Dissertations / Theses on the topic 'Propagative regime'

L'objectif de cette thèse est de clarifier les conditions d'émergence en métaux liquides des ondes d'Alfvén dans un domaine géométriquement contraint. La première partie de ce travail de recherche est consacrée à une étude linéaire des ondes d'Alfvén dans l'approximation bas-Rm et en régime non inertiel. La seconde partie porte sur l'étude expérimentale d'un écoulement oscillant forcé électriquement, soumis à un champ magnétique axial, statique et uniforme, et confiné entre deux parois horizontales rigides, sans glissement et électriquement isolantes.Dans l'étude théorique menée, une première partie vise à discuter la relation de dispersion pour la dynamique des ondes d'Alfvén. Elle présente les conséquences liées à des gradients (mécaniques et magnétiques) perpendiculaires au champ magnétique imposé, plus particulièrement la manière dont la propagation de l'onde est ainsi modifiée. Dans la deuxième partie, un vortex axisymétrique confiné entre deux parois horizontales isolées électriquement et sans glissement est magnétiquement forcé à une fréquence donnée. Ce forçage prend en compte le rayon du vortex afin d'étudier l'impact des gradients transversaux sur la dynamique de l'écoulement. Une étude semi-analytique de la dynamique de l'écoulement est à nouveau réalisée dans un cadre bas-Rm et non inertiel. Cette étude, réalisée en faisant varier la fréquence de forçage et l'intensité du champ magnétique, met en évidence deux régimes très distincts, à savoir un premier régime oscillant-diffusif, régi par la compétition entre l'effet pseudo-diffusif de la force de Lorentz et le terme instationnaire de la quantité de mouvement, et un second régime, propagatif, régi par les ondes d'Alfvén et obtenu pour des fréquences de forçage plus élevées. L'étude met également en évidence l'impact des gradients transversaux sur ce régime propagatif. En plus de sur-amortir les ondes, les gradients transversaux modifient les fréquences naturelles des pics de résonance d'ondes, lesquels résultent de la superposition d'ondes incidentes et réfléchies entre les parois du domaine d'étude.Parallèlement à ce travail théorique, un dispositif a été conçu afin d'étudier expérimentalement la dynamique d'écoulements oscillants sous un champ magnétique (jusqu'à 10T). Un écoulement est forcé dans un récipient cubique de 15 cm x 15 cm x 10 cm au moyen d'un courant alternatif injecté à l'aide de quatre électrodes situées sur la plaque inférieure. En utilisant une instrumentation basée sur les différences locales de potentiel électrique aux niveau des plaques (d'Hartmann) supérieure et inférieure, nous validons les prédictions du modèle. Plus précisément, nous retrouvons un régime propagative modifié par les gradients transversaux ainsi que le régime oscillant-diffusif, obtenu pour des fréquences de forçage plus faibles.En plus des résultats obtenus à la fréquence de forçage, un premier aperçu des signaux obtenus à d'autres fréquences est présenté. Certains des pics de fréquence obtenus ne pouvant pas être expliqués par une approche linéaire, nous suggérons qu'ils sont générés par des interactions non linéaires d'ondes d'Alfvén. En outre, une étude préliminaire sur le pic à la première harmonique de la fréquence de forçage montre qu'il est très probablement associé à des ondes d'Alfvén
The thesis aims to clarify the conditions for Alfvén waves to propagate in a closed liquid metal domain. A first part of the research work presented is dedicated to a linear study of Alfvén waves in the low-Rm approximation and under the inertia-less limit. The second part is the experimental investigation of an electrically-induced oscillating flow subjected to an axial, static and uniform magnetic field and confined between two electrically insulating and no-slip horizontal walls.The theoretical study is itself split into two sub-parts. The first one aims to discuss the dispersion relation which contains the Alfvén wave dynamics. It presents the consequences of (mechanical and magnetic) gradients perpendicular to the imposed magnetic field. As such transverse gradients tend to impede the wave propagation. In the second sub-part an axisymmetric vortex confined between to electrically insulated and no-slip horizontal walls is magnetically forced at a given frequency. This forcing is radially dependent so as to study the impact of transverse gradients on the flow dynamics. A semi-analytical investigation of the flow dynamics is again carried out in the low-Rm approximation and under the inertia-less limit. This investigation is performed by varying the forcing frequency and the magnetic field intensity. This brings to emphasize two very distinct regimes for the oscillating vortex:- an oscillating-diffusive regime governed by the competition between pseudo-diffusive effects of the Lorentz force and the unsteady term of the momentum- a truly propagative regime, obtained for higher forcing frequencies, found definitelygoverned by Alfvén waves.The study also highlights how the propagative regime can be affected by transverse gradients. In addition to over-damping the waves, transverse gradients are found to modify the natural frequencies for which wave resonance peaks result from the superimposition of incident and reflected waves in the container.Beside this theoretical work, a setup has been designed in order to experimentally investigate the dynamics of oscillating flows under a strong magnetic field (up to 10T). A flow was forced in a cuboid vessel 15 cm x 15 cm x 10 cm by means of AC currents injected through a cartesian grid of four electrodes located at the bottom plate. Using instrumentation based on the measurement of local electric potential differences at the top and bottom horizontal (Hartmann) plates, we validate model's prediction. More precisely, a propagative dynamics in the presence of transverse gradients is recovered. The oscillating-diffusive regime is also recovered from experiments performed at small enough forcing frequency.In addition to results obtained at the forcing frequency, a first insight of signals obtained at other frequencies is shown. Frequency peaks obtained, eg the harmonics of the forcing frequency, are demonstrated not to be explained by a linear approach. We suggest that Alfvén wave non-linear interactions are a good candidate to explain these peaks. A preliminary study further shows that peaks at the first harmonic are likely to be Alfvén waves

Nous étudions la propagation d'une impulsion lumineuse à
travers un milieu diffusant. La propagation peut être décrite
par l'équation de transfert radiatif en régime dynamique
(ETR). Nous proposons une méthode de résolution en
géometrie plane. Elle consiste à appliquer la méthode
des ordonnées discrètes dans le domaine fréquentiel de
l'ETR. Nous calculons ainsi le flux d'énergie directionnel
transmis et rétrodiffuse en fonction du temps et de l'angle.

Dans la seconde partie, nous étudions la validité de
l'approximation de la diffusion. Cependant, l'expression théorique
du coefficient D en milieu absorbant varie selon l'approche
adoptée. Une nouvelle approche fondée sur l'analyse du mode
fondamental de l'ETR est développée. Sans faire aucune
hypothèse concernant le niveau d'absorption, nous obtenons une
nouvelle définition et interprétation de D.

La troisième partie est consacrée à l'étude de la
transition entre le regime balistique et le regime
diffusif. Nous montrons que les reflexions internes jouent un
rôle important pour les systèmes à faible épaisseur
optique. Nous montrons ainsi que le régime diffusif est atteint
aux temps longs, pour des systèmes de taille 8L ou L est la
longueur de transport.

Enfin, dans la derniere partie, nous modélisons la
corrélation temporelle du signal diffuse. Deux théories
(QELS et DWS) permettent de modéliser respectivement le signal en
diffusion simple et en régime diffusif. Nous avons
développe un modèle décrivant les deux régimes
ainsi que le régime intermédiaire. Le modèle est fondé
sur une approche de type marche au hasard et sur la résolution de
l'équation de transfert radiatif dynamique. Ce modèle a permis
de décrire des expériences récentes de mesures de
fluctuations temporelles de champs et d'intensite dans le
régime de diffusion multiple intermédiaire.

