Dissertations / Theses on the topic 'High frequency gravity waves'

Small-scale structure of the thermosphere is studied at high-latitudes for its important role in ion-neutral coupling. Four Fabry-Perot Interferometers (FPIs) in Scandinavia are primarily used. These are supplemented by a range of other instruments, including the Spectrograph Imaging Facility, radars, magnetometers, all-sky cameras, and satellite data. The FPIs measure the atomic oxygen emission line at 6300 A, from 240 km altitude. Emission intensities, thermospheric line of sight wind speeds, and neutral temperatures are obtained. Comparisons of electron densities from tomography data and EISCAT (European Incoherent SCATter) radar with FPI intensities allow the investigation whether dissociative recombination is the dominant production mechanisms of the nighttime 6300 A oxygen line. Cross correlations indicate that the thermosphere varies on short temporal scales. Altitude variations have less effect due to the broad (-50 km) emission height band. Atmospheric gravity waves in the thermosphere have been detected for the first time in ground-based FPI data using Lomb-Scargle analysis. Joule heating from electrojet currents, and particle precipitation in the auroral oval, have been identified as the primary source mechanisms using two case study nights. High time resolution data shows a limit to the variability of the thermosphere to be approximately 1-minute. Statistical studies of the gravity waves from 567 nights of FPI data show that the length of the night and time resolution are the most important influences on the number and periods of waves detected. Greater numbers of short period waves are detected in the rapidly responding intensities than in the winds and temperatures. Little variation with geomagnetic activity or solar cycle is observed. Periods at particular harmonics of the length of the night are preferred between October and February. Comparisons of mainland and Svalbard data show that the shorter period waves that are formed equatorward in the auroral oval mostly dissipate before reaching Svalbard.

The aim of this research is to investigate the feasibility of using stress waves for condition monitoring of gears. The project involved setting up an experimental rig, carrying out experimental work, acquiring stress waves signatures, and processing the signals. It has been shown that stress waves can successfully be employed for early detection of incipient gear failure. A experimental gearbox was employed during the experiments. Miniature ultrasound transducers, both sensitive and sufficiently small, were manufactured and installed on the stationary outer race of the rolling element bearing of the gearbox to detect stress waves from the meshing gears. The stress waves signals from the transducers were digitised and digitally processed to extract relevant information. The signatures were high-pass filtered at a cut-off frequency of 200 kHz, thus representing exclusive ultrasonic frequencies. A new statistical parameter, Energy Index, was developed and performed on the stress wave signatures which were segmented to represent individual gear teeth. Along with this new parameter, the classical statistical parameters, (Peaks, RMS, Standard Deviation, Kurtosis, etc.) were also performed. Conclusive results are presented in graphical form in terms of Cumulative Energy Indices' and Energy Indices in polar form for individual gear teeth. A new algorithrn was also developed and presented for the envelope detection of signal by iterative peak detection. Although no direct comparison was made between condition monitoring of gears using stress waves and methods such as low frequency vibration analysis and wear debris analysis, it is apparent that stress waves monitoring offers a much earlier warning of incipient gear failure because the technique can detect material defonnations which are precursors to changes in the dynamic properties of gears and the occurrence of wear debris. The technique, therefore, can predict incipient failure much earlier, extending the lead-time before failure, and as a result, minimising sudden failures which may have catastrophic consequences.

The aim of this research is to investigate the feasibility of using stress waves for condition monitoring of gears. The project involved setting up an experimental rig, carrying out experimental work, acquiring stress waves signatures, and processing the signals. It has been shown that stress waves can successfully be employed for early detection of incipient gear failure. A experimental gearbox was employed during the experiments. Miniature ultrasound transducers, both sensitive and sufficiently small, were manufactured and installed on the stationary outer race of the rolling element bearing of the gearbox to detect stress waves from the meshing gears. The stress waves signals from the transducers were digitised and digitally processed to extract relevant information. The signatures were high-pass filtered at a cut-off frequency of 200 kHz, thus representing exclusive ultrasonic frequencies. A new statistical parameter, Energy Index, was developed and performed on the stress wave signatures which were segmented to represent individual gear teeth. Along with this new parameter, the classical statistical parameters, (Peaks, RMS, Standard Deviation, Kurtosis, etc.) were also performed. Conclusive results are presented in graphical form in terms of Cumulative Energy Indices' and Energy Indices in polar form for individual gear teeth. A new algorithrn was also developed and presented for the envelope detection of signal by iterative peak detection. Although no direct comparison was made between condition monitoring of gears using stress waves and methods such as low frequency vibration analysis and wear debris analysis, it is apparent that stress waves monitoring offers a much earlier warning of incipient gear failure because the technique can detect material defonnations which are precursors to changes in the dynamic properties of gears and the occurrence of wear debris. The technique, therefore, can predict incipient failure much earlier, extending the lead-time before failure, and as a result, minimising sudden failures which may have catastrophic consequences.

An important property of the Earth's atmosphere is its ability to support wave motions, and indeed, waves exist throughout the Earth's atmosphere at all times and all locations. What is the importance of these waves? Imagine standing on the beach as water waves come crashing into you. In this case, the waves transport energy and momentum to you, knocking you off balance. Similarly, waves in the atmosphere crash, known as breaking, but what do they crash into? They crash into the atmosphere knocking the atmosphere off balance in terms of the winds and temperatures. Although the Earth's atmosphere is full of waves, they cannot be observed directly; however, their effects on the atmosphere can be observed. Waves can be detected in the winds and temperatures, as mentioned above, but also in pressure and density. In this dissertation, three different studies of waves, known as gravity waves, were performed at three different locations. For these studies, we investigate the size of the waves and in which direction they move. Using specialized cameras, gravity waves were observed in the middle atmosphere (50-70 miles up) over Alaska (for three winter times) and Norway (for one winter time). A third study investigated gravity waves at a much higher altitude (70 miles on up) using radar data from Alaska (for three years). These studies have provided important new information on these waves and how they move through the atmosphere. This in turn helps to understand in which direction these waves are crashing into the atmosphere and therefore, which direction the energy and momentum are going. Studies such as these help to better forecast weather and climate.

