Geopolymer is a novel cementitious material which can be a potential alternative to ordinary Portland cement (OPC) for all practical applications. However, until now research on this revolutionary material is limited mainly to experimental studies, which have the limitations in considering the details of the atomic- and meso-scale structure and atomic scale mechanisms that govern the properties at the macro-scale. Most experimental studies on geopolymer have been conducted focusing only on the macroscopic properties and considering it as a single-phase material. However, research has shown that geopolymer is a composite material consisting of geopolymer binder (GB), unreacted source material, and, in the presence of Ca in the source material, calcium silicate hydrate (CSH). Therefore, in this research, a multiscale/multiphysics modeling approach has been taken to understand geopolymer structure and mechanical properties under varying conditions and at different length scales. First, GB was prepared at the atomic scale using molecular dynamics (MD) simulations with varying Si/Al ratios and water contents within the nano voids. The MD simulated geopolymer structure was validated based on comparison with experiments using X-ray pair distribution function (PDF), infra-red (IR) spectra, coordination of atoms, and density. The results indicate that the highest strength occurs at a Si/Al ratio of 2-3 and the presence of molecular water negatively affects the mechanical properties of GB. The loss of strength for GB with increased water content is linked to the diffusion of Na atoms and subsequent weakening of Al tetrahedra. The GB was also subjected to nanoindentation using MD and the effect of indenter size and loading rate was investigated at an atomic scale. A clear correlation between the indenter size and observed hardness of GB was observed which proves indentation size effects (ISE). Realizing the composite nature of geopolymer, the presence of unreacted and secondary phases such as quartz and CSH in geopolymer was also investigated. To do that, the mechanical properties of GB, the secondary phases and their interfaces was first determined from MD simulations. Using the MD generated properties, a meso-scale model of geopolymer composite was prepared in Peridynamics (PD) framework which considered large particles of GB and secondary phases of nanometers in size which cannot be easily modeled in MD. The meso-scale model provides a larger platform to study geopolymer in the presence of large nano-voids and multiple phases. Results from the PD simulations were directly comparable to experimentally observed mechanical properties. Findings of this study can be directly used in future to construct more advanced and sophisticated models of geopolymer and will be instrumental in designing the synthesis condition for geopolymer with superior mechanical properties.

Transport phenomena at the nanoscale can differ from that at the continuum because the large surface area to volume ratio significantly influences material properties. While the modeling of many such transport processes have been reported in the literature, a few examples exist that integrate molecular approaches into the more typical macroscale perspective. This thesis extends the understanding of nanoscale transport governed by charge, mass and energy transfer, comparing these phenomena with the corresponding continuum behavior where applicable. For instance, molecular simulations enable us to predict the solvation structure around ions and describe the diffusion of water in salt solutions. In another case, we find that in the absence of interfacial effects, the stagnation flow produced by two opposing nanojets can be suitably described using continuum relations. We also examine heat conduction within solids of nanometer dimensions due to both the ballistic propagation of lattice vibrations in small confined dimensions and a diffusive behavior that is observed at larger length scales. Our simulations determine the length dependence of thermal conductivity for these cases as well as effects of isotope substitution in a material. We find that a temperature discontinuity at interfaces between dissimilar materials arises due to interfacial thermal resistance. We successfully incorporate these interfacial nanoscale effects into a continuum model through a modified heat conduction approach and also by a multiscale computational scheme. Finally, our efforts at integrating research with education are described through our initiative for developing and implementing a nanotechnology module for freshmen, which forms the first step of a spiral curriculum. / Ph. D.

