Dissertationen zum Thema „Granular media“
Um die anderen Arten von Veröffentlichungen zu diesem Thema anzuzeigen, folgen Sie diesem Link: Granular media.
Geben Sie eine Quelle nach APA, MLA, Chicago, Harvard und anderen Zitierweisen an
Machen Sie sich mit Top-50 Dissertationen für die Forschung zum Thema "Granular media" bekannt.
Neben jedem Werk im Literaturverzeichnis ist die Option "Zur Bibliographie hinzufügen" verfügbar. Nutzen Sie sie, wird Ihre bibliographische Angabe des gewählten Werkes nach der nötigen Zitierweise (APA, MLA, Harvard, Chicago, Vancouver usw.) automatisch gestaltet.
Sie können auch den vollen Text der wissenschaftlichen Publikation im PDF-Format herunterladen und eine Online-Annotation der Arbeit lesen, wenn die relevanten Parameter in den Metadaten verfügbar sind.
Sehen Sie die Dissertationen für verschiedene Spezialgebieten durch und erstellen Sie Ihre Bibliographie auf korrekte Weise.
1
Kanzaki, Cabrera Takeichi. „Numerical modeling of anisotropic granular media“. Doctoral thesis, Universitat de Girona, 2013. http://hdl.handle.net/10803/133834.
Annotation:
Granular materials are multi-particle systems involved in many industrial process and everyday life. The mechanical behavior of granular media such as sand, coffee beans, planetary rings and powders are current challenging tasks. In the last years, these systems have been widely examined experimentally, analytically and numerically, and they continue producing relevant and unexpected results. Despite the fact that granular media are often composed of grains with anisotropic shapes like rice, lentils or pills, most experimental and theoretical studies have concerned spherical particles. The aim of this thesis has been to examine numerically the behavior of granular media composted by spherical and non-spherical particles. Our numerical implementations have permitted the description of the macroscopic properties of mechanically stable granular assemblies, which have been experimentally examined in a framework of the projects "Estabilidad y dinámica de medios granulares anisótropos" (FIS2008- 06034-C02-02) University of Girona and "Interacciones entre partículas y emergencia de propiedades macroscópicas en medios granulares" (FIS2008-06034-C02-01) University of NavarraEls materials granulars són sistemes de moltes partícules implicats en diversos processos industrials i en la nostra vida quotidiana. El comportament mecànic de conjunts granulars, com la sorra, grans de cafè, anells o pols planetàries, representa actualment un repte per a la ciència. En els últims anys aquests sistemes s’han estudiat àmpliament de forma experimental, analítica i numèrica. De totes maneres, avui dia es continuen obtenint resultats rellevants, i en moltes ocasions, inesperats. Malgrat el fet que els materials granulars sovint estan compostos per grans amb forma anisotròpica, com l’arròs, les llenties o les píndoles, la majoria dels estudis experimentals i teòrics se centren en partícules esfèriques. L’objectiu d’aquesta tesi ha estat analitzar numèricament el comportament dels mitjans granulars compostos per partícules esfèriques i no esfèriques. Els mètodes numèrics implementats han permès la descripció de les propietats macroscòpiques de piles i columnes granulars, que s’han estudiat experimentalment en el marc dels projectes "Estabilidad y dinámica de medios granulares anisótropos" (FIS2008-06034-C02- 02) de la Universitat de Girona i "Interacciones entre partículas y emergencia de propiedades macroscópicas en medios granulares" (FIS2008-06034-C02- 01) de la Universitat de Navarra
2
Sharrock, Glenn. „Tool shearing of granular media /“. St. Lucia, Qld, 2003. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe17559.pdf.
3
Olivera, Bonilla Roberto Rafael. „Numerical Simulations of Undrained Granular Media“. Thesis, University of Waterloo, 2004. http://hdl.handle.net/10012/915.
Annotation:
The objective of the present study was to develop a fluid flow-coupled distinct element model capable of capturing the undrained behaviour of granular soils by considering fundamental physical mechanisms that involve fluid flow and particle interaction. The method considers granular media as assemblies of ellipsoidal particles arranged on a plane and interacting by means of contact forces. Saturation effects are incorporated by assuming that particles are immersed in fluid, the flow of which is simulated as occurring through a network of conduits. The flow through conduits is according to a Hagen-Poiseuille relation; a transient solution is obtained by solving a system of differential equations. The developed fluid-flow coupled distinct element was used to conduct various numerical simulations and the mechanisms of undrained deformations were examined from a micromechanical point of view. The dissertation begins with a literature review on the undrained behaviour of granular materials as observed in laboratory experiments. A review of previous attempts to simulate undrained tests micromechanically is also presented, and the advantages and disadvantages of various methods are examined. The capability of the developed model to simulate two-dimensional fluid-flow and pressure dissipation problems is demonstrated by means of comparisons with analytical solutions. Fluid pressure dissipation problems are qualitatively compared with Terzaghi's one-dimension theory of consolidation. It is shown that transient flow problems are accurately modelled by the fluid flow network approach. Simulated compression tests were carried out to examine the effects of different confining pressures and initial densities on the macroscopic response. The results compare favorably with those commonly observed in undrained laboratory experiments. Simulated tests are analyzed from a micromechanical point of view. It is shown that macroscopic behaviour can be traced to changes in micromechanical fabric descriptors. The effects of the interparticle friction angle on the undrained behaviour of the assemblies are investigated. The undrained strength is considerably increased by increasing interparticle friction. The main mechanism found to be responsible for the development of higher strength is the tendency of the specimens to dilate during shear distortion. The effects of the principal stress direction on the macroscopic response are examined. The behaviour of initially anisotropic samples is significantly altered by the direction of the principal stresses relative to the anisotropy direction. It is demonstrated that macroscopic permeability of the media has a considerable effect on the strength. This behaviour is attributed to the inhomogeneity of pore pressure distributions which increases with decreased permeability. The results presented are generally in agreement with observations previously reported from laboratory experiments. The possible applications of the model for future research are also discussed.
4
Shin, Hosung. „Development of discontinuities in granular media“. Diss., Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29653.
Annotation:
Thesis (Ph.D)--Civil and Environmental Engineering, Georgia Institute of Technology, 2010.Committee Chair: Santamarina, J. Carlos; Committee Member: Bachus, Robert C.; Committee Member: Burns, Susan E.; Committee Member: Cartwright, Joseph A.; Committee Member: Goldsztein, Guillermo; Committee Member: Mayne, Paul W. Part of the SMARTech Electronic Thesis and Dissertation Collection.
5
Callahan, Thomas Patrick. „Non-Newtonian fluid injection into granular media“. Thesis, Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/39618.
Annotation:
The process of fluid injection into granular media is relevant to a wide number of applications such as enhanced oil recovery, grouting, and the construction of permeable reactive barriers. The response of the subsurface is dependent on multiple factors such as in-situ stresses, fluid properties, flow rate, and formation type. Based on these conditions a variety of response mechanisms can be initiated ranging from simple porous infiltration to hydraulic fracturing.
Currently, the mechanics of fluid injection into competent rock are well understood and can be sufficiently modeled using linear elastic fracture mechanics. Because the grains in rock formations are individually cemented together, they exhibit cohesion and are able to support tensile stresses. The linear elastic method assumes tensile failure due to stress concentrations at the fracture tip. A fracture propagates when the stress intensity factor exceeds the material toughness (Detournay, 1988)
However, understanding fluid injection in cohesionless granular media presents a much larger obstacle. Currently, no theoretical models have been developed to deal with granular media displacements due to fluid injection. Difficulty arises from the complexity of fluid rheology and composition used in engineering processes, the strong coupling between fluid flow and mechanical deformation, the non-linear response of subsurface media, and the multi-scale nature of the problem.