The main focus of this thesis is on fast numerical methods, where adaptivity is an important mechanism to lowering the methods' complexity. The application of the methods are in the areas of wireless communication, antenna design, radar signature computation, noise prediction, medical ultrasonography, crystal growth, flame propagation, wave propagation, seismology, geometrical optics and image processing.   We first consider high frequency wave propagation problems with a variable speed function in one dimension, modeled by the Helmholtz equation. One significant difficulty of standard numerical methods for such problems is that the wave length is very short compared to the computational domain and many discretization points are needed to resolve the solution. The computational cost, thus grows algebraically with the frequency w. For scattering problems with impenetrable scatterer in homogeneous media, new methods have recently been derived with a provably lower cost in terms of w. In this thesis, we suggest and analyze a fast numerical method for the one dimensional Helmholtz equation with variable speed function (variable media) that is based on wave-splitting. The Helmholtz equation is split into two one-way wave equations which are then solved iteratively for a given tolerance. We show rigorously that the algorithm is convergent, and that the computational cost depends only weakly on the frequency for fixed accuracy.  We next consider interface tracking problems where the interface moves by a velocity field that does not depend on the interface itself. We derive fast adaptive  numerical methods for such problems. Adaptivity makes methods robust in the sense that they can handle a large class of problems, including problems with expanding interface and problems where the interface has corners. They are based on a multiresolution representation of the interface, i.e. the interface is represented hierarchically by wavelet vectors corresponding to increasingly detailed meshes. The complexity of standard numerical methods for interface tracking, where the interface is described by marker points, is O(N/dt), where N is the number of marker points on the interface and dt is the time step. The methods that we develop in this thesis have O(dt^(-1)log N) computational cost for the same order of accuracy in dt. In the adaptive version, the cost is O(tol^(-1/p)log N), where tol is some given tolerance and p is the order of the numerical method for ordinary differential equations that is used for time advection of the interface.   Finally, we consider time-dependent Hamilton-Jacobi equations with convex Hamiltonians. We suggest a numerical method that is computationally efficient and accurate. It is based on a reformulation of the equation as a front tracking problem, which is solved with the fast interface tracking methods together with a post-processing step.  The complexity of standard numerical methods for such problems is O(dt^(-(d+1))) in d dimensions, where dt is the time step. The complexity of our method is reduced to O(dt^(-d)|log dt|) or even to O(dt^(-d)).<br><p>QC 20121116</p>

Devices were developed to explore the perturbation of graphene using high-frequency signals. Two kinds of effects were studied: the interaction of graphene with surface acoustic waves (SAWs) and the propagation of picosecond pulses. The coupling of graphene with SAWs was first studied using flip-chip devices, which employed an evanescent field extending across the gap between the chips. A later design directly integrated layers of graphene grown by chemical vapour deposition (CVD) on piezoelectric 5ubstrates, containing interdigital transducers (IDTs) for SAW generation and detection. Graphene devices were characterised using Raman spectroscopy and atomic force microscopy; the electronic properties of CVD-graphene were investigated by performing magnetoresistance measurements. The measurements of the acoustically-generated current in the direct-coupling devices closely followed the SAW response of the IDTs, demonstrating the acousto-electric effect in graphene for the first time . . In a second class of devices, graphene was transferred to a quartz substrate, and, using on-chip waveguides, integrated with photoconductive switches capable of generating and detecting sub-picosecond pulses, which allow studying THz-frequency transmission in the system. Pulses containing frequency components of up to 2.5 THz were generated in these devices. The demonstration of the acousto-electric effect in graphene paves the way for SAW-based charge manipulation in graphene, such as singleelectron transport. The propagation of picosecond pulses in graphene could be used to further investigate the properties of graphene in the terahertz-frequency range.

En metod för att beräkna energin hos ett högfrekvent vågfält inuti en begränsad tvådimensionell domän presenteras. Metoden är baserad på att approximera energin medelst strålföljning. Strålarna startar i en punktkälla och reflekteras (perfekt) vid domänens gräns. Energin i en del av domänen är summan av energierna i de strålar som passerar igenom den. Metoden har implementerats i datorprogrammet MATLAB. Numeriska experiment har gjorts för olika fall. Bland dessa är ett enkelt fall där den korrekta lösningen är känd och det har konstaterats att metoden då konvergerar mot denna rätt lösning. Resultat för ett fall där den korrekta lösningen är okänd har jämförts med en simulering av Helmholtz ekvation med hjälp av programmet COMSOL Multiphysics.<br>A method for computing the energy of the high frequency wave field inside a bounded two-dimensional domain is presented. The method is based on approximating the energy using ray tracing. The rays originate from a point source and are reflected (perfectly) at the boundary. The energy in a part of the domain is the sum of the energies in the rays passing through it. The method has been implemented in the computing program MATLAB. Numerical experiments have been made for various cases. Among these is a simple case where the correct solution is known and it has been verified that the method converges towards this correct solution. Results for a case where the correct solution is unknown have been compared with a simulation of Helmholtz equation using the program COMSOL Multiphysics.

2011/2012<br>Il presente lavoro di tesi presenta diverse applicazioni e sviluppi del metodo modale: in particolare, immaginando un percorso logico dalle basse alle alte frequenze, si sono considerate le onde di tsunami, onde sismiche propagantesi in strutture a forte eterogeneità laterale, fino a giungere all’applicazione del metodo al dominio acustico. Nel primo capitolo l’argomento trat- tato è quello degli tsunami (onde di gravità a lungo periodo), estendendo la tecnica modale agli tsunami e mostrando come questa metodologia consenta diversi approcci alla modellazione sia delle eterogeneità del modello strutturale che del processo di rottura sismico che genera l’onda di tsunami. Queste tecniche sono state applicate in diversi contesti, sia al fine di validarne i rela- tivi codici di calcolo (Mar Mediterraneo Occidentale, terremoto e tsunami di Tohoku del marzo 2011) che per valutare scenari di pericolosità da tsunami per le zone considerate (costa del Viet- nam, Mare Adriatico). L’argomento del secondo capitolo è la modellazione di onde sismiche che si propagano in modelli a forte eterogeneità laterale, in particolare con interfacce verticali liquido-solido. Dopo una breve introduzione teorica, vengono mostrati i risultati di alcuni test di modellazione che utilizzano vari modelli strutturali e diversi domini di frequenza (in parti- colare si arriverà a una frequenza massima di 10Hz). Quindi, andando a frequenze sempre più alte, fino a 100Hz, il pacchetto di programmi sviluppato originariamente per le onde sismiche è stato testato e, dopo gli opportuni miglioramenti, utilizzato per studiare la propagazione di onde acustiche in strutture oceaniche. Il fine di questi test, argomento del terzo e conclusivo capitolo di questo lavoro, è capire la sensibilità delle onde acustiche alle caratteristiche della struttura in cui si propagano, come ad esempio la presenza di strati solidi sedimentari a bassa veloc- ità o di un canale a bassa velocità nel liquido. Nel secondo e nel terzo capitolo lo studio dei segnali ottenuti è stata approfondita mediante un’analisi congiunta tempo-frequenza (FTAN) che ha notevolmente aiutato a capirne le caratteristiche salienti, correlate sia alla sorgente che alla struttura, confermando ulteriormente la validità e la poliedricità dell’approccio modale in questo genere di studi.<br>In this thesis work, we presents several applications and developments of the modal method: in particular, imagining a logical path from low to high frequencies, are considered the tsunami waves, seismic waves propagating in structures with strong lateral heterogeneity, to conclude with the application of the method to the acoustic range. The topic treated in the first chapter is the extension of the modal technique to the tsunami wavefield (long period gravity waves), showing how this methodology allows different approaches to the modeling of both heteroge- neous structural models and of the seismic rupture process that generates the tsunami. These techniques have been applied in different contexts, both in order to validate the computer codes (Western Mediterranean Sea, the Tohoku earthquake and tsunami of March 2011) and to as- sess tsunami hazard scenarios for the selected target zones (Vietnam Coasts, Adriatic Sea). The topic of the second chapter is the modeling of seismic waves propagating in strong laterally het- erogeneous models, in particular with vertical liquid-solid interfaces. After a short theoretical introduction, we shows the results of some modelling tests adopting several structural models and different frequency ranges (in particular reaching a maximum frequency of 10 Hz). Thus, going to higher frequencies, up to 100Hz, the package of programs originally developed for the seismic waves has been tested and, after appropriate improvements, used to study the propa- gation of acoustic waves in the oceanic structures. The purpose of these tests, subject of the third and final chapter of this work, is to understand the sensitivity of the acoustic waves to the characteristics of the structure in which they propagate, such as the presence of low velocity sedimentary layers or a low velocity channel in the liquid. In the second and third chapter, the study of the computed signals was complemented through a joint time-frequency analisys (FTAN) which greatly helped to understand their salient features, related to both the source and the structure, further confirming the validity and the polyhedric nature of the modal approach.<br>XXV Ciclo<br>1978

