In this thesis, field oriented control of permanent magnet synchronous motors using three-level neutral-point-clamped inverter is studied. Permanent magnet synchronous motors are used in high performance drive applications. In this study, the permanent magnet synchronous motor is fed by three-level neutral-point-clamped inverter. For three-level neutral-point-clamped inverter different space vector modulation algorithms, which are reported in literature, are analyzed and compared via computer simulations. The voltage balance on dc-link capacitors is also analyzed and a software control method is implemented in conjunction with the space vector PWM modulation, utilized. Nonlinear effects such as dead-time, semiconductor voltage drop and delays in gate drive circuitries also present in neutral-point-clamped inverter. The effects of these nonlinearities are studied and a compensation method for these nonlinear effects is proposed. The theoretical results are supported with computer simulations and verified with experimental results.

L'objectif de cette thèse est d'étudier la faisabilité de l'interférométrie par rétro-injection optique (IRO) pour la mesure de vitesse en dynamique des matériaux. La dynamique des matériaux est l'étude du comportement des matériaux soumis à des chocs (chocs laser, chocs plan,compression isentropique, projectiles, etc.). Dans ces conditions, nous cherchons à mesurer des vitesses pouvant aller jusqu'à 10 km/s. La technique IRO est couramment utilisée comme capteur embarqué pour mesurer de faibles vitesses dans divers domaines. Cependant, très peu d'études ont été réalisées sur les limites hautes vitesses de ce type de capteur. La rétro-injection optique provoque des effets non linéaires dans la cavité laser : elle perturbe la puissance d'émission optique. Suivant le taux de rétro-injection optique, le laser peut présenter un comportement chaotique et il n'est alors plus possible de récupérer l'information de la vitesse et/ou du déplacement de la cible à partir des signaux. Nous avons pour cela développé des modèles mathématiques et réalisé un grand nombre de simulations numériques afin d'évaluer les performances et les limitations de ce système. Nous avons notamment étudié l'influence de la réflectivité de la cible, de la longueur et de la fréquence de la modulation de la cavité externe. / The purpose of this thesis is to study the feasibility of optical feedback interferometry (OFI) for measuring velocities for dynamical material's behaviour studies. Dynamical material's behaviour studies permit to analyse the shocked material when subjects to shocks (laser shocks, isentropic compression, projectiles, etc.). In these conditions, we seek to measure velocities up to 10 km/s. The OFI technique is regularly used as an embedded system to measure slow velocities in various fields. However, very few studies have been performed for determining velocities measurement limits for this system. As a matter of fact, the optical feedback induces nonlinear effects into the laser's cavity : it disrupts the laser's emitted optical power. Depending on the optical feedback strength, the laser can show chaotic behaviour, then it is no longer possible to get the information for the target's velocity or displacement regarding the signal. In this study, we have been developing mathematical models and performing a wide range of numerical simulations to study the performances and the limits of the OFI technique. We have been also studying the influence of the targets reflectivity, the length and the modulation frequency of the external cavity.

