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Epiphaniou, Nicholas. « Modelling of Dynamic Friction Across Solid Material Interfaces Using Molecular Dynamics Techniques ». Thesis, Cranfield University, 2009. http://hdl.handle.net/1826/4458.
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The topic of this PhD is to investigate materials interfaces under the application of com-pressive forces and dynamic friction. Friction studies are important in applications for high-speed machining and ballistic penetration modelling, two areas where it is important to understand the behaviour of rapidly moving interfaces. Gaining insight into the velocity dependence of the effective tangential force, and its time-evolution, under various external loads is also of particular interest. It is important to understand on an atomic and/or molec-ular level the fundamentals of tribological processes. Some of the processes investigated in this thesis include plastic deformation due to high compression, the response of materials when sliding occurs in terms of temperature variation across the interface and its relation-ship with atomic diffusion. Moreover, the materials dependence on operating conditions of temperature, loading and dynamic friction are factors that ultimately determine the design of tribological systems.
In the last few years it has been shown that materials properties depend on the size, as smaller specimens are relatively stronger than larger ones. This thesis is aiming to em-ploy state of the art numerical and theoretical methods, which are vital to give a significant insight and understanding of the fundamental issues concerning dynamic friction of tribo-logical processes at the atomic scale. The mechanical behaviour is investigated in detail to reveal an accurate theoretical description of the frictional force at metallic surfaces. Special consideration is taken into account for the mechanism that causes dissipation in the form of heat. The strong deformation when materials undergo dynamic friction causes energy to dissipate away from the interface at a high rate.
Additionally, investigation of the plastic deformation and its variation under conditions prevalent at high speed sliding is carried out. Knowledge of the yield point under these conditions is important to obtain accurate constitutive models for the shear stresses. In-vestigating how the material strength varies under sliding friction and obtaining accurate evaluation of the stresses involved has proved difficult and time consuming. This is primar¬ily attributed to the fact that experiments are difficult to conduct and expensive facilities are required. This thesis focuses on aspects of this complex process with the aid of molecular dynamic simulations.
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Sonwalkar, Nishikant. « Molecular dynamics of ice-solid bi-material interfaces ». Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/12916.
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Banerjee, J. R. « Advances in structural dynamics, aeroelasticity and material science ». Thesis, City University London, 2015. http://openaccess.city.ac.uk/14901/.
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This submission for the degree of Doctor of Science includes all the publications by the author and a description of his research, covering the period 1969-2015. The main contributions to knowledge made by the author concern his new approaches to structural dynamics, aeroelasticity, material science and related problems. In particular, the major activities of his research relate to the (i) free vibration and buckling analysis of structures, (ii) dynamic stiffness formulation, (iii) response of metallic and composite structures to deterministic and random loads, (iv) aeroelasticity of metallic and composite aircraft, (v) a unified approach to flutter, dynamic stability and response of aircraft, (vi) aeroelastic optimisation and active control, (vii) application of symbolic computation in structural engineering research, (viii) development of software packages for computer aided structural analysis and design and (ix) thermal properties of polymer nanocomposites and hot ductility of steel. The free vibration analysis of structures is a research topic which has been an age old companion of the author ever since he was working for his Master’s degree in Mechanical Engineering in the early 1970s, when he chose a crankshaft vibration problem of the Indian Railways as the research topic for his Master’s thesis. With increasing maturity and experience, he provided solutions to vibration and buckling problems ranging from a simple single structural element to a high capacity transport airliner capable of carrying more than 500 passengers and a large space platform with a plan dimension of more than 30 metres. To provide these solutions, he resorted to an elegant, accurate, but efficient method, called the dynamic stiffness method, which uses the so-called dynamic stiffness matrix of a structural element as the basic building block in the analysis. The author has developed dynamic stiffness matrices of a large number of structural elements including beams, plates and shells with varying degrees of complexity, particularly including those made of composite materials. Recently he published the dynamic stiffness matrices of isotropic and anisotropic rectangular plates for the most general case when the plate boundaries are free at all edges. Computation of natural frequencies of isotropic and anisotropic plates and their assemblies for any boundary conditions in an exact sense has now become possible for the first time as a result of this development. This ground-breaking research has opened up the possibility of developing general purpose computer programs using the dynamic stiffness method for computer-aided structural analysis and design. Such computer programs will be vastly superior to existing computer programs based on the finite element method, both in terms to accuracy and computational efficiency. This is in line with the author’s earlier research on free vibration and buckling analysis of skeletal structures which led to the development of the computer program BUNVIS (Buckling or Natural Vibration of Space Frames) and BUNVIS-RG (Buckling or Natural Vibration of Space Frames with Repetitive Geometry) which received widespread attention. Numerous research papers emerged using BUNVIS and BUNVIS-RG as research tools. The author’s main contributions in the Aeronautical Engineering field are, however, related to the solutions of problems in aeroelasticity, initially for metallic aircraft and in later years for composite aircraft. He investigated the aeroelastic problems of tailless aircraft for the first time in his doctoral studies about 40 years ago. In this research, a unified method combining two major disciplines of aircraft design, namely that of stability and control, and that of flutter and response, was developed to study the interaction between the rigid body motions of an aircraft and its elastic modes of distortion. The computer program CALFUN (CALculation of Flutter speed Using Normal modes) was developed by the author for metallic aircraft and later extended to cover composite aircraft. The associated theories for composite aircraft were developed and the allied problems of dynamic response to both deterministic and random loads were solved. With the advent of advanced composite materials, the author’s research turned to aeroelasticity of composite aircraft and then to optimization studies. New, novel and accurate methods were developed and significant inroads were made. The author broke new ground by applying symbolic computation as an aid to the solution of his research problems. The computational efficiency of this new approach became evident as a by-product of his research. The development of software based on his theories has paved the way for industrial applications. His research works on dynamic stiffness modelling of composite structures using layer-wise and higher order shear deformation theory are significant developments in composites engineering. Such pioneering developments were necessitated by the fact that existing methodologies using classical lamination theory are not sufficiently accurate, particularly when the structural components made from composite materials are thick, e.g. the fuselage of a transport airliner. Given the close relationship between structural engineering and material science, the author’s research has broadened into polymers and nano-composites, functionally graded materials and hot ductility of steel. His research activities are continuing and expanding with further diversification of his interests.
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NEGRONI, MATTIA. « Dynamics in Porous Materials ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2020. http://hdl.handle.net/10281/263115.
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Il mio lavoro di tesi si è basato sulla caratterizzazione dei materiali porosi rivolgendo particolare attenzione alla ricerca di elementi dinamici all’interno delle strutture e allo studio dei gas adsorbiti. Sono riuscito a rilevare la presenza di rotori parafenilenici ultraveloci sia in cristalli molecolari porosi che in metal-organic framework (MOF). Uno studio più approfondito ha inoltre rivelato come questi moti siano influenzati dal gas adsorbito. Nello specifico l’energia di attivazione della rotazione aumenta in funzione della quantità di gas nei pori. Per meglio capire questa interazione è però fondamentale la conoscenza del comportamento dei gas nei materiali porosi. Ho pertanto rivolto la mia attenzione allo studio del moto di xeno e CO2 in diversi materiali. L’utilizzo combinato di NMR e calcoli ab initio si è rivelato fondamentale per la comprensione di questi fenomeni ed è stato possibile rivelare particolari caratteristiche tanto dei gas quanto dei materiali stessi. La complessità della diffusione all’interno dei canali si è anche presentata in modi insoliti come il moto elicoidale dell’anidride carbonica imposto dal potenziale elettrostatico. Volendo continuare lo studio dei gas nei pori, ho caratterizzato diversi porous aromatic framework (PAF) con la tecnica dello xeno iperpolarizzato. Questo non mi ha consentito solo di misurare con accuratezza le dimensioni dei pori ma anche calcolare l’energia di interazione tra lo xeno e le pareti dei canali. Desiderando espandere le mie conoscenze sull’iperpolarizzazione come tecnica NMR, ho passato sei mesi presso il gruppo del Prof. L. Emsley a Losanna imparando la dynamic nuclear polarization (DNP) nonché la sua applicazione a diversi materiali.My thesis work was based on the characterization of porous materials, paying particular attention to the research of dynamic elements within the structures and to the study of adsorbed gases. I was able to detect the presence of ultrafast paraphenylenic rotors in both porous molecular crystals and metal-organic frameworks (MOFs). A more detailed study has also revealed how these motions are influenced by the adsorbed gas. Specifically, the activation energy of the rotation increases as a function of the quantity of gas in the pores. To better understand this interaction, the knowledge of the behavior of gases in porous materials is fundamental. I turned my attention to the study of xenon and CO2 motion in different materials. The combined use of NMR and ab initio calculations proved to be fundamental for understanding these phenomena and it was possible to reveal particular characteristics both of the gases and of the materials. The complexity of the diffusion within the channels has also been presented in unusual ways as the helicoidal motion of carbon dioxide imposed by the electrostatic potential. To continue the study of pore gases, I characterized several porous aromatic frameworks (PAFs) with the hyperpolarized xenon technique. This not only allowed me to accurately measure the pore size but also to calculate the interaction energy between the xenon and the channel walls. To expand my knowledge on hyperpolarization as an NMR technique, I spent six months at the group of Prof. L. Emsley in Lausanne learning dynamic nuclear polarization (DNP) as well as its application to different materials.
