Dissertations / Theses on the topic 'CIRCULAR TUNNELS'

In many tunnel designs, reinforced concrete tunnel lining design is selected based on construction requirements rather than design loads. A constant cross-section is typically used along a tunnel even if the design loads change from one location to another, especially for tunnels constructed by tunnel boring machines (TBM). Factor of safety against failure is not constant along the length of tunnel and is typically high at shallow depth regions. Factor of safety during a rare event is usually much less than the ones set for service load states. Rare events such as earthquake, train derailment, explosion and long duration fires do not happen daily and generally a minor reparable damage is targeted at the structure during those types of events. The focus of this study is to analytically investigate structural fire safety of reinforced concrete circular tunnel linings in terms of reduction in service load safety and to develop recommendations for preliminary assessment of structural fire endurance of circular tunnel linings. Analytical methods accounting for thermal non-linearity, material degradation, tunnel lining-ground interaction and fire time stages are available to assess the structural fire safety of the concrete tunnel linings. Analytical results are determined to be in good agreement with tunnel key segment hydrocarbon fire test.

This dissertation is concerned with investigating the efficiency of dynamic vibration absorbers (DVAs) as measures to mitigate ground-borne vibrations induced by railway traffic in double-deck tunnels. The main topics of the dissertation are the coupling of a set of longitudinal distributions of DVAs to the interior floor of a double-deck tunnel dynamic model, the computation of the response of this coupled system due to train traffic and obtaining the optimum design parameters of the DVAs to minimize this response. To address the first concern, a methodology for coupling a set of longitudinal distributions of DVAs to any railway subsystem in the context of a theoretical dynamic model of railway infrastructure is developed. The optimum design parameters of the DVAs are obtained using an optimization process based on a genetic algorithm. The effectiveness of the DVAs is assessed by two response parameters, which are used as objective functions to be minimized in the optimization process: the energy flow radiated upwards by the tunnel and the maximum transient vibration value (MTVV) in the building near the tunnel. The model used to compute the former is a two-and-a-half dimensional (2.5D) semi-analytical model of a train-track-tunnel-soil system that considers a full-space soil model, and the one used to compute the latter is a hybrid experimental-numerical model of a train-track-tunnel-soil-building system. In the hybrid model, a numerical model of the track-tunnel system based on 2.5D coupled finite element-boundary element formulation along with a dynamic rigid multi-body model of the vehicle is used to compute the response in the tunnel wall, and then, the response in the building is computed using experimentally obtained transfer functions between the tunnel wall and the building. The triaxial response in the building is used to compute the MTVV. An alternative option to evaluate the MTVV in a building is to use a fully theoretical model of the train-track-tunnel-soil-building system. In the context of this modeling strategy, a computationally efficient method to calculate the 2.5D Green's functions of a layered soil is also presented. The results show that the DVAs would be an effective mitigation measure for railway-induced vibrations in double-deck tunnels as reductions up to 6.6 dB in total radiated energy flow and up to 3.3 dB in the vibration inside a nearby building are achieved in the simulations presented in this work.
En esta tesis se estudia la eficiencia de los absorbedores de vibraciones dinámicos (DVAs) como medidas de mitigación de las vibraciones inducidas por infraestructuras ferroviarias aplicados a túneles ferroviarios de dos niveles. Los principales desarrollos de la tesis son el acoplamiento de un conjunto de distribuciones longitudinales de DVAs a la losa intermedia de un modelo dinámico de túnel de dos niveles, el cálculo de la respuesta de este sistema acoplado debido al paso del tren y la obtención de los parámetros óptimos de los DVAs para minimizar esta respuesta. Para abordar la primer punto, se ha desarrollado una metodología con el fin de acoplar un conjunto de distribuciones longitudinales de DVAs a cualquier subsistema ferroviario en el contexto de modelos teóricos de la dinámica de infraestructura ferroviarias. Los parámetros óptimos de los DVAs han sido obtenidos mediante un proceso de optimización basado en un algoritmo genético. La eficiencia de los DVAs se evalúa mediante dos quantificadores de la respuesta dinámica del sistema, los cuales se utilizan como funciones objetivo a minimizar en el proceso de optimización: el flujo de energía total radiado hacia arriba desde el túnel y el valor máximo de vibración transitoria (MTVV) en el forjada de un edificio cercano al túnel. El modelo utilizado para calcular el primero es un modelo semi-analítico del sistema vehículo-vía-túnel-terreno que considera un modelo de terreno de espacio completo, y el que se utiliza para calcular el segundo es un modelo híbrido experimental-numérico del sistema vehículo-vía-túnel-terreno-edificio. En el modelo híbrido, se utiliza un modelo numérico del sistema vía-túnel basado en la formulación acoplada de elementos finitos-elementos de contorno acoplados, formulada en el dominio del número de onda y la frecuencia, junto con un modelo dinámico multicuerpo del vehículo con el objetivo de calcular la respuesta en la pared del túnel. Luego, la respuesta en el edificio se calcula utilizando funciones de transferencia obtenidas experimentalmente entre la pared del túnel y el edificio. Para calcular el MTVV, se utiliza la respuesta triaxial en el edificio. Una opción alternativa para evaluar el MTVV en un edificio es utilizar un modelo totalmente teórico del sistema vehículo-vía-túnel-terreno-edificio. En el contexto de esta estrategia de modelado, también se presenta un método computacionalmente eficiente para calcular las funciones de Green de un terreno en capas en el dominio 2.5D. Los resultados muestran que los DVAs pueden ser una medida de mitigación efectiva para las vibraciones inducidas por infraestructuras ferroviarias en el marco de un túnel ferroviario de dos niveles, ya que en las simulaciones presentadas en esta tesis se alcanzan reducciones de hasta 6.6 dB en el flujo de energía total radiado y hasta 3.3 dB en la vibración dentro de un edificio cercano.

An experimental study of active control of fully separated flow over a symmetrical circular-arc airfoil at high angles of attack was performed. The experiments were carried out in a low-speed, open circuit wind tunnel. Angles of attack from 10 to 40 degrees were tested. Low-power input, unsteady excitation was applied to the leading or trailing edge shear layers. The actuation was provided by the periodic oscillation of a 4-percent-chord flap placed on the suction side of the airfoil and facing the sharp edge. Vortex-shedding frequencies were measured and harmonic combinations selected as the applied actuator frequencies. Pressure measurements over the airfoil show that the control increased the normal force coefficient by up to 70%. This supports the idea of vortex capture in the time-averaged sense, enhancing the lift on the airfoil by managing the shear layer roll up. The results indicate the viability of the control of large-scale flow fields by exploiting the natural amplification of disturbances triggered by small-scale actuators. The application of flow control on sharp-edged aircraft wings could lead to improved maneuverability, innovative flight control and weight reduction. These can be achieved by inexpensive, low-power, rugged actuators.
Master of Science

This thesis presents a three-dimensional dynamic model of a double-deck circular tunnel embedded in a full-space. The model uses the receptance method to obtain the response of the complete structure from the response of its parts. The considered subsystems are the interior floor and the tunnel-soil coupled system. The classical thin plate theory is considered to represent the behaviour of the first and the Pipe in Pipe model is chosen to describe the second. Because the complete model is assumed to be geometrically invariant in the train circulation direction, the coupling of both systems is performed in the wavenumber-frequency domain. After the model formulation, some important issues about its numerical computation are detailed and the obtained results are discussed. The response of a double-deck tunnel to a dynamic and to a quasistatic excitation is compared to the response obtained for a simple tunnel. The first comparison is done performing a power flow study of both tunnel structures when a harmonic line load is applied on them. The main differences between their radiation magnitudes and patterns are identified and discussed. The second comparison is done calculating the total amount of energy crossing a certain surface when a static load moving at a constant speed is considered. Results for a wide range of load speeds and radial distances are presented. A complete track-tunnel-soil model is finally obtained coupling a superstructure model to the interior floor model previously presented.

