Dissertations / Theses on the topic 'Transmission and reflection coefficients'

The interpretation of reflection, transmission and conversion (RTC) coefficients in fluid-saturated porous rock is of utmost importance for the characterisation of hydrocarbon reservoirs. It has also been suggested that high-pressure pore-fluid is responsible for anomalously high (up to 0.15-0.25) values of reflection coefficients (deep crustal reflectors) in the lower crust, subduction zones and subglacial deforming sediments. In my thesis I consider the interaction of seismic waves at the interface between fluid-saturated poroelastic media, taking properly into account the effects of fluid-solid interaction. I derive dynamic equations for wave propagation in poro-elastic media and obtain an explicit wave decomposition in up- and down-going components. I then develop an algorithm to compute RTC coefficients at the interface between two arbitrary poro-elastic media, when possible pore-pressure discontinuity at the interface is taken into account. My algorithm is written in a matrix form allowing me to compute RTC coefficients for plane waves at all frequencies and all angles of incidence. This algorithm encompasses both visco-elastic and poro-elastic cases including the effects caused by partially sealed interface. Using numerical examples I show that my algorithm is consistent with (visco-)elastic case. It is designed in a matrix form suitable for conventional computations of multilayered stacks as used in the reflectivity method. A range of possible applications and extensions such as wave propagation in finely-layered fluid-saturated sediments are discussed.

The dynamic modelling of one-dimensional jointed structures is relevant to many engineering applications, such as pipe systems and beam networks in constructions. Currently available techniques are undermined by inadequate ability to model the joints and other discontinuities due to uncertainty in their properties. Measured modal data can be used to update joint models, but often with limited success. In this thesis a wave approach is employed to investigate the reflection and transmission coefficients of various joint models in structural waveguides. The reflection and transmission coefficients are potentially more sensitive to the parameters of the joint models. Numerical simulations and experiments have been performed on three types of jointed waveguides. Appropriate models have been identified for these cases and sensitivities of the scattering coefficients to joint parameters have been investigated. Accurate measurement of the reflection and transmission coefficients is desired in order to estimate joint parameters. A noise model is developed and a perturbation method is used to study the influence of measurement noise on the estimated reflection and transmission coefficients. An iterative method is examined to solve the non-linear problem of estimating the parameters of a joint from measured reflection and transmission coefficients, in a leastsquares sense. Issues concerning the iteration process, such as the selection of objective functions and frequency ranges, are examined in accordance with the sensitivity of the objective function to unknown parameters. The parameter identification method is validated by numerical simulation case studies and then verified by using measured data for mass discontinuities on beams, a supported straight pipe and a right-angled pipe bend. The case studies demonstrate that parameter identification of discontinuities in waveguides by using the wave approach is a success where modal methods are inappropriate.

Using the method of matched asymptotic expansions the reflection and transmission coefficients are calculated for scattering of oblique water waves by a vertical barrier. Here an assumption is made that the barrier is small compared to the wavelength and the depth of water. A number of sloshing problems are considered. The eigenfrequencies are calculated when a body is placed in a rectangular tank. Here the bodies considered are a vertical surface-piercing or bottom-mounted barrier, and circular and elliptic cylinders. When the body is a vertical barrier, the eigenfunction expansion method is applied. When the body is either a circular or elliptic cylinder, and the motion is two-dimensional, the boundary element method is applied to calculate the eigenfrequencies. For comparison, two approximations, "a wide-spacing", and "a small-body" are used for a vertical barrier and circular cylinder. In the wide-spacing approximation, the assumption is made that the wavelength is small compared with the distance between the body and walls. The small-body approximation means that a typical dimension of the body is much larger than the cross-sectional length scale of the fluid motion. For an elliptic cylinder, the method of matched asymptotic expansions is used and compared with the result of the boundary- element method. Also a higher-order solution is obtained using the method of matched asymptotic expansions, and it is compared with the exact solution for a surface-piercing barrier. Again the assumption is made that the length scale of the motion is much larger than a typical body dimension. Finally, the drift force on multiple bodies is considered the ratio of horizontal drift force in the direction of wave advance on two cylinders to that on an isolated cylinder is calculated. The method of matched asymptotic expansions is used under the assumption that the wavelength is much greater than the cylinder spacing.

An experimental investigation of the interaction of water waves with flexible structures acting as breakwaters was carried out. Wave profiles, mapped out by water level measuring transducers, were studied to provide information on the performance of different breakwater models. A new signal analysis procedure for determining reflection coefficients based on wavelet theory was developed and compared to a conventional method. The reliability of using wavelet analysis to separate a partial standing wave into incident and reflected wave components was verified with a numerical example. It was also verified by the small variance in the estimates of the incident wave height from independent experimental measurements. Different geometries of rigid and flexible structures were constructed and examined. Reflection, transmission and energy loss coefficients were obtained over them. The influence of various properties of the models, such as the width and the internal pressure, on the effectiveness in reflecting or absorbing the incident wave energy was determined. Various factors which affect the performance of the breakwater, including the water depth, the wave length and the wave amplitude, were measured and documented. Suspended and bottom-mounted models were considered. The flow field over and near a hemi-cylindrical breakwater model was also examined using a flow visualization technique. An overall comparison among the models has also been provided. The results showed that the rectangular models, rigid and flexible, are the most effective structures to dissipate wave energy. The flow visualization technique indicated that the flow conforms with the circular geometry of a hemi-cylindrical breakwater model, yielding no flow separation.
Ph. D.

The deployment of dedicated short-range communications (DSRC) roadside units (RSUs) allows a connected or automated vehicle to acquire information from the surrounding environment using vehicle-to-infrastructure (V2I) communication. However, wireless communication using DSRC has shown to exhibit null points, at repeatable distances. The null points are significant and there was unexpected loss in the wireless signal strength along the pathway of the V2I communication. If the wireless connection is poor or non-existent, the V2I safety application will not obtain sufficient data to perform the operation services. In other words, a poor wireless connection between a vehicle and infrastructure (e.g., RSU) could hamper the performance of a safety application. For example, a designer of a V2I safety application may require a minimum rate of data (or packet count) over 1,000 meters to effectively implement a Reduced Speed/Work Zone Warning (RSZW) application. The RSZW safety application is aimed to alert or warn drivers, in a Cooperative Adaptive Cruise Control (CACC) platoon, who are approaching a work zone. Therefore, the packet counts and/or signal strength threshold criterion must be determined by the developer of the V2I safety application. Thus, we selected an arbitrary criterion to develop an empirical method of ascertaining the null points from a DSRC RSU. The research motivation focuses on developing an empirical method of calculating the null points of a DSRC RSU for V2I communication at a highway on/off-ramp. The intent is to improve safety, mobility, and environmental applications since a map of the null points can be plotted against the distance between the DSRC RSU and a vehicle's onboard unit (OBU). The main research question asks: 'What is a more robust empirical method, compared to the horizontal and vertical laws of reflection formula, in determining the null points from a DSRC RSU on a highway on/off ramp?' The research objectives are as follows: 1. Explain where and why null points occur from a DSRC RSU (Chapter 2) 2. Apply the existing horizontal and vertical polarization model and discuss the limitations of the model in a real-world scenario for a DSRC RSU on a highway on/off ramp (Chapter 3 and Appendix A) 3. Introduce an extended horizontal and vertical polarization null point model using empirical data (Chapter 4) 4. Discuss the conclusion, limitations of work, and future research (Chapter 5). The simplest manner to understand where and why null points occur is depicted as two sinusoidal waves: direct and reflective waves (i.e., also known as a two-ray model). The null points for a DSRC RSU occurs because the direct and reflective waves produce a destructive interference (i.e., decrease in signal strength) when they collide. Moreover, the null points can be located using Pythagorean theorem for the direct and reflective waves. Two existing models were leveraged to analyze null points: 1) signal strength loss (i.e., a free space path loss model, or FSPL, in Appendix A) and 2) the existing horizontal and vertical polarization null points from a DSRC RSU. Using empirical data from two different field tests, the existing horizontal and vertical polarization null point model was shown to contain limitations in short distances from the DSRC RSU. Moreover, the existing horizontal and vertical polarization model for null points was extremely challenging to replicate with over 15 DSRC RSU data sets. After calculating the null point for several DSRC RSU heights, the paper noticed a limitation of the existing horizontal and vertical polarization null point model with over 15 DSRC RSU data sets (i.e., the model does not account for null points along the full length of the FSPL model). An extended horizontal and vertical polarization model is proposed that calculates the null point from a DSRC RSU. There are 18 model comparisons of the packet counts and signal strengths at various thresholds as perspective extended horizontal and vertical polarization models. This paper compares the predictive ability of 18 models and measures the fit. Finally, a predication graph is depicted with the neural network's probability profile for packet counts =1 when greater than or equal to 377. Likewise, a python script is provided of the extended horizontal and vertical polarization model in Appendix C. Consequently, the neural network model was applied to 10 different DSRC RSU data sets at 10 unique locations around a circular test track with packet counts ranging from 0 to 11. Neural network models were generated for 10 DSRC RSUs using three thresholds with an objective to compare the predictive ability of each model and measure the fit. Based on 30 models at 10 unique locations, the highest misclassification was 0.1248, while the lowest misclassification was 0.000. There were six RSUs mounted at 3.048 (or 10 feet) from the ground with a misclassification rate that ranged from 0.1248 to 0.0553. Out of 18 models, seven had a misclassification rate greater than 0.110, while the remaining misclassification rates were less than 0.0993. There were four RSUs mounted at 6.096 meters (or 20 feet) from the ground with a misclassification rate that ranged from 0.919 to 0.000. Out of 12 models, four had a misclassification rate greater than 0.0590, while the remaining misclassification rates were less than 0.0412. Finally, there are two major limitations in the research: 1) the most effective key parameter is packet counts, which often require expensive data acquisition equipment to obtain the information and 2) the categorical type (i.e., decision tree, logistic regression, and neural network) will vary based on the packet counts or signal strength threshold that is dictated by the threshold criterion. There are at least two future research areas that correspond to this body of work: 1) there is a need to leverage the extended horizontal and vertical polarization null point model on multiple DSRC RSUs along a highway on/off ramp, and 2) there is a need to apply and validate different electric and magnetic (or propagation) models.
Ph. D.