Nous presentons dans ce manuscrit une etude de l'interaction d'impulsions laser ultra-intenses avec un plasma sous-dense. Nous nous sommes plus particulierement interesses a l'influence des effets relativistes sur la propagation et a la mise en evidence de l'autofocalisation relativiste. Celle-ci intervient lorsque la puissance du laser est superieure a une puissance critique d'autofocalisation. Pour une puissance laser inferieure a cette puissance critique, la divergence du faisceau est reduite apres interaction avec le plasma. Une modelisation analytique et numerique de l'interaction montre que la non-linearite relativiste est responsable de cet effet observe experimentalement. Lorsque la puissance laser depasse la puissance critique, l'autofocalisation relativiste a pu etre clairement mise en evidence. Alors que la focalisation du faisceau dans le vide permet d'atteindre un eclairement de 3x10#1#8 w/cm#2, la simulation des experiences montrent qu'un eclairement de 5x10#1#9 w/cm#2 est atteint dans le plasma apres autofocalisation. D'autre part, des foyers multiples apparaissent et l'impulsion est alors guidee sur une distance tres superieure a la longueur de rayleigh. Pour des impulsions d'une picoseconde de duree et un plasma d'hydrogene, les ions ont le temps de bouger pendant la duree de l'impulsion, favorisant l'apparition d'un canal de densite electronique. Celui-ci est alors responsable d'une accentuation du phenomene de reduction de divergence et la puissance critique d'autofocalisation peut etre reduite dans nos experiences d'un facteur deux

Nous etudions, theoriquement, numeriquement et experimentalement, la propagation d'ondes acoustiques dans des systemes unidimensionnels desordonnes constitues par un empilement alterne de couches metalliques d'impedance acoustique differente et d'epaisseur aleatoire. Pour etudier le regime de propagation lineaire, nous avons utilise un formalisme de matrice de transfert qui permet de reduire la recherche du coefficient de transmission d'un empilement a l'etude des proprietes de produits de matrices 2 2 unimodulaires et aleatoires. Contrairement a un milieu parfaitement periodique qui presente des bandes interdites en frequences, un milieu desordonne ne permet pas la propagation des ondes qui sont exponentiellement attenuees. Cette decroissance exponentielle est definie par la longueur de localisation qui est deduite de l'equation self-consistente de dyson-scmidt ; nous avons resolu cette equation par un schema perturbatif qui nous permet de donner la valeur de l'amortissement des ondes dans la limite du faible de desordre (regime de diffusion rayleigh). Nous avons montre numeriquement que pour un empilement realiste sur le plan experimental le comportement du coefficient de transmission est eloigne de la prediction theorique obtenue dans la limite asymptotique. Nous prevoyons l'ouverture des bandes interdites secondaires entre celles de l'empilement periodique que nous avons faiblement desordonne. Pour etudier la propagation en regime non-lineaire nous avons integre numeriquement l'equation differentielle hyperbolique qui regit une telle propagation en utilisant un algorithme d'analyse base sur un schema des differences finies. Sur le plan experimental nous fabriquons les echantillons en chauffant a plusieurs centaines de degres un empilement de feuillets fer et cuivre ou aluminium et cuivre d'une epaisseur respective moyenne de 100 m, enserre dans un cylindre de molybdene. Il est necessaire de determiner une temperature et un temps de recuit ideal pour obtenir une bonne qualite d'interfaces car les metaux choisis forment des phases supplementaires aux interfaces pendant l'interdiffusion thermique. Les echantillons ont ete caracterise par microscopie electronique a balayage et spectrometrie a selection en energie. Les etudes acoustiques ont ete realise selon des techniques hyperfrequences classiques, bases sur la methode pulse-echo en mode transmission. Nous determinons ainsi le spectre de transmission de la structure moyenne sous-jacente qui presente des bandes interdites dans un domaine frequentiel qui s'etale entre 5-30 mhz. Nous mesurons la vitesse de phase en utilisant la methode zero-crossing permettant de mettre en evidence le caractere dispersif du milieu. La mesure de l'attenuation intrinseque d'un materiau multicouche desordonne et la comparaison avec celle d'un echantillon periodique nous a permis de constater l'existence d'une absorption supplementaire due au desordre. Les experiences en regime non-lineaire que nous avons entreprises ont montre que la forte attenuation, liee a la structure granulaire des feuillets, inhibe dans ces empilements les phenomenes non-lineaires, tels que la generation d'harmoniques superieurs.

The federally endangered tan riffleshell (Epioblasma florentina walkeri) is restricted to only one known reproducing population, in Indian Creek, Tazewell County, Virginia. Attempts to recover this species by augmenting relic populations throughout its historic range are aided through knowledge of its population biology and requirements in culture environments. Infestations of host fish (fantail darters, Etheostoma flabellare), obtained from four river drainages, with tan riffleshell glochidia showed that significantly more juveniles transformed per fish from infestations on fantail darters from Indian Creek (mean = 59.22 ± 10.01) than on fantail darters from the Roanoke River (mean = 9.45 ± 10.64) (p = 0.024). Number of juveniles from fantail darters collected from Elk Garden and the South Fork Holston River were not significantly different from those of either Indian Creek fish or Roanoke River fish. These results support the hypothesis that mussel-host fish relationships are likely mediated by fish immune responses. Furthermore, this study suggests that this compatibility has resulted from coadaptation between the tan riffleshell and fantail darter populations in Indian Creek.

The tan riffleshell population in Indian Creek was estimated to be 1078 adults (95% CI= 760 - 1853), using Schumacher's modification of Schnabel's maximum likelihood estimator. The sex ratio and size distribution of males and females were approximately equal. Specimen ages, determined from thin-sections of shells, showed that mussels aged by external annuli on shells likely underestimates the true ages of individuals.

Appropriate culture conditions for this species were examined using juveniles of the wavyrayed lampmussel (Lampsilis fasciola) as a surrogate. In the first experiment, juvenile growth and survival was compared between four substratum types (fine sediment, < 120μm; fine sand, 500 μm-800 μm; coarse sand, 1000 μm-1400 μm; and mixed sediment, < 1400 μm) and two light treatments in open versus covered recirculating troughs (2.8 m). Juveniles in fine sediment substratum and covered troughs fared poorest, with 7% survival and growth to only 0.86 mm in length after 16 wk. Juveniles in mixed sediment and open troughs fared best, with 26% survival and growth to 1.09 mm after 16 wk. Additionally, juveniles in fine sand in covered troughs had significantly higher survival (23.1%) than juveniles in fine sediment (p = 0.04), and juveniles in fine sand survived consistently better between light treatments than in the other substrata. There were no significant differences among the other treatments.

