المؤلفون: Fernández Núñez, Isabel

المساهمون: University/Department: Universitat de Barcelona. Departament de Física Quàntica i Astrofísica

مرشدي الرسالة: Verdaguer Oms, Enric, 1950-, Bulashenko, Oleg, Espriu, D. (Domènec)

المصدر: TDX (Tesis Doctorals en Xarxa)

مصطلحات موضوعية: Forats negres (Astronomia), Agujeros negros (Astronomía), Black holes (Astronomy), Astrofísica, Astrophysics, Difracció, Difracción, Diffraction, Propagació d'ones, Propagación de ondas, Wave propagation, Ciències Experimentals i Matemàtiques

الوصف: In physics, it is common to find different phenomena being described by similar equations. A good analogy can make us look at a problem from a different point of view. In that way, ideas may be transferred from one field of science to another, allowing to model new phenomena after previous, well-studied ones. In the case of the field of analogue gravity, systems that mimic certain aspects of the physics of curved spacetimes are studied. In this thesis, we are interested in the analogy between geometry and media. It has been known for several decades that light propagation in a gravitational field is formally equivalent to that in a bianisotropic medium. On the one hand, ray paths are bent due to spacetime curvature. On the other hand, spatial variations of the permittivity and permeability of a material can make light follow curved trajectories. These two phenomena can be related mathematically in the context of transformation optics, which provides the tools to determine the medium parameters necessary to mimic a certain coordinate transformation. Materials with these specific properties are not naturally occurring, therefore, the emergence of metamaterial science at the beginning of the century was needed to realize them. Metamaterials are artificial composite materials with sub-wavelength constitutive elements that exhibit exotic properties. They have been one of the hot topics of the past years given the variety of opportunities they offer: negative refraction, superlenses, indefinite dispersion, invisibility, among many others. In this thesis we study the analogues of two static spacetimes from the point of view of transformation optics: one with spherical symmetry and one with conical geometry. Both cases are inspired by solutions to Einstein’s equations: the Schwarzschild black hole and the cosmic string, respectively. For each case, we derive the permittivity and permeability of the analogous material using Plebanski’s formulation of the electromagnetic constitutive equations. We solve numerically the wave equation in the metamaterial and compare the results with analytical theories. We find that the spherically symmetric spacetime can be mimicked by either an anisotropic or isotropic medium due to its rotational symmetries. This is achieved by performing a coordinate transformation of the general metric to a conformally flat form. We obtain the medium parameters for both cases and apply the results to the case of the Schwarzschild black hole. We simulate the propagation of a Gaussian beam in the two materials and compare the numerical results with the null-geodesics in the Schwarzschild spacetime, finding a good agreement. The cosmic string is an example of a topological defect with conical geometry. A conical space can be interpreted as flat space with a wedge removed. We make use of this transformation to study the wave equation in the cosmic string background. We apply asymptotic diffraction theories to obtain analytical models that describe wave propagation of electromagnetic or gravitational waves (in a certain gauge). We find that our expressions reproduce accurately the results of the numerical simulations in the analogous metamaterial. Moreover, with our models, we can understand the observed diffraction pattern as the interference of four characteristic waves. With this interpretation we can introduce the Fresnel observation zones, which are related to the diffraction maxima. They help localize the regions – in either space or frequency – where the wave effects are more significant. In fact, in the diffraction by a non-compact object such as the cosmic string, we find that the contribution to the field of wave effects such as interference or diffraction can be of the same order as the geometrical optics terms. Furthermore, the conical topology also appears in condensed matter systems as disclinations or wedge dislocations, therefore we expect our results to be applicable in those systems as well.