Generation of electricity from ocean waves has seen increasing research and commercial interest in recent years. The development of projects of several hundred megawatts rated capacity is now being considered. There is a clear need for improved understanding of the environmental impact of large-scale wave energy extraction, particularly in nearshore regions where sediment transport and cliff erosion may be affected. This thesis investigates the change in nearshore wave conditions and sediment transport due to energy extraction by long strings of wave energy devices. The influence of wave energy converter (WEC) arrays has been studied using transmission coefficients implemented within a spectral wave model. It is shown that the breaking wave height nearshore is larger (5%) if transmission is defined as frequency dependent. This is due to the energy dissipation processes associated with different wave frequencies. Linear wave theory is employed to determine frequency dependent transmission and reflection coefficients across a line of wave energy devices based onthe amplitude of scattered and radiated waves. This approach is compared with experimental measurements of the wave field in the vicinity of an array of five heaving floats. The transmitted wave amplitude is predicted with reasonable accuracy but additional numerical damping is required to predict the measured float response amplitude. This comparison indicates that linear analysis is an acceptable approach for predicting float response and wave field in the vicinity of the array for a certain range of conditions. Linear wave analysis is subsequently applied to investigate the variation of transmission coefficients with distance inshore of a long array of heaving WECs undergoing free response and with damping specified to optimise power extraction. A method is presented for identifying representative transmission and reflection coefficients such that change in wave energy is equal to energy extraction by the devices. These coefficients are employed to quantify the change in nearshore conditions due to deployment of a long line of wave devices at a site near the East Anglian coastline. Wave conditions are modelled at 12 points along the shoreline over a 140 year period and significant wave height reductions up to 30% were obtained. Importantly, changes in nearshorewave direction are also observed. Analysis using the sediment transport model SCAPE (Soft Cliff and Platform Erosion model) indicates that the introduction of the array reduces both the sediment transport rate and cliff recession rate by an average of 50%.

Dans ce travail de thèse, nous nous sommes intéressés à la modélisation et la simulation de la propagation d'ondes ultrasonores dans l'os cortical. Plus précisément, nous avons étudié et analysé la technique dite des ultrasons quantitatifs (Quantitative Ultrasound, QUS) pour l'évaluation de la qualité du tissu osseux. Il s'agit d'une technique émergente dont l'application aux tissus osseux suscite un intérêt particulier dans la communauté scientifique. Le tissu osseux étant un tissu vivant, il est sujet au vieillissement et à divers pathologies parmi lesquelles on peut citer ostéoporose, ostéomalacie, ostéoporomalacie, ou encore, la maladie dite de Paget. Pour accompagner les soins à prodiguer au tissu osseux, une surveillance de sa qualité s'avère indispensable. Dans ce contexte, les méthodes ultrasonores sont réputées être intéressantes, de par leurs caractères non-invasif, peu coûteux, portable et non-ionisant. Cependant, utiliser des ultrasons dans le cadre de la caractérisation du tissu osseux, suppose une compréhension profonde des différents phénomènes physiques mis en jeu lors de leur propagation. Dans cette optique, notre travail est développé dans la thématique de la modélisation dédiée à la propagation des ondes ultrasonores dans des guides d'ondes multidimensionnels, hétérogènes, anisotropes, et composés de matériaux dont l'hétérogénéité peut être qualifiée d'aléatoire. Une des originalités de cette thèse concerne l'étude des coefficients de réflexion et de transmission et des courbes de dispersion en présence d'incertitudes dues aux propriétés matérielles. Dans une première partie, nous étudions les phénomènes de réflexion/transmission via un modèle tri-couches bidimensionnels prenant en compte les tissus mous et l'hétérogénéité aléatoire du tissu osseux. Nous avons pu analyser l'impact de ces caractéristiques sur les coefficients de réflexion et de transmission. Un gradient de propriétés matérielles de l'os est introduit, et son impact sur les coefficients d'intérêt est examiné. L'aspect modal des ondes est exploré, en étudiant la dispersion des ondes de Lamb. Les résultats obtenus dans une configuration géométrique bidimensionnelle ont permis de discuter l'influence des divers paramètres, en terme de propriétés mécaniques et/ou géométriques, sur la propagation des ondes ultrasonores dans le tissu cortical. Dans une deuxième partie, le modèle est étendu pour une configuration géométrique cylindrique. La discussion est menée afin d'analyser l'influence de la géométrie tridimensionnelle de l'os sur les phénomènes de propagation
In this thesis, we are interested in the modeling and simulation of the propagation of ultrasonic waves in the cortical bone. Precisely, we have studied and analyzed the Quantitative Ultrasound (QUS) technique for the evaluation of the quality of bone tissue. It is an emerging technique those the application to bone tissue arouses particular interest in the scientific community. Since bone tissue is a living tissue, it is subject to aging and various pathologies, such osteoporosis, osteomalacia, osteoporomalacia, or the so-called Paget disease. To assist in therapeutic follow-up of the bone, monitoring of quality of bone tissue is essential. In this context, methods based on QUS technique are deemed to be interesting, due of their non-invasive, inexpensive, portable and non-ionizing characteristics. However, use the ultrasound in the context of characterization of bone tissue, requires a deep understanding of the different physical phenomena involved in their propagation. In this perspective, our work is developed in the modeling theme dedicated to the propagation of ultrasonic waves in multidimensional, heterogeneous, anisotropic waveguides, constituted of materials whose heterogeneity can be qualified as random. One of the originalities of this thesis concerns the study of the reflection and transmission coefficients and the dispersion curves in the presence of uncertainties in the material properties. In a first part, we study the reflection/transmission phenomena via a two-dimensional tri-layer model taking into account the soft tissues and the random heterogeneity of the bone tissue. We analyzed the impact of these characteristics on the reflection and transmission coefficients. A gradient of material properties is introduced, and its effect on the coefficients of interest is examined. The modal aspect of the waves is explored, by studying the dispersion of Lamb waves. The results obtained in a two-dimensional geometrical configuration made it possible to discuss the influence of the various parameters, in terms of mechanical and/or geometric properties, on the propagation of the ultrasonic waves in the cortical tissue. In a second part, the proposed model is extended for a cylindrical geometric configuration. The discussion is carried out in order to analyze the influence of the three-dimensional geometry of the bone on the phenomena of propagation

Cette thèse propose un formalisme général pour modéliser la propagation des ondes acoustiques dans une multicouche composée de toute combinaison de couches liquides, solides élastiques isotropes et poro-élastiques isotropes, la méthode ayant la flexibilité d'être développée pour inclure d'autres natures de couches. Dans un premier lieu, un algorithme stable est développé, basé sur l'approche récursive de la matrice de rigidité, pour modéliser la propagation d'une onde plane incidente sur la multicouche en fonction de son angle d'incidence et de sa fréquence. Cet algorithme fusionne de manière récursive les matrices de rigidité des couches individuelles de la structure en une matrice de rigidité totale et permet ensuite le calcul des coefficients de réflexion et de transmission, ainsi que les composantes de déplacement et de contrainte à l'intérieur de la multicouche pour chaque direction d'incidence des ondes planes. Deuxièmement, pour modéliser la propagation d'un faisceau délimité d'ondes incidentes, la technique du spectre angulaire est utilisée, basée sur la décomposition de ce faisceau en un spectre d'ondes planes se propageant dans des directions différentes. Par la suite, le faisceau d'onde réfléchi dans le milieu d'incidence et le faisceau d'onde transmis dans le milieu de transmission, ainsi que la distribution des champs (composantes de déplacement et de contrainte) à l'intérieur de la multicouche sont obtenus en superposant la contribution de toutes les ondes planes se propageant dans les différentes directions. Comme application numérique, une tri-couche solide-poreuse-solide immergée dans l'eau est simulée. La réflexion et la transmission qui en résultent, ainsi que les composantes de déplacement et de contrainte dans la multicouche, correspondants à l’onde plane incidente et au faisceau limité incident, révèlent la stabilité du procédé et la continuité des déplacements et des contraintes aux interfaces
This thesis proposes a general formalism to model the acoustic wave propagation in a multilayer consisting of any combination of fluid, isotropic elastic solid, and isotropic poroelastic layers, the method having the flexibility to be extended to include other layer-natures. At a first stage, a stable algorithm is developed, based on the recursive stiffness matrix approach, to model the propagation of a plane wave incident on the multilayer as a function of its incidence angle and frequency. This algorithm merges recursively the structureindividual layers stiffness matrices into one total stiffness matrix and allows then the calculation of the reflection and transmission coefficients, as well as the displacement and stress components inside the multilayer for every incident plane wave direction. Secondly, to model the propagation of a bounded incident wave beam, the angular spectrum technique is used which is based on the decomposition of this beam into a spectrum of plane waves traveling in different directions. The corresponding reflected wave beam in the incidence medium, and the transmitted wave beam in the transmission medium, as well as the fields distributions (displacement and stress components) inside the multilayer are obtained by summing the contribution of all the plane waves traveling in different directions. As a numerical application, a three-layered solid-porous-solid structure immersed in water is simulated. The resulting reflection and transmission as well as the displacement and stress components in the multilayer corresponding to both, the incident plane wave in different directions and the incident bounded beam reveal the stability of the method and the continuity of the displacements and stresses at the interfaces

