Motion Of Grains Research Articles

Stochastic motion of grains with charge gradients in external electric fields

We consider a theoretical model describing the “anomalous heating” of charged grains due to their stochastic motion in the volume of a spatially inhomogeneous plasma. On the basis of this model for the first time we propose the analytical relations for conditions of the heating of grains due to the gradient of their charge in the electric field of a trap. The obtained relations were tested by numerical simulations of the problem for one and two charged particles.

Cycloid motions of grains in a dust plasma**Project supported by the National Natural Science Foundation of China (Grant Nos. 11205044 and 11405042), the Natural Science Foundation of Hebei Province, China (Grant Nos. A2011201006 and A2012201015), the Research Foundation of Education Bureau of Hebei Province, China (Grant No. Y2012009), the Program for Young Principal Investigators of Hebei Province, and the Midwest

Hypocycloid and epicycloid motions of irregular grains (pine pollen) are observed for the first time in a dust plasma in a two-dimensional (2D) horizontal plane. These cycloid motions can be regarded as a combination of a primary circle and a secondary circle. An inverse Magnus force originating from the spin of the irregular grain gives rise to the primary circle. Radial confinement resulting from the electrostatic force and the ion drag force, together with inverse Magnus force, plays an important role in the formation of the secondary circle. In addition, the cyclotron radius is seen to change periodically during the cycloid motion. Force analysis and comparison experiments have shown that the cycloid motions are distinctive features of an irregular grain immersed in a plasma.

Flow and fracture in water-saturated, unconstrained granular beds

The injection of gas in a liquid-saturated granular bed gives rise to a wide variety of invasion patterns. Many studies have focused on constrained porous media, in which the grains are fixed in the bed and only the interstitial fluid flows when the gas invades the system. With a free upper boundary, however, the grains can be entrained by the ascending gas or fluid motion, and the competition between the upward motion of grains and sedimentation leads to new patterns. We propose a brief review of the experimental investigation of the dynamics of air rising through a water-saturated, unconstrained granular bed, in both two and three dimensions. After describing the invasion pattern at short and long time, a tentative regime-diagram is proposed. We report original results showing a dependence of the fluidized zone shape, at long times, on the injection flow rate and grain size. A method based on image analysis makes it possible to detect not only the fluidized zone profile in the stationary regime, but also to follow the transient dynamics of its formation. Finally, we describe the degassing dynamics inside the fluidized zone, in the stationary regime. Depending on the experimental conditions, regular bubbling, continuous degassing, intermittent regime or even spontaneous flow-to-fracture transition are observed.

Process-scale modelling of microstructure in direct chill casting of aluminium alloys

The mechanical properties of an alloy being related to its microstructure, the understanding of the mechanisms responsible for the grain structure formation in direct chill casting is crucial. However, the grain size prediction by modelling is difficult since a variety of multi-scale coupled phenomena have to be considered. Nucleation and growth of the grains are interrelated, and the macroscopic transport phenomena such as the motion of grains and inoculant particles with the flow impact the nucleation-gowth competition. Thus we propose to study the grain size distribution of a 5182 alloy industrial scale slab of 510 mm thickness, both non-inoculated and inoculated with Al-3Ti-1B, for which experimental grain size measurements are available. We use a volume-averaged two-phase multi-scale model that describes nucleation from inoculant particles and grain growth, fully coupled with macroscopic transport phenomena: fluid flow induced by natural convection and solidification shrinkage, heat, mass and solute mass transport, grains and inoculant particles motion. We analyze the effect of liquid and grain motion as the effect of grain morphology on microstructure formation and we show in which extent those phenomena are responsible for the grain size distribution observed experimentally. The effect of the refiner level is also studied.

Experimental study and analysis on the rising motion of grains in a vertically-vibrated pipe

Previous experimental investigations have shown that when a narrow pipe is inserted into a granular bed and is vibrated vertically but the granular bed is kept still, the grains in the bed can enter the pipe and rise against gravity along the pipe and finally stabilized at a certain height. The growth velocity and final stable height of the grain column inside the pipe can be controlled by varying the vibration conditions. In this paper, we discuss those experimental findings. We establish a mathematic relation between the grain column height (h) and time (t), and by using the relation we discuss the change of the growth velocity (dh/dt) and acceleration (d2h/dt2) with t and h, respectively. We also analyze the mechanism of the rising motion of the grains during vibration. Furthermore, we derive a theoretical expression for describing the final stable height (hst), which shows that the main factors influencing the height are vibration strength (Γ ), bulk density of grains, inner diameter of the pipe, and vibration frequency, and that hst increases nonlinearly in the presence of air and linearly in a vacuum environment with increasing Γ.

