Study Of Granular Flows Research Articles

Planar column collapse of elongated grains

The granular column collapse is a benchmark configuration for the study of granular flows in dry, saturated, and submerged conditions. The collapse sequence and resultant mobility is acknowledged to be controlled by the column aspect ratio, while grain properties define the relative transition of each stage. Grain shape effects are found to modify the global shear resistance of granular media, with a strong and coupled interaction when interacting with a fluid. In this work, we present the first steps towards the study of grain shape effects in a column collapse when interacting with an ambient fluid. For this purpose, we use a planar configuration and explore the collapse of a column consisting of rod-like grains and study the initial and after collapse grain orientations. On it, the mobilized grains deposit in a preferential horizontal orientation, but further experiments are required to confirm if a nematic configuration can be achieved.

A study of granular flow through horizontal wire mesh screens for concentrated solar power particle heating receiver applications – Part II: Parametric model predictions

Numerical methods are increasingly being used to study granular flow phenomena, given the difficulty in extracting data from physical studies. Two common methods are the discrete element method (DEM), and a two fluid computational fluid dynamics (CFD) method. A companion paper (Sandlin and Abdel-Khalik, 2018) compared a DEM model and a CFD model of a granular flow through horizontal wire mesh screens, with experimental data. The aim of this study is to assess the sensitivity of both models to various input parameters. The results of this investigation will guide future modelers of particle heating receivers and other flowing particulate systems in selecting the appropriate modeling options and parameters to enhance the models ability to predict the actual particulate flow characteristics. For the DEM model, it was found that the granular material properties, especially the values for normal and rolling friction, had the largest impact on simulation results. For the CFD model, it was found that the constitutive relationships for frictional pressure, viscosity, and the treatment of physical boundary conditions had the largest impact on simulation outcomes. In addition, both numerical models exhibit a non-monotonic relationship between mass flux and the granular coefficient of restitution, and show reduced mass flux when using a simulation domain with offset wire meshes. The influence of other material properties and sub-modeling options is less pronounced. Methods of obtaining appropriate material properties and sub-modeling options are discussed.

A study of granular flow through horizontal wire mesh screens for concentrated solar power particle heating receiver applications – Part I: Experimental studies and numerical model development

A proposed design for a concentrating solar power (CSP) receiver uses granular material – such as sand – as the heat transfer and energy storage medium. Early designs of particle heating receivers (PHR) utilize a falling curtain of particles which directly absorbs the concentrated solar radiation. However, falling curtain receivers have several disadvantages including significant heat and particle losses and short residence time within the irradiation zone. One design proposal which overcomes these challenges is the so called “impeded flow PHR design”, in which the particles flow over, around, or through a series of obstacles in the flow path. This reduces the average velocity of the particles, thereby increasing their residence time in the irradiation zone of the receiver. It also reduces heat and particle losses from the receiver. However, granular flows through complex structures are not well understood, rendering a priori design of impeded flow PHR geometries difficult. To better understand these flows, lab scale models of a PHR design variant using horizontal wire mesh screens have been constructed, allowing granular flows through the receiver geometry to be experimentally analyzed. In addition, two different numerical modeling approaches – the discrete element method (DEM) model, and a two-fluid computational fluid dynamics (CFD) model – have been developed to model the flow of particles through the specified receiver geometry. The results of the DEM model are in reasonable agreement with the experimental data with respect to mass flux, and better matches the experimental data than the CFD model. A companion paper presents parametric studies to assess the sensitivity of model predictions to various design and modeling parameters (Sandlin and Abdel-Khalik, 2017).

Study of granular flow in silo based on electrical capacitance tomography and optical imaging

This paper presents a comparison between optical images and two different modalities of electrical capacitance tomography (ECT), namely an off-the-shelf ECT system and a model one combining an inductance (L) - capacitance (C) - resistance (R) meter with a multiplexer. This comparison was performed to analyse the degree of measurement accuracy and to estimate the spatial resolution of the ECT systems. In this case, the detection of the position of flow boundaries during silo discharging and its translation when discharging changes from concentric to eccentric were investigated. A versatile rectangular silo model was used to generate different types of funnel flow of rice material, which was designated as the most adapted granular material to create both optical and electrical permittivity contrasts between stagnant and flowing zones. When a high temporal resolution is not requested, the model ECT system reduces the measurement error of flow boundary detection between 3% and 9% when compared with optical measurements. Moreover, the system is able to capture boundary translation of about 5% of the silo bin width, which is below the assumed spatial resolution of standard commercial ECT systems.

Transport analogy for segregation and granular rheology.

Here, we show a direct connection between density-based segregation and granular rheology that can lead to insight into both problems. Our results exhibit a transition in the rate of segregation during simple shear that occurs at I∼0.5 and mimics a coincident regime change in flow rheology. We propose scaling arguments that support a packing fraction criterion for this transition that can both explain our segregation results as well as unify existing literature studies of granular rheology. By recasting a segregation model in terms of rheological parameters, we establish an approach that not only collapses results for a wide range of conditions, but also yields a direct relationship between the coordination number z and the segregation velocity. Moreover, our approach predicts the precise location of the observed regime change or saturation. This suggests that it is possible to rationally design process operating conditions that lead to significantly lower segregation extents. These observations can have a profound impact on both the study of granular flow or mixing as well as industrial practice.

