3D DEM Research Articles

Lateral solid mixing in gas-fluidized beds: CFD and DEM studies

We investigated lateral solid mixing in gas-fluidized beds using CFD and DEM. We ran 3D CFD simulations, comparing the results with those formerly obtained in 2D CFD simulations. We observed that the frictional stress model affects the numerical results. This was also observed in 2D simulations, but in 3D simulations the effect is less pronounced. The 3D simulations described the lateral solid mixing process more accurately than the 2D simulations, simulation dimensionality being an important factor. To analyse further the role of frictional stress models, we ran 3D DEM simulations, employing the soft-sphere approach to model the particle–particle contact forces. The simulation results agreed reasonably well with the empirical data, but their accuracy depended on the values used for the collision parameters; also, the 3D CFD simulations matched the empirical data more closely. Altogether, we concluded that the simulation dimensionality plays the dominant role in predicting lateral solid mixing accurately.

A METHOD OF GENERATING PANORAMIC STREET STRIP IMAGE MAP WITH MOBILE MAPPING SYSTEM

Abstract. This paper explores a method of generating panoramic street strip image map which is called as “Pano-Street” here and contains both sides, ground surface and overhead part of a street with a sequence of 360° panoramic images captured with Point Grey’s Ladybug3 mounted on the top of Mitsubishi MMS-X 220 at 2m intervals along the streets in urban environment. On-board GPS/IMU, speedometer and post sequence image analysis technology such as bundle adjustment provided much more accuracy level position and attitude data for these panoramic images, and laser data. The principle for generating panoramic street strip image map is similar to that of the traditional aero ortho-images. A special 3D DEM(3D-Mesh called here) was firstly generated with laser data, the depth map generated from dense image matching with the sequence of 360° panoramic images, or the existing GIS spatial data along the MMS trajectory, then all 360° panoramic images were projected and stitched on the 3D-Mesh with the position and attitude data. This makes it possible to make large scale panoramic street strip image maps for most types of cities, and provides another kind of street view way to view the 360° scene along the street by avoiding the switch of image bubbles like Google Street View and Bing Maps Streetside.

A METHOD OF GENERATING PANORAMIC STREET STRIP IMAGE MAP WITH MOBILE MAPPING SYSTEM

This paper explores a method of generating panoramic street strip image map which is called as “Pano-Street” here and contains both sides, ground surface and overhead part of a street with a sequence of 360° panoramic images captured with Point Grey’s Ladybug3 mounted on the top of Mitsubishi MMS-X 220 at 2m intervals along the streets in urban environment. On-board GPS/IMU, speedometer and post sequence image analysis technology such as bundle adjustment provided much more accuracy level position and attitude data for these panoramic images, and laser data. The principle for generating panoramic street strip image map is similar to that of the traditional aero ortho-images. A special 3D DEM(3D-Mesh called here) was firstly generated with laser data, the depth map generated from dense image matching with the sequence of 360° panoramic images, or the existing GIS spatial data along the MMS trajectory, then all 360° panoramic images were projected and stitched on the 3D-Mesh with the position and attitude data. This makes it possible to make large scale panoramic street strip image maps for most types of cities, and provides another kind of street view way to view the 360° scene along the street by avoiding the switch of image bubbles like Google Street View and Bing Maps Streetside.

3D particle-scale modeling of gas–solids flow and heat transfer in fluidized beds with an immersed tube

In this work, a fully three-dimensional (3D) model of combined computational fluid dynamics and discrete element method (CFD–DEM) is for the first time developed to study the gas–solids flow and heat transfer in fluidized beds with an immersed tube. A critical bed thickness is first determined at which the bed can be regarded as fully 3D. Then the validity of the model using the critical bed thickness is tested both qualitatively and quantitatively. It is shown that the model can successfully reproduce the typical relationship between pressure drop and gas velocity, and flow and heat transfer characteristics such as the four distinct stages of bubble transit through the tube and the peak of heat transfer coefficient between tube and the bed for certain gas velocity (which are however not well-predicted by previous 2D CFD–DEM and 2D CFD–3D DEM models). Finally the results are analyzed to improve the fundamental understanding of the system. It is demonstrated that both the gas and solids phases have 3D flow characteristics including the unique feature of 3D orientations of gas velocity vector field around the bubble. It is predicted that the maximum heat transfer coefficient is a result of the competition between surface–particle conduction and surface–fluid convection. The obtained results should be useful to the development of the fundamental understanding of the flow and heat transfer characteristics in a fluidized bed with immersed tubes.

Does Elevation Play a Role in the Transmission of Pulmonary Tuberculosis in South Africa?