The sound generated by high speed trains can be exacerbated by the presence of trackside structures. Tunnels are the principal structures that have a strong influence on the noise produced by trains. A train entering a tunnel causes air to flow in and out of the tunnel portal, forming a monopole source of low frequency sound ["infrasound"] whose wavelength is large compared to the tunnel diameter. For the compact case, when the tunnel diameter is small, incompressible flow theory can be used to compute the Green's function that determines the monopole sound. However, when the infrasound is "shielded" from the far field by a large "flange" at the tunnel portal, the problem of calculating the sound produced in the far field is more complex. In this case, the monopole contribution can be calculated in a first approximation in terms of a modified Compact Green's function, whose properties are determined by the value at the center of a. disk (modelling the flange) of a diffracted potential produced by a thin circular disk. In this thesis this potential is calculated numerically. The scattering of sound by a thin circular disk is investigated using the Finite Difference Method applied to the three dimensional Helmholtz equation subject to appropriate boundary conditions on the disk. The solution is also used to examine the unsteady force acting on the disk.

La magnétorésistance des jonctions tunnel magnétiques(TMR) est d'autant plus grande que la polarisation en spin des électrodes est importante. L'utilisation de demi-métaux ferromagnétiques devrait théoriquement permettre d'obtenir des TMR infinies. La double pérovskite Sr2FeMoO6 (SFMO) est un demi-métal ferromagnétique présentant une température de Curie de 415K bien supérieure à la température ambiante. L'étude que nous avons menée sur ce matériau comporte deux volets. Dans un premier temps, nous avons mesuré par dichroi͏̈sme magnétique les moments portés par le fer et le molybdène et mis en évidence leur configuration antiparallèle ainsi que la valence mixte du fer dans ce composé. Ces résultats sont en accord avec les calculs de structure de bande existants. En parallèle, nous avons élaboré des couches minces de ce matériau par ablation laser. Par une caractérisation structurale et magnétique approfondie, nous avons mis en évidence l'existence de phases parasites riches en fer qui apparaissent dans les premiers stades de la croissance à haute température. Afin d'éviter leur apparition, nous avons mis au point un procédé de croissance en trois étapes. Les couches ainsi obtenues présentent une faible rugosité et une aimantation de surface identique à celle du SFMO massif. Des jonctions tunnel SFMO/SrTiO3/Co ont également été élaborées. Elles présentent des courbes courant tension non linéaires caractéristiques de l'effet tunnel. Les premiers résultats sur les jonctions sont donc encourageants et les études sur celles-ci se poursuivent
For magnetoresistance of magnetic tunnel junctions (TMR), the bigger is the spin polarization of the electrodes, the more important is the magnetoresistance. Use of ferromagnetic half metals should give infinite TMR. Double perovskite Sr2FeMO6 (SFMO) is a ferromagnetic half metal having a Curie temperature of 415K far higher than room temperature. The study we made on this material is in, two parts. First, we measured by magnetic dichroism the iron and molybdenum spin moments and we made obvious their antiparallel configuration and the mixed valence of iron in this compound. These results agree the existing structure band calculations. In the same time, we elaborated thin films of this material by pulsed laser deposition. By an advanced structural and magnetic characterization, we brought to the fore the existence of iron rich parasitic phases which were developing in the early stage of the high temperature growth. In order to avoid their apparition, we developed a three steps method. Films obtained in this way are presenting a low roughness and a surface magnetization close to the bulk material. Tunnel junctions SFMO/SrTiO3/Co have also been elaborated. These junctions present non-linear current-voltage curves which are characteristics of tunnel effect. First results on junctions are promising and studies on it are going on

The purpose of the study was to evaluate the merits of the DiscRotor concept that combine the features of a retractable rotor system for vertical take-off and landing (VTOL) with an integral, circular wing for high-speed flight. Tests were conducted to generate basic aerodynamic characteristics of the DiscRotor in hover and in fixed-wing flight.

To assess the DiscRotor during hover, small scale tests were conducted on a 3ft diameter rotor without the presence of a fuselage. A â hover rigâ was constructed capable of rotating the model rotor at speeds up to 3,500 RPM to reach tip speeds of 500fps. Thrust and torque generated by the rotating model were measured via a two-component load cell, and time averaged values were obtained for various speeds and pitch angles. It has been shown that the DiscRotor will perform well in hover. Ground Effects in hover were examined by simulating the ground with a movable, solid wall. The thrust was found to increase by 50% compared to the ground-independent case. Pressure distributions were measured on the ground and disc surfaces. Velocity measurements examined the flow field downstream of the rotor by traversing a seven hole velocity probe. A wake behind the rotor was shown to contract due to a low pressure region that develops downstream of the disc.

Wind tunnel experimentation was also performed to examine the fixed wing flight of the DiscRotor. These experiments were performed in the VA Tech 6â X6â Stability Tunnel. A model of the fuselage and a circular wing was fabricated based upon an initial sizing study completed by our partners at Boeing. Forces were directly measured via a six degree of freedom load cell, or balance, for free stream velocities up to 200fps. Reynolds numbers of 2 and 0.5 million have been investigated for multiple angles of attack. Low lift-to-drag ratios were found placing high power requirements for the DiscRotor during fixed-wing flight. By traversing a seven-hole velocity probe, velocities in a 2-D grid perpendicular to the flow were measured on the model. The strengths of shed vortices from the model were calculated. A method to improve fixed-wing performance was considered where two blades were extended from the disc. An increase of 0.17 in the CL was measured due to the interaction between the disc and blades.

This research utilized a wide range of experiments, with the aim of generating basic aerodynamic characteristics of the DiscRotor. A substantial amount of quantitative data was collected that could not be included in this document. Results aided in the initial designs of this aircraft for the purpose of evaluating the merit of the DiscRotor concept.
Master of Science

Cette thèse présente une étude des interfaces métal de transition ferromagnétique/oxyde et leur importance dans les mécanismes de transport tunnel polarisé en spin. Le travail a porté sur des bicouches ultraminces NiMnSb/MgO(001), Fe/ MgO(001), Co/ MgO(001) et Mn/ MgO(001) élaborées par épitaxie par jets moléculaires. L'originalité de ce travail réside dans l'étude approfondie des propriétés électroniques du matériau magnétique en contact avec la barrière d'oxyde : hybridation, polarisation et magnétisme à l'interface ont été étudiés en utilisant les techniques de laboratoire ainsi que le rayonnement synchrotron. Nous avons montré que les interfaces réelles Fe et Co /MgO sont des modèles du point de vue structural et magnétique, contrairement à Mn qui s'oxyde en contact avec la barrière MgO. Parallèlement, l'étude des effets magnétorésistifs dans des jonctions tunnel totalement épitaxiées Fe/MgO/Fe(001) a apporté des preuves du filtrage par la barrière MgO des ondes de Bloch en fonction de leurs symétries. Enfin, nous avons mis en évidence l'influence de la qualité structurale et chimique de l'interface Fe/MgO et des électrodes sur ces mécanismes
The quality of ferromagnetic metal/oxide interfaces and its influence on spin dependent tunneling processes are studied in this thesis. The work was dedicated to ultrathin bilayers NiMnSb/MgO(001), Fe/ MgO(001), Co/ MgO(001) and Mn/ MgO(001) elaborated by Molecular Beam Epitaxy. The originality of this work lies on the study of the electronic properties of the magnetic material in contact with the oxide (interfacial hybridization, polarization, and magnetism, using regular characterization means as well as synchrotron radiation sources). Fe/MgO and Co/MgO are shown to be model interfaces from the growth and the chemical point of view. On the contrary, Mn is oxidized when it is in contact with MgO. In addition, tunnel magnetoresistance are probed in totally magnetic tunnel junctions Fe/MgO/Fe(001). Transport measurements evidence spin filtering effects due to the MgO barrier depending on the symmetry of the Bloch waves. Moreover, we show that the structural quality of the bottom Fe/MgO interface and of the Fe electrode have non negligeable influence on these spin dependent mechanisms

