with refraction of light beams in optics. This phenomenon can be obtained as a consequence of a recent variational model of shear zones. The predictions of the model are tested and confirmed by 3D discrete element simulations. We found that shear zones follow Snell's law of light refraction.

Experimental Evidence for Molecular Chaos in Granular Gases

GW Baxter — 2008-03-18T22:03:56-00:00

Physical Review Letters, Vol. 99, No. 2. (2007)

We present measurements showing the presence and the absence of molecular chaos in a two-layer vertically vibrated granular media where a plate drives a horizontal layer of massive grains, which, in turn, drives a second horizontal layer of lighter grains above the first. In the first layer driven by the plate, the velocities are spatially correlated. In the second layer, we find uncorrelated velocities consistent with the presence of molecular chaos. In this experiment, energy injection that is randomized in both space and time throughout the shaking cycle is necessary for observing molecular chaos and “kinetic theory”-like behavior. At higher densities, excluded volume effects force velocity correlations in the system which is no longer “gaslike” in behavior.

Swarming and Swirling in Self-Propelled Polar Granular Rods

Arshad Kudrolli — 2008-02-26T22:07:35-00:00

Physical Review Letters, Vol. 100, No. 5. (2008)

Using experiments with anisotropic vibrated rods and quasi-2D numerical simulations, we show that shape plays an important role in the collective dynamics of self-propelled (SP) particles. We demonstrate that SP rods exhibit local ordering, aggregation at the side walls, and clustering absent in round SP particles. Furthermore, we find that at sufficiently strong excitation SP rods engage in a persistent swirling motion in which the velocity is strongly correlated with particle orientation.

Orientational Correlation and Velocity Distributions in Uniform Shear Flow of a Dilute Granular Gas

Bishakdatta Gayen — 2008-03-18T00:55:16-00:00

Physical Review Letters, Vol. 100, No. 6. (2008)

Using particle simulations of the uniform shear flow of a rough dilute granular gas, we show that the translational and rotational velocities are strongly correlated in direction, but there is no orientational correlation-induced singularity at perfectly smooth (=-1) and rough (=1) limits for elastic collisions (e=1); both the translational and rotational velocity distribution functions remain close to a Gaussian for these two limiting cases. Away from these two limits, the orientational as well as spatial velocity correlations are responsible for the emergence of non-Gaussian high-velocity tails. The tails of both distribution functions follow stretched exponentials, with the exponents depending on normal (e) and tangential () restitution coefficients.

Temperature Oscillations in a Compartmentalized Bidisperse Granular Gas

Meiying Hou — 2008-03-18T00:51:22-00:00

Physical Review Letters, Vol. 100, No. 6. (2008)

A granular clock is observed in a vertically vibrated compartmentalized granular gas composed of two types of grains with the same size. The dynamics of the clock is studied in terms of an unstable evaporation or condensation model for the granular gas. In this model, the temperatures of the two types of grains are considered to be different, and they are functions of the composition of the gas. Oscillations in the system are driven by the asymmetric collisions properties between the two types of grains. Both our experiments and model show that the transition of the system from a homogeneous state to an oscillatory state is via a Hopf bifurcation.

Velocity fluctuations in a low-Reynolds-number fluidized bed

Shang-You Tee — 2008-04-17T18:54:33-00:00

Journal of Fluid Mechanics, Vol. 596, No. -1. (2008), pp. 467-475.

The velocity fluctuations of particles in a low-Reynolds-number fluidized bed have important similarities and differences with the velocity fluctuations in a low-Reynolds-number sedimenting suspension. We show that, like sedimentation, the velocity fluctuations in a fluidized bed are described well by the balance between density fluctuations due to Poisson statistics and Stokes drag. However, unlike sedimentation, the correlation length of the fluctuations in a fluidized bed increases with volume fraction. We argue that this difference arises because the relaxation time of density fluctuations is completely different in the two systems.

Granular materialsPacking grains by thermal cycling

K Chen — 2006-07-20T21:49:03-00:00

Nature, Vol. 442, No. 7100. (19 July 2006), pp. 257-257.

Eliminating Segregation in Free-Surface Flows of Particles

Deliang Shi — 2008-03-18T19:31:43-00:00

Physical Review Letters, Vol. 99, No. 14. (2007)

By introducing periodic flow inversions, we show both experimentally and computationally that forcing with a value above a critical frequency can effectively eliminate both density and size segregation. The critical frequency is related to the inverse of the characteristic time of segregation and is shown to scale with the shear rate of the particle flow. This observation could lead to new designs for a vast array of particle processing applications and suggests a new way for researchers to think about segregation problems.

Granular Rayleigh-Taylor Instability: Experiments and Simulations

Jan Vinningland — 2008-03-18T21:42:16-00:00

Physical Review Letters, Vol. 99, No. 4. (2007)

A granular instability driven by gravity is studied experimentally and numerically. The instability arises as grains fall in a closed Hele-Shaw cell where a layer of dense granular material is positioned above a layer of air. The initially flat front defined by the grains subsequently develops into a pattern of falling granular fingers separated by rising bubbles of air. A transient coarsening of the front is observed right from the start by a finger merging process. The coarsening is later stabilized by new fingers growing from the center of the rising bubbles. The structures are quantified by means of Fourier analysis and quantitative agreement between experiment and computation is shown. This analysis also reveals scale invariance of the flow structures under overall change of spatial scale.

