Motion Of Individual Particles Research Articles

A characterisation of the magnetically induced movement of NdFeB-particles in magnetorheological elastomers

Magnetorheological elastomers are a type of smart hybrid material where elastic properties of a soft elastomer matrix are combined with magnetic properties of magnetic micro particles. This combination leads to a complex interplay of magnetic and elastic phenomena, of which the magnetorheological effect is the best described. In this paper, magnetically hard NdFeB-particles were used to obtain remanent magnetic properties. X-ray microtomography has been utilised to analyse the particle movement induced by magnetic fields. A particle tracking was performed; thus, it was possible to characterise the movement of individual particles. Beyond that, a comprehensive analysis of the orientation of all particles was performed at different states of magnetisation and global particle arrangements. For the first time, this method was successfully applied to a magnetorheological material with a technically relevant amount of magnetic NdFeB-particles. A significant impact of the magnetic field on the rotation and translation of the particles was shown.

Particles climbing along a vertically vibrating tube: numerical simulation using the Discrete Element Method (DEM)

It has been reported experimentally that granular particles can climb along a vertically vibrating tube partially inserted inside a granular silo. In this paper, we use the Discrete Element Method (DEM) available in the Multiphase Flow with Interphase eXchanges (MFIX) code to investigate this phenomenon. By tracking the movement of individual particles, the climbing mechanism was illustrated and analyzed. The numerical results show that a sufficiently high vibration strength is needed to form a low solids volume fraction region inside the lower end of the vibrating tube, a dense region in the middle of the tube, and to bring the particles outside from the top layers down to fill in the void. The results also show that particle compaction in the middle section of the tube is the main cause of the climbing. Consequently, varying parameters which influence the compacted region, such as the restitution coefficient, change the climbing height.

Three-dimensional deformations in transparent soil using fluorescent markers

This paper presents details of a new, simple and low-cost technique that allows three-dimensional (3D) movements of individual particles, and particle aggregations, to be observed in geotechnical physical model experiments. This has been achieved by seeding a transparent granular mass formed from sand-sized fused silica particles and a pore fluid with matching refractive index with crushed uranium glass particles, which do not significantly impact transparency. When illuminated by low-power lasers, the particles fluoresce and appear bright green, which greatly increases the sample's texture, and can be easily detected by photography. These tracer particles have been used to obtain two-dimensional soil deformations using both digital image correlation and particle tracking; and by using stereo-photogrammetry, 3D positions and displacements can also be analysed. Displacements were measured through 85 mm of the transparent model, and the technique should allow a single plane to be illuminated and measured at much greater depths. Fluorescent uranium glass tracer particles enable both individual glass particles and particle aggregations to be tracked. The stereo-photogrammetric system developed is shown to be a cost-effective and viable method of quantifying 3D deformations within a transparent soil model.

Time Evolution of Tsallis Distribution in Paul Trap

Motion of individual charged particles inside a Paul trap is very well understood, but the understanding of collective dynamics offers a challenge and opens up avenues for investigation. This problem has been addressed earlier by solving the 1-D Vlasov equation assuming the initial distribution to be Gaussian. In this paper, we generalize this solution by taking a Tsallis distribution, since in some Paul trap experiments, the plasma distribution has been reported to have power law tails. The time averaged distribution function is found to be doubled humped in velocity beyond a spatial threshold and the hump moves away from the bulk as the Tsallis parameter is increased from 1. The plasma temperature is found to vary quadratically with the spatial coordinate.

