Behavior Of Granular Media Research Articles

Impact of lifters on velocity and residence time distributions of fabrics in a tumble dryer

The impact of lifters on fabric motion in tumble dryers was examined in this study. A trajectory tracking system based on visual imaging techniques was used to investigate the transverse motion of tracer fabric. From the observation of velocity and residence time distributions of tracer fabric, fabric transverse motion can be divided into passive and active regions. These are entirely consistent with existing forms of bed motion describing the behavior of granular media in rotating cylinders in other fields such as chemical engineering, food drying, and clothes washing processes. Experimental and theoretical analyses indicated that the number of lifters had appreciable impacts on the dynamics of fabric motion, manifested in differences in motion pattern and lift height, thereby enhancing the heat and mass transfer inside the drying drum. This can be seen by comparing drying performance under different drying conditions. The findings in this study shed more light on the impact of lifters on fabric transverse motions which has significant implications for the overall design of tumble dryers.

Key connection between gravitational instability in physical gels and granular media

We study gravitationally-driven (Rayleigh–Taylor-like) instability in physical gels as a model for the behavior of granular media falling under gravity; physical gels have a structural elasticity and may be fluidized, capturing both the solid and liquid properties of granular systems. Though ubiquitous in both industrial and natural contexts, the unique static and dynamic properties of granular media remain poorly understood. Under the action of a gravitational force, granular materials may flow while exhibiting heterogeneous rigidity, as seen during e.g., avalanches or landslides. Though the onset of this gravitational “instability” has been addressed, the mechanism behind its incidence is not yet understood. We find key quantitative similarities between Rayleigh–Taylor-like instability in physical gels and granular systems. In particular, we identify a common scaling law, showing that the instability is chiefly governed by the thickness of the flowable region.

Thermophysical Characteristics of a Cold Compressed Granular Medium: An Experimental Investigation

To our knowledge, very few models have described the thermal behavior of granular media or powders as a function of the mechanical stresses to which they are subjected. In recent years, many researchers have expressed great interest in establishing laws that can show the relationship between the apparent thermal conductivity and the mechanical behavior of granular media. This paper presents an experimental study of heat transfer enhancement using granular blocks. A number of experiments were carried out in the test section of a wind tunnel where velocity and temperature measurements were performed. The velocity profiles permitted us to have an idea on the flow structure in the presence of granular blocks. The temperature measurements under various operating conditions (various Reynolds numbers and granular blocks structures) led us to the conclusion that using granular blocks can improve the thermal transfer in comparison with the case of a smooth channel and this enhancement is strongly dependent on the geometrical and thermo-physical properties of these blocks.

Quasistatic kinetic avalanches and self-organized criticality in deviatorically loaded granular media.

The behavior of granular media under quasistatic loading has recently been shown to attain a stable evolution state corresponding to a manifold in the space of micromechanical variables. This state is characterized by sudden transitions between metastable jammed states, involving the partial micromechanical rearrangement of the granular medium. Using numerical simulations of two-dimensional granular media under quasistatic biaxial compression, we show that the dynamics in the stable evolution state is characterized by scale-free avalanches well before the macromechanical stationary flow regime traditionally linked to a self-organized critical state. This, together with the nonuniqueness and the nonmonotony of macroscopic deformation curves, suggests that the statistical avalanche properties and the susceptibilities of the system cannot be reduced to a function of the macromechanical state. The associated scaling exponents are nonuniversal and depend on the interactions between particles. For stiffer particles (or samples at low confining pressure) we find distributions of avalanche properties compatible with the predictions of mean-field theory. The scaling exponents decrease below the mean-field values for softer interactions between particles. These lower exponents are consistent with observations for amorphous solids at their critical point. We specifically discuss the relationship between microscopic and macroscopic variables, including the relation between the external stress drop and the internal potential energy released during kinetic avalanches.

An experimental and analytical study of the effect of cold compression on the thermophysical properties of a granular medium

Based on the previous literature, very few models have described the thermal behavior of granular media or powders as a function of the mechanical stresses to which they are subjected. In recent years, many researchers have been interested in establishing laws that can express the relationship between the apparent thermal conductivity and the mechanical behavior of granular media. The present paper seeks to present a simple model that describes the variation of the apparent thermal conductivity of a granular medium as a function of the mechanical stress. One of the main objectives of this paper is to produce a tool for calculating the thermal conductivity of heterogeneous media, especially that of granular media. For the resolution of the problem, it was decided to use an experimental method recently developed in the laboratory and which is due to be the basis of our calculations. This method is called the hot rod method. It was initially developed to evaluate the damage to a soil subject to superficial heat shock (fires, burns). The results show that for short times and a distance between two measurement points large enough, the 2D transfer can be reduced to a 1D transfer which for long times is hybridized to a hot wire transfer. The modeling of thermal transfers within the environment makes it possible to know the temperature field of soil under the effect of a thermal accident.