The solar atmosphere is a highly dynamic environment in which a huge diversity of waves and instabilities has been detected. The matter in that region is in plasma state, and thus is affected by the presence of electromagnetic fields. To understand its dynamics, a theory that combines the equations describing the properties and evolution of fluids with those for electric and magnetic fields is required. Among the several available alternatives that fulfill the mentioned conditions, ideal magnetohydrodynamics (MHD) is a useful description when the phenomena of interest are associated with low frequencies. For long temporal scales, all the species that compose a plasma are strongly coupled and they can be treated as a single fluid. However, when the temporal scales are shorter, the coupling is weaker and collisions between the different species produce a deviation on the properties of waves from those predicted by ideal MHD. Consequently, a more complex and accurate theory is needed. In this Thesis, a multi-fluid theory that takes into account the effects of ion-neutral collisions, Coulomb collisions and magnetic diffusivity, and makes use of a generalized Ohm's law that includes Hall's term is presented. Then, it is applied to the investigation of waves and instabilities in several layers and structures of the solar atmosphere, such as the fully ionized solar corona and solar wind, and the partially ionized chromosphere and quiescent prominences or filaments. By means of numerical simulations and the analysis of the dispersion relation for small-amplitude transverse perturbations, the impact of collisions on the properties of the low-frequency Alfvén waves and the high-frequency ion-cyclotron and whistler modes is studied. It is shown that the damping caused by collisional friction is dominated by the ion-neutral interaction at low frequencies and by Coulomb collisions and magnetic diffusivity at high frequencies. Moreover, the cut-off regions and resonances that the ion-cyclotron waves have in collisionless fluids are removed when collisions are taken into account. It is also demonstrated that the consideration of Hall's term in the induction equation is fundamental for the proper description of high-frequency waves in weakly ionized plasmas. Non-linear effects, such as heating, and perturbations of large-amplitude are also studied. On the one hand, it is shown that the ponderomotive force generated by non-linear Alfvén waves, which induces variations of density and pressure of the plasma, is greatly affected by the interaction of ions with neutrals. On the other hand, friction due to collisions causes dissipation of the energy of the perturbations. A fraction of that energy is transformed into heat and rises the temperature of the fluid. In this way, the plasma in quiescent prominences or in the chromosphere may be heated by ion-neutral collisions. Finally, the effect of shear flows at the interface between two partially ionized media are also investigated. The presence of a shear flow velocity leads to the development of the Kelvin-Helmholtz instability. Here, the onset of such instability is studied for partially ionized magnetic flux tubes and an application to cylindrical filament threads is given. It is found that the collisional coupling between ions and neutrals reduces the growth rates of the instability for sub-Alfvénic shear flows but cannot completely suppress it, which means that partially ionized plasmas are unstable for any value of the shear flow. The comparison of the analytical results with observations performed by other authors show that, for a range of parameters of the perturbations, the computed growth rates are compatible with the typical lifetimes of threads.<br>La atmósfera solar es un ambiente altamente dinámico en el que se ha detectado una gran variedad de ondas e inestabilidades. La materia en tal región se encuentra en estado de plasma, por lo que es afectada por la presencia de campos electromagnéticos. Para comprender su dinámica, se requiere una teoría que combine las ecuaciones que describen las propiedades y evolución de los fluidos con las de los campos eléctricos y magnéticos. Entre las diferentes alternativas disponibles que cumplen las condiciones mencionadas, la magnetohidrodinámica (MHD) ideal es una descripción útil cuando los fenómenos de interés están asociados a frecuencias bajas. Para escalas temporales largas, las especies componentes del plasma están fuertemente acopladas y pueden ser tratadas como un fluido único. Para escalas temporales más cortas, el acoplamiento es más débil y las colisiones entre las distintas especies producen un desvío en las propiedades de las ondas respecto a las predichas por la MHD ideal. Consecuentemente, se necesita una teoría más compleja y precisa. En esta Tesis se presenta una teoría multi-fluido que tiene en cuenta los efectos de las colisiones ión-neutro, las colisiones de Coulomb y la difusividad magnética, y usa una ley de Ohm generalizada que incluye el término de Hall. Tal teoría es luego aplicada a la investigación de ondas e inestabilidades en varias capas y estructuras de la atmósfera solar, como la corona y el viento solar, que están completamente ionizados, y la cromosfera y protuberancias, que se hayan parcialmente ionizadas. Mediante simulaciones numéricas y el análisis de la relación de dispersión para perturbaciones transversales de pequeña amplitud, se estudia el impacto que las colisiones tienen en las propiedades de las ondas de Alfvén, de baja frecuencia, y los modos ión-ciclotrón y \textit{whistler}, de alta frecuencia. El atenuamiento causado por la fricción debida a las colisiones está dominado por la interacción ión-neutro a bajas frecuencias y por las colisiones de Coulomb y la difusividad magnética a altas frecuencias. Además, las regiones de corte y resonancias que las ondas ión-ciclotrón tienen en fluidos sin colisiones desaparecen cuando éstas son tenidas en cuenta. También se muestra que la inclusión del término de Hall es fundamental para describir correctamente las ondas de alta frecuencia en plasmas débilmente ionizados. También se estudian efectos no lineales, como el calentamiento, y perturbaciones de gran amplitud. Por una parte, se demuestra que la fuerza ponderomotriz generada por ondas de Alfvén no lineales, que causan variaciones en la densidad y presión del plasma, es fuertemente afectada por la interacción de iones con neutros. Por otra, la fricción debida a colisiones causa la disipación de la energía de las perturbaciones. Una fracción de esa energía es transformada en calor y aumenta la temperatura del fluido. Así, el plasma en una protuberancia quiescente o en la cromosfera puede ser calentado mediante las colisiones ión-neutro. Finalmente, también se investiga el efecto de flujos de cizalladura en la interfaz entre dos medios parcialmente ionizados. La presencia de dichos flujos lleva al desarrollo de la inestabilidad de Kelvin-Helmholtz. Aquí, se estudia la fase inicial de dicha inestabilidad, con la aplicación al caso particular de hilos cilíndricos de filamentos solares. El acoplamiento mediante colisiones entre iones y neutros reduce los ritmos de crecimiento de la inestabilidad para flujos sub-Alfvénicos pero no evita por completo su aparición, lo que significa que los plasmas parcialmente ionizados son inestables para cualquier valor del flujo de cizalladura. La comparación de los resultados analíticos con observaciones realizadas por otros autores muestra que, para un rango de parametros de las perturbaciones, los ritmos de crecimiento calculados son compatibles con la vida media típica de los hilos.<br>L'atmosfera solar és un ambient altament dinàmic en el que s'ha detectat una gran varietat d'ones i inestabilitats. La matèria en aquesta regió es troba en estat de plasma, i per tant es veu afectada per la presència de camps electromagnètics. Per comprendre la seva dinàmica es requereix una teoria que combini les equacions que descriuen les propietats i l'evolució dels fluids amb les del camps elèctrics i magnètics. De les diverses alternatives disponibles que compleixen els requeriments anteriorment citats, la magnetohidrodinàmica (MHD) ideal és una descripció útil quan els fenòmens d'interès estan associats a freqüències baixes. Per escales temporals llargues, les espècies que componen el plasma es troben fortament acoblades i poden ser tractades com a un únic fluid. Pel contrari, quan les escales temporals són més curtes, l'acoblament és més feble i les col·lisions entre les distintes espècies produeixen desviacions en les propietats de les ones respecte a les esperades en MHD ideal. En conseqüència, és necessàri una teoria més complexa i precisa. En aquesta Tesi es presenta una teoria multi-fluid que té en compte els efectes de les col·lisions ió-neutre, les col·lisions de Coulomb i la difusivitat magnètica, i utilitza una llei d'Ohm generalitzada que inclou el terme de Hall. Aquesta teoria s'aplica a la invesigació d'ones i inestabilitats en diverses capes i estructures de l'atmosfera solar, com són la corona i el vent solar, que estan completament ionitzats, i la cromosfera i protuberàncies, que es troben parcialment ionitzats. Mitjançat les simulaciones numèriques i l'anàlisi de la relació de dispersió per pertorbacions transversals de petita amplitud, s'estudia l'impacte que les col·lisions tenen en les propietats de les ones d'Alfvén, de baixa freqüència i els modes ió-ciclotró i \textit{whistler}, d'alta freqüència. L'atenuació produïda per la fricció deguda a les col·lisions està dominada per la interacció ió-neutre a baixes freqüències i per les col·lisions de Coulomb i la difusivitat magnètica a altes freqüències. A més, les regions de tall i ressonàncies que les ones ió-ciclotró tenen en els fluids sense col·lisions desapareixen quan aquestes s'inclouen al model. També s'ha trobat que l'efecte del terme de Hall és fonamental per descriure correctament les ones d'alta freqüència en plasmes dèbilment ionitzats. També s'estudien efectes no lineals, com és l'escalfament, i pertorbacions de gran amplitud. Per una banda, se demostra que la força ponderomotriu generada per ones d'Alfvén no lineals, que causen variacions en la densitat i pressió del plasma, està fortament afectada per la interacció del ions amb els neutres. Per altra banda, la fricció deguda a les col·lisions causa la dissipació de l'energia de les pertorbacions. Una fracció d'aquesta energia és transformada en calor i augmenta la temperatura del fluid. D'aquesta manera, el plasma en una protuberància quiescent o en la cromosfera pot ser escalfat mitjançant les col·lisions ió-neutre. Finalment, també s'investiga l'efecte d'un flux amb cisalladura en l'interfase entre dos medis parcialment ionitzats. La presència del flux dona lloc al desenvolupament de l'inestabilitat de Kelvin-Helmholtz. Aquí, s'estudia la fase inicial d'aquesta inestabilitat, aplicada al cas particular de fils cilíndrics en filaments solars. L'acoblament a través de les col·lisions entre ions i neutres redueix el ritme de creixement de l'inestabilitat per fluxos sub-Alfvénics però no evita per complet la seva aparició, el que significa que els plasmes parcialment ionitzats són inestables per qualsevol valor del flux de cisalladura. La comparació dels resultats analítics amb observacions realitzades per altres autors mostra que, per un rang de paràmetres de les pertorbacions, els ritmes de creixement calculats són compatibles amb la vida mitja típica dels fils a protuberàncies.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.<br>Includes bibliographical references (p. [85]-87).<br>Traditionally, the high frequency components of earthquake loading are disregarded as a source of structural damage because of their small energy content and because their frequency is too high to resonate with the natural frequencies of structures. This thesis argues that higher frequency waves travelling through stiff masonry structures can trigger two types of failure mechanisms that have not yet been taken into account. First, the high frequencies can cause small vertical inter-stone vibrations that result in irreversible relative displacements of the stones, which may ultimately lead to collapse. The energy needed to cause this deformation and failure comes largely from gravitational forces. The second failure mechanism is associated with the increase of the outward thrust that results from the partial fluidization and densification of the loose granular inner core of some unreinforced masonry walls. Preliminary results of a series of static and dynamic tests, as well as of numerical models, demonstrate the potentially destructive effects of high frequency/low energy seismic waves on unreinforced masonry structures.<br>by Patrik K. Meyer.<br>S.M.