Pulmonary Tuberculosis is the number one killer in South Africa. Although this is helped spurred on by the HIV/AIDS co-epidemic. We explored the effect of elevation in Tuberculosis transmission and mortality. Landsat 8 imageries were downloaded through earth-explorer and processed in Arc map 10.3. LULC maps generated from the satellite images were used for land characterization of provinces in South Africa. 3D DEM in Arc scene was utilized in displaying the elevation dataset. Our findings on the aggregate data at the provincial level could not show the association between elevation and Tuberculosis transmission and mortality.

EVALUATION OF MATCHING STRATEGIES FOR IMAGE-BASED MOBILE MAPPING

Abstract. The paper presents the implementation of a dense multi-view stereo matching pipeline for the evaluation of image sequences from a camera-based mobile mapping system. For this purpose the software system SURE is taken as a basis. Originally this system was developed to provide 3D point clouds or DEM from standard airborne and terrestrial image blocks. Since mobile mapping scenarios typically include stereo configurations with camera motion predominantly in viewing direction, processing steps like image rectification and structure computation of the existing processing pipeline had to be adapted. The presented investigations are based on imagery captured by the mobile mapping system of the Institute of Geomatics Engineering in the city center of Basel, Switzerland. For evaluation, reference point clouds from terrestrial laser scanning are used. Our first results already demonstrate a considerable increase in reliability and completeness of both depth maps and point clouds as result of the matching process.

Comparison of gross tractive effort of a single grouser in two-dimensional DEM and experiment

In the development of DEM programs for interaction studies, 2D DEM analysis is often compared to the results of experiments done in 3D conditions. From an alternative viewpoint, a quasi-2D experiment was designed for this study to collect results of gross tractive effort and to examine the accuracy of 2D DEM applied to soil–grouser systems. To intensify the understanding and comparison of the grouser reaction and soil behavior, a soil model incorporating aluminum cylinders was used in the experiment instead of real soil. The particle size distribution of model soil was approximated by the combination of cylinders of two radii. The bulk density was set to be identical in DEM and the experiment. The normal spring constant and the coefficient of rolling friction in the contact model of DEM were adjusted using trial and error, considering the allowable time step, sinkage of the grouser, and the slope angle of the cavity behind the grouser. The developed 2D DEM program was confirmed to have sufficient capability to reproduce the experimentally obtained results of gross tractive effort with nine combinations of conditions of grouser height and grouser thickness.

DEM Simulations of Granular Soils under Undrained Triaxial Compression and Plane Strain

The paper presents 3D DEM simulation results of undrained tests for loose assemblies with varied porosities under both triaxial compression and plane strain conditions, using a periodic cell. The undrained tests were modelled by deforming the samples under constant volume conditions, which corresponds to saturated soil samples. The undrained stress paths are shown to be qualitatively similar to physical experimental results. The triggering of liquefaction and temporary liquefaction is identified by a microscopic parameter with redundancy factor (RF) equal to unity, which defines the transition from “solid-like” to “liquid-like” behaviour. The undrained behaviour of granular soils is found to be mainly governed by the evolution of redundancy factor, and a reversal of deviatoric stress in stress path (temporary liquefaction) is found to be due to temporary loss of contacts forming a structural mechanism in the system where RF is smaller than unity during the evolution.

A new suggestion for determining 2D porosities in DEM studies

In discrete element modeling, 2D software has been widely used in order to gain further insights into the fundamental mechanisms with less computational time. The porosities used in 2D DEM studies should be determined with appropriate approaches based on 3D laboratory porosities. This paper summarizes the main approaches for converting porosities from 3D to 2D for DEM studies and theoretical evaluations show that none of the current approaches can be widely used in dealing with soil mechanical problems. Therefore, a parabolic equation and a criterion have been suggested for the determination of 2D porosities in this paper. Moreover, a case study has been used to validate that the 2D porosity obtained from the above suggestion to be rational with both the realistic contact force distribution in the specimen and the good agreement of the DEM simulation results of direct shear tests with the corresponding experimental data. Therefore, the parabolic equation and the criterion are suggested for the determination of 2D porosities in a wide range of polydisperse particle systems, especially in dealing with soil mechanical problems.

Rheological and 3D DEM characterization of potential rutting of cold bituminous mastics

Cold recycling with bitumen emulsion represents one of the most attractive pavements rehabilitation methods. This technique has increased its popularity because it combines environmental and economical efficiency to the production of bituminous layers of adequate strength.The binder matrix obtained from this application is generally represented by mastic of bitumen, cement and filler. The percentage of these three elements affects the mixture performance in terms of stiffness, fatigue and rutting resistance as well as durability.The main purpose of this study is to evaluate how cement and limestone filler contents affect the rutting response of mastics. Laboratory testing and 3D Discrete Element Method were performed using different percentages of filler and cement, keeping constant the overall volume fraction of both compared to the total volume of the mastic samples.Temperature sweep and Multiple Stress Creep Recovery tests were implemented to study the behavior of mastics in terms of permanent deformation by shear stress.

