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Antarctic Wind Engineering deals with the effects of wind on the built environment. The assessment of wind induced forces, wind resource and wind driven snowdrifts are the main tasks for a wind engineer when participating on the design of an Antarctic building. While conventional Wind Engineering techniques are generally applicable to the Antarctic environment, there are some aspects that require further analysis due to the special characteristics of the Antarctic wind climate and its boundary layer meteorology. <p>The first issue in remote places like Antarctica is the lack of site wind measurements and meteorological information in general. In order to complement this shortage of information various meteorological databases have been surveyed. Global Reanalyses, produced by the European Met Office ECMWF, and RACMO/ANT mesoscale model simulations, produced by the Institute for Marine and Atmospheric Research of Utrecht University (IMAU), have been validated versus independent observations from a network of 115 automatic weather stations. The resolution of these models, of some tens of kilometers, is sufficient to characterize the wind climate in areas of smooth topography like the interior plateaus or the coastal ice shelves. In contrast, in escarpment and coastal areas, where the terrain gets rugged and katabatic winds are further intensified in confluence zones, the models lack resolution and underestimate the wind velocity. <p>The Antarctic atmospheric boundary layer (ABL) is characterized by the presence of strong katabatic winds that are generated by the presence of surface temperature inversions in sloping terrain. This inversion is persistent in Antarctica due to an almost continuous cooling by longwave radiation, especially during the winter night. As a result, the ABL is stably stratified most of the time and, only when the wind speed is high it becomes near neutrally stratified. This thesis also aims at making a critical review of the hypothesis underlying wind engineering models when extreme boundary layer situations are faced. It will be shown that the classical approach of assuming a neutral log-law in the surface layer can hold for studies of wind loading under strong winds but can be of limited use when detailed assessments are pursued. <p>The Antarctic landscape, mostly composed of very long fetches of ice covered terrain, makes it an optimum natural laboratory for the development of homogeneous boundary layers, which are a basic need for the formulation of ABL theories. Flux-profile measurements, made at Halley Research Station in the Brunt Ice Shelf by the British Antarctic Survery (BAS), have been used to analyze boundary layer similarity in view of formulating a one-dimensional ABL model. A 1D model of the neutral and stable boundary layer with a transport model for blowing snow has been implemented and verified versus test cases of the literature. A validation of quasi-stationary homogeneous profiles at different levels of stability confirms that such 1D models can be used to classify wind profiles to be used as boundary conditions for detailed 3D computational wind engineering studies. <p>A summary of the wind engineering activities carried out during the design of the Antarctic Research Station is provided as contextual reference and point of departure of this thesis. An elevated building on top of sloping terrain and connected to an under-snow garage constitutes a challenging environment for building design. Building aerodynamics and snowdrift management were tested in the von Karman Institute L1B wind tunnel for different building geometries and ridge integrations. Not only for safety and cost reduction but also for the integration of renewable energies, important benefits in the design of a building can be achieved if wind engineering is considered since the conceptual phase of the integrated building design process.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Cette thèse présente une étude empirique et théorique approfondie de l'évolution spectrale des étoiles naines blanches, avec un accent particulier sur les naines blanches chaudes. La composition atmosphérique (et donc l'apparence spectrale) de ces cadavres stellaires peut changer drastiquement avec le temps à mesure qu'ils se refroidissent. Ce phénomène est généralement interprété comme le résultat d'une compétition entre divers mécanismes de transport des éléments dans l'enveloppe stellaire (tels que la diffusion, la convection, les vents et l'accrétion), mais demeure mal compris à plusieurs égards. Il est impératif de remédier à cette situation pour être en mesure d'exploiter le potentiel immense des naines blanches pour notre compréhension du passé de la Galaxie. Pour mieux caractériser l'incidence de l'évolution spectrale, nous effectuons tout d'abord une analyse spectroscopique exhaustive de près de 2000 naines blanches chaudes (Teff > 30,000 K) observées par le relevé SDSS. Nous déterminons les propriétés atmosphériques (notamment la température effective et la composition de surface) de ces objets à l'aide d'un nouvel ensemble de modèles d'atmosphère calculé spécifiquement à cet effet. En examinant la fréquence relative des étoiles riches en hydrogène et riches en hélium en fonction de la température, nous obtenons pour la première fois un portrait empirique détaillé de l'évolution spectrale des naines blanches chaudes. Plus spécifiquement, nous déduisons (1) qu'environ une étoile sur quatre arrive sur la séquence de refroidissement