Les risques d'éboulements doivent être évalués et surveillés afin de prévenir les pertes de vies humaines et dommages aux infrastructures. A cet égard, il est important de créer des catalogues d'événements et de comprendre la dynamique des éboulements. Les ondes sismiques peuvent être utiles à cette fin, car elles transmettent des informations précieuses sur l'événement. Elles sont générées lorsque des éboulements touchent le sol et peuvent être utilisées pour détecter, classer et localiser des événements. Plus encore, on peut déduire des propriétés des éboulements telles que leur volumes et leur comportement dynamique. Cependant, les signaux sismiques hautes fréquences (&gt;1Hz) sont mal compris. En effet, ils sont associés à des sources sismiques complexes qui sont réparties dans l'espace et peuvent varier rapidement dans le temps. De plus, les ondes sismiques hautes fréquences sont susceptibles d'être diffusées et diffractées en raison des interactions avec les hétérogénéités du sol ou la topographie de surface. Cette thèse franchit une étape importante dans la compréhension des signaux sismiques hautes fré-quences des éboulements en simulant la propagation des ondes sismiques en utilisant la méthode des éléments spectraux (SEM) avec des profils de vitesse réalistes et des topographies de surface 3D. L'influence de la topographie sur le champ des ondes sismiques est étudiée. On constate que l'ampli-fication induite par la topographie est sensiblement différente entre les sources situées en profondeurs et celles situées en surface. En effet, les ondes de surface générées par des sources peu profondes sont exposées à une diffusion et à une diffraction constantes lorsqu'elles se déplacent le long de la surface. La désintégration de l'énergie le long de la surface est étudiée pour différents modèles de vitesse et des équations sont dérivées pour calculer rétroactivement l'énergie sismique totale rayonnée par la source. Ceci est intéressant du fait du lien entre l'énergie sismique et le volume d'éboulement. Afin de tenir compte des effets topographiques, il est proposé un facteur de correction qui peut être introduit dans le calcul de l'énergie. Les signaux sismiques générés par les éboulements du cratère Dolomieu du Piton de la Fournaise, à La Réunion, sont analysés. Les sismogrammes synthétiques sont utilisés pour identifier et interpréter les signaux observés qui sont générés par des impacts uniques. L'influence de la topographie sur les formes d'onde est démontrée et la sensibilité avec l'emplacement et la direction de la source est évaluée. Les caractéristiques du signal telles que les amplitudes et le contenu fréquentiel sont expliquées sur la base de la théorie du contact de Hertz. De plus, les rapports spectraux entre stations, calculés à partir des signaux sismiques d'éboulement, sont considérés comme caractéristiques de la position de la source. La comparaison avec les rapports spectraux simulés suggère qu'ils sont dominés par la propagation le long de la topographie plutôt que par le mécanisme de la source. Sur la base de ces résultats, une méthode est proposée pour la localisation des éboulements à l'aide de rapports énergétiques simulés entre stations. La méthode est appliquée pour localiser les éboulements dans le cratère de Dolomieu. La mise en œuvre de la méthode implique une fenêtre temporelle glissante qui permet une application simple sur des signaux sismiques continus. L'accent est mis sur la capacité de la méthode à surveiller l'activité des éboulements en temps réel<br>Rockfall hazard has to be evaluated and monitored in order to prevent loss of life and infrastructure. In this regard it is important to create event catalogs and understand rockfall dynamics. Seismic waves can help for this purpose as they carry valuable information of the event. They are generated when rockfalls impact the ground and can be used to detect, classify and locate events. Beyond that, rockfall properties such as their volume and their dynamic behavior can be inferred. Yet, high frequency seismic signals (&gt;1Hz) are poorly understood. This is because they are associated to complex seismic sources which are spatially distributed and can rapidly vary over time. On top of this, high frequency seismic waves are prone to be scattered and diffracted due to interactions with soil heterogeneities or surface topography. This thesis takes an important step forward to enhance understanding of high frequency rockfall seismic signals by simulating seismic wave propagation on domains with realistic velocity profiles and 3D surface topographies using the Spectral Element Method (SEM). The influence of the topography on the seismic wave field is investigated. It is found that topography induced amplification is substantially different between deep sources and sources located at the surface. This is because surface waves generated by shallow sources are exposed to constant scattering and diffraction when traveling along the surface. The energy decay along the surface is investigated for different velocity models and equations are derived to back-calculate the total seismic energy radiated by the source. This is of interest as the rockfall seismic energy is related to the rockfall volume. In order to account for topography effects, a correction factor is proposed which can be introduced in the energy calculation. Observed seismic signals generated by rockfall at Dolomieu crater on Piton de la Fournaise volcano, La Réunion, are analyzed. Synthetic seismograms are used to identify and interpret observed signals generated by single impacts. The influence of topography on the waveforms is demonstrated and the sensitivity on source location as well as source direction is evaluated. Signal characteristics such as amplitudes and frequency content are explained based on Hertz contact theory. Additionally, inter-station spectral ratios computed from rockfall seismic signals are shown to be characteristic of the source position. Comparison with simulated spectral ratios suggest that they are dominated by the propagation along the topography rather than the mechanism of the source. Based on these findings, a method is proposed for the localization of rockfalls using simulated inter-station energy ratios. The method is applied to localize rockfalls at Dolomieu crater. The implementation of the method involves a sliding time window which allows a straightforward application on continuous seismic signals. The potential of the method to monitor rockfall activity in real-time is emphasized

