No description available.

Shear banding is a commonly observed yet complex form of instability in granular media by which the deformation is localized in a narrow zone along a certain path. The aim of this study is to investigate the micromechanics of shear banding using the discrete element method (DEM). For this purpose, a model was developed and calibrated to simulate the macroscale behavior of sand under plane strain conditions. Upon validation against laboratory experiments, two types of confining boundaries, displacement- and force-controlled, were examined to study the kinematics of shear bands. A constant volume test was then used to investigate the evolution of antisymmetric stresses before, during, and after shear band formation. The results indicate that the antisymmetric stresses significantly increase within the shear band throughout the loading history, but may not describe the precursory shear band conditions. The DEM model is shown to properly capture the micromechanics of shear bands.

Cementation is known to significantly influence the mechanical and hydrologic properties of granular porous media by increasing the stiffness of the elastic response to stress and reducing permeability. The relationship between the changes in cementation and changes in permeability are well documented in literature. However, limited quantitative data exists on the relationship between changes in the amount of cementation and changes in the mechanical response of granular media. The goal of this research is to quantify the effects of cementation on the mechanical properties of granular porous media at the meso-scale and investigate the influence of the competing roles of mechanical and hydrologic properties on fluid flow and deformation at the macro-scale. To accomplish this goal, we developed a multiple scale approach that utilizes the parameterization control of meso-scale Discrete Element Method (DEM) models and the ability to couple fluid flow and solid deformation physics with macro-scale poro-elasticity models. At the meso-scale, a series of DEM models are designed to simulate biaxial tests of variably cemented sandstone in order to investigate the effects of cementation on the elastic and inelastic response of the porous media. The amount of cementation in the DEM model is quantified using a bond to grain ratio (BGR). The BGR is the number of bonds (the bonds represent the cement) divided by the number of grains in each model. The BGRs of the DEM models correlate to BGRs of natural samples and allow constraint of the percent cementation in the DEM models. A decrease in BGR from 2.25 to 1.00 results in a two fold decrease in shear modulus. The resulting shear moduli from the DEM models are used as input properties into two dimensional, axial symmetric poroelastic models of an isotropic confined aquifer. The poroelastic models address the implications of changes in mechanical properties and hydrologic properties on large scale fluid removal and deformation as well as address the importance of the competing roles of hydrologic and mechanical properties.

The discrete-element method for predicting skin friction for turbulent flow over rough surfaces considers the drag on the surface to be the sum of the skin friction on the flat part of the surface and the drag on the individual roughness elements that protrude into the boundary layer. The discrete-element method considers heat transfer from a rough surface to be the sum of convection through the fluid on the flat part of the surface and the convection from each of the roughness elements. The discrete-element method has been widely used and validated for roughness composed of sparse, ordered, and deterministic elements. Modifications made to the discrete-element roughness method to extend the validation to real surface roughness are detailed. These modifications include accounting for the deviation of the roughness element cross sections from circular configurations, determining the location of the computational "surface" that differs from the physical surface, and accounting for temperature changes along the height of the roughness elements. Two randomly-rough surfaces found on high-hour gas-turbine blades were characterized using a Taylor-Hobson Form Talysurf Series 2 profilometer. A method for using the three-dimensional profilometer output to determine the geometry input required in the discrete-element method for randomly-rough surfaces is presented. Two randomly-rough surfaces, two elliptical-analog surfaces, and two cone surfaces were generated for wind-tunnel testing using a three-dimensional printer. The analog surfaces were created by replacing each random roughness element from the original randomly-rough surface with an elliptical roughness element with the equivalent planorm area and eccentricity. The cone surfaces were generated by placing conical roughness elements on a flat plate to create surfaces with equivalent values of centerline-averaged height or root-mean-square (RMS) height as the randomly-rough surfaces. The results of the wind tunnel skin friction coefficient and Stanton number measurements and the discrete-element method predictions for each of the six surfaces are presented and discussed. For the randomly-rough surfaces studied, the discrete-element method predictions are within 7% of the experimentally measured skin friction coefficients. The discrete-element predictions are within 16% of the experimentally measured Stanton numbers for the randomly-rough surfaces.

No description available.

