Granular Flow System Research Articles

Study of oblique granular impingement flow through CFD-DEM simulations and energy consumption statistics

This paper investigates an experimental granular impingement flow system through simulations and energy consumption statistics. With the increase of solid concentration, flow patterns undergo cross flow (dense edge and dilute center), dispersed flow, and mixed-jet flow (dilute edge and dense center) in turn. The influence of gas phase on the flow pattern was investigated. According to the analysis of force and time-averaged velocity, it is revealed that gas-solid interaction will cause energy dissipation of granular flow, which cannot be ignored in specific cases. Then, a flow map is proposed according to both the Archimedes number and solid concentration. In addition, the flow pattern transition and local structure variations can be reflected by energy consumption statistics. Based on the simulations and methods developed in this work, it is promising to deepen the understanding and study of granular impingement flow from the viewpoints of energy consumption and new-type flow map.

Two‐ and three‐dimensional multiphase mesh‐free particle modeling of transitional landslide withμ(I) rheology

AbstractLandslides, which are the sources of most catastrophic natural disasters, can be subaerial (dry), submerged (underwater), or semi‐submerged (transitional). Semi‐submerged or transitional landslides occur when a subaerial landslide enters water and turns to submerged condition. Predicting the behavior of such a highly dynamic multi‐phase granular flow system is challenging, mainly due to the water entry effects, such as wave impact and partial saturation (and resulted cohesion). The mesh‐free particle methods, such as the moving particle semi‐implicit (MPS) method, have proven their capabilities for the simulation of the highly dynamic multiphase systems. This study develops and evaluates a numerical model, based on the MPS particle method in combination with theμ(I) rheological model, to simulate the morphodynamic of the granular mass in semi‐submerged landslides in two and three dimensions. An algorithm is developed to consider partial saturation (and resulting cohesion) during the water entry. Comparing the numerical results with the experimental measurements shows the ability of the proposed model to accurately reproduce the morphological evolution of the granular mass, especially at the moment of water entry.

The Study of Three-Dimensional Granular Stream Flowing through the Test Hopper-Shaped Target

The experiments are carried out in a three-dimensional channel with a screw conveyor, which plays the role of granular drives for the granular flow system and determines the injection of granular in the test target section. The jam-to-dense transition of granular flow is studied with the different inclination angle. The results show that, with a fixed diameter of hopper orifice and initial filling position, there is a change from jam to dense when the inclination angle larger than 22°. Variation of the flow rate with elevated frequency of the screw conveyor is further studied. The flow pattern is changed from dilute to dense with increasing rotation frequency of the screw rod. When the rotation frequency is larger than 5 Hz, the flow is dense. The dynamic balance of the interface between dilute to dense granular is observed in the main target section. We further research the dynamic interface by measuring the highest and lowest location with time and also simulate the gravity flow rate and screw conveyor flow rate with EDEM. From the results, we find that the interface between dilute flow and dense flow is influenced by the combined action of crew conveyor flow and dense gravity flow.

A drag force formula for heterogeneous granular flow systems based on finite average statistical method

The existing drag models are mostly based on the assumption of homogenous fluidization. However, the use of a homogeneous drag model to predict a heterogeneous granular flow system will cause a deviation. In this study, we developed a drag force model based on the assumption of heterogeneous fluidization. To prevent weakening of the heterogeneous characteristics in the drag force formula, we propose a finite average statistical method to filter the information of the heterogeneous granular cluster. The filtered information was used to fit the modified drag formula, which can reflect the heterogeneity of the granular cluster considering different configurations. A comparison shows that the new proposed drag formula filtered by the finite average statistical method fits well with energy minimization multi-scale simulation results.

Size segregation in compressible granular shear flows of binary particle systems

This paper deals with the modelling and simulation of segregation in granular materials. The basis is a hydrodynamic model for granular material flows, which is extended to capture the dynamic process of segregation in shear flows of systems with small and large particles. The granular flow equations consist of a set of compressible Navier–Stokes-like equations as well as an equation for the granular temperature. With the help of the granular temperature equation, the granular flow equations are able to cover a wide range of regimes, starting from dilute to arresting flows. However, this paper focuses on dry granular shear flows. It extends this hydrodynamic system in a dense shear flow regime by a segregation equation using the framework of mixture theory. Special focus is lain on the segregation direction. A procedure from mechanics is adapted to obtain the segregation direction from the granular flow system independent of the choice of the coordinate system. In particular, this is done in three-dimensional space. Due to the compressibility of the granular flow system and the structure of the derived segregation equation, solving the segregation equation requires special numerical treatment. Therefore, a suitable numerical scheme is presented which prevents the system from reaching unphysical states.

Multiphase mesh-free particle modeling of local sediment scouring with μ(I) rheology

Abstract Sediment scouring is a common example of highly dynamic sediment transport. Considering its complexities, the accurate prediction of such a highly dynamic multiphase granular flow system is a challenge for the traditional numerical techniques that rely on a mesh system. The mesh-free particle methods are a newer generation of numerical techniques with an inherent ability to deal with the deformations and fragmentations of a multiphase continuum. This study aims at developing and evaluating a multiphase mesh-free particle model based on the weakly compressible moving particle semi-implicit (WC-MPS) formulation for simulation of sediment scouring. The sediment material is considered as a non-Newtonian viscoplastic fluid, whose behavior is predicted using a regularized μ(I) rheological model in combination with pressure-dependent yield criteria. The model is first validated for a benchmark problem of viscoplastic Poiseuille flow. It is then applied and evaluated for the study of two classical sediment scouring cases. The results show that the high-velocity flow currents and the circulations can create a low-viscosity region on the surface of the sediment continuum. Comparing the numerical results with the experimental measurements shows a good accuracy in prediction of the sediment profile, especially the shape and dimensions of the scour hole.