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Dovstam, Krister. « On material damping modelling and modal analysis in structural dynamics / ». Stockholm, 1998. http://www.lib.kth.se/abs98/dovs1216.pdf.
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Ding, Lifeng. « A molecular dynamics study of material behavior controlled by interface ». Thesis, University of Leicester, 2010. http://hdl.handle.net/2381/8916.
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In this work, the behaviour of nano-structured materials that is controlled by the interface is studied using Molecular Dynamics (MD). Four different types of nano-structured materials were investigated: (1) the sintering behaviour of nanoparticle; (2) the evolution of bamboo-like nanowires; (3) the mechanical property of the interlamellar phase of semicrystalline polymers; and (4) the mechanical property of the interlamellar phase of biodegradable polymers. In the MD simulation of nanoparticle sintering, it is observed that the particles can reorient themselves to match their crystalline orientations at the beginning of the sintering and thereby form different types of necks between different particles. This leads to different mechanisms of matter redistribution at the different necks. It has also been observed that the particles switch the mechanism of matter transportation halfway through the sintering process. None of these can be handled by the continuum model. However, assuming the right scenario, the continuum theory does agree with the MD simulation for particles consisting of just a few thousand atoms. In the multi-scale MD simulation of the evolution of bamboo-like nanowires, the microstructure evolution behaviour of the bamboo nanowire is observed very different to the conventional bamboo structure polycrystals. When the materials reduce to the nano-size, different evolution behaviour occurs: the low angle tilt grain boundary (GB) tends to be eliminated by forming a bending crystal form and dislocation slip might occur when raise the temperature; the large tilt GB is found stable at low temperature but the GB diffusion is very sensitive to the temperature; An interesting microstructure evolution behaviour of the nanowire with the small radius starting with the large angle GB is observed. A new hcp grain is nucleated from the triple point of the bamboo structure. In the multi-scale MD study of the mechanical property of the interlamellar phase of semicrystalline biodegradable polymers, it is found that the mechanical stiffness of interlamellar phase below Tg is mainly governed by the LJ interaction along the polymer backbone. Therefore, good polymer chain entanglement enhances the LJ interaction and increases the mechanical strength. Although the amorphous interlamellar phase is not the idea elastomer when temperature is above the glass transition temperature, it also shows the elastomer behaviour above Tg when we examine the number of long chains inside the amorphous interlamellar phase. The results of this study further support Pan's entropy spring theory by showing the Young's modulus drop lags behind the biodegradation process at temperatures above the glass transition temperature. For the amorphous interlamellar phase below the glass transition temperature, the Young's modulus drops quickly as the chain scissions quickly reduce the polymer chain entanglement.
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Gacek, Sobieslaw Stanislaw. « Molecular dynamics simulation of shock waves in laser-material interaction ». [Ames, Iowa : Iowa State University], 2009.
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Marks, Benjamin. « Grainsize dynamics of granular flows ». Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/9372.
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This dissertation deals with the description of a granular material as a continuum with an internal coordinate that represents the grainsize distribution. The inclusion of this internal coordinate allows us to describe polydispersity in a natural and simple manner. The bulk of this dissertation is built on four published papers. Each paper is prefaced by an introductory section, where the motivation for the paper is presented. In the first paper, I show how the fundamental mechanism of granular segregation can be represented in a cellular automaton. An equivalent continuum model is derived from the rules of the cellular automaton, similar to previous theories. The second paper extends this mechanism to include arbitrary grainsize distributions in a continuum framework. This continuum description predicts not only the evolution of the grainsize distribution in space and time, but also kinematics. I show an extension of the theory in Chapter 5 so that it can be included in a numerical continuum solver. This is then used to predict steady state grainsize distributions in Chapter 6, which are shown to be a function of only the stress gradient and diffusivity. This new continuum theory predicts that segregation will create a lubrication effect that accelerates the flow. In the third paper, I show experimentally how this effect creates additional forces when a granular avalanche impacts an obstacle. At experimental scale, a 20% increase in force is measured. In the final paper, comminution is added to the grainsize framework in a new cellular automaton, allowing me to model crushable flows. I show how the grainsize distributions measured in confined comminution can be predicted from this model. Additionally, when segregation is introduced log-normal grainsize distributions develop as in avalanche flow. The transition from power law to log-normal grainsize distributions is explained as an interaction between comminution and segregation.
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Pennell, Sara. « The material culture of food in early modern England, circa 1650-1750 ». Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302127.
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Tränkle, Marion. « Material agency and performative dynamics in the practices of media art ». Thesis, Brunel University, 2011. http://bura.brunel.ac.uk/handle/2438/8767.
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This dissertation identifies a strategy of artistic inquiry within contemporary media art practice. It applies the concept of material that acts in an agential capacity, generating performative acts. It argues that the emergent potentials of materials and their interconnectedness with the compositional layers of a work can facilitate modes of effecting change in the artistic system. Through the theoretical investigation of the production processes of physical structures and environments, the thesis focuses on the compositional dynamics within which materials actively perform. It examines how Lars Spuybroek’s architectural design method of Material Machines (2004), and both the tactile potential as well as tactical uses of materials as generators to the formtaking process, might describe an open and active artistic strategy for employing the experimental capacities of such materialization processes. Building on philosophical and conceptual arguments that trace concepts of agency (Bruno Latour’s Actant-Network theory) and enactment (Karen Barad’s concept of intra-acting), the thesis introduces the two installation works ANI_MATE (described as a performative pneumatic stage machine) and ON TRACK (described as a mechanic-robotic installation). These apply the introduced artistic strategies. The analyses of these two artworks traces the particular capacities of the materials involved (respectively, their elasticity or viscosity) to negotiate forces of physical movement, which effect the system to transiently or irreversibly transform. ANI_MATE is a machine that is artist-operated and that explores the relationship between liveanimation procedures and the transformability and flexibility of its material environment. In contrast, ON TRACK’s performative machine ecology removes human agency. The machines act autonomously, giving rise to chance in the artistic system and allowing agency to emerge from the dynamic interconnectivity between materials, parts, and processes, eventually producing an entropic scenario of spilling resources. The thesis concludes that, in the context of a post digital paradigm in-development, such artistic practice offers a new strategy for an emergent aesthetics within contemporary physical-digital performance.
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Hofmann, Marc [Verfasser], et Jürgen [Akademischer Betreuer] Blum. « Dynamics of Granular Material on Small Bodies / Marc Hofmann ; Betreuer : Jürgen Blum ». Braunschweig : Technische Universität Braunschweig, 2014. http://d-nb.info/1175820334/34.
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Steer, Jonathan. « Influences of plant growth and root material on soil microbial community dynamics ». Thesis, University of East London, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298081.
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Lam, Stephen Tsz Tang. « Accelerated atomistic prediction of structure, dynamics and material properties in molten salts ». Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/129108.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, September, 2020Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 122-142).
Various advanced nuclear reactors including fluoride high-temperature salt-cooled reactors (FHRs), molten salt reactors (MSRs) and fusion devices have proposed to use molten salt coolants. However, there remain many uncertainties in the chemistry, dynamics and physicochemical properties of many salts, especially over the course of reactor operation, where impurities are introduced, and compositional and thermodynamic changes occur. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) were used for property, structure and chemistry predictions for a variety of salts including LiF, KF, NaF, BeF2, LiCl, KCl, NaCl, prototypical Flibe (66.6%LiF-33.3%BeF2), and Flinak (46.5%LiF-11.5NaF-42%KF). Predictions include thermophysical and transport properties such as bulk density, thermal expansion coefficient, bulk modulus, and diffusivity, which were compared to available experimental data.
DFT consistently overpredicted bulk density by about 7%, while all other properties generally agreed with experiments within experimental and numerical uncertainties. Local structure was found to be well predicted where pair distribution functions showed similar first peak distances (+ 0.1 A) and first shell coordination numbers (+ 0.4 on average), indicating accurate simulation of chemical structures and atomic distances. Diffusivity was also generally well predicted within experimental uncertainty (+20%). Validated DFT and AIMD methods were applied to study tritium in prototypical salts since it is an important corrosive and diffusive impurity found in salt reactors. It was found that tritium species diffusivity depended on its speciation (TF vs. T2), which was related to chemical structures formed in Flibe and Flinak salts. Further, predictions allowed comparison with and interpretation of past contradictory experimental results found in the literature.