[Truncated abstract] Interferometric gravitational wave detectors are being built around the world with continually improving measurement sensitivities. Noise levels from sources that are intrinsic to these detectors must be reduced to a level below the gravita- tional wave signal. Seismic noise in the low frequency range, which is within the gravitational wave detection bandwidth, is a concern for earth-based detectors. This thesis presents research and development of a high performance vibration isolation system that is designed to attenuate seismic noise. The final design will be used as part of a fully working interferometer at the Australian International Gravitational Observatory (AIGO). Pendulums and springs are conventionally used for the horizontal and vertical vibration isolation components respectively. A complete system comprises of a cascade of these components, each stage dramatically improving the level of isola- tion. The residual motion at the test mass level is thus reduced but is dominated by the normal mode resonances of the chain. A simple and effective method to reduce residual motion further is to add ultra-low frequency pre-isolation stages which suspend the chain. The Roberts Linkage is a relatively new and simple geometrical structure that is implemented in the pre-isolation stages. Here we present experimental results of improving isolation based on mathematical mod- elling. The attenuation of seismic noise in the vertical direction is almost as important as that in the horizontal direction, due to cross-coupling between the two planes. To help improve the vertical performance a lightweight Euler spring that stores no static energy was implemented into the AIGO suspension system. ... Theoretical and experimental results are presented and discussed. Currently the AIGO laboratory consists of two 80 m length arms. They are aligned along the east and south directions. One of AIGO's top priorities is the installation of two complete vibration isolators in the east arm to form a Fabry-Perot cavity. Assembling two suspension systems will enable more accurate performance measurements of the tuned isolators. This would significantly reduce the measurement noise floor as well as eliminate the seismic noise spectrum due to referencing with the ground motion. The processes involved in preparing such a task is presented, including clean room preparation, tuning of each isolator stage, and local control schematics and methods. The status of the AIGO site is also presented.

Une combinaison entre l’assemblage de monocouches au sommet de nano-cristaux d’or et la microscopie à sonde locale permet d’explorer l’interaction entre les molécules actives et son milieu, mais aussi démontrer expérimentalement une diode moléculaire peut travailler à haute fréquence. La fabrication des nanocristaux d’or par lithographie électronique rapide nous a permis leur caractérisation par des techniques nécessitant des surfaces millimétrique, mais également par des techniques en champ proche. La détection electrochimique d’un petit nombre de molécules par nanocrystal prévoit d’intéressantes recherches futures sur l’attache et la détection de molécules uniques sur la nano-particule d’or. D’un côté, cette étude confirme une théorie récente: les effets coopératifs entre les molécules ont un effet d’asymétrie sur « la courbe d’histogramme de conductance ». D’un autre côté, des théories d’électrochimie ont été utilisées pour investiguer les effets d’interaction entre molécules avec groupe redox et l’écart-type de l’énergie moléculaire. Cette étude nous a permis d’extraire des gammes d’énergie de couplage entre molécules, un premier pas vers une estimation quantitative expérimentale de ce paramètre clé pour les applications en électronique organique. A l’aide d’un AFM couplé à un système de mesure en hyperfréquences, nous mesurons les propriétés électroniques à l’échelle nanométrique. Nous prouvons une diode moléculaire à 18 GHz avec un ratio de redressement de 12 dB (facteur 4) à cette fréquence. Des petites capacitances, dans la gamme de l’aF ont en particulier permis d’observer ce comportement à hautes fréquences<br>An attractive combination of self-assembled monolayers on top of “Au” single crystal Nanoparticles (AuNp) and Scanning Probe Microscopies permits to explore the interaction between active molecules in the junction, as well as with the media. At the same time, we demonstrate the experimental proof of a molecular rectifying diode working at gigahertz frequency. Device fabrication by fast e-beam lithography allows their characterization by techniques that may need millimeter scale surfaces as well as by near field Scanning Probe Microscopies. Detection of a little number of molecules per AuNP promises interesting future research in the challenge of grafting and detecting single molecules per nanoparticle. On the one hand, this investigation confirms a recent theoretical prediction that cooperative effects between molecules may have an effect on the asymmetry of the conductance histogram line shape. On the other hand, established electrochemical theories are exploited to investigate similar factors such as interaction between redox molecules and the modification of the energy level of molecular orbitals. This study permits extracting a range of coupling energies between molecules that may be a first step towards the quantitative experimental estimation of this key parameter in molecular electronics. Thanks to an AFM connected to Network analyzer, we characterize a molecular diode operating at high frequency to 18 GHz with a rectification ratio of 12 dB (factor 4) at this frequency. Small capacitances in the order of few aF permit to see this behavior at high frequencies