Force Chain Characteristics and Effects of a Dense Granular Flow System in a Third Body Interface During the Shear Dilatancy Process

In order to investigate the characteristics of force chains in a granular flow system, a parallel plate shear cell is constructed to simulate the shear movement of an infinite parallel plate and observe variations in relevant parameters. The shear dilatancy process is divided into three stages, namely, plastic strain, macroscopic failure, and granular recombination. The stickslip phenomenon is highly connected with the evolution of force chains during the shear dilatancy process. The load–distribution rate curves and patterns of the force chains are utilized to describe the load-carrying behaviors and morphologic changes of force chains separately. Force chains, namely, “diagonal gridding,” “tadpole-shaped,” and “pinnate” are defined according to the form of the force chains in the corresponding three stages.

Force Chain Characteristics and Effects of a Dense Granular Flow System in a Third Body Interface during the Shear Dilatancy Process

Force Chain Characteristics and Effects of a Dense Granular Flow System in a Third Body Interface during the Shear Dilatancy Process

Mesh-free SPH modeling of sediment scouring and flushing

A two-phase mesh-free Lagrangian model, based on the Smoothed Particle Hydrodynamics (SPH) method, is developed and evaluated for the continuum-based study of sediment-water system (a dense submerged granular flow system). The case studies are (a) sediment scouring downstream of a wall-jet, and (b) reservoir sediment flushing, both of great interest of engineers and researchers. The mesh-free Lagrangian nature of the SPH method makes the model capable of dealing with the large interfacial deformations involved in these complex water-sediment flow systems. Water and sediment materials are both treated as continuum and the governing equation of their motion are solved on a Lagrangian frame using SPH formulation. A pressure-dependent viscoplastic rheological model is used to describe the behavior of the sediment phase. A combination of Mohr–Coulomb and Shields yielding criteria (in place of commonly used Mohr–Coulomb yield criteria) is employed to determine the critical stress above which the sediment material behaves as a viscous fluid. Effect of this combination on the model results is investigated. To validate the developed numerical model, this study also provides some original experimental measurements for the test cases of this study. The results show the good compatibility of the numerical and experimental water and bed profiles. The sediment feature such as scouring and deposition areas (for wall-jet scouring case) and the sediment failure slope (for the flushing test case) are reproduced by the developed SPH model, with a good degree of accuracy. The numerical results as well as experimental measurements of this study not only provide an insight into better understanding of the complicated mechanism of sediment transport induced by rapid flow discharge, but also provide a reliable numerical tool and benchmark for simulation of such problems.

Phase transition and bistable phenomenon of granular flows down a chute with successive turnings

This paper studies the granular flow down a chute with two successive turnings, which play the role of bottlenecks for the granular flow system and determine the granular flow state in main section between them. With the increase of main section width D , phase transition from dilute to dense granular flow is observed: When the main section width D is small (large), the granular flow at upper (lower) bottleneck is dense and the granular flow is dilute (dense) in the main section. More interestingly, a bistable region is exhibited, in which either dilute flow or dense flow may occur and continue for the entire run. In this region, the packing in the reservoir will affect initial flow rate and then affect the flow pattern. This study can be viewed as a paradigm for the jamming and unjamming transitions under shear due to gravity.

Simulated and measured flow of granules in a bladed mixer—a detailed comparison

Experimental measurements and simulations of a granular flow system have been compared in detail. Positron emission particle tracking (PEPT) was used to study the motion of glass beads in a vertical axis mixer with slowly rotating flat blades. Unlike conventional techniques which are restricted to measuring the flow at the surface or a transparent wall, PEPT revealed the motion of material throughout the entire bed. The flow produced was three dimensional with vortices, and is more complicated than other granular flows that have been described previously. Discrete element method (DEM) was employed to simulate the same system using various sets of parameters for the bed material. As there are extensive three-dimensional flow data from both the experiment and the simulations, it has been possible to make comprehensive quantitative comparisons. This enabled the accuracy of the sets of assumptions for the DEM simulations to be examined. The simulations predicted the overall motion of the bed well. However no one set of assumptions was best, but that different sets predicted the detailed motion more accurately in different parts of the bed. None the less, it is evident that DEM models can now be used with some confidence to explore mixer design and performance.

Correlation time and diffusion coefficient imaging: application to a granular flow system.

A parametric method for spatially resolved measurements for velocity autocorrelation functions, R(u)(tau) = <u(t)u(t + tau)>, expressed as a sum of exponentials, is presented. The method is applied to a granular flow system of 2-mm oil-filled spheres rotated in a half-filled horizontal cylinder, which is an Ornstein-Uhlenbeck process with velocity autocorrelation function R(u)(tau) = <u(2)>e(- ||tau ||/tau(c)), where tau(c) is the correlation time and D = <u(2)>tau(c) is the diffusion coefficient. The pulsed-field-gradient NMR method consists of applying three different gradient pulse sequences of varying motion sensitivity to distinguish the range of correlation times present for particle motion. Time-dependent apparent diffusion coefficients are measured for these three sequences and tau(c) and D are then calculated from the apparent diffusion coefficient images. For the cylinder rotation rate of 2.3 rad/s, the axial diffusion coefficient at the top center of the free surface was 5.5 x 10(-6) m(2)/s, the correlation time was 3 ms, and the velocity fluctuation or granular temperature <u(2)> was 1.8 x 10(-3) m(2)/s(2). This method is also applicable to study transport in systems involving turbulence and porous media flows.