Do granular systems obey statistical mechanics? A review of recent work assessing the applicability of equilibrium theory to vibrationally excited granular media

Driven granular media — assemblies of discrete, macroscopic elements exposed to a source of mechanical energy — represent inherently out-of-equilibrium systems. Although granular media are ubiquitous in both nature and industry, due to their dissipative nature and resultant complex behaviors they remain startlingly poorly understood as compared to classical, thermodynamic systems. Nonetheless, in recent years it has been observed that the behaviors of granular media can, under certain circumstances, closely resemble those of equilibrium systems. One of the most important contemporary questions in the field of granular physics is whether these similarities are merely superficial, or whether the parallels run deep enough that the behaviors of these nonequilibrium systems can in fact be successfully captured using analogs to existing theoretical models developed for classical systems. In this review, we draw together the findings of a variety of recent studies where this question has been addressed, comparing and contrasting the results and conclusions presented. We focus our attention on vibrated and vibrofluidized granular beds, which provide a canonical system representative of various equilibrium and nonequilibrium physical systems, and whose simple dynamics offer a valuable testing ground for exploring the fundamental physics of the granular state.

The behaviour of free-flowing granular intruders

Particle shape affects both the quasi-static and dynamic behaviour of granular media. There has been significant research devoted to the flowability of systems of irregularly shaped particles, as well as the flow of grains around fixed intruders, however the behaviour of free flowing intruders within granular flows remains comparatively unexplored. Here, the effect of the shape of these intruder particles is studied, looking at the kinematic behaviour of the intruders and in particular their tendency of orientation. Experiments are carried out within the Stadium Shear Device, which is a novel apparatus able to continuously apply simple shear conditions to two-dimensional grain analogues. It is found that the intruder shows different behaviour to that of the bulk flow, and that this behaviour is strongly shape dependent. These insights could lead to the development of admixtures that alter the flowability of granular materials.

Assessment of the Mechanical Behaviour of Granular Media by DEM-Based True Triaxial Tests

This study presents the numerical simulation of a true triaxial test by means of the discrete element method (DEM). Experimental results performed on Toyoura sand are employed as reference and the calibration methodology is explained. Physical aspects of the real soil, such as the grain size distribution and the relative density, are considered during the generation of the virtual sample. It is shown that the main aspects of the macro-mechanical behaviour of granular soils during compression loading can be fairly represented by the idealised simulations with particles.

Features of the deformation behavior of granular media under a load

The piecewise linear character of the load–strain dependences of a granular medium consisting of fragile particles was experimentally established and theoretically substantiated. A model for grinding a granular medium in a conic or roller inertia mill was developed taking into consideration the features of its deformation behavior.

Numerical analysis of plastic behavior of granular media exposed to deformation under broken path loading

Based on DEM, the problem on uniform shearing of granular material specimen is considered, the shape of the particles was chosen to be spherical, with assigned distribution per radii. At interfaces of the particles, dry friction and viscosity were taken into account. Loading was implemented as simple and broken trajectory shearing. The load trajectory was broken by means of sudden change in the shearing direction. It has been found that in this case, dilatancy rate and angle between axes of tensors of stresses and strain rates change jump-wise. The discrete medium shows the properties of the continuum model of granular material with internal friction and dilatancy. The authors discuss a criterion of the continuum model to be equivalent to the initial discrete model. Applicability of the numerical results to the continuum modeling of deformation is demonstrated.

Particle Finite Element Simulation of Granular Media

Despite their ubiquity, the simulation of granular materials poses a continuing challenge in computational mechanics, as these materials can behave like solid, liquid and gas. In this paper, a recently developed version of the Particle Finite Element Method is applied for analyzing the behaviour of granular media under very large deformations. Both quasi-static and dynamic problems are considered. It is shown that this continuum approach is applicable to general large deformation problems of granular materials, including liquid-like flows.

Coupled CFD–DEM simulation of fluid–particle interaction in geomechanics

This paper presents a coupled Computational Fluid Dynamics and Discrete Element Method (CFD–DEM) approach to simulate the behaviour of fluid–particle interaction for applications relevant to mining and geotechnical engineering. DEM is employed to model the granular particle system, whilst the CFD is used to simulate the fluid flow by solving the locally averaged Navier–Stokes equation. The particle–fluid interaction is considered by exchanging such interaction forces as drag force and buoyancy force between the DEM and the CFD computations. The coupled CFD–DEM tool is first benchmarked by two classic geomechanics problems where analytical solutions are available, and is then employed to investigate the characteristics of sand heap formed in water through hopper flow. The influence of fluid–particle interaction on the behaviour of granular media is well captured in all the simulated problems. It is shown in particular that a sand pile formed in water is more homogeneous in terms of void ratio, contact force and fabric anisotropy. The central pressure dip of vertical stress profile at the base of sandpile is moderately reduced, as compared to the dry case. The effects of rolling resistance and polydispersity in conjunction with the presence of water on the formation of sandpile are also discussed.