Lastly, robust neural network interatomic potentials (NNIPs) were developed for LiF and Flibe. The LiF NNIP accurately reproduced DFT calculations for pair interactions, solid LiF and liquid molten salt. The Flibe NNIP was developed for molten salt at the reactor operating temperature of 973K and was found to reproduce local structures calculated from DFT and showed good stability and accuracy during extended MD simulation. Ab initio methods and NNIPs can play a major role in advanced reactor development. Combined with experiments, these methods can greatly improve fundamental understanding and accelerate materials discovery, design and selection.
by Stephen Tsz Tang Lam.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering
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Farooque, Tanya Mahbuba. « Biochemical and mechanical stimuli for improved material properties and preservation of tissue-engineered cartilage ». Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/26710.
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Thesis (Ph.D)--Chemical Engineering, Georgia Institute of Technology, 2009.Committee Chair: Boyan, Barbara; Committee Chair: Wick, Timothy; Committee Member: Brockbank, Kelvin; Committee Member: Nenes, Athanasios; Committee Member: Sambanis, Athanassios. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Pourmand, Payam. « NMR detection of liquid dynamics in porous matrices ». Thesis, KTH, Skolan för kemivetenskap (CHE), 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-145864.
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Porous materials or a porous media can be encountered in our everyday life, both in industrial and household systems and in the nature. Generally speaking all solid and semisolid materials are porous to some degree e.g. different dense rock types, plastics etc. Porous materials are constantly finding more and more applications, both in industry and research. Many commercially important process in the industry utilize porous media e.g. flow of fluids through porous media for separation process and porous catalyst supports. This has strongly contributed to the development of porous media with controlled properties, which can be utilized for understanding the behavior of liquids confined in the material, and the morphology of these synthetic materials. This thesis work brings some insight and understanding of porous materials i.e. Controlled Pore Glass (CPG). Report also contains a brief explanation of Nuclear Magnetic Resonance (NMR) spectroscopy, diffusion NMR and other techniques such as Mercury porosimetry. The first part of the thesis is focused on determining the required amount of liquid i.e. octanol needed to achieve full pore saturation for different CPGs with varying pore sizes. This was achieved by taking into account that the transverse relaxation time T2 is sensitive in the ms-ns of motional correlation times, and that there are physical factors in porous material which affect the T2. Second part, diffusion NMR is used to study self-diffusion of octanol confined in CPG, thus bringing some insight on mass transfer limitations within porous systems. The report present results obtained from experiments with NMR and Diffusion NMR, discusses the issues that can arise when investigating porous materials and suggest solutions
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Zhang, Fei. « Adsorption of Small Molecules in Advanced Material Systems ». Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/89917.
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Adsorption is a ubiquitous phenomenon that plays key roles in numerous applications including molecule separation, energy storage, catalysis, and lubrications. Since adsorption is sensitive to molecular details of adsorbate molecule and adsorbent materials, it is often difficult to describe theoretically. Molecular modeling capable of resolving physical processes at atomistic scales is an effective method for studying adsorption. In this dissertation, the adsorption of small molecules in three emerging materials systems: porous liquids, room-temperature ionic liquids, and atomically sharp electrodes immersed in aqueous electrolytes, are investigated to understand the physics of adsorption as well as to help design and optimize these materials systems.
Thermodynamics and kinetics of gas storage in the recently synthesized porous liquids (crown-ether-substituted cage molecules dispersed in an organic solvent) were studied. Gas molecules were found to store differently in cage molecules with gas storage capacity per cage in the following order: CO2>CH4>N2. The cage molecules show selectivity of CO2 over CH4/N2 and demonstrate capability in gas separation. These studies suggest that porous liquids can be useful for CO2 capture from power plants and CH4 separation from shale gas.
The effect of adsorbed water on the three-dimensional structure of ionic liquids [BMIM][Tf2N] near mica surfaces was investigated. It was shown that water, as a dielectric solvent and a molecular liquid, can alter layering and ordering of ions near mica surfaces. A three-way coupling between the self-organization of ions, the adsorption of interfacial water, and the electrification of the solid surfaces was suggested to govern the structure of ionic liquid near solid surfaces.
The effects of electrode charge and surface curvature on adsorption of N2 molecules near electrodes immersed in water were studied. N2 molecules are enriched near neutral electrodes. Their enrichment is enhanced as the electrode becomes moderately charged but is reduced when the electrode becomes highly charged. Near highly charged electrodes, the amount of N2 molecules available for electrochemical reduction is an order of magnitude higher near spherical electrodes with radius ~1nm than near planar electrodes. The underlying molecular mechanisms are elucidated and their implications for development of electrodes for electrochemical reduction of N2 are discussed.Doctor of Philosophy
Adsorption is a ubiquitous phenomenon that plays key roles in numerous applications including molecule separation, energy storage, catalysis, and lubrications. Since adsorption is sensitive to molecular details of adsorbate molecule and adsorbent materials, it is often difficult to describe theoretically. Molecular modeling capable of resolving physical processes at atomistic scales is an effective method for studying adsorption. In this dissertation, the adsorption of small molecules in three emerging materials systems: porous liquids, room-temperature ionic liquids, and atomically sharp electrodes immersed in aqueous electrolytes, are investigated to understand the physics of adsorption as well as to help design and optimize these materials systems. Thermodynamics and kinetics of gas storage in the recently synthesized porous liquids (crown-ether-substituted cage molecules dispersed in an organic solvent) were studied. Gas molecules were found to store differently in cage molecules with gas storage capacity per cage in the following order: CO2>CH4>N2. The cage molecules show selectivity of CO2 over CH4/N2 and demonstrate capability in gas separation. These studies suggest that porous liquids can be useful for CO2 capture from power plants and CH4 separation from shale gas. The effect of adsorbed water on the three-dimensional structure of ionic liquids [BMIM][Tf2N] near mica surfaces was investigated. It was shown that water, as a dielectric solvent and a molecular liquid, can alter layering and ordering of ions near mica surfaces. vi A three-way coupling between the self-organization of ions, the adsorption of interfacial water, and the electrification of the solid surfaces was suggested to govern the structure of ionic liquid near solid surfaces. The effects of electrode charge and surface curvature on adsorption of N2 molecules near electrodes immersed in water were studied. N2 molecules are enriched near neutral electrodes. Their enrichment is enhanced as the electrode becomes moderately charged but is reduced when the electrode becomes highly charged. Near highly charged electrodes, the amount of N2 molecules available for electrochemical reduction is an order of magnitude higher near spherical electrodes with radius ~1nm than near planar electrodes. The underlying molecular mechanisms are elucidated and their implications for development of electrodes for electrochemical reduction of N2 are discussed.
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Tomar, Vikas. « Atomistic modeling of the AL and FE2O3 material system using classical molecular dynamics ». Available online, Georgia Institute of Technology, 2005, 2005. http://etd.gatech.edu/theses/available/etd-10172005-130638/.
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Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2006.Sathya Hanagud, Committee Member ; Min Zhou, Committee Chair ; David McDowell, Committee Member ; Jianmin Qu, Committee Member ; Naresh Thadhani, Committee Member ; Karl Jacob, Committee Member. Vita. Includes bibliographical references.
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Atlar, Sibel. « Modelling Part Dynamicsin Machining Processes Considering Material Removal ». Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/12609154/index.pdf.
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Self-excited chatter vibration in machining is one of the most important limitations on utilizing the increasing productivity of modern machine tools. In order to predict stable depth of cuts at high cutting speeds, the stability lobe diagram for a spindle-tool holder-tool combination must be developed. The frequency response function (FRF) of the system must be known for analytical prediction of the stability lobe diagrams. When the flexibility of the workpiece is important, the workpiece itself should be included in the system model by considering the variation of its dynamics at different stages of the machining process.
In this thesis, an exact structural modification method is used to find the frequency response functions of the workpiece to be machined at every stage of the machining process. In order to obtain the system matrices and the modal parameters of the original structure, a commercial finite element program MSC. Marc©is used. The frequency response functions of workpiece are calculated by using the computer program developed in this thesis, and are compared with the ones found by MSC. Marc©
. The stability lobe diagram of the system is obtained by combining the FRFs of the tool with those of the workpiece. The effects of the dynamic of the workpiece on the stability lobe diagrams are studied extensively by using the results of case studies presented in this thesis. In order to increase productivity, minimum chatter-free machining times are also calculated for different cases. For this purpose the effects of the different radial depth of cuts and different cutting strategies on the stability and the machining time are examined with various case studies.
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Koči, Love. « Studies of Material Properties using Ab Initio and Classical Molecular Dynamics ». Doctoral thesis, Uppsala universitet, Kondenserade materiens teori (Fysik IV), 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8626.