Simulation of shale–proppant interaction in hydraulic fracturing by the discrete element method

In this paper, a three dimensional discrete element method (3D DEM) was proposed and deployed to simulate shale–proppant interaction in hydraulic fracturing. Shale is represented by particles with cement bond, and proppant is represented by particles without a cement layer. The velocity Verlet method is implemented to substitute the traditional central time integration scheme. The proposed DEM is used to investigate the shale–proppant interactions and evaluate the fracture aperture under different proppant sizes, Young׳s moduli and pressure levels. The results reveal that, the more soft shale particle, the higher pressure and the larger proppant size imply smaller crack aperture and larger plastic zone for other given conditions.

3D DEM simulation and analysis of void fraction distribution in a pebble bed high temperature reactor

The current work analyzes the radial and axial distributions of void fraction of a pebble bed high temperature reactor. A three-dimensional pebble bed corresponding to our test facility of pebble bed type gas-cooled high temperature reactor (HTR-10) in Tsinghua University is simulated via discrete element method, and the radial and axial void fraction profiles are calculated. It validates the oscillating characteristics of radial void fraction near the wall. Detailed calculations show the differences of void fraction profiles between the stationary packing bed and the dynamically discharging bed. Based on the vertically and circumferentially averaged radial distribution and horizontally averaged axial distribution of void fraction, a fully three-dimensional analytical distribution of void fraction throughout the bed is established. The results show the combined effects of gravity and void variation in the pebble bed caused by the pebble discharging. It indicates the linearly increased packing effect caused by gravity in the vertical (axial) direction and the normal distribution of void in the horizontal (radial) direction by pebble drainage. These two effects coexist in the conical base of the bed whereas only the former effect exists in the cylindrical volume of the bed.

Steady state of solid-grain interfaces during simulated CPT

Abstract It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers filled with sand. However, the cost of each simulation is an important factor. Hence, to achieve manageable simulation times the discrete material representing the sand was scaled up to sizes that were more typical of gravel than sand. A side effect of the scaled-up discrete material size employed in the model was an increased fluctuation of the macro-response that can be filtered away to observe a macroscopic steady-state cone resistance. That observation is the starting point of this communication, where a series of simulations in which the size ratio between penetrometer and particles is varied are systematically analyzed. A micromechanical analysis of the penetrometer-particle interaction is performed. These curves reveal that a steady state is arrived also at the particle-cone contact level. The properties of this dynamic interface are independent of the initial density of the granular material.

DEM Study on Energy Allocation Behavior in Crushable Soils

Detailed knowledge of particle-scale energy allocation behavior under the influence of particle breakage is of fundamental importance to the development of micromechanics-based constitutive models of sands. This paper reports original results of the energy input/dissipation of an idealized crushable soil using 3D DEM simulations. Particle breakage is modeled as the disintegration of synthetic agglomerate particles which are made up of parallel-bonded elementary spheres. Simulation results show that the initial specimen density and crushability strongly affect the energy allocation of the soil both at small and large strains. The major role of particle breakage, which itself only dissipates a negligible amount of input energy, is found to advance the soil fabric change and promote the interparticle friction dissipation. Particularly, at small strains, particle breakage disrupts the strain energy buildup and thus reduces the mobilized shear strength and dilatancy of a granular soil. At large strains where particle breakage is greatly reduced, a steady energy dissipation by interparticle friction and mechanical damping is observed. Furthermore, it is found that shear bands develop in most dense crushable specimens at large strains, but they are only weakly correlated to the anisotropy of the accumulated friction dissipation.

2D DEM analyses for T-M coupling effects of extreme temperatures on surrounding rock-supporting system of a tunnel in cold region

Taking the Kunlunshan Tunnel on Qinghai-Tibet Railway as an engineering background, the curved wall-inverted arch lining of the tunnel was simplified into the straight wall-umbrella arch one, and the fractured rock mass with developed joints was treated as a discrete medium in the calculation. Using the UDEC code, the numerical simulations for thermo-mechanical coupling processes in the surrounding rock mass-supporting system were carried out aiming at the conditions of mean temperature, extreme highest temperature and extreme lowest temperature in one year. The distributions and changes of stresses, displacements, plastic zones, temperatures in the rock mass of near field, as well as the loading states in the model-building concrete and bolting were investigated and compared for these three computation cases. The results show that compared with the case of mean temperature, the ranges, where the temperatures of surrounding rock mass change obviously, are 6.0 m and 6.5 m, respectively, for the cases of extreme highest temperature and extreme lowest temperature; the displacements of tunnel are raised by 3.2–9.3 and 5.7–12.7 times, and the thicknesses of plastic zones reach 1.5–2.5 m and 2.0–4.5 m for case 2 and 3, respectively; the extreme temperatures of air have strong effects on the stress, deformation and failure states of supporting structure of tunnel in cold region, and the influence degree of extreme lowest temperature is the highest.