Complex motions of grains in dusty plasma with nonuniform magnetic field

We have studied various complex motions of the irregular dust grains immersed in non-uniformly magnetized plasma. The cylindrical magnet that we used for experiments significantly alters the radial distribution of the sheath potential which confines the negatively charged grains. Grains are horizontally illuminated by a 50 mW, 532 nm laser sheet and imaged by a CCD camera from the upper transparent electrode. Hypocycloid and epicycloid motions of grains are observed for the first time as far as we know. Cuspate cycloid motions, circle motion, wave motion, and stationary grains are also observed. Their trajectories can be obtained by using long-time exposure, and the characteristic parameters of the grain movement are measured by using the image processing with MATLAB. Though the dust grains can move around the magnet steadily in various trajectories, the induced magnetic field is too weak to give rise to cycloid motions of grains. Then we propose a new mechanism that an inverse Magnus force induced by the spin of the irregular grains plays an important role in their cycloid motions. The pollen pini we used for experiment is not a regular microsphere, there is a symmetry in the shape. On the basis of Bernoulli principle, the pressure difference between the left and right side of the forward moving grains produces the inverse Magnus effect. Additional comparison experiments with regular microspheres are also performed to confirm that the cycloid motions are distinctive features of an irregular dust grain immersed in the plasma. The periodical change of the cyclotron radius as the grain travels would result in the (cuspate) cycloid motions, and the maximal value of angular velocity of spin is about 105 rad/s. Our experimental observations can be well explained based on the force analysis in 2D horizontal plane.

Precursors and triggering mechanisms of granular avalanches

Exploration of the dynamical response of an inclined granular packing before the avalanche starts allows a study of the dynamical transition from a static packing to a flowing one and brings information which may be helpful for the prediction of the occurrence of the avalanche. During inclination, small rearrangements implying only a few grains are first detected. The size and the rate of these rearrangements increase with the inclination angle. At some stage, large amplitude and quasi periodical events are observed. These events, called precursors, consist of collective motions of grains. Other studies showed that the precursors are bulk phenomena and allowed to interpret these events as reorganizations of the weak-contact sub network occurring in the packing. Besides optical imaging techniques which give access to grain motion and rearrangements during the tilting, acoustic methods can be experimentally used to detect precursors and to probe internal rearrangements in the bulk of the granular layer. Because part of the acoustic wave energy transports through the contact network and the elastic beads of the medium, the probing methods based on the monitoring of acoustic signatures are sensitive to changes in the elastic properties of the granular layer. We give an overview of the acoustic probing methods implemented previously in the experiments of granular layer destabilization. They can be divided in two groups, the passive methods where acoustic sensors are used to listen to the sounds emitted by the destabilized layer itself and the active methods where an acoustic signal with desired properties is generated and detected by transducers in the medium. Systematic experiments of granular layer destabilization for various granular media and external conditions are compared and allow better understanding of the mechanisms responsible for the appearance, periodicity and intensity of precursors.

Localisation in granular media: Particle approach, homogenisation and continuum modelling

AbstractThe individual motion of grains in granular material has a strong influence on the macroscopic material behaviour, which is in particular the case for the phenomena of strain localisation in shear zones and justifies the need for techniques that incorporate a micro‐macro transition. In this contribution, granular media are investigated in three steps. Firstly, a microscopic particle‐based modelling is set up, where individual grains are considered as rigid uncrushable particles while their motion is obtained through Newton's equations of state. The inter‐particle contact forces are thereby determined via constitutive contact‐force formulations, which have to account for the envisaged material behaviour. The second step is the homogenisation of the obtained particle's displacements and contact forces through a particle‐centre‐based strategy towards continuum quantities. Therefore, Representative Elementary Volumes (REV) are introduced on the mesoscale and the specific construction of the REV boundary leads to the understanding of granular media as a micropolar continuum. Finally, in order to verify the homogenisation strategy, a continuum based micropolar model is applied to model localisation phenomena and a comparative study of the results is carried out in a qualitative way. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical simulations of dust charging and wakefield effects