avec une atmosphère d'hélium, et (2) qu'environ deux tiers de ces objets développent ultérieurement une atmosphère d'hydrogène. En outre, nous accordons une attention particulière aux naines blanches hybrides (qui montrent à la fois des traces d'hydrogène et d'hélium) de notre échantillon et à ce que ces objets distinctifs nous apprennent sur l'évolution spectrale. Nous étudions ensuite l'évolution spectrale d'un point de vue théorique en modélisant les transformations chimiques qui s'opèrent dans les naines blanches. Pour ce faire, nous utilisons le code d'évolution stellaire STELUM, qui inclut un traitement cohérent et réaliste du transport des éléments et nous permet donc de réaliser les simulations numériques d'évolution spectrale les plus sophistiquées à ce jour. Nous modélisons la diffusion de l'hydrogène résiduel dans une enveloppe d'hélium à haute température, qui mène ultimement à la formation d'une atmosphère d'hydrogène. Nous simulons également le mélange convectif de cette couche superficielle d'hydrogène avec la couche sous-jacente d'hélium à basse température, qui produit à nouveau une surface dominée par l'hélium. En outre, nous étudions le transport du carbone dans les étoiles riches en hélium, incluant à la fois le tri gravitationnel à haute température et le dragage convectif à basse température. Ces calculs donnent lieu à plusieurs résultats astrophysiques d'intérêt. Nous obtenons notamment une contrainte inédite sur la quantité d'hydrogène résiduel contenue dans les naines blanches chaudes dominées par l'hélium. Nous démontrons aussi que la bifurcation observée dans le diagramme couleur-magnitude des naines blanches découvertes par le satellite Gaia est une signature du processus de mélange convectif à basse température. Par ailleurs, nos modèles fournissent de précieuses informations sur les propriétés des étoiles polluées par le carbone, en particulier sur leur masse et leur zone convective. Enfin, le résultat le plus important de cette thèse est la résolution définitive du problème le plus sérieux de la théorie de l'évolution spectrale, soit le problème de l'origine de l'hydrogène à la surface des naines blanches de type DBA. / This thesis presents an in-depth empirical and theoretical study of the spectral evolution of white dwarf stars, with a particular focus on hot white dwarfs. The atmospheric composition (and thus the spectral appearance) of these stellar remnants can change drastically over time as they cool. This phenomenon is generally interpreted as the result of an interplay between various element transport mechanisms in the stellar envelope (such as diffusion, convection, winds, and accretion), but remains poorly understood in several respects. It is imperative to remedy this situation to be able to exploit the immense potential of white dwarfs for our understanding of the past of the Galaxy. To better characterize the incidence of spectral evolution, we first carry out an exhaustive spectroscopic analysis of nearly 2000 hot white dwarfs (Teff > 30,000 K) observed by the SDSS survey. We determine the atmospheric properties (in particular the effective temperature and surface composition) of these objects using a new set of model atmospheres calculated specifically for this purpose. By examining the relative frequency of hydrogen-rich and helium-rich stars as a function of temperature, we obtain for the first time a detailed empirical picture of the spectral evolution of hot white dwarfs. More specifically, we infer (1) that about one in four stars enters the cooling sequence with a helium atmosphere, and (2) that about two-thirds of these objects eventually develop a hydrogen atmosphere. Furthermore, we pay special attention to the hybrid white dwarfs (which exhibit traces of both hydrogen and helium) in our sample and to what can be learned about spectral evolution from these distinctive objects. We then study spectral evolution from a theoretical point of view by modeling the chemical transformations that take place in white dwarfs. To do this, we use the stellar evolution code STELUM, which includes a consistent and realistic treatment of element transport and therefore allows us to perform the most sophisticated numerical simulations of spectral evolution to date. We model the diffusion of residual hydrogen in a helium envelope at high temperature, which ultimately leads to the formation of a hydrogen atmosphere. We also simulate the convective mixing of this superficial hydrogen layer with the underlying helium layer at low temperature, which once again produces a helium-dominated surface. Furthermore, we study the transport of carbon in helium-rich stars, including both gravitational settling at high temperature and convective dredge-up at low temperature. These calculations give rise to several astrophysical results of interest. In particular, we obtain an unprecedented constraint on the amount of residual hydrogen contained within hot helium-dominated white dwarfs. We also demonstrate that the bifurcation observed in the color-magnitude diagram of white dwarfs discovered by the Gaia satellite is a signature of the convective mixing process at low temperature. Furthermore, our models provide valuable information on the properties of carbon-polluted stars, in particular on their mass and convective zone. Finally, the most important result of this thesis is the definitive resolution of the most serious problem of the theory of spectral evolution, namely the problem of the origin of hydrogen at the surface of DBA-type white dwarfs.