The structure of this thesis is intended to first give the reader a basic background of some of the fundamental concepts for non-Newtonian fluid flow in granular media. Fluid properties as well as some interaction mechanisms are described in relation to the injection process. Next, the results from an experimental series of injection tests are presented with a discussion of the failure/flow processes taking place.
We developed a novel technique which allows us to visualize the injection process by use of a transparent Hele-Shaw cell. Specifically, we will be using polyacrylamide solutions at a variety of concentrations to study non-Newtonian effects on the response within the Hele-Shaw cell. By performing tests at a range of solution concentrations and injection rates we are to be able to identify a transition from an infiltration dominated flow regime to a fracturing dominated regime.
6
Woudberg, Sonia. „Laminar flow through isotropic granular porous media“. Thesis, Link to the online version, 2006. http://hdl.handle.net/10019/1320.
7
Clément, Christian Paul André René. „Multiscale modelling of fluid-immersed granular media“. Thesis, University of Nottingham, 2010. http://eprints.nottingham.ac.uk/10956/.
Annotation:
In this thesis we present numerical simulation studies of fluid-immersed granular systems using models of varying scales and complexities. These techniques are used to examine the effects of an interstitial fluid on the dynamics of dense granular beds within a number of vibrated systems. After an introduction to the field of granular materials, we present the techniques used to model both the granular dynamics and the fluid flow. We introduce various multiscale techniques to couple the motion of the granular and fluid phases. An extensive comparison between these techniques is conducted for some well-known systems. The fluid-grain coupling techniques are applied to some larger systems in order to determine under what situations the approaches are most suitable. An investigation concerning three-dimensional fluid-driven convection within vertically vibrated fluid-immersed granular beds is then presented. Here we observe granular piling and determine that this is a result of fluid-driven convective cycles within the bed which may be strengthened through the presence of wall friction. Our simulations capture this convective behaviour and lead to a detailed understanding of the mechanisms behind the phenomenon. Under a wide range of conditions a system of fluid-immersed fine grains within a vibrated partitioned cell will transfer in their entirety into just one of the segments through a linking channel at the cell base. We perform an experimental and numerical study in order to understand the principle mechanisms behind the “partition instability”. We determine that the instability arises due to the fluid experiencing less resistance to its motion when flowing through the shallower granular column during vibration. A simple analytical model is developed which captures this behaviour. It is commonly known that large dense intruders may rise rapidly to the surface of a granular bed when subjected to a vertical vibrational force. We next present an experimental and numerical study to determine the principle mechanism associated when the granular bed is immersed in a fluid, the fluid-enhanced Brazil nut effect. Our key finding is that the behaviour of the intruder is sensitive to the detailed fluid and particle flow in its vicinity. An analytical approach is developed to model the rising of a dense intruder in a vibrating fluid-immersed porous bed. Finally a brief study is presented into the behaviour of a vibrated system of fluid-immersed particles held within a zero-gravity environment. We conduct simulations which are able to reproduce the behaviour of a collection of particles suspended magnetically in a vibrating fluid.
8
Clewett, James. „Emergent surface tension in boiling granular media“. Thesis, University of Nottingham, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604898.
Annotation:
In this thesis we present findings from an experimental and numerical study of loosely confined, dry granular gases subject to vertical vibration. We found that the system phase separates into a high-density, liquid-like phase and a low-density gas-like phase. The phase separation was shown to occur at a critical driving amplitude but is independent of frequency. To introduce our work, we give an overview of phase separation in driven granular gases. Ofpatiicular interest are: a solid-liquid-like phase separation in tightly confined, dry granular mono layers and a liquid-gas-like phase separation in loosely confined, wet granular gases. Our system differs from the above examples in two significant ways: our cell is deeper than that used to tightly confine the granular mono layers, so that we avoid the formation of a solid-like phase; our patiicles are dry and as such there are no cohesive forces between the particles. The liquid-gas phase separation is a useful system in which to study far-from-equilibrium phenomena because the particles are easily homogenised and then quenched into the phase-separating state. The system also allows us to smoothly approach the phase-transition boundaries. The phase separation was shown to be spinodal driven, with a region of negative compressibility due to an excess in the granular temperature of the particles in the dilute phase. The origin of the excess temperature was traced to the coherent motion of particles above a critical driving amplitude. By switching to a frequency modulated driving signal the phase separation was suppressed, demonstrating the requirement for coherent motion. The experiment shows the importance of using realistic driving motion in simulations. The phase-separation coarsening dynamics were shown to be similar to that of thermodynamic systems evolving under curvature driven diffusion (model B). Using the Cahn-Hilliard equation we accurately predict the dominant length scale in the early-time dynamics. In thelIDodynamics the Cahn-Hilliard equation desclibes the minimisation of an excess interfacial energy. This suggests that we might define an effective free energy for our granular system, however, as yet it is not clear what is meant by free energy in the context of a far-fi·om-equilibrium system. Finally, by studying the surface tension of quasi-2D liquid-like droplets in the steady state, we found behaviour consistent with Laplace's equation, demonstrating that the surface tension is real. Detailed measurements of the pressure in the interfacial region show that the surface tension results predominantly from an unexpected anisotropy in the kinetic energy part of the pressure tensor, in contrast to thelIDodynamic systems where surface tension arises from either the attractive interaction between pmticles or entropit considerations. The general nature of our argument for the Oligin of the surface tension means that it should apply to other granular phase separations and segregations in granular rnixhlres. As such this might be a new general mechanism in far-from-equiliblium thermodynamics. Throughout the thesis we use numerical simulations, configured with a geometry matching that of the experimental cell. To simulate the two million particles required we created a bespoke molecular dynamics code to execute using GPGPU hardware. The peliormance of our simulations was comparable to the state of the art in the literature, approximately twenty times faster than simulations on modern CPU processors.
9
Slade, R. E. „Wave propagation in fluid saturated granular media“. Thesis, University of Bath, 1995. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760666.
10
Stefanovska, Emilija. „Simulation of granular media using parallel processing“. Thesis, Kingston University, 1995. http://eprints.kingston.ac.uk/20598/.
Annotation:
A numerical simulation method in which quasi static mechanical equilibrium is achieved in distinct elements has been developed for two dimensional applications with elasto- frictional interactions. The method works on a step- by-step basis, using small strain steps, achieving equilibrium by means of a least squares routine in which particle displacements and spins are updated to minimise the sum of forces and the sum of moments on all elements. An error analysis is carried out which shows that for small strain steps a stable, robust routine is obtained. Some elements of the routine can be run in parallel and an implementation on a parallel processing system of 17 Transputers (including the Master Processor) is discussed. Various aspects of parallel implimentation are discussed. The language OCCAM is used. The routine uses a novel nonlinear elasto-frictional interaction, which can be integrated analytically. A number of simulations with discs at high packing density is shown. Dominant mechanisms are detected, both for isotropic compression and for a deviatoric stress path. These mechanisms include the making and breaking of contacts and the formation of force bridges. Observed macroscopic effects are dilatancy and reduced stiffness at higher stress ratio, followed by failure. The failure phenomena are identified indirectly by identification with a theoretical model. Simulations of metal-matrix composites are presented as mixtures of circular discs and rectangular fibres. Loose assemblies are considered. Similar mechanisms as in the case of circular discs are found and in addition the bending moment on the fibres is calculated. Great heterogeneity characterises all simulations.
11
Snoeijer, Jacobus Hendrikus. „Statistics of force networks in granular media /“. s. l. : s. n, 2003. http://catalogue.bnf.fr/ark:/12148/cb39298967h.