Applications of geomorphology, tectonics, geology and geophysical interpretation of, East Kom Ombo depression, Egypt, using Landsat images

In the Southern Eastern Desert of Egypt, A NW–SE oriented structural graben extends from the North of Aswan to the Red Sea coast with a length of about 400km and an average width of 30km. The area has significant potential for development as it may be provided with water from surface and subsurface sources and is the site of prospection for petroleum. The present paper is an attempt to understand the structural evolution and genetic development of the geomorphologic features of the area and constructing presently a new geomorphological map at a scale of 1:250,000 using Landsat ETM images and field checks. Available geological maps and the produced geomorphological map are digitized by using the ARC-GIS software. The same program is also used to produce a 3D DEM for surface and subsurface features. Based on new interpretations of aeromagnetic and radiometric data, the subsurface features of the basement cover were illustrated on a 3D map. Geological–geomorphological profiles have been constructed in different directions in the area to identify present and ancient geomorphologic features. The place and shape of subsurface deep seated NW–SE trending faults have been determined. The faults, which generated the graben have downthrows in the order of 900–5800m. The surface and subsurface observations delineate the dominant downthrow of about 3750m. Three E–W subsurface faults have been detected under Nubia sandstone, one of them, displaying a downthrow of about 845m, cuts through the basement rocks.

802 付加体形成過程のリアル砂粒子寸法での2次元DEMシミュレーション

802 付加体形成過程のリアル砂粒子寸法での2次元DEMシミュレーション

On a Gross Traction Generated at Grouser for Tracked Agricultural Vehicles

Abstract The cross-section shape of a single grouser shoe is known as one of governing factors for generating gross traction of tracked vehicles working on deformable terrain. Previous studies revealed that not only the height of grouser but also the thickness of grouser affected the gross traction. In this study, to prepare for the development of predicting the gross traction of rubber track system, an in-house 2D DEM program was developed and refined for the analysis of gross traction generated by a single rigid grouser. The purpose of this study is to investigate the effect of grouser height and width on the gross traction of a grouser, and to numerically observe the source of gross traction of a grouser. Numerical results confirmed larger gross traction for smaller thickness ratio of 0.1, which was observed in the 2D experimental results using artificial model soil. For large thickness ratios of 0.3 and 0.5, deteriorating behavior of gross traction can be seen in DEM. Moreover, it was confirmed that gross traction of a grouser in DEM was mostly generated by the action of vertical face of grouser for thickness ratios of 0.1, 0.3, and 0.5.

Generating realistic 3D sand particles using Fourier descriptors

This paper presents a novel method of generation of random realistic sand grains for use in three dimensional (3D) DEM simulations. Based on the concept of Fourier descriptors for sand grains proposed recently by the same authors, we first randomly generate three 2D contours of cross-section for a real sand particle in three orthogonal planes, and then develop a morphing technique to construct the external 3D surface of the particle to match these cross-sections. The proposed method is examined by application to the generation of six sands reported in the literature using the Fourier spectrums available for these sands. We show that with a proper correction on the smoothness and roundness of the orthogonal projection calibrated from the six sands, the method can generate fairly consistent results as compared to the real sands. Further validation of the proposed method on another three sands shows satisfactory performance. The advantages and limitations of the method, as well as relevant future applications of the work to granular material modelling are discussed.

DEM analysis of energy dissipation in crushable soils

It is well known that particle crushing plays a critical role in the mechanical behavior of granular soils. Understanding energy dissipation under the influence of particle breakage is of key importance to the development of micromechanics-based constitutive models for sands. This paper reports the original results of the energy input/dissipation of an idealized crushable soil using 3D DEM simulations. Particle breakage is modeled as the disintegration of the synthetic agglomerate particles which are made up of parallel-bonded elementary spheres. A parametric study is performed to fully investigate the effects of initial specimen density and crushability on the energy allocation of the crushable soil.The simulation results show that the initial specimen density and the crushability strongly affect the energy allocation of the soil both at small and large strains. The major roles of particle breakage, which itself only dissipates a negligible amount of input energy, are to advance changes in the soil fabric and to promote the interparticle friction dissipation. Particularly, at small strains, particle breakage disrupts the strain energy buildup, and thus, reduces the mobilized shear strength and dilatancy of a granular soil. At large strains, where particle breakage is greatly reduced, steady energy dissipation by interparticle friction and mechanical damping is observed. Furthermore, it is found that shear bands develop in most dense crushable specimens at large strains, but they are only weakly correlated to the anisotropy of the accumulated friction dissipation.