Presented is an overview of a semi-analytical solution which was developed to study the response of internally pressurized elliptical composite cylinders with clamped boundaries. Using a geometrically linear analysis and the solution scheme, the response of a quasi-isotropic elliptical cylinder is compared with the response of a quasi-isotropic circular cylinder in order to study the effects of elliptical geometry. The distinguishing features of the response of an elliptical cylinder are the inward normal displacement of the cross section at the ends of the major diameter that occur despite the outward force of the internal pressure, the presence of circumferential displacements, and the presence of inplane shear strains. These effects lead to spatial variations, including sign reversals, of a number of displacement, strain, and curvature responses. The responses of a quasi-isotropic elliptical cylinder evaluated using a geometrically linear analysis are then compared to the responses evaluated using a geometrically nonlinear analysis. It is shown that geometric nonlinearities tend to flatten certain responses at the ends of the minor diameter, and reduce the magnitude of certain responses in the boundary region. To study the influence of material orthotropy, the responses of axially-stiff and circumferentially-stiff elliptical cylinders evaluated using geometrically nonlinear analyses are examined. It is shown that in some instances material orthotropy can be used to mitigate the influence of the elliptical geometry and make particular responses look like those of a circular cylinder. An evaluation of failure using the maximum stress and Hashin failure criteria and geometrically linear and nonlinear analyses is presented for elliptical cylinders. These failure criteria involve interlaminar shear stresses which are computed by integrating the equilibrium equations of elasticity through the thickness of the cylinder wall. The failure criteria are used to assess the mode of failure (e.g., tensile or compressive fiber or matrix modes), the location of failure, and the pressure at failure. Both criteria predict first failure to occur at the clamped boundaries because of matrix cracking. The predicted failure pressures and circumferential locations are very similar for the two criteria, and the nonlinear analyses predict slightly higher pressures at somewhat different circumferential locations. First fiber failure is also considered. For this failure the two criteria predict similar failure scenarios for the linear analyses, but they differ in their predictions for the nonlinear analyses. Specifically, using the maximum stress criterion, the circumferentially-stiff elliptical cylinder is predicted to fail due to fiber compression, but the Hashin criterion predicts failure to be due to fiber tension, and at a different circumferential location. Also, first fiber failure pressures are at least a factor of two greater than the first matrix failure pressure. / Master of Science

In the atomic, molecular, optical physics field, processes involving two photons are very well understood and used in applications ranging from spectroscopy to laser cooling technics. In this thesis is presented the study and experimental realization of two two-photon processes. Using sodium atoms trapped in a magnetic optical trap we could demonstrate two-photon cooperative absorption, and that the creation of a molecular bound state using only light fields, or photonic bound state, is possible. Two-photon cooperative absorption, very common in solid-state physics, is a process where a pair of atoms initially in the ground state is excited to the double excited state, via absorption of two photons with frequecy that is not ressonant with any excited state. Its experimental realization with cold atoms may open new and exciting possibilities to better understand nonlinear effects, and it is a new way to create correlated atoms and photons in cold atomic physics. This absorption was observed by ionization of the pair after the excitation. A simple model that considers only dipole-dipole interactions between the atoms allows us to understand the basic features observed in the experimental data. A photonic bound state uses two photons to create the two basic features of a molecular bound state: a repulsive part and an attractive part. A blue photon, blue detuned from the atomic transition, connects the ground state of the pair to the repulsive part of the first excited molecular state 1, and a red photon, red detuned from the atomic transition, connects the connects the ground state of the pair to the attractive part of the first excited molecular state. In the dressed state picture, when the light fields are strong, this three-states-two-photon system creates adiabatic bound potentials that are strongly dependent of the photon properties. Using a theoretical model we could study how this bound energies changes when we change the photon properties, and the experimental data shows that this photos are indeed dressing the potentials with a efficiency that would enable the creation of photonic molecules. / No campo da física atômica, molecular e ótica processos envolvendo dois fótons são bem compreendidos e usados em diversas aplicações. Nesta tese apresentamos o estudo e a realização experimental de dois processos de dois-fótons. Usando átomos de sódio aprisionados em uma armadilha magneto ótica, demonstramos a absorção cooperativa de dois fótons e que a criação de um estado ligado molecular usando somente campos de luz, ou estado ligado fotônico, é possível. Absorção cooperativa de dois fótons, um processo bem comum em física de estado sólido, acontece quando um par de átomos inicialmente no estado fundamental é excitado para o estado duplamente excitado, via absorção de dois fótons de frequência não ressonante com a dos estados excitado. A realização experimental deste processo em um sistema de átomos frios pode abrir novas, e excitantes possibilidades para entender melhor processos não lineares, e é um novo método de criar átomos e fótons correlacionados. Essa absorção foi observada através da ionização do par depois da absorção, e um modelo simples que considera somente interação dipolo-dipolo entre os dois átomos nós ajuda entender as características básicas dos dados obtidos. Um estado ligado fotônico usa dois fótons para criar as duas características básicas dos estados ligados moleculares: a parte repulsiva e a parte atrativa. Um fóton azul, deslocado para o azul da transição atômica, conecta o estado fundamental do par a parte repulsiva do primeiro estado excitado molecular 1, e um fóton vermelho, deslocado para o vermelho da transição atômica, conecta o estado fundamental a parte atrativa do deslocado para o azul da transição atômica. No contexto de estados vestidos, quando os campos de luz são intensos, esse sistema de três estados e dois fótons cria potenciais ligantes adiabáticos que são fortemente dependentes das propriedades desses fótons. Usando um modelo teórico para esses potenciais pudemos estudar como é essa dependência, com as características do fótons, e os dados experimentais mostram que esses fótons estão de fato vestindo os estados com uma eficiência que viabiliza a criação de moléculas fotônicas.