Resolved Simulation of a Granular-Fluid Flow with a Coupled SPH-DCDEM Model

Debris flows represent some of the most relevant phenomena in geomorphological events. Because of the potential destructiveness of such flows, they are the subject of a vast amount of research. This paper addresses the need for a numerical model applicable to granular-fluid mixtures featuring high spatial and temporal resolution, and thus capable of resolving the motion of individual particles, including all interparticle contacts. The DualSPHysics meshless numerical implementation based on smoothed particle hydrodynamics (SPH) is expanded with a distributed-contact discrete-element method (DCDEM) in order to explicitly solve the fluid and solid phases. The specific objective is to test the SPH-DCDEM approach by comparing its results with experimental data. An experimental setup for stony debris flows in a slit check dam is reproduced numerically, where solid material is introduced through a hopper, assuring a constant solid discharge for the considered time interval. With each sediment particle possibly undergoing several simultaneous contacts, thousands of time-evolving interactions are efficiently treated because of the model’s algorithmic structure and the HPC implementation of DualSPHysics. The results, composed of mainly retention curves, are in good agreement with the measurements, correctly reproducing the changes in efficiency with slit spacing and density.

Decorrelation correction for nanoparticle tracking analysis of dilute polydisperse suspensions in bulk flow.

Nanoparticle tracking analysis, a multiprobe single particle tracking technique, is a widely used method to quickly determine the concentration and size distribution of colloidal particle suspensions. Many popular tools remove non-Brownian components of particle motion by subtracting the ensemble-average displacement at each time step, which is termed dedrifting. Though critical for accurate size measurements, dedrifting is shown here to introduce significant biasing error and can fundamentally limit the dynamic range of particle size that can be measured for dilute heterogeneous suspensions such as biological extracellular vesicles. We report a more accurate estimate of particle mean-square displacement, which we call decorrelation analysis, that accounts for correlations between individual and ensemble particle motion, which are spuriously introduced by dedrifting. Particle tracking simulation and experimental results show that this approach more accurately determines particle diameters for low-concentration polydisperse suspensions when compared with standard dedrifting techniques.

Probabilistic approach to nonlinear wave-particle resonant interaction.

In this paper we provide a theoretical model describing the evolution of the charged-particle distribution function in a system with nonlinear wave-particle interactions. Considering a system with strong electrostatic waves propagating in an inhomogeneous magnetic field, we demonstrate that individual particle motion can be characterized by the probability of trapping into the resonance with the wave and by the efficiency of scattering at resonance. These characteristics, being derived for a particular plasma system, can be used to construct a kinetic equation (or generalized Fokker-Planck equation) modeling the long-term evolution of the particle distribution. In this equation, effects of charged-particle trapping and transport in phase space are simulated with a nonlocal operator. We demonstrate that solutions of the derived kinetic equations agree with results of test-particle tracing. The applicability of the proposed approach for the description of space and laboratory plasma systems is also discussed.

Viscosity of particulate soap films: approaching the jamming of 2D capillary suspensions.

We compute the effective viscosity of particulate soap films thanks to local velocity fields obtained by Particle Image Velocimetry (PIV) during film retraction experiments. We identify the jamming of these 2D capillary suspensions at a critical particle surface fraction (≃0.84) where effective viscosity diverges. Pair correlation function and number of neighbors in contact or close to contact reveal the cohesive nature of this 2D capillary granular media. The experimental 2D dynamic viscosities can be predicted by a model considering viscous dissipation at the liquid interfaces induced by the motion of individual particles.

Discrete element modeling of 3D media motion in vibratory finishing process

A model has been developed to investigate the three-dimensional media motion during the vibratory finishing processes. This work presents a vibratory finishing machine model using a discrete element method (DEM) that calculates the media interactive normal and tangential contact forces among the media particles. The DEM model predicts the dynamic motion of individual particles inside the vibratory machine container based on Hertzian contact mechanics. The influence of contact parameters such as contact stiffness, friction and damping on media motion has been investigated to determine the critical operating parameters for the vibratory finishing process. The simulation results have been validated with experimental data. This model provides an understanding of vibratory finishing process fundamentals, guidelines for vibratory finishing machine design and optimal operating conditions.