В даній магістерській дисертаційній роботі проведено аналітичне дослідження ультразвукового засобу технологічного контролю поверхневої густини тканин. Проведений аналіз показав, що для забезпечення випуску якісних тканин необхідно проведення оперативного технологічного контролю їх поверхневої густини. В теперішній час застосовуються переважно руйнівні контактні методи контролю поверхневої густини тканин, які засновані на вирізанні та зважуванні зразків тканин, тоді як безконтактні не використовуються хоча мають ряд суттєвих переваг у порівнянні з контактними. Як показав проведений у першому розділі дисертації аналіз, для оперативного технологічного контролю поверхневої густини тканин, доцільним є застосування ультразвукових методів контролю. У другому розділі дисертації розглянуті особливості розповсюдження ультразвукових хвиль в тканинах, які пов’язані з розмірами пор та іншими структурними показниками тканин, які впливають на проходження ультразвукових хвиль скрізь тканину та відбиття від неї. Проведено дослідження проходження ультразвукової хвилі крізь контрольовані тканини з різними розмірами пор і відбиття від них та отримані аналітичні залежності для розрахунку та аналізу взаємодії ультразвукових хвиль з нитками тканин з різними акустичними опорами. Отримано аналітичні залежності, які пов’язують амплітудні співвідношення ультразвукових хвиль як із зміною самих діаметрів ниток основи та утоку, так і безпосередньо з поверхневою густиною тканини. Доведено, що згасанням ультразвукових коливань для більшості тканин можна знехтувати, а вибором співвідношення об’ємної густини тканини та довжиною ультразвукової хвилі в тканині можна знизити вплив згасання на амплітудні співвідношення ультразвукових хвиль. Показано, що при збільшенні тривалості ультразвукового імпульсного сигналу зменшуються амплітудна та фазова похибки в порівнянні з безперервним сигналом. Тому необхідно вибирати тривалість ультразвукового імпульсного сигналу такою, щоб не відбувалось перевідбиттів ультразвукових хвиль від поверхні тканини та поверхонь п’єзоперетворювачів. В третьому проведена розробка ультразвукового засобу технологічного контролю поверхневої густини тканин та його експериментальні дослідження.
In this master's dissertation an analytical study of the ultrasonic means of technological control of tissue surface density. The analysis showed that to ensure the release of quality fabrics it is necessary to carry out operational technological control of their surface density. Currently, mainly destructive contact methods of tissue surface density control are used, which are based on cutting and weighing tissue samples, while non-contact ones are not used, although they have a number of significant advantages over contact ones. As shown by the analysis conducted in the first section of the dissertation, for the operational technological control of tissue surface density, it is advisable to use ultrasonic control methods. The second section of the dissertation discusses the peculiarities of the propagation of ultrasonic waves in tissues, which are related to the pore size and other structural parameters of tissues that affect the passage of ultrasonic waves through the tissue and reflection from it. A study of the passage of ultrasonic waves through controlled tissues with different pore sizes and reflections from them and obtained analytical dependences for the calculation and analysis of the interaction of ultrasonic waves with tissue threads with different acoustic resistances. Analytical dependences are obtained, which relate the amplitude ratios of ultrasonic waves both with the change of the diameters of the warp and weft threads, and directly with the surface density of the fabric. It has been shown that the attenuation of ultrasonic vibrations can be neglected for most tissues, and the choice of the ratio of the bulk density of the tissue and the length of the ultrasonic wave in the fabric can reduce the effect of attenuation on the amplitude ratio of ultrasonic waves. It is shown that as the duration of the ultrasonic pulse signal increases, the amplitude and phase errors decrease in comparison with the continuous signal. Therefore, it is necessary to choose the duration of the ultrasonic pulse signal so that there are no reflections of ultrasonic waves from the surface of the fabric and the surfaces of the piezoelectric transducers. In the third development of ultrasonic means of technological control of surface density of fabrics and its experimental researches is carried out.

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Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
In this work, the transmission line method is explored on the study of the propagation phenomenon in nonhomogeneous walls with finite thickness. It is evaluated the efficiency and applicability of the method, considering materials like gypsum, wood and brick, found in the composition of the structures of walls in question. The results obtained in this work are compared to those available in the literature, for several particular cases. A good agreement is observed, showing that the performed analysis is accurate and efficient in modeling, for instance, the wave propagation through building walls and integrated circuit layers in mobile communication and radar system applications. Later, simulations of resistive sheets devices such as Salisbury screens and Jaumann absorbers and of transmission lines made of metal-insulator-semiconductor (MIS) are made. Thereafter, it is described a study on frequency surface selective structures (FSS). It is proposed the development of devices and microwave integrated circuits (MIC) of such structures, for the accomplishment of experiments. Finally, future works are suggested, for instance, on the development of reflectarrays, frequency selective surfaces with dissimilar elements, and coupled frequency selective surfaces with elements located on different layers
Neste trabalho, ? utilizado o M?todo da Linha de Transmiss?o, para a investiga??o do fen?meno de propaga??o em paredes n?o-homog?neas e de espessura finita. A avalia??o da efici?ncia e aplicabilidade do m?todo da linha de transmiss?o ? realizada, considerando materiais como gesso, madeira e tijolo, encontrados na composi??o das estruturas de paredes em quest?o. Posteriormente, s?o apresentadas simula??es para superf?cies resistivas, como telas de Salisbury e absorvedores Jaumann, e para linhas de transmiss?o do tipo metal-isolante-semicondutor (MIS), al?m do estudo sobre superf?cies seletivas de freq??ncia (FSS). Em seguida, ? proposto o desenvolvimento de dispositivos e circuitos integrados de microondas (MIC) de tais estruturas, para a realiza??o de experimentos. Os resultados obtidos demonstram que a an?lise efetuada neste trabalho ? eficiente e precisa. Para diversas estruturas e aplica??es em circuitos, os valores num?ricos obtidos para os par?metros analisados foram comparados com os valores te?ricos e experimentais, inclusive de outros autores. Nestes casos, observa-se uma excelente concord?ncia. Estes resultados indicam o potencial da t?cnica adotada para a an?lise da propaga??o de ondas eletromagn?ticas atrav?s de estruturas de camadas m?ltiplas, com aplica??es em sistemas de comunica??es m?veis e radar. Finalmente, s?o apresentadas propostas para a realiza??o de trabalhos futuros relacionados, por exemplo, com o desenvolvimento de reflectarrays, superf?cies seletivas de freq??ncia com elementos dissimilares, localizados na mesma interface, e superf?cies seletivas de freq??ncias acopladas, com elementos localizados sobre camadas distintas

In search for quieter engines there is a need for a better understanding of the acoustic properties of engine intake and exhaust system components. Besides mufflers which have the purpose of reducing pressure pulses originating from the internal combustion (IC) engine, there are many components in a modern car exhaust and intake system, e.g., air-filters, coolers, catalytic converters, particulate filters - all having an effect on the pressure pulses or sound field in the system. In this work the focus is on the turbocharged IC-engine where both, sound scattering (reflection and transmission) and sound generation from automotive turbochargers are studied. In addition, sound reflection from an open ended pipe, such as the tailpipe of an IC-engine exhaust is investigated.             Accurate and efficient methods to fully characterize turbochargers by measuring the acoustic two-port have been developed.  Compared to earlier work, a number of modifications are suggested for improving the quality of the results. A study on three different automotive turbochargers is also presented, including data for sound scattering for both the compressor and turbine. The results for the transmission of sound, which is of interest for the ability of a turbocharger to reduce noise coming from the engine, is plotted for all tested cases against a dimensionless frequency scale (Helmholtz-number). This makes it possible to generalize the result in order to draw conclusions about the behavior for any turbocharger.              The sound generation was also studied and three different methods to estimate the sound power are suggested. The methods were used to investigate sound generation at different operating points and identify source mechanisms for a turbocharger compressor.             An accurate method for measuring the reflection of plane acoustic waves from a pipe termination in a duct with hot gas flow has been developed and tested. Representing the acoustical conditions at an exhaust tail-pipe, the data obtained is important for effective modeling of exhaust systems. The experimental results of the reflection coefficient were compared with Munt`s theory on flow duct openings. The measurements were carried out for air jet velocities up to Mach 0.4 and for flow temperatures up to 100°C in order to study temperature effects on the reflection properties. It was concluded, that the experimental results agree well with the Munt theory.