No description available.

The vast and rich body of literature covering the numerical modelling of hydrodynamic floating body systems has demonstrated their great power and versatility when applied to offshore marine energy systems. It is possible to model almost any type of physical phenomenon which could be expected within such a system, however, limitations of computing power continue to restrict the usage of the most comprehensive models to very narrow and focused design applications. Despite the continued evolution of parallel computing, one major issue that users of computational tools invariably face is how to simplify their modelled systems in order to achieve practically the necessary computations, whilst capturing enough of the pertinent physics, with great enough ‘resolution’, to give robust results. The challenge is, in particular, to accurately deliver a complete spectrum of results, that account for all of the anticipated sea conditions and allow for the optimisation of different control scenarios. This thesis examines the uncertainty associated with the effects of viscosity and nonlinear behaviour on a small scale model of an oscillating system. There are a wide range of Computational Fluid Dynamics (CFD) methods which capture viscous effects. In general however, the oscillating, six degree-of-freedom floating body problem is best approached using a linear potential flow based Boundary Element Method (BEM), as the time taken to process an equivalent model will differ by several orders of magnitude. For modelling control scenarios and investigating the effects of different sea states, CFD is highly impractical. As potential flows are inviscid by definition, it is therefore important to know how much of an impact viscosity has on the solution, particularly when different scales are of interest during device development. The first aim was to develop verified and validated solutions for a generic type decaying system. The arrangement studied was adapted from an array tank test experiment which was undertaken in 2013 by an external consortium (Stratigaki et al., 2014). Solutions were delivered for various configurations and gave relatively close approximations of the experimental measurements, with the modelling uncertainties attributed to transient nonlinear effects and to dissipative effects. It was not possible however to discern the independent damping processes. A set of CFD models was then developed in order to investigate the above discrepancies, by numerically capturing the nonlinear effects, and the effects of viscosity. The uncontrolled mechanical effects of the experiment could then be deduced by elimination, using known response patterns from the measurements and derived results from the CFD simulations. The numerical uncertainty however posed a significant challenge, with the outcomes supported by verification evidence, and detailed discussions relating to the model configuration. Finally, the impact of viscous and nonlinear effects were examined for two different interacting systems – for two neighbouring devices, and an in-line array of five devices. The importance of interaction behaviour was tested by considering the transfer of radiation forces between the model wave energy converters, due to the widely accepted notion that array effects can impact on energy production yields. As there are only very limited examples of multi-body interaction analysis of wave energy devices using CFD, the results with this work provide important evidence to substantiate the use of CFD for power production evaluations of wave energy arrays. An effective methodology has been outlined in this thesis for delivering specific tests to examine the effects of viscosity and nonlinear processes on a particular shape of floating device. By evaluating both the inviscid and viscous solutions using a nonlinear model, the extraction of systematic mechanical effects from experimental measurements can be achieved. As these uncontrolled frictional effects can be related to the device motion in a relatively straightforward manner, they can be accommodated within efficient potential flow model, even if it transpires that they are nonlinear. The viscous effects are more complex; however, by decomposing into shear and pressure components, it may in some situations be possible to capture partially the dynamics as a further damping term in the efficient time-domain type solver. This is an area of further work.