Gravity-driven granular flow in a silo: Characterizing local forces and rearrangements

While the gravity-driven flow of a granular material in a silo geometry can be modeled by the Beverloo equation, the mesoscale-level particle rearrangements and interactions that drive this flow are not wellunderstood. We have constructed a quasi-two-dimensional system of bidisperse, millimeter-scale disks with photoelastic properties that make force networks within the material visible. The system is contained in an acrylic box with an adjustable bottom opening. We can approach the clogging transition by adjusting this opening. By placing the system between cross-polarizers, we can obtain high-speed video of this system during flow, and extract intensity signals that can be used to identify and quantify localized, otherwise indeterminate forces. We can simultaneously track individual particle motions, which can be used to identify shear transformation zones in the system. In this paper, we present our results thus far.

Analysis of the incipient motion of spherical particles in an open channel bed, using a coupled computational fluid dynamics–discrete element method model

The measurement of liquid-induced erosion using CFD–DEM (computational fluid dynamics–discrete element method) models has been studied in detail, particularly in rough pipes. Some studies have provided measurements of the erosion rate in open liquid–solid systems, but there is less information on the incipient motion of individual particles since it is difficult to design test beds that can provide reliable results. This work compares the fluid flow velocity required to initiate incipient motion of a particle predicted by a coupled CFD–DEM model with measurements obtained during an experiment in an open channel under laboratory conditions. The experiment was designed to obtain a continuous flow with a slow and gradual increase in water velocity. The bed was made using two rows of spheres fixed in staggered positions, and a test sphere resting on top of the three neighbouring fixed spheres (i.e. nestling in the space between the surfaces of the fixed spheres). A 50 mm-high spillway gate was located downstream of the test sphere in order to obtain deeper water upstream, and provide more easily monitored and controllable water flows. The critical flow velocity required to initiate incipient motion in the five test spheres of different dimensions was measured by acoustic Doppler velocimetry. The difference in the results provided by the two methods was

Hydrodynamic drift ratchet scalability

The hydrodynamic drift ratchet provides a novel means to continuously separate particles at the microscale, based on particle size. Separation arises from a combination of diffusion and particle‐wall hydrodynamic interactions. As there are currently no verified experiments, our aim is to determine numerically how these systems scale so that appropriate experiments can be designed. Using nondimensional variables, we derive the correct scaling parameters governing drift ratchets by simulating individual particle motion using a model that treats the particle dynamics at pore walls as elastic reflections. While our model does not quantitatively resolve the detailed hydrodynamic interactions, we show that it does recover the correct scaling behavior for these interactions. Our simulations demonstrate that the drift velocity relative to the characteristic pore size is independent of pore size if all the relevant nondimensional groups remain constant. Dynamic similarity can be used to facilitate the appropriate design and testing protocols for experiments. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2358–2366, 2017

Using real time particle tracking to understand soil particle movements during rainfall events

Very little is known about how individual soil particles move over a soil surface as a result of rainfall. Specifically there is virtually no information about the pathway a particle takes, the speed at which it travels and when it is in motion. Here we present a novel technique that can give insight into the movement of individual soil particles. By combining novel fluorescent videography techniques with custom image processing and a fluorescent soil tracer we have been able to trace the motion of soil particles under simulated rainfall in a laboratory soil flume. The system is able track multiple sub-millimeter particles simultaneously, establishing their position 50 times a second with sub-millimeter precision. An analysis toolkit has been developed enabling graphical and numerical analysis of the data obtained. For example, we are able to visualise and quantify parameters such as distance and direction of travel. Based on our observations we have created a conceptual model (Stop, hop, roll) which attempts to present a unified model for the movement of soil particles across a soil surface. It is hoped that this technology will open up new opportunities to create, parameterise and evaluate soil models as the motion of individual soil particles can now be easily monitored.

The Frame of Fixed Stars in Relational Mechanics

Relational mechanics is a gauge theory of classical mechanics whose laws do not govern the motion of individual particles but the evolution of the distances between particles. Its formulation gives a satisfactory answer to Leibniz's and Mach's criticisms of Newton's mechanics: relational mechanics does not rely on the idea of an absolute space. When describing the behavior of small subsystems with respect to the so called "fixed stars", relational mechanics basically agrees with Newtonian mechanics. However, those subsystems having huge angular momenta will deviate from the Newtonian behavior if they are described in the frame of fixed stars. Such subsystems naturally belong to the field of astronomy; they can be used to test the relational theory.