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In this thesis, material properties have been examined under extreme conditions in computer-based calculations. The research on iron (Fe), nickel (Ni), and ferropericlase (Mg1-xFexO) are not only important for our understanding of the Earth, but also for an improved knowledge of these materials per se. An embedded-atom model for Fe demonstrated to reproduce properties such as structure factors, densities and diffusion constants, and was employed to evaluate temperature gradients at Earth core conditions. A similar interaction together with a two-temperature method was applied for the analysis of shock-induced melting of Ni. For Mg1-xFexO, the magnetic transition pressure was shown to increase with iron content. Furthermore, the C44 softening with pressure and iron composition supports the experimentally observed phase transition for Mg0.8Fe0.2O at 35 GPa. The properties of high density helium (He) is of great interest as the gas is one of the most abundant elements in the solar system. Furthermore, He and neon (Ne) are often used as pressure media in diamond anvil cells. The melting of He showed a possible fcc-bcc-liquid transition starting at T=340 K, P=22 GPa with a Buckingham potential, whereas the bcc phase was not seen with the Aziz form. For Ne, Monte Carlo calculations at ambient pressure showed very accurate results when extrapolating the melting temperatures to an infinite cluster limit. At high pressure, a one-phase ab initio melting curve showed a match with one-phase L-J potential results, which could imply a correspondence between ab initio/classical one-phase/two-phase calculations. In the search for hard materials, ab initio calculations for four TiO2 phases were compared. Just as imposed by experiment, the cotunnite phase was found to be very hard. The anomalous elastic behavior of the superconducting group-V metals V, Nb, Ta was found to be related to shrinking nesting vectors and the electronic topological transition (ETT).
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Dunn, A. « A molecular dynamics study of diamond as a plasma facing material for fusion ». Thesis, University College London (University of London), 2011. http://discovery.ucl.ac.uk/1136851/.
Texte intégralRésumé :
Fusion power offers a promising source of clean energy for the future, however, one of the greatest challenges in tokamak reactor design is developing materials suitable to withstand the intense plasma-material interactions. Carbon, mostly in its graphitic form, is currently a favorite plasma facing material in many reactors. Diamond, however, offers many advantages over other materials but is not widely accepted. Although diamond exhibits excellent structural and thermal properties, tritium retention is a major concern for carbon. However, recent experimental evidence suggests that diamond might fare better than other carbon structures as a plasma facing material. This thesis investigates the the cumulative effect of exposing diamond to high thermal shock and tritium bombardment using classical molecular dynamics simulations. Of interest is diamond's resistance to graphitisation and the mechanisms behind tritium retention. Surfaces of different lattice orientation and level of hydrogen termination were incrementally heated to temperatures in excess of 2000 K. Generally, these diamond structures appeared to be stable up to temperatures of about 1000 K. Orientation did play a large part in determining the temperature of phase change, as did the level of hydrogen termination. Greater hydrogen coverages mimicked bulk continuation and increased resistance to graphitisation. These diamond surfaces, as well as a graphite and a diamond grain-boundary surface, were bombarded at a range of temperatures (300-2100 K) with high fluxes (1029 m-2s-1) of 15 eV tritium atoms in studying relative tritium retention at and below the surface as well as sputtered hydrocarbon yields. Below temperatures of graphitisation the diamond structure confined tritium, and thus further structural damage, to the upper surface. The graphitic surface allowed for deeper tritium penetration and retention. The presence of a grain boundary in the diamond slab allowed small amounts of tritium to penetrate deep into the bulk. Diamond surfaces were also bombarded at 300 K whilst independently varying incident ion energy (7.5-30 eV) and incident interval time (0.3-1.2 ps). Greater ion energies caused proportionally greater damage as well as reducing the ability of the structure to disperse incident thermal energy. At these extremely high fluxes sputter yield appeared to not vary with flux but was found to be proportional to fluence.
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Tomar, Vikas. « Atomistic modeling of the AL and Fe₂O₃ material system using classical molecular dynamics ». Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7502.
Texte intégralRésumé :
In the current research, a framework based on classical molecular dynamics (MD) is developed for computational mechanical analyses of complex nanoscale materials. The material system of focus is a combination of fcc-Al and and #945;-Fe₂O₃. The framework includes the development of an interatomic potential, a scalable parallel MD code, nanocrystalline composite structures, and methodologies for the quasistatic and dynamic strength analyses. The interatomic potential includes an embedded atom method (EAM) cluster functional, a Morse type pair function, and a second order electrostatic interaction function. The framework is applied to analyze the nanoscale mechanical behavior of the Al+Fe₂O₃ material system in two different settings. First, quasistatic strength analyses of nanocrystalline composites with average grain sizes varying from 3.9 nm to 7.2 nm are carried out. Second, shock wave propagation analyses are carried out in single crystalline Al, Fe₂O₃, and one of their interfaces. The quasistatic strength analyses reveal that the deformation mechanisms in the analyzed nanocrystalline structures are affected by a combination of factors including high fraction of grain boundary atoms and electrostatic forces. The slopes as well as the direct or inverse nature of observed Hall-Petch (H-P) relationships are strongly dependent upon the volume fraction of the Fe₂O₃ phase in the composites. The compressive strengths of single phase nanocrystalline structures are two to three times the tensile strengths owing to the differences in the movement of atoms in grain boundaries during compressive and tensile deformations. Analyses of shock wave propagation in single crystalline systems reveal that the shock wave velocity (US) and the particle velocity (UP) relationships as well as the type and the extent of shock-induced deformation in single crystals are strongly correlated with the choice of crystallographic orientation for the shock wave propagation. Analyses of shock wave propagation through an interface between Al and Fe2O3 point to a possible threshold UP value beyond which a shock-induced structural transformation that is reactive in nature in a region surrounding the interface may be taking place. Overall, the framework and the analyses establish an important computational approach for investigating the mechanical behavior of complex nanostructures at the atomic length- and time-scales.
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Salahshoor, Pirsoltan Hossein. « Nanoscale structure and mechanical properties of a Soft Material ». Digital WPI, 2013. https://digitalcommons.wpi.edu/etd-theses/924.
Texte intégralRésumé :
"Recently, hydrogel have found to be promising biomaterials since their porous structure and hydrophilicity enables them to absorb a large amount of water. In this study the role of water on the mechanical properties of hydrogel are studied using ab-initio molecular dynamics (MD) and coarse-grained simulations. Condensed-Phased Optimized Molecular Potential (COMPASS) and MARTINI force fields are used in the all-atom atomistic models and coarse-grained simulations, respectively. The crosslinking process is modeled using a novel approach by cyclic NPT and NVT simulations starting from a high temperature, cooling down to a lower temperature to model the curing process. Radial distribution functions for different water contents (20%, 40%, 60% and 80%) have shown the crosslinks atoms are more hydrophilic than the other atoms. Diffusion coefficients are quantified in different water contents and the effect of crosslinking density on the water diffusion is studied. Elasticity parameters are computed by constant strain energy minimization in mechanical deformation simulations. It is shown that an increase in the water content results in a decrease in the elastic. Finally, continuum hyper elastic model of contact lens is studied for three different loading scenarios using Finite Element Model. "
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Swoger, Maxx Ryan. « Computational Investigation of Material and Dynamic Properties of Microtubules ». University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1532108320185937.
Texte intégral
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Stoian, Razvan. « Investigations of the dynamics of material removal in ultrashort pulsed laser ablation of dielectrics ». [S.l.] : [s.n.], 2000. http://www.diss.fu-berlin.de/2001/67/index.html.
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Brading, Melanie Gayle. « The influence of fluid dynamics and surface material on pure and binary culture biofilms ». Thesis, University of Exeter, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307314.
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Zimmermann, Dominik. « Material forces in finite inelasticity and structural dynamics topology optimization, mesh refinement and fracture / ». [S.l. : s.n.], 2008. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-34667.
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Brockmann, Tobias H. « Theory of adaptive fiber composites from piezoelectric material behavior to dynamics of rotating structures ». Dordrecht Heidelberg London New York, NY Springer, 2009. http://d-nb.info/997517948/34.
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Arepalli, Uma Maheswar. « A Study of Moisture Induced Material Loss of Hot Mix Asphalt (HMA) ». Digital WPI, 2017. https://digitalcommons.wpi.edu/etd-dissertations/409.