El fenomen atmosf��ric de les brises marines adopta a l���illa de Mallorca caracter��stiques protot��piques, i la seva efectivitat permet una f��cil constataci�� com�� i la subseq��ent verificaci�� cient��fica. Operant simult��niament des de les diferents costes, les brises adquireixen un car��cter unitari i centr��pet que ocasiona converg��ncies d���aire a ��rees de l���interior, a on la precipitaci�� estival tendeix a focalitzar-se. L���espai agrari de Mallorca s���ha conformat hist��ricament amb el massiu desplegament territorial d���infrastructures de base cereal��stica, fonamentalment eres de batre. El principal factor de la seva localitzaci�� ��s la disponibilitat de vent, tot constituint unitats de significaci�� e��lica susceptibles d���interpretar-se en clau clim��tica. La seva georeferenciaci�� per fotointerpretaci�� de l���ortofotografia de l���any 1956 s���ha tradu��t en un mapa sint��tic d���eres, m��s una metodologia in��dita per a determinar indirectament les direccions del vent a cada empla��ament. El model ���anemogr��fic��� resultant s���ajusta amb relativa bondat amb les simulacions del sistema de brises precedents. / El fen��meno atmosf��rico de las brisas marinas adopta en la isla de Mallorca caracter��sticas protot��picas, y su efectividad permite una f��cil constataci��n com��n y la subsiguiente verificaci��n cient��fica. Operando simultaneamente desde las diferentes costas, las brisas adquieren un car��cter centr��peto que ocasiona convergencias de aire en ��reas del interior, donde la precipitaci��n estival tiende a focalizarse. El espacio agrario de Mallorca se ha conformado hist��ricamente con el masivo despliegue territorial de infraestructuras de base cereal��stica, fundamentalmente eras de trillar. El principal factor de su localizaci��n es la disponibilidad de viento, constituyendo unidades de significaci��n e��lica susceptibles de interpretarse en clave clim��tica. Su georeferenciaci��n por fotointerpretaci��n de la ortofotografia del a��o 1956 se traduce en un mapa sint��tico de eras, m��s una metodolog��a in��dita para determinar indirectamente las direcciones del viento en cada emplazamiento. El modelo ���anemogr��fico��� resultante se ajusta con relativa bondad con las simulaciones del sistema de brisas precedentes. / In Mallorca, sea breeze presents prototypical characteristics. It is so consistent that its effectiveness allows for easy common as well as scientifc verification. Daytime breezes operate simultaneously from different coasts, and penetrate inland. Its centripetal nature causes wind convergences in certain inland areas, where summer rainfall tend to focus. The agricultural area has historically made with massive territorial deployment of cereal production infrastructure, mainly threshing floors, the main factor of its location being the quantity of the wind resources available. These are small circular spaces that can be interpreted in relation to climate. A synthetic chart of threshing floors and a methodology designed to indirectly determine the dominant wind directions at each site have been developed, based on their geo-referencing by photo-interpretation of the ortho-photographs of 1956. The resulting ���anemographic��� model fits nicely with the main features of the sea breezes identifed by the theoretical-experimental and numerical simulations already tested.