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Experimental and numerical analyses by DEM of the range of a probe in a granular medium

Experimental and numerical analyses by DEM of the range of a probe in a granular medium

and the solid fraction of the granular medium. It allows us to explore all the contacts network between the particles by the calculus of (i) normal and tangential forces, (ii) moment, (iii) mobility of each particle. 3. Results and discussion

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Contribution to certain physical and numerical aspects of the study of the heat transfer in a granular medium

Contribution to certain physical and numerical aspects of the study of the heat transfer in a granular medium

Figure 2.2: Simplication of the grains' assembly (with a periodic pattern). The dashed rectangle is the computational domain used in this work. The present model implies many assumptions; there is no doubt that some important characteristics of a real granular medium (especially the randomness of both shape and position of the grains, and the roughness of their surface) should aect the results obtained in this work. On the other hand, we can state that gravity doesn't aect a lot the shape of the liquid meniscus (the Bond number, based on a reference length chosen as the height of the meniscus, is of the order of one thousandth for a grain of radius 1 mm ). Nevertheless, we think that our simple model has the worthiness to reveal a very interesting behavior, that is the hysteresis during the change in the liquid volume. Lastly, although the true temperature eld is 3D, we suppose that it is well described by the local axisymmetric eld around one symmetry line joining two spheres.
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Bending transition in the penetration of a flexible intruder in a 2D dense granular medium

Bending transition in the penetration of a flexible intruder in a 2D dense granular medium

interaction between an elastic beam and a granular flow, we report our experimental observation of bistability be- tween two different regimes (straight or bent fiber), we identify the control parameters (rigidity of the fiber and packing fraction of the granular medium) and propose a simple model ; in section IV we give a specific focus on the development of the bending transition and show that it is associated with a clear symmetry breaking of both the geometry of the fiber and the density of the granular material in the vicinity of the fiber; we then propose in section V a model of bending induced compaction that allows us to reasonably reproduce the evolution of the fiber deflection in the course of the bending transition; a summary of our main results is finally given in section VI.
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Experimental and numerical analyses by DEM of the range of a probe in a granular medium

Experimental and numerical analyses by DEM of the range of a probe in a granular medium

Flahault, 34093, Montpellier cedex 5, France *Corresponding author: julien.lehuen@supagro.fr Highlights • There is vertical stratification within a granular medium located in a reactor. • The range of force imposed by the blade is all the lower when the speed is high. • DEM simulations give the structural rearrangement which occurs during trials. • Spatiotemporal correlations of particle motions observed by PIV could be

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Drag force applied to an intruder during its extraction from an ensiled granular medium: experiments and 2D DEM simulations

Drag force applied to an intruder during its extraction from an ensiled granular medium: experiments and 2D DEM simulations

In this work we investigate, by means of experiments and Discrete Element Method (DEM) simulations, the drag force acting on an intruder vertically pulled up from an ensiled granular medium. In the experiments, the intruder is first positioned at the bottom of a cell filled with glass beads and then moved upward at constant velocity. The fluctuations of the drag force acting on the moving intruder is monitored by a force sensor while the particle flow around it is captured using a high-speed camera. The images are then processed using a Particle Image Velocimetry (PIV) code. The force profiles and the flow of particles around the intruder are characterized as a function of the intruder velocity. A 2D DEM code was developed in order to simulate this problem with similar boundary conditions. With this code, it is possible to better analyzed the perturbation of the granular network in the vicinity of the intruder. The simulated particle flows and the drag force show a good agreement with the experimental results. Three flow regimes can be distinguished depending on the depth of the intruder: quasistatic, frictional and collisional. These regimes depend to the velocity of the intruder scaled by the characteristic particle speed ( 𝑔𝑑) of the particles.
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Mixing of a wet granular medium: Effect of the particle size, the liquid and the granular compacity on the intensity consumption

Mixing of a wet granular medium: Effect of the particle size, the liquid and the granular compacity on the intensity consumption

Otherwise, these measurements during the mixing show that the porosity is not constant from the dry to the capillary state. A dilation of the granular bed takes place in the pendular state, and a densification occurs in the capillary state. So, the compacity depends on the added liquid amount, influences the shear resis- tance (and consequently the specific intensity consumed by the mixer), and is an important parameter to take into account in the study of the granular medium behaviour during mixing.