12
Scheffler, Tim Niclas. „Kollisionskühlung in elektrisch geladener granularer Materie - Collisional cooling in electrical charged granular media“. Gerhard-Mercator-Universitaet Duisburg, 2001. http://www.ub.uni-duisburg.de/ETD-db/theses/available/duett-05222001-111655/.
Annotation:
Particles in granular systems collide inelasticly and kinetic energy is dissipated in the granular system. Granular temperature measures the unordered relative motion of the particles. As a result of the inelastic collisions granular temperature decreases, this process is called collisional cooling. In most cases granular particles are charged. This thesis studies the influence of electrical charges on the collisional cooling by using computer simulations and kinetic theory. It is shown, that electrical charge modifies the dissipation rate by a Boltzmann-factor. The energy barrier of the Boltzmann-factor is given by the electrostatic interaction of two colliding particles. In dense systems this energy barrier is reduced due to the interactions with the particles, that do not take part in the collision. A quantitative expression is given for the effective reduction of the energy barrier. The results found for homogeneous systems is expanded for the local description of inhomogeneous systems.
13
Tsukahara, Michel. „Jamming in granular media : modeling of experimental data /“. [S.l.] : [s.n.], 2009. http://library.epfl.ch/theses/?nr=4490.
14
Kuczynski, Leszek W. „Evaluation of the flocculation process in granular media“. Thesis, University of Ottawa (Canada), 1985. http://hdl.handle.net/10393/4801.
15
Emelko, Monica Beata. „Removal of Cryptosporidium parvum by granular media filtration“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/NQ60533.pdf.
16
Bizon, Christopher Andrew. „Simulations of wave patterns in oscillated granular media /“. Digital version accessible at:, 1998. http://wwwlib.umi.com/cr/utexas/main.
17
Wang, Zhongzheng. „Capillary Effects on Fluid Transport in Granular Media“. Thesis, The University of Sydney, 2021. https://hdl.handle.net/2123/25895.
Annotation:
Fluid transport phenomena in granular media are of great importance due to various natural and industrial applications, including CO2 sequestration, enhanced oil recovery, remediation of contamination, and water infiltration into soil. Although numerous studies exist in the literature with aims to understand how fluid properties and flow conditions impact the transport process, some key mechanisms at microscale are often not considered due to simplifications of physical phenomenon and geometry, limited computational resources, or limited temporal/spatial resolution of existing imaging techniques.
In this Thesis, we investigate fluid transport phenomena in granular media with a focus on the capillary effects. We move from relatively simple scenario on patterned surfaces to more complex granular media, tackling a variety of liquid-transport related problems that all have extensive industrial applications. The bulk of this Thesis is composed of six published papers. Each chapter is prefaced by an introductory section presenting the motivation for the corresponding paper and its context within the greater body of work.
This Thesis reveals the impact of some previously neglected physical phenomena at microscale on the fluid transport in granular materials, providing new insights and methodology for describing and modelling fluid transport process in porous media.
18
Zhang, Fengshou. „Pattern formation in fluid injection into dense granular media“. Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/43716.
Annotation:
Integrated theoretical and experimental analysis is carried out in this work to investigate the fundamental failure mechanisms and flow patterns involved in the process of fluid injection into dense granular media. The experimental work is conducted with aqueous glycerin solutions, utilizing a novel setup based on a Hele-Shaw cell filled with dense dry sand. The two dimensional nature of the setup allows direct visualization and imaging analysis of the real-time fluid and grain kinematics. The experimental results reveal that the fluid flow patterns show a transition from simple radial flow to a ramified morphology while the granular media behaviors change from that of rigid porous media to localized failure that lead to development of fluid channels. Based on the failure/flow patterns, four distinct failure/flow regimes can be identified, namely, (i) a simple radial flow regime, (ii) an infiltration-dominated regime, (iii) a grain displacement-dominated regime, and (iv) a viscous fingering-dominated regime. These distinct failure/flow regimes emerge as a result of competition among various energy dissipation mechanisms, namely, viscous dissipation through infiltration, dissipation due to grain displacements, and viscous dissipation through flow in thin channels and can be classified based on the characteristic times associated with fluid injection, hydromechanical coupling and viscoelastoplasticity.
The injection process is also analyzed numerically using the discrete element method (DEM) coupled with two fluid flow scheme, a fixed coarse grid scheme based on computational fluid dynamics (CFD) and a pore network modeling scheme. The numerical results from the two complementary methods reproduce phenomena consistent with the experimental observations and justify the concept of associating the displacement regimes with the partition among energy dissipation mechanisms. The research in this work, though fundamental in nature, will have direct impacts on many engineering problems in civil, environmental and petroleum engineering such as ground improvement, environmental remediation and reservoir stimulation.
19
Duffaut, Kenneth. „Stress sensitivity of elastic wave velocities in granular media“. Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for petroleumsteknologi og anvendt geofysikk, 2011. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-16174.
Annotation:
Knowledge of the stress sensitivity of elastic-wave velocities in sedimentary rocks with different consolidation is important for several geophysical applications such as seismic pore pressure prediction, time-lapse seismic analysis and not least in seismic exploration. Since clays or shales are more compactable than sand or sandstones they often experience large plastic deformation causing both clay platelet orientation and porosity reduction of the order of 50% or more. The interplay between stress and porosity effects may mask the stress sensitivity of velocities in clays or shales. The porosity loss occurring in sandstones during mechanical compaction is generally slower and hence the stressinduced velocity changes are better preserved at least prior to the onset of grain cementation. Small amounts of contact cement, 1 to 2% of the total rock volume, can have a dramatic effect on velocities and their corresponding stress dependence. We demonstrate this reduction in stress sensitivity of elastic-wave velocity between sandstones of different consolidation through comparison of rock physics contact model-predictions of compressional to shear velocity ratio (Vp/Vs), with similar estimates obtained from time-lapse seismic amplitude versus offset (AVO) data from two oil producing fields. Furthermore, modified contact models are proposed to investigate the role of the combined effects of intergrain contact friction, increasing differential stress and stress ratio in relation to the propagation direction of the elasticwaves in granular media. Model-predictions are compared with ultrasonic measurements obtained from experimental compression tests on unconsolidated sands.
20
Mazouchova, Nicole. „Principles of fin and flipper locomotion on granular media“. Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/47645.
Annotation:
Locomotion of animals, whether by running, flying, swimming or crawling, is crucial
to their survival. The natural environments they encounter are complex containing
fluid, solid or yielding substrates. These environments are often uneven and inclined, which
can lead to slipping during footsteps presenting great locomotor challenges. Many animals
have specialized appendages for locomotion allowing them to adapt to their environmental
conditions. Aquatically adapted animals have fins and flippers to swim through the water,
however, some species use their paddle-like appendages to walk on yielding terrestrial substrates
like the beach. Beach sand, a granular medium, behaves like a solid or a fluid when
stress is applied. Principles of legged locomotion on yielding substrates remain poorly understood,
largely due to the lack of fundamental understanding of the complex interactions
of body/limbs with these substrates on the level of the Navier-Stokes Equations for fluids.
Understanding of the limb-ground interactions of aquatic animals that utilize terrestrial
environments can be applied to the ecology and conservation of these species, as well as
enhance construction of man-made devices.
In this dissertation, we studied the locomotion of hatchling loggerhead sea turtles on
granular media integrating biological, robotic, and physics studies to discover principles
that govern fin and flipper locomotion on flowing/yielding media. Hatchlings in the field
modified their limb use depending on substrate compaction. On soft sand they bent their
wrist to utilize the solid features of sand, whereas on hard ground they used a rigid flipper
and claw to clasp asperities during forward motion. A sea turtle inspired physical model in
the laboratory was used to test detailed kinematics of fin and flipper locomotion on granular
media. Coupling of adequate step distance, body lift and thrust generation allowed the robot
to move successfully forward avoiding previously disturbed ground. A flat paddle intruder
was used to imitate the animal's flipper in physics drag experiments to measure the forces
during intrusion and thrust generation.