A second experiment was performed to determine whether juveniles were responding directly to the presence of light or whether only the increased autochthonous production improved growth and survival. One-half of each of three 2.8 m troughs were covered with 50% shade cloth, while the other sides were left open to ambient light. Additionally, the best and worst sediments from the first experiment (fine sand and fine sediment) were used again to verify the results from the previous experiment. In this case, juveniles in both sides of the troughs grew equally well, but juveniles in the open sides had significantly poorer survival (open mean: 1.78%, sd = 5.01; covered mean: 7.4%, sd = 5.01) (p = 0.046). Fine sediment yielded significantly higher growth of juveniles than fine sand (p = 0.009), with shell lengths of 2.63 mm (sd = 0.075) in fine sediment and 1.94 mm (sd = 0.102) in fine sand. The differences in survival and growth between the two experiments were attributed to differential numbers of chironomids and platyhelminths, which are predators of young juveniles. Additionally, the fine sediment was more tightly packed in the first experiment than in the second, which may have restricted movement and subsequently reduced survival. Light alone likely did not affect juvenile survival and growth; rather, it was seemingly the greater abundance of aufwuchs available as food. This hypothesis was corroborated by a juvenile behavior experiment, which showed that juveniles did not act differently when in tanks not exposed to light versus those open to ambient light.
Master of Science

Dans le procede silva (separation isotopique par laser en vapeur atomique), la photo-ionisation de l'uranium 235 est realisee selectivement a l'aide de plusieurs lasers. Pour utiliser au mieux les photons, les impulsions laser se propagent sur de grandes distances au sein de la vapeur atomique. Au cours de leur trajet, elles subissent eventuellement des deformations spatiales et temporelles qui peuvent abaisser l'efficacite de l'excitation. Pour optimiser celle ci, il est necessaire de savoir calculer l'evolution des caracteristiques des faisceaux laser et de la vapeur sur toute la zone d'interaction. L'objectif de ce travail est de contribuer a la validation d'une approche simplifiee (formulation cinetique) pour decrire les phenomenes lies a la propagation resonante d'une impulsion laser en regime semi-coherent. A cette fin, la demarche qui ete suivie se base sur une etude numerique et une etude experimentale sur l'europium. De leur confrontation est nee la necessite de developper un nouveau code de calcul, euro, dont l'originalite tient a la prise en compte complete de la structure hyperfine et qui reduit significativement l'ecart entre les previsions des codes et les resultats experimentaux. Le code euro sert alors de reference pour etudier dans un certain nombre de situation la pertinence du calcul cinetique.

Thesis (M.S. in Engineering Acoustics and M.S. in Electrical Engineering)--Naval Postgraduate School, March 2007.
Thesis Advisor(s): John Colosi. "March 2007." Includes bibliographical references (p. 81-82 ). Also available in print.

Transmissible spongiform encephalopathies (TSEs) or prion diseases are caused by a post-translational conversion of the normal cellular form of the prion protein (PrPC) into the pathological and infectious isoform denoted as prion or PrPSc. PrPC has been shown as a high-affinity copper-binding protein, and to a lesser extent binding to other divalent cations through the octarepeat region (OR) and the non-OR copper binding sites located in the disordered N-terminal domain. Studies on the role of copper in promoting prion conversion and infectivity yielded controversial results. In this work, we explored the role of histidine residues which are crucial for copper coordination in prion conversion using a combination of cell culture and cell-free approaches. The first evidence was derived from chronically prion-infected neuronal murine cells (ScN2a) transiently expressed in murine PrPC carrying artificial mutations at histidines located both at the OR and non-OR regions. We found that the lack of each histidine in the OR has neither effect on prion replication nor protein maturation and trafficking. Intriguingly, mutagenesis of histidine 95 (H95Y) does enhance prion conversion leading to de novo infectious material formation and cause aberrant accumulation during protein trafficking. Thus, we hypothesize that H95 could function as molecular switch for prion conversion, and copper bound to this residue may function in protein conformation stabilization. We also propose a cellular model for prion formation in cells expressing the H95Y mutant. Interestingly, our data may establish a platform for rationally designed experiments aimed at elucidating whether the H95Y mutation may cause de novo prion diseases in transgenic mice.

This thesis addresses the problem of inference in factor graphs, especially the LDPC codes, almost solved by message-passing algorithms. In particular, the Belief Propagation algorithm (BP) is investigated as a particular message-passing algorithm whose suboptimality is discussed in the case where the factor graph has a loop-like topology. From the equivalence between the BP and the Bethe approximation in statistical physics that is generalized to the region-based approximation, is detailed the Generalized Belief Propagation algorithm (GBP), a message-passing algorithm between clusters of the factor graph. It is experimentally shown to surpass the BP in the cases where the clustering deals with the harmful topological structures that prevents the BP from rightly decoding any LDPC code, namely the trapping sets. We do not only confront the BP and the GBP algorithms according to their performance from the point of view of the channel coding with the error-rate, but also according to their dynamical behaviors for non-trivial error-events for which both algorithms can exhibit chaotic beahviors. By means of classical and original dynamical quantifiers, it is shown that the GBP algorithm can overcome the BP algorithm.

L'objectif de ce travail est d'étudier la propagation des précurseurs du claquage ("streamers" en anglais) et leurs propriétés électriques dans l'huile minérale à grande distance sous tension impulsionnelle (jusqu'à 480 kV) dans diverses conditions de tension appliquée (de la tension minimum de propagation jusqu'aux fortes surtensions), de distance inter¬électrode (de 2,5 à 35 cm), de géométrie des électrodes (pointe-plan, semi-uniforme, sphère-plan) et en présence de solides isolants. Ce travail apporte une contribution sur la caractérisation des phénomènes de claquage et des mécanismes mis en jeu dans les streamers positifs. Pour des tensions inférieures à la tension de claquage, l'arrêt, la transition au claquage et la conductibilité des streamers sont décrits. En particulier, il a été montré que le streamer n'est conducteur que durant une réillumination totale de sa branche principale. En présence de surtensions, l'existence d'une tension d'accélération corrélée à l'apparition des streamers rapides est mise en évidence. Trois modes de propagation différents sont identifiés et caractérisés: 2e™ mode (2 à 4 km/s), 3eme mode (10 à 20 km/s) et 4cme mode (>100 km/s). Une corrélation entre la forme, la vitesse et la charge des streamers a été mise en évidence et la chute de tension dans le streamer a été déterminée. Les calculs de la charge et du champ sur l'électrode plane réalisés par la Méthode de Simulation de Charge (CSM) en considérant différents modèles macroscopiques en présence d'une chute de tension ont permis de conclure que les streamers sont comparables à des objets assez conducteurs de forme simple (cylindre ou sphère). Une corrélation qualitative entre le champ macroscopique à l'extrémité des streamers calculé par CSM, et la vitesse a été obtenue. E existe un champ critique =400 kV/cm, au-delà duquel les streamers sont toujours rapides (> 10 km/s). Ce travail propose deux mécanismes complémentaires permettant d'expliquer la constance de la vitesse des streamers positifs dans l'huile minérale: l'effet de la chute de tension et/ou l'effet électrostatique dû à la ramification des streamers. H a été montré que la forme globale des streamers détermine le champ à leur extrémité et par conséquent les modes de propagation. Ceci a été vérifié en étudiant l'influence de la géométrie des électrodes (effet d'écran), des solides isolants (effet de la position) et de la nature du liquide (effet des additifs)
The aim of this work is to study the propagation of the "streamers" and their electrical properties in mineral oil at long distances under pulsed voltage (up to 480 kV) under various voltage conditions (from the minimum propagation voltage to the high overvoltages), inter-electrode distance (2. 5 to 35 cm), electrode geometry (plane-to-plane, semi-uniform, planar sphere) of solid insulators. This work makes a contribution on the characterization of the breakdown phenomena and the mechanisms involved in the positive streamers. For voltages below the breakdown voltage, stopping, transition to breakdown and conductivity of the streamers are described. In particular, it has been shown that the streamer is conductive only during a total re-ignition of its main branch. In the presence of overvoltages, the existence of an acceleration voltage correlated with the appearance of the fast streamers is highlighted. Three different propagation modes are identified and characterized: 2nd mode (2 to 4 km / s), 3rd mode (10 to 20 km / s) and 4th mode (> 100 km / s). A correlation between the shape, speed and load of the streamers was highlighted and the voltage drop in the streamer was determined. Calculations of the charge and field on the plane electrode carried out by the Load Simulation Method (CSM), considering different macroscopic models in the presence of a voltage drop, have led to the conclusion that the streamers are comparable to objects conductors of simple shape (cylinder or sphere). A qualitative correlation between the macroscopic field at the end of the streamers calculated by CSM, and the speed was obtained. E there exists a critical field = 400 kV / cm, beyond which the streamers are always fast (> 10 km / s). This work proposes two complementary mechanisms allowing to explain the constancy of the speed of the positive streamers in the mineral oil: the effect of the voltage drop and / or the electrostatic effect due to the branching of the streamers. It has been shown that the overall shape of the streamers determines the field at their end and hence the propagation modes. This has been verified by studying the influence of electrode geometry (shielding effect), insulating solids (positional effect) and the nature of the liquid (additive effect)