Les matériaux absorbants des ondes électromagnétiques sont des éléments importants pour l'évaluation de nombreux systèmes électroniques militaires mais également civils. Ces matériaux sont utilisés, par exemple, pour la réduction des interférences électromagnétiques (EMI) dans divers composants sans fils, la réduction de la surface équivalente radar (SER) ou comme absorbants à l'intérieur des chambres de mesures. C’est cette dernière application qui est visée par les travaux de cette thèse. L’objectif de mes travaux de thèse est d’optimiser des matériaux absorbants utilisés dans les chambres anéchoïques. La géométrie et la composition du matériau absorbant sont les deux paramètres qui influencent la capacité d’absorption de l’onde électromagnétique par un matériau. Ce seront donc les deux pistes d’optimisation explorés durant cette thèse. Notre but est d’obtenir les absorbants présentant les plus faibles coefficients de réflexion et de transmission, soit une absorption élevée, ceci dans une large bande de fréquence
The electromagnetic absorber materials are important elements for evaluating various electronic and civil systems. These materials are used, for example, for minimizing electromagnetic interferences (EMI) in different wireless components, for minimizing the radar cross section (RCS) or for usage in anechoic chambers. The latter application is the targeted work in this thesis. The objective of this work is to optimize the absorber materials used in anechoic chambers. The geometry and the material composition are the two parameters that influence the absorption of the electromagnetic wave inside the material itself. This are the two topics of optimization explored in this thesis. Our objective is to obtain material absorbers that have low reflection and transmission coefficients and high absorption in a large frequency band

Cette thèse s'intéresse à la modélisation mathématique et à la simulation numérique du transport électronique dans des dispositifs semiconducteurs nanométriques. Différents modèles de transport, destinés à la description des diverses régions d'un transistor MOSFET, sont introduits et analysés. Une attention particulière est portée sur la modélisation des effets quantiques ayant lieu dans ces dispositifs (système auto-consistant de Schrödinger/Poisson avec des conditions aux bords ouvertes)
The present PhD thesis is concerned with the mathematical modelling and the numerical simulation of the electron transport in nanoscale semiconductor devices. Different transport models are introduced and analyzed, aimed to describe the various regions of a MOSFET transistor. We focus our attention particularly on the modelling of quantum effects taking place in such devices (self-consistent Schrödinger-Poisson system with open boundary conditions)

Thesis: Ph. D. in Geophysics, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references.
The aim of seismic tomography is to determine a model of Earth properties that best explain observed seismic data. In practice, the limitations placed on our observations and computational capabilities force us to make a number of decisions about the scales and parameterizations of models, the nature of the data considered, and the approximations to wave propagation that connect the two. This thesis will consider three divergent approaches to seismic tomography spanning different representations of Earth structure at different scales, using different parts of the teleseismic wavefield, and solving the inverse problem with different approximations to the wave equation and different optimization methods. In choosing each of these approaches, we address two major decisions that influence the tomographic process: First, what relative value do we place on an less approximate treatment of wave physics versus the ability to incorporate as much information as possible in our inversion? Second, how can we use novel data to better constrain smooth seismic structure in regions that were previously unresolved? The first project presents a global ray-theoretical P-wave model that encompasses millions of traveltime picks. In this inversion, the addition of data from the dense USArray Transportable Array to global catalog data allows us to image the structure of the Eastern United States with unprecedented resolution and make a robust evaluation of the spatial scales of the heterogeneity. The second project develops a finite frequency approach to turning wave transmission tomography using a computationally efficient one-way wave propagation on curvilinear coordinates. The use of overturning coordinate systems allows for the application of wave equation tomography to phases previously unused in other oneway schemes. The final project presents a novel approach to wave-equation teleseismic reflection tomography using free surface multiples. The use of these multiply reflected phases helps to localize heterogeneity in the model to within layers of Earth structure. This project spans the final two chapters and includes the theoretical developments and an inaugural application to SsPmp data from the Hi-CLIMB array in Tibet.
by Scott A. Burdick.
Ph. D. in Geophysics

This study is concerned with the reflection and transmission of spherical waves at a plane interface between two different media. The phenomenon of the reflection and transmission of spherical waves has been studied by means of analytical methods, numerical computation, and experimental tests. A new integral representation for a spherical wave is obtained by transforming Lamb/Sommerfeld's integral representation. The new integral has no singularity so it allows more accurate numerical integration. A new proof of Lamb/Sommerfeld's integral representation for a spherical wave is presented based on the new integral. By using the new form of solutions for reflected waves and existing solutions for transmitted waves, numerical studies have been carried out to examine. the phenomenon of reflection and transmission. of spherical waves at plane surfaces of discontinuity in material properties. It is shown that the effective critical angle for the total reflection of a spherical wave is greater than that of a plane wave at a hard boundary, and that when the source height increases the effective critical angle for the total reflection of a spherical wave tends to that of a plane wave. It is shown that recent predictions of spherical wave reflection and transmission coefficients greater than 1 at normal incidence under certain condition are probably due to numerical integration error. It also has been found that for spherical wave reflection and transmission, the time average energy flux, normal to a plane parallel to the plane of discontinuity, may locally be in the direction opposite to that of the direction of energy transmission over the plane as a whole. This so-called "backward wave" occurs in an interference between the direct and reflected waves, as well as in a transmitted wave. An indirect test on the theory has been performed to check the pressure field, above a rigid boundary, predicted by the spherical wave theory. Theoretical and experimental results were in good agreement.

A number of experimental and theoretical studies, performed in the field of technical flow duct acoustics are presented in this thesis. The acoustical methods treated are implemented on turbocharged IC-engines and engine gas exchange system components. A new method based on the well-known two-load technique has been developed. The method was applied to characterise the source data of various piston-engines with non-linear behaviour including a 6 cylinder turbo-charged truck diesel engine. The source characterisation results were compared to the results obtained using the linear two-load technique. It was demonstrated that the new non-linear multi-load technique gives improved results when the source is slightly non-linear. The use of active one-port models has been tested to characterize an air terminal device (ATD) as a source of flow generated noise. In order to predict the noise generation at different operating points of the device a scaling law was derived and verified. In the experimentally derived scaling law a flow speed dependence of 3 was found for the narrow band spectra, corresponding to a dipole-like behavior of the source in the plane wave range. The proposed technique was validated successfully and the results indicated a good prediction of in-duct sound generation by the air terminal device. Sound reflection from hot flow duct openings has been investigated experimentally. The reflection coefficient was measured for flow temperatures up to 500 ºC and jet velocities up to 108m/s. The results have been compared with famous Munt’s theory. It was concluded that at low Mach number and Helmholz number cases the results agree well with the Munt’s model. This was the first experimental validation of the theory for hot flow conditions. Experimental procedures to determine the sound transmission through automotive turbo-charger compressors were developed and described in detail. An overview of a unique turbocharger testing facility established at KTH CICERO in Stockholm is given. The facility can be used to measure acoustic two-port data for turbo-compressors. Results from measurements on a passenger car turbo-compressor are presented and the influence of operating conditions on the sound transmission is discussed. Current wave action models developed in CMT for computation of the gas exchange processes in I.C. engines have been implemented to determine the acoustic wave transmission through the turbo- compressor. The models are validated with the experimental data and the results are presented for different operating conditions of a Volvo passenger car turbo-compressor.
QC 20100809

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2007.
Includes bibliographical references (p. 179-189).
This thesis is concerned with the reflection and transmission properties of layered left-handed materials (LHM). In particular, the reflection properties of (LHM) slabs are studied for the Goos-Hanchen (GH) lateral shift phenomenon. We demonstrate a unique GH lateral shift phenomenon, which shows that both positive and negative shifts can be achieved using the same LHM slab configuration. This phenomenon is different from previously established cases where the GH lateral shift can be only negative or only positive when different LHM slab configurations are used. We also show that there exist two distinct cases with this unique phenomenon. One case has two regions of incident angles where the GH lateral shift directions are different, while another case has three regions with alternated GH shift directions. A generalized analytical formulation for analyzing the GH lateral shift direction is provided, which reveals that this unique phenomenon is related to the relative amplitudes of the growing and decaying evanescent waves inside the LHM slabs. The energy flux patterns within LHM slabs are further studied to show the influence of the evanescent waves on the GH shift direction change.
(cont.) Furthermore, the transmission property of LHM slabs are studied on the finite slabs' maging capability. First, the development of the numerical simulation tool - the Finite-Difference Time-Domain method (FDTD) - investigates the ability of the method to model a perfect lens made of a slab of homogeneous LHM. It is shown that because of the frequency dispersive nature of the medium and the time discretization, an inherent mismatch in the constitutive parameters exists between the slab and its surrounding medium. This mismatch in the real part of the permittivity and permeability is found to have the same order of magnitude as the losses typically used in numerical simulations. Hence, when the LHM slab is lossless, this mismatch is shown to be the main factor contributing to the image resolution loss of the slab. In addition, finite-size LHM slabs are studied both analytically and numerically since they have practical importance in the actual experiments. The analytical method is based on Huygens' principles using truncated current sheets that cover only the apertures of the slabs. It is shown that the main effects on the images' spectra due to the size of the slabs can be predicted by the proposed analytical method, which can, therefore, be used as a fast alternative to numerical simulations.
(cont.) Furthermore, the property of negative energy streams at the image plane is also investigated. This unique property is found to be due to the interactions between propagating and evanescent waves and can only occur with LHM slabs, of both finite-size and infinite size. The last part of the thesis deals with multi-layered media for the application to antenna isolations. The setup is with two horn antennas located beneath the ground plane with 10 A distance apart. In order to reduce the coupling between antennas, multi-layered media placed on top of the ground plane need to be designed to suppress the fields. After the problem is simplified to the dipole antenna coupling in infinite slabs, the method to evaluate the fields inside layered media is presented. This method obtains the spectral domain Green's function first and then transforms the fields to the spatial domain using the Sommerfeld-type integration. After the method is validated using right-handed materials (RHM) from references, it is extended to include media like LHM as well as p. negative material and : negative material . The validation with these materials are done by comparing the results with CST microwave studio simulations. The first configuration for the antenna isolation design if one layer slab backed by the grounded plane. Two different approaches are used to find the optimum slab parameters for the isolation.
(cont.) One approach is to use Genetic Algorithm (GA) to optimize the slab's constitutive parameters and the thickness for a minimum coupling level. The other approach is to develop an analytic asymptotic expression for the field, and then used the expression to design the slab parameters for the best isolation. We conclude that both approaches yield the same design for the given configuration. The effectiveness of the design is also validated on a grounded finite slab, which is the representation of the actual application. Finally, multi-layered media for the antenna isolation is studied. GA method is applied with an optimization scheme tailed for a five layered structure. We show that GA converges very fast to the solution and the result yields satisfactory isolation between the antennas.
by Jianbing James Chen.
Ph.D.