Texte intégralRésumé :
"Susceptibility of Hot Mix Asphalt (HMA) mixes to moisture induced damage is one of the main reasons for premature failures of asphalt pavements. Hence, the evaluation of mixes for the moisture susceptibility is an essential part of the mix design. The existing methods are found to be in-sufficient to characterize mixes in terms of their moisture damage potential, and many studies have been conducted to establish an improved methodology that can better address the issue. Most of these methods involve the determination of changes in mix properties due to moisture conditioning in the laboratory or to verify the mix performance in the field or the laboratory. In the field moisture susceptible mixes are also found to lose material to extents that are dependent upon the properties of the mix and materials. So far, there has been no comprehensive study to investigate the loss of materials due to moisture induced damage. The objective of this study was to identify and evaluate a conditioning and a test method that can be used on a regular basis to detect moisture susceptible mixes and to understand the combined problem of moisture induced material loss and change in strength/stiffness of the mix. The Moisture Induced Stress Tester (MIST), Ultrasonic Pulse Velocity (UPV), Dynamic Modulus in Indirect tensile mode, and Indirect Tensile Strength (ITS) tests were utilized in the study. The effluent from the MIST was checked for the gradation of dislodged aggregates and the Dissolved Organic Carbon (DOC) content. A system dynamics (SD) approach was also adopted to investigate the problem and establish a model to reproduce field observations. The results showed that the use of MIST in combination with UPV or ITS is able to identify moisture susceptible mixes, in particular for mixes with the potential of aggregate breakdown. The mixes with a higher loss of asphalt binder during conditioning exhibit higher tensile strengths, and those with a loss of finer materials, which is indicative of aggregate breakdown, show a lower tensile strength. For the mixes used in this study, the rate of change in indirect tensile strength during moisture conditioning was found to be strongly correlated to the pre-conditioning modulus of the mix. A step-by-step framework to characterize the moisture susceptible mixes was presented."
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Deng, Haiyan. « Analysis and Synthesis of Fixturing Dynamic Stability in Machining Accounting for Material Removal Effect ». Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14038.
Texte intégralRésumé :
A fixture is a critical link in a machining system. The majority of prior work treats the fixture-workpiece system as quasi-static and ignores the system dynamics. In addition, material removal effect (MRE) on fixture-workpiece dynamics is generally ignored. The primary goal of this thesis is to develop a model-based framework for analysis and synthesis of the fixturing dynamic stability of a machining fixture-workpiece system accounting for the MRE. Five major accomplishments of this thesis are summarized as follows: First, a systematic procedure for analysis of fixturing dynamic stability of an arbitrarily configured machining fixture-workpiece system is developed. Second, models and approaches developed in this work are experimentally validated. It is found that consideration of dynamics and characterization of system dynamic properties are crucial for an accurate analysis. Third, an in-depth investigation of the MRE on fixture-workpiece dynamics is performed. The results show that material removal can significantly change the system characteristics and behavior and approaches developed are capable of capturing the change. Fourth, roles of important fixture design and machining process parameters in affecting fixturing dynamic stability are studied and understood via a parameter effect analysis. Additionally, fixturing dynamic stability is found to be sensitive to the parameter imprecision. Finally, a generic approach for determination of minimum clamping forces that ensure fixturing dynamic stability is developed. Because of MRE, dynamic clamping is found to be an option to achieve the best possible system performance. Models and approaches developed in this thesis are generic and can be used as simulation tools in fixture design. Insights obtained from this research advance the knowledge base of machining fixtures and provide general fixture design guidelines.
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Fu, Kai. « Growth Dynamics of Semiconductor Nanostructures by MOCVD ». Doctoral thesis, KTH, Teoretisk kemi (stängd 20110512), 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11447.
Texte intégralRésumé :
Semiconductors and related low-dimensional nanostructures are extremely important in the modern world. They have been extensively studied and applied in industry/military areas such as ultraviolet optoelectronics, light emitting diodes, quantum-dot photodetectors and lasers. The knowledge of growth dynamics of semiconductor nanostructures by metalorganic chemical vapour deposition (MOCVD) is very important then. MOCVD, which is widely applied in industry, is a kind of chemical vapour deposition method of epitaxial growth for compound semiconductors. In this method, one or several of the precursors are metalorganics which contain the required elements for the deposit materials. Theoretical studies of growth mechanism by MOCVD from a realistic reactor dimension down to atomic dimensions can give fundamental guidelines to the experiment, optimize the growth conditions and improve the quality of the semiconductor-nanostructure-based devices. Two main types of study methods are applied in the present thesis in order to understand the growth dynamics of semiconductor nanostructures at the atomic level: (1) Kinetic Monte Carlo method which was adopted to simulate film growths such as diamond, Si, GaAs and InP using the chemical vapor deposition method; (2) Computational fluid dynamics method to study the distribution of species and temperature in the reactor dimension. The strain energy is introduced by short-range valence-force-field method in order to study the growth process of the hetero epitaxy. The Monte Carlo studies show that the GaN film grows on GaN substrate in a two-dimensional step mode because there is no strain over the surface during homoepitaxial growth. However, the growth of self-assembled GaSb quantum dots (QDs) on GaAs substrate follows strain-induced Stranski-Krastanov mode. The formation of GaSb nanostructures such as nanostrips and nanorings could be determined by the geometries of the initial seeds on the surface. Furthermore, the growth rate and aspect ratio of the GaSb QD are largely determined by the strain field distribution on the growth surface.QC 20100713
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Buzogany, Raquel Froese. « Modelagem da convergência de materiais em desastres por meio de dinâmica de sistemas ». Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/3/3148/tde-13042017-111909/.
Texte intégralRésumé :
O número de desastres vem aumentando a cada ano e a quantidade de pessoas afetadas por estes é cada vez maior. A convergência de materiais, que é o afluxo de grande quantidade de material ao local do desastre, é observada em muitos eventos e tem grande impacto sobre operações humanitárias; em especial, nos processos de logística humanitária, como transporte, processamento e distribuição de itens necessários no auxílio às vítimas. Devido a diversos efeitos negativos da convergência - por exemplo, o emprego de recursos humanos em tarefas não essenciais e a maior demora para distribuição de material prioritário -, é necessário estudar os fatores que a influenciam e, também, como suas consequências afetam o sistema em que se insere. Desta forma, utilizou-se o método de dinâmica de sistemas para desenvolver um modelo que permita, pela análise de diversos cenários, alcançar uma melhor compreensão sobre o fenômeno. O modelo, além de permitir testar políticas abordadas na literatura, também proporciona uma simulação para o preparo em desastres reais e, no âmbito acadêmico, é o primeiro modelo de convergência de materiais a integrar tanto variáveis quantitativas quanto qualitativas e propiciar uma análise de forma dinâmica no tempo. A partir do cenário base, o caso das inundações em São Luiz do Paraitinga em 2010, foram simulados 13 cenários com o objetivo de identificar quais políticas aumentariam a quantidade de material prioritário distribuído para suprir as necessidades das vítimas. No cenário base foram encontrados gargalos no transporte e na distribuição de materiais e a análise de sensibilidade ficou restrita a uma política, limitando a generalização do trabalho. Na implementação de políticas uma a uma, observa-se que a não admissão de material não prioritário atinge os melhores resultados, seguida pela política de realocação de recursos humanos para processamento de material prioritário. Já no caso de políticas mistas, o controle de admissão com uma maior disponibilidade de recursos é o cenário que mais se sobressai no auxílio às vítimas.The number of disasters is increasing each year and the amount of people affected by them is also getting bigger. Material convergence, which is the influx of large amounts of material to the disaster site, is observed in many events and has a large impact on humanitarian operations; in particular in humanitarian logistics processes, such as transportation, processing and distribution of necessary items to assist victims. Due to various negative effects of the convergence - for example, the use of human resources in non-core tasks and the longer delay for priority material distribution - it is necessary to study the factors that influence it and how its consequences affect the system. In this way, the method of system dynamics was used to develop a model that allows, through the analysis of several scenarios, to reach a better understanding about the phenomenon. The model, besides allowing to test policies approached in the literature, also provides a simulation for the preparation in real disasters and, in the academic scope, it is the first model of material convergence to integrate both quantitative and qualitative variables and to provide a dynamic analysis. Based on the baseline scenario, the case of the floods in São Luiz do Paraitinga in 2010, 13 scenarios were simulated to identify which policies would increase the amount of priority material distributed to meet the needs of the victims. Analyzing the base scenario there were found bottlenecks both in the transportation and the distribution of material and, as the sensitivity analysis was restricted to only one policy, the generalization of the presented work has been limited. In implementing policies one by one, it is observed that the non-admission of non-priority material achieves the best results, followed by the policy of reallocating human resources to processing priority material. In the case of mixed policies, admission control with greater availability of resources is the most prominent scenario in assisting victims.
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Lutfurakhmanov, Artur. « Fluid Dynamics of Material Micro-Deposition : Capillary-Based Droplet Deposition and Aerosol-Based Direct-Write ». Diss., North Dakota State University, 2012. https://hdl.handle.net/10365/26820.
Texte intégralRésumé :
With rapid development of the direct-write technology, in addition to requirement of non-destructive printing, there is a need for non-expensive, robust, and simplified techniques of micro/nano fabrication. This dissertation proposes a new technique of non-invasive lithography called Capillary-Based Droplet Deposition and suggests improvements to existing Aerosol-Jet Direct-Write method that leads to deposition of thinner lines.