الوصف (مترجم): La investigación en gravitación análoga consiste en el estudio de sistemas físicos donde se pueden reproducir algunos de los fenómenos propios de relatividad general. Esta tesis se centra en la conocida propagación análoga de las ondas electromagnéticas en un espacio-tiempo curvado y en un medio con una permitividad y permeabilidad generalmente anisótropas. Para la realización de este tipo de medios, fue necesaria la aparición de los metamateriales, materiales artificiales diseñados para tener propiedades electromagnéticas fuera de lo común. En este contexto, se estudian modelos análogos a dos objetos estáticos con distinta simetría: uno con simetría esférica y otro con topología cónica. Ambos casos están motivados por soluciones a las ecuaciones de Einstein: el agujero negro de Schwarzschild y la cuerda cósmica, respectivamente. A través de las ecuaciones de óptica de transformación, determinamos los parámetros de los medios análogos a estos espacio-tiempos. Estudiamos la propagación de ondas electromagnéticas en los materiales obtenidos mediante simulaciones numéricas y comparamos los resultados con teorías analíticas, encontrando muy buen acuerdo. Por un lado, los contrastamos con las geodésicas en los espacio-tiempos considerados a través del formalismo Hamiltoniano. Por otro lado, desarrollamos modelos analíticos para describir la difracción de una onda (en principio tanto electromagnética como gravitatoria) debido a la cuerda cósmica. Para ello, usamos teorías asintóticas de difracción en un espacio virtual plano con un déficit de ángulo, ya que es una representación equivalente a la geometría cónica de la cuerda. De este modo, se obtienen expresiones que nos permiten explicar detalladamente los fenómenos ondulatorios de interferencia y difracción que se producen en este espacio-tiempo. Observamos que estos efectos pueden ser comparables a los términos de óptica geométrica: añaden una modulación en la amplificación del campo relacionada con la formación de imágenes dobles propia de la topología de la cuerda. Cabe destacar que estos fenómenos son conceptualmente distintos a los que se podrían esperar en la difracción sobre un objeto sólido como una barra. Utilizando nuestros modelos analíticos, obtenemos el patrón de difracción característico de la cuerda cósmica, que podría ser de interés para su detección.

وصف الملف: application/pdf

الوصول الحر: http://hdl.handle.net/10803/663393Test

المؤلفون: Padullés Rulló, Ramon

المساهمون: University/Department: Universitat de Barcelona. Departament de Física Aplicada

مرشدي الرسالة: Cardellach Galí, Estel, Soler i Duffour, M. Rosa

المصدر: TDX (Tesis Doctorals en Xarxa)

مصطلحات موضوعية: Astronomia, Astronomía, Astronomy, Ocultacions, Ocultaciones, Occultations, El·lipsometria, Elipsometría, Ellipsometry, Radioastronomia, Radioastronomía, Radio astronomy, Precipitacions (Meteorologia), Precipitaciones atmosféricas, Precipitations (Meteorology), Propagació d'ones, Propagación de ondas, Wave propagation, Electrodinàmica còsmica, Electrodinámica cósmica, Cosmic electrodynamics, Ciències Experimentals i Matemàtiques`

الوصف: In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric (Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims to capture ROs using a two orthogonal linear polarization antenna. The experiment has been named Radio Occultations and Heavy Precipitation with PAZ (ROHP-PAZ). The objective is to measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced when heavy precipitation flattened raindrops are present in the ray-path. This effect, widely studied in weather radar community, will be measured from space using GNSS signals for the first time with PAZ, which is planned to be launched in 2017. The main objective of this new concept is to enhance the RO capabilities by providing vertical precipitation information along with the current standard RO thermodynamic products (i.e. temperature, pressure and moisture). Until now, no other observing system has been able to provide simultaneous thermodynamic and precipitation information under extreme conditions. The high vertical resolution, global coverage and all-weather capability properties of the RO observations combined with vertical indication of precipitation intensity can be of great value for heavy rain characterization, and therefore for climate and weather forecast and research. The theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. The theoretical basis has been developed combining electromagnetic propagation theory and cloud and precipitation microphysics. Forward scattering simulations at L-band have been obtained in order to relate the microphysics parameters with the expected Pol-RO observables. The feasibility has been addressed using coincident (in space and time) RO profiles and space-based precipitation observations. Such simultaneous observations allow for the characterization of actual RO measurements according to the coincident precipitation information. Finally, the applications have been investigated through realistic end-to-end simulations of the Pol-RO observations, which provide the anticipated Pol-RO products for different precipitation situations, regions, and seasons. Before the launch of the satellite, a field campaign has been conducted with the aim of starting the characterization of the polarimetric measurements. The engineering model of the PAZ antenna was placed at the top of a mountain peak in order to capture, for the first time, linear polarimetric GNSS signals at low grazing angle. This campaign has been useful to start identifying the hardware internal effects and unexpected precipitation features that will be affecting the Pol-RO observations. These effects have been incorporated to the simulations, hence providing valuable feedback to obtain more realistic Pol-RO products. Besides feedback, the data from the field campaign have shown the first observational evidence that precipitation and other hydrometeors induce a noticeable effect on the GNSS polarimetric signals. All these exercises yielded several relevant results. The noise level analysis from actual RO observations sensing precipitation scenarios has allowed to set a detectability threshold for the technique, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. Nevertheless, the integrated nature of the Pol-RO observable does not allow to distinguish between the contributions from the rain's intensity and extension, leaving an ambiguity in the provided product. In an attempt to solve such ambiguity, a tomographic approach has been proposed, which has yielded promising theoretical results. Moreover, it has been shown how the Pol-RO observables can be linked to physical precipitation parameters, such as the along-ray averaged rain rate, in a probabilistic way. The end-to-end simulation has also revealed that the ionosphere will induce a non-negligible depolarization, that will require calibration. Finally, the collocated data has shown the potential applications for Pol-ROs products.