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Bending transition in the penetration of a flexible intruder in a two-dimensional dense granular medium

Bending transition in the penetration of a flexible intruder in a two-dimensional dense granular medium

However the compaction of a cluster of grains upstream of the fiber induces long range effects in the granular medium [37, 49]. In particular, it is accompanied here by recirculation flows on both sides of the intruder that tend to erode the cluster and convect grains downstream of the fiber. These recirculations are visible in the map of displacement fields of Fig. 4 in the plate frame, when the displacement of the plate has been subtracted from each grain displacement. In our previous work on the penetration of rigid intruders [28], it was observed that the lateral extent of recirculations was proportional to the diameter of the cylindrical intruder. Adapting this argument to the flexible intruder of spanwise length δ leads to a grain flux recirculating on both sides of the fiber which is proportional to δ. The number of grains eroded from the cluster for each penetration step is thus expected to scale like dN c −
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Mechanical feedback between a growing root and a deformable granular medium: extracting physical laws from numerical simulations

Mechanical feedback between a growing root and a deformable granular medium: extracting physical laws from numerical simulations

We started answering these questions in a theoretical way, developing a numerical model of root growth in a granular medium. The model is based on the discrete-element method (DEM). Single roots are modelled using chains of connected spheroline elements. The growth is initiated from a circular element placed at the free surface of a granular bed prepared by random pluviation. This circle plays the role of a meristem, which is constantly replicated at a given rate and pushed forward under the action of elastic forces, generating a line of fixed thickness equal to the diameter of the circle and with prescribed stiffness and bending moment. The orientation of the meristem at every growth step is driven by the dynamics of the whole root under the action of its internal elastic forces and reaction forces exerted by the grains. The preliminary model is two-dimensional, which not allows the pore space to be opened as in a 3D situation, consequently limiting root the penetration. To overcome this limitation, we introduce two different diameters for the grains, i.e. a “real” diameter that is considered to calculate grain-grain mechanical interactions, and a smaller “virtual” diameter to take into account root-grains interactions. The difference between the two diameters corresponds to the width of a gap at contact points through which the roots can pass. The ratio of this gap to the root diameter is considered as a model parameter.
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Drag force applied to an intruder during its extraction from an ensiled granular medium: experiments and 2D DEM simulations

Drag force applied to an intruder during its extraction from an ensiled granular medium: experiments and 2D DEM simulations

In this work we investigate, by means of experiments and Discrete Element Method (DEM) simulations, the drag force acting on an intruder vertically pulled up from an ensiled granular medium. In the experiments, the intruder is first positioned at the bottom of a cell filled with glass beads and then moved upward at constant velocity. The fluctuations of the drag force acting on the moving intruder is monitored by a force sensor while the particle flow around it is captured using a high-speed camera. The images are then processed using a Particle Image Velocimetry (PIV) code. The force profiles and the flow of particles around the intruder are characterized as a function of the intruder velocity. A 2D DEM code was developed in order to simulate this problem with similar boundary conditions. With this code, it is possible to better analyzed the perturbation of the granular network in the vicinity of the intruder. The simulated particle flows and the drag force show a good agreement with the experimental results. Three flow regimes can be distinguished depending on the depth of the intruder: quasistatic, frictional and collisional. These regimes depend to the velocity of the intruder scaled by the characteristic particle speed ( 𝑔𝑑) of the particles.
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Stress distribution inside a powder bed: does Janssen’s model applicable locally in a granular medium

Stress distribution inside a powder bed: does Janssen’s model applicable locally in a granular medium

Stress distribution inside a powder bed: does Janssen’s model applicable locally in a granular medium.. Agnès Duri-Bechemilh, Sandra Mandato, Bernard Cuq, Thierry Ruiz.[r]

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Vertical slow drag of an intruder in a laterally confined granular medium

Vertical slow drag of an intruder in a laterally confined granular medium

maximum vertical force value (mean value ≈ 2.59 ± 0.26 N) and its position (mean = 18.68 ± 2.41 mm) are quite uncorrelated with the dimensionless speed. However, the position of this peak corresponds to the position of the highest force of the static profile. During this loading phase, a rearrangement of the inter-granular voids above the blade is observed and corresponds to the creation of a void volume taking place just below the blade (Fig. 1b). The void volume comes from the compaction of the granular layers upon the blade. As the initial compactness of the powder bed (0.45) is lower than the critical compactness above which the granular medium dilates to allow particle rearrangements, it is relevant to assume that a compacted zone is formed just above the blade. During the formation of this compacted zone and all along the blade displacement, particles above the blade are evacuated by micro-avalanches in the free volume and form a concave heap (crater-like) on both sides of the blade. Whatever v*, the height of the concave heap depends on the blade width (1.97 cm) and the angle of repose of semolina (43.8°). The formation of this hole takes place before the force reaches its first peak and corresponds to a small inflexion in the force profile.
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Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods

Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods

We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed.
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Mixing of a wet granular medium: Influence of the liquid addition method

Mixing of a wet granular medium: Influence of the liquid addition method

At the beginning of the mixing process (I), the first amount of liquid added is distributed at the surface of the particles without the formation of liquid bridges. Then, in the pendulary state (II), the liquid amount at the particle surface is high enough to form capillary bridges between them, while the medium becomes more cohesive and there is a rapid increase in energy consumption. During the funicular state (III), the porous volume of the previously formed agglomerates is fi lled by the additional liquid. In this state, the increase in the intensity is much lower because the majority of the capillary bridges have already been formed in the pendulary state. When the porosity in the agglomerates is completely filled by the liquid (end of III), the additional liquid forms new liquid bridges between agglomerates until the formation of a single agglomerate. This is the capillary state (IV), characterized by the appearance of a rapid increase in energy consumption (capillary peak). Finally, in the droplet state, the volume of liquid becomes higher than the bulk porosity of the granular medium and the energy consumption decreases.
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Lift forces in granular media

Lift forces in granular media

When studying the forces experienced by an object in a flow, the total force is usually split in two contributions: a drag force parallel to the direction of the mean flow, and a lift force perpen- dicular to the flow. The drag force in granular media has been studied in detail by many authors in different configurations, with the obstacle dragged either horizontally 11 , 12 or vertically as in impact problems. 13 – 15 A main result is that at low speeds a quasi static regime exists, for which the drag is independent of the velocity and proportional to the hydrostatic pressure times the surface area of the object. 16 – 19 The robustness of this frictional scaling has been tested by changing the geometry of the objects, 20 by probing the effect of the boundaries of the vessel containing the medium, 21 by chang- ing the effective gravity 22 or by varying the initial volume fraction. 19 At higher velocities such as encountered in impact problems, an inertial contribution is observed in addition to the frictional one, which scales as the square of the velocity. 15 By contrast, very few studies address the question of the lift force in granular media. Percier et al. 23 studied the lift experienced by an inclined plate moving at the free surface of a granular medium. The case of an object entirely plunged into the packing has been studied first by Ding et al. 24 in the quasi-static regime and more recently by Potiguar and Ding 25 at higher velocities. In their experiments and simulations, Ding et al. 24 have dragged different objects horizontally under the free surface. They have shown that even for a symmetric object like a cylinder, a strong lift force exists. A main result of their study is that the lift increases linearly with the depth, like the drag force. The authors have proposed a phenomenological model, in order to compute the lift on obstacles of various geometries. They assume that the lift created by each
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Capillary Fracturing in Granular Media

Capillary Fracturing in Granular Media

In a granular medium, fractures open when forces ex- erted by the fluids exceed the mechanical forces that resist particle rearrangements. In cohesionless granular material, these forces include elastic compression and friction. For systems with densely packed, highly compliant particles, pore opening occurs by means of particle deformation [ 24 ]. However, for many types of particles including most min- eral grains and manufactured beads, the high particle stiff- ness limits interparticle deformation, making frictional sliding the dominant deformation mechanism that alters the pore geometry.
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Destabilization of immersed granular avalanches

Destabilization of immersed granular avalanches

* Corresponding author: patrick.mutabaruka@univ-montp2.fr By means of 3D coupled molecular dynamics/Lattice Boltzmann simulations, we analyze the destabilization process of a granular bed of spherical particles inclined above its angle of repose and immersed in a viscous fluid [1]. Extensive simulations were performed for different values of the packing fraction and slope angle. We study the evolution of macroscopic observables such as shear strain, packing fraction and excess pore pressure. We then analyze the contact network anisotropy. Two regimes are evidenced as in experiments [2,3]: a loose regime where the slope fails spontaneously and a dense regime where the failure is delayed as a result of negative excess pore pressure built up in reaction to the dilation of the bed. The two regimes belong to the packing fractions below and above 0.59, respectively. We focus in more detail on the creep-like deformation of the inclined bed in the dense regime. The time evolution of the packing fraction and shear strain scale with a characteristic time obtained from a model based on the balance of granular stresses in the presence of a pore excess pressure and the relation of the latter with dilatancy controlled by Darcian drag forces. The cumulative shear strain at failure is found to be around 0.2, close to the experimental value [2], irrespective of the initial packing fraction and inclination angle. In the same way, the time and packing fraction at failure are correctly predicted by the model. A noticeable finding is that the network deforms by distortion at nearly constant connectivity. The contact network anisotropy grows with shear strain, and slope failure is triggered when the anisotropy levels off. The anisotropy thus appears as an internal variable, reflecting the distortion of the contact network. The independence of the internal friction angle with respect to the initial packing fraction and its dependence on the slope angle were studied and shown to be a consequence of slope stabilization by the cohesive-like effect of negative excess pore pressure. It is also interesting to note that the transition from stable equilibrium to inertial flow in the presence of a fluid is accompanied by large fluctuations. As soon as the capacity of volume change by distortion is nearly exhausted, slope instability is triggered by the fluctuations and amplified by lubrication forces as the avalanches proceeds.
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Interrupted shear of granular media