In the atomic, molecular, optical physics field, processes involving two photons are very well understood and used in applications ranging from spectroscopy to laser cooling technics. In this thesis is presented the study and experimental realization of two two-photon processes. Using sodium atoms trapped in a magnetic optical trap we could demonstrate two-photon cooperative absorption, and that the creation of a molecular bound state using only light fields, or photonic bound state, is possible. Two-photon cooperative absorption, very common in solid-state physics, is a process where a pair of atoms initially in the ground state is excited to the double excited state, via absorption of two photons with frequecy that is not ressonant with any excited state. Its experimental realization with cold atoms may open new and exciting possibilities to better understand nonlinear effects, and it is a new way to create correlated atoms and photons in cold atomic physics. This absorption was observed by ionization of the pair after the excitation. A simple model that considers only dipole-dipole interactions between the atoms allows us to understand the basic features observed in the experimental data. A photonic bound state uses two photons to create the two basic features of a molecular bound state: a repulsive part and an attractive part. A blue photon, blue detuned from the atomic transition, connects the ground state of the pair to the repulsive part of the first excited molecular state 1, and a red photon, red detuned from the atomic transition, connects the connects the ground state of the pair to the attractive part of the first excited molecular state. In the dressed state picture, when the light fields are strong, this three-states-two-photon system creates adiabatic bound potentials that are strongly dependent of the photon properties. Using a theoretical model we could study how this bound energies changes when we change the photon properties, and the experimental data shows that this photos are indeed dressing the potentials with a efficiency that would enable the creation of photonic molecules. / No campo da física atômica, molecular e ótica processos envolvendo dois fótons são bem compreendidos e usados em diversas aplicações. Nesta tese apresentamos o estudo e a realização experimental de dois processos de dois-fótons. Usando átomos de sódio aprisionados em uma armadilha magneto ótica, demonstramos a absorção cooperativa de dois fótons e que a criação de um estado ligado molecular usando somente campos de luz, ou estado ligado fotônico, é possível. Absorção cooperativa de dois fótons, um processo bem comum em física de estado sólido, acontece quando um par de átomos inicialmente no estado fundamental é excitado para o estado duplamente excitado, via absorção de dois fótons de frequência não ressonante com a dos estados excitado. A realização experimental deste processo em um sistema de átomos frios pode abrir novas, e excitantes possibilidades para entender melhor processos não lineares, e é um novo método de criar átomos e fótons correlacionados. Essa absorção foi observada através da ionização do par depois da absorção, e um modelo simples que considera somente interação dipolo-dipolo entre os dois átomos nós ajuda entender as características básicas dos dados obtidos. Um estado ligado fotônico usa dois fótons para criar as duas características básicas dos estados ligados moleculares: a parte repulsiva e a parte atrativa. Um fóton azul, deslocado para o azul da transição atômica, conecta o estado fundamental do par a parte repulsiva do primeiro estado excitado molecular 1, e um fóton vermelho, deslocado para o vermelho da transição atômica, conecta o estado fundamental a parte atrativa do deslocado para o azul da transição atômica. No contexto de estados vestidos, quando os campos de luz são intensos, esse sistema de três estados e dois fótons cria potenciais ligantes adiabáticos que são fortemente dependentes das propriedades desses fótons. Usando um modelo teórico para esses potenciais pudemos estudar como é essa dependência, com as características do fótons, e os dados experimentais mostram que esses fótons estão de fato vestindo os estados com uma eficiência que viabiliza a criação de moléculas fotônicas.