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, February 2007.<br>Includes bibliographical references (p. 53).<br>(cont.) Investigated in this thesis are the excitation and observation of Langmuir wave turbulence caused by the parametric decay instability (PDI) in high-frequency space plasma heating experiments conducted at the NSF/DoD High Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska during the spring and summer of 2006. The PDI is the decay of an electromagnetic (EM) wave into an electron plasma wave (i.e., Langmuir wave) and an ion acoustic wave. When the excited Langmuir wave parametrically decays into another Langmuir and ion acoustic wave pair, a cascade of Langmuir waves can occur provided that the instability threshold is satisfied. According to recently advanced theory by Kuo and Lee [2005], there are two possible methods of cascade: non-resonant and resonant. While the non-resonant cascade proceeds at the location of excitation, the resonant process occurs at lower altitudes to minimize losses that the non-resonant process incurs by remaining at the excitation altitude. Such losses are caused by the frequency mismatch effect, as the decay ion acoustic wave frequency becomes much less than that of the normal ion acoustic waves. In their downward propagation the Langmuir waves in the resonant cascade suffer from propagation losses, however these losses are less than those associated with the non-resonant process. The resonant process is therefore expected to have a lower threshold. Theoretical claims and calculations are compared to observations made at Arecibo, Puerto Rico and Tromso, Norway. Claims are also are supported by incoherent backscatter radar observations made at the HAARP facility in Gakona.<br>by Daniel L. Rokusek.<br>S.M.

The effects of interplanetary shock impact angles have the potential to have far reaching consequences. By their nature, interplanetary shocks are a direct consequence of a variety of solar events including both Coronal Mass Ejections (CMEs) and Co-rotating Interaction Regions (CIRs). They have the ability to move the magnetopause, the boundary between the Earth's magnetosphere and the surrounding plasma, leading to ionospheric current systems and an enhanced ring current. Their association with a time-varying EMF also makes them potentially dangerous at a human level. This EMF can couple to electrical currents in technological infrastructure that can overload transformers, communication cables, and power grids. As IP shocks have the potential to have a large impact on our society, research to further our understanding of these events is prudent. We know that shocks can couple to currents and ULF waves in the magnetosphere-ionosphere system. Much of the current research into their behaviors has been focused on models and simulations and has indicated that the shock impact angle should affect the properties of the waves. To investigate the potential influence of the impact angle, data from a series of Antarctic magnetometers was collected and compared to a database of known interplanetary shocks to determine when the response to different shocks was detected at the magnetometer. For this investigation, we were concerned with determining what impact if any, the impact angle of the IP shock had on the generation of Pc5 waves. To that end, the power spectra both before and after the shock was calculated. This information was then combined with the shock impact angle to determine what effects if any, the shock impact angle had on Pc5 wave occurrence rates. From our research, it was determined that the impact angle of the interplanetary shock had a significant impact on the occurrence rate and properties of Pc5 waves observed by high-latitude ground magnetometers.<br>Master of Science<br>Interplanetary shocks, drive interactions between the solar wind and the Earth’s atmosphere, and they have the potential to have far reaching consequences. Caused by a variety of solar events including both Coronal Mass Ejections (CMEs) and Co-rotating Interaction Regions (CIRs), they have the ability to physically move the locations of regional boundaries of the ionized part of Earth’s atmosphere, leading to a variety of electromagnetic effects. They also pose a danger at the human level by generating electrical currents in technological infrastructure that can overload transformers, communication cables, and power grids. As they pose a danger to our society, understanding them is prudent. A large portion of the current research into their behaviors has been focused on models and simulations and has shown that the shock impact angle should affect the properties of the waves. For this investigation, data from a series of Antarctic sensors was collected and compared to a database of known interplanetary shocks to determine when different shocks were detected. Specifically, for our investigation, we were concerned with determining what impact if any, the impact angle of the IP shock had on the generation of Pc5 waves, a specific type of ULF wave. This was accomplished by calculating the power level at different frequencies both before and after the shock. This information was then combined with the shock impact angle to determine what effects if any, the shock impact angle had on Pc5 wave occurrence rates. From our research we found that the impact angle of the interplanetary shock had a significant impact on the generation of Pc5 waves.

A Rayleigh-scatter lidar operated from Utah State University (41.7°N, 111.8°W) for a period spanning 11 years ― 1993 through 2004. Of the 900 nights observed, data on 150 extended to 90 km or above. They were the ones used in these studies related to atmospheric gravity waves (AGWs) between 45 and 90 km. This is the first study of AGWs with an extensive data set that spans the whole mesosphere. Using the temperature and temperature gradient profiles, we produced a climatology of the Brunt-Väisälä (buoyancy) angular frequency squared, N2 (rad/s)2. The minimum and maximum values of N2 vary between 2.2×10-4 (rad/s)2 and 9.0×10-4 (rad/s)2. The corresponding buoyancy periods vary between 7.0 and 3.5 minutes. While for long averages the atmosphere above Logan, Utah, is convectively stable, all-night and hourly profiles showed periods of convective instability (i.e., negative N2). The N2 values were often significantly different from values derived from the NRL-MSISe00 model atmosphere because of the effects of inversion layers and semiannual variability in the lidar data. Relative density fluctuation profiles with 3-km altitude resolution and 1-hour temporal resolution showed the presence of monochromatic gravity waves on almost every night throughout the mesosphere. The prevalent values of vertical wavelength and vertical phase velocity were 12-16 km and 0.5-0.6 m/s, respectively. However, the latter has the significant seasonal variation. Using these two observed parameters, buoyancy periods, and the AGW dispersion relation, we derived the ranges of horizontal wavelength, phase velocity, and source distance. The prevalent values were 550-950 km, 32-35 m/s, and 2500-3500 km, respectively. The potential energy per unit mass Ep showed great night-to-night variability, up to a factor of 20, at all heights. Ep grew at approximately the adiabatic rate below 55-65 km and above 75-80 km. Step function decreases in Ep imply that the AGWs in between gave up considerable energy to the background atmosphere. In addition, Ep varies seasonally. Below 70 km, it has a semiannual variation with a maximum in winter and minima in the equinoxes. At the highest altitudes it has an annual variation with a maximum in winter and a minimum in summer.