In heavily settled areas, deformations induced by the tunnel excavation may cause serious damage to nearby structures. In this study it is aimed to model ground deformations induced by main tunnels and connection tunnels excavations as well as groundwater drainage. Therefore, it is necessary to study effective means of controlling tunnel induced deformations. The main parameters affecting the failure and deformation state of the soil around a circular underground opening are the physical characteristics of the soil, the diameter of the opening, and the support pressure. During the construction stage of Necatibey Station of KizilayÇ
ayyolu metro line (Ankara, Turkey), challenging ground conditions involving highly heterogeneous and locally water saturated foundation soils have been encountered. Possibility of damage at the surface and/or on the underground structures can be estimated using finite difference method (FDM) of analysis. In this study, two geophysical methods namely Electrical Resistivity Imaging (ERI) and Ground Penetrating Radar (GPR) were utilized to distinguish soil types at the study area. By correlating these geophysical survey results with the boring v logs, 3-Dimensional soil profile was revealed at the study area to build up a basis for numerical models. 3-Dimensional (3D) FDM analyses were conducted to assess tunneling induced deformations, along with movements around shallow soft ground main tunnels and connection tunnels. During sequential excavations, temporary and permanent shotcrete lining was also simulated. The soil behavior is assumed to be governed by an elastic-perfectly plastic constitutive relation based on the Mohr&ndash
Coulomb criterion. The computed deformations around these openings have been compared with the in-situ measurements. The results of the study revealed that the 3-D elasto-plastic analyses yield comparably good correlation with the in-situ measurements. Also, in this study, the effects of main tunnels excavations on each other and the effects of connection tunnels excavations on main tunnels were identified in terms of ground deformations. In order to simulate induced surface settlement due to groundwater withdrawal at the site 3-D fully coupled (fluidmechanical) numerical models were run using different time durations. The model studies revealed that deformations monitored at the ground surface are directly related with the tunnel construction practice. Pumping groundwater has very little or no effect on the measured deformations.

Les premiers stades de la croissance du NiO/Cu(111) ont été caractérisés du point de vue chimique, morphologique et structurale à une échelle microscopique. Pour cela, nous avons utilisé différentes méthodes d'élaboration et combiné différentes techniques de laboratoire in situ ainsi que le rayonnement synchrotron. Les bicouches à couplage d'échange NiO/FeNi/Cu(111) ont aussi été étudiées. L'interface Ni/Cu(111) a été étudié en mettant en évidence les liens entre la morphologie, la structure et les propriétés magnétiques. Les valeurs réduites du moment de spin sont reliées à l'hybridation 3d à l'interface Ni-Cu et au recouvrement de Cu. Dans toute la gamme des épaisseurs étudiées l'anisotropie du moment orbital est dans le plan, déterminant l'orientation de l'axe de facile aimantation. Cette axe peut être reliée à la tétragonalisation du réseau cristallin du Ni. Des couches ultraminces de NiO ont été obtenues par évaporation à partir de pépites de NiO à température ambiante ainsi qu'à 250ʿC. Les couches minces peuvent être décrite par une bicouche NiO/Ni/Cu(111). Le GISAXS permettent de confirmer les observations STM et de mettre en évidence l'auto-organisation des îlots de NiO. Due à la faible efficacité d'oxydation, les couches minces de NiO déposées à partir du Ni métallique sous oxygène, présentent quelques différences par rapport aux couches déposées à partir des pépites. Le mécanisme de formation des îlots proposé est basé sur un processus de nucléation/agrégation de clusters. Les résultats structurales (LEED et GIXD) peuvent être attribués soit à la formation d'une phase hexagonale a-Ni2O3, soit à une distorsion structurale de la maille NiO(111)( )R30ʿ. Le système à couplage d'échange NiO/FeNi/Cu(111) a été élaboré en utilisant les deux méthodes d'élaboration du NiO. L'interface NiO/FeNi est très abrupte, présentant une texturation suivant les axes cristallographiques. Les analyses structurales montrent une bonne épitaxie du NiO sur les films d'alliage FeNi
The very first stages of the growth of NiO/Cu(111) interface, were characterized on a microscopic scale including chemical, morphological, and structural aspects. Different elaboration procedures were used and we combined in-situ laboratory and synchrotron radiation techniques. Complete exchange coupled NiO/FeNi/Cu(111) bilayer system have also been investigated. The metallic Ni/Cu(111) interface has been studied, evidencing the close relationship between morphology, structure and magnetic properties. The reduced spin magnetic moments were correlated with the Ni-Cu 3d hybridization and with the Cu capping. The in-plane orbital moment anisotropy, related with the Ni tetragonalization, confirms that the easy axis of magnetization is in the plane for all measured thin nickel films. Ultra-thin NiO films, obtained through MBE evaporation of NiO nuggets, were grown at room temperature and at 250ʿC. The resulting system can globally be described in terms of a spontaneous NiO/Ni/Cu(111) layering. Confirming the STM real-space observations, GISAXS experiments and calculations allowed evidencing the self-organized nature of the NiO islands obtained at room temperature. Due to the poor oxidizing efficiency of the molecular oxygen, the NiO films deposited from metallic Ni at 250ʿC, exhibit several differences with respect to those obtained by NiO nuggets evaporation at room temperature. A cluster nucleation/aggregation mechanism was proposed, based on the STM observations. Formation of a-Ni2O3 hexagonal phase, or structural distortion of the NiO(111)( )R30ʿ structure could both explain the LEED and GIXD results. Exchange coupled NiO/FeNi/Cu(111) bilayer was elaborated using both, NiO nuggets and metallic Ni in oxygen partial pressure evaporation. Sharp NiO/FeNi interfaces were obtained, with textured NiO. Structural analysis of the NiO films deposited on FeNi/Cu(111) films evidenced twinned NiO(111), in much better epitaxy than the oxides deposited on the Cu(111) substrate