Over the last two decades, advanced fabrication techniques have enabled the fabrication of materials and devices at sub-micron length scales. For heat conduction, the conventional Fourier model for predicting energy transport has been shown to yield erroneous results on such length scales. In semiconductors and dielectrics, energy transport occurs through phonons, which are quanta of lattice vibrations. When phase coherence effects can be ignored, phonon transport may be modeled using the semi-classical phonon Boltzmann transport equation (BTE). The objective of this thesis is to develop an efficient computational method to solve the BTE, both for single-material and multi-material systems, where transport across heterogeneous interfaces is expected to play a critical role. The resulting solver will find application in the design of microelectronic circuits and thermoelectric devices. The primary source of computational difficulties in solving the phonon BTE lies in the scattering term, which redistributes phonon energies in wave-vector space. In its complete form, the scattering term is non-linear, and is non-zero only when energy and momentum conservation rules are satisfied. To reduce complexity, scattering interactions are often approximated by the single mode relaxation time (SMRT) approximation, which couples different phonon groups to each other through a thermal bath at the equilibrium temperature. The most common methods for solving the BTE in the SMRT approximation employ sequential solution techniques which solve for the spatial distribution of the phonon energy of each phonon group one after another. Coupling between phonons is treated explicitly and updated after all phonon groups have been solved individually. When the domain length is small compared to the phonon mean free path, corresponding to a high Knudsen number ([mathematical equation]), this sequential procedure works well. At low Knudsen number, however, this procedure suffers long convergence times because the coupling between phonon groups is very strong for an explicit treatment of coupling to suffice. In problems of practical interest, such as silicon-based microelectronics, for example, phonon groups have a very large spread in mean free paths, resulting in a combination of high and low Knudsen number; in these problems, it is virtually impossible to obtain solutions using sequential solution techniques. In this thesis, a new computational procedure for solving the non-gray phonon BTE under the SMRT approximation is developed. This procedure, called the coupled ordinates method (COMET), is shown to achieve significant solution acceleration over the sequential solution technique for a wide range of Knudsen numbers. Its success lies in treating phonon-phonon coupling implicitly through a direct solution of all equations in wave vector space at a particular spatial location. To increase coupling in the spatial domain, this procedure is embedded as a relaxation sweep in a geometric multigrid. Due to the heavy computational load at each spatial location, COMET exhibits excellent scaling on parallel platforms using domain decomposition. On serial platforms, COMET is shown to achieve accelerations of 60 times over the sequential procedure for Kn<1.0 for gray phonon transport problems, and accelerations of 233 times for non-gray problems. COMET is then extended to include phonon transport across heterogeneous material interfaces using the diffuse mismatch model (DMM). Here, coupling between phonon groups occurs because of reflection and transmission. Efficient algorithms, based on heuristics, are developed for interface agglomeration in creating coarse multigrid levels. COMET is tested for phonon transport problems with multiple interfaces and shown to outperform the sequential technique. Finally, the utility of COMET is demonstrated by simulating phonon transport in a nanoparticle composite of silicon and germanium. A realistic geometry constructed from x-ray CT scans is employed. This composite is typical of those which are used to reduce lattice thermal conductivity in thermoelectric materials. The effective thermal conductivity of the composite is computed for two different domain sizes over a range of temperatures. It is found that for low temperatures, the thermal conductivity increases with temperature because interface scattering dominates, and is insensitive to temperature; the increase of thermal conductivity is primarily a result of the increase in phonon population with temperature consistent with Bose-Einstein statistics. At higher temperatures, Umklapp scattering begins to take over, causing a peak in thermal conductivity and a subsequent decrease with temperature. However, unlike bulk materials, the peak is shallow, consistent with the strong role of interface scattering. The interaction of phonon mean free path with the particulate length scale is examined. The results also suggest that materials with very dissimilar cutoff frequencies would yield a thermal conductivity which is closest to the lowest possible value for the given geometry. / text

This thesis covers the simulation and modelling of the transesterification of triglyceride oils to make biodiesel, using heterogeneous catalysts. Initially, data fitting was performed to fit overall kinetic rate equations to experimental data, ignoring diffusional behaviour. Additionally, experiments were undertaken to investigate the influence of feed ratio on the reaction kinetics. A single site mechanism with surface reaction as the rate limiting step was found to most closely match the experimental conversion profiles for the operating conditions studied. To incorporate diffusional behaviour into the modelling a multicomponent diffusion methodology was adapted for use within this system. To verify transport properties of the system and the suitability of this theoretical diffusion calculation, measurement of density and viscosity for a range of mixtures was undertaken, along with molecular dynamics simulation to produce diffusion coefficients. Finally, a novel algorithm was developed to simulate coupled diffusion and reaction within the pores of the catalyst and the subsequent bulk concentration changes this produced.