Charging of dust grains and related phenomena are fundamental problems in the physics of complex plasmas. The relative motion of grains and plasma breaks the symmetry in dust charging and gives rise to the wake in plasma density and potential, which can significantly influence the dynamics of other grains. This paper gives an overview of dust charging in two- and multi-component plasma flows and related wake effects, and presents recent results from self-consistent particle-in-cell (PIC) simulations. The role of wakefields is considered in the context of charging of downstream grains.

Time-resolved X-ray diffraction studies of solidification microstructure evolution in welding

High-brilliance and high-energy polychromatic X-ray diffraction (XRD) has been used for time-resolved studies of rapid solidification microstructure evolution in situ during simulated spot welding in austenitic steels. Weld pools of 5.0mm diameter and 0.6–1.5mm depth were formed at the steel plate surfaces by radiation heating from halogen lamps. Solidification was initiated by powering off the lamps, and completed within 1.5s while measuring diffraction from the solidifying grains of the pool at a 1kHz frame rate. The data contain time-resolved information on individual grain growth and overall solid fraction evolution, and furthermore reveal prominent individual and collective motion of grains during early stages of solidification, presumably caused by convective currents in the pool. Ultrafast polychromatic high-energy XRD is novel in studies of microstructure evolution during welding. The experimental technique could quite readily be used for similar studies in real welding.

The rising motion of grains in a vibrating pipe

The rising motion of grains in a vibrating pipe is studied experimentally and by DEM (discrete element method) simulations. A pipe is partially inserted into a static granular layer. With the pipe initially filled with a certain height of grains, when the pipe is set into vertical vibrations, grains in the pipe will rise along the pipe and the motion will be finally stabilized at a certain height. The rising height and the speed of motion are strongly dependent on the vibration strength. By using a high speed camera and through DEM simulation, the trajectory of individual grains and the force acting on the grains in each vibration cycle can be obtained. A force-based mechanism for this rising motion is proposed. During vibrations of the pipe, the grains in the pipe are compacted and loosened alternately, and the force of the pipe and the grains varies periodically, thus leading to the rising motion of grains. A new possible way of continuously transporting grains is presented in this work.

Effect of Interstitial Media on Segregation in Vertically Vibrated Granular Mixtures

Vertically vibrated segregation behaviors of binary granular mixtures with different interstitial media are experimentally investigated. To study the role of interstitial media on the segregation, two types of interstitial fluids are adopted and the resulting phase diagrams are compared. The water-immersed granular mixture exhibits two kinds of complete segregation behaviors: Brazil nut effect and sandwich patterns, at least the latter is absent in the same air-immersed mixture. Additionally, the segregation extent is improved remarkably for the water-immersed mixture. The experimental observation further confirms that the effect of interstitial media on the relative motion of grains is one of the predominant mechanisms for granular segregation.

Numerical study of the impact of inoculant and grain transport on macrosegregation and microstructure formation during solidification of an Al-22%Cu alloy

We investigate the impact of the nucleation law for nucleation on Al-Ti-B inoculant particles, of the motion of inoculant particles and of the motion of grains on the predicted macrosegregation and microstructure in a grain-refined Al-22 wt.% Cu alloy casting. We conduct the study by numerical simulations of a casting experiment in a side-cooled 76×76×254 mm sand mould. Macrosegregation and microstructure formation are studied with a volume-averaged two-phase model accounting for macroscopic heat and solute transport, melt convection, and transport of inoculant particles and equiaxed grains. On the microscopic scale it accounts for nucleation on inoculant particles with a given size distribution (and corresponding activation undercooling distribution)and for the growth of globular solid grains. The growth kinetics is described by accounting for limited solute diffusion in both liquid and solid phases and for convective effects. We show that the consideration of a size distribution of the inoculants has a strong impact on the microstructure(final grain size) prediction. The transport of inoculants significantly increases the microstructure heterogeneities and the grain motion refines the microstructure and reduces the microstructure heterogeneities.