L'intérêt pour les propriétés magnétiques des nanostructures de métaux de transition et de lanthanides de faible dimensionnalité n’a cessé de croitre au cours des deux dernières décennies, tant pour leur intérêt en recherche fondamentale que pour la perspective d’applications technologiques. De manière remarquable, les propriétés magnétiques des nanostructures peuvent être ajustées en contrôlant leur géométrie, leur structure atomique et leur environnement chimique. Dans cette thèse, un gabarit 1D composé de nanorubans de Si auto-organisés est utilisé pour guider la croissance de nanolignes de métaux de transition dans le but d’étudier leurs propriétés magnétiques. La géométrie et la structure atomique des nanorubans de Si et des nanolignes de métaux ont été étudiées in situ par microscopie par effet tunnel. Concernant le silicium, notre étude montre qu’une température de 490 K est nécessaire pour obtenir un gabarit 1D hautement ordonné. Les résultats obtenus sur les métaux de transition ont permis de déterminer la géométrie et la structure des nanolignes. Pour accéder aux propriétés magnétiques des nanolignes de Co, des mesures par XMCD ont été effectuées en température, en utilisant différentes orientations du champ magnétique. Les résultats montrent que les deux premières couches de Co adsorbées sur les nanorubans présentent une réponse magnétique faible, tandis que les couches supérieures présentent une aimantation exaltée. Deux axes d’anisotropie dans le plan ont été mis en évidence. Les moments magnétiques et l'énergie d’anisotropie magnétique ont été déterminés quantitativement. Les études en température suggèrent un comportement superparamagnétique
Interest in the magnetic properties of low dimensional transition metal and lanthanide nanostructures has seen an unprecedented rise in the last two decades due to both their fundamental interest and perspectives of technological applications. Remarkably, the magnetic properties of nanostructures can be tuned by controlling their geometry, atomic structure and chemical environments. In this thesis, a one-dimensional template composed of self-organized Si nanoribbons is used to grow transition metal nanolines, prior to the characterization of their magnetic properties. The geometries and the atomic structure of both the Si nanoribbons and the metal nanolines were investigated in situ by scanning tunneling microscopy. The growth mechanisms were investigated by exploring a large set of growth conditions. Regarding the Si growth, our study shows that a temperature of 490 K is necessary to obtain a long-range ordered one-dimensional template. Concerning the transition metal study, the results resolved the nanoline geometries and atomic structures.To access the magnetic properties of the Co nanolines on Si, XMCD measurements were performed using different magnetic field orientations and temperatures.The results show that the first two Co layers directly adsorbed onto the Si nanoribbons present a weak magnetic response while the upper Co layers exhibit an enhanced magnetization. Remarkably, two in-plane easy axes of magnetization were evidenced.The magnetic moments and the magnetic anisotropic energy are determined quantitatively.Temperature-dependent investigations strongly suggest a superparamagnetic behavior

Cette thèse présente une étude des interfaces métal de transition ferromagnétique/oxyde et leur importance dans les mécanismes de transport tunnel polarisé en spin. Le travail a porté sur des bicouches ultraminces NiMnSb/MgO(001), Fe/ MgO(001), Co/ MgO(001) et Mn/ MgO(001) élaborées par épitaxie par jets moléculaires. L'originalité de ce travail réside dans l'étude approfondie des propriétés électroniques du matériau magnétique en contact avec la barrière d'oxyde : hybridation, polarisation et magnétisme à l'interface ont été étudiés en utilisant les techniques de laboratoire ainsi que le rayonnement synchrotron. Parallèlement, l'étude des effets magnétorésistifs dans des jonctions tunnel totalement épitaxiées Fe/MgO/Fe(001) a apporté des preuves du filtrage par la barrière MgO des ondes de Bloch en fonction de leur symétrie. Enfin, nous avons mis en évidence l'influence de la qualité structurale et chimique de l'interface Fe/MgO et des électrodes sur ces mécanismes.

Comprendre les propriétés des interfaces molécules/métaux est d’une importance capitale pour la spintronique organique. La première partie porte sur l’étude des propriétés magnétiques de molécules de phtalocyanine de manganèse. Nous avons montré que les premières couches moléculaires forment des colonnes avec un arrangement antiferromagnétique sur la surface de Co(100). Ces dernières mènent à de l’anisotropie d’échange. La seconde partie porte sur l’étude d’une molécule à transition de spin, la Fe(phen)2(NCS)2, sublimée sur différentes surfaces. Nous avons identifié les états de spin d’une molécule unique sur du Cu(100). De plus, nous avons commuté l’état de spin d’une molécule unique pourvu qu’elle soit suffisamment découplée du substrat
Understanding the properties of molecules at the interface with metals is a fundamental issue for organic spintronics. The first part is devoted to the study of magnetic properties of planar manganese-phthalocyanine molecules and Co films. We evidenced that the first molecular layers form vertical columns with antiferromagnetic ordering on the Co(100) surface. In turn, these molecular columns lead to exchange bias. The second part is focused on the study of a spin-crossover complex, Fe(phen)2(NCS)2 sublimed on different metallic surfaces. We identified the two spin states of a single molecules on Cu(100). By applying voltages pulses, we switched the spin state of a single molecule provided that it is sufficiently decoupled from the substrate

碩士
大葉大學
機械工程學系碩士班
91
An experimental investigation of the drag curve of the two-dimensional (2-D) flow around a circular cylinder at low to medium Reynolds numbers (ranging from 55 to 560 based on the diameter of the circular cylinder) is described. Both horizontal and vertical soap film tunnels were used to set up a truly 2-D experiment. These films, which have a thickness ranging from 1∼10μm, provide an excellent means for studying hydrodynamics in two-dimensions. The ratio of the flow field characteristic length (cylinder diameter or vortex size) to the thickness routinely exceeds 104. Momentum defect measurements were used to obtain the time averaged drag coefficient, Cd, via laser Doppler velocimetry (LDV). It is found that the drag curve is in good agreement with the 2-D computations of Henderson [11]. Two 3-D instabilities of the natural wake: Mode A and Mode B instability disappear. This result implies that the intrinsic 3-D motions in the previously nominally 2-D experiments are not present in these novel devices. The separation angle of the flow around a circular cylinder is also investigated in the regime of Reynolds number less than 280. The time-averaged separation angles were obtained by averaging the instantaneous images from the soap-film flow visualization instead of from the commonly used streakline images from finite time exposures. Excellent agreement has achieved between the present experimental result and the numerical simulations by Wu et al. [29]. High-speed CCD was also used in this study, and found that the separation angle would oscillate with time.