A Study of Granular Flow in a Conical Hopper Discharge Using Discrete and Continuum Approach

Discrete and continuum modelling of granular flow in a conical hopper discharge is presented. The Discrete Element Method (DEM) is used for discrete modelling and Finite Element Method based on an Arbitrary Lagrangian-Eulerian (ALE) formulation for continuum modelling. The ALE model has shown its capability to model granular flow and macroscopic behaviour in silos in the authors’ previous work (Wang et al., 2013). The use of ALE overcomes mesh distortion problem due to material large deformation which is often encountered in standard finite element modelling of silo discharge. However, there is still limitation when a continuum method is used to model silo discharge, in particular, to pursue particle-scale features in such granular flow. In this study, wall pressure, flow pattern and flow rate during silo discharge are investigated using both ALE and DEM simulation. Classical theories are used as a comparison, to verify the two numerical models. By comparison, it is indicated that both FE model using ALE formulation and DEM model have the capacity to model the granular behaviour and wall pressure for granular process in silos. The conclusion that ALE has advantages over DEM in wall pressure prediction giving stable pressure profile with smooth trend; while DEM has the ability to evaluate the role of particle-scale parameters on macro-scale response in such granular flow after carefully smoothing out the results has been drawn in the present study.

Chute flow as a means of segregation characterization

Handling of granular materials involves the use of many solids processing devices such as chutes, hoppers, tumblers or other transfer equipment. Unfortunately, granular materials segregate if they are subjected to flow or external agitation in the presence of a gravitational field. Thus, operation of many of these devices is prone to segregation. Many studies of granular flow have focused on gravity driven chute flows owing to their practical importance in granular processing and to the fact that the relative simplicity of this type of flow allows for development and testing of new theories/equipment. In the present work, we observe the deposition behavior of both mono-sized and polydisperse dry granular materials in an inclined chute flow, both experimentally and computationally. We investigate the effects on the mass fraction distribution of granular materials of different parameters such as chute angle, particle size, falling height and charge amount. The simulation results obtained using the Discrete Element Method (DEM) are compared with the experimental findings and a high degree of agreement is observed. Tuning of the underlying contact force parameters allows realistic results and is used as a means of validating the simulation model against available experimental data. The tuned simulation is then used to test predictions of granular segregation theories outlined in previous studies. That is, it has been predicted that a system can maintain a mixed configuration when L<Uavgts, where L, Uavg, and ts denote the length of the chute, the average stream-wise flow velocity of the particles, and the characteristic time of segregation, respectively. Our results with the tuned simulation support these conclusions for chute flows.

Control of the fluctuation in the uniform granular flow by a random force field

A normal distribution random force field is introduced into the study of granular flow, and its effect is observed by computer simulations. The results show that the random force almost does not cause changes in average density and velocity in a uniform granular flow, and affects little the fluctuation of the density. The main effect of the random force field is that it increases the fluctuation of the granular velocity and maintains the granular flow at a certain dispersed kinetic energy by competing with the dissipation of the granular system. It is also shown that the dispersed kinetic energy obtained by the random force field is not equally distributed in each degree of freedom, and that the equipartition of energy is difficult to realize in granular system because of its dissipation property.

Bedload: a granular phenomenon

AbstractBedload, the transport of sediment remaining in contact with the stream bed, has mainly been studied from the perspective of the correlation between fluid driving forces and the responding sediment flux. Yet grain–grain interactions are important and bedload should also be considered as a granular phenomenon. We review progress made recently in the study of granular flows, especially on segregation and rheology, that better illuminates the nature of bedload. Granular flows may exhibit gas‐like or fluid‐like flow, or quasi‐solid deformation. All three conditions might be duplicated in bedload. Understanding of intense bedload transport occurring continuously in a layer several grains deep – typical of sand beds – might greatly benefit from results in granular physics, as illustrated by grain‐inspired bedload results. However, processes restricted to the surface of the bed, when particles move intermittently and the bed becomes structured, while characteristic in gravel‐bed channels, are not well addressed in granular physics. Mutual study of these phenomena may benefit both physics and fluvial geomorphology. We intend, therefore, to contribute to an enhanced dialogue between granular physics and bedload science communities. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Homogeneous Cooling States¶are not always¶Good Approximations to Granular Flows

A widespread belief in the study of granular flow is the existence of “homogeneous cooling states”, i.e., self-similar solutions which would attract all solutions, faster than the equilibrium solution does. In most cases, the existence of these self-similar solutions is an open problem. Here we consider a one-dimensional model, which has been used for some years, and for which simple self-similar solutions do exist. However, we prove that the approximation is quite poor. Our proof makes use of the powerful and simple tools of mass transportation, and exploits the structure of the evolution equation, seen as a nonlinear transport equation.