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A família Bromeliaceae apresenta distribuição essencialmente neotropical, contando com 52 gêneros que abrangem 3.320 espécies. Das espécies de bromélias, 34% são exclusivas da Mata Atlântica, sendo que Portea petropolitana e Billbergia horrida se enquadram nessa categoria. O acelerado processo de destruição dos ecossistemas, somado ao extrativismo predatório, tem levado a reduções drásticas nas populações. Apesar de sua importância ecológica, medicinal e ornamental, a realização de pesquisas fisiológicas com bromélias ainda é rara. A cultura de tecidos apresenta-se como uma alternativa bastante atrativa para a conservação de espécies ameaçadas, possibilitando a obenção de taxas elevadas de multiplicação a partir de diferentes explantes. Visando promover a conservação in vitro de plantas de Billbergia horrida, Portea petropolitana e Tillandsia polystachia assim como a multiplicação em larga escala, este estudo teve por objetivos o estabelecimento de protocolos eficientes de multiplicação utilizando diferentes reguladores de crescimento (BAP, CIN, TDZ e GA3) buscando identificar as concentrações ideais dos mesmos para o cultivo de cada uma das espécies. O tratamento que promoveu as maiores taxas de multiplicação in vitro de P. petropolitana foi o suplementado com BAP a 7,5 μM. As maiores taxas de enraizamento foram observadas no tratamento suplementado com 5 μM de GA3. Para B. horrida o tratamento que promoveu as maiores taxas de propagação in vitro foi o suplementado com BAP a 2,5 μM. Plântulas com maior número de raízes foram obtidas em resposta à suplementação do meio de cultura com GA3 a 2,5 μM. O tratamento que promoveu a melhor taxa de propagação in vitro de T. polystachia foi o suplementado com BAP a 2,5 μM. Um maior número de raízes foi obtido em resposta ao GA3 a 10 μM. Plântulas das três espécies foram eficientemente aclimatizadas ex vitro. Devido à presença do O2 na atmosfera terrestre, a formação de espécies reativas de oxigênio (EROs) é um processo inevitável em eventos metabólicos essenciais como a fotossíntese e a respiração. Os sistemas de defesa que promovem a eliminação das EROs envolvem mecanismos enzimáticos e não enzimáticos. Embora alguns estudos tenham investigado a separação funcional existente ao longo das folhas de bromélias epífitas com tanque, alguns deles inclusive envolvendo enzimas essenciais ao metabolismo vegetal, pesquisas relacionando os sistemas enzimáticos e isoenzimáticos antioxidativos nas diferentes regiões das folhas de bromélias inexistem até o momento. No presente estudo, em plantas mantidas em condições de campo, foram avaliadas as atividades enzimáticas brutas e os perfis isoenzimáticos das enzimas catalase (CAT), peroxidase (POD) e superóxido dismutase (SOD) em diferentes regiões das folhas (apical, mediana e basal) de Portea Petropolitana e Billbergia horrida, assim como em diferentes estratos de altura de um forófito (inferior, mediano e superior) em Tillandsia polystachia. Padrões específicos de expressão das isoenzimas SOD e POD foram observados nas diferentes porções das folhas de P. petropolitana. A atividade da CAT não foi detectada em nenhuma das porções das folhas das três espécies. O maior número de isoformas da SOD e a maior intensidade da isoforma da POD, observados na região apical das folhas de P. petropolitana, estão, aparentemente, relacionados à atividade metabólica mais intensa resultante da maior atividade fotossintética e do nível de maturidade dessa porção da folha. Esses resultados foram corroborados pelas maiores atividades dessas enzimas também observadas na porção apical das folhas por meio de métodos espectrofotométricos. A região apical da folha é o principal sítio de assimilação fotossintética e de biossíntese de parede celular e lignina nas bromélias, envolvendo processos metabólicos normalmente relacionados à geração de EROs. Nos estudos de eletroforese realizados com B. horrida um mesmo padrão de isoformas da SOD foi observado em todas as regiões foliares avaliadas. Todavia, para a POD, diferentes padrões de intensidade na expressão isoenzimática foram detectados nas diferentes partes da folha. A maior intensidade das isoformas da POD observada na região apical das folhas também parece estar relacionada à maior maturidade fisiológica dessa região, resultado corroborado pela atividade enzimática mais intensa observada para essa enzima na análise espectrofotométrica realizada com materiais provenientes da região apical da folha. Durante a senescência foliar, as PODs, juntamente com outras enzimas, estão envolvidas na biossíntese de parede celular e de lignina, através de mecanismos que envolvem a produção e o acúmulo de EROs. Em T. polystachia, um mesmo padrão de isoenzimas da SOD foi observado nos diferentes estratos do forófito. Nessa espécie, a atividade da POD não foi observada nas análises de eletroforese, embora tenha sido detectada por espectrofotometria, especialmente nas posições medianas e mais elevados do forófito. Na análise espectrofotométrica, as atividades das CATs e das SODs mostraram padrões de expressão similares, com valores mais elevados encontrados nas posições mais baixas e mais elevadas do forófito. Esses resultados revelaram a ocorrência de variação na atividade enzimática em plantas de T. polystachia em resposta à posição (altura) no forófito. O teor de proteínas também seguiu o mesmo padrão em resposta ao estrato de localização das plantas no forófito. Os resultados encontrados no presente estudo permitem concluir que, para as três espécies estudadas, o BAP foi o regulador de crescimento que promoveu as maiores taxas de multiplicação in vitro, sendo o GA3 o regulador de crescimento que possibilitou a obtenção de plântulas enraizadas e prontas para a fase de aclimatização. As três espécies de bromélias estudadas foram eficientemente aclimatizadas após três meses de cultivo in vitro. O perfil de expressão das isoenzimas da SOD e da POD variou conforme as regiões das folhas em P. petropolitana. Em B. horrida, a SOD exibiu o mesmo padrão de isoformas em todas as regiões foliares, embora diferentes padrões de intensidade na expressão isoenzimática da POD tenham sido detectados. Em T. polystachia, a SOD exibiu o mesmo perfil de isoenzimas nos diferentes estratos do forófito. Esses resultados são os primeiros relatos sobre a variação espacial da atividade enzimática antioxidativa e da expressão isoenzimática em folhas de espécies de bromélias.