الوصف (مترجم): Obtenir mesures simultànies de l’estat termodinàmic de l’atmosfera i de precipitació ha esdevingut un repte per la comunitat científica. Les missions espacials dedicades a obtenir perfils termodinàmics de l’atmosfera tenen problemes amb la presència de núvols gruixuts, ja que el medi esdevé opac a la radiació infraroja (que és la banda de l’espectre electromagnètic en la qual operen). Alternativament, es poden utilitzar radiosondes. Les radiosondes obtenen perfils termodinàmics de l’atmosfera amb molt alta resolució vertical, però tenen l’inconvenient que el seu llançament necessita certa infraestructura, i per tant les zones més remotes en queden al marge. Això inclou pràcticament la totalitat dels mars i oceans, i moltes zones sub-desenvolupades. Per tant, moltes de les zones amb precipitació extrema no poden ser caracteritzades amb aquesta tècnica. A més a més, la resolució temporal acostuma a ser molt baixa, ja que no se’n poden llençar moltes al dia degut a l’elevat cost econòmic que suposaria. Per altra banda, els radars que mesuren les estructures en tres dimensions de la precipitació no tenen la capacitat d’obtenir perfils de temperatura o pressió. Les estacions meteorològiques, que poden ser molt nombroses en segons quins territoris, estan limitades a mesures en superfície, i altra vegada, mars, oceans i regions sub-desenvolupades en queden al marge. Amb tot, les Radio Ocultacions Polarimètriques emergeixen com una tècnica a tenir en compte a l’hora de caracteritzar precipitació extrema. La seva cobertura global, alta resolució vertical i la capacitat de penetrar en núvols i precipitació la fa una tècnica molt atractiva en aquest sentit. Cada cop més estudis científics coincideixen en apuntar un augment en la freqüència d’aquests fenòmens extrems, i una caracterització acurada és necessària per millorar els models de predicció.

وصف الملف: application/pdf

الوصول الحر: http://hdl.handle.net/10803/404380Test

المؤلفون: Bergadà Caramés, Pau

المساهمون: University/Department: Universitat Ramon Llull. EALS - Electrònica

مرشدي الرسالة: Regué, Joan Ramon, Pijoan Vidal, Joan Lluís

المصدر: TDX (Tesis Doctorals en Xarxa)

مصطلحات موضوعية: High Frequency, Ionosphere, Digital communications, Direct Sequence Spread Spectrum, Sky-wave propagation, Orthogonal Frequency Division Multiplexing, Multipath spread, Doppler spread, Signal to Noise Ratio, Enginyeria i Arquitectura

الوقت: 621.3

الوصف: El sistema de comunicació ràdio d’alta freqüència (HF, en anglès) és usat arreu del món per agències governamentals i no governamentals sempre que calgui una alternativa a les comunicacions via satèl•lit: vaixells a alta mar, avions fora de cobertura de xarxes ràdio amb visió directa, operacions militars, zones on la infraestructura ha estat destruïda per algun tipus de desastre o bé zones llunyanes sense cap altre tipus de comunicació. La ràdio HF representa una alternativa, o un sistema de backup al satèl•lit per a comunicacions de llarg abast i en redueix els costos, evita la vulnerabilitat i els problemes de sobirania. En aquesta tesi s’ha estudiat l’enllaç HF entre la base antàrtica espanyola Juan Carlos I, situada a l’illa Livingston a l’arxipèlag de les Shetland del Sud, i Espanya. L’objectiu d’aquest treball és estudiar els problemes que afecten la propagació; és a dir, la relació senyal a soroll i interferència, la dispersió multicamí i la dispersió per efecte Doppler, i dissenyar la capa física d’un enllaç HF de baixa velocitat, poca potència i llarg abast. Pel que fa aquest últim punt es fan un parell de propostes: espectre eixamplat per seqüència directa (DSSS, en anglès) i multiplexació per divisió en freqüència ortogonal (OFDM, en anglès). El repte que es planteja és el de la definició de les característiques dels símbols que millor encaixen en aquest canal per tal d’obtenir un benefici de la diversitat temporal i freqüencial que ofereix el canal. Des de l’any 2003 diverses campanyes han permès estudiar aquest canal HF, però no va ser fins la campanya 2009/2010 que s’obtingué un foto de les característiques, diürnes i nocturnes, de la ionosfera. En els articles que es presenten en aquesta tesi hem estès el rang freqüencial d’estudi respecte a investigacions prèvies i hem mostrat diferències de comportament entre el dia i la nit. Hem usat els resultats de la caracterització del canal per a dissenyar i comparar la bondat dels símbols DSSS i OFDM. Ambdues possibilitats han resultat ser candidates a implementar l’enllaç HF entre l’Antàrtida i Espanya. Tot i així, ambdues tècniques representen visions diferents de la implementació del mòdem: mentre que DSSS obté bons resultats a baixa velocitat en entorns amb baixa relació senyal a soroll, OFDM aconsegueix tasses de velocitat més elevades en canals més benignes.