Esta dissertação de mestrado investiga os formatos de modulação óptica alternativos ao tradicional formato binário de intensidade (NRZ - Non-return-to-zero), ditos avançados, como tecnologia chave para atender às exigências do mercado atual de telecomunicações por transporte de grande volume de dados e alta largura de banda, associados com a redução do custo da informação transmitida por bit. Os formatos avançados de modulação óptica fazem parte das tecnologias que estão sendo investigadas intensamente com o objetivo de superar os desafios impostos pela nova geração de sistemas ópticos com taxas a partir de 40 Gb/s. Este trabalho focaliza o estudo dessa tecnologia, apresentando uma visão geral dos formatos mais discutidos atualmente, suas características e tecnologias de implementação. Foram realizadas simulações utilizando o software comercial Optisystem versão 7.0 da empresa canadense Optiwave Systems Inc., para analisar o desempenho de alguns formatos de modulação de intensidade e de fase com respeito à sensibilidade do receptor, resistência ao cascateamento de filtragem óptica e a efeitos dispersivos e não-lineares. Procurou-se avaliar os formatos em enlaces reais, com características necessárias à sua possível utilização. Também é feita uma breve consideração dos custos associados aos formatos de modulação analisados neste trabalho. Dentre os formatos analisados, os RZ (Return-to zero) apresentaram maior sensibilidade do receptor e melhor tolerância aos efeitos não-lineares que os seus equivalentes NRZ, porém baixa tolerância aos efeitos dispersivos e ao cascateamento de filtragem óptica. O formato duobinário mostrou fraco desempenho de sensibilidade, mas provou ser o mais tolerante à dispersão cromática e à concatenação de filtragem óptica. Os formatos de modulação de fase mostraram ser bem resistentes aos efeitos não-lineares e aos efeitos dispersivos. / This MSc thesis investigates optical modulation formats as an alternative to the use of the traditional intensity binary format NRZ (Non-return-to-zero). Those formats, classified as advanced, represent one of the key technologies aimed to enable the high data volume and broadband, demanded by the present telecommunication market, with cost reduction of transmitted information per bit. Those technologies have been intensevely studied with purpose of overcoming the challenges imposed by the new generation of optical systems, with bit rates from 40 Gb/s. This work focuses on the study of modulation formats most discussed currently, presenting an overview of their main characteristics and implementation techniques. The performance of some intensity and phase formats, in terms of receiver sensitivity, optical filtering cascading, dispersion and nonlinear effects robustness, have been theoretically analyzed from simulations carried out with the commercial software Optisystem v. 7.0, of the Canadian company Optiwave Systems Inc. The analysis has been applied to real optical links, with characteristics that, potentially, demand their use. A cost analysis, based on estimated values associated to devices required for implementing the formats investigated in this work, is also presented. Among the formats studied, those based on RZ (Return-to zero) presented the highest receiver sensitivity and best tolerance to nonlinear effects, in comparison to their equivalent formats, based on NRZ. However, they are less tolerant to dispersive effects and optical filtering cascading. The duobinary format (DB) has shown a weak performance in terms of sensitivity, though has been proved to be the most tolerant to chromatic dispersion and filtering cascading effects. Among all formats, the phase modulation ones presented the highest resistance against nonlinear and dispersive effects.

The growth that aviation has seen in the last decades has drawn the attention on the environmental impact of aircraft. An important part of this environmental impact is the noise emitted by air vehicles, which is considered rather significant for community annoyance. The generation and propagation of air vehicle noise are two different areas of interest, which require accurate prediction in order to control the emitted noise levels. The present thesis employs numerical methods in order to investigate various air vehicle noise propagation effects. It is divided in two parts: the far field and the near field study. Each of these studies is concentrated on the sound propagation mechanisms that are dominant for each case and uses a numerical method that is best suited for it in terms of mechanisms incorporated and cost effectiveness. The