This thesis presents the implementation of the upper bound limit analysis in combination with finite elements and linear optimization for solving different stability problems in geomechanics under plane strain conditions. Although the nonlinear optimization techniques are becoming quite popular, the linear optimization has been adopted due to its simplicity in implementation and ease in attaining the convergence while performing the analysis. The objectives of the present research work are (i) to reduce the computational effort while using an upper bound finite element limit analysis with linear programming in dealing with geotechnical stability problems, and (ii) to obtain solutions for a few important geotechnical stability problems associated with reinforced earth, unsupported tunnels and a group of anchors. It is also intended to examine the developments of the failure patterns in all the cases. For carrying out the analysis for different stability problems, three noded triangular elements have been used throughout the thesis. The nodal velocities are treated as basic unknown variables and the velocity discontinuities are employed along the interfaces of all the elements. The soil mass is assumed to obey the Mohr-Coulomb’s failure criterion and an associated flow rule. The Mohr-Coulomb yield surface is linearized by means of an exterior regular polygon circumscribing the actual yield circle so that the finite element formulation leads to a linear programming problem. A simple technique has been proposed for reducing the computational effort while solving any geotechnical stability problem by using the upper bound finite element limit analysis and linear optimization. In the proposed method, the problem domain has been discretized into a number of different regions in which a particular order (number of sides) of the polygon has been specified to linearize the Mohr-Coulomb yield criterion. A greater order of the polygon needs to be chosen only in that part of the domain wherein the rate of the plastic strains becomes higher. The computational effort required to solve the problem with this implementation reduces considerably. By using the proposed method, the bearing capacity has been computed for smooth as well as rough strip footings and the results obtained are found to be quite satisfactory. The ultimate bearing capacity of a rigid strip footing placed over granular, cohesive-frictional and purely cohesive soils, reinforced with single and a group of two horizontal layers of reinforcements has been determined. The necessary formulation has been introduced to incorporate the inclusion of reinforcement in the analysis. The efficiency factors, and , to be multiplied with Nc and Nγ for finding the bearing capacity of reinforced foundations, have been established. The results have been obtained (i) for different values of soil friction angles in case of granular and cohesive-frictional soils, and (ii) for different rates at which the cohesion increases with depth for purely cohesive soil under undrained condition. The optimum positions of the reinforcements' layers corresponding to which and becomes maximum, have been established. The effect of the length of the reinforcements on the results has also been analyzed. As compared to cohesive soil, the granular soils, especially with greater values of frictional angle, cause much more predominant increase in the bearing capacity. The stability of a long open vertical trench laid in a fully cohesive and cohesive-frictional soil has been determined with an inclusion of single and a group of two layers of horizontal reinforcements. For different positions of the reinforcement layers, the efficiency factor (ηs), has been determined for several combinations of H/B, m and where H and B refer to height and width of the trench, respectively, and m accounts for the rate at which the cohesion increases linearly with depth for a fully cohesive soil with = 0. The effect of height to width of the long vertical trench on the stability number has been examined for both unreinforced and reinforced soils. The optimal positions of the reinforcements layers, corresponding to which becomes maximum, have been established. The required length of reinforcements to achieve maximum efficiency factor corresponding to optimum depth of reinforcement has also been determined. The magnitude of the maximum efficiency factor increases continuously with an increase in both m and . The effect of pseudo-static horizontal earthquake body forces on the stability of a long unsupported circular tunnel (opening) formed in a cohesive frictional soil has been determined. The stability numbers have been obtained for various values of H/D (H = tunnel cover, D = diameter of the tunnel), internal friction angle of soil, and the horizontal earthquake acceleration coefficient The computations revealed that the values of the stability numbers (i) decreases quite significantly with an increase in , and (ii) become continuously higher for greater values of H/D and . The failure patterns have also been drawn for different combinations of H/D, and . The geometry of the failure zone around the periphery of the tunnel becomes always asymmetrical with an inclusion of horizontal seismic body forces. The interference effect on the stability of two closely spaced parallel (twin) long unsupported circular tunnels formed in fully cohesive and cohesive-frictional soils has been evaluated. The variation of the stability number with S/D has been established for different combinations of H/D, m and ; where D refers to the diameter of each tunnel, S is the clear spacing between the tunnels, and is the internal friction angle of soil and m accounts for the rate at which the cohesion increases linearly with depth for a soil with = 0. On account of the interference of two tunnels, the stability number reduces continuously with a decrease in the spacing between the tunnels. The minimum spacing between the two tunnels required to eliminate the interference effect increases with (i) an increase in H/D and (ii) a decrease in the values of both m and . The failure patterns have also been generated for a few cases with different values of S/D. The size of the failure zone is found to become smaller for greater values of m and . The horizontal pullout capacity of a group of two vertical strip anchors embedded, along the same vertical plane in sand, at shallow depths has been determined. At collapse, it is assumed that the anchor plates are subjected to the same uniform horizontal velocity without any bending or tilt. The pullout resistance increases invariably with increases in the values of embedment ratio, friction angle of the sand mass and anchor-soil interface friction angle. The effect of spacing (S) between the anchors on their group collapse load is examined in detail. For a given embedment ratio, the total group failure load becomes maximum corresponding to a certain optimal spacing (Sopt). The values of Sopt increases with an increase in the value of , but the changes in the value of H/B and do not have any significant effect on Sopt. The vertical uplift capacity of a group of two horizontal strip plate anchors with the common vertical axis buried in purely cohesive as well as in cohesive frictional soil has been computed. The variation of the uplift factors Fc, Fq and F , due to the contributions of soil cohesion, surcharge pressure and unit weight, respectively, has been evaluated for different combinations of S/B and H/B. As compared to a single isolated anchor, the group of two anchors generates significantly greater magnitude of Fc. On the other hand, the factors Fq and F , for a group of two anchors are found to become almost equal to that of a single isolated anchor as long as the levels of the lower plate in the group and the single isolated anchor are kept the same. For the group of two horizontal strip plate anchors in purely cohesive soil, an increase of cohesion of soil mass with depth and the effect of self weight of the soil have been incorporated. The uplift factor Fcy both due to cohesion and unit weight of the soil has also been computed for the anchors embedded in clay under undrained condition. For given embedment ratios, the factor Fcy increases linearly with an increase in the normalized unit weight of soil mass upto a certain value before attaining a certain maximum magnitude. The computational results obtained for different research problems would be useful for design.

This thesis presents the implementation of the upper bound limit analysis in combination with finite elements and linear optimization for solving different stability problems in geomechanics under plane strain conditions. Although the nonlinear optimization techniques are becoming quite popular, the linear optimization has been adopted due to its simplicity in implementation and ease in attaining the convergence while performing the analysis. The objectives of the present research work are (i) to reduce the computational effort while using an upper bound finite element limit analysis with linear programming in dealing with geotechnical stability problems, and (ii) to obtain solutions for a few important geotechnical stability problems associated with reinforced earth, unsupported tunnels and a group of anchors. It is also intended to examine the developments of the failure patterns in all the cases. For carrying out the analysis for different stability problems, three noded triangular elements have been used throughout the thesis. The nodal velocities are treated as basic unknown variables and the velocity discontinuities are employed along the interfaces of all the elements. The soil mass is assumed to obey the Mohr-Coulomb’s failure criterion and an associated flow rule. The Mohr-Coulomb yield surface is linearized by means of an exterior regular polygon circumscribing the actual yield circle so that the finite element formulation leads to a linear programming problem. A simple technique has been proposed for reducing the computational effort while solving any geotechnical stability problem by using the upper bound finite element limit analysis and linear optimization. In the proposed method, the problem domain has been discretized into a number of different regions in which a particular order (number of sides) of the polygon has been specified to linearize the Mohr-Coulomb yield criterion. A greater order of the polygon needs to be chosen only in that part of the domain wherein the rate of the plastic strains becomes higher. The computational effort required to solve the problem with this implementation reduces considerably. By using the proposed method, the bearing capacity has been computed for smooth as well as rough strip footings and the results obtained are found to be quite satisfactory. The ultimate bearing capacity of a rigid strip footing placed over granular, cohesive-frictional and purely cohesive soils, reinforced with single and a group of two horizontal layers of reinforcements has been determined. The necessary formulation has been introduced to incorporate the inclusion of reinforcement in the analysis. The efficiency factors, and , to be multiplied with Nc and Nγ for finding the bearing capacity of reinforced foundations, have been established. The results have been obtained (i) for different values of soil friction angles in case of granular and cohesive-frictional soils, and (ii) for different rates at which the cohesion increases with depth for purely cohesive soil under undrained condition. The optimum positions of the reinforcements' layers corresponding to which and becomes maximum, have been established. The effect of the length of the reinforcements on the results has also been analyzed. As compared to cohesive soil, the granular soils, especially with greater values of frictional angle, cause much more predominant increase in the bearing capacity. The stability of a long open vertical trench laid in a fully cohesive and cohesive-frictional soil has been determined with an inclusion of single and a group of two layers of horizontal reinforcements. For different positions of the reinforcement layers, the efficiency factor (ηs), has been determined for several combinations of H/B, m and where H and B refer to height and width of the trench, respectively, and m accounts for the rate at which the cohesion increases linearly with depth for a fully cohesive soil with = 0. The effect of height to width of the long vertical trench on the stability number has been examined for both unreinforced and reinforced soils. The optimal positions of the reinforcements layers, corresponding to which becomes maximum, have been established. The required length of reinforcements to achieve maximum efficiency factor corresponding to optimum depth of reinforcement has also been determined. The magnitude of the maximum efficiency factor increases continuously with an increase in both m and . The effect of pseudo-static horizontal earthquake body forces on the stability of a long unsupported circular tunnel (opening) formed in a cohesive frictional soil has been determined. The stability numbers have been obtained for various values of H/D (H = tunnel cover, D = diameter of the tunnel), internal friction angle of soil, and the horizontal earthquake acceleration coefficient The computations revealed that the values of the stability numbers (i) decreases quite significantly with an increase in , and (ii) become continuously higher for greater values of H/D and . The failure patterns have also been drawn for different combinations of H/D, and . The geometry of the failure zone around the periphery of the tunnel becomes always asymmetrical with an inclusion of horizontal seismic body forces. The interference effect on the stability of two closely spaced parallel (twin) long unsupported circular tunnels formed in fully cohesive and cohesive-frictional soils has been evaluated. The variation of the stability number with S/D has been established for different combinations of H/D, m and ; where D refers to the diameter of each tunnel, S is the clear spacing between the tunnels, and is the internal friction angle of soil and m accounts for the rate at which the cohesion increases linearly with depth for a soil with = 0. On account of the interference of two tunnels, the stability number reduces continuously with a decrease in the spacing between the tunnels. The minimum spacing between the two tunnels required to eliminate the interference effect increases with (i) an increase in H/D and (ii) a decrease in the values of both m and . The failure patterns have also been generated for a few cases with different values of S/D. The size of the failure zone is found to become smaller for greater values of m and . The horizontal pullout capacity of a group of two vertical strip anchors embedded, along the same vertical plane in sand, at shallow depths has been determined. At collapse, it is assumed that the anchor plates are subjected to the same uniform horizontal velocity without any bending or tilt. The pullout resistance increases invariably with increases in the values of embedment ratio, friction angle of the sand mass and anchor-soil interface friction angle. The effect of spacing (S) between the anchors on their group collapse load is examined in detail. For a given embedment ratio, the total group failure load becomes maximum corresponding to a certain optimal spacing (Sopt). The values of Sopt increases with an increase in the value of , but the changes in the value of H/B and do not have any significant effect on Sopt. The vertical uplift capacity of a group of two horizontal strip plate anchors with the common vertical axis buried in purely cohesive as well as in cohesive frictional soil has been computed. The variation of the uplift factors Fc, Fq and F , due to the contributions of soil cohesion, surcharge pressure and unit weight, respectively, has been evaluated for different combinations of S/B and H/B. As compared to a single isolated anchor, the group of two anchors generates significantly greater magnitude of Fc. On the other hand, the factors Fq and F , for a group of two anchors are found to become almost equal to that of a single isolated anchor as long as the levels of the lower plate in the group and the single isolated anchor are kept the same. For the group of two horizontal strip plate anchors in purely cohesive soil, an increase of cohesion of soil mass with depth and the effect of self weight of the soil have been incorporated. The uplift factor Fcy both due to cohesion and unit weight of the soil has also been computed for the anchors embedded in clay under undrained condition. For given embedment ratios, the factor Fcy increases linearly with an increase in the normalized unit weight of soil mass upto a certain value before attaining a certain maximum magnitude. The computational results obtained for different research problems would be useful for design.

