Granular Segregation Research Articles

Origin of granular axial segregation bands in a rotating tumbler: An interface-mixing driven Rayleigh-Taylor instability

The origin of large and small particle axial bands in long rotating tumblers is a long-standing question. Using DEM simulations, we show that this axial segregation is due to a Rayleigh-Taylor instability which is characterized by the fact that the density of a granular medium increases with mixing and decreases with segregation. For initially mixed particles, segregation and collisional diffusion in the flowing layer balance and lead to a three-layer system, with a layer of large particles over a layer of small particles, and, interposed between these layers, a layer of more densely packed mixed particles. The higher density mixed particle layer over the lower density small particle layer induces a Rayleigh-Taylor instability, evident as waviness in the interface between the layers. The waviness destabilizes into ascending plumes of small particles and descending plumes of mixed particles with large particles enriched near the surface, which become evident as small and large particle bands visible at the free surface. Rolls driven by segregation at the tilted interface between plumes maintain the pattern of frozen plumes. Published by the American Physical Society 2024

Microscopic mechanism of squeeze expulsion in granular size segregation

Microscopic mechanism of squeeze expulsion in granular size segregation

Identification of dominant DEM parameters for multi-component segregation during heap formation, hopper discharge and chute flow

Segregation of granular materials is a critical phenomenon in various industries, such as food processing, pharmaceuticals, and mining. The Discrete Element Method (DEM) is an effective tool for gaining insight into granular segregation by providing particle-level information and the freedom to model mixtures that are often difficult or impossible to achieve through experiments. To ensure realistic material behaviour and correct representation of segregation, it is essential to calibrate the model parameters systematically. However, in the context of multi-component segregation, it is extremely challenging and computationally expensive to consider all parameters in the calibration procedure since interaction parameters between components must also be considered. This work aims to identify the dominant DEM parameters for modelling multi-component segregation during hopper discharge, chute flow and heap formation in a mixture of pellet and sinter. Utilising a representative example of a multi-component mixture with different sizes, densities and shapes used in blast furnaces, the investigation is done for various initial configurations as well as various mass ratios of the mixture. Our findings revealed that, while only particle-geometry interaction parameters dominate the segregation after the hopper and chute flow, particle-particle parameters are also significant for segregation in the heap. We also demonstrated that the downstream segregation is significantly influenced by the segregation upstream. Moreover, we found that the effect of pellet-sinter interactions is negligible. This research provides insights into the dominant DEM parameters, facilitating more efficient and robust calibration of multi-component models in future research endeavours.

Self-replicating segregation patterns in horizontally vibrated binary mixture of granules

Fluidized granular mixtures of various particle sizes exhibit intriguing patterns as different species segregate and condense. However, understanding the segregation dynamics is hindered by the inability to directly observe the time evolution of the internal structure. We discover self-replicating bands within a quasi-2D container subjected to horizontal agitation, resulting in steady surface waves. Through direct observation of surface flow and evolving internal structures, we reveal the crucial role of coupling among segregation, surface flow, and hysteresis in granular fluidity. We develop Bonhoeffer-van der Pol type equations grounded in experimental observations, reproducing complex band dynamics, such as replication, oscillation, and breathing. It suggests the similarity between pattern formation in granular segregation and that in reaction–diffusion systems.

Experimental investigation into segregation behavior of spherical/non-spherical granular mixtures in a thin rotating drum

This paper uses physical experiments to investigate the segregation behavior of binary granular mixtures in a quasi-two-dimensional rotating drum. Spherical polyformaldehyde (POM) beads and cylindrical red beans constitute the granular mixtures. The effects of particle size, particle density, and particle shape interplay during the segregation process in the spherical/non-spherical particulate system. A long-axis ratio (LAR), the ratio of the spherical POM beads' diameter to the red beans' primary dimension, was defined to explore the particle shape effect. The experimental results show that the long-axis ratio and the rotation speed play substantial roles in the granular segregation behavior. As the long-axis ratio increases, the steady-state segregation intensity decreases. An increase in the rotation speed enhances the segregation of the binary granular mixtures for each long-axis ratio studied here. In addition, the average velocity and granular temperature of spherical POM beads increase as the long-axis ratio increases. Both properties also increase as the rotation speed increases. The dynamic angle of repose for the binary mixtures increases with the increase in the long-axis ratio. Most interestingly, reverse granular segregation does occur at a long-axis ratio of 0.70 with the cylindrical red beans in the core and the spherical POM beads at the periphery for each rotation speed studied here. This reverse segregation has not been observed in previous studies. This highlights the substantial impact of particle shape on the granular segregation in binary granular mixtures.