With the increasing demand on wireless and portable devices, the radio frequency front end blocks are required to feature properties such as wideband, high frequency, multiple operating frequencies, low cost and compact size. However, the current radio frequency system blocks are designed by combining several individual frequency band blocks into one functional block, which increase the cost and size of devices. To address these issues, it is important to develop novel approaches to further advance the current design methodologies in both space and spectrum domains. In recent years, the concept of artificial materials has been proposed and studied intensively in RF/Microwave, Terahertz, and optical frequency range. It is a combination of conventional materials such as air, wood, metal and plastic. It can achieve the material properties that have not been found in nature. Therefore, the artificial material (i.e. meta-materials) provides design freedoms to control both the spectrum performance and geometrical structures of radio frequency front end blocks and other high frequency systems. In this dissertation, several artificial materials are proposed and designed by different methods, and their applications to different high frequency components and circuits are studied. First, quasi-conformal mapping (QCM) method is applied to design plasmonic wave-adapters and couplers working at the optical frequency range. Second, inverse QCM method is proposed to implement flattened Luneburg lens antennas and parabolic antennas in the microwave range. Third, a dual-band compact directional coupler is realized by applying artificial transmission lines. In addition, a fully symmetrical coupler with artificial lumped element structure is also implemented. Finally, a tunable on-chip inductor, compact CMOS transmission lines, and metamaterial-based interconnects are proposed using artificial metal structures. All the proposed designs are simulated in full-wave 3D electromagnetic solvers, and the measurement results agree well with the simulation results. These artificial material-based novel design methodologies pave the way toward next generation high frequency circuit, component, and system design.

The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing. With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and six-hourly momentum and heat fluxes. While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and six-hourly cases), then Vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers.

The ionosphere is an important factor in high frequency (HF) radio propagation providing an opportunity to study ionospheric variability as well as the space weather conditions under which HF communication can take place. This thesis presents the validation of HF propagation conditions for the Ionospheric Communication Enhanced Profile Analysis and Circuit (ICEPAC) and Advanced Stand Alone Prediction System (ASAPS) models over Africa by comparing predictions with the measured data obtained from the International Beacon Project (IBP). Since these models were not developed using information on the African region, a more accurate HF propagation prediction tool is required. Two IBP transmitter stations are considered, Ruaraka, Kenya (1.24°S, 36.88°E) and Pretoria, South Africa (25.45°S, 28.10°E) with one beacon receiver station located in Hermanus, South Africa (34.27°S, 19.l2°E). The potential of these models in terms of HF propagation conditions is illustrated. An attempt to draw conclusions for future improvement of the models is also presented. Results show a low prediction accuracy for both ICEPAC and ASAPS models, although ICEPAC provided more accurate predictions for daily HF propagation conditions. This thesis suggests that the development of a new HF propagation prediction tool for the African region or the modification of one of the existing models to accommodate the African region, taking into account the importance of the African ionospheric region, should be considered as an option to ensure more accurate HF Propagation predictions over this region.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, February 2006.<br>"February 2006."<br>Includes bibliographical references (p. 87-88).<br>Astronauts are exposed to hazards unique to space travel. These hazards include radiation exposure and adaptation of the human body to the microgravity environment. For lunar and low earth orbital missions, the exposure period is typically less than six months and return to Earth is less than two weeks away. For travel beyond the Earth's moon, the microgravity exposure time will increase from months to years and return time will increase from weeks to months. Current countermeasures employ impact and high force loading to maintain bone health. An astronaut runs on a treadmill to impact load the weight bearing components of the musculoskeletal system. Elastic bands provide the "down" force for the astronaut while running. For high force loading, the astronaut performs a specified regimen of weight lifting exercises using resistive devices. The resistive devices provide a load in microgravity similar to that of free weights on Earth. These countermeasures have been beneficial in slowing bone adaptation, but have not stopped it. The imperceptible muscle contractions required for posture maintenance may be the absent load that the skeletal system requires to maintain bone health. Unlike the muscles that are required for impact and high force loading, the postural muscles work continuously to keep humans balanced and upright in a gravity environment.<br>(cont.) Jumping, running and even sitting require posture maintenance. Studies have shown that low magnitude loads applied at a high frequency to the weight bearing bones have not only maintained the bone mineral density, but also more importantly, maintained the structure of the bones. This thesis demonstrates the design of a vibrating pedal that delivers a perceptible, low magnitude load at a high frequency ([approx.]30 Hz) to the foot. This design required no external power and was implemented on a Human Powered Artificial Gravity (HPAG) cycle. A device similar to the vibrating pedal device created for this research could benefit society by providing an effective therapy against the disease of osteoporosis. A vibrating pedal could easily be mounted on a stationary cycle, possibly even standard bicycle, and provide a beneficial therapy to the user.<br>by Bruce Naakaii Ts'oh Webster.<br>S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1985.<br>Microfiche copy available in Archives and Science.<br>Bibliography: leaves 178-185.<br>by William Scott Phillips.<br>Ph.D.