Many engineering and scientific applications of flow in porous media are characterized by transport phenomena at multiple spatial scales, including pollutant transport, groundwater remediation, and acid injection to enhance well production. Carbon sequestration in particular is a multiscale problem, because the trapping and leakage mechanisms of CO2 in the subsurface occur from the sub-pore level to the basin scale. Quantitative and predictive pore-scale modeling has long shown to be a valuable tool for studying fluid-rock interactions in porous media. However, due to the size limitation of the pore-scale models (10-4-10-2m), it is impossible to model an entire reservoir at the pore scale. A straightforward multiscale approach would be to upscale macroscopic parameters (e.g. permeability) directly from pore-scale models and then input them into a continuum-scale simulator. However, it has been found that the large-scale models do not predict in many cases. One possible reason for the inaccuracies is oversimplified boundary conditions used in this direct upscaling approach. The hypothesis of this work is that pore-level flow and upscaled macroscopic parameters depends on surrounding flow behavior manifested in the form of boundary conditions. The detailed heterogeneity captured by the pore-scale models may be partially lost if oversimplified boundary conditions are employed in a direct upscaling approach. As a result, extracted macroscopic properties may be inaccurate. Coupling the model to surrounding media (using finite element mortars to ensure continuity between subdomains) would result in more realistic boundary conditions, and can thus improve the accuracy of the upscaled parameters. To test the hypothesis, mortar coupling is employed to couple pore-scale models and also couple pore-scale models to continuum models. Flow field derived from mortar coupling and direct upscaling are compared, preferably against a true solution if one exists. It is found in this dissertation that pore-scale flow and upscaled parameters can be significantly affected by the surrounding media. Therefore, using arbitrary boundary conditions such as constant pressure and no-flow boundaries may yield misleading results. Mortar coupling captures the detailed variation on the interface and imposes realistic boundary conditions, thus estimating more accurate upscaled values and flow fields. An advanced upscaling tool, a Super Permeability Tensor (SPT) is developed that contains pore-scale heterogeneity in greater detail than a conventional permeability tensor. Furthermore, a multiscale simulator is developed taking advantage of mortar coupling to substitute continuum grids directly with pore-scale models where needed. The findings from this dissertation can significantly benefit the understanding of fluid flow in porous media, and, in particular, CO2 storage in geological formations which requires accurate modeling across multiple scales. The fine-scale models are sensitive to the boundary conditions, and the large scale modeling of CO2 transport is sensitive to the CO2 behavior affected by the pore-scale heterogeneity. Using direct upscaling might cause significant errors in both the fine-scale and the large-scale model. The multiscale simulator developed in this dissertation could integrate modeling of CO2 physics at all relevant scales, which span the sub-pore or pore level to the basin scale, into one single simulator with effective and accurate communication between the scales. The multiscale simulator provides realistic boundary conditions for the fine scales, accurate upscaled information to continuum-scale, and allows for the distribution of computational power where needed, thus maintaining high accuracy with relatively low computational cost. / text

The continuing trends of miniaturisation, mass customisation, globalisation and wide use of the Internet have great impacts upon manufacturing in the 21st century. Micro manufacturing will play an increasingly important role in bridging the gap between the traditional precision manufacturing and the emerging technologies like MEMS/NEMS. The key requirements for micro manufacturing in this context are hybrid manufacturing capability, modularity, reconfigurability, adaptability and energy/resource efficiency. The existing design approaches tend to have narrow scope and are largely limited to individual manufacturing processes and applications. The above requirements demand a fundamentally new approach to the future applications of micro manufacturing so as to obtain producibility, predictability and productivity covering the full process chains and value chains. A novel generic modular reconfigurable platform (GMRP) is proposed in such a context. The proposed GMRP is able to offer hybrid manufacturing capabilities, modularity, reconfigurablity and adaptivity as both an individual machine tool and a micro manufacturing system, and provides a cost effective solution to high value micro manufacturing in an agile, responsive and mass customisation manner. An integrated framework has been developed to assist the design of GMRPs due to their complexity. The framework incorporates theoretical GMRP model, design support system and extension interfaces. The GMRP model covers various relevant micro manufacturing processes and machine tool elements. The design support system includes a user-friendly interface, a design engine for design process and design evaluation, together with scalable design knowledge base and database. The functionalities of the framework can also be extended through the design support system interface, the GMRP interface and the application interface, i.e. linking to external hardware and/or software modules. The design support system provides a number of tools for the analysis and evaluation of the design solutions. The kinematic simulation of machine tools can be performed using the Virtual Reality toolbox in Matlab. A module has also been developed for the multiscale modelling, simulation and results analysis in Matlab. A number of different cutting parameters can be studied and the machining performance can be subsequently evaluated using this module. The mathematical models for a non-traditional micro manufacturing process, micro EDM, have been developed with the simulation performed using FEA. Various design theories and methodologies have been studied, and the axiomatic design theory has been selected because of its great power and simplicity. It has been applied in the conceptual design of GMRP and its design support system. The implementation of the design support system is carried out using Matlab, Java and XML technologies. The proposed GMRP and framework have been evaluated through case studies and experimental results.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22463号 / 工博第4724号 / 新制||工||1738(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 梶 弘典, 教授 佐藤 啓文, 教授 関 修平 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