How particle shape affects granular segregation in industrial and geophysical flows

Industrial and environmental granular flows commonly exhibit a phenomenon known as "granular segregation," in which grains separate according to physical characteristics (size, shape, density), interfering with industrial applications (cement mixing, medicine, and food production) and fundamentally altering the behavior of geophysical flows (landslides, debris flows, pyroclastic flows, riverbeds). While size-induced segregation has been well studied, the role of grain shape has not. Here we conduct numerical experiments to investigate how grain shape affects granular segregation in dry and wet flows. To isolate the former, we compare dry, bidisperse mixtures of spheres alone with mixtures of spheres and cubes in a rotating drum. Results show that while segregation level generally increases with particle size ratio, the presence of cubes decreases segregation levels compared to cases with only spheres. Further, we find differences in the segregation level depending on which shape makes up each size class, reflecting differences in mobility when smaller grains are cubic or spherical. We find similar dynamics in simulations of a shear-driven coupled fluid-granular flow (e.g., a simulated riverbed), demonstrating that this phenomenon is not unique to rotating drums; however, in contrast to the dry system, we find that the segregation level increases in the presence of cubic grains, and fluid drag effects can qualitatively change segregation trends. Our findings demonstrate competing shape-induced segregation patterns in wet and dry flows that are independent from grain size controls, with implications for many industrial and geophysical processes.

Segregation of binary particles in pulsed gas-solid fluidized bed

Pulsating airflow-driven gas-solid fluidization is an effective method to segregate particles of different densities. The current article presents an elaborate computational study on the development of simulation methodology, and parametric trend identification of pulsating airflow-driven granular segregation for the first time using computational fluid dynamics (CFD), discrete element modeling (DEM) coupled analysis. The rigorous validation of the computational prediction has been produced against the experimental result reported by Li et al. (2021) and an excellent agreement has been observed. Segregation behavior for different density ratios of particles, different patterns of pulsation, and different particle shapes is presented in the current article. The segregation index improves with the density ratio of the binary mixture of particles, but the rate of increment of the segregation index is highly dependent on the airflow rate. Near the optimal value of airflow rate, the rate of increment of the segregation index reduces with the density ratio. Keeping the bed height and material property constant, different patterns of pulsation namely—sinusoidal, triangular, and exponential have been adopted. The segregation index is higher for sinusoidal patterns, whereas it reduces when the pattern is changed to triangular and exponential. Particles of constant volume with different shapes are also considered to understand the effect of particle shape on segregation. The segregation index deteriorates with the increasing surface area of the particle at constant volume.

The significance of the trapped kink in granular segregation pattern formation

In binary-size granular segregation, when the avalanche stops, the surface of the granular bed is raised upstream of the flow layer, forming a local wedge structure, called trapped kink, which is important for segregation pattern formation. In this paper, we established the relationship between the position distribution of trapped kink head and granular segregation patterns. The experimental result shows that the position distribution of the trapped kink head is a log-normal distribution. The core pattern appeared when the standard deviation of the distribution is smaller than 0.4. When the standard deviation of the distribution is larger than 0.4, the segregation pattern would be from the core pattern to the streak pattern. The position distribution of the trapped kink head is affected by factors, the repose angle and the avalanche time. This provides an important theoretical basis for describing the formation of the segregation pattern and improving the segregation model. • The position distribution of the trapped kink head was measured and counted. • The relationship between the granular segregation patterns and the position distribution of trapped kink head was established. • The factors affecting the trapped kink were analyzed.

Transient segregation of ternary granular mixtures simultaneously charged into and discharged from a shaft furnace

The simultaneous charging at the top and discharging at the bottom inevitably have the combined effects on granular segregation in the shaft furnace. Herein, we first establish a 1:1 full mathematical model of shaft furnace and validate the parameters for the discrete element method (DEM) simulation. Then a quantified method based on an equilateral triangle is updated to characterize the ternary-component (coke, sinter, and pellet) granular segregation. The results show that compared with the sole operation, the simultaneous charging and discharging operations make the overall ternary segregation worse in the central and peripheral regions of the shaft furnace. Furthermore, transient granular behaviors in terms of the granular profile, the instantaneous segregation, the particle-scale trajectory and vertical velocity, and the granular number distribution are given full discussions to provide a data-driven insight into the combined effects on granular segregation.