Bromeliaceae has essentially neotropical distribution, with 52 genera covering 3,320 species. The bromeliad species, 34% are unique to the Atlantic Forest, Portea petropolitana and Billbergia horrida fall into this category. The accelerated process of destruction of ecosystems, coupled with the predatory extraction has led to drastic reductions in populations. Although its ecological importance, medicinal and ornamental, conducting physiological studies with bromeliads is still rare. Tissue culture is presented as a very attractive alternative for the conservation of endangered species, allowing obenção of high multiplication rates from different explants. To promote in vitro conservation Billbergia horrida, Portea petropolitana and Tillandsia polystachia plants as multiplication on a large scale, this study aimed to establish efficient multiplication protocols using different growth regulators (BAP, KIN, TDZ and GA3 ) seeking to identify the optimal concentrations there of for the cultivation of individual species. The treatment produced the greatest in vitro multiplication rates of P. petropolitana was supplemented with BAP at 7.5 uM. The highest rooting rates were observed in the treatment supplemented with 5 uM of GA3. To B. horrida treatment which produced the greatest of in vitro propagation rates was supplemented with 2.5 uM BAP. Seedlings with larger roots were obtained in response to the supplementation of culture medium with GA3 at 2.5 uM. The treatment promoted the best in vitro propagation rate T. polystachia was supplemented with 2.5 uM BAP. A higher number of roots was obtained in response to GA3 10 uM. Seedlings of the three species were efficiently acclimatized ex vitro. Due to the presence of O2 in the atmosphere, the formation of reactive oxygen species (ROS) is an inevitable process in essential metabolic events such as photosynthesis and respiration. Defense systems that promote the elimination of ROS involving enzymatic and non-enzymatic mechanisms. Although some studies have investigated the existing functional compartmentalization along the leaves of epiphytic bromeliad tank, including some involving enzymes essential to plant metabolism, research relating the enzyme systems and antioxidative isozyme in different regions of the bromeliad leaves do not exist yet. In this study, in plants grown under field conditions, gross enzymatic activities and enzyme profiles of catalase enzymes were evaluated (CAT), peroxidase (POD) and superoxide dismutase (SOD) in different regions of the leaves (apical, median and basal ) of Portea petropolitana and Billbergia horrida, and in different strata of height of a host tree (lower, middle and higher) in Tillandsia polystachia specific patterns of expression of SOD isoenzymes and POD were observed in different portions of the sheets P. petropolitana. The CAT activity was not detected in any of the portions of the sheets of the three species. The largest number of isoforms of SOD and the greater intensity of isoform of POD, observed in the apical region of P. petropolitana leaves are apparently related to more intense metabolic activity resulting from higher photosynthetic activity and the level of maturity of this portion of the leaf. These results were supported by higher activity of these enzymes also observed in the apical portion of the leaves through spectrophotometric methods. The apical region of the leaf is the primary photosynthetic assimilation site and cell wall biosynthesis and lignin in bromeliads, involving metabolic processes typically related to the generation of ROS. In electrophoresis studies with B. horrida the same pattern of SOD isoforms was observed in all evaluated foliar regions. However, for the POD different intensity patterns in the isozyme expression were detected in different parts of the sheet. The greatest intensity of POD isoforms observed in the apical region of the leaves also seems to be related to increased physiological maturity of the region, a result confirmed by more intense enzymatic activity observed for this enzyme in the spectrophotometric analysis with materials from the apical region of the sheet. During senescence, the pods along with other enzymes, are involved in cell wall biosynthesis and lignin, through mechanisms that envovem the production and accumulation of ROS. In T. polystachia, the same pattern of SOD isoenzymes was observed in different strata of the host tree. In this species, the POD activity was not observed in the electrophoresis analysis, although it has been detected by spectrophotometry, especially in middle positions and higher phorophyte. In the spectrophotometric analysis, the activities of CAT and SODs showed similar patterns of expression, with highest values observed in lower positions and higher phorophyte. These results revealed the occurrence of variation in the enzymatic activity in T. polystachia plants in response to the position (height) in phorophyte. The protein content also followed the same pattern in response to the stratum location of the plants in phorophyte. The results of this study allow us to conclude that, for the three species studied, the BAP was the growth regulator which promoted the highest in vitro multiplication rates, and the GA3 growth regulator which enabled obtaining rooted and ready seedlings for the acclimatization phase. The three species of bromeliads studied were efficiently acclimatized after three months of in vitro culture. The expression profile of the isoenzymes of SOD and POD varied as the regions of the leaves of P. petropolitana. In B. horrida, SOD exhibited the same pattern of isoforms in all foliar regions, although different intensity patterns in the isozyme expression of POD were detected. In T. polystachia, SOD showed the same profile of isozymes in different strata of the host tree. These results are the first reports of the spatial variation of antioxidative enzyme activity and isoenzyme expression in leaves of bromeliads species.