[EN] Phononic crystals are artificial materials formed by a periodic arrangement of inclusions embedded into a host medium, where each of them can be solid or fluid. By controlling the geometry and the impedance contrast of its constituent materials, one can control the dispersive properties of waves, giving rise to a huge variety of interesting and fundamental phenomena in the context of wave propagation. When a propagating wave encounters a medium with different physical properties it can be transmitted and reflected in lossless media, but also absorbed if dissipation is taken into account. These fundamental phenomena have been classically explained in the context of homogeneous media, but it has been a subject of increasing interest in the context of periodic structures in recent years as well. This thesis is devoted to the study of different effects found in sonic and phononic crystals associated with transmission, reflection and absorption of waves, as well as the development of a technique for the characterization of its dispersive properties, described by the band structure. We start discussing the control of wave propagation in transmission in conservative systems. Specifically, our interest is to show how sonic crystals can modify the spatial dispersion of propagating waves leading to control the diffractive broadening of sound beams. Making use of the spatial dispersion curves extracted from the analysis of the band structure, we first predict zero and negative diffraction of waves at frequencies close to the band-edge, resulting in collimation and focusing of sound beams in and behind a 3D sonic crystal, and later demonstrate it through experimental measurements. The focusing efficiency of a 3D sonic crystal is limited due to the strong scattering inside the crystal, characteristic of the diffraction regime. To overcome this limitation we consider axisymmetric structures working in the long wavelength regime, as a gradient index lens. In this regime, the scattering is strongly reduced and, in an axisymmetric configuration, the symmetry matching with acoustic sources radiating sound beams increase its efficiency dramatically. Moreover, the homogenization theory can be used to model the structure as an effective medium with effective physical properties, allowing the study of the wave front profile in terms of refraction. We will show the model, design and characterization of an efficient focusing device based on these concepts. Consider now a periodic structure in which one of the parameters of the lattice, such as the lattice constant or the filling fraction, gradually changes along the propagation direction. Chirped crystals represent this concept and are used here to demonstrate a novel mechanism of sound wave enhancement based on a phenomenon known as "soft" reflection. The enhancement is related to a progressive slowing down of the wave as it propagates along the material, which is associated with the group velocity of the local dispersion relation at the planes of the crystal. A model based on the coupled mode theory is proposed to predict and interpret this effect. Two different phenomena are observed here when dealing with dissipation in periodic structures. On one hand, when considering the propagation of in-plane sound waves in a periodic array of absorbing layers, an anomalous decrease in the absorption, combined with a simultaneous increase of reflection and transmission at Bragg frequencies is observed, in contrast to the usual decrease of transmission, characteristic in conservative periodic systems at these frequencies. For a similar layered media, backed now by a rigid reflector, out-of-plane waves impinging the structure from a homogeneous medium will increase dramatically the interaction strength. In other words, the time delay of sound waves inside the periodic system will be considerably increased resulting in an enhanced absorption, for a broadband spectral range.
[ES] Los cristales fonónicos son materiales artificiales formados por una disposición periódica de inclusiones en un medio, pudiendo ambos ser de carácter sólido o fluido. Controlando la geometría y el contraste de impedancias entre los materiales constituyentes se pueden controlar las propiedades dispersivas de las ondas. Cuando una onda propagante se encuentra un medio con diferentes propiedades físicas puede ser transmitida y reflejada, en medios sin pérdidas, pero también absorbida, si la disipación es tenida en cuenta. La presente tesis está dedicada al estudio de diferentes efectos presentes en cristales sónicos y fonónicos relacionados con la transmisión, reflexión y absorción de ondas, así como el desarrollo de una técnica para la caracterización de sus propiedades dispersivas, descritas por la estructura de bandas. En primer lugar, se estudia el control de la propagación de ondas en transmisión en sistemas conservativos. Específicamente, nuestro interés se centra en mostrar cómo los cristales sónicos son capaces de modificar la dispersión espacial de las ondas propagantes, dando lugar al control del ensanchamiento de haces de sonido. Haciendo uso de las curvas de dispersión espacial extraídas del análisis de la estructura de bandas, se predice primero la difracción nula y negativa de ondas a frecuencias cercanas al borde de la banda, resultando en la colimación y focalización de haces acústicos en el interior y detrás de un cristal sónico 3D, y posteriormente se demuestra mediante medidas experimentales. La eficiencia de focalización de un cristal sónico 3D está limitada debido a las múltiples reflexiones existentes en el interior del cristal. Para superar esta limitación se consideran estructuras axisimétricas trabajando en el régimen de longitud de onda larga, como lentes de gradiente de índice. En este régimen, las reflexiones internas se reducen fuertemente y, en configuración axisimétrica, la adaptación de simetría con fuentes acústicas radiando haces de sonido incrementa la eficiencia drásticamente. Además, la teoría de homogenización puede ser empleada para modelar la estructura como un medio efectivo con propiedades físicas efectivas, permitiendo el estudio del frente de ondas en términos refractivos. Se mostrará el modelado, diseño y caracterización de un dispositivo de focalización eficiente basado en los conceptos anteriores. Considérese ahora una estructura periódica en la que uno de los parámetros de la red, sea el paso de red o el factor de llenado, cambia gradualmente a lo largo de la dirección de propagación. Los cristales chirp representan este concepto y son empleados aquí para demostrar un mecanismo novedoso de incremento de la intensidad de la onda sonora basado en un fenómeno conocido como reflexión "suave". Este incremento está relacionado con una ralentización progresiva de la onda conforme se propaga a través del material, asociado con la velocidad de grupo de la relación de dispersión local en los planos del cristal. Un modelo basado en la teoría de modos acoplados es propuesto para predecir e interpretar este efecto. Se observan dos fenómenos diferentes al considerar pérdidas en estructuras periódicas. Por un lado, si se considera la propagación de ondas sonoras en un array periódico de capas absorbentes, cuyo frente de ondas es paralelo a los planos del cristal, se produce una reducción anómala en la absorción combinada con un incremento simultáneo de la reflexión y transmisión a las frecuencias de Bragg, de forma contraria a la habitual reducción de la transmisión, característica de sistemas periódicos conservativos a estas frecuencias. En el caso de la misma estructura laminada en la que se cubre uno de sus lados mediante un reflector rígido, la incidencia de ondas sonoras desde un medio homogéneo, cuyo frente de ondas es perpendicular a los planos del cristal, produce un gran incremento de la fuerza de
[CAT] Els cristalls fonònics són materials artificials formats per una disposició d'inclusions en un medi, ambdós poden ser sòlids o fluids. Controlant la geometría i el contrast d'impedàncies dels seus materials constituents, és poden controlar les propietats dispersives de les ondes, permetent una gran varietatde fenòmens fonamentals interessants en el context de la propagació d'ones. Quan una ona propagant troba un medi amb pèrdues amb propietats físiques diferents es pot transmetre i reflectir, però també absorbida si la dissipació es té en compte. Aquests fenòmens fonamentals s'han explicat clàssicament en el context de medis homogenis, però també ha sigut un tema de creixent interés en el context d'estructures periòdiques en els últims anys. Aquesta tesi doctoral tracta de l'estudi de diferents efectes en cristalls fonònics i sònics lligats a la transmissió, reflexió i absorció d'ones, així com del desenvolupament d'una tècnica de caracterització de les propietats dispersives, descrites mitjançant la estructura de bandes. En primer lloc, s'estudia el control de la propagació ondulatori en transmissió en sistemes conservatius. Més específicament, el nostre interés és mostrar com els cristalls sonors poden modificar la dispersió espacial d'ones propagants donant lloc al control de l'amplària per difracció dels feixos sonors. Mitjançant les corbes dispersió espacial obtingudes de l'anàlisi de l'estructura de bandes, es prediu, en primer lloc, la difracció d'ones zero i negativa a freqüències próximes al final de banda. El resultat és la collimació i focalització de feixos sonors dins i darrere de cristalls de so. Després es mostra amb mesures experimentals. L'eficiència de focalització d'un cristall de so 3D està limitada per la gran dispersió d'ones dins del cristall, que és característic del règim difractiu. Per a superar aquesta limitació, estructures axisimètriques que treballen en el règim de llargues longituds d'ona, i es comporten com a lents de gradient d'índex. En aquest règim, la dispersió es redueix enormement i, en una configuració axisimètrica, a causa de l'acoblament de la simetría amb les fonts acústiques que radien feixos sonors, l'eficiència de radiació s'incrementa significativament. D'altra banda, la teoria d'homogeneïtzació es pot utilitzar per a modelar, dissenyar i caracteritzar un dispositiu eficient de focalització basat en aquests conceptes. Considerem ara una estructura periòdica en la qual un dels seus paràmetres de xarxa, com ara la constant de xarxa o el factor d'ompliment canvia gradualment al llarg de la direcció de propagació. Els cristalls chirped representen aquest concepte i s'utilitzen ací per a demostrar un mecanisme nou d'intensificació d'ones sonores basat en el fenòmen conegut com a reflexió "suau". La intensificació està relacionada amb la alentiment progressiva de l'ona conforme propaga al llarg del material, que està associada amb la velocitat de grup de la relació de dispersió local en els diferents plànols del cristall. Es proposa un model basat en la teoria de modes acoblats per a predir i interpretar este efecte. Dos fenòmens diferents cal destacar quan es tracta d'estructures periòdiques amb dissipació. Per un costat, al considerar la propagació d'ones sonores en el plànol en un array periòdic de capes absorbents, s'observa una disminució anòmala de l'absorció i es combina amb un augment simultani de reflexió i transmissió en les freqüències de Bragg que contrasta amb la usual disminució de transmissió, característica dels sistemes conservatius a eixes freqüències. Per a un medi similar de capes, amb un reflector rígid darrere, les ones fora del pla incidint l'estructura des de un medi homogeni, augmentaran considerablement la interacció. En altres paraules, el retràs temporal de les ones sonores dins del sistema periòdic augmentarà significativament produint un augmen
Cebrecos Ruiz, A. (2015). Transmission, reflection and absorption in Sonic and Phononic Crystals [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/56463
TESIS
Premiado