far-field study of this thesis focuses on the nonlinear propagation of helicopter rotor noise using the Burgers equation, a well known one direction propagation method. The Burgers equation incorporates geometrical spreading, atmospheric absorption and nonlinear distortion effects. Towards this study, the HELISHAPE descent case experimental database is used. Blade Vortex Interaction (BVI) noise, the dominant noise contributor during descent, is mainly examined. It is shown that advancing side BVI noise is affected by nonlinear distortion, while retreating side BVI noise is not. For some frequency bands the difference between linear and nonlinear calculations can be as high as 7 dB. Based on signal characteristics at source, two quantities are derived. The first quantity (termed polarity) is based on the pressure gradient of the source signal and can be used to determine whether a BVI signal will evolve as an advancing or a retreating side signal. The second quantity (termed weighted rise time) is a measure of the impulsiveness of the BVI signal and can be used to determine at which frequency nonlinear effects start to appear. Finally, polarity and weighted rise time are shown to be applicable in cases of BVI noise generated from different blade tips, as well as, in cases of non-BVI noise. However, employment of the Burgers equation can be time consuming to be included in routine calculations. It also requires knowledge of the initial pressure time signal. The power spectrum alone, which is usually known, is not sufficient. In order to overcome these difficulties, three prediction methods are presented that are based on the Burgers equation. These are: i) a numerically generated database, ii) correlation equations and iii) the phase assignment method. Near field propagation of air vehicle noise requires different treatment than far field. The effects which are mainly affecting the propagation are geometrical spreading, convection and refraction effects due to the flow field, as well as reflections and diffraction on the air vehicle surfaces. Towards these objectives, a new low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics field described by the Nonlinear Perturbation equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter. Proof of concept for the new method is provided by its application to the case of a conventional jet transport airplane, examining the effect of flow field and wing shielding on the near field noise levels. During the aforementioned noise investigation and analysis, results on Blade Wake Interaction (BWI) noise were also reached. Presently, the mechanism of BWI noise generation, as well as the corresponding prediction model, are still under consideration. Helicopter rotor BWI noise is known to be significant during take-off and level flight, while less attention has been given to descent flight conditions, where BVI noise is dominant. Through signal analysis of the HELISHAPE descent case acoustic database, the rotor azimuthal region responsible for BWI noise is localized and the dominance of BVI noise in the BWI frequency region is shown. Coherence analysis of the blade pressure data indicate significant chordwise coherence in the 3 to 4 Struhal number range and absence of acoustic dipoles in the BWI frequency range. The findings of this study support BWI prediction models based on Amiet’s theory and suggest that BWI noise can be ignored for predictions of rotor noise in descent flight conditions. / Η ανάπτυξη που συναντάται στην αεροπλοΐα τις τελευταίες δεκαετίες έχει τραβήξει την προσοχή στην περιβαλλοντολογική επίδραση των αεροσκαφών. Ένα σημαντικό μέρος αυτής της επίδρασης είναι ο θόρυβος των αεροχημάτων ο οποίος ευθύνεται κατά ένα μεγάλος μέρος για την ενόχληση του πληθυσμού στις περιοχές των αεροδρομίων και όχι μόνο. Η δημιουργία και η διάδοση του θορύβου αεροχημάτων είναι δύο διαφορετικές περιοχές ενδιαφέροντος, οι οποίες απαιτούν ακριβή πρόβλεψη στην προσπάθεια ελέγχου των εκπεμπόμενων ηχητικών επιπέδων. Η παρούσα διατριβή χρησιμοποιεί αριθμητικές μεθόδους για τη μελέτη της διάδοσης του θορύβου αεροχημάτων. Χωρίζεται δε σε δύο μέρη: τη μελέτη του μακρινού και του κοντινού πεδίου. Καθεμία από αυτές τις μελέτες επικεντρώνεται στους μηχανισμούς ηχητικής διάδοσης που είναι κυρίαρχοι σε κάθε περίπτωση και χρησιμοποιεί μια αριθμητική μέθοδο που ταιριάζει καλύτερα όσο αναφορά την απόδοσή της και τους μηχανισμούς που εξετάζονται. Η μελέτη μακρινού πεδίου