The collapse of a mine pillar is a catastrophic event with great consequences for a mining operation. These events are not uncommon, and have been reported to produce air blasts able to knock down, seriously injury or kill miners; cause cascade pillar failures which involve the collapse of neighboring pillars; produce surface subsidence; and sterilize valuable reserves. In spite of the low probability of occurrence for a pillar collapse in comparison to other ground control instability issues, these consequences make these events high risk. Therefore, the design of these structures should be considered from a risk perspective rather than from a factor-of-safety deterministic approach, as it has been traditionally done. Discontinuities are one of the main failure drivers in underground stone pillars. Regardless of this, traditional pillar strength equations do not consider the effect of these. Recently, the NIOSH pillar strength equation introduced a Large Discontinuity Factor that acknowledges the effect of discontinuities in pillar strength. However, this parameter only considers "averaged" parameters in a deterministic way, failing to account for the spatial variability of fracture networks. This work presents a risk-based pillar design framework that enables to characterize the effect of discontinuities in pillar strength, as well as account for the possible range of stresses that will be acting on pillars. The proposed method was evaluated in an underground dipping stone mine. Discontinuities were characterized by integrating Laser Scanning and virtual discontinuity mapping. Information obtained from the discontinuity mapping process was used to generate discrete fracture networks (DFNs) for each discontinuity set. The Discrete Element Modeling Software 3DEC was used along with the DFNs to simulate fractured rock pillars. Different fractured pillar strength modeling approaches were evaluated, and the most adequate in terms of pillar strength values, failure mechanisms representation, and processing times, was selected. The selected model was tested stochastically, and these results were used to characterize pillar strength variability due to the presence of discontinuities. Pillar stress distributions were estimated using an stochastic finite volume continuous numerical model that accounted for the dipping nature of the deposit and the case study mine design. A pillar probability of failure baseline was defined by contrasting resulting pillar strength and stress distributions using the reliability method. Results from this design framework provide additional decision-making tools to prevent pillar failure from the design stages by reducing the uncertainty. The proposed method enables the integration of pillar design into the risk analysis framework of the mining operation, ultimately improving safety by preventing future pillar collapses. / Doctor of Philosophy / Underground mining operations involve the removal of rock material from the ground. Engineers are required to design structural elements to ensure the stability of the openings as the material is extracted. These structural elements are known as pillars, and are usually carefully-designed regular chunks of rock left unmined. The pressures that the mined rock was carrying are shed to these pillars, which sizes and dimensions must provide enough strength to ensure the overall stability of the mine and avoid a collapse. Failure of mine pillars are events that have occurred, causing serious consequences such as injuring and killing mine workers, producing ground surface sinking affecting neighboring communities, and halting the regular mine operation. Due to the severity of the consequences of pillar collapses, these events are classified as high risk. Therefore, pillar design should be addressed from a perspective that estimates the likelihood of pillar failure given all possible hazards during their design process. The rock material that composes mine pillars present fractures and weakness planes that have an influence on pillar strength. Even though it has been widely demonstrated that these features have a direct impact on pillar strength, most of the commonly used pillar design methods fail to consider such effect, producing uncertainty about the possible range of values for the actual strength of the pillars. This work introduces a pillar design framework that enables to characterize the effect of discontinuities in pillar strength, as well as account for the possible range of stresses that will be acting on pillars. The proposed method was evaluated in an underground inclined stone mine. Laser scanning was used to map and characterize rock fractures. Fracturing information was used to generate virtual three-dimensional fracture models referred to as discrete fracture networks (DFNs). A computational mechanical model of the mine pillar was done using the software 3DEC to evaluate the compressive strength of the fractured pillar. Multiple fracturing scenarios were tested and distributions of possible pillar strengths were estimated from these tests. An additional computational model to estimate the distribution of the stresses in the pillar was performed considering the mine designs and geological conditions. Results from both analyses allowed to estimate a pillar probability of failure baseline. This design framework provides additional decision-making tools to prevent pillar failure from the design stages by reducing uncertainty. The proposed method enables the integration of pillar design into the risk analysis framework of the mining operation, ultimately improving safety by preventing future pillar collapses.