Reynolds-number dependence of turbulence enhancement on collision growth

Abstract. This study investigates the Reynolds-number dependence of turbulence enhancement on the collision growth of cloud droplets. The Onishi turbulent coagulation kernel proposed in Onishi et al. (2015) is updated by using the direct numerical simulation (DNS) results for the Taylor-microscale-based Reynolds number (Reλ) up to 1140. The DNS results for particles with a small Stokes number (St) show a consistent Reynolds-number dependence of the so-called clustering effect with the locality theory proposed by Onishi et al. (2015). It is confirmed that the present Onishi kernel is more robust for a wider St range and has better agreement with the Reynolds-number dependence shown by the DNS results. The present Onishi kernel is then compared with the Ayala–Wang kernel (Ayala et al., 2008a; Wang et al., 2008). At low and moderate Reynolds numbers, both kernels show similar values except for r2 ∼ r1, for which the Ayala–Wang kernel shows much larger values due to its large turbulence enhancement on collision efficiency. A large difference is observed for the Reynolds-number dependences between the two kernels. The Ayala–Wang kernel increases for the autoconversion region (r1, r2 < 40 µm) and for the accretion region (r1 < 40 and r2 > 40 µm; r1 > 40 and r2 < 40 µm) as Reλ increases. In contrast, the Onishi kernel decreases for the autoconversion region and increases for the rain–rain self-collection region (r1, r2 > 40 µm). Stochastic collision–coalescence equation (SCE) simulations are also conducted to investigate the turbulence enhancement on particle size evolutions. The SCE with the Ayala–Wang kernel (SCE-Ayala) and that with the present Onishi kernel (SCE-Onishi) are compared with results from the Lagrangian Cloud Simulator (LCS; Onishi et al., 2015), which tracks individual particle motions and size evolutions in homogeneous isotropic turbulence. The SCE-Ayala and SCE-Onishi kernels show consistent results with the LCS results for small Reλ. The two SCE simulations, however, show different Reynolds-number dependences, indicating possible large differences in atmospheric turbulent clouds with large Reλ.

Laboratory-scale validation of a DEM model of screening processes with circular vibration

In this work, DEM results and experimental data from a specially designed circularly vibrating screening model are compared quantitatively under a range of operating conditions. The validation is carried out in two aspects. First, the motion of tracked particles is recorded using high-speed dynamic analysis system and their velocities are compared with DEM results. Second, the total screening performance indexes are compared to those predicted by DEM. Overall agreements for both spherical and non-spherical particle models are observed in both the motion of individual particles and the screening performance quantities. Generally, better agreements are found for non-spherical particle model with the particle shape reasonably considered. Quantitatively, spherical particle model is less accurate because the simplification of particle shape leads to good mobility and over-prediction of velocity and screening efficiency. The validity of the DEM model of the screening process is hence approved. The validated DEM model can be applied to perform extensive numerical studies to achieve optimal performance and better operation of such complicated and important processes in mineral processing.

Two-dimensional systems with competing interactions: dynamic properties of single particles and of clusters

Systems with short-range attractive and long-range repulsive interactions are able to form mesophases at sufficiently low temperatures. In two dimensions, such mesophases emerge as clusters, stripes or bubbles. Using extensive Monte Carlo simulations we investigate the static and the dynamic properties of such a cluster-forming system over a broad temperature range and for different densities. Via the static properties we analyse how ordering into close packed configurations sets in both at the level of the particles as well as at the level of the clusters. The dynamic properties provide information on how, at low temperature, the motion of individual particles is influenced by the dynamic slowing down of the clusters. Finally, we discuss the different diffusion mechanisms at play at low and intermediate densities.

Nucleation and structural growth of cluster crystals.