Une formulation de la densite electronique en fonction de l'altitude et de l'angle solaire est etablie pour la basse ionosphere, cette formulation est basee sur la resolution des equations differentielles relatives aux densites des charges caracteristiques du milieu ionise traite. La relation entre les problemes aeronomiques et les problemes de propagation des ondes electromagnetiques est etudiee en reliant la vitesse de phase des ondes tbf a certains parametres representatifs de l'atmosphere moyenne. Cette relation est ensuite utilisee pour evaluer les variations de l'altitude de reflexion des ondes tbf ainsi que pour la determination de la temperature dans la mesosphere

Ce sujet de thèse s’inscrit dans le contexte du contrôle intégré des structures, ou « Structural Health Monitoring » (SHM). Cette technique de contrôle non-destructif vise à utiliser un ou plusieurs capteurs, installés dans ou sur la structure d’intérêt. Le contrôle se fait in-situ et de façon périodique, afin d’obtenir des informations sur l’éventuelle apparition de défauts, tels que les défauts de corrosion pour les matériaux métalliques ou les défauts de délaminage pour les matériaux composites. Les données recueillies par les capteurs alimentent une analyse statistique dont le but est d’évaluer la santé de la structure au moment du contrôle, d’estimer son temps de vie restant et de faciliter la prise de décision quant à sa maintenance. Le SHM est de plus en plus présent dans de nombreux domaines industriels, en particulier dans le secteur aéronautique. Aussi le développement de modèles numériques pertinents comme performants constitue un atout majeur dans la conception de ces systèmes. Grâce à leur capacité à se propager sur de très grande distance, l’utilisation de capteurs ultrasonores générant des ondes guidées élastiques est une solution attirante. En pratique, des capteurs piézo-électriques fins, disposés à la surface de la structure, ou éventuellement enfouis pendant le procédé de fabrication, sont utilisés. Ils permettent l’émission et la réception des perturbations ultrasonores. Cependant, la nature dispersive des ondes guidées, combinée avec l’anisotropie inhérente aux matériaux composites rend difficile l’analyse des signaux obtenus lors du contrôle. De plus, proposer une modélisation fine de la propagation de ce type d’onde dans des configurations industrielles faisant intervenir des géométries complexes est une tâche difficile. Ces deux points constituent des obstacles non négligeables au développement de la méthodologie SHM, et l’objectif de cette thèse est de constituer l’ensemble des outils numériques qui permettront de proposer des solutions concrètes à ces problèmes
This thesis is embedded in the context of « Structural Health Monitoring ». This method of non-destructive testing aiming at monitoring in-situ an engineered structure is increasingly used in numerous industrial fields, e.g. the aeronautics industry. It is based upon elastic guided waves propagating over large distances. The interactions between incident wave fields and structural defects are gathered through a network of receiving sensors. The dispersive nature of guided waves, combined with the inherent anisotropy of some industrial materials, such as composites, makes the interpretation of the output signals difficult. The goal of this thesis is to provide meaningful numerical tools enhancing the understanding and analysis of propagation and interaction phenomena, appearing during the control experiment. The thesis lies between the physical and mathematical modelling of elastic waves and the construction of relevant numerical schemes, altogether in an innovating industrial context involving complex geometries and materials

Dans cette thèse est étudiée la propagation dans les guides d’ondes désordonnés. Dans un premier temps, les concepts et les outils de base pour l'étude de la propagation en guide d'onde complexe sont introduits. Nous présentons les différents régimes de transport et les phénomènes liés aux effets cohérents (comme les fluctuations universelles de conductance, l'augmentation de la rétrodiffusion ou la localisation d’Anderson). Outre les phénomènes remarquables associés au transport diffusif, les effets cohérents des ondes dans les milieux désordonnés se manifestent aussi lorsque la distribution spatiale des diffuseurs présente des symétries. Une observation d'un gain significatif et large bande de la transmission au travers de barrières opaques fait l'objet du deuxième chapitre. Nous en profitons pour introduire la théorie du transfert radiatif et l'équation de diffusion pour résoudre le problème de propagation en milieux désordonnés d'un point de vue énergétique. Puis la sensibilité du gain en transmission aux défauts de symétrie est étudiée dans le troisième chapitre. Les limites liées à l'absorption, toujours présente dans un contexte expérimental, sont inspectées. Le quatrième chapitre se focalise sur l'une des signatures du régime de localisation : la propagation à l'intérieur des milieux isolants est insensible à la source d'excitation. Nous présentons quelques cas particuliers ou cette propriété est rompue. Enfin, plusieurs études préliminaires liées à d'autres phénomènes de propagation en milieu complexe sont discutées comme perspectives à ce travail
This thesis focuses on the wave propagation in disordered waveguides. First, we introduce the basic tools and concepts in order to present the different transport regimes and phenomena related to coherent effects (such as universal conductance fluctuations, enhanced backscattering or Anderson's localization). Beside the remarkable phenomena associated to diffuse transport, the coherent effects are also noticeable when the spatial distribution of the scatterers displays symmetries. The second part is dedicated to the observation of a significant and broadband enhancement in transmission through opaque barriers. The sensitivity of this phenomenon to symmetry defects is discussed in Chapter 4. Limitations due to absorption, which are always present in an experimental context, and localization are also inspected. The fourth chapter focuses on one of the signatures of the localization regime: the propagation within these insulating media is insensitive to the source. We present some special cases in which this property is broken. Finally, several preliminary studies related to other propagation phenomena in complex media are discussed as perspectives for this work

First, we investigate the Bouguer-Lambert-Beer (BLB) law as applied to the transmission of ultrashort pulses through water in the linear absorption regime. We present a linear theory for propagation of ultrashort laser pulses, and related experimental results are in excellent agreement with this theory. Thus we conclude that recent claims of the BLB law violations are inconsistent with the experimental data obtained by our group. Second, we study the dynamics of ultrashort pulses in a Lorentz medium and in water via the saddle point method. It shows that the saddle point method is a more efficient and faster method than the direct integration method to study one-dimensional pulse propagation over macroscopic distances (that is, distance comparable to the wavelength) in a general dielectric medium. Comments are also made about the exponential attenuation of the generalized Sommerfeld and Brillouin precursors. By applying the saddle point method, we also determined that the pulse duration estimated by the group velocity dispersion (GVD) approximation is within 2% of the value computed with the actual refractive index for a propagation distance of 6 m in water.

Structural concrete is the most widely used material in the construction of bridges, pave-ments, runways, dams and other infrastructures which are subjected to uctuating loads during its service period. Concrete contains internal aws in the form of micro-cracks as an inherent property. When subjected to fatigue loading, distributed micro-cracks are formed at the sites of pre-existing aws, which subsequently, localize to form a major crack and propagates. The crack growth curve of a structural component when subjected to fatigue loading depicts a sigmoidal pattern. This curve is divided into three distinct regions namely sub-threshold crack propagation (short crack), stable crack propagation (long crack) and unstable crack propagation depending on the crack propagation rate. Most of the fatigue life is spent in the sub-critical stage (small crack) before the for-mation of long cracks. Hence, from the view of estimating the fatigue life, the crack initiation and early crack propagation (short crack stage) phase are the most important and correct concepts need to be developed. Hence, in this work, the behavior of propa-gation and non-propagationof short cracks in concrete when subjected to fatigue loading is addressed. Small non-propagating cracks are usually found at notch roots when the nominal stress range is below certain limits that depend on the notch sensitivity. Analysis is performed on geometrically similar three-point bend beams of three di erent sizes and subjected to fatigue loading in order to determine the important factors that a ect the notch sensitivity and to determine the minimum stress range required for the initiation and propagation of short cracks. A criterion for crack initiation and propagation is proposed based on linear elastic fracture mechanics. Using this criterion, the maximum length of non-propagating crack that can be formed from fatigue loading alone and the minimum stress range required to propagate a crack without arrest are computed. It is observed that the notch sensitivity increases with increase in beam size, decrease in notch-tip aspect ratio and increase in the fatigue limit of the material. Since the probability of formation of a non-propagating crack at a notch tip decreases with increase in notch sensitivity, and since it is desirable not to have a non-propagating crack in experimental investigations, it is essential to design a specimen with higher notch sensitivity. A crack spends a considerable amount of time in the short crack regime. The short cracks are found to propagate at higher rates than the long cracks at the same nominal stress intensity factor which is known as the short crack anomaly. It is important to consider this anomaly in the prediction of the residual life of damaged concrete structures. Hence, in the present work, an analytical model is developed using the principles of dimensional analysis and self-similarity in order to estimate the rate of short crack growth in concrete. The important parameters such as load range, threshold value of stress intensity factor range, modulus of elasticity, tensile strength, fracture energy, stress ratio, crack size and the maximum aggregate size are considered in the development of the short crack growth model. The model is calibrated and validated using the experimental results that are available in the literature. A probabilistic analysis is carried out to determine the sensitivity of each of the di erent parameters that has been considered on the crack growth rate using the coe cient of variation method. It is found that the crack length is the most sensitive parameter to short crack growth rate followed by the load range. A term called `characteristic fatigue life of short crack' is de ned as the number of fatigue cycles that can be applied such that not more than ve percent of the short cracks is expected to proceed to the long crack regime. Furthermore, the fatigue life of a crack spent in the short crack regime is determined through a reliability based study using the Monte Carlo technique. It is found that the smaller sized specimens have larger fatigue life in the short crack regime than the larger specimens.