A hollow capillary filled with liquid is a dispensing tool employed for the Capillary-Based Droplet Deposition method. Due to pressure applied from one side of the capillary, a liquid meniscus is formed at the opposite side of the capillary. After the meniscus touches the substrate, a liquid bridge between the capillary and substrate is formed. The capillary retraction causes the bridge rupturing and liquid droplet deposition. In the first part of this dissertation, the Capillary-Based Deposition method is considered both theoretically and experimentally. From bridge modeling, it is found that the droplet size is dependent on pressure applied, inner radius and wall thickness of the capillary, and liquid-capillary and liquid-substrate equilibrium contact angles. Three deposition scenarios are identified showing that minimum deposited droplet size is about 15% of the capillary inner diameter. Modeling results are verified in experiments with different water-glycerol solutions used as test liquid and with capillaries of wide range of inner diameters.
The second part of the dissertation is devoted to theoretical investigation of the Aerosol-Jet Direct-Write method where few micron width lines are created from aerosol droplets that move along with the gas flowing through a converging micro-nozzle. Gas velocity and density profiles inside and outside of the nozzle are obtained from
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ANSYS/CFX simulation. Aerosol droplet trajectories and velocity components are calculated using all forces acting on the particles in the flow. Comparing all forces, it is found that only Stokes and Saffman forces are relevant for simulation of the gas-particle interaction. Original 1D equation for Saffman force is extended to two dimensional gas flows. For some parameter ranges, Saffman force is found to be negligibly small. Based on simulation results, two nozzle designs are proposed in order to collimate aerosol particles with diameters of 1.5-5.0 microns toward the nozzle centerline.
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Bergh, Magnus. « Interaction of Ultrashort X-ray Pulses with Material ». Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis Acta Universitatis Upsaliensis, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8274.
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Kappiyoor, Ravi. « Mechanical Properties of Elastomeric Proteins ». Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/54563.
Texte intégralRésumé :
When we stretch and contract a rubber band a hundred times, we expect the rubber band to fail. Yet our heart stretches and contracts the same amount every two minutes, and does not fail. Why is that? What causes the significantly higher elasticity of certain molecules and the rigidity of others? Equally importantly, can we use this information to design materials for precise mechanical tasks? It is the aim of this dissertation to illuminate key aspects of the answer to these questions, while detailing the work that remains to be done.
In this dissertation, particular emphasis is placed on the nanoscale properties of elastomeric proteins. By better understanding the fundamental characteristics of these proteins at the nanoscale, we can better design synthetic rubbers to provide the same desired mechanical properties.Ph. D.
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Lee, Seung Han. « Material property estimation method using a thermoplastic pyrolysis model ». Link to electronic thesis, 2005. http://www.wpi.edu/Pubs/ETD/Available/etd-121905-033150/.
Texte intégralRésumé :
Thesis (M.S.)--Worcester Polytechnic Institute.Keywords: material property; thermometer; cone calorimeter; finite difference method; thermoplastic; pyrolysis model; fire dynamics simulators Includes bibliographical references. (p.162-163)
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Aeberhard, Philippe C. « Computational modelling of structure and dynamics in lightweight hydrides ». Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:bfaf28b1-da03-4ce9-8577-5e8c18eb05ae.
Texte intégralRésumé :
Hydrogen storage in lightweight hydrides continues to attract significant interest as the lack of a safe and efficient storage of hydrogen remains the major technological barrier to the widespread use of hydrogen as a fuel. The metal borohydrides Ca(BH₄)₂ and LiBH₄ form the subject of this thesis; three aspects of considerable academic interest were investigated by density functional theory (DFT) and molecular dynamics (MD) modelling. (i) High-pressure crystal structures of Ca(BH₄)₂ were predicted from a structural analogy between metal borohydrides and isoelectronic metal oxides. The structural stability of hydrogen storage materials under high pressure is an important aspect, as high-pressure polymorphs may provide structures with better hydrogen desorption properties. The isoelectronic analogue of Ca(BH₄)₂ is TiO₂, and structural equivalents of Ca(BH₄)₂ in the baddeleyite, columbite and cotunnite structures of TiO₂ were found to be stable at elevated pressure. Thermodynamic stability was evaluated by computing the Gibbs energy with respect to pressure and temperature. The pressure-dependence of the Helmholtz energy was determined to described a third-order Birch-Murnaghan equation of state, and the harmonic approximation was used to compute the vibrational energy levels and the Helmholtz energy as a function of temperature. The proposed structures are consistent with reports of two hitherto unidentified high-pressure phases observed experimentally. (ii) The disordered structure of the high-temperature phase of LiBH4 was studied by ab initio molecular dynamics (MD) at temperatures ranging from 200-535 K. It was found that the model emerging from analysis of the MD simulations properly accounts for dynamical disorder and fundamentally differs from the published experimental and theoretical structures. The validity of the MD model was corroborated by comparison of calculated pair distribution functions, vibrational spectra and a crystallographic model with neutron diffraction data; good agreement was found. A reassignment of the space group from P63mc to P63/mmc is proposed based on evidence for additional symmetry from MD simulations. (iii) Finally, a new MD-based method was developed to simulate fast ionic diffusion in LiBH₄. The colour diffusion algorithm - a nonequilibrium molecular dynamics method originally developed for the study of model fluids - was adapted and applied to self-diffusion of atoms in a solid for the first time. Calculated diffusion coefficients agreed very well with published measurements, and diffusion pathways that include collective particle effects were determined directly from the simulation results, thereby opening up a promising and efficient new method for the study of phenomena such as superionic conduction.
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Kraus, Zachary. « Computational tools for preliminary material design of metals and polymer-ceramic nano composites ». Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/51795.
Texte intégralRésumé :
In this dissertation, algorithms for creating estimated potentials for metals and modeling of nano composites are developed. The efficacy of the algorithms for estimated potentials were examined. The algorithm was found to allow molecular dynamic and Monte Carlo modeling to be included in the potential building process. Additionally, the spline based equations caused issues with the elastic constants and Young’s modulus due to extra local minima. Two algorithms were developed for improved modeling of nano composites: one was a random number generation algorithm for initializing polymer, second was a bonding algorithm for controlling bonds between polymer and nano particle. Both algorithms were effective in their tasks. Additionally, the algorithms for improved nano composite modeling were used for preliminary material design of PMMA metal oxide nano composite systems. The results from the molecular dynamic simulations show the bonding between polymer matrix and nanoparticle has a large effect on the Young’s modulus and if this bonding could be controlled, the tensile properties of PMMA-metal oxide nano composites could be tailored to the applications’ requirements. The simulations also showed bonding had caused changes in the density of the material which than effected the energy on the polymer chain and the Young’s modulus. A model was than developed showing the relationship between density and the chain energy, and density and the Young’s modulus. This model can be used for a better understanding and further improvement of PMMA-metal oxide nano composites.
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Walters, Thomas E. « Development Of A Smart Material Electrohydrostatic Actuator Considering Rectification Valve Dynamics And In Situ Valve Characterization ». The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1211996027.
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Larson, John P. « Design of a Magnetostrictive-Hydraulic Actuator Considering Nonlinear System Dynamics and Fluid-Structure Coupling ». The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1402566309.
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ESTEVES, ARMANDO M. « Modelamento do contínuo de simulações micromecânicas com base em novas teorias de comportamento plástico do material ». reponame:Repositório Institucional do IPEN, 2005. http://repositorio.ipen.br:8080/xmlui/handle/123456789/11277.
Texte intégralRésumé :
Made available in DSpace on 2014-10-09T12:50:11Z (GMT). No. of bitstreams: 0Made available in DSpace on 2014-10-09T13:58:45Z (GMT). No. of bitstreams: 1 10556.pdf: 3868334 bytes, checksum: db3b8774b1e81550d9d9483f00d514f7 (MD5)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
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Nair, Arun Krishnan. « Modeling nonlinear material behavior at the nano and macro scales ». Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/28432.
Texte intégralRésumé :
Theoretical and computational methods have been used to study nonlinear effects in the mechanical response of materials at the nano and macro scales. These methods include, acoustoelastic theory, molecular dynamics and finite element models.
The nonlinear indentation response of Ni thin films of thicknesses in the nano scale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases, larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The presence of grain boundaries in the films leads to the emission of dislocations at a lower applied stress. For a single crystal Ni thin film of a thickness of 20 nm a direct comparison of simulation and experimental results is presented, showing excellent agreement in hardness values. The effects of using different interatomic potentials and indentation rates for the simulations are also discussed. Dynamic indentation of the Ni thin film was also carried out for different frequencies. It has been found that there is a 12% increase in dislocations compared to quasi static indentation and the results are consistent with experiments.