We study the nucleation of crystalline cluster phases in the generalized exponential model with exponent n = 4. Due to the finite value of this pair potential for zero separation, at high densities the system forms cluster crystals with multiply occupied lattice sites. Here, we investigate the microscopic mechanisms that lead to the formation of cluster crystals from a supercooled liquid in the low-temperature region of the phase diagram. Using molecular dynamics and umbrella sampling, we calculate the free energy as a function of the size of the largest crystalline nucleus in the system, and compare our results with predictions from classical nucleation theory. Employing bond-order parameters based on a Voronoi tessellation to distinguish different crystal structures, we analyze the average composition of crystalline nuclei. We find that even for conditions where a multiply occupied fcc crystal is the thermodynamically stable phase, the nucleation into bcc cluster crystals is strongly preferred. Furthermore, we study the particle mobility in the supercooled liquid and in the cluster crystal. In the cluster crystal, the motion of individual particles is captured by a simple reaction-diffusion model introduced previously to model the kinetics of hydrogen bonds.

Sediment micromechanics in sheet flows induced by asymmetric waves: A CFD–DEM study

Understanding the sediment transport in oscillatory flows is essential to the investigation of the overall sediment budget for coastal regions. This overall budget is crucial for the prediction of the morphological change of the coastline in engineering applications. Since the sediment transport in oscillatory flows is dense particle-laden flow, appropriate modeling the particle interaction is critical. Although traditional two-fluid approaches have been applied to the study of sediment transport in oscillatory flows, the approaches do not capture the interaction of the particles. The study of the motion of individual sediment particles and their micromechanics (e.g., packing and contact force) in oscillatory flows is still lacking. In this work, a parallel CFD–DEM solver SediFoam that can model the inter-particle collision is applied to study the granular micromechanics of sediment particles in oscillatory flows. The results obtained from the CFD–DEM solver are validated by using the experimental data of coarse and medium sands. The comparison with experimental results suggests that the flow velocity, the sediment flux and the net sediment transport rate predicted by SediFoam are satisfactory. Moreover, the micromechanic quantities of the sediment bed are presented in detail, including the Voronoi concentration, the coordination number, and the particle interaction force. It is demonstrated that the variation of these micromechanic quantities at different phases in the oscillatory cycle is significant, which is due to different responses of the sediment bed. To investigate the structural properties of the sediment bed, the correlation of the Voronoi volume fraction and coordination number is compared to the results from the fluidized bed simulations. The consistency in the comparison indicates the structural micromechanics of sediment transport and fluidized bed are similar despite the differences in flow patterns. From the prediction of the CFD–DEM model, we observed that the coordination number in rapid sheet flow layer is larger than one, which indicates that a typical particle in the sediment layer is in contact with more than one particles, and thus the binary collision model commonly used in two-fluid approaches may underestimate the contact between the particles.

Macroscopic characteristics of heavy particle resuspension obtained from direct numerical simulations of pressure driven channel flow

Three-dimensional direct numerical simulations are presented of the flow of fluid containing heavy spherical particles in a channel. The simulations resolve the motion of individual particles and the details of the flow at the particle level. They are based on the finite element method and the Distributed Lagrange Multiplier/ Fictitious Domain Method. The problem is characterized by four parameters, namely a Reynolds and an Archimedes number, a density ratio and a dimensionless particle size. The last three are fixed and the effects of the Reynolds number are studied over a range covering the onset of particle resuspension and the transition of flow to significant unsteadiness. Encouraging estimates for the onset of resuspension and the particle flux as a function of the excess Shields number are found. The results are analyzed in terms of the vertical distribution of horizontally averaged macroscopic quantities, obtained by applying integral momentum balances. On the basis of such balances, particle stresses and interfacial forces between the fluid and particle phases are estimated and compared to existing models, developed mainly for Stokes flow conditions. The qualitative agreement is encouraging but points to the need for taking into account several effects, notwithstanding those of finite particle Reynolds numbers. Provided that such information becomes available the set of four equations derived from the suspension and particle phase momentum equations could describe the average equilibrium distribution, the flow of both phases and the pressure in the fluid.