<p>In this study we propose a fast hybrid spectral-element time-domain (SETD) / finite-element time-domain (FETD) method for transient analysis of multiscale electromagnetic problems, where electrically fine structures with details much smaller than a typical wavelength and electrically coarse structures comparable to or larger than a typical wavelength coexist.</p><p>Simulations of multiscale electromagnetic problems, such as electromagnetic interference (EMI), electromagnetic compatibility (EMC), and electronic packaging, can be very challenging for conventional numerical methods. In terms of spatial discretization, conventional methods use a single mesh for the whole structure, thus a high discretization density required to capture the geometric characteristics of electrically fine structures will inevitably lead to a large number of wasted unknowns in the electrically coarse parts. This issue will become especially severe for orthogonal grids used by the popular finite-difference time-domain (FDTD) method. In terms of temporal integration, dense meshes in electrically fine domains will make the time step size extremely small for numerical methods with explicit time-stepping schemes. Implicit schemes can surpass stability criterion limited by the Courant-Friedrichs-Levy (CFL) condition. However, due to the large system matrices generated by conventional methods, it is almost impossible to employ implicit schemes to the whole structure for time-stepping.</p><p>To address these challenges, we propose an efficient hybrid SETD/FETD method for transient electromagnetic simulations by taking advantages of the strengths of these two methods while avoiding their weaknesses in multiscale problems. More specifically, a multiscale structure is divided into several subdomains based on the electrical size of each part, and a hybrid spectral-element / finite-element scheme is proposed for spatial discretization. The hexahedron-based spectral elements with higher interpolation degrees are efficient in modeling electrically coarse structures, and the tetrahedron-based finite elements with lower interpolation degrees are flexible in discretizing electrically fine structures with complex shapes. A non-spurious finite element method (FEM) as well as a non-spurious spectral element method (SEM) is proposed to make the hybrid SEM/FEM discretization work. For time integration we employ hybrid implicit / explicit (IMEX) time-stepping schemes, where explicit schemes are used for electrically coarse subdomains discretized by coarse spectral element meshes, and implicit schemes are used to overcome the CFL limit for electrically fine subdomains discretized by dense finite element meshes. Numerical examples show that the proposed hybrid SETD/FETD method is free of spurious modes, is flexible in discretizing sophisticated structure, and is more efficient than conventional methods for multiscale electromagnetic simulations.</p> / Dissertation

A l'échelle macroscopique, la mécanique des milieux continus (MMC) rencontre parfois des difficultés à représenter correctement le comportement d'un système physique, du fait d'une modélisation insuffisante des phénomènes. Ces faiblesses sont particulièrement marquées dans les systèmes où les interfaces, qui font apparaître des échelles d'espace très différentes, jouent un rôle prépondérant : microfluidique, écoulements polyphasiques etc.. Or, dans de nombreux domaines, et notamment dans le milieu pétrolier, les modèles macroscopiques existants semblent insuffisants pour pouvoir traiter correctement les cas proposés. Par ailleurs, la méconnaissance des paramètres d’entrée d'une simulation macroscopique tels que les propriétés de transport, introduit parfois une mauvaise représentation de l’ensemble des processus diffusifs. La simulation à l'échelle microscopique, en l'occurrence la dynamique moléculaire classique (DM), peut pallier certains problèmes rencontrés par les approches macroscopiques, en permettant de mieux appréhender les divers processus physiques, notamment aux interfaces. Elle permet également de suppléer l’expérimentation, en permettant de calculer pour un fluide modèle les propriétés physiques du mélange étudié. Ainsi, à partir des ces données générées, il est possible de construire des corrélations palliant aux différents manques. Néanmoins, de par son caractère microscopique, cette approche ne permet de simuler que des échelles sub-micrométriques qui sont bien éloignées de la taille indispensable à la plupart des cas réalistes, qu’ils soient académiques ou industriels. En couplant les deux démarches, macroscopique et microscopique, de manière directe ou indirecte, il est donc envisageable d’accéder à des informations que l’une ou l’autre des ces approches ne peut fournir seule. / Hybrid atomistic-continuum methods allow the simulation of complex flows, depending on the intimate connection of many spatiotemporal scales : from the nanoscale to the microscale and beyond. By limiting the molecular description within a small localized region, for example near fluid/fluid or fluid/solid interfaces (breakdown of the continuum), these methods are useful to study large systems for reasonable times. Besides, there is a wide variety of applications for such hybrid methods, ranging from the micro- or nano-scale devices, and other industrial processes such as wetting, droplet formation, and biomolecules near interfaces. In this work, we present one scheme for coupling the Navier-Stokes set of equations with Molecular Dynamics. Among the existing alternatives to couple these two approaches, we have chosen to implement a domain decomposition algorithm based on the alternating Schwarz method. In this method, the flow domain is decomposed into two overlapping regions : an atomistic region described by molecular dynamics and a continuum region described by a finite volume discretization of the incompressible Navier-Stokes equations. The fundamental assumption is that the atomistic and the continuum descriptions match in the overlapping region, where the exchange of information is performed. The information exchange, requires the imposition of velocity from one sub-domain in the form of boundary conditions (Dirichlet)/constraints on the solver of the other subdomain and vice versa. The spatial coupling as well as the temporal coupling of the two approaches has been investigated in this work. To show the feasibility of such a coupling, we have applied the multiscale method to a classical fluid mechanics problems.