الوصف (مترجم): Los sistemas de radio de alta frecuencia (HF, en inglés) son usados por agencias gubernamentales y no gubernamentales en todo el mundo siempre que se necesite una alternativa a las comunicaciones por satélite: barcos en alta mar, aviones fuera del rango de cobertura de las redes radio de visión directa, operaciones militares, zonas donde la infraestructura ha sido destruida por algún desastre. Ésta ofrece una alternativa, o representa un sistema de backup, a las comunicaciones vía satélite, evitando los costes, la vulnerabilidad y los problemas de soberanía de las comunicaciones por satélite. En esta tesis se ha estudiado el enlace HF entre la base antártica española Juan Carlos I en la isla Livingston, en las Shetland del sur y España. El objetivo de este trabajo es el estudio de las limitaciones de la propagación ionosférica (como la relación señal a ruido e interferencia, la dispersión multicamino y la dispersión por efecto Doppler) y el diseño de la capa física de un enlace HF de baja velocidad, baja potencia y largo alcance. Se han estudiado un par de propuestas para este enlace, como son el espectro ensanchado por secuencia directa (DSSS, en inglés) y la multiplexación por división en frecuencia ortogonal (OFDM, en inglés). El reto ha sido definir las características que mejor se adecuan a este enlace para poder aprovechar la diversidad temporal y frecuencial que ofrece el canal HF. Desde el año 2003 diversas campañas de sondeo han permitido estudiar el canal HF pero no es hasta la campaña 2009/2010 que se consigue una fotografía de la actividad ionosférica tanto nocturna como diurna. En los artículos que se presentan en esta tesis hemos extendido los estudios previos a todo el rango de frecuencias HF y hemos mostrado las diferencias entre el día y la noche. Hemos usado estos resultados de caracterización del canal para diseñar y comparar símbolos DSSS y símbolos OFDM. Ambas posibilidades han resultado ser posibles candidatas para implementar un enlace HF de baja velocidad entre la Antártida y España. Sin embargo ambas técnicas representan dos aproximaciones distintas a la implementación del módem. Mientras que DSSS consigue un buen funcionamiento a baja velocidad en escenarios con baja relación señal a ruido, OFDM consigue tasas de transmisión más altas en escenarios más benignos.
High Frequency (HF) radio is used by governmental and non nongovernmental agencies worldwide whenever an alternative to satellites for sky wave communication is needed: ships at sea, aircraft out of range of line-of-sight radio networks, military operations, disaster areas with communication infrastructure destroyed or distant regions lacking other communications. It offers an alternative to satellites, or a backup, for long-haul communications, thus avoiding the costs, vulnerabilities and sovereignty concerns of satellite communications. In this thesis the HF link between the Antarctic Spanish Station Juan Carlos I in Livingston Island, South Shetlands and Spain is studied. The aim of this study is to address the impairments that affect HF propagation (i.e., signal-to-noise plus interference ratio, multipath and Doppler shift and spread) and to design the physical layer of a low rate, low power and long-haul HF link. Some proposals regarding this last issue are addressed, i.e., direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM). The challenge is to define the symbol characteristics that best fit the link to benefit from time and frequency diversity that offers the HF channel. Since 2003 several transmission campaigns have allowed to study the HF channel but it is not until the 2009/2010 campaign that we have achieved a whole picture of both diurnal and nocturnal ionospheric activity. In the papers presented in this thesis we have extended the previous research to the whole range of HF frequencies and we have shown the differences on performance between day and night. We have used the results from channel characterization to design and compare the performance of DSSS and OFDM symbols. Both techniques have turned out to be possible candidates to implement a low rate HF link between Antarctica and Spain. However, both techniques stand for different approaches of the modem: DSSS achieves good performance at low data rate in low SNR scenarios, whereas OFDM achieves higher data rate in benign channels

وصف الملف: application/pdf

الوصول الحر: http://hdl.handle.net/10803/285837Test

المؤلفون: Yarahmadi, Somayeh

مرشدي الرسالة: Electrical Engineering, Mili, Lamine M., von Spakovsky, Michael R., Centeno, Virgilio A., Liu, Chen-Ching, Boker, Almuatazbellah M.