21
SaÌnchez, Lana Diego Paul. „Fluid driven separation and pattern formation in granular media“. Thesis, University of Nottingham, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.417966.
22
Barnes, D. J. „A study of the micro-mechanics of granular media“. Thesis, Aston University, 1985. http://publications.aston.ac.uk/14251/.
Annotation:
The development of more realistic constitutive models for granular media, such as sand, requires ingredients which take into account the internal micro-mechanical response to deformation. Unfortunately, at present, very little is known about these mechanisms and therefore it is instructive to find out more about the internal nature of granular samples by conducting suitable tests. In contrast to physical testing the method of investigation used in this study employs the Distinct Element Method. This is a computer based, iterative, time-dependent technique that allows the deformation of granular assemblies to be numerically simulated. By making assumptions regarding contact stiffnesses each individual contact force can be measured and by resolution particle centroid forces can be calculated. Then by dividing particle forces by their respective mass, particle centroid velocities and displacements are obtained by numerical integration. The Distinct Element Method is incorporated into a computer program 'Ball'. This program is effectively a numerical apparatus which forms a logical housing for this method and allows data input and output, and also provides testing control. By using this numerical apparatus tests have been carried out on disc assemblies and many new interesting observations regarding the micromechanical behaviour are revealed. In order to relate the observed microscopic mechanisms of deformation to the flow of the granular system two separate approaches have been used. Firstly a constitutive model has been developed which describes the yield function, flow rule and translation rule for regular assemblies of spheres and discs when subjected to coaxial deformation. Secondly statistical analyses have been carried out using data which was extracted from the simulation tests. These analyses define and quantify granular structure and then show how the force and velocity distributions use the structure to produce the corresponding stress and strain-rate tensors.
23
Ellis, Matthew. „Simulations of magnetic reversal properties in granular recording media“. Thesis, University of York, 2015. http://etheses.whiterose.ac.uk/12215/.
Annotation:
With increasing demand for high density magnetic recording devices a paradigm shift is required to overcome the super-paramagnetic limit. By using a high anisotropy material, such as L1_0 FePt, and heat assisted magnetic recording the areal density can be taken well beyond 1 Tbit/in^2. For FePt the grains on which the data is stored can be reduced to 3nm in size before thermal noise becomes an issue. Therefore, the understanding of how magnetic grains of only a few nanometres in size behave and switch is highly important. Here an atomistic model of magnetic materials, based on the Heisenberg exchange interaction and localised classical atomic moments, is used to investigate the magnetic reversal properties of granular recording media. A detailed model for FePt, parametrised from ab-initio, is presented and this is used to investigate the finite size effects in nanometre grains. Using this model, the finite size effects on the linear reversal regime is investigated. Comparing a model using the Landau-Lifshitz-Bloch macrospin equation to the atomistic results show that multi-scale modelling may be valid down to the 3 nm length scale. The inter-granular exchange interaction between neighbouring grains is investigated using a model invoking the presence of magnetic impurities in the grain inter-layer. Using constrained Monte-Carlo methods the effective exchange is calculated for different impurity density, grain separation and temperature. Following this, helicity dependent all optical switching in granular FePt is investigated. The phase space for switching through the Inverse Faraday effect is explored. Using the Master equation, a model for thermal switching with magnetic circular dichroism is explored which qualitatively explains the induced magnetisation observed experimentally. Finally, the coupling of the spin and lattice system is modelled by combining spin and molecular dynamics. The coupling to the lattice excites magnons which appear to decay implying that some damping processes are occurring.
24
Goel, Ashish. „Dynamic loading of structures by high speed granular media“. Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/277444.
Annotation:
This thesis analyses the impact of granular aggregates with structures using experiments and numerical simulations. Original contributions include an insight into multiple factors affecting the loading and damage to the structures, along with study of numerical parameters important for realistic prediction of the interaction between the granular media and structures. It extends the current understanding related to such interactions, with an underlying motivation to guide strategies in order to reduce the structural damage. The response of structures impacted by granular media (sand or soil) is of significant research interest for many applications. One of the applications is for landmine explosions which causes ejection of soil from ground and damage to structures impacted by this ejected soil. Experimentation is done in a laboratory setting where the cylindrical sand slugs are generated at high speed using an impulse provided by a piston. This induces a velocity gradient along the slug, because of which the slug expands during the flight before impacting the target. Deformable as well as rigid flat targets are considered in two orientations relative to the incoming slug: perpendicular (i.e. normal orientation) and inclined at an angle of 45°. The targets are supported by force transducers to capture the loading from the slug. Simulations are performed using a combination of discrete particle and finite element schemes, which enables the analysis of the fully coupled interaction between the flowing granular media and the structure. A contact model involving multiple parameters is used for inter-particle and particle-target contact. Firstly, a numerical analysis is performed to characterise the temporal evolution of slugs and their impact on monolithic beams constrained at the ends. Out of all the parameters used for inter-particle contact definition in discrete particle method, only the contact stiffness is found to effect the velocity gradient in the slug before it impacts the target. Other factor influencing the gradient is the acceleration provided by the piston. A strong dependence of beam deflection on the stand-off distance is observed due to the velocity gradient in the slugs. As the second step, the effect of target surface properties on the transmitted momentum is analysed. Experiments are done by applying coatings of different hardness and roughness on the target surface impacted by sand slugs. For normally oriented targets, the transmitted momentum is observed to be insensitive to the change in surface coating. In contrast, for inclined targets, a significant influence of coatings is observed. Additionally, the momentum transmitted to the inclined targets is always less than that for normal targets. Numerical analysis of this surface effect reveals that assuming the slug particles to be spherical shape in simulations does not capture the particle/target interactions accurately and under-predicts the frictional loading on the target. Following this, a detailed numerical study is done to understand the effect of the shape of particles in the slug. Simple shaped non-spherical particles are constructed by combining spherical sub-particles. With increasing angularity of particles in the slug, the frictional loading on the target is shown to increase. This results in an increase of momentum transmitted to inclined targets. For normally oriented targets however, the particle shape does not affect the overall transmitted momentum, which is a behaviour similar to that observed when studying the effect of target surface properties. In addition, effect of fracture of particles in the slug is analysed by using beam connections between sub-particles that break during the impact with the target. If the fracture results in increasing particle angularity, the transmitted momentum increases, whereas the situation reverses if fracture results on more spherical shaped particles. Lastly, a strategy to reduce the loading on the targets is analysed by using sacrificial coating on the target surface. In experiments, this coating is placed on the rigid target surface using a lubricant at their interface. When impacted by the slug, this coating slides on the target surface, resulting in a reduction of frictional loading on the target. If the friction at the coating/target interface vanishes, the transmitted momentum approaches the theoretical minimum value. Simulations are used to first validate the experimental observations and then to extend the concept of sliding coatings using deformable targets. Both the transmitted momentum and deflections depended on the thickness of the target and coating. When a coating is used, the deflections increase due to reduction in target thickness. It is found that the best strategy to reduce the damage to the target is to use least possible thickness of the coating and minimise the friction at the interface between the coating and the target. The presented work examines many of the factors that affect the loading on the target impacted by granular slugs, in addition to characterising the expansion of slugs before the target impact. The analysed factors include those already known such as target stand-off distance, inclination and unveils others such as target surface properties and granular properties. The numerical analysis discloses important parameters and shows the effect of particle shape, highlighting the shortcomings of widely used spherical particle assumption in the numerical studies. A strategy using a sacrificial coating to reduce damage to the target is also analysed.