[EN] Nature is nonlinear. The linear description of physical phenomena is useful for explain observations with the simplest mathematical models, but they are only accurate for a limited range of input values. In the case of intense acoustics waves, linear models obviate a wide range of physical phenomena that are necessary for accurately describe such high-amplitude waves, indispensable for explain other exotic acoustic waves and mandatory for developing new applied techniques based on nonlinear processes. In this Thesis we study the interactions between nonlinearity and other basic wave phenomena such as non-classical attenuation, anisotropic dispersion and periodicity, and diffraction in specific configurations. First, we present intense strain waves in a chain of cations coupled by realistic interatomic potentials. Here, the nonlinear ionic interactions and lattice dispersion lead to the formation of supersonic kinks. These intrinsically-nonlinear localized dislocations travel long distances without changing its properties and explain the formation of dark traces in mica crystals. Then, we analyze nonlinear wave processes in a system composed of multilayered acoustic media. The rich nonlinear dynamics of this system is characterized by its strong dispersion. Here, harmonic generation processes and the relation with its band structure are presented, showing that the nonlinear processes can be enhanced, strongly minimized or simply modified by tuning the layer parameters. In this way, we show how the dynamics of intense monochromatic waves and acoustic solitons can be controlled by artificial layered materials. In a second part, we include diffraction and analyze four types of singular beams. First, we study nonlinear beams in two dimensional sonic crystals. In this system, the inclusion of anisotropic dispersion is tuned for obtain simultaneous self-collimation for fundamental and second harmonic beams. The conditions for optimal second harmonic generation are presented. Secondly, we present limited diffraction beam generation using equispaced axisymmetric diffraction gratings. The obtained beams are truncated version of zero-th order Bessel beams. Third, the grating spacing can be modified to achieve focusing, where the generated nonlinear beams presents high gain, around 30 dB, with a focal width which is between the diffraction limit and the sub-wavelength regime, but with its characteristic high amplitude side lobes strongly reduced. Finally, we observe that waves diffracted by spiral-shaped gratings generate high-order Bessel beams, conforming nonlinear acoustic vortex. The conditions to obtain arbitrary-order Bessel beams by these passive elements are presented. Finally, the interplay of nonlinearity and attenuation in biological media is studied in the context of medical ultrasound. First, a numerical method is developed. The method solves the constitutive relations for nonlinear acoustics and the frequency power law attenuation of biological media is modeled as a sum of relaxation processes. A new technique for reducing numerical dispersion based on artificial relaxation is included. Second, this method is used to study the harmonic balance as a function of the power law, showing the role of weak dispersion and its impact on the efficiency of the harmonic generation in soft-tissues. Finally, the study concerns the nonlinear behavior of acoustic radiation forces in frequency power law attenuation media. We present how the interplay between nonlinearity and the specific frequency power law of biological media can modify the value for acoustic radiation forces. The relation of the nonlinear acoustic radiation force with thermal effects are also discussed. The broad range of nonlinear processes analyzed in this Thesis contributes to understanding the behavior of intense acoustic waves traveling trough complex media, while its implications for enhancing existent applied acoustics techniques are presented.<br>[ES] La Naturaleza es no lineal. La descripción lineal de los fenómenos físicos es de gran utilidad para explicar nuestras observaciones con modelos matemáticos simples, pero éstos sólo son precisos en un limitado rango de validez. En el caso de onda acústica de alta intensidad, los modelos lineales obvian un amplio rango de fenómenos físicos que son necesarios para describir con precisión las ondas de gran amplitud, pero además son necesarios para explicar otros procesos más exóticos e indispensables para desarrollar nuevas aplicaciones basadas en propagación no lineal. En esta Tesis, estudiamos las interacciones entre no linealidad y otros procesos complejos como atenuación no-clásica, dispersión anisotrópica y periodicidad, y difracción en configuraciones específicas. En primer lugar, presentamos ondas de deformación en una cadena de cationes acoplados por potenciales realísticas. Aquí, las interacciones no lineales entre iones, producen la conformación de kinks supersónicos. Estas dislocaciones localizadas intrínsecamente no lineales viajan por la red largas distancias sin variar sus propiedades, y pueden explicar la formación de trazas en minerales como la mica. Aumentando la escala del problema, estudiamos los procesos acústicos no lineales en medios multicapa. La rica dinámica de estos medios está caracterizada por la fuerte dispersión debido a la periodicidad del sistema. Aquí, estudiamos los procesos de generación de harmónicos, mostrando como modificando la estructura podemos potenciar, minimizar, o simplemente modificar artificialmente la transferencia de energía entre las componentes espectrales, y de esta manera controlar la dinámica de las ondas y solitones en el interior de la estructura. En la segunda parte, incluimos difracción y analizamos cuatro tipos de haces singulares. En primer lugar, analizamos haces ultrasónicos no lineales en cristales de sonido bidimensionales. En este sistema, las propiedades de anisotropía del medio son ajustadas para obtener la auto-colimación simultánea del primer y segundo harmónico. Así, se obtiene la propagación no difractiva para las dos componentes. En segundo lugar, presentamos haces de difracción limitada empleando rejillas de difracción axisimétricas. Por último, demostramos la generación de haces de Bessel de orden superior mediante estructuras en espiral. En la última parte, estudiamos la competición entre no linealidad y la atenuación y dispersión observable en medios biológicos en el contexto de las aplicaciones de biomédicas de los ultrasonidos. En primer lugar desarrollamos un nuevo método computacional para la dependencia frecuencial en forma de ley de potencia de la absorción característica de los tejidos. Este método en dominio temporal es usado posteriormente para revisar los procesos básicos no lineales prestando especial interés en el paper de la dispersión del tejido. Por último, la resolución de las ecuaciones constitutivas nos permite abordar la descripción no lineal de la fuerza de radiación acústica producida en tejidos biológicos, y las implicaciones existentes con la deposición de energía y transferencia de momento para ondas ultrasónicas de alta intensidad. El amplio abanico de procesos no lineales analizados en esta tesis contribuye a una mejor comprensión de la dinámica de las ondas acústicas de alta intensidad en medios complejos, donde las implicaciones existentes en cuanto a la mejora de sus aplicaciones prácticas son puestas de manifiesto.<br>[CAT] La Naturalesa és no lineal. La descripció lineal dels fenòmens físics és de gran utilitat per a explicar les nostres observacions amb models matemàtics simples, però aquests sol són precisos en un limitat rang de validesa. En el cas d'ona acústica d'alta intensitat, els models lineals obvien un ampli rang de fenòmens físics que són necessaris per a descriure amb precisió les ones de gran amplitud, però a més són necessaris per a explicar altres processos més exòtics i indispensables per a desenvolupar noves aplicacions basades en propagació no lineal. En aquesta Tesi, estudiem les interaccions entre no-linealitat i altres processos complexos com atenuació no-clàssica, dispersió anisotròpica i periodicitat, i difracció en configuracions específiques. En primer lloc, presentem ones de deformació en una cadena de cations acoblats per potencials realistes. Ací, les interaccions no lineals entre ions, produeixen la conformació de kinks supersònics. Aquestes dislocacions localitzades intrínsecament no lineals viatgen per la xarxa llargues distàncies sense variar les seues propietats, i poden explicar la formació de traces en minerals com la mica. Augmentant l'escala del problema, estudiem els processos acústics no lineals en mitjans multicapa. La rica dinàmica d'aquests mitjans es caracteritza per la forta dispersió a causa de la periodicitat del sistema. Ací, estudiem els processos de generació d'harmònics, mostrant com modificant l'estructura podem potenciar, minimitzar, o simplement modificar artificialment la transferència d'energia entre les components espectrals, i d'aquesta manera controlar la dinàmica de les ones i solitons a l'interior de l'estructura. En la segona part, incloem difracció i analitzem quatre tipus de feixos singulars. En primer lloc, analitzem feixos ultrasònics no lineals en cristalls de so bidimensionals. En aquest sistema, les propietats d'anisotropia del medi són ajustades per a obtenir l'acte-col·limació simultània del primer i segon harmònic. Així, s'obté la propagació no difractiva per a les dues components. En segon lloc, presentem feixos de difracció limitada emprant reixetes de difracció axisimètriques. Per últim, vam demostrar la generació de feixos de Bessel d'ordre superior mitjançant estructures en espiral. En l'última part, estudiem la competició entre no linealitat i l'atenuació i dispersió observable en medis biològics en el context de les aplicacions biomèdiques dels ultrasons. En primer lloc desenvolupem un nou mètode computacional per a la dependència freqüencial en forma de llei de potència de l'absorció característica dels teixits biològics. Aquest mètode en domini temporal és usat posteriorment per a revisar els processos bàsics no lineals prestant especial interés en el paper de la dispersió del teixit. Per últim, la resolució de les equacions constitutives ens permet abordar la descripció no lineal de la força de radiació acústica produïda en teixits biològics, i les implicacions existents amb la deposició d'energia i transferència de moment per a ones ultrasòniques d'alta intensitat. L'ampli ventall de processos no lineals analitzats en aquesta tesi contribueix a una millor comprensió de la dinàmica de les ones acústiques d'alta intensitat en medis complexos, on les implicacions existents quant a la millora de les seues aplicacions practiques són posades de manifest.<br>Jiménez González, N. (2015). Nonlinear Acoustic Waves in Complex Media [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/53237<br>TESIS<br>Premiado

Electromagnetic wave scattering from a semi-infinite array of dipoles in free space is described by using asymptotic high frequency methods. An electric field integral expression is obtained and solved with asymptotic high frequency methods. An asymptotic field expression is obtained for a finite &times<br>infinite array of dipoles in free space. The analytical closed form expression for the array guided surface wave launching coefficient is obtained via a combination of an asymptotic high frequency analysis of a related reciprocal problem and Lorentz reciprocity integral formulation for the semi-infinite planar dipole array in which modified Kirchhoff approximation is used. The accuracy and the validity of the asymptotic analytical solutions are compared with the numerical solutions available in the literature before.