مصطلحات موضوعية: Electromechanical wave propagation, Non-homogeneous EMW modeling, Disturbance propagation, Rotor angle instability, Dynamic stability analysis

الوصف: When a power system is subjected to a disturbance, the power flow changes, leading to deviations in the synchronous generator rotor angles. The rotor angle deviations propagate as electromechanical waves (EMWs) throughout the power system. These waves became observable since the development of synchrophasor measurement instruments. The speed of EMW propagation is hundreds of miles per second, much less than the electromagnetic wave propagation speed, which is the speed of light. Recently, with the development of renewable energy resources and a growth in using HVDC and FACTS devices, these waves are propagating slower, and their impacts are more considerable and complicated. The protection system needs a control system that can take suitable action based on local measurements to overcome the results of power system faults. Therefore, the dynamic behavior of power systems should be properly observed. The EMW propagation in the literature was studied using assumptions such as constant voltage throughout the entire power system and zero resistances and equal series reactances for the transmission lines. Although these assumptions help simplify the power system study model, the model cannot capture the entire power system's dynamic behaviors, since these assumptions are unrealistic. This research will develop an accurate model for EMW propagation when the system is facing a disturbance using a continuum model. The model includes a novel inertia distribution. It also investigates the impacts of voltage changes in the power system on EMW behaviors and when these impacts are negligible. Furthermore, the impacts of the internal reactances of synchronous generators and the resistances of transmission lines on EMW propagation are explored.
Doctor of Philosophy
Power systems, essential for electricity supply, undergo disturbances causing changes in power flow and synchronous generator behavior. These disturbances create electromechanical waves (EMWs) that influence system dynamics. Recent advancements, including renewable energy integration and new technologies, alter EMW behavior, posing challenges for control and protection systems. Existing studies simplify models, limiting their accuracy. This research aims to develop a realistic EMW propagation model considering factors like novel inertia distribution, voltage changes, and internal generator properties. This work addresses the evolving power landscape, enhancing our understanding of power system dynamics for improved control and reliability.

وصف الملف: ETD; application/pdf

الإتاحة: https://hdl.handle.net/10919/117327Test

المؤلفون: O'Kane, Aisling

المساهمون: Copley, Alexander

مصطلحات موضوعية: Earthquake Ground Motions, Earthquake Hazards, Foreland Basins, Himalayas, Mountains, Seismic-Wave-Propagation Modelling, Seismology, Tectonics

الوصف: The regions adjacent to tectonically active mountain belts are exposed to significant earthquake hazard, since the range-bounding faults produce large earthquakes, and the underlying geological structure amplifies the resulting ground shaking. The aim of this dissertation is to investigate the regional-scale controls on earthquake ground motions and seismic hazard in these settings. The first part of this dissertation describes models of the seismic wavefield produced by thrust-faulting earthquakes on mountain range fronts. The earthquake source characteristics and foreland basin structure were varied within reasonable geological bounds, and the earthquake-induced ground shaking was calculated. The earthquake source parameters were determined to be the dominant control on the amount of near-source ground shaking. However, the foreland basin structure, in particular the basin depth relative to the dominant wavelength of the seismic waves, determines the importance of dispersion as the waves propagate through the basin. These results highlight the importance of accurately determining earthquake source characteristics (particularly depth), and the underlying geological structure, during hazard assessment. These principles were then applied to study the active tectonics and seismic hazard in the north-west Himalayas. Field, satellite, and seismological observations were used to determine the fault geometry beneath the NW Himalayas and investigate the relationship between thrust faulting and folding. These results were used to construct seismic-wavefield models, to determine earthquake ground motion estimates if the Main Himalayan Thrust in the region were to rupture. These models show that peak ground velocities are extremely sensitive to minor variations in the fault geometry. Finally, the earthquake-induced building damage in foreland basins was investigated. Using seismic-wavefield modelling, alongside fragility curves for generic building types, the relationships between earthquake location, characteristics, and building damage were investigated. The results quantify the previously poorly known trade-off between earthquake location and magnitude in determining damage distributions. Additionally, the results quantify the factors that can cause over- or under-estimates of the magnitudes of historical earthquakes based on reported damage distributions, with important implications for understanding the accumulated slip deficit in continental collision zones.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.883740Test

المؤلفون: Garcia Neefjes, Erik

المساهمون: Parnell, William, Assier, Raphael

مصطلحات موضوعية: Visco-thermal losses, Fluid-structure interaction, Scholte waves, Stress relaxation, Acoustics, Wave propagation, Dissipation, Elasticity, Viscosity