25
Sundberg, Garth. „Numerical Modeling of Electromagnetic Scattering in Explosive Granular Media“. PDXScholar, 2010. https://pdxscholar.library.pdx.edu/open_access_etds/865.
Annotation:
Terahertz (THz) reflection and transmission spectroscopy is a promising new field with applications in imaging and illicit material detection. One particularly useful application is for the detection of improvised explosive devices (IEDs) which is a favorite weapon of global terrorists. Explosive materials have been shown to have a unique spectral signature in the THz band which can be used to identify the explosives. However, the initial measurements performed on the explosive samples do not account for the modulation of the spectral features by random scattering that will be prevalent with actual samples encountered in applications. The intent of this work is to characterize and quantify the effects of random scattering that may alter the spectral features. Specifically, the effect that a randomly rough surface and granular scattering has on the scattered THz wave (T-Rays) will be investigated and characterized using the Finite-Difference Time-Domain (FDTD) simulation method. The FDTD method is a natural choice for this work as it can handle complicated geometries (i.e., multiple scatterers, arbitrarily rough interfaces, etc.) arbitrary materials (i.e., dispersive media, etc.) and provides broadband frequency data with one simulation pass. First, the effect that the randomly rough surface of the sample explosive has on the extracted spectral signature will be studied using a Monte-Carlo analysis. Then the effect of the complex structure inside the explosive material (the granular scatterers) will be considered. Next, when the physics of the rough surface and granular scattering are understood, a robust method to extract the spectral signature from the reflected T-rays will be developed.
26
Bijeljic, Branko. „Flow and mass transfer in fibrous and granular porous media“. Thesis, Imperial College London, 2000. http://hdl.handle.net/10044/1/8005.
27
Mohammad, Vali Samani Abbas. „Travel-time tomography for stress reconstruction in granular soil media“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq22221.pdf.
28
Fekete, N. „Prediction of stresses in granular media by an integral method“. Thesis, McGill University, 1990. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=59280.
Annotation:
The Method of Integral Relations is used to predict stress distributions in a granular half-space due to a normally applied two-dimensional surface load. The resulting stress profiles are compared with Boussinesq's solution for a two-dimensional elastic medium and with the results of experiments which were carried out in sand using an axi-symmetric geometry. The theory predicts that the behaviour of the solutions depends primarily on the shape of the surface loading profiles. A parabolic loading profile equivalent to that observed experimentally yields vertical normal stress profiles which have good qualitative agreement with the experimental results and with the Diffusion Equation solution of Harr et al. As the surface pressure distribution approaches a uniform one, the theory predicts the existence of double-peaked vertical normal stress profiles similar to those observed in experiments in rock masses under equivalent loading conditions.
29
Haeberle, Jan [Verfasser], und Matthias [Gutachter] Sperl. „Properties of Tailored Granular Media / Jan Haeberle ; Gutachter: Matthias Sperl“. Köln : Universitäts- und Stadtbibliothek Köln, 2019. http://d-nb.info/1179356039/34.
30
Becker, Christopher R. (Christopher Ryan). „Drag coefficients on razor clams in slightly fluidized granular media“. Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45783.
Annotation:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references.
Razor clams are able to burrow deeply into granular media with only a small fraction of force required by traditional anchoring devices. It is hypothesized that the collapse of their shell and subsequent localized fluidization of the media is responsible for a large reduction in drag, thereby allowing the clam to burrow. A test setup comprised of a fluidized bed connected to a pump with an attached ball valve for flow regulation is constructed which allows testing of drag force in conditions similar to that the clam experiences, as well as in an environment void of wall effects. Testing is done using a dead clam attached to a stainless steel rod which is passed through the fluidized bed to obtain accurate data for the razor clam. The result is that a slight fluidization to a void fraction between 40-45% gives a drag reduction which is more than sufficient for a clam to burrow at velocities seen in nature.
by Christopher R. Becker.
S.B.
31
Wang, Jianfeng. „Micromechanics of Granular Media: A Fundamental Study of Interphase Systems“. Diss., Virginia Tech, 2006. http://hdl.handle.net/10919/27216.
Annotation:
The interphase is a localized region adjacent to a manufactured inclusion that is surrounded by granular soil. These regions are ubiquitous in civil infrastructure and often are components of large-scale composite systems. The interphase region influences load-deformation behavior of the entire composite system. However, mechanisms that control the mechanical behavior of the interphase region and, in turn, control the composite structure behavior, are not clearly understood. Few relationships exist for predicting interphase behavior from properties of granular materials and the inclusion surface that can be measured in the laboratory.
A two dimensional discrete element model of a general interphase system was developed and validated against laboratory data. Numerical experiments are conducted with varying soil to inclusion relative geometry. A new micromechanics-based approach, which utilizes microscopic quantities to explain the mechanics of granular media from a continuum point view, is adopted to investigate the mechanisms that underlie the interphase behavior.
It is shown that the grain to inclusion surface relative geometry controls the degree of granular media strength mobilization by controlling development of fabric and contact force anisotropy inside the interphase region. A unique bilinear relationship exists between the mobilized granular media strength and the principal direction of average contact force anisotropy at the interface between the particles touching the surface and the inclusion. These findings suggest the problem is one of contact and can not be solved using purely geometric correlations, as past research presumed. A fundamental mechanism of behavior, long sought in geomechanics problems, is presented. Publications resulting from this research are significant and original contributions to the geoengineering, material science, geophysics and granular physics literature.Ph. D.
32
Meng, Yue S. M. Massachusetts Institute of Technology. „Jamming transition and emergence of fracturing in wet granular media“. Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/128983.
Annotation:
Thesis: S.M., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, September, 2020Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages 43-48).
The interplay between multiphase flow in a granular medium and the displacement of the grain particles generates a wide range of patterns, including fractures, desiccation cracks, and labyrinth structures. There are several controlling parameters behind the morphodynamics that govern the transition between different regimes. A modified capillary number characterizes the crossover from capillary fingering to viscous fingering, and a transition from fingering to fracturing can be achieved either by decreasing frictional resistance. The balance between frictional, viscous, and capillary forces has been studied in experiments and simulations, and has helped understanding the underlying mechanisms for a wide range of phenomena, including fractures in drying colloidal suspensions, and methane migration in lake sediments. In this thesis we study fluid-induced deformation of granular media, and the fundamental role of capillarity and wettability on the emergence of fracture patterns. We develop a hydromechanical computational model, coupling a "moving capacitor" dynamic network model of two-phase flow at the pore scale with a discrete element model of grain mechanics. We simulate the slow injection of a less viscous fluid into a frictional granular pack initially saturated with a more viscous, immiscible fluid. We study the impact of wettability and initial packing density, and find four different regimes of the fluid invasion: cavity expansion and fracturing, frictional fingers, capillary invasion, and capillary compaction. We explain fracture initiation as emerging from a jamming transition, and synthesize the system's behavior in the form of a novel phase diagram of jamming for wet granular media.
by Yue Meng.
S.M.
S.M. Massachusetts Institute of Technology, Department of Civil and Environmental Engineering
33
Lätzel, Marc. „From microscopic simulations towards a macroscopic description of granular media“. [S.l. : s.n.], 2003. http://www.bsz-bw.de/cgi-bin/xvms.cgi?SWB10447201.
34
Head, David Andrew. „Topics in non-equilibrium systems theory“. Thesis, Brunel University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246169.
35
Chornewich, Cristina. „Bacterial transport in granular porous media: the effects of cell concentration and media pre-coating“. Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=67039.