Suction of splash after impact on dry quick sand

It is well known that a splash occurs when an object impacts at high velocity on a liquid's surface. If the impact is fast enough, surface tension and air pressure gradi- ents pull the crown-shape splash all the way towards the axis of symmetry, making it to collapse and seal the surface. In this paper we show that splash and surface sealing are also observed in impacts on soft, dry sand. We observe influence of air pressure and grains size on the shape of the splash. By tracking individual grains using high-speed imaging we calculate their acceleration, which results from gravity and drag forces. Assuming friction drag parallel, and pressure drag perpendicular to the direction of motion of grains we estimate the friction and pressure drag contributions to the drag force. Our results support the idea that pressure drag from Bernoulli effect is at the origin of the surface seal.

Dynamical regimes of a granular gas in microgravity : a molecular dynamics study

We propose a numerical model, based on molecular dynamics, which is able to reproduce the behavior of a dissipative granular gas in microgravity. Granular material is confined in a cubic box of side length L following a sinusoidal motion of amplitude A and angular velocity ω = 2πf along the z-axis. The simulations are performed using the parameters of earlier Texus experiments [15]. Our results are in excellent agreement with experimental data. Moreover, we discovered various dynamical regimes and the physical conditions for their appearance : a gaseous state, the formation of either small or large clusters, the collective motion of grains. Phase diagrams are drawn where transitions between these different granular states are emphasized. Transitions are discussed by considering Statistical Physics models.

Phase field crystal simulations of nanocrystalline grain growth in two dimensions

We study two-dimensional grain growth at the nanoscale using the phase field crystal (PFC) model. Our results show that for circular grains with large misorientations the grain area decreases linearly with time, in good agreement with classical grain growth theory. For circular grains with small initial misorientations, grain rotation occurs as a result of the coupled motion between the normal motion of the grain boundary and the tangential motion of the adjacent grains. Despite this rotation and its effect on the grain boundary energy, the grain area decreases linearly with time. In addition, for intermediate initial grain misorientations, we find a repeating faceting–defaceting transition during grain shrinkage and a different relationship between the grain area and time, which suggests a different grain growth mechanism than that for small and large misorientations. For a circular grain embedded between a bicrystal with a symmetric tilt boundary, we find that the evolution of the embedded grain closely depends on dislocation reactions at triple junctions.

Comment on “A random kinetic energy model for rock avalanches: Eight case studies” by T. Preuth et al.

Comment on “A random kinetic energy model for rock avalanches: Eight case studies” by T. Preuth et al.

Experimental study on the vertical motion of grains in a vibrated U-tube

Weinvestigateexperimentallythebehaviorofthe exponential growth rate of a column of grains, in a partially filled vertically shaken U-tube. For the set of frequencies used we identify three qualitatively different behaviors for the exponential growth rate γ as a function of the dimen- sionless acceleration Γ : (1) an interval of zero growth for low Γ with a smooth change to nonzero growth, analogous to a continuous phase transition; (2) a sigmoidal region for γ at intermediate values of the dimensionless acceleration Γ ; and(3)anabruptchangefromhighvaluesof γ tozerogrowth at high values of Γ , similar to a first order phase transition. We obtain that our experimental data is well described by a Boltzmann Sigmoidal funtion, for the change of the expo- nential growth rate with the dimensionless acceleration of the vertical vibrations.

Motion of Grain on the Screen Surface under Circular Motion (III)

Motion of Grain on the Screen Surface under Circular Motion (III)

Dust coagulation in star formation with different metallicities

Dust grains coagulate into larger aggregates in dense gas. This changes their size distribution and possibly affects the thermal evolution of star-forming clouds. We here investigate dust coagulation in collapsing pre-stellar cores with different metallicities by considering the thermal motions of grains. We show that coagulation does occur even at low metallicity $\sim 10^{-6} \Zsun$. However, we also find (i) that the H$_2$ formation rate on dust grains is reduced only after the majority of H$_2$ is formed; and (ii) that the dust opacity is modified only after the core becomes optically thick. Therefore, we conclude that the effects of dust coagulation can safely be neglected in discussing the temperature evolution of the pre-stellar cores for any metallicity as long as the grain motions are thermal.