الوصف: This thesis is focused around the theoretical study of attenuation of acoustic and elastic waves due to viscous and thermal effects. The initial focus is on fluid acoustic media, where we employ the well known theory of linear thermo-visco-acoustics (TVA) to study the influence of boundary layer effects on the propagation of sound in narrow channels filled by air and water. In the latter case, the effects of fluid-structure interaction are taken into account by assuming the neighbouring solid is elastic, but only acoustically hard solids are analysed. On an attempt to generalise the type of media in consideration, the possible advantages arising from the development of a theory for thermo-visco-elasticity (TVE) in this context are noticed. We propose a TVE framework which incorporates more general material behaviour such as creep compliance and stress relaxation, and can be reduced to several other physically relevant theories like TVA for Newtonian fluids, in a way that we can accurately study a diverse class of materials ranging from metals and polymers to air and water in a large number of conditions. As for TVA fluids, TVE media accept three families of modes in free-space, namely two coupled thermo-compressional waves and a shear wave whose phase speed and attenuation differ significantly depending on the specific material. Accurate asymptotic approximations to thermo-compressional coupling are provided which highly simplify the initial expressions for the wavenumbers. We consider a canonical scattering problem consisting of a compressional plane wave incident on two TVE half-spaces in perfect contact, where the thermo-viscous effects on reflection/transmissions and conveniences of the developed framework as opposed to standard approaches in the literature are highlighted. We make use of the above framework to extend the initial study by examining fluid-filled slits within soft viscoelastic media, which we find gives rise to very different results to those obtained for hard solids in the initial work. We show that this can partly be attributed to the properties of the Scholte mode which propagates in the interface of a fluid-solid half-space and is analysed thoroughly. Particular emphasis is put on how the stress relaxation effects can influence the results, which we find to be significant under certain conditions that are discussed in detail. Furthermore, given the generality of the framework, we can analyse the problem of fluid-loaded viscoelastic plates under the same set of dispersion equations obtained for the slit. In particular, we find that for sufficiently soft media so that the phase speed of the symmetric coupled plate-Scholte mode becomes dispersive, the mode experiences a global maximum in attenuation which may be of physical interest, particularly if stress relaxation can be exploited.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.854463Test

المؤلفون: Crawford, Alasdair Ruairidh

المساهمون: Droubi, M. G., Faisal, N. H.

مصطلحات موضوعية: Acoustic emission, Adhesively-bonded joints, Defect detection, Wave propagation

الوصف: The aim of this study is to investigate the application of acoustic emission (AE) techniques to the defect detection and monitoring of adhesively-bonded joints. Pencil Lead Breaks (PLBs) have been used as a simulated AE source to experimentally investigate the characteristics of AE wave-propagation in adhesively-bonded joints, and have been combined with Artificial Neural Networks (ANNs) to provide a novel method of defect detection and sizing. Modal AE analysis has been applied to destructive testing of adhesively-bonded specimens as a novel method to differentiate between fracture modes. Dynamic Finite Element Analysis (FEA) has been utilised to simulate the AE generation and propagation to further investigate the findings of the experimental studies and to assess the applicability of the findings over a broader range of conditions than could be achieved experimentally. PLB tests have been conducted on large (500mm x 500mm x 1mm) aluminium sheet specimens to identify the effects of an adhesive layer on AE wave-propagation. Three specimens were considered: a single sheet; two sheets placed together without adhesive; and an adhesively-bonded specimen. The simulated AE source was applied to the specimens at varying propagation distances and orientations. The acquired signals were processed using wavelet transforms to explore time-frequency features, and compared with modified group velocity curves based on the Rayleigh-Lamb equations to allow identification of wave modes and edge reflections. The effects of propagation distance and source orientation were investigated, while comparison has been made between the three specimens. PLB tests were also used to detect, size and investigate the effect of void-type adhesive defects. Defect-free specimens were used for reference, and specimens with two different void sizes were tested. The PLB source was used to generate simulated AE which would propagate through the defect region and then be recorded with the AE system. Four configurations were tested to assess the effects of source-sensor propagation distance, and source and sensor proximity to the defect. Typical AE parameters of peak amplitude, rise time, decay time, duration, number of counts and AE energy were investigated. Frequency analyses by Fast Fourier Transformation (FFT), partial powers and wavelet transform (WT) were also implemented. ANNs, using the raw Time-Domain signal as an input, were successfully trained and tested to differentiate between the tested specimen types and to estimate the defect sizes. AE-instrumented Double Cantilever-Beam (Mode-I fracture) and Lap-Shear (Mode-II fracture) tests were conducted on similar adhesively-bonded aluminium specimens. Linear source location was used to identify the source-to-sensor propagation distance of each recorded hit. Theoretical dispersion curves were used to identify regions of the signal corresponding to the symmetric and asymmetric wave-modes. Additionally, peak wavelet transform coefficients for the wave modes were compared between the two fracture modes and assessed as an indicator of fracture mode. It was concluded that there is a relationship between the fracture mode and the generated wave modes, with Mode-II fracture typically generating a relatively greater symmetric wave mode than Mode-I fracture. Dynamic FEA was used to replicate both the PLB tests and the destructive tests, and to investigate the effects of a range of parameters that could not all be practically varied in experimental work. Adhesive Young's modulus (representative of different adhesive types), adhesive layer thickness and adhesive void size were varied in the simulated PLB tests. FEA was also used to investigate the effects of fracture mode on the generated acoustic emissions in simulated mixed mode-bending tests, conducted over a range of mode mixities. The FEA results were found to corroborate the results of the experimental work and support a relationship between fracture mode and generated wave modes. It was also identified that a variety of other parameters may also affect the wave modes, and thus need to be considered to achieve effective use of modal analysis to differentiate between fracture modes.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.851571Test