εστιάζεται στη μη-γραμμική διάδοση θορύβου ρότορα ελικοπτέρου με τη χρήση της εξίσωσης Burgers, μιας γνωστής μεθόδου υπολογιστικής διάδοσης ήχου σε μία διεύθυνση. Η εξίσωση Burgers περικλείει τα φαινόμενα της γεωμετρικής εξασθένισης, ατμοσφαιρικής απορρόφησης και μι-γραμμικής στρέβλωσης. Σε αυτή τη μελέτη χρησιμοποιήθηκαν τα δεδομένα του πειράματος HELISHAPE. Ο θόρυβος από αλληλεπίδραση πτερυγίου-στροβίλου (BVI noise), ο οποίος είναι ο κυρίαρχος αεροδυναμικός θόρυβος στην περίπτωση της καθοδικής πτήσης, είναι αυτός που εξετάστηκε κυρίως. Αποδείχθηκε ότι ο θόρυβος BVI στην πλευρά του προχωρούντος πτερυγίου (advancing side) επηρεάζεται σημαντικά από τα μη-γραμμικά φαινόμενα σε αντίθεση με την πλευρά του υποχωρούντος πτερυγίου (retreating side). Μερικές μπάντες συχνοτήτων έδειξαν διαφορές μεταξύ γραμμικής και μη-γραμμικής διάδοσης έως και 7dB. Βασιζόμενοι στα χαρακτηριστικά των σημάτων, δύο νέα μεγέθη ορίστηκαν. Το πρώτο από αυτά, με το όνομα πολικότητα (polarity) βασίζεται στην παράγωγο της ακουστικής πίεσης του αρχικού σήματος και μπορεί να προσδιορίσει αν το σήμα θα διαδοθεί σαν προχωρούν ή υποχωρούν. Το δεύτερο μέγεθος, με το όνομα ζυγισμένος χρόνος ανόδου (weighted rise time) είναι μια εκτίμηση της παλμικότητας του σήματος BVI και μπορεί να χρησιμοποιηθεί για να προσδιορίσει την περιοχή συχνοτήτων όπου τα μη-γραμμικά φαινόμενα θα ξεκινήσουν να εμφανίζονται. Αυτά τα μεγέθη αποδείχθηκε ότι μπορούν να χρησιμοποιηθούν και σε περιπτώσεις θορύβου BVI άλλων πτερύγων, καθώς και σε μη-BVI σήματα. Ωστόσο, η χρήση της εξίσωσης Burgers μπορεί να είναι χρονοβόρα αν συμπεριληφθεί σε υπολογισμούς ρουτίνας. Ακόμη προϋποθέτει γνώση του αρχικού σήματος ακουστικής πίεσης. Το ηχητικό φάσμα από μόνο του, που συνήθως είναι γνωστό, δεν είναι αρκετό. Για να ξεπεραστούν αυτές οι δυσκολίες, παρουσιάζονται τρεις μέθοδοι πρόβλεψης οι οποίες βασίζονται στην εξίσωση Burgers. Αυτές είναι: i) μια υπολογιστικά δημιουργημένη βάση δεδομένων ii) εξισώσεις συσχετίσεων και iii) η μέθοδος της ανάθεσης φάσης. Η μελέτη διάδοσης θορύβου στο κοντινό πεδίο απαιτεί διαφορετική μεταχείριση από το μακρινό πεδίο. Τα φαινόμενα που την επηρεάζουν περισσότερο είναι αυτά της γεωμετρικής εξασθένισης, μεταφοράς και περίθλασης λόγω του ροϊκού πεδίου, καθώς και ανάκλαση και διάθλαση στις επιφάνειες του αεροχήματος. Για το λόγο αυτό μελετάται μια νέα μέθοδος χαμηλής τάξης ακρίβειας, αλληλεπίδρασης ροϊκού-ακουστικού πεδίου για την διάδοση και διάθλαση ήχου σε ασταθή συμπιεστή ροή με τη χρήση 3D υβριδικού πλέγματος. Το ολικό πεδίο διαχωρίζεται στο ροϊκό πεδίο περιγραφόμενο από τις εξισώσεις Euler και το ακουστικό πεδίο από τις μη-γραμμικές εξισώσεις διαταραχών. Η μέθοδος αποδεικνύεται ικανή να προβλέψει την ηχητική διάδοση μονοπόλου, ενώ η χρήση προσαρμοσμένου πλέγματος βελτιώνει την ακρίβεια του ηχητικού πεδίου. Η αλληλεπίδραση ήχου και στερεών επιφανειών εξετάζεται επίσης σχετικά με τα φαινόμενα ανάκλασης και διάθλασης. Εξετάζεται ακόμα η διάδοση ήχου σε ασταθές ροϊκό πεδίο χρησιμοποιώντας στατική και δυναμική σύζευξη και αποδεικνύεται η σημαντικότητα της δεύτερης. Απόδειξη χρηστικότητας της νέα μεθόδου επιδεικνύεται με την εφαρμογή για την περίπτωση συμβατικού επιβατηγού αεροσκάφους, όπου εξετάζονται τα φαινόμενα της επίδρασης της ροής και της ηχητικής κάλυψης της πτέρυγας στα ηχητικά επίπεδα του κοντινού πεδίου. Κατά τη διάρκεια των παραπάνω ακουστικών διερευνήσεων, επιτεύχθηκαν ακόμα κάποια αποτελέσματα πάνω στον θόρυβο λόγω αλληλεπίδρασης πτερυγίου-απορεύματος (BWI noise). Μέχρι τώρα ο ακριβής μηχανισμός του θορύβου BWI, όπως και το αντίστοιχο μοντέλο πρόβλεψης, αποτελεί θέμα μελετών. Ο θόρυβος BWI στα ελικόπτερα είναι γνωστό ότι είναι σημαντικός στις περιπτώσεις ανοδικής και επίπεδης πτήσης, ενώ μικρή βαρύτητα έχει δοθεί στην περίπτωση της καθοδικής πτήσης λόγω της κυριαρχίας του BVI θορύβου. Μέσω ανάλυσης σημάτων του HELISHAPE για τη περίπτωση της καθοδικής πτήσης, εντοπίστηκε η περιοχή του δίσκου του ρότορα υπεύθυνη για τον θόρυβο BWI και δείχθηκε η επίδραση του θορύβου BVI στην περιοχή συχνοτήτων BWI. Ανάλυση συνοχής (coherence analysis) των αεροδυναμικών πιέσεων των πτερυγίων έδειξε σημαντικές τιμές στη περιοχή τιμών 3 με 4 αριθμών Struhal και απουσία ακουστικών διπόλων στην περιοχή συχνοτήτων του BWI. Τα ευρήματα της παρούσας μελέτης υποστηρίζουν το μοντέλο πρόβλεψης θορύβου BWI που βασίζεται στη θεωρία του Amiet και προτείνουν ότι ο θόρυβος BWI μπορεί να αγνοηθεί στις προβλέψεις θορύβου ρότορα σε συνθήκες καθοδικής πτήσης.