Experimental investigation into the wet non-spherical granular segregation and mixing in rotating drums

This paper experimentally investigates the wet non-spherical granular segregation in a quasi-2D rotating drum. The granular mixtures consisted of spherical POM beads and non-spherical red beans. To explore the interlocking effect resulting from particle shape, a long axis ratio was defined. Experimental results reveal that both the long axis ratio and the liquid content play crucial roles in wet non-spherical granular segregation. The steady-state segregation index decreases monotonically with the increase of long axis ratio. As the long axis ratio increases, the average velocity and granular temperature of spherical POM beads increase. Both properties also increase monotonically with liquid content. The dynamic angle of repose increases with the increasing liquid content, whereas the steady-state segregation index decreases. Reverse segregation does occur in a dry non-spherical mixture with a size ratio of 1.42, indicating that the shape effect appears to enhance the degree of reverse segregation appreciably. • Shape effect has a significant influence on granular segregation. • Final segregation index decreases monotonically with the long axis ratio. • Reverse granular segregation occurs at a specific LAR = 0.704. • Reverse granular segregation was mitigated in wet granular systems.

Investigation of local process in granular segregation based on discrete element method

The phenomenon of granular size segregation under vibration has plagued researchers due to the unsteady and high particle number concentration. Discrete element method (DEM) is employed to simulate the process of intruder rising through small background particles from the bottom to the top in a cylindrical container. To describe the segregation level during this process, a segregation evaluation parameter, α, is introduced. It is found that the granular process can be divided into two stages according to the evaluation of α, an initial stage (IS) and a stable intermediate stage (SIS). For a better understanding of the segregation process in these different stages, a new viewpoint focused on the local processes is presented in this article. The segregation mechanism is clarified through numerical simulation of local processes and reference to previous research results.

Genetic relationship of microstructure and mechanical properties between ferrite/pearlite steel plate and billet

This paper proposed the double-pass compression thermal/force simulation experiment to in situ analyze the phase transformation of continuous casting billet during the controlled rolling and controlled cooling process. The genetic relationship between the central segregation of the billet and solute element distribution of the steel plate, and also the genetic relationship of microstructure and mechanical properties between them were experimentally studied. The results show that the microstructure and mechanical properties between the billet and corresponding batch of steel plate have the genetic relationship, and the link of the relationship is central segregation. The phase transformation of supercooling austenite will be affected by the central segregation of Mn and C elements, and the granular bainite abnormal segregation band is the transformation product of Mn and C element segregation region.

Prediction of segregation characterization based on granular velocity and concentration in rotating drum

In a binary granular system composed of two types of particles with different granule sizes and the same density, particle sorting occurs easily during the flow process. The segregation pattern structure is mainly affected by the granular velocity and granular concentration in the flow layer. This paper reports on the experimental velocity and concentration measurement results for spherical particles in a quasi-two-dimensional rotating drum. The relationship between the granular velocity along the depth direction of the flow layer and granular concentration was established to characterize structures with different degrees of segregation. The corresponding relationships between the granular velocity and concentration and the segregation pattern were further analyzed to improve the theoretical models of segregation (convection–diffusion model and continuous flow model) and provide a reference for granular segregation control in the production process.

Multifluid model simulations of gravitational instabilities in fluidized binary granular materials

AbstractPredicting granular flow using continuum approaches is fundamental to advance granular physics and industrial applications. Previous studies have demonstrated that continuum modeling can capture some hydrodynamic instability analogs in monodisperse granular materials. Recently, a family of gravitational instabilities was found between two types of grains, including a Rayleigh–Taylor analog, a granular bubble rising, a granular droplet splitting and particle segregation. The high packing fraction and bi‐disperse nature of these flows allow for new challenges to continuum models. Herein, we applied multifluid modeling (MFM) that treats the gas phase and two separate granular phases as fully interpenetrating continua to simulate these instabilities and compared them with newly conducted experiments. Simulation results reproduced all of the four instabilities and demonstrated that frictional particle stress and solids–solids drag are needed to capture critical qualitative flow features. This study can serve as a basis for challenging future particle stress and solids–solids drag models.

Surface band segregation and internal convection in rotating sphere densely filled with granular material: Experiments

While granular segregation in partially filled containers has been studied extensively, granular dynamics in densely filled spheres is not fully understood. Here, surface band segregation and granular convection are reported in a rotating sphere of highly compacted glass beads. Distinct from the Rayleigh–Bénard convection, granular convection has a butterfly shaped structure with vortexes of alternating layers of small/large beads, which is stable and independent of the sphere size. Two concentric interfaces at the zero tangential/norm flux are discovered, which divide the sphere into three layers from the surface to the core. The law that governs the jamming dynamics in rotating spheres remains an open question.