المؤلفون: Haindl, Claudia

المساهمون: Nissen-Meyer, Tarje

مصطلحات موضوعية: Earthquake hazard analysis, Seismic prospecting, Equations--Numerical solutions, California--Hayward Fault, Seismic wave propagation

الوصف: Owing to continual improvements in computation power, we are getting closer to modelling seismic waves in ways that reflect the realistic complexity of the Earth. Nevertheless, modelling the true multi-scale structure of the Earth at large scale and/or high frequencies is currently still beyond our reach from a computational cost perspective. Hence, we rely on efficient modelling software to increase the scope of seismic simulations. Azimuthal Complexity Adaptation (ACA) is an innovative way of reducing the computational cost of seismic simulations by exploiting inherent sparsity of the wavefield. In this thesis, AxiSEM3D, an ACA-based modelling solver, is expanded to model wave propagation in local-scale settings. I find that the sparsity of wavefields persists in the presence of complex local-scale structures such as thrust faults and salt bodies and in models with full anisotropy. Further, I develop 3D wavefield scanning to locate complex areas of the wavefield and to investigate how they relate to structures in the subsurface model. The results suggest that high wavefield complexity is typically localised near abrupt wavespeed contrasts, on the low-wavespeed side thereof. I attempt to develop a new modelling solver which overcomes some of the limitations of AxiSEM3D. The resulting method is unstable, but it demonstrates the algorithmic complexity required to perform ACA in settings like coast lines. I conclude that a hybrid method which combines classic 3D modelling and ACA may be a simpler solution for increasing the scope of AxiSEM3D. Finally, I implement fault ruptures to simulate an earthquake scenario in the San Francisco Bay Area. Using ACA, I can include low-wavespeed bay muds which are usually neglected to save computational cost. Neglecting these muds leads to an underestimate of the shaking in the San Francisco Bay, even at low frequencies.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.860092Test

المؤلفون: Caçoilo, Andreia Patrícia Gandarinho

المساهمون: Teixeira-Dias, Filipe, Rush, David

مصطلحات موضوعية: 620.8, protective shelters, safety requirements, shelter configuration, sea containers, blast waves, shock waves, confined spaces, blast wave propagation, pressure history profile, structural analysis, blast loading, pressure-impulse diagrams

الوصف: The safety of both military personnel and equipment in unstable regions has for a long time been a major issue and concern. Protective shelters with multiple configurations have been widely used to meet safety requirements. Since military compounds are subjected to di↵erent types of threats, such as explosive devices, it is of utmost importance a good understanding of the response of such shielding structures to blast waves. Accordingly, propagation of shock waves in partially- or fully-confined environments is a complex phenomenon due to the possibility of multiple reflections, di↵ractions and superposition of waves. Yet, being able to derive valid predictions of such phenomena is highly relevant, e.g. when it comes to the assessment of protection of personnel. This study looks at the propagation of blast waves in confined spaces and its action on structures, such as compound survival shelters. Whilst full-scale tests o↵er useful insights, the time and expenses associated with such experiments renders then unpracticable. Small-scale experimental models in combination with the Hopkinson- Cranz scaling laws, however, represent a viable alternative to the study of blast wave evolution. The experimental set-up was designed as a rigid structure and built to have a geometrical reduction factor across all dimensions. Experimental analyses were performed on a small-scale model of the actual configuration of the compound survival shelter subjected to the detonation of an explosive charge at di↵erent locations close to its entrance. Pressure-time signals were recorded on a number of locations in the model and a numerical model, based on the explicit finite element code LS-DYNA, was also developed to complement the experimental programme. The recorded experimental data were compared with the numerical predictions to validate the FE model. The proposed numerical model predicts and captures the relevant stages of the propagation of the shock wave. The study of blast wave propagation, which is di↵erent from the propagation of a shock wave in free-field scenarios, is not completely described in literature, especially when it comes to structural response. A numerical analysis of a single corrugated member was performed to evaluate the influence of several wave related parameters on its structural response, e.g. impulse, multiple positive and negative pressure profiles and signal simplifications. Results indicate that the negative impulse train in the pressure-time history plays a significant role in obtaining an accurate performance of the structure. It was also found that a complex pressure history profile can be reduced to a simplified pulse for structural analysis purposes. The consequence of blast events, namely terrorist attacks, warfare scenarios or accidental explosions, usually means severe damage of structures and loss of life. Pressure-impulse diagrams are widely used as a rapid and intuitive tool to investigate the blast response of structural elements under a number of di↵erent blast scenarios. In this study, a numerical model of a 20 ft steel ISO container was developed using LS-DYNA and the accuracy of its response to blast loading is verified against experimental full-scale test data available in the literature. The results show a good agreement between the experimental data and numerical results. Pressure-impulse diagrams were also derived to correlate the damage criteria under di↵erent blast loading scenarios.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.799046Test