Structural concrete is the most widely used material in the construction of bridges, pave-ments, runways, dams and other infrastructures which are subjected to uctuating loads during its service period. Concrete contains internal aws in the form of micro-cracks as an inherent property. When subjected to fatigue loading, distributed micro-cracks are formed at the sites of pre-existing aws, which subsequently, localize to form a major crack and propagates. The crack growth curve of a structural component when subjected to fatigue loading depicts a sigmoidal pattern. This curve is divided into three distinct regions namely sub-threshold crack propagation (short crack), stable crack propagation (long crack) and unstable crack propagation depending on the crack propagation rate. Most of the fatigue life is spent in the sub-critical stage (small crack) before the for-mation of long cracks. Hence, from the view of estimating the fatigue life, the crack initiation and early crack propagation (short crack stage) phase are the most important and correct concepts need to be developed. Hence, in this work, the behavior of propa-gation and non-propagationof short cracks in concrete when subjected to fatigue loading is addressed. Small non-propagating cracks are usually found at notch roots when the nominal stress range is below certain limits that depend on the notch sensitivity. Analysis is performed on geometrically similar three-point bend beams of three di erent sizes and subjected to fatigue loading in order to determine the important factors that a ect the notch sensitivity and to determine the minimum stress range required for the initiation and propagation of short cracks. A criterion for crack initiation and propagation is proposed based on linear elastic fracture mechanics. Using this criterion, the maximum length of non-propagating crack that can be formed from fatigue loading alone and the minimum stress range required to propagate a crack without arrest are computed. It is observed that the notch sensitivity increases with increase in beam size, decrease in notch-tip aspect ratio and increase in the fatigue limit of the material. Since the probability of formation of a non-propagating crack at a notch tip decreases with increase in notch sensitivity, and since it is desirable not to have a non-propagating crack in experimental investigations, it is essential to design a specimen with higher notch sensitivity. A crack spends a considerable amount of time in the short crack regime. The short cracks are found to propagate at higher rates than the long cracks at the same nominal stress intensity factor which is known as the short crack anomaly. It is important to consider this anomaly in the prediction of the residual life of damaged concrete structures. Hence, in the present work, an analytical model is developed using the principles of dimensional analysis and self-similarity in order to estimate the rate of short crack growth in concrete. The important parameters such as load range, threshold value of stress intensity factor range, modulus of elasticity, tensile strength, fracture energy, stress ratio, crack size and the maximum aggregate size are considered in the development of the short crack growth model. The model is calibrated and validated using the experimental results that are available in the literature. A probabilistic analysis is carried out to determine the sensitivity of each of the di erent parameters that has been considered on the crack growth rate using the coe cient of variation method. It is found that the crack length is the most sensitive parameter to short crack growth rate followed by the load range. A term called `characteristic fatigue life of short crack' is de ned as the number of fatigue cycles that can be applied such that not more than ve percent of the short cracks is expected to proceed to the long crack regime. Furthermore, the fatigue life of a crack spent in the short crack regime is determined through a reliability based study using the Monte Carlo technique. It is found that the smaller sized specimens have larger fatigue life in the short crack regime than the larger specimens.

Flame propagation through a channel equipped with obstacles was studied experimentally. Two types of obstacle geometries were investigated, i.e., wall-mounted cross-flow cylinders and fence-type obstacles mounted on the top and bottom channel surfaces. The motivation for this research is its applications to both high-speed propulsion and industrial explosion safety. The effect of obstacle distribution and blockage ratio on flame acceleration was investigated in a 2.54cm x 7.6cm “narrow” channel with wall-mounted cross-flow cylindrical obstacles. The cylinders were arranged in a “staggered” or “inline” pattern, with blockage ratios of 0.5 and 0.67. Schlieren images were used to study the flame shape and its leading edge velocity for a range of fuel-air mixtures compositions. It was determined that initial flame propagation occurs faster in higher blockage ratios due to the higher frequency perturbation to the flow. Flame acceleration led to different quasi-steady flame and detonation propagation regimes. In general, higher final steady flame velocities were reached in the lower blockage ratios, and detonation limits were found to be influenced by the geometry. The influence of channel width on flame acceleration was also determined using fence-type obstacles with a single blockage ratio. Experiments were performed in a 2.54cm x 7.6cm and 7.6cm x 7.6cm channel. Schlieren images were again used to study the flame shape and to obtain leading edge velocity. The flame tip was found to have a parabolic profile across the channel width for the narrower channel and flatter profile in the wider channel. It was determined that the channel width has a weak effect on the flame velocity down the channel length. As such, flame acceleration was initially only slightly more pronounced in the narrow channel before the reverse became true later in the wide channel.
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-07-18 21:13:40.436

Ce travail concerne l'étude de la résistance à la fatigue dans le domaine gigacyclique de deux matériaux. Le premier, AS7G06 est utilisé pour la fabrication des carters de turbines pour hélicoptères, le second, l'acier R5, est utilisé pour la fabrication des chaînes pour plates-formes pétrolières. L'alliage d'aluminium AS7G06 moulé utilisé dans les carters de turbines est soumis à un très grand nombre de cycles en raison de la durée d'utilisation et de la vitesse de rotation de la turbine, environ 30.000 tr/min. Les échantillons provenant de vraies pièces ont été testés à température ambiante et à 150 ° C, et sou s trois rapports de charge. La résistance à la fatigue à très grand nombre de cycles au delà du milliard de cycles a été étudié, l'amorçage des fissures est systématiquement dues aux retassures. L'acier R5, selon la dénomination internationale des Sociétés de la Classification Internationale des Systèmes Offshore est un acier faiblement allié. Cet acier à haute résistance est habituellement utilisé pour la fabrication de chaînes d'amarrage pour les plates-formes pétrolières offshore dans la Mer du Nord. Cet acier possède une haute résistance, et une bonne résistance à la corrosion. Les chaînes sont conçues pour 30 ans, et sont chargées à basse fréquence (~ 0,5 Hz) en raison des vagues dans en mer ; ceci représente plus de 108 cycles. Les échantillons provenant de chaînes en acier R5 ont été testés en fatigue à des rapports R différents, sous trois conditions différentes : éprouvettes vierges dans l'air ou pré-corrodées en brouillard salin puis cyclées à l'air ou bien éprouvettes cyclées in situ dans l'eau de mer synthétique. La résistance à la fatigue dans le régime gigacyclique et les causes de l'amorçage des fissures ont été étudiées. En particulier, nous avons cherché à comprendre l'effet des défauts sur la résistance à la fatigue gigacyclique. La rupture en fatigue dans l'alliage AS7G06 est souvent due aux porosités. Pour l'acier R5, des piqûres de corrosion sont souvent la cause de rupture par fatigue. Une modélisation de la durée de la phase de propagation des fissures de fatigue est proposé et montre qu'au delà de 10^7 cycles, la durée de vie est essentiellement majoritairement due à la phase d'amorçage 10des fissures et non a la phase de propagation.