Annotation:
Column transport experiments were conducted under saturated conditions to examine the effects of cell concentration and media pre-coating. Two strains of E. coli were used in the study; the commonly studied laboratory organism E. coli K12 D21 and a mutant of the waterborne pathogen E. coli O157:H7. Column experiments were conducted with both clean sand and sand that was pre-coated with bacteria. The influent concentration of the E. coli strains was varied over several orders of magnitude to examine the effect of cell concentration. Concentration dependent removal rates were observed for both organisms in both the clean and media pre-coated sand columns. It was also found that the media pre-coating either does not influence the transport behavior or it decreases the attachment efficiency. Although differences in transport are observed, these differences are not large enough to have a significant influence on the predicted travel distances.Des expériences de transport par colonne ont été menées afin d'examiner les effets de la concentration des cellules et du pré-revêtement de média. Deux souches de bactéries ont été utilisées: E. coli K12 D12 et une souche mutante E. coli O157:H7. Les expériences par colonne ont été menées avec du sable propre et du sable qui a été préalablement enduit de bactéries. La concentration de l'influent en bactérie a été variée sur plusieurs ordres de grandeur pour examiner l'effet de la concentration cellulaire. Une dépendance du taux d'élimination à la concentration a été observée pour les deux souches de bactéries dans les deux types de sable. De plus, le pré-revêtement de média n'influence d'aucune façon le comportement du transport ni en réduit l'efficacité d'adhésion. Bien que des différences dans le transport ont été observées, celles-ci n'ont eu aucun effet significatif sur la prédiction de la distance à parcourir.
36
Collinson, Roger. „Mathematical models and numerical techniques for plasticity flows of granular media“. Thesis, Curtin University, 1998. http://hdl.handle.net/20.500.11937/1930.
Annotation:
A mathematical study has been undertaken to model various kinds of granular flows including the perfect plasticity flow and the viscous elasto-plasticity flow. The work is mainly based on the double-shearing theory originated by Spencer and developed by many others. The focus of the project is on the formulation of the theory, the construction of mathematical models and the development of robust simulation techniques.Based on a general formulation of the double-shearing theory, the perfect plasticity flow is shown to be governed by a set of highly nonlinear first order hyperbolic partial differential equations with two distinct characteristics. A sophisticated numerical algorithm is then developed based on the method of characteristics to determine the stress discontinuity and the velocity and stress fields. With the method developed, a numerical study is then undertaken to model the flow of granular materials in a hopper in the presence of stress discontinuity and to investigate the influence of various parameters on the distribution of hopper wall pressures.Utilising the double shearing theory, a set of stress-strain constitutive equations in explicit form has been derived, which makes it possible to formulate the double-shearing theory within the framework of the finite element method. Thus, consequently, a sophisticated finite element technique has been developed to solve the general boundary value problem governing the viscous elasto-plasticity flows obeying the double-shearing theory. Numerical implementation of the frictional boundary condition is also presented. The model is then illustrated with a numerical example demonstrating the influence of wall friction on the distribution of pressures on silo walls throughout the dynamic process of material discharge from silos.
37
Newlon, Scott. „INTRUDER DYNAMICS RESPONSE OF GRANULAR MEDIA WITH NON-LINEAR INTERACTION POTENTIALS“. OpenSIUC, 2017. https://opensiuc.lib.siu.edu/theses/2252.
Annotation:
An investigation into the intruder dynamics of dry dimensionless, frictionless discs in bidispersed, disordered systems is carried out using computer simulations. The velocity of an intruder particle driven under constant force is used as a tool to determine scaling of velocity as a function of packing pressure. Using these velocity for a range of pressures, $4 \times 10^{-7}\leq P \leq 4 \times 10^{-2}$. A universal scaling relation is proposed and plotted. The force required to cause the packing to yield to the driven intruder is determined and plotted as function of pressure. Power law exponents were extracted for the yielding force vs. the pressure. The extracted values were used to study the micro-rheology of the intruder particle. Grain scale characteristics are used to infer global elastic modulus properties.
38
Dai, Weijing. „THERMAL CONDUCTION IN GRANULAR MEDIA: FROM INTERFACE, TOPOLOGY TO EFFECTIVE PROPERTY“. Thesis, The University of Sydney, 2018. http://hdl.handle.net/2123/19838.
Annotation:
Granular media are particulate substance featured by their unique discrete structure, which are commonly seen in daily life and extensively used in industry. Differently from those continuum materials whose properties are mostly defined by their chemical formulas and status, granular media further require clarification about the effects of their topology on their properties. Therefore, effective properties are used to emphasise this distinction in measuring and describing granular media. In this study, we focus on the effective thermal conductivity of generalised gas-filled granular media, which is highly related to energy technologies and advanced fabrication processes. With particularly concentration on the topological transitions in vibrated granular media, how the topology influences the effective thermal conductivity is explored. Aiming at revealing the mechanisms governing the heat conduction of granular media, a bottom-up consequence scheme is employed in this study by decomposing the macroscale phenomena into grain-scale interactions. Under such scheme, the objectives of this work are further divided into (1) investigating the heat conduction mechanisms at inter-grain contact interfaces and (2) integrating the thermal contact units based on the topology of granular media. To accomplish the former investigation, the finite element analysis is implemented to model the gas-solid thermal interaction contributed by the Smoluschowski effect that gives rise to coupling dependence of gas pressure and grain size. With a systematic study on the heat conduction of individual units, the later objective is tackled by introducing the grain-scale thermal interaction into discrete element methods. With the combination of these cross-scale studies, a numerical framework is established. Furthermore, the thermal measurement system based on transient plane source techniques is applied to experimentally characterise correlations between the effective thermal conductivity and external mechanical loading. These experimental results as well as available literature data are used to quantitatively verify the proposed numerical method. In order to figure out the topological influence on the effective thermal conductivity, the discrete element method is further employed to examine the mesoscale behaviours of agitated granular media. The grain-scale structural characterisation unravels the topological transitions in vibration. Granular crystallisation, a process prompting the disorder-to-order transition, is identified as the major phenomenon and its boundary dependent mechanisms are iii proposed. Moreover, the topological influence on the effective thermal conductivity can be assessed with respect to the crystallisation, i.e., the degree of structure order, of granular media. With the fundamental research in this thesis on the heat conduction mechanisms and the granular crystallisation, the effective thermal conductivity is studied in a full range of scales from individual grains to bulk media. In summary, we demonstrate and experimentally validate a multiscale framework to solve the thermal problems in granular media that can also be applied to other effective conduction properties.
39
Zaccherini, Rachele. „Surface waves attenuation in granular media through a small-scale Metabarrier“. Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017.
Annotation:
The present thesis describes a small-scale experiment, carried out in the laboratory of the Swiss Federal Institute of Technology of Zurich (ETH). The research focuses on metamaterials, locally resonant structures able to affect the propagation of waves passing through them.
The present thesis investigates an innovative method to attenuate Rayleigh waves through the insertion of a barrier of scaled resonators into the soil, capable of generating a bandgap in the dispersion relation. Waves, whose frequency fall within the bandgap, cannot propagate through the resonant structure. Each resonator is constituted by a steel mass mounted on top of a spring made with 16 beams forming a truss.
Taking advantage of the results of A. Palermo et al [1] as a starting point, we carried out a small-scale experiment in a big wooden box filled with glass beads in order to investigate the effectiveness of the designed metabarrier in attenuating surface waves generated by a metal rod exciting the surface every 300 ms. We found a stop-band in the dispersion relation inside the metabarrier, generated by the coupling between the vertical component of Rayleigh waves and the longitudinal resonances of the resonators.
In parallel with the laboratory experiment, some numerical simulations have been performed with the software Comsol Multiphysics in order to compare the results obtained experimentally.
40
Noorzad, Ali. „Cyclic behavior of cohesionless granular media using the compact state concept“. Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0004/NQ40317.pdf.