Characteristic modes (CMs) provide deep insight into the electromagnetic behaviour of any arbitrarily shaped conducting structure because the CMs are unique to the geometry of the object. We exploit this very fact by predicting a perhaps surprising number of important antenna metrics such as resonance frequency, radiation efficiency and antenna Q (bandwidth) without needing to specify a feeding location. In doing so, it is possible to define a collection of objective functions that can be used in an optimizer to shape-synthesize antennas without needing to define a feed location a priori. We denote this novel form of optimization “feedless” or “excitation-free” antenna shape synthesis. Fundamentally, we are allowing the electromagnetics to dictate how the antenna synthesis should proceed and are in no way imposing the physical constraints enforced by fixed feeding structures. This optimization technique is broadly applied to three major areas of antenna research: electrically small antennas, multi-band antennas and reflectarrays. Thus, the scope of applicability ranges from small antennas, to intermediate sizes and concludes with electrically large antenna designs, which is a testament to the broad applicability of characteristic mode theory. Another advantage of feedless electromagnetic shape synthesis is the ability to synthesize antennas whose desirable properties approach the fundamental limits imposed by electromagnetics. As an additional benefit, the feedless optimization technique is shown to have greater computational efficiency than traditional antenna optimization techniques.

Thesis (Ph.D.)--Indiana University, Dept. of Geological Sciences, 2005.
Source: Dissertation Abstracts International, Volume: 66-11, Section: B, page: 5867. Adviser: Gary L. Pavlis. Title from dissertation home page (viewed Oct. 11, 2006).

The work presented within this thesis details the experimental investigation of the surface waves supported on metasurfaces. Particular attention has been given to the reflection of these surface waves from planar discontinuities associated with these metasurfaces. Various experimental techniques have been developed throughout this work to characterise surface wave supporting metasurfaces. These include a new technique for measuring the dispersion of surface waves supported on metasurfaces, characterisation of the near-field associated with the surface waves, a device for launching planar phase front surface waves and finally a technique for measuring the surface wave reflection coefficient. The dispersion of surface waves on a square array of square cross-section metal pillars has been fully characterised and compared to FEM modelling. The results show that a family of surface waves may be supported by pillar or crossed slit structures rather than just holes even though there is now no lowest cut-off frequency. A family of TM surface modes have been shown to exist with dispersions which asymptote to frequencies defined by the pillar heights (slit depth) and the refractive index of the material filling the slits. Primarily this work focussed on the surface wave properties associated with a square array of square metal patches on a dielectric coated ground plane and a Sievenpiper `mushroom' metasurface. The amplitude reflection coefficient of these surface waves has been studied for three distinct systems: Firstly for surface waves incident upon the termination of a these metasurfaces to free space, secondly for surface waves incident upon the interface between a dielectric coated and uncoated metasurface and thirdly for surface waves incident on the boundary between two metaurfaces. The reflection coefficient of surface waves incident upon the termination of the metasurface to free space is found to increase significantly with the confinement of the surface mode. This confinement, and therefore the form of the reflection coefficient, is significantly different for the two metasurfaces considered due to their dispersions. This increase in the reflection coefficient is caused by both the momentum mismatch of the surface wave compared to the freely propagating modes and the different field distributions of the two modes. The reflection coefficient of surface waves incident upon the boundary between a coated and uncoated metasurface has been experimentally characterised for the metal patch array and Sievenpiper `mushroom' metasurfaces. It is shown that the addition of a thin, significantly subwavelength, dielectric overlayer onto the metasurface vastly perturbs the surface wave dispersion. The reflection coefficient of the surface waves is found to depend on the dispersion of the mode supported on the coated and uncoated metasurface and the overlayer thickness. Most noticeably the thickness of the overlayer, by comparison to the surface wave decay length, has a significant effect on scattering to free space associated with the surface wave reflection. The final system considered was designed to investigate the impedance approximation, often used to describe metasurfaces, and found it to be an incomplete description of the surface waves supported on the metasurfaces used within this study. In the impedance approximation the two surfaces considered are said to be `impedance matched` at certain frequencies. It is demonstrated that the failure of the impedance approximation to accurately describe this system is due to the behaviour of the electric field within the metasurfaces. These are not analytically described in the impedance approximation and are required for an accurate description of the surface waves supported on these metasurfaces.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Architecture, 2007.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 194-201).
Advanced fenestration systems are increasingly being used to distribute solar radiation purposefully in buildings. Distribution of visible light and near infrared radiation can be optimized to enhance daylighting and reduce thermal loads. Light redirecting window systems are one of many innovative fenestration systems available for improving the daylighting and thermal performance of buildings. Many emerging and existing light redirecting systems have both spectrally and angularly selective optical properties. To study these properties, a device that measures the spectral, bi-directional transmission and reflection distribution functions of complex fenestration systems is being developed at the Massachusetts Institute of Technology. This device, a goniophotometer, will measure photometric and radiometric BT(R)DFs for radiation of 380 to 1700 nanometer wavelengths, encompassing much of the solar spectrum. The device incorporates spectroradiometrically calibrated digital cameras and absorption filters to gather quasi-spectral information about reflection and transmission by complex fenestration systems. It relies on a half-mirrored, aluminum coated acrylic hemi-ellipsoid to project reflected or transmitted light towards a digital camera.
(cont.) The device will be able to characterize BT(R)DFs for a variety of fenestration system materials, assemblies, and building materials. The goal of this research is to support the development of innovative, spectrally and angularly selective window systems that can improve daylighting and comfort and/or reduce cooling and heating loads in buildings. This thesis focuses on calibrating digital cameras to measure radiances with unknown spectra, developing the hemi-ellipsoid for the new goniophotometer, and developing methods for constructing quasi-spectral BT(R)DFs using this new device. The calibrated cameras also have potential for use in other applications, for example, as radiometers and photometers in rooms with light of known spectra.
by Nicholas Gayeski.
S.M.