Iterative finite element model of nonlinear viscoplastic analyses for blended granular porous media

The iterative finite element model, in which an element is used to represent a single particle, is generated to analyze the global behavior of multiple-material aggregates of materially nonlinear viscoplastic particles. The generalized Maxwell model is used to define four types of specific nonlinear viscoplastic materials, which are the elasto visco-plastic matter with linear viscosity, the plasto visco-plastic material with linear viscosity, the elasto visco-plastic media with nonlinear viscosity, and the plasto visco-plastic media with nonlinear viscosity. The theory and relevant penalty iterative algorithm are developed to analyze the four representative mixed granular systems consisting of materially nonlinear viscous particles. To verify precision of stress calculation, solutions of an axis-symmetric radial flow problem are compared with results of the literature and they are in a great agreement. The results present here provide significant insight into the fundamental behavior of granular media under compaction conditions, including prediction of the overall aggregates stress-strain response.

Role of contact couples and couple stress in the behaviour of granular media

This paper analyses the interest of taking into account contact couples in the granular material description as well as considering the inadequacy of the micropolar description. The study is made on two types of samples: One which takes into account contact couples, and the other which does not. The response of these two media, which are submitted to a biaxial test, is analysed from both the micromechanic and macroscopic viewpoints. A numerical study which is performed on these two samples shows the influence of the presence of couples on the local static variables as well as on the macroscopic behaviour. A statistical homogenization approach is analysed to simulate the effects of couples. Due to the presence of an internal variable, a numerical study proves that this approach is relevant. This internal variable allows the taking into consideration of the influence of contact couples. A first step in the description of couples versus contact orientation is made by the introduction of the standard deviation. Finally, the inadequacy of the micropolar description which takes into account micropolar stresses is pointed out.

Local friction effect of the global behaviour of granular media

In this study it has first been put in evidence, that the privileged paths of transmission of forces pass by the most resistant kernels and, after the latter are broken by shear, the net of forces paths is modified. and fluctuations appear in each curve. As in this type of material, the solicitations in the shear plane used entirely the internal friction angle, when the latter takes its limit state. Second, the global friction is rather independent of local friction, and when the local friction of a material is lower, its global friction is greater than the local one and vice versa. Science Ltd.

A continuum theory of granular media partially saturated with a viscous fluid

AbstractThis paper is concerned with the behaviour of granular media containing voids penetrated by a mixture of viscous liquid and gas (air). It is assumed that, for such materials, the skeleton may be considered as rate‐independent and the sensitivity to the applied loading rate may be attributed to the viscosity of the liquid filling the void space. Within the context of geomechanics, such assumptions appear to be relevant to all coarse‐grained cohesionless materials, including saturated sands. A mathematical formulation for the description of undrained response is presented. The macroscopic properties are estimated based on volume averaging, which is applied to both stress and strain measures within the heterogeneous constituent medium. Numerical examples are provided illustrating the influence of the viscosity of the liquid on the liquefaction potential of a loose granular assembly.

Simulating the effects of gravitational field and atmosphere on behavior of granular media

Designs for structures, vehicles, and equipment to be used on the surfaces of the moon and Mars cannot be optimized without a knowledge of how soils in various gravitational fields and atmospheric conditions behave when subjected to static and dynamic loading. Strengths of the lunar and Martian gravitational fields (0.17 and 0.38 times terrestrial, respectively) can be simulated for approximately 30 sec by flying the KC-135 aircraft through appropriate parabolas. Effects of gravitational field on explosion crater formation and projectile penetration have been investigated. Techniques devised for use in these tests are discussed. Diameters of explosion craters and depths of projectile penetration in sand were found to increase as the gravitational field strength decreased. Atmospheric pressure variations down to ultra-high vacuum are significant in the behavior of rock powders. Approaches used in ultra-high vacuum tests on adhesion and compressibility of basalt powders are discussed. The forces of adhesion between rock powders and metal alloy surfaces were noted to be greater in ultra-high vacuum than in air. The forces required to compress slowly a basalt powder are greater after bakeout and exposure to ultra-high vacuum than when this rock powder is tested at atmospheric pressure.

Analytical Description of Behavior of Granular Media

Normality of the plastic strain increment vector to the loading surface is not always observed for granular media. Beginning from a thermodynamic basis and considering only isothermal processes, a series of physically appealing assumptions is employed to increasingly delimit the possible directions of the plastic strain increment vector. The results of the analysis indicate that on certain portions of the loading surface normality should hold while on the remainder, envelopes or fans are possible. Finally, several alternative restrictions are offered which provide a unique stress-strain relation at every point on the loading surface.