41
Alkhaldi, Hashem. „Contact investigations of granular mechanical media in a tumbling sorting machine“. [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-31752.
42
Nam, Kyung Moon. „Modeling Terahertz Diffuse Scattering from Granular Media Using Radiative Transfer Theory“. PDXScholar, 2010. https://pdxscholar.library.pdx.edu/open_access_etds/380.
Annotation:
Terahertz (THz) spectroscopy can potentially be used to probe and characterize inhomogeneous materials, however spectroscopic identification of such materials from spectral features of diffuse returns is a relatively underdeveloped area of study. In this thesis, diffuse THz scattering from granular media is modeled by applying radiative transfer (RT) theory for the first time in THz sensing. Both classical RT theory and dense media radiative transfer (DMRT) theory based on the quasi-crystalline approximation (QCA) are used to calculate diffuse scattered intensity. The numerical solutions of the vector radiative transfer equations (VRTE) were coded and calculated in MATLAB. The diffuse scattered field from compressed Polyethylene (PE) pellets containing steel spheres was measured in both transmission and reflection using a THz time domain spectroscopy (THz-TDS) system. Measurement results showed energy redistribution by granular media due to volume scattering as well as angle dependent spectral features due to Mie scattering. The RT model was validated by successfully reproducing qualitative features observed in experimental results. Diffuse intensity from granular media containing Teflon, lactose sugar, and C4 explosive was then calculated using the RT models. Simulation results showed the amplitude of diffuse intensity is affected by factors such as grain size, fractional volume of grains, thickness of scattering layer, and scattering angles. Spectral features were also observed in the diffuse intensity spectra from media containing grains with THz spectral signatures. The simulation results suggest the possibility of identifying materials from diffuse intensity spectra.
43
Collinson, Roger. „Mathematical models and numerical techniques for plasticity flows of granular media“. Curtin University of Technology, School of Mathematics and Statistics, 1998. http://espace.library.curtin.edu.au:80/R/?func=dbin-jump-full&object_id=10697.
Annotation:
A mathematical study has been undertaken to model various kinds of granular flows including the perfect plasticity flow and the viscous elasto-plasticity flow. The work is mainly based on the double-shearing theory originated by Spencer and developed by many others. The focus of the project is on the formulation of the theory, the construction of mathematical models and the development of robust simulation techniques.Based on a general formulation of the double-shearing theory, the perfect plasticity flow is shown to be governed by a set of highly nonlinear first order hyperbolic partial differential equations with two distinct characteristics. A sophisticated numerical algorithm is then developed based on the method of characteristics to determine the stress discontinuity and the velocity and stress fields. With the method developed, a numerical study is then undertaken to model the flow of granular materials in a hopper in the presence of stress discontinuity and to investigate the influence of various parameters on the distribution of hopper wall pressures.Utilising the double shearing theory, a set of stress-strain constitutive equations in explicit form has been derived, which makes it possible to formulate the double-shearing theory within the framework of the finite element method. Thus, consequently, a sophisticated finite element technique has been developed to solve the general boundary value problem governing the viscous elasto-plasticity flows obeying the double-shearing theory. Numerical implementation of the frictional boundary condition is also presented. The model is then illustrated with a numerical example demonstrating the influence of wall friction on the distribution of pressures on silo walls throughout the dynamic process of material discharge from silos.
44
Ding, Yang. „Simulation and theoretical study of swimming and resistive forces within granular media“. Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/45880.
Annotation:
Understanding animal locomotion requires modeling the interaction of the organism with its environment. Locomotion within granular media like sand, soil, and debris that display both solid and fluid-like behavior in response to stress is less studied than locomotion within fluids or on solid ground. To begin to reveal the secrets of movement in sand, I developed models to explain the subsurface locomotion of the sand-swimming sandfish lizard. I developed a resistive force theory (RFT) with empirical force laws to explain the swimming speed observed in animal experiments. By varying the amplitude of the undulation in the RFT, I found that the range of amplitude used by the animal predicted the optimal swimming speed. I developed a numerical model of the sandfish coupled to a discrete element method simulation of the granular medium to test assumptions in the RFT and to study more detailed mechanics of sand-swimming. Inspired by the shovel-shaped head of the sandfish lizard, I used the simulation to study lift forces in granular media: I found that when a submerged intruder moved at a constant speed within a granular medium it experienced a lift force whose sign and magnitude depended on the intruder shape. The principles learned from the models guided the development of a biologically inspired robot that swam within granular media with similar performance to the lizard.
45
Ruta, Sergiu. „Study of interaction effects in magnetic granular systems for recording media application“. Thesis, University of York, 2017. http://etheses.whiterose.ac.uk/20272/.
Annotation:
Magnetic nano-systems have a wide scope of applications ranging from data storage technologies to biomedicine. In data storage devices the information is encoded in the grain magnetisation corresponding to bit "0" and "1". The recording media industry is driven by increasing the areal density of stored data and decreasing cost, while in the same time maintaining the thermal stability and signal-to-noise ratio. For this, close-packed 2-dimensional granular systems are used, with nanometre grain size. The quality of such magnetic recording media depends on the intrinsic material properties and on the inter-granular coupling via exchange and magnetostatic interaction. The work presented here studies the effects of inter-granular coupling and investigates different approaches to extract intrinsic properties from the bulk measurements. Due to the irregular shape of the grains, the dipole approximation for magnetostatic interaction is inaccurate. For higher accuracy, a 5-fold numerical integral is required for each pair of grains. Analytical integration over the grains height is possible reducing the numerical calculation to a 3-fold integral. The competition between the exchange and magnetostatic interactions leads to complex magnetic structures and correlated behaviour, where groups of grains behave collectively. The effects are observed and studied here based on the magnetic radial correlation function which shows a damped oscillatory form as a function of grain separation. The correlation length increases with increasing exchange interaction. An important consequence of correlated behaviour is that it alters the intrinsic switching field distribution (SFD), leading to an effective SFD. The intrinsic SFD is a fundamental characteristic of granular magnetic materials, defined as the distribution of irreversible switching events of magnetic grains in the absence of inter-granular interactions. Separating the intrinsic SFD from the effective SFD remains a challenge. Two methods that have been widely used to extract the intrinsic SFDs from hysteresis based measurements, the so-called FORC method and the Delta H(M, Delta M)-method are compared. It is shown here that the FORC diagrams contain useful information about the interactions in the system, but the ability to extract the intrinsic SFD is limited to the system in which the magnetic correlations can be neglected. Identifying the SFD from hysteresis loop measurements in the parameter range relevant for applications, requires applying the inverse problem solving techniques such as the Delta H(M, Delta M)-method.
46
Chantharayukhonthorn, Maytee. „A hybrid discrete element and continuum method for multiscale granular media modeling“. Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122146.
Annotation:
Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 95-98).
Capturing the propagation of microscale physics to macroscale phenomena is intractable for many large systems. Scale propagation is a major issue in granular media, wherein two extremes are often taken. In one, granular materials are modeled as a continuum, which greatly reduces the number of degrees of freedom that describe the system and can thus be simulated relatively quickly. However continuum models are not always precise and have difficulty capturing certain effects such as particle size dependence. In discrete element methods (DEM), every grain and the interactions between them are simulated. DEM is accurate but solve time scales poorly with large grain numbers. Here, we present a hybrid simulation scheme, which seeks a best-of-both-worlds solution by bridging these two approaches. A mass of granular media is partitioned into three domains: a continuum domain represented using the material point method (MPM), discrete grains using DEM, and a transition zone of both MPM and DEM that are coupled via kinematic constraints. An "oracle" determines which areas of the domain are MPM and which are DEM, and converts between the two. In the canonical example of silo flow, flow with a sufficiently small orifice jams, resolving length scale dependent effects. Collapse of granular columns modeled with the hybrid method compare quantitatively well with pure discrete simulation and experiments in literature. A significant speedup is seen with the hybrid method over a similar domain of pure discrete grains.
by Maytee Chantharayukhonthorn.