Jamming Transition in Granular Systems

TS Majmudar — 2007-12-31T21:59:04-00:00

Physical Review Letters, Vol. 98, No. 5. (2007)

Recent simulations have predicted that near jamming for collections of spherical particles, there will be a discontinuous increase in the mean contact number Z at a critical volume fraction c. Above c, Z and the pressure P are predicted to increase as power laws in -c. In experiments using photoelastic disks we corroborate a rapid increase in Z at c and power-law behavior above c for Z and P. Specifically we find a power-law increase as a function of -c for Z-Zc with an exponent around 0.5, and for P with an exponent around 1.1. These exponents are in good agreement with simulations. We also find reasonable agreement with a recent mean-field theory for frictionless particles.

A Theory of Cooperative Diffusion in Dense Granular Flows

Martin Bazant — 2007-04-05T21:54:49-00:00

(8 May 2004)

Dilute granular flows are routinely described by collisional kinetic theory, but dense flows require a fundamentally different approach, due to long-lasting, many-body contacts. In the case of silo drainage, many continuum models have been developed for the mean flow, but no realistic statistical theory is available. Here, we propose that particles undergo cooperative displacements in response to diffusing “spots” of free volume. The typical spot size is several particle diameters, so cages of nearest neighbors tend to remain intact over large distances. The spot hypothesis relates diffusion and cage-breaking to volume fluctuations and spatial velocity correlations, in agreement with new experimental data. It also predicts density waves caused by weak spot interactions. Spots enable fast, multiscale simulations of dense flows, in which a small, internal relaxation enforces packing constraints during spot-induced motion. In the continuum limit of the model, tracer diffusion is described by a new stochastic differential equation, where the drift velocity and diffusion tensor are coupled non-locally to the spot density. The same mathematical formalism may also find applications to glassy relaxation, as a compelling alternative to void (or hole) random walks.

Collective Behavior in a Granular Jet: Emergence of a Liquid with Zero Surface Tension

Xiang Cheng — 2008-03-18T02:05:40-00:00

Physical Review Letters, Vol. 99, No. 18. (2007)

We perform the analog to “water bell” experiments with granular jets. Rebounding from cylindrical targets, wide granular jets produce sheets or cones with shapes that mimic a zero-surface-tension liquid. The jets' particulate nature appears when the number of particles in the cross section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment has a counterpart in the behavior of quark-gluon plasmas generated by colliding heavy ions. There, a high collision density gives rise to collective behavior also described as a liquid.

Role of interparticle forces and interparticle friction on the bulk friction in charged granular media subjected to shearing

SJ Antony — 2008-06-11T22:25:54-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

We study the consequences of the interplay between electrostatic forces, mechanical contact forces, and frictional properties of grains upon the bulk frictional properties of charged granular media subjected to quasistatic shearing. We show that, the variations in short-range electrostatic forces between the grains (which are often ignored in the existing studies) dominantly affect the bulk friction. Charging enhances the fabric anisotropy of heavily loaded contacts—this enhances the bulk friction, more significantly, in the case of low frictional granular systems.

Influences of the interstitial liquid on segregation patterns of granular slurries in a rotating drum

Tilo Finger — 2008-06-11T22:24:18-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 3. (2007)

Granular mixtures immersed in a liquid (slurries) show segregation dynamics which are quantitatively and qualitatively different from those of dry systems. The principal mechanisms of the segregation dynamics in slurries, as well as the relevant material parameters that must be taken into account in a dynamic description are not sufficiently understood so far. We investigate experimentally the influence of the viscosity of the interstitial liquid on the coarsening of axial segregation patterns in a horizontally rotating mixer. It is found that not only the characteristic time scales but also fundamental structural features of these patterns are influenced by the viscous properties of the liquid component.

The Spot Model for random-packing dynamics

Martin Bazant — 2008-03-24T22:29:29-00:00

Mechanics of Materials, Vol. 38, No. 8-10. ( 2006), pp. 717-731.

The diffusion and flow of amorphous materials, such as glasses and granular materials, has resisted a simple microscopic description, analogous to defect theories for crystals. Early models were based on either gas-like inelastic collisions or crystal-like vacancy diffusion, but here we propose a cooperative mechanism for dense random-packing dynamics, based on diffusing "spots" of interstitial free volume. Simulations with the Spot Model can efficiently generate realistic flowing packings, and yet the model is simple enough for mathematical analysis. Starting from a non-local stochastic differential equation, we derive continuum equations for tracer diffusion, given the dynamics of free volume (spots). Throughout the paper, we apply the model to granular drainage in a silo, and we also briefly discuss glassy relaxation. We conclude by discussing the prospects of spot-based multiscale modeling and simulation of amorphous materials.