Pavement is usually composed of aggregate, asphalt binder, and air voids; rigid pavement is built with hydraulic cement concrete; reinforced pavement contains steel. With these wide ranges of materials, different mechanical behaviors need to be defined in the pavement simulation. But so far, there is no research providing a comprehensive introduction and comparison between various contact models. This paper will give a detail exploration on the contact models that can be potentially used in DEM pavement simulation; in the analysis, it includes both a theoretical part, simulation results and computational time cost, which can reveal the fundamental mechanical behaviors for the models, and that can be a reference for researchers to choose a proper contact model. A new contact model—the power law viscoelastic contact model is implemented into software PFC 3D and is numerically verified. Unlike existing linear viscoelastic contact models, the approach presented in this thesis provides a detailed exploration of the contact model for thin film power-law creeping materials based on C.Y Chueng's work. This model is aimed at simulating the thin film asphalt layer between two aggregates, which is a common structure in asphalt mixtures. Experiments with specimens containing a thin film asphalt between two aggregates are employed to validate the new contact model. / Master of Science

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The increased necessity to obtain power from other sources than conventional fossil fuels has led to the growing interest in solar power. The problem with the proposed technology is that it can only provide power during the day and therefore requires some sort of storage system, if power is to be supplied throughout the day and night. A number of storage systems exist, but the one of particular interest for this research, is packed rock beds. Rock beds have the advantage that if designed right, they have the potential to be one of the most cost effective means of storing thermal energy for solar power plants. Discrete Element Models (DEM) of rock beds were therefore developed through both experimental and numerical procedures, by conducting a series of sensitivity, calibration and verification studies. The developed models were then used to study various aspects associated with rock beds, which were either too impractical, impossible or too expensive to conduct through actual experimental work. This research focused specifically on the potential of constructing self-supporting tunnels within the rock beds in order to improve the air flow uniformity through the bed, while minimizing the pressure drop. It was observed that if the appropriate steps were followed, stable self-supporting tunnels could be formed. Valuable information such as the rock orientations resulting from different packing directions could also be derived from the models and finally, a method to convert the DEM models into the appropriate format such that it could be imported into a CFD preprocessor for future CFD studies, was developed. / AFRIKAANSE OPSOMMING: Die verhoogde noodsaaklikheid om energie te verkry uit ander bronne as konvensionele fossielbrandstowwe, het gelei tot die groeiende belangstelling in sonkrag energie. Die probleem met die voorgestelde tegnologie is dat dit net energie gedurende die dag kan voorsien en dus word daar ’n stoorstelsel benodig indien energie deur beide die dag en nag voorsien moet word. Tans bestaan daar wel ’n aantal van hierdie stoorstelsels, maar die een wat van besondere belang is in hierdie navorsing, is verpakte klip beddens. Klip beddens het die voordeel dat, indien dit reg ontwerp is, dit oor die potensiaal beskik om een van die mees koste-doeltreffende middels te wees vir die stoor van termiese energie vir sonkragstasies. Diskreet Element Modelle (DEM) van die klip beddens is ontwikkel deur gebruik te maak van beide experimentele en numeriese metodes waartydens ’n reeks sensitiwiteits-, kalibrasie- en verifiëring studies uitgevoer is. Die ontwikkelde modelle is gebruik om verskeie aspekte van klip beddens te ondersoek, wat of te onprakties, onmoontlik of te duur is vanuit ’n eksperimentele oogpunt. Hierdie navorsing het spesifiek gefokus op die potensiaal om self-ondersteunende tonnels binne in die klip beddens te vorm, ten einde die egaligheid van die lugvloei deur die bed te verbeter, terwyl die drukval geminimeer word. Daar is waargeneem dat stabiele self-ondersteunende tonnels wel gevorm kon word indien die toepaslike stappe gevolg is. Waardevolle inligting soos die klip oriëntasies wat as gevolg van die verskillende verpakkings rigtings onstaan kon ook vanuit die model verkry word. Ten slotte is ’n metode ontwikkel om die DEM modelle na die toepaslike formaat te omskep sodat dit ten einde gebruik kan word in numeriese vloeidinamika studies.