The purpose of this case study was to explore and describe the process of reflection as it developed during a school year for one cooperating teacher engaged in the coaching process with two student teachers. My interest was driven by the need currently expressed by researchers (Munby, 1989; Russell, 1988; Schon, 1987; Shulman, 1987; Weiss & Louden, 1989) to extend our understanding of reflection to the work of individual teachers. Using Schon's (1983, 1987) account of reflection as the theoretical framework, this study describes the evolution of one teacher's reflective practice as it occurred within reflective interviews, dialogues with her student teachers, and during classroom instruction, tracing the development and changes that occurred in her perspective on her work before, during, and as a result of events of practice. Anomalies in that process--occasions typified by reversions to earlier stages of development--were examined, and factors of influence on her reflection, perceptions, and actions were determined. This work establishes a general structure of the developmental reflective process of a cooperating teacher and identifies emergent patterns in the way she spoke of her work, interacted with her student teachers, and transmitted knowledge. Patterns of control, patterns in the way that problems were framed, and patterns of the use of metaphors in the language manifested themselves through the teacher's own words which captured her thinking, beliefs, and knowledge of practice. Factors that appeared to influence those patterns include (1) the constraints of time and of the school environment, (2) the cooperating teacher's own personal history and educational experiences, and (3) the cooperating teacher's participation in a clinical faculty project, a program for cooperating teachers that provided the opportunity for reflective interaction and guided teachers through the process of collaborative inquiry and joint experimentation. This description may clarify the notion of reflection, and may help develop principles of good practice and more clear-cut strategies in the coaching process of students and cooperating teachers and in the continuing professional development of experienced teachers.
Ed. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.
Includes bibliographical references (p. 175-185).
The effective use of complex fenestration systems (CFS) in buildings requires a detailed knowledge of their optical spectral and directional properties. Bidirectional scattering functions (BSDFs), either in transmission (BTDFs) or reflection (BRDFs), are used to assess these properties and gather information vital to CFS design and analysis. To enable this analysis in a time and budget conscious manner, an innovative video goniospectrometer, called the Heliodome, has been developed. The Heliodome relies on filtered digital imaging, mathematical methods, and the use of a unique semi-transparent hemispheroidal light collection system to investigate the spectral and angular selectivity of CFS across the visible and near infrared portions of the solar spectrum. This thesis seeks to describe the most recent advancements in the development of the Heliodome-the completion and characterization of the spheroidal mirror component, the integration of a spectral estimation method, the photometric calibration of the camera, and the final automation and validation-- enabling the measurement of both spectral radiometric BSDFs and photometric BSDFs. The completion of this research should support the advancement of CFS that improve the use of daylighting in a space, reducing energy consumption and managing solar gains while improving visual comfort in buildings.
by Eleanor Stokes.
S.M.

Different types of breakwaters have been developed in the past for the protection of valuable coastal property, commercial activity and beach morphology. Among these, gravity-type breakwaters are the most common and provide good surface wave attenuation. However, these breakwaters are not always suitable due to their adverse impact on the coastal environment. To alleviate the problem, free surface breakwaters with a variety of caisson designs have been proposed and developed. The main advantages of such breakwaters are low capital cost, freedom from silting and scouring, short construction period, circulation of water beneath the breakwater and exertion of relatively low hydrodynamic forces on the structure as compared to conventional breakwaters. However, complete tranquillity on the lee side is not likely to occur due to wave energy transfer through the permeable parts of the breakwater. The degree of wave attenuation primarily depends on the configuration of the breakwater, the water depth and the incident wave conditions. The hydrodynamic performance of such free surface breakwaters is the subject of this thesis. Semicircular breakwaters mounted on a low-crested rubble mound structure were successfully built for harbour protection in Japan and China. However, the concept of having semicircular structures as free surface breakwaters has not yet been explored by the research community. As a result, this research is initiated with the aim of developing a free surface semicircular breakwater (SCB) that would serve as an anti-reflection barrier and provide reasonably good wave protection to coastal and marine infrastructures. To meet this research goal, a free surface SCB models were constructed and tested in a wave flume under various wave conditions. The experiments were conducted in three stages. For the first stage, the SCB model was initially tested without any perforations on the curved surface (i.e. a solid SCB) for different depths of immersion from the still water level in the wave flume. For the second stage, the front curved wall of the model was subsequently perforated with rectangular openings of different dimensions, producing front wall porosity of 9, 18 and 27%. Following this, two rows of rectangular openings near the crest of the rear curved wall were provided so as to facilitate water infiltration and escape of the run-up waves. For the third stage, additional effort was made to extend the draft of the breakwater by adding a wave screen at the front or/and rear. The screen porosity was 25, 40 and 50%. The hydrodynamic characteristics of the SCB models were investigated in both regular and irregular seas through a series of systematic experimental programme. The water surface elevations were measured at different locations upstream and downstream of the models to determine the coefficients of wave transmission (CT), reflection (CR) and energy dissipation (CL) as well as the wave climate coefficients in front and inside the breakwater chamber. The horizontal wave forces exerted on the SCB models and the wave screen(s) were also measured and subsequently normalised to yield the force coefficients in the analysis. These hydrodynamic coefficients for the respective test cases are presented and discussed in this thesis. The experimental results revealed that even though the solid SCB was a better wave attenuator than the perforated ones, it produced a considerable amount of wave reflection. The perforated SCB with 9% porosity of the front wall (denoted as SCB9) outperformed the other perforated breakwater models; however, it produced high wave transmission when the draft was limited and subjected to longer period waves. Hence, wave screens were added to further enhance the performance of the SCB9. The SCB9 with double screens of 25% porosity was found to provide the highest hydraulic performance. Empirical equations were developed using a multiple regression technique to provide design formulae for wave transmission, wave reflection and horizontal wave forces. The proposed empirical equations showed good agreement with the experimental data. These equations are intended to be of direct use to engineers in predicting the hydrodynamic performance of free surface SCBs. However, sensible engineering judgement must be taken while using these equations as they are based on small scale laboratory tests.

Une méthode originale de transmission a permis d'obtenir les coefficients d'atténuation. Le phénomène d'auto-atténuation a ensuite été pris en compte par la méthode de Bellini, puis a été validé sur des simulations, sur des objets et finalement sur des patients présentant une pathologie neurologique. Les résultats ont montré une amélioration notable de la quantification des acquisitions obtenues en SPECT.

The acoustical reflection and transmission properties of flat plates in water are of relevance to the behaviour of submerged structures exposed to waterborne sound waves. The reflection and transmission coefficients are generally defined for plane wave incidence. Where the plate is simple and uniform these coefficients may be estimated from its elastic properties. When confirmation of the estimates is required or the plate is complex in its construction then an experimental measurement of these must be made. It is shown that the plane wave reflection and transmission coefficients may be estimated using an arbitrary field incident upon a panel of the material under test. The incident and reflected or transmitted field is mathematically modelled as the sum of a continuum of plane harmonic waves. The amplitude of each component may be found by spatially and temporally sampling the fields, and Fourier Transforming the results. Advantage is taken of the properties of circularly symmetric fields to simplify the three dimensional Fourier Transform to a two dimensional Fourier/Hankel Transform. Simple division of the amplitudes of the transmitted plane components by the amplitudes of the corresponding incident plane components yields the transmission coefficient as a function of wavenumber and frequency. The numerical implementation of a suitable transform is discussed, and the effects of mis-positioning of the measurement hydrophone upon the results considered. Acoustically compact transient sources are shown to be suitable for plate insonification, in that they are wideband in both wavenumber and frequency. Several types of sources are discussed and details of an underwater spark source given. Experimental results are presented for a 16 mm thick and a 32 mm thick aluminium plate. The measured position and amplitude of dominant features of the transmission coefficient, such as the grazing and coincident peaks, agree reasonably well with theory.

Vienas iš efektyviausių medžiagos savybių tyrimo būdų yra praėjusios pro medžiagą ir atsispindėjusios nuo jos šviesos parametrų analizė. Darbe išnagrinėtas šviesos sklidimas anizotropine medžiaga,analizuojami būdai, suskaičiuojantys atsispindėjusios, praėjusios pro anizotropinę plėvelę atspindžio ir pralaidumo matricų elementai.
On purpose about structure of the light, you must to make measurement of light reflections and transmission parameters.In this work we finding following parameters with the help of the program “Mathematica 6”. I with “Mathematica 6 “can find the light reflection from anisotropic media and transmission through anisotropic media slab coefficient by the anisotropic media to matrixs.

La mesure en cuve des coefficients complexes de reflexion r et de transmission t d'un panneau excite en incidence oblique permet de determiner les coefficients de masquage c#m et d'anechoisme c#a des materiaux utilises en acoustique sous-marine. Ces parametres definissent l'utilisation la plus adaptee en acoustique sous-marine de ces materiaux en tant que reflecteurs dur, reflecteurs mous, materiaux de masquage ou anechoiques. La premiere partie de ce travail repose sur la determination theorique en incidence oblique des coefficients c#a et c#m suivant r et t ou suivant les modules mecaniques qui caracterisent le materiau teste. Nous etudions la precision experimentale attendue sur c#a et c#m en utilisant soit des mesures en cuve de r et t, soit des mesures rheologiques a l'aide d'un viscoelasticimetre et d'un k-metre. La seconde partie de l'etude est consacree a l'optimisation des mesures experimentales de r et t en utilisant des traitements holographiques. En pratique, si l'excitation est une unique onde plane d'incidence normale, la reponse de l'antenne, principalement concentree dans le domaine propagatif du spectre, peut perturber la mesure. Cependant, l'utilisation d'une excitation a bande large dans l'espace des nombres d'onde permet de reduire les effets de la diffraction. A cet effet, nous etudions l'utilisation d'une antenne cylindrique calibree en champ proche et qui genere plusieurs ondes planes d'incidences specifiees. L'antenne cylindrique est synthetisee a partir d'une colonne generant une onde cylindrique. Les coefficients de ponderations optimaux sont calcules pour chaque position de la colonne autour du panneau. La methode est ensuite testee avec un panneau dont les coefficients sont prealablement determines a partir de mesures rheologiques. Les resultats sont satisfaisants pour un angle d'incidence n'excedant pas 53. Lieu de 40 avec les methodes holographiques usuelles.