Fatigue is a phenomenon caused by dynamic loads which is characterised by the propagation of cracks and can culminate in the complete fracture of a material. In the military aeronautical sector, particularly in pilot training, this problem takes on special importance given the diversity of missions and manoeuvres that are carried out. The Epsilon TB-30 aircraft is used in the Portuguese Air Force for basic piloting instruction in the 101st Fleet, at the Air Base 11 in Beja. Within the scope of Structural Integrity monitoring, a fleet aircraft was recently instrumented with a system that includes the installation of an accelerometer at the aircraft’s centre of gravity and five strain gauges in critical locations of the aircraft, ,which led to the definition of the main objectives of this work: the development of an automatic crack propagation tool for a critical location in the aircraft structure, as well as the comparison of the crack growth rate when subjected to a severe and non-severe flight regime, typical flights in the operation of the Epsilon fleet. In order to simulate the crack propagation, this work included a finite element model developed in Ansys software to obtain the stress intensity factor values for several crack sizes. It was also carried out, with the resource to Matlab, the crack propagation modeling using the Walker and NASGRO propagation laws to get an expression for the geometric factor Y. Subsequently, a comparison analysis of crack growth behaviour for different flight severity regimes, propagation laws and initial crack sizes was conducted. Finally, a web application was developed to straightforwardly and automatically simulate the crack propagation for a real-world flight.
A fadiga é um fenómeno causado por cargas dinâmicas que se caracteriza pela propagação de fissuras e pode culminar na rotura total de um material. No sector aeronáutico militar, particularmente na formação de pilotos, este problema assume especial importância dada a diversidade de missões e manobras que são levadas a cabo. O avião Epsilon TB-30 é utilizado na Força Aérea Portuguesa para instrução básica de pilotagem na Frota 101, na Base Aérea nº11 em Beja. No âmbito da monitorização da integridade estrutural, uma aeronave da frota foi recentemente instrumentada com um sistema que inclui a instalação de um acelerómetro no centro de gravidade da aeronave e cinco extensómetros em locais críticos da aeronave, o que levou à definição dos principais objectivos deste trabalho: o desenvolvimento de uma ferramenta de propagação automática de fenda para um local crítico na estrutura da aeronave, bem como a comparação na propagação de uma fenda quando sujeita a um regime de voo severo e não-severo, voos típicos na operacção da frota Epsilon. A fim de simular a propagação da fenda, este trabalho incluiu um modelo de elementos finitos desenvolvido no software Ansys para a obtenção dos valores do factor de intensidade de tensão para vários tamanhos de fenda. Foi também realizado, com o recurso a Matlab, a modelação da propagação da fenda utilizando as leis de propagação Walker e NASGRO para a obtenção de uma expressão para o factor geométrico, Y. Posteriormente, foi realizada uma análise comparativa do comportamento de crescimento da fenda para diferentes regimes de severidade de voo, leis de propagação e tamanhos iniciais da fenda. Finalmente, foi desenvolvida uma aplicação web que permite simular de forma simples e automática a propagação da fenda para um voo real.

L'objet de cette thèse se rapporte principalement au problème de la transmission d'ondes spériques à travers une interface plane séparant deux milieux fluides, la source se trouvant dans le milieu de plus faible célérité. Sous certaines conditions, ce modèles physique simple présente un intérêt particulier dû à l'existence d'une onde de "surface", appelée onde latérale ou inhomogène.
Dans un premier temps, nous avons traité le cas s'une source ponctuelle monochromatique à l'aide de méthodes asymptotiques, de façon à vérifier l'existence physique de cette contribution. Dans le cas du dioptre plan air-eau, nous avons pu séparer expérimentalement la contribution latérale de la contribution géométrique, et mettre en évidence le comportement et les propriétés de celles-ci. Le champ réfracté total fait alors apparaître des zones d'interférences en parfait accord avec l'étude théorique et numérique. La contribution latérale présentant un caractère "dispersif", montre l'intérêt dans le cas du régime transitoire, d'utiliser une méthode de type temps-échelle.
La fonction de Green peut être décomposée de façon naturelle en trois contributions analogues à celles du régime harmonique. La transformée en ondelettes permet alors de calculer de façon exacte ces différentes contributions et d'en étudier leur comportement. L'orginalité des résultats obtenus est la mise en évidence à certaines échelles, de phénomènes transitoires très brefs (échos) qui permettent d'engager une discussion nouvelle de ce type de problème. Une expérimentation combinée à une analyse temps-échelle (ondelettes) a confirmé ces observations.
Par analogie à la formule de reconstitution simple de la transformée en ondelettes inverse, nous avons pu élaborer, pour de grandes distances radiales, une formule de reconstruction de la dépendance temporelle du signal-source (problème inverse) à partir de la mesure de la pression transmise (jouant le rôle de "pseudo-coefficients d'ondelettes") sur les profondeurs.
Enfin, l'application de cette transformation à un problème de rétrodiffusion acoustique par des coques sphériques élastiques (interface fluide/solide) a montré qu'il est possible d'accéder à certaines caractéristiques physiques de la cible.

"July, 2003"
Includes bibliographical references (leaves 177-183)
xxii, 183 leaves : ill. (some col.) ; 30 cm.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Thesis (Ph.D.)--University of Adelaide, School of Chemistry and Physics, Discipline of Physics and Mathematical Physics, 2004

This thesis has two major objectives. The first objective is to investigate the seasonal and diurnal variations in occurrence of HF coherent echoes. We assess F-region echo occurrence rates for the PolarDARN HF radars at Inuvik (INV) and Rankin Inlet (RKN) and the auroral zone SuperDARN radars at Saskatoon (SAS) and Prince George (PGR) for the period of 2007-2010. We show that the INV and RKN PolarDARN radars show comparable rates of echo occurrence all the time and they detect 1.5-2.5 times more echoes through ½-hop propagation mode (MLATs=80°-85°) than the SAS and PGR SuperDARN radars through 1½-hope propagation mode (MLATs=75°-80°). For all four radars, the winter occurrence rates are about ~2 times higher than the summer rates. For observations in the dusk, midnight and dawn sectors, equinoctial maxima are evident. The pattern of echo occurrence in terms of MLT/season is about the same for all radars with clear maxima near noon during winters and summers and enhanced (as compared to other time of the day) occurrence rates during equinoctial dusk and dawn hours. Additionally, to investigate the effect of solar cycle on occurrence of F-region echoes, we consider the near noon and near midnight echo occurrence rates for the Saskatoon radar over the period of 1994-2010. We show that there is a strong, by a factor of ~10, increase in SAS night-side echo occurrence towards solar maximum. The effect does not exist for the dayside echoes; moreover, a decrease in number of echoes, by a factor of ~2, was discovered for the declining phase of the solar cycle. The second objective is to evaluate the electron density and the electric field as factors controlling the occurrence of F-region echoes. We use observations of these two ionospheric parameters measured by CADI ionosonde and RKN observations of echo occurrence rates over Resolute Bay (MLAT=83°). We show that there is a correlation in changes of echo occurrence and electron density changes for 3 years of radar-ionosonde joint operation (2008-2010). The comparison of radar-ionosonde data shows that the enhanced echo occurrence at near noon hours during summer months correlate with the enhanced electric field during these periods.