In a proposed design for a concentrated solar power tower, sand is irradiated by solar energy and transfers its energy to another fluid stream by means of a finned tube heat exchanger. To maximize heat transfer and minimize potential damage to the heat exchanger, it is desired to have a very uniform flow through the heat exchanger. However, performing full scale flow tests can be expensive, impractical, and depending upon the specific quantities of interest, unsuitable for revealing the details of what it happening inside of the flow stream. Thus, the discrete element method has been used to simulate and study particulate flows. In this project, the flow of small glass beads through a square pyramid shaped hopper and a wedge shaped hopper were studied at the lab scale. These flows were also simulated using computers running two versions of discrete element modeling software – EDEM and LIGGGHTS. The simulated results were compared against the lab scale flows and against each other. They show that, in general, the discrete element method can be used to simulate lab scale particulate flows as long as certain material properties are well known, especially the friction properties of the material. The potential for increasing the accuracy of the simulations, such as using better material property data, non-uniform particle size distributions, and non-spherical particle shapes, as well as simulating heat transfer within a granular flow are also discussed.

Ce travail de thèse concerne la prédiction des structures en béton soumises à des chargements extrêmes. Il s'intéresse plus particulièrement au comportement du béton sous fort confinement où la contrainte peut atteindre des niveaux de l'ordre du giga Pascal. La modélisation de ce comportement doit être capable de reproduire la compaction irréversible. Pour ce faire, deux lois de comportement élasto-plastique - endommageable ont été développées et implantées dans un code aux éléments discrets. Les paramètres utilisés dans ces lois sont calibrés par les simulations des essais de traction/compression uniaxial, des essais hydrostatiques et triaxiaux. Une fois les paramètres calibrés, la loi montrant le meilleur agrément avec l'expérience a été choisie pour la prédiction de la réponse du béton sous différents niveaux de confinement. Les résultats du modèle sont analysés non seulement à l'échelle macroscopique mais également à l'échelle de l'élément discret. La nécessité de prendre en compte une loi d'interaction de type élasto-plastique-endommageable est aussi montrée. La deuxième partie du travail de thèse développe une méthode de couplage entre le modèle éléments discrets et un modèle d'écoulement compressible en tenant compte des mécanismes physiques fondamentaux d'interaction entre l'écoulement interne et les particules solides d'un matériau poreux. Le problème d'écoulement est résolu par une méthode en volumes finis, où le volume est discrétisé en tétraèdres issus d'une triangulation régulière de Delaunay. Notre modèle est une adaptation aux fluides compressibles d'un modèle développé initialement pour les écoulements incompressibles. Ce couplage a été utilisé pour simuler le comportement triaxial des bétons humides et saturés sous différents niveaux de confinement. Les résultats nous montrent une bonne reproduction du comportement non-drainé du béton saturé sous faible confinement. Pour fort confinement, les simulations ne se rapprochent des résultats expérimentaux qu'au prix d'une compressibilité du fluide plus faible que celle de l'eau. Par ailleurs, la contrainte effective était une variable pertinente pour décrire le comportement du béton humide par un état limite intrinsèque indépendant du degré de saturation. / This thesis work deals with the predicting of concrete structures submitted to some extreme loadings, and, more particularly, focuses on behavior of concrete under a high-confining pressure. At this range of pressures, irreversible compaction of the material occurs and needs to be considered. Doing so, two elasto-plastic-damaged constitutive laws have been developed and implanted into a discrete element numerical code. Local parameters to be used in these constitutive laws are identified by simulating reference uniaxial traction/compression tests and triaxial compression tests. Once these parameters have been obtained, the law showing the best agreement with the experimental data has been chosen to predict the reponse of concrete sample for triaxial compressive tests at different levels of confinement. The numerical results have been analyzed not only at macroscopic scale but also at discrete element scale. The need of a constitutive law taking into account the elasto-plastic-damaged behavior has been also proved. The second objective of the thesis work was to develop a fluid flow – coupled discrete element model by considering fundamental physical mechanisms of the interaction between the internal fluide flow and the solid particles of a porous material. The flow problem is solved by the finite volume method, where the volume is discretized into tetrahedra issue of a regular Delaunay triangulation. Our model is an adaptation for elastic fluids of a model originally developed for incompressible flows. The developed fluid-flow coupled discrete element has been used to simulate the undrained triaxial behavior of concrete under different levels of confinement. The results show a good reproduction of undrained behavior of saturated concrete under low confinement. For high confinement, the simulations only resemble the experimental results when the fluid compressibility is lower than that of water. Moreover, the effective stress was a relevant variable to describe the behavior of the wet concrete by an intrinsic limit state independent of the degree of saturation.