Orientador: Lucio Tunes Dos Santos
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica e Instituto de Geociências
Made available in DSpace on 2018-08-11T01:05:43Z (GMT). No. of bitstreams: 1 Grosfeld_Valeria_D.pdf: 2985169 bytes, checksum: 82ddf2042ee5ce573f870e3deebfa469 (MD5) Previous issue date: 2007
Resumo: Neste trabalho estudamos os coeficientes de reflexão RPP e RPS para ondas elásticas. Introduzimos uma nova aproximação tipo impedância para o coeficiente RPS, baseados no êxito deste tipo de aproximações para o RPP na região de ângulos críticos e pós-críticos. Apesar de não ter-se mostrado tão eficiente quanto a aproximação tipo impedância de reflexão para RPP na região de interesse, se comparamos nossa aproximação com algumas já existentes, o comportamento é um pouco melhor que as aproximações precedentes. Esta análise foi feita mediante uma nova metodología, baseada em curvas de desempenho de algoritmos. Também mostramos que aproximando o coeficiente de reflexão RPP por uma aproximação tipo impedância é possível obter um indicador da presença de hidrocarbonetos sem necessidade de inverter os dados. Por outro lado discutimos como, com algumas hipóteses adicionais, se podem estimar alguns parâmetros elásticos das rochas diretamente dos dados. Por último, analisamos o efeito de parte do processamento sísmico na obtenção de nossas estimações em dados sintéticos
Abstract: In this work we are concerned with the reflection coefficients RPP and RPS for elastic waves. Based on the success of an impedance type approximation for the coefficient RPP in the critical region, we introduce a new approximation for the coefficient RPS. Although the new approximation was not so good as for the previous in the RPP case, comparing our approach with the existing ones we found that its behaviour is a little better than the preceding approaches. This comparison was made using a new methodology based on performace profile curves. Moreover, we show that, using the impedance type approximation for the reflection coeficient RPP , it is possible to get an indicator that reacts to the presence of hydrocarbons without the necessity of the invert the data. We also demostrate that under suitable conditions some elastic parameters of the rocks can be estimated directly from the data. Finally, we analyse the effect of seismic processing in order to obtain our estimate in synthetic data
Doutorado
Reservatórios e Gestão
Doutor em Ciências e Engenharia de Petróleo

This work studies guided wave propagation in pipes coated with thick highly viscoelastic coating materials. The main motivation for this study is the problems associated with Long Range Ultrasonic Testing (LRUT) of coated pipelines. The results reported in the literature show that the proper determination of the optimum LRUT parameters depends strongly on the bulk acoustic properties of the coating materials. The bulk acoustic properties of coating materials reported in the literature show that they could vary significantly depending on the coating material age, temperature and bonding level. The methods for acoustic characterisation of coating materials reported by other researchers, have been studied and it was found that they do not take into account the temperature changes and bonding level variation. In this work, the bulk acoustic properties of two highly viscoelastic bitumen based coating materials are investigated. The conventional methods for acoustic characterisation are studied and a new method for independent measurement of bulk shear properties of bitumen is developed. The bulk acoustic properties of bitumen based coating materials are also studied by two new characterisation methods. The first method derives the bulk coating material properties from experimental data on guided wave reflection coefficients. The second method derives the coating material bulk properties from experimentally measured guided wave attenuation data. It is demonstrated that these new methods deliver much more accurate values for the bulk acoustic properties when compared to the data measured by conventional methods. The second method is used to study the temperature effect on the bulk acoustic properties and it is demonstrated that temperature has a significant effect. The validity of the acoustic properties for the two bitumen materials is investigated through comparison between numerical predictions and experimental data measured for guided wave reflection coefficients and attenuation of the torsional T(0,1) and longitudinal L(0,2) guided wave modes. Good agreement is achieved in the frequency range between 20 kHz and 100 kHz, which is typical for LRUT of pipes.

This study reports on the physical and optical characterization of scandium oxide thin films. Thin films of scandium oxide, 20-40 nm thick, were deposited on silicon wafers, quartz slides, and silicon photodiodes by reactively sputtering scandium in an oxygen environment. These samples were characterized using ellipsometry, high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis. A 28.46 nm thick scandium oxide thin film was measured in the Extreme Ultraviolet (EUV) from 2.7 to 50 nm (459.3 to 24.8 eV) using synchrotron radiation at the Advanced Light Source Beamline 6.3.2 at the Lawrence Berkeley National Laboratory. In these measurements, a new method for data collection was used, in which the reflection and transmission data were collected simultaneously. Analysis of the EUV reflection and transmission data was performed using a front-side reflection, matrix-multiplication technique, which is novel among EUV analytical practice. During data analysis, a new weighting scheme was used, named "adaptive weighting". This analysis provides the first experimentally determined optical constants n and k for scandium oxide thin films from 4.5-30 nm. Also, the positions of the L2 and L3 electronic transitions of scandium oxide have been measured, at 3.069 and 3.101 nm (404.0 and 399.9 eV), respectively, while the measurements near the M transition suggest it to be at approximately 31.5 nm (39.4 eV). Comparing the electronic transition positions of scandium oxide to those of scandium show that the oxidation of scandium shifts the positions to lower energies. For L2 the shift is about 1.8 eV, for L3 the shift is about 1.4 eV, and for M the shift is about 1.9 eV. The binding energies of scandium oxide are greater than those of scandium, as is expected for an oxide compared to its parent metal. This trend in the shift of the transition positions is unexpected, and warrants further investigation.

A fundamental step in the further developments of comprehensive modelling of the diffusive processes in liquids requires the possibility of obtaining reliable and accurate experimental data of the diffusion and thermodiffusion coefficients of multicomponent liquid systems. In the present work, we perform an experimental investigation of the diffusive properties of binary and ternary liquid systems. Two experimental techniques, the ‘Open Ended Capillary’ technique and the ‘Transient Interferometric Technique’ have been developed. Those techniques have been used for the experimental characterization of several systems composed of 1,2, 3,4-Tetrahydrnaphtalene, Isobutylbenzene and Dodecane at ambient temperature. Those particular species were selected as a simplified multicomponent system modelling the fluids contained in natural crude oils reservoirs.

For each of these techniques, experimental set-ups were designed, implemented and calibrated. The procedures for identifying the ternary diffusion coefficients from the measured compositions fields were studied in details.

The Open Ended Capillary Technique was applied under gravity condition to study isothermal diffusion binary and ternary systems. Difficulties related to a new procedure for interpreting the data collected at short times of the experiments are highlighted and its implication in the generalization of the technique for the study of multicomponent systems is discussed.

The Transient Interferometric Technique was used to perform an experimental study of three binary systems under gravity conditions. It was also applied for the investigation of ternary systems under microgravity condition in the frame of the DSC on SODI experiment, which took place aboard the International Space Station in 2011. The experimental results are reported and the analysis of the accuracy of the technique is presented. The TIT is the first technique ever providing accurate experimental measurements of the complete set of diffusion and thermodiffusion coefficients for ternary liquid systems.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Presentation de trois methodes obtenues a partir d'algorithmes classiques pour accroitre la robustesse intrinseque de la quantification vectorielle lorsque les indices sont perturbes par des erreurs isolees

To effectively design systems of microchannels it is necessary for scientists and engineers to understand thermal transport characteristics of microchannels. To experimentally determine the convective heat transfer coefficient of microchannels it is necessary to measure both the bulk and surface temperature fields. This investigation aims to develop a technique, named Total Internal Reflection Fluorescent Micro-Thermometry (TIR-FMT), to measure the temperature of water within several hundred nanometers of a wall--effectively, the surface temperature of the wall. In TIR-FMT, an evanescent-wave is generated in the water near the wall. The intensity of this evanescent-wave decays exponentially with distance from the wall. A fluorophore if illuminated by the evanescent-wave can absorb a photon. Excited fluorophores subsequently emit red-shifted photons, which are called fluorescence. The probability of a fluorescent emission is temperature-dependent. Therefore, by monitoring the intensity of the fluorescence a correlation can be made to the temperature of the region of illumination. Using the TIR-FMT technique the temperature dependence of the fluorescence intensity from buffered fluorescein (pH=9.2) was determined to be 1.35%/C. TIR-FMT can be used to measure the temperature of a fluorophore solution within 600 nm of a wall across a temperature range of 12.5-55C. The average uncertainties (95% confidence) of the temperature measured was determined to be 2.3C and 1.5C for a single 1.5x1.5 and #956;m pixel and the entire 715x950 and #956;m viewfield. By spatial averaging, average uncertainties of 2.0C and 1.8C were attained with spatial resolutions of 16x16 and 100x100 and #956;m, respectively.

Cílem mé diplomové práce bylo studium existujících metod pro detekci a odstranění odlesků ze sekvence snímků, nalezení jejich omezení a návrh možných vylepšení. Konkrétně jsme se zaměřili na rovinné spekulární povrchy, jejichž vzhled může být modelován superpozicí odrážené a přenášené vrstvy. Prozkoumali jsme především metody využívající vzájemný pohyb vrstev jako hlavní klíč k jejich oddělení. Popsali jsme jejich společný případ selhání, který spočívá v neschopnosti správného oddělení oblastí s nevýraznou texturou ve směru pohybu kamery. Výsledkem našeho úsilí je metoda řešící tento problém. Jejím hlavním přínosem je nový způsob odhadu hran obou vrstev, kdy důraz je kladen na správné oddělení hran zmíněných problematických oblastí. Věnovali jsme se také následnému odhadu barev jednotlivých vrstev, kdy se vedle hran vrstev využívá i odhad jejich hloubkových map, a popsali jsme alternativní přístup ke klasické kvadratické optimalizaci.