碩士
國立臺灣科技大學
營建工程系
106
In this study, the PFC2D code was used to explore the circular tunnel and horseshoe tunnel with four different overburden depths (H=300, 400, 500, 600 m).The tunnel was excavated in a hard rock (granite), and the whole section was excavated without support. The stress redistribution, displacement distribution and microcrack development during tunneling were analyzed under three different initial stresses (K=0.5, 1, 2). The results show that the horseshoe main tunnel excavation, change of the micro bonding stress and particle displacement during tunneling are highly related different initial stresses and geometric linear shapes of Excavation face . The most need to pay attention to the roof of tunnel under K=2, the microcrack development in the three-stage tunnel excavation, has the maximum destruction area.

碩士
國立臺灣科技大學
營建工程技術學系
86
The behavior of tunnel cross connections between tubes should be three-dimensional. The cross connections mechanisms can not be fully understood if two-dimensional numerical simulation is used, even if empirical judge aids and safety factor amplifies. Three-dimensionalanalyses focus on circular cross connections under different rock massclassification RMR and lateral stress ratio k. The diameters of the maintunnel and cross connection are 12 and 9 meters respectively, and the circular centers are on the same level. The research results show that the stress concentration is significant near the penetration region of cross connections, and the concentration ishigher in the acute side than obtuse side for 45 degree inclined crossconnections. The influential region depends on the rock condition, the region increases if RMR decreases, the tendency is significant if cross connections are located on poor rock.

碩士
國立臺灣科技大學
營建工程系
103
Tunneling is the work in civil engineering that surrounding geological condition plays a very important role. Plane of weakness or joints dominate the construction behavior in rock mass. It is always assumed that tunneling rock is homogeneous and isotropic for most theoretic formulas such as Kirsch formula, and tunnel cross-section shape is circle. The ideal assumption can’t fit most practical conditions in construction design and analysis. Therefore, it has to stimulate in other ways. This study use two-dimensional numerical software UDEC to stimulate the excavation of circular tunnel. Single joint cutting circular tunnel with different orientations is simulated based on single joint has the strongest anisotropy. The tunneling mechanical behavior both single joint condition and no joint condition are analyzed and compared. Some conclusions and suggestion are presented for the practical design, constructions, and monitoring.

碩士
國立臺灣科技大學
營建工程系
90
ABSTRACT This research investigates the relationship between longitudinal ground arch and the deformation of tunnel excavation to evaluate the possibility of using measurements of tunnel three-dimensional deformations to predict the geological conditions beforehand. A three-dimensional numerical code, FLAC-3D, was used to analyze the excavation of tunnel through the interface of different geological conditions. Different lateral stress coefficients and the erection of support were studied. The three-dimensional deformation behavior around the tunnel excavation face and the development of stresses and deformations ahead the excavation face were investigated. The following conclusions were obtained from this research. (1)Longitudinal ground arch would induce a stress concentration zone ahead the excavation face no matter the deformability of rock mass or the lateral stress coefficient. (2)Longitudinal ground arch has obvious effect on the longitudinal deformations and very little effect on the vertical deformations. (3)The ratios of the longitudinal deformations (L) and the radial deformations (S) at the crown of the excavation face would increase while the tunnel is excavated from competent rock to weak rock, and vice versa. (4)The changes of ratios L/S would more clearly reflect the geological interface if the differences of the rock mass deformability are larger, and vice versa. (5)At the condition of larger lateral stress coefficients, the ratios of L/S at the crown would more clearly reflect the geological differences than the ratios of L/S at the side walls, and vice versa. (6)The ratios of L/S are more sensitive to the changes of geology when supports are applied to the tunnel as compared with no support conditions.

碩士
國立臺灣科技大學
營建工程技術學系
86
The behavior of cross connections between tunnel tubes should be three-dimensional, and is not suitable using two-dimensional numerical simulation, no matter how empirical judge or safety factor includes. Three-dimensional analysis of under different RMR and lateral stress ratio K. The cross section shape of main tunnel is triple-circular-arcs for main tunnel, and horse-shoe for cross connections. The widths of the main tunnel and cross connection are 17 and 7 meters respectively, and the circular centers The research results show that stress concentration is significant around the penetration region of cross connections, and the concentration is higher in the acute side than obtuse side for inclined cross connections. The influential region of cross connections depends on the rock condition, the region increases if RMR decreases, the tendency is significant if cross connections are located on poor rock.