Acoustoelastic theory was used to study how nonlinear elastic properties of unidirectional graphite/epoxy (gr/ep) effect the energy flux deviation due to an applied shear stress. It was found that the quasi-transverse wave (QT) exhibits more flux deviation compared to the quasi-longitudinal (QL) or the pure transverse (PT) due to an applied shear stress. The flux shift in QT wave due to an applied shear stress is higher than that for an applied normal stress along laminate stacking direction for the same magnitude. The QT wave has energy flux deviation due to shear stress at 0o and 90o fiber orientations as compared to normal stress case where the flux deviation is zero. It was found that the energy flux shift of QT wave in gr/ep varies linearly with applied shear stress. The Finite element model of the equations of motion combined with the Newmark method in time was used to confirm the flux shift predicted by theory.Ph. D.
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Stoian, Razvan. « Adaptive techniques for ultrafast laser material processing ». Habilitation à diriger des recherches, Université Jean Monnet - Saint-Etienne, 2008. http://tel.archives-ouvertes.fr/tel-00352662.
Texte intégralRésumé :
Le besoin d'une très grande précision lors du traitement des matériaux par laser a fortement encouragé le développement des études de l'effet des impulsions ultra brèves pour la structuration des matériaux à une échelle micro et nano métrique. Une diffusion d'énergie minimale et une forte non linéarité de l'interaction permet un important confinement énergétique à des échelles les plus petites possibles. La possibilité d'introduire des changements de phases rapides et même de créer de nouveaux états de matière ayant des propriétés optimisées et des fonctions améliorées donne aux impulsions ultra brèves de sérieux arguments pour être utilisées dans des dispositifs très précis de transformation et de structuration des matériaux. L'étude de ces mécanismes de structuration et, en particulier, de leurs caractéristiques dynamiques, est une clé pour l'optimisation de l'interaction laser-matière suivant de nombreux critères utiles pour les procédés laser : efficacité, précision, qualité. Ce mémoire synthétise les travaux de l'auteur sur l'étude statique et dynamique du dépôt d'énergie ultra rapide, avec application aux procédés laser. La connaissance de la réponse dynamique des matériaux après irradiation laser ultra brève montre que les temps de relaxation pilotent l'interaction lumière-matière. Il est alors possible d'adapter l'énergie déposée à la réponse du matériau en utilisant les toutes récentes techniques de mise en forme spatio temporelle de faisceaux. Un couplage optimal de l'énergie donne la possibilité d'orienter la réponse du matériau vers un résultat recherché, offrant une grande flexibilité de contrôle des procédés et, sans doute, la première étape du développement de procédés « intelligents ».
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Fontes, Yuri Correa. « Resposta ao desbalanço de rotor com absorvedor dinâmico rotativo com elemento viscoelástico ». Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/18/18149/tde-25072016-170222/.
Texte intégralRésumé :
O presente trabalho consiste no projeto de um modelo em elementos finitos de um absorvedor dinâmico rotativo utilizando-se um material viscoelástico como componente dissipador do sistema. O absorvedor é composto por um anel de material viscoelástico interposto entre dois anéis de aço, o qual é fixado na extremidade livre de um sistema rotativo representado por um eixo flexível, suportado por dois rolamentos, no qual estão fixos dois discos igualmente espaçados do centro entre os dois mancais. O modelo em elementos finitos do sistema rotativo é validado com os dados experimentais do modelo real e suas velocidades críticas são determinadas baseadas no diagrama de Campbell e na resposta ao desbalanço em um dos discos. O modelo inicial do absorvedor dinâmico rotativo é replicado de um modelo da literatura e as respostas a uma excitação na forma de impulso são comparadas. O modelo desenvolvido equipara-se ao da literatura para frequências até 600 Hz, intervalo que compreende as velocidades críticas a serem amortecidas. A otimização do absorvedor é realizada através de variações da geometria do mesmo e são traçadas curvas de influência de cada parâmetro sobre suas frequências naturais. Com base nestas curvas são realizadas análises de influência conjunta dos parâmetros geométricos sobre tais frequências. Pelos resultados obtidos verifica-se a possibilidade da obtenção de um modelo que atue sobre modos de flexão específicos do sistema rotativo, atenuando as amplitudes de vibração das velocidades críticas correspondentes a cada modo. Uma vez obtidos os modelos de absorvedores dinâmicos correspondentes aos dois primeiros modos de flexão do sistema rotativo, ambos são acoplados ao sistema e se observa grande redução dos picos de amplitude do primeiro modo de flexão, enquanto os picos do segundo modo sofrem baixa alteração.The present work concerns the development, optimization and validation of a finite element model of a dynamic vibration absorber using a viscoelastic material as the damping component. The dynamic absorber consists of a ring of viscoelastic material interposed between two rings of steel, which is fixed to the free end of a rotary system represented by a flexible shaft supported by two bearings, on which are fixed two discs equally spaced in the center of both bearings. The finite element model of the rotating system is validated with experimental data from the actual model and its critical speeds are determined based on the Campbell diagram and in its response to the imbalance. The initial model of the dynamic absorber is replicated from a model of the literature and the responses to an impulse excitation are compared. The developed model matches the literature one for frequencies up to 600 Hz, range comprising the critical speeds to be damped. The absorber\'s optimization is accomplished through variations of its geometry and influence curves of each parameter over its natural frequencies are drawn. Based on these curves, combined influence analyzes of the geometrical parameters over such frequencies are performed. From the results obtained, it can be seen the possibility of achieving a model that acts on specific bending modes of the rotation system, reducing the vibration amplitudes of the critical speeds corresponding to each mode. Once obtained the dynamic absorbers models corresponding to the first two modes of vibration of the rotatative system, both models are coupled to the system and it is observed great reduction of the amplitude of the first bending mode peaks, while the second mode suffer low peaks reduction.
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Hedman, Stefan. « Smooth and non-smooth approaches to simulation of granular matter ». Thesis, Umeå universitet, Institutionen för fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-50584.
Texte intégralRésumé :
Granular matter is defined as a collection of particle grains, such as sand.This type of matter have different characteristics (solid, liquid and gas) depending on the energy level per grain. There are several approaches to modeling and numerical simulations of granular matter. They are used by different groups for different purposes, and the choice between the approaches is based on knowledge and tradition rather than what might be best for the purpose. The key questions are when to use what method and what physical quality is lost depending on the choice.Two regimes of discrete element granular simulations emerge: smooth and non-smooth. To compare the efficiency and physical quality of the two approaches, four physics softwares are examined including Bullet Physics, LMGC90, AgX and LIGGGHTS. Test scenes are setup in each software and the results are compared to each other or to the results of other work.The thesis is performed at UMIT Research Lab at Umeå University.
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Brockmann, Tobias H. [Verfasser]. « Theory of adaptive fiber composites : from piezoelectric material behavior to dynamics of rotating structures / Tobias H. Brockmann ». Dordrecht, 2009. http://d-nb.info/997517948/34.
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Aladool, Azzam Salahuddin Younus. « Investigation of crystallization dynamics in phase-change material using the Master rate equation at ultrafast heating rates ». Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/29434.