S.M.
S.M. Massachusetts Institute of Technology, Department of Mechanical Engineering
47
Wang, Da. „Accelerated granular matter simulation“. Doctoral thesis, Umeå universitet, Institutionen för fysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-110164.
Annotation:
Modeling and simulation of granular matter has important applications in both natural science and industry. One widely used method is the discrete element method (DEM). It can be used for simulating granular matter in the gaseous, liquid as well as solid regime whereas alternative methods are in general applicable to only one. Discrete element analysis of large systems is, however, limited by long computational time. A number of solutions to radically improve the computational efficiency of DEM simulations are developed and analysed. These include treating the material as a nonsmooth dynamical system and methods for reducing the computational effort for solving the complementarity problem that arise from implicit treatment of the contact laws. This allow for large time-step integration and ultimately more and faster simulation studies or analysis of more complex systems. Acceleration methods that can reduce the computational complexity and degrees of freedom have been invented. These solutions are investigated in numerical experiments, validated using experimental data and applied for design exploration of iron ore pelletising systems.This work has been generously supported by Algoryx Simulation, LKAB (dnr 223-
2442-09), Umeå University and VINNOVA (2014-01901).
48
Warnakulasooriya, Niranjan Mahaguruge. „Intruder Dynamic Response in Particulate Media“. OpenSIUC, 2017. https://opensiuc.lib.siu.edu/dissertations/1395.
Annotation:
Many everyday materials, broadly classified as ``particulate media'', are at the heart of many industries and natural phenomena. Examples range from the storage and transport of bulk foods and aggregates such as grains and coal; the processing of pharmaceutical pills and the grinding coffee beans; to the mitigation and cost control of life-threatening events like landslides, earthquakes, and silo failures. The common theme connecting all these phenomena is the mechanical stability of the granular material that arises from interactions at the microscopic level of the grain scale, and how this influences collective properties at the bulk, macroscopic scale. In this dissertation, we present an extensive study of the mechanical properties of a physics-based model of granular particle systems in two dimensions using computer simulations. Specifically, we study the dynamics of an intruder particle that is driven through a dense, disordered packing of particles. This practical technique has the benefit of being amenable to experimental application which we expect will motivate future studies in the area. We find the `microrheology' of the intruder can be traced back to the properties of underlying, original, unperturbed packing, thereby providing a method to characterize the mechanical properties of the material that may otherwise be unavailable. To perform this study, we initially created mechanically stable granular packings of bidisperse discs, for several orders of magnitude of particle friction coefficient $\mu$, over a range in packing densities, or packing fractions $\phi$, in the vicinity of the critical packing fraction $\phi_c$, the density below which the packing is no longer stable. This range in $\phi$ translates to a range in packing pressures $P$, spanning several orders of magnitude down to the $P\rightarrow 0$ limit. For each packing, we apply a driving force to the intruder probe particle and find the critical force $F_{c}$, the minimum force required to induce motion of the probe as it is dragged through the system. We find that $F_{c}(\mu)$ for the different friction packings, scales with the packing pressure $P$ as a power-law according to: $F_{c}(\mu) - F_{c}^{o}(\mu) \sim P^{\beta(\mu)}$. The power-law exponent, $\beta(\mu)$ becomes friction dependent, but approaches the value, $\beta(\mu\to0) = 1.0 \pm 0.1$ in the zero-friction limit. $F_{c}^{o}(\mu)$ is the value of $F_{c}$ in the limit $P \to 0$, that similarly depends on the friction coefficient as, $F_{c}^{o}(\mu) \to 0$, when $\mu \to \infty$. We use this property of $F_{c}^{o}(\mu)$ to characterize the mechanical properties of different frictional packings. Another focus of this study is the `microrheology' of the intruder through force-velocity dependencies in $\mu=0$ systems at different $P$. For this case, the intruder is driven through the packing at a steady-state velocity $$, for driving forces above the critical force $F_D > F_c$. We introduce a scaling function that collapses the force-velocity curves onto a single master curve. This power law scaling of the collapsed curve as $P\rightarrow 0$ is reminiscent of a continuous phase transition, reinforcing the notion that the mechanical state of the system exhibits critical-like features. Furthermore, we also find an alternative scaling collapse of the form: $- \sim (F_{D} - F_{c})^{\alpha}$, where $$ represents a constant velocity term in the limit of small excess forcing, and the critical force $F_{c}$ now appears as fitting parameter that matches our explicit calculations. Thence, we are able to extract $F_{c}$ from a driven probe without a-priori having any knowledge about the state of the system. To further investigate the transition of the system through the different intruder force perturbations, we implemented a coarse graining (CG) technique that transforms our discrete particle interaction force information into continuous stress fields. Through this methodology, we are able to calculate the kinetic and contact stresses as the intruder is driven through the system. We are able to qualify and quantify the directional and distance dependencies of the stress response of the packing due to the driven probe via radial and azimuthal stress calculations. In particular, we find how the stress response not only captures the wake region behind the driven intruder, but also how the stress decays in the forward direction of the intruder, which follows universal behavior.
49
Procopio, Adam T. Zavaliangos Antonios. „On the compaction of granular media using a multi-particle finite element model /“. Philadelphia, Pa. : Drexel University, 2006. http://hdl.handle.net/1860/1297.
50
Maladen, Ryan Dominic. „Biological, simulation, and robotic studies to discover principles of swimming within granular media“. Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/42852.
Annotation:
The locomotion of organisms whether by running, flying, or swimming is the result of multiple degree-of-freedom nervous and musculoskeletal systems interacting with an environment that often flows and deforms in response to movement. A major challenge in biology is to understand the locomotion of organisms that crawl or burrow within terrestrial substrates like sand, soil, and muddy sediments that display both solid and fluid-like behavior. In such materials, validated theories such as the Navier-Stokes equations for fluids do not exist, and visualization techniques (such as particle image velocimetry in fluids) are nearly nonexistent.
In this dissertation we integrated biological experiment, numerical simulation, and a physical robot model to reveal principles of undulatory locomotion in granular media. First, we used high speed x-ray imaging techniques to reveal how a desert dwelling lizard, the sandfish, swims within dry granular media without limb use by propagating a single period sinusoidal traveling wave along its body, resulting in a wave efficiency, the ratio of its average forward speed to wave speed, of approximately 0.5. The wave efficiency was independent of the media preparation (loosely and tightly packed). We compared this observation against two complementary modeling approaches: a numerical model of the sandfish coupled to a discrete particle simulation of the granular medium, and an undulatory robot which was designed to swim within granular media. We used these mechanical models to vary the ratio of undulation amplitude (A) to wavelength (λ) and demonstrated that an optimal condition for sand-swimming exists which results from competition between A and λ. The animal simulation and robot model, predicted that for a single period sinusoidal wave, maximal speed occurs for A/ λ = 0.2, the same kinematics used by the sandfish. Inspired by the tapered head shape of the sandfish lizard, we showed that the lift forces and hence vertical position of the robot as it moves forward within granular media can be varied by designing an appropriate head shape and controlling its angle of attack, in a similar way to flaps or wings moving in fluids. These results support the biological hypotheses which propose that morphological adaptations of desert dwelling organisms aid in their subsurface locomotion. This work also demonstrates that the discovery of biological principles of high performance locomotion within sand can help create the next generation of biophysically inspired robots that could explore potentially hazardous complex flowing environments.