Impact of a Projectile on a Granular Medium Described by a Collision Model

Jérôme Crassous — 2008-03-18T01:50:04-00:00

Physical Review Letters, Vol. 99, No. 24. (2007)

We propose a model for the propagation of energy due to the impact of a granular projectile on a dense granular medium. Energy is transferred from grain to grain during binary collision events. The transport of energy may then be viewed as a random walk with a split of energy during successive collisions. There is a qualitative and quantitative agreement between this simple description and experimental results.

DEBRIS-FLOW MOBILIZATION FROM LANDSLIDES

Richard Iverson — 2008-04-26T23:17:17-00:00

Annual Review of Earth and Planetary Sciences, Vol. 25, No. 1. (1997), pp. 85-138.

Abstract Field observations, laboratory experiments, and theoretical analyses indicate that landslides mobilize to form debris flows by three processes: (a) widespread Coulomb failure within a sloping soil, rock, or sediment mass, (b) partial or complete liquefaction of the mass by high pore-fluid pressures, and (c) conversion of landslide translational energy to internal vibrational energy (i.e. granular temperature). These processes can operate independently, but in many circumstances they appear to operate simultaneously and synergistically. Early work on debris-flow mobilization described a similar interplay of processes but relied on mechanical models in which debris behavior was assumed to be fixed and governed by a Bingham or Bagnold rheology. In contrast, this review emphasizes models in which debris behavior evolves in response to changing pore pressures and granular temperatures. One-dimensional infinite-slope models provide insight by quantifying how pore pressures and granular temperatures can influence the transition from Coulomb failure to liquefaction. Analyses of multidimensional experiments reveal complications ignored in one-dimensional models and demonstrate that debris-flow mobilization may occur by at least two distinct modes in the field.

Spontaneous Separation of Charged Grains

Amit Mehrotra — 2007-08-29T15:18:03-00:00

Physical Review Letters, Vol. 99, No. 5. (2007)

In 1867, Lord Kelvin described an experiment in which two streams of water droplets were connected so that each stream amplified the charge on the second stream [W. Thomson, Proc. R. Soc. London 16, 67 (1867).]. We present here a complementary effect in flowing grains that spontaneously separates similar and well-mixed grains into two charged streams of demixed grains. This effect has important consequences for industrial and natural processes.

Cleavage cracking resistance of high angle grain boundaries in Fe-3%Si alloy

Y Qiao — 2007-08-06T14:02:14-00:00

Mechanics of Materials, Vol. 35, No. 3-6. ( 2003), pp. 313-331.

Geotechnical Properties of Low Calcium and High Calcium Fly Ash

Das — 2006-06-02T14:58:00-00:00

Geotechnical and Geological Engineering, Vol. 24, No. 2. (April 2006), pp. 249-263.

Comparison between kinetic and thermodynamic effects on grain growth

Feng Liu — 2006-01-10T19:52:41-00:00

Thin Solid Films, Vol. 466, No. 1-2. (1 November 2004), pp. 108-113.

A detailed comparison between the kinetic and the thermodynamic effects on grain growth was given, and the corresponding models were fitted to the experimental data, respectively. It was found that both models can explain the derivation from the normal parabolic growth under ideal condition. According to the kinetic model, a single isothermal grain growth can be understood in terms of a single, thermally-activated rate process with constant Q and grain boundary (GB) energy, [sigma]b; impurity atoms accumulated in the GBs might exert a retarding force on GB migration, but do not change the grain growth activation energy, Q. On the other hand, it is necessary to invoke variable Q and [sigma]b according to the thermodynamic model, where the probably existing impurities and/or surface oxide seem to block surface and/or GB diffusion path, thus increasing Q and reducing [sigma]b.

Segregation and diffusion of hydrogen in grain boundaries of palladium

T Mutschele — 2006-01-10T19:50:27-00:00

Scripta Metallurgica, Vol. 21, No. 2. (February 1987), pp. 135-140.

Atomistic theory and computer simulation of grain boundary structure and diffusion

Diana Farkas — 2006-01-31T18:59:08-00:00

Journal of Physics: Condensed Matter, Vol. 12, No. 42. (2000), pp. R497-R516.

We present a review of the current state of atomistic theory and computer simulation methods in the study of grain boundary structure and diffusion properties. We review the multiplicity of possible local minimum energy structures that arise for the structure of the same grain boundary, particularly in the case of a grain boundary in an ordered alloy. We also review recent structural studies performed for randomly generated grain boundaries. The basic features of the interaction of vacancies with the grain boundary are reviewed for the case of special boundaries. We describe the use of a combination of molecular statics/Monte Carlo techniques for the calculation of diffusion properties along grain boundaries based on many-body interatomic potentials. The method is exemplified in the results obtained for a special grain boundary in the intermetallic compound NiAl. Finally, we describe the studies that have been carried out using molecular dynamics for special grain boundaries in fcc metals, showing that both vacancy and interstitial mechanisms may be important. The advantages and disadvantages of these techniques for the study of grain boundary diffusion are discussed.

Hydrogen diffusion in nanocrystalline nickel indicating a structural change within the grain boundaries after annealing

C Marte — 2007-05-31T15:53:45-00:00

pp. 1171-1175.