碩士
國立臺灣大學
機械工程學研究所
100
In high population density areas, the risks from overhead electric cables to human health are increasing. As a result, replacing traditional overhead cables with underground cables are needed. However, the cables will produce large amount of heat in the underground tunnel. The research of cooling down the sewer system is still lacking in Taiwan. Therefore, this research utilizes a cement tube to simulate the environment of a sewer system, heat sticks which work as the cables in the system, and water for indirect cooling in the sewer system. Controls in the experiment are five positions of the water pipe in the cement tube (A, B, C, D, E), range of room temperature (17℃ to 31.1℃), site of the heat stick(30ﾟ and 120ﾟ), power of the stick - 700W, flow rate (7.5LPM), and the numbers of the pipe (two, three, four ). After these controls have been adjusted, thermocouple wires that are installed in the cement tube detect the temperature of three different sections: a random cross section, the entry, and the exit of the water pipe. We then analyze the plot which is built from the data from the sensors. In different room temperatures we found that the higher the room temperature the more heat the water pipe can cool, we also found that the heat which is dissipated by the air surrounding the tube is diminished. Thus, for different room temperatures and the same pipe placement, we cannot optimize the placement of the cooling pipes. It must remain within the certain range of room temperature, so the temperature which is measured from a random cross section will not be affected by room temperature. Within the specific temperature range, the number of pipes is three, the heat stick is placed approximately 120ﾟ from the top of the tube, and we found that the optimal positions of the pipes is ACE. We then changed the number of the pipe to four and placed heat sticks in position 120 and 30 degrees from the top of the tube. In this condition, the optimizations of four pipes are ACDE and BCDE. Next, we continue to place four pipes in the tube, but within different range of room temperatures. We got the same optimizations which are ACDE and BCDE. We used the data from the experiment which is using less pipes to simulate and prove the optimizations of multiple cooling pipes, because of the experiment which is utilizing multiple pipes needs to spend a lot time to obtain all data.

碩士
國立臺灣科技大學
營建工程系
103
This study is using numerical analysis software PFC2D to simulate a circular tunnel with overburden depth 300 meters, considering two different circular tunnel sections are 4 meters and 8meters. Under no jointed rock and single joint cutting circular tunnel with different places, such as roof, right side wall and center, parallel bond stress redistribution, development of micro crack and displacement of roof were compared with these combinations. The analytic results show that the micro crack development under no jointed rock in two different circular tunnel sections, within 2 meters thickness of surrounding rock near excavation surface. The beginning of the micro crack development in single jointed rock , it follows along the main jointed. Then, the micro crack spread around the excavation surface. Finally, discussion the stress distribution and deformation in order to know the micro crack development after the tunnel excavation.

碩士
國立臺灣大學
機械工程學研究所
97
In near future, the governments energetically beautify cities, and the engineering design of large underground tunnel becomes more general and worthy to take seriously. We don’t have enough engineering experiences of building large underground tunnel in Taiwan and lack for related literarure of a large underground tunnel. This research proceeds in a large circular tunnel model, in which air is cooled by indirect water cooling system. The linear heat source in the tunnel is simulated by electric heaters. The effect of the position of the heat source, the power of the heat source(600W,800W,100W), the flow rate of cooling water(2,4,8LPM), and the position and number of cooling-water pipes were investigated. Air temperature and temperature of axial direction in the tunnel were obtained with thermocouples, and used them to plot the temperature distribution of cross section of tunnel. The research shows that the temperature distribution of cross section didnot have strong change with changing the flow rate of single pipe of cooling water and power of heaters. The temperature of air of tunnel decreased when the flow rate is minima due to the effect of natural convection was increased. As the pipes of cooling water moved left side of tunnel from right side, the area of local higher temperature will spread. The application of engineering has to focus on it. When single cooling pipe fixed at left side, it have the most largest efficiency of heat exchange rate. In the experiment of fixed flow rate of the number of cooling pipes, the result shows that using multi-pipe to reach the lower temperature of air in tunnel than using single pipe. The vortex by cold air and hot air violently mixed due to the cross section of temperature distribution had area of local higher temperature. If the linear heat source is located the bottom of tunnel space, we have more uniform and lower air temperature in the tinnel. Using multi-pipe set up top of tunnel space and linear heat sources is located the bottom of tunnel is the best layout of indirect water cooling system. Moreover, the special phenomenon of temperature oscillation between cooling water inlet and outlet was found, which is due to the energy exchanges between the air of tunnel and wall of tunnel, and the linear heat sources do not keep the same temperature.

碩士
國立臺灣大學
機械工程學研究所
98
In recent years, people demands the use of space needs and beautifying the landscape of cities. The underground tunnel development and engineering design get much attention. We don’t have enough engineering experiences of building large underground tunnel in Taiwan and lack for related literarures of a large underground tunnel. Therefore , this research proceeds in a large circular tunnel model, in which air is cooled by indirect water cooling system. Using two linear heat sources in the tunnel is simulated by electric heaters. In order to investigate the related heat transfer issues. The manipulated variables of this research are the positions of two heat sources( 30°, 60°, 90°, and 120°), the powers of heat source ( 2600W , 1920W , and 1300W ) , the flow rates of cooling water (4LPM , 8 LPM , and 12LPM) , the number of cooling-water pipes( single , double , multi-pipe ) , and cooling-water pipes with five positions. Air temperatures , temperatures of axial direction and cooling-water pipes in the tunnel are obtained with thermocouples, then useing them to plot the temperature distribution of cross section in tunnel. The research shows that if the position of the lower heaters , the absorption of heat transfer rate and the cooling rate will be better in single pipe. The disadvantage is that the overall temperature of the tunnel will become high. If the location of pipes are closer to the heaters , The high temperatures at the top of heaters significantly affected by inhibition. When increasing the flow rate of the pipe , the heat transfer rate will increase , but it’s not proportional to increase. When the positions of the two heat sources( 30°, 90°, and 120° ) , the D pipe is optimun cooling rate in single pipe. But the optimum cooling rate of heat sources in 60° is the C pipe. In the experiment of changing the number of cooling pipes with fixed flow rate, the distributed installation of pipes is better then the concentrated installation of pipes. Installing multi-pipe is useful for cooling the temperatures of cross section and the temperatures of axial direction. In the experiment of changing the positions of heaters , moving from the bottom to top with the heaters , the area of lower temperatures becomes larger. When the power of the heater is decreased , the temperatures of cross section will be lower. The optimum layout of indirect water cooling system is using multi-pipe and the electric heaters placement in the top of the tunnel is optimum.

碩士
中原大學
土木工程研究所
86
Tunnel excavations are among one of the most challenging constructions in civil engineering, due to variable ground conditions, few available measurements, difficulty in installation, etc. The in-situ rock masses possess many different formations, discontinuities of various forms, the state of initial stress, and other environmental impacts. Therefore, determining the most relevant stability factors for tunnel excavation is always an essential issue for civil engineers. Previous experiences indicated that the dip of the major discontinuity (joint) is the most important factor affecting the stability of a tunnel roof. In this paper, a set of joint properties, lateral pressure coefficient, and the state of gravity are adopted to study their influences on the stability of a circular tunnel. The two-dimensional discontinuous deformation analysis (DDA) is used for the numerical analysis.DDA developed by Dr. Shi in 1988 has become another powerful tool to analyze the complex behavior of jointed rocks or discontinuous media.The method is formulated according to the minimization of potential energy and the open-close iterations for contacts, and it has the following features: 1) providing the closed-forms of block integration; 2) fulfilling the non-penetration and no-tension contact constraints (complete block kinematics); 3) satisfying the dynamic equilibrium condition every step; 4) performing pseudo-static or dynamic computations.The site conditions under study are a circular tunnel of 10m diameter, depth of 500m, no existence of water, and regular pattern of jointing. After establishing the problem geometry layout, a series of DDA runs are performed according to a set of values of joint properties (like dip , friction angle, spacing S, set number #), lateral pressure coefficient K, and the state of gravity g. The influences of these factors on the stability of the circular tunnel are evaluated in term of the radial displacement of tunnel wall. According to the analysis results under the given condition, it is concluded that: 1) the gravity effect can not be neglected, and it results in a higher value of radial displacement on the tunnel roof; 2) one joint set exerts high anisotropy, however, for the case of two joint sets, anisotropic behavior does not prevail; 3) the spacing and geometric location of joints also affect the stability of the tunnel walls; 4) as the joint friction angle increases, the radial displacement of the tunnel wall decreases to a certain value; 5) as the lateral pressure coefficient increases, the radial displacement of the tunnel roof remarkably declines.