Esta dissertação de mestrado investiga os formatos de modulação óptica alternativos ao tradicional formato binário de intensidade (NRZ - Non-return-to-zero), ditos avançados, como tecnologia chave para atender às exigências do mercado atual de telecomunicações por transporte de grande volume de dados e alta largura de banda, associados com a redução do custo da informação transmitida por bit. Os formatos avançados de modulação óptica fazem parte das tecnologias que estão sendo investigadas intensamente com o objetivo de superar os desafios impostos pela nova geração de sistemas ópticos com taxas a partir de 40 Gb/s. Este trabalho focaliza o estudo dessa tecnologia, apresentando uma visão geral dos formatos mais discutidos atualmente, suas características e tecnologias de implementação. Foram realizadas simulações utilizando o software comercial Optisystem versão 7.0 da empresa canadense Optiwave Systems Inc., para analisar o desempenho de alguns formatos de modulação de intensidade e de fase com respeito à sensibilidade do receptor, resistência ao cascateamento de filtragem óptica e a efeitos dispersivos e não-lineares. Procurou-se avaliar os formatos em enlaces reais, com características necessárias à sua possível utilização. Também é feita uma breve consideração dos custos associados aos formatos de modulação analisados neste trabalho. Dentre os formatos analisados, os RZ (Return-to zero) apresentaram maior sensibilidade do receptor e melhor tolerância aos efeitos não-lineares que os seus equivalentes NRZ, porém baixa tolerância aos efeitos dispersivos e ao cascateamento de filtragem óptica. O formato duobinário mostrou fraco desempenho de sensibilidade, mas provou ser o mais tolerante à dispersão cromática e à concatenação de filtragem óptica. Os formatos de modulação de fase mostraram ser bem resistentes aos efeitos não-lineares e aos efeitos dispersivos. / This MSc thesis investigates optical modulation formats as an alternative to the use of the traditional intensity binary format NRZ (Non-return-to-zero). Those formats, classified as advanced, represent one of the key technologies aimed to enable the high data volume and broadband, demanded by the present telecommunication market, with cost reduction of transmitted information per bit. Those technologies have been intensevely studied with purpose of overcoming the challenges imposed by the new generation of optical systems, with bit rates from 40 Gb/s. This work focuses on the study of modulation formats most discussed currently, presenting an overview of their main characteristics and implementation techniques. The performance of some intensity and phase formats, in terms of receiver sensitivity, optical filtering cascading, dispersion and nonlinear effects robustness, have been theoretically analyzed from simulations carried out with the commercial software Optisystem v. 7.0, of the Canadian company Optiwave Systems Inc. The analysis has been applied to real optical links, with characteristics that, potentially, demand their use. A cost analysis, based on estimated values associated to devices required for implementing the formats investigated in this work, is also presented. Among the formats studied, those based on RZ (Return-to zero) presented the highest receiver sensitivity and best tolerance to nonlinear effects, in comparison to their equivalent formats, based on NRZ. However, they are less tolerant to dispersive effects and optical filtering cascading. The duobinary format (DB) has shown a weak performance in terms of sensitivity, though has been proved to be the most tolerant to chromatic dispersion and filtering cascading effects. Among all formats, the phase modulation ones presented the highest resistance against nonlinear and dispersive effects.