The effect of recoater geometry and speed on granular convection and size segregation in powder bed fusion

In metal additive manufacturing (AM), powder bed fusion technologies such as selective laser melting and binder jetting rely on the spreading of fine metal powder to build up the layers of a part. This work studies the complex interaction between metal powder particles and the recoater. We apply the Discrete Element Method (DEM) to a calibrated cohesive Ti-6Al-4V powder model, matching the particle properties to experimental measurements of size, shape and inter-particle forces. We simulate powder bed fusion recoating at varying speeds and layer thicknesses with two different recoater geometries: a toothed rake and a solid blade. Our results demonstrate that the recoating mechanism induces significant granular convection, with the recoater velocity and geometry strongly influencing the degree of particle circulation and size segregation. Notably a toothed rake recoater improved particle circulation – while the solid blade recoater increases size segregation within the heap. Furthermore the recoater was found to filter fine and coarse particles, allowing smaller particles to flow underneath the recoater, and larger particles through its teeth. Our results demonstrate the key mechanisms which drive granular convection during recoating and how the fine details of recoater geometry impacts surface layer roughness and final part quality.

Experimental investigations for the effect of fine powders on size-induced segregation in binary granular mixtures

This paper presents experimental investigations on the size-induced segregation behavior of binary granular mixtures in a rotating drum by adding a small amount of fine powders. Four different fine powder contents were selected, and three different rotation speeds were adopted. Experimental results reveal that the flowability of granular mixtures can be enhanced by a lubrication effect induced by a proper amount of fine powders. The segregation index and rate increase with the increasing fine powder content and rotation speed. The local-ensemble average velocity and granular temperature for large main particles also increase with the increase of fine powder content and rotation speed. The increase of fine powder content provides stronger lubrication effects, and the increase of rotation speed offers more energy into the granular mixtures. The dynamic properties proposed herein can characterize the size-induced granular segregation behavior. The increasing fine powder content and rotation speed facilitate size-induced granular segregation.

Microscopic structure and dynamics study of granular segregation mechanism by cyclic shear.

Granular mixtures with size difference can segregate upon shaking or shear. However, the quantitative study of this process remains difficult because it can be influenced by many mechanisms. Conflicting results on similar experimental systems are frequently obtained when the experimental conditions are not well controlled, which is mainly due to the fact that many mechanisms can be at work simultaneously. Moreover, it is often that macroscopic or empirical measures, which lack microscopic physical bases, are used to explain the experimental findings and therefore cannot provide an accurate and complete depiction of the overall process. Here, we carry out a detailed and systematic microscopic structure and dynamics study of a cyclically sheared granular system with rigorously controlled experimental conditions. We find that both convection and arching effect play important roles in the segregation process in our system, and we can quantitatively identify their respective contributions.

Granular segregation on the rubble-pile asteroid Itokawa

We investigate the dynamics of regolith on rubble-pile asteroids to explain granular processes observed in reality. In particular, we explain how the appearance of boulders on the surface of asteroid Itokawa could have resulted from a size sorting process in granular media called the Brazil Nut Effect (BNE). The Discrete Element Method (DEM) is implemented to perform numerical simulations of the BNE in a micro-gravity environment caused by inter-particle collisions during seismic vibrations. Firstly, we present the results of how the BNE depends on the magnitude of surface gravity. It is estimated that segregation processes on Itokawa occur over much longer time-scales (in the order of a few hundred years) than the same processes would require in the presence of a strong gravitational field, like on Earth. Secondly, we also find that the size sorting could also result from kinetic sieving encountered during granular avalanches. Finally, we discuss how the void-filling mechanism becomes more efficient when there is a higher relative size difference between the boulders and the surrounding grains. Our model has important implications in understanding the resurfacing of Itokawa by trying to explain one of the many complex geophysical processes that occur in such unique conditions.

How universal is the vibration-velocity controlled granular convection?

Recently, a number of articles have reported that granular convection induced by continuous vibration is controlled by vibration velocity, in contrast with some previous studies. We have reported such an example for the Brazil nut effect when the vibration is given discontinuously, using a one-layer granular bed in a cell with down-facing side walls. Here, we report the effect of vibration phase and wall friction using the same experimental system, to confirm rising motion of an intruder induced by granular convection is again governed by vibration velocity. We compare two different cases of vibration phase for giving intermittent vibration cycles, and found one, in which granular packing is well established before grains start to lose contacts due to vibration, provides distinctly high reproducibility. We further control the side wall friction using a microfabrication technique, and found that significantly high reproducibility is attained in a cell with vertical side walls when a millimeter texture is introduced on the side walls. Our results indicate that the granular convection is universally controlled by vibration velocity. The present study opens a way to conduct highly reproducible experiments on granular dynamics, which is indispensable for deep physical understanding of granular flow and segregation.