Interrupted shear of granular media

Size-independent behaviour These results may be described on the basis of the Dieterich-Rice-Ruina model for solid friction. The coef- ficient a of logarithmic variation of the friction coefficient with the sliding velocity and the coefficient b of logarith- mic variation of the friction coefficient with the ageing time are found to be of the same order of magnitude as in solid friction. They both have very a slight dependence on the normal loading force, which has not been reported for solid friction. Another difference with solid friction is that the creep response depends on the packing fraction of the granular media.
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Granular compaction and stretched exponentials

Granular compaction and stretched exponentials

?? e-mail: jean.eric.mathonnet@gmail.com ??? e-mail: blanche.dalloz@univ-amu.fr ???? e-mail: philippe.sornay@cea.fr the KWW relaxation expression may be questioned when the granular material exhibits a cohesive property. Widely used in the industry, cohesive powders are often di fficult to handle and transport. A cohesive powder usually presents a high angle of repose and a low bulk volume fraction. The cohesiveness of a powder may be measured through its ability to flow under gravity, and this ill-defined "flowa- bility" is often described by the Hausner ratio I H , the ratio
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Initiation of immersed granular avalanches

Initiation of immersed granular avalanches

By means of coupled molecular dynamics–computational fluid dynamics simulations, we analyze the initiation of avalanches in a granular bed of spherical particles immersed in a viscous fluid and inclined above its angle of repose. In quantitative agreement with experiments, we find that the bed is unstable for a packing fraction below 0.59 but is stabilized above this packing fraction by negative excess pore pressure induced by the effect of dilatancy. From detailed numerical data, we explore the time evolution of shear strain, packing fraction, excess pore pressures, and granular microstructure in this creeplike pressure redistribution regime, and we show that they scale excellently with a characteristic time extracted from a model based on the balance of granular stresses in the presence of a negative excess pressure and its interplay with dilatancy. The cumulative shear strain at failure is found to be 0.2, in close agreement with the experiments, irrespective of the initial packing fraction and inclination angle. Remarkably, the avalanche is triggered when dilatancy vanishes instantly as a result of fluctuations while the average dilatancy is still positive (expanding bed) with a packing fraction that declines with the initial packing fraction. Another nontrivial feature of this creeplike regime is that, in contrast to dry granular materials, the internal friction angle of the bed at failure is independent of dilatancy but depends on the inclination angle, leading therefore to a nonlinear dependence of the excess pore pressure on the inclination angle. We show that this behavior may be described in terms of the contact network anisotropy, which increases with a nearly constant connectivity and levels off at a value (critical state) that increases with the inclination angle. These features suggest that the behavior of immersed granular materials is controlled not only directly by hydrodynamic forces acting on the particles but also by the influence of the fluid on the granular microstructure. DOI: 10.1103/PhysRevE.89.052203 PACS number(s): 45.70. −n, 61.43.−j, 47.57.Gc
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Granular Computing Methods in Bioinformatics.

Granular Computing Methods in Bioinformatics.

process of knowledge discovery. In this sense, data mining on the constantly growing bioinformatics databases is possibly the only way to achieve that goal. One of the most important fields of modern bioinformatics where granular computing methods have a large potential and where successful applications have already been made is genomics; in particular, the analysis of DNA microarrays. DNA is the molecule that encodes genetic information. In Eukaryotes (all organisms except viruses, bacteria, and bluegreen algae), it is a double#stranded molecule held together by weak bonds between base pairs of nucleotides, namely adenine (A), guanine (G), cytosine (C), and thymine (T). Base pairs form between A and T and between G and C; thus the base sequence of each single strand can be obtained from that of the other. RNA is the molecule found in the nucleus and cytoplasm of cells; and it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is related to that of DNA. There are several RNA molecules: messenger RNA, transfer RNA, ribosomal RNA, and others.
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