المؤلفون: Abdullahi, Mustapha

المساهمون: Oyadiji, Sunday

مصطلحات موضوعية: 621.8, FEA, Acoustic Reflectometry, Time of Flight, CFD, Computational Fluid Dynamics, Finite Element Analysis, Pipeline integrity testing, Non-Destructive Testing, Acoustic, Pipeline damage, Leakage detection, Blockage detection, Acoustic Wave Propagation, NDT

الوصف: Pipelines are considered to be the safest and most economical way of transporting liquid and gas over a long distance. They are used for transporting crude oil and natural gas in process industries, for water transportation and distribution systems in urban and rural areas, for transporting aviation fuel and hydraulic piping as well as air pressure chambers in aircraft. Leaking pipelines can lead to spillage of hydrocarbon fluid into the surrounding environment, and consequently result in fire or outright explosion of the pipeline system as well as production loss, damage to the environment and potential loss of lives. Therefore, accurately identifying and predicting the location and growth of defects are essential and enable the operator to assess and actively manage the integrity of the pipelines. Current leakage detection methods use several non-destructive testing methods such as magnetic particle inspection, radiographic, ultrasonic, pressure transient and acoustic signal methods. This thesis presents acoustic wave propagation-based methods to identify leakage and blockage in pipe systems. In the study, the acoustic finite-element analysis (AFEA) method is employed to simulate acoustic wave propagation in fluid-filled pipes with leakage and blockage but without flow. Computational fluid dynamics (CFD) analysis was also employed to simulate acoustic wave propagation in fluid-filled pipes with leakage and blockage under flow conditions. In addition, experimental testing was conducted to validate some of the numerical results. The experiments consisted of the measurement of acoustic wave propagation in an air-filled pipe with leakage and blockage. The main investigations and contributions of the thesis are summarised as follows, 1) An efficient finite element analysis (FEA) procedure to simulate acoustic wave propagation in fluid-filled pipes is developed. The developed technique overcomes the large errors that occur in the numerical predictions of acoustic wave propagation. The use of different acoustic element types of 2D-like and 3D formulations, and of linear and quadratic interpolations, are investigated in order to deduce the most accurate acoustic FEA approach, and thereby, to minimise the errors. 2) The CFD technique is applied to simulate acoustic wave propagation and reflectometry in a fluid-filled pipe with flow. This is a novel approach of using CFD to predict the acoustic wave propagation in a pipeline in the presence of flow. 3) AFEA and CFD were employed to simulate acoustic wave propagation and reflectometry in a fluid-filled pipe with and without flow. The Time of Flight approach was used on the predicted acoustic waveforms for the pipes with leakage in order to identify the presence of leakage and to predict the size and location of the leakage in the fluid-filled pipe in the presence of flow. 4) Detection of blockage from the predicted data obtained from the CFD and FEA simulation of acoustic wave propagation/reflection in air-filled pipelines without mean flow. Also, the CFD technique was also used to simulate the acoustic wave propagation/reflection in air-filled pipelines with mean flow. Furthermore, the Time of Flight approach was used to identify size and locate the blockage from the simulated data. 5) The experimental measurements of acoustic wave propagation in a straight air-filled intact pipe and air-filled pipes with leakage. The Time of Flight approach was employed to identify size and locate the blockage from the simulated data. Furthermore, the measured and simulated results are compared to validate the simulated results.

الوصول الحر: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.816305Test