Texte intégralRésumé :
Phase-change materials are widely used in non-volatile computer memories, and in arithmetic and logic processing applications. Phase-change based devices are also required to operate at different and high heating rates in response to electrical or optical excitations to achieve the required read-write rates. Crystallization is a fundamental and complex process involved in the phase transition operation in phase-change materials. It is sensitive to the nature of the phase-change material, its thermodynamic and kinetic parameters, geometric and interface effects, and thermal history. Thus, crystallization is the time limiting process in phase-change technologies. This work is concerned with theoretically understanding the crystallization dynamics of the Ge2Sb2Te5 (GST) phase-change material under different heating regimes and at the micro-structure level of the material to reduce crystallization times and increase the operating speed of phase-change devices and memories. A review and comparison of crystallization models was carried out to distinguish the more physically realistic Master rate equation method's ability to naturally trace both the nucleation and growth processes during crystallization, through the attachment and detachment of monomers to calculate the distribution of nano-cluster size distributions necessary to achieve the aims of this research. Full mathematical derivations and numerical implementation details of both the original discrete form of the Master rate equation and its approximate form were provided. Error analysis and computational experiments illustrated the limitations of the approximate form of the rate equation, and its detrimental sensitivity to the model parameters to justify the use of the discrete rate equation throughout this work. The crystallization rate is a strong function of the material's viscosity, and hence the physically realistic Mauro−Yue−Ellison−Gupta−Allan (MYEGA) model of the temperature dependence of viscosity was implemented in the Master rate equation. Crystallization simulations were carried out under ramped annealing conditions with heating rates from 50 K/s to 40,000 K/s to study the role of the viscosity model parameters (including the fragility index, glass transition temperature, and infinite temperature viscosity) on the crystallization dynamics. Those simulations showed, for high and low heating rates, the influence of the increasing fragility index on reducing the cluster nucleation time and increasing the crystallization speeds. Moreover, the increase of the glass transition temperature made a corresponding shift in crystallization temperature towards higher values. Furthermore, at low heating rates, infinite temperature viscosity parameter (i.e. extrapolated value of viscosity at temperature = ∞) has negligible effect on the crystallization dynamics while, at higher heating rates, smaller values of infinite temperature viscosity parameter increase the crystallization rate and final crystalline volume. Due to the relatively low computational cost of the Master rate equation method (compared to atomistic level computations), an iterative numerical algorithm was developed to fit Kissinger plots simulated with the Master rate equation system to experimental Kissinger plots from ultrafast calorimetry measurements at increasing heating rates. The simulations and analysis revealed the strong coupling between the glass transition temperature and fragility index, and highlighted the often ignored role of the dependence of the glass transition temperature on heating rate for the accurate estimation of the fragility index from analysis of experimental measurements. The extracted fragility indices in this work were lower than published values, highlighting the limitations of existing methods of extracting the viscosity parameters (using oversimplified analytical models with disparity in model parameters), and the importance of using detailed crystallization models for analysis of experimental measurements. Moreover, and for the first time, the variation of glass transition temperature with heating rate for GST was extracted from Kissinger measurements, in agreement with the values reported in the literature. The influence of the preparation conditions of amorphous GST on the crystallization dynamics was theoretically investigated using the Master rate equation by systematically implementing initial distributions of cluster sizes resulting from different thermal treatments such as melt-quenching and pre-annealing, and theoretical Gaussian initial cluster size distributions. Simulations of ramped pre-annealing to temperatures much lower than the crystallization temperature showed distributions of nano-clusters sizes of 2 - 8 nm in agreement with recently published high-resolution transmission electron microscopy measurements. Furthermore, the simulations explicitly showed the marked decrease in crystallization temperature (and therefore increase in crystallization speed) when there is predominately a narrow distribution of smaller crystalline clusters embedded in the initial amorphous phase.
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Volpi, Silvia. « High-fidelity multidisciplinary design optimization of a 3D composite material hydrofoil ». Diss., University of Iowa, 2018. https://ir.uiowa.edu/etd/6325.
Texte intégralRésumé :
Multidisciplinary design optimization (MDO) refers to the process of designing systems characterized by the interaction of multiple interconnected disciplines. High-fidelity MDO usually requires large computational resources due to the computational cost of achieving multidisciplinary consistent solutions by coupling high-fidelity physics-based solvers. Gradient-based minimization algorithms are generally applied to find local minima, due to their efficiency in solving problems with a large number of design variables. This represents a limitation to performing global MDO and integrating black-box type analysis tools, usually not providing gradient information. The latter issues generally inhibit a wide use of MDO in complex industrial applications.
An architecture named multi-criterion adaptive sampling MDO (MCAS-MDO) is presented in the current research for complex simulation-based applications. This research aims at building a global derivative-free optimization tool able to employ high-fidelity/expensive black-box solvers for the analysis of the disciplines. MCAS-MDO is a surrogate-based architecture featuring a variable level of coupling among the disciplines and is driven by a multi-criterion adaptive sampling (MCAS) assessing coupling and sampling uncertainties. MCAS uses the dynamic radial basis function surrogate model to identify the optimal solution and explore the design space through parallel infill of new solutions.
The MCAS-MDO is tested versus a global derivative-free multidisciplinary feasible (MDF) approach, which solves fully-coupled multidisciplinary analyses, for two analytical test problems. Evaluation metrics include number of function evaluations required to achieve the optimal solution and sample distribution. The MCAS-MDO outperforms the MDF showing a faster convergence by clustering refined function evaluations in the optimum region.
The architecture is applied to a steady fluid-structure interaction (FSI) problem, namely the design of a tapered three-dimensional carbon fiber-reinforced plastic hydrofoil for minimum drag. The objective is the design of shape and composite material layout subject to hydrodynamic, structural, and geometrical constraints. Experimental data are available for the original configuration of the hydrofoil and allow validating the FSI analysis, which is performed coupling computational fluid dynamics, solving the Reynolds averaged Navier-Stokes equations, and finite elements, solving the structural equation of elastic motion. Hydrofoil forces, tip displacement, and tip twist are evaluated for several materials providing qualitative agreement with the experiments and confirming the need for the two-way versus one-way coupling approach in case of significantly compliant structures.
The free-form deformation method is applied to generate shape modifications of the hydrofoil geometry. To reduce the global computational expense of the optimization, a design space assessment and dimensionality reduction based on the Karhunen–Loève expansion (KLE) is performed off-line, i.e. without the need for high-fidelity simulations. It provides with a selection of design variables for the problem at hand through basis rotation and re-parametrization. By using the KLE, an efficient design space is identified for the current problem and the number of design variables is reduced by 92%.
A sensitivity analysis is performed prior to the optimization to assess the variability associated with the shape design variables and the composite material design variable, i.e. the fiber orientation. These simulations are used to initialize the surrogate model for the optimization, which is carried out for two models: one in aluminum and one in composite material. The optimized designs are assessed by comparison with the original models through evaluation of the flow field, pressure distribution on the body, and deformation under the hydrodynamic load. The drag of the aluminum and composite material hydrofoils is reduced by 4 and 11%, respectively, increasing the hydrodynamic efficiency by 4 and 7%. The optimized designs are obtained by evaluating approximately 100 designs. The quality of the results indicates that global derivative-free MDO of complex engineering applications using expensive black-box solvers can be achieved at a feasible computational cost by minimizing the design space dimensionality and performing an intelligent sampling to train the surrogate-based optimization.
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Musuva, Mutinda. « The multiscale wavelet finite element method for structural dynamics ». Thesis, Brunel University, 2015. http://bura.brunel.ac.uk/handle/2438/12468.
Texte intégralRésumé :
The Wavelet Finite Element Method (WFEM) involves combining the versatile wavelet analysis with the classical Finite Element Method (FEM) by utilizing the wavelet scaling functions as interpolating functions; providing an alternative to the conventional polynomial interpolation functions used in classical FEM. Wavelet analysis as a tool applied in WFEM has grown in popularity over the past decade and a half and the WFEM has demonstrated potential prowess to overcome some difficulties and limitations of FEM. This is particular for problems with regions of the solution domain where the gradient of the field variables are expected to vary fast or suddenly, leading to higher computational costs and/or inaccurate results. The properties of some of the various wavelet families such as compact support, multiresolution analysis (MRA), vanishing moments and the “two-scale” relations, make the use of wavelets in WFEM advantageous, particularly in the analysis of problems with strong nonlinearities, singularities and material property variations present. The wavelet based finite elements (WFEs) presented in this study, conceptually based on previous works, are constructed using the Daubechies and B-spline wavelet on the interval (BSWI) wavelet families. These two wavelet families possess the desired properties of multiresolution, compact support, the “two scale” relations and vanishing moments. The rod, beam and planar bar WFEs are used to study structural static and dynamic problems (moving load) via numerical examples. The dynamic analysis of functionally graded materials (FGMs) is further carried out through a new modified wavelet based finite element formulation using the Daubechies and BSWI wavelets, tailored for such classes of composite materials that have their properties varying spatially. Consequently, a modified algorithm of the multiscale Daubechies connection coefficients used in the formulation of the FGM elemental matrices and load vectors in wavelet space is presented and implemented in the formulation of the WFEs. The approach allows for the computation of the integral of the products of the Daubechies functions, and/or their derivatives, for different Daubechies function orders. The effects of varying the material distribution of a functionally graded (FG) beam on the natural frequency and dynamic response when subjected to a moving load for different velocity profiles are analysed. The dynamic responses of a FG beam resting on a viscoelastic foundation are also analysed for different material distributions, velocity and viscous damping profiles. The approximate solutions of the WFEM converge to the exact solution when the order and/or multiresolution scale of the WFE are increased. The results demonstrate that the Daubechies and B-spline based WFE solutions are highly accurate and require less number of elements than FEM due to the multiresolution property of WFEM. Furthermore, the applied moving load velocities and viscous damping influence the effects of varying the material distribution of FG beams on the dynamic response. Additional aspects of WFEM such as, the effect of altering the layout of the WFE and selection of the order of wavelet families to analyse static problems, are also presented in this study.
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Yang, Xiaofan. « Multi-scale simulation of filtered flow and species transport with nano-structured material ». Diss., Manhattan, Kan. : Kansas State University, 2010. http://hdl.handle.net/2097/4271.
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Zimmermann, Dominik [Verfasser]. « Material forces in finite inelasticity and structural dynamics : topology optimization, mesh refinement and fracture / vorgelegt von Dominik Zimmermann ». Stuttgart : Inst. für Mechanik (Bauwesen), 2008. http://d-nb.info/997054255/34.
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