碩士
國立中央大學
土木工程研究所
91
This study experimentally investigates the pressure distribution on the surface of three-dimensional circular cylinder in a uniform flow. The experiments were carried out in a wind tunnel. The flow conditions include three different kinds of surface roughness. Instantaneous fluctuating wind pressures were measured by an electronic pressure scanner. Based on the pressure measurement, the distributions of mean pressure coefficient Cp, rms Cprms, and peak pressure Cpp were calculated. Also, the separation point and the location of minimum negative pressure will move toward upstream direction as surface roughness increase. Compared to the results of Lee (2002), the moving range of separation point will decrease as surface roughness increase. The experimental results also revealed that the probability of pressure fluctuations for positive pressure is close to the Gaussian distribution function. On the other hand, negative pressure side was skewed and did not necessary follow Gaussian distribution.

The dissertation addresses the design of ultra-high towers under the wind action and has a special application for Solar Updraft Power Plants. They are a highly sustainable natural resource for electric power generation, based on a combined sun-wind energy solution. The object of the investigation is a 1-km tall solar tower, made of reinforced concrete and stiffened along the height by stiffening rings. They are usually introduced in the design of solar towers in order to reduce the structural vulnerability to the wind action by enhancing a beam-like behaviour. However, the wind tunnel tests performed within this research showed that the presence of rings along the height of the tower modifies the aerodynamics of the flow around the circular cylinder and creates a bi-stable and asymmetric load condition, even at moderately high Reynolds numbers. This phenomenon is new and unknown. Similar effects were observed in the critical range of Re number and around two side-by-side cylinders, but the conditions of occurrence and the physical reasons are profoundly different. The discovery of the existence of such a bistable and asymmetric load condition induced by rings along the height of a finite length circular cylinder, its interpretation, as well as the cross-checked experimental evidence in different wind-tunnels confirmed also by numerical simulations, are the original contributions of this work. Then, the effect is quantified on the structural response. The bistable asymmetric load on the structure did not result to be a prohibitive load condition for solar towers and the magnitude of the effect depends on the number and on the size of the rings. Mitigation strategies are then proposed in the work. Furthermore, the dissertation evaluates the shell response to the stochastic wind loading process and provides to the designer a general unified simple tool to define design wind loads for quasi-static calculations of ultra-high towers in any atmospheric boundary layer flow.

碩士
臺灣大學
機械工程學研究所
98
Natural convection heat transfer in a circular enclosure has been developed for a long time. One of many applications is cooling system of underground cable tunnels. For Taiwan, We lack for experiences of building underground cable tunnels. We need more literatures to provide a lot of information for structuring underground cable tunnels. In underground cable tunnels, a part of electric energy is dissipated by electrical cable so the cooling-water system is needed. In this research the circular tunnel made of concrete was used to simulate underground cable tunnels. Tube-shaped heater was simulated electrical cables. Five pipes made of stainless structured indirect cooling water system. The purpose of this research is to optimize cooling efficiency. The effect of the position of cooling water pipes(A,B,C,D,E), the angle of the heat source(30°,60°,90°,120°), the power of the heat source(1300, 975, and 650 W), the flow rate of the cooling water(4, 8, 12, and 15LPM), the number of the cooling water pipes were discussed. T-type Thermocouples installed on cross section of downstream section of underground cable tunnel to measure air temperature. In the experiment of single pipe of cooling water, the smallest distance between single pipe of cooling water and heater was the highest efficiency but local relative high temperature was higher than any other case. When the position of single pipe of cooling water is fixed, the power of heater didn’t influence the distribution of temperature of air significantly. The average Temperature profile was like a parabolic line when the flow rate of cooling water increased from 4 LPM to 15 LPM and strong fluctuations of temperature of air occurred on the section of entrance of the tunnel as the flow rate of cooling water more than 12 LPM. The level of heat source was lower then the temperature of air was more uniform. In the experiment of number of pipes of cooling water, the result shows that more pipes of cooling water were activity then the temperature of cross section of the tunnel is lower due to increase the total area of cooling water pipes. The best layout is that Multi-pipes are arranged on the top of the tunnel and heater is the same but below the pipes.

碩士
國立臺灣大學
機械工程學研究所
99
In recent years, for safety reasons and the consideration of landscape, the development of underground cables tunnel becomes a very important part in urban planning. In Taiwan, we have less experience in this area and lack of related research. Therefore, in this research, we use a large circular tunnel as a model, in which we use double linear heat sources to simulate the heat lost of high-voltage cables at work. In the same time, with the indirect water cooling system, the lost heat will be taken out of the tunnel. Through this way, we hope we can understand the mechanism of heat transfer in the tunnel. The effects of the power of the heat sources (250, 600, 1100, 1400, and 1800W), the numbers of cooling pipes (two or four), the flow rate of cooling water (4, 6, 8, 10, 12, and 14 LPM), and the temperature of cooling water (11, 14, 17, and 20℃) will be discussed. In the tunnel, the thermocouples were installed to measure the temperature of the cross section of the tunnel, the cooling water at inlet and outlet of the water pipes, and inside and outside the tunnel wall. Then, we made drafting and analysis. In the experiment without cooling system, we found that when the heat sources at the angle of 120° in the tunnel and the power is not too high, the related temperature will below 15℃ in the area below the tunnel. But at the top of the tunnel, the related temperature will be over 38℃, and the area above the heat sources will be over 50℃. In the experiment of changing the numbers of the water pipes and water flow rate, we found that the one with more water pipes will obviously suppress the area of high temperature in the tunnel at the same water flow rate, but to the one with less water pipes, its cross section of temperature will be more sensitive to the water flow rate. As the water flow rate increasing, the suppression of the area of high temperature in the tunnel will be more evident at lower heat sources power, but to the area of low temperature in the tunnel, the one with higher heat sources power will be more evident. There is a marked difference between the temperature inside and outside the tunnel wall whether the water cooling system is working. But after the cooling system turning on, the difference will be unapparent as the water flow rate increasing. In the experiment of different cooling water temperature, we found that as the temperature of water increasing, the phenomenon of heat accumulation at the top of the tunnel will be more significant, and then increasing the water flow rate will modify this phenomenon especially for the lower heat sources power. As the water temperature increasing, the difference of temperature between inlet and outlet of the water pipes will decrease, and the one with higher water flow rate will decrease slowly.

This work develops a discontinuous Galerkin finite element discretization of non- linear hyperbolic conservation equations with efficient and robust high order stabilization built on an entropy-based artificial viscosity approximation. The solutions of equations are represented by elementwise polynomials of an arbitrary degree p > 0 which are continuous within each element but discontinuous on the boundaries. The discretization of equations in time is done by means of high order explicit Runge-Kutta methods identified with respective Butcher tableaux. To stabilize a numerical solution in the vicinity of shock waves and simultaneously preserve the smooth parts from smearing, we add some reasonable amount of artificial viscosity in accordance with the physical principle of entropy production in the interior of shock waves. The viscosity coefficient is proportional to the local size of the residual of an entropy equation and is bounded from above by the first-order artificial viscosity defined by a local wave speed. Since the residual of an entropy equation is supposed to be vanishingly small in smooth regions (of the order of the Local Truncation Error) and arbitrarily large in shocks, the entropy viscosity is almost zero everywhere except the shocks, where it reaches the first-order upper bound. One- and two-dimensional benchmark test cases are presented for nonlinear hyperbolic scalar conservation laws and the system of compressible Euler equations. These tests demonstrate the satisfactory stability properties of the method and optimal convergence rates as well. All numerical solutions to the test problems agree well with the reference solutions found in the literature. We conclude that the new method developed in the present work is a valuable alternative to currently existing techniques of viscous stabilization.