La réduction des émissions polluantes est actuellement un enjeu majeur au sein du secteur aéronautique. Parmi les solutions développées par les motoristes, la combustion en régime pauvre apparaît comme une technologie efficace pour réduire l’impact de la combustion sur l’environnement.Or, ce type de technologie favorise l’apparition d’instabilités de combustion issues d’un couplage thermo-acoustique. Des études expérimentales précédemment menées à l’ONERA ont mis en évidence l’importance de l’atomisation au sein d’un injecteur multipoint sur le phénomène d’instabilités de combustion. L’objectif de cette thèse est de mettre en place la méthodologie multi-échelle pour reproduire les phénomènes de couplage entre l’atomisation du jet liquide en présence d’un écoulement gazeux transverse (configuration simplifiée d’un point d’injection d’un injecteur multipoint) et d’une perturbation acoustique imposée, représentative de l’effet d’une instabilité de combustion. Ce type d’approche pourra, à terme, être utilisé pour la simulation instationnaire LES d’un système de combustion, et permettra de déterminer les temps caractéristiques de convection du carburant liquide pouvant affecter les phénomènes d’évaporation et de combustion, et donc l’apparition des instabilités de combustions. Afin de valider cette approche,les résultats issus des simulations sont systématiquement comparés aux observations expérimentales obtenues dans le cadre du projet SIGMA. Dans un premier temps, une simulation du jet liquide en présence d’un écoulement gazeux transverse est réalisée. Cette simulation a permis de valider l’approche multi-échelle : pour cela, les grandes échelles du jet, ainsi que les mécanismes d’atomisation reproduits par les simulations, sont analysés. Ensuite, l’influence d’une perturbation acoustique sur l’atomisation du jet liquide est étudiée. Les comportements instationnaires du jet et du spray issu de l’atomisation sont comparés aux résultats expérimentaux à l’aide des moyennes temporelles et des moyennes de phase. / The reduction of polluting emissions is currently a major issue in the aeronautics industry.Among the solutions developed by the engine manufacturers, lean combustion appears as an effectivetechnology to reduce the impact of combustion on the environment. However, this type oftechnology enhances the onset of combustion instabilities, resulting from a thermo-acoustic coupling.Experimental studies previously conducted at ONERA have highlighted the importanceof atomization in a multipoint injector to the combustion instabilities. The aim of this thesis isto implement the multi-scale methodology to reproduce the coupling phenomena between theatomization of the liquid jet in the presence of a crossflow (which is a simplified configuration ofan injection point of a multipoint injector) and an imposed acoustic perturbation, representativeof the effect of combustion instabilities. This type of approach can ultimately be used for the unsteadysimulation of a combustion system, and will determine the characteristic convection timesof the liquid fuel that can affect the phenomena of evaporation and combustion, and therefore theappearance of combustion instabilities. In order to validate this approach, the results obtainedfrom the simulations are systematically compared with the experimental observations obtainedwithin the framework of the SIGMA project. First, a simulation of the liquid jet in gaseous crossflowis performed. This simulation enabled us to validate the multi-scale approach : to this end,the large scales of the jet, as well as the atomization mechanisms reproduced by the simulations,are analyzed. Then, the influence of an acoustic perturbation on the atomization of the liquidjet is studied. The unsteady behavior of the jet and the spray resulting from the atomization arecompared with the experimental results using time averages and phase averages.