Dissertations / Theses on the topic 'Start modelling'

Modern heavy trucks tend to get more and more equipment demanding electric power. As a result, the electric power left for startingbecome more and more limited. If a complete view of the entire starting system-battery, starter and the combustion engine - is used, the total system can be investigated and optimized. This thesis is a study of the starting system and its components. Theories for each component are presented and models are derived for a complete starting system. Focus lies on the battery and starter motor. The purpose of the modelling work is to gain knowledge of the starting system. Some results can also be obtained from the simulations - it is very important to keep the electrical resistance as low as possible and the differences between battery types are surprisingly big.

Controlled Start Transmissions (CST) can be described as a mechanical transmission combined with a wet clutch for controlled torque output. CST:s are commonly used to start up heavy loads for example mining conveyors. Several CST:s can work together to share the load. The transferred torque is controlled by a hydraulic wet clutch which is a proven technology for high torque transfer and low wear. This thesis is a part of a project to increase the knowledge and improve the CST design and control performance. The heat generation in the wet clutch is the focus of this thesis. Literature review shows that most research is done in order to get high accuracy for smaller clutches and most of the high torque engagements have very short transients.Models for the CST clutch heat generation together with thermal behavior have been developed and investigated. This includes a temperature model together with a kinetic model of the gearbox and a clutch torque model. Validation of separate model components and sensitivity analysis of the parameters are made. The developed model is then analyzed by comparing measurements from a commission site and simulations to get an idea of how much heat is generated.

The workflow of different game modelling artists is explored, with the goal of creating a melee weapon designed for games. A low polygon weapon was created using the research into the most common components from professional workflows of creating game models. One of the goals for the paper was to obtain a greater understanding of professional workflows when creating models for games. Other goals were to utilize the research and design a finished game model from concept to end product.

In our research, we set out to model, understand and evaluate the business process at a start-up actuarial firm which employs Report Writers (RWers) who specialise in quantifying actuarial matters. We simulated various "what-if" and extreme scenarios relating to (1) the impact of qualitative variables (stress, morale and health) on RWer productivity, (2) hiring policies for RWers who have various skills sets, (3) the allocation of RWers to various roles within the process, (4) the impact that a high turnover of experienced RWers has on productivity, (5) the impact of introducing a flexible working arrangement (flexitime). This was done through business process modelling and simulations. The business process we modelled was governed by numerous potentially complex inter-relationships between variables and inter-relationships, which we believed could lead to potentially significant feedback loops. The models we built were then simulated over a period of 3 to 7 years to gain insights into the behavioural trends of the firm's business process over time when subject to "what-if" scenarios and policy implementations. The model simulations allowed us to get an understanding of the behaviour of processes over time, and the key variables and relationships involved in bringing about such behaviour as certain variables were subjected to changes in levels, as set out in our objectives. We made use of relevant literature, expert opinion, past data, questionnaires and cognitive mapping techniques to build simulation models. Guided by methodologies used in literature on modelling qualitative variables, bearing in mind the dangers in modelling for them, we modelled for the complex inter-relationships between qualitative and quantitative variables.

Published Article
The simulation of an exhaust manifold's thermal behaviour is an important concern for various reasons. Amongst them is the need to minimise catalyst light-offtime as significant exhaust emissions are generated within this period. Modelling such behaviour is not simplistic as it is governed by complex interactions between exhaust gas flow and the manifold itself. Computational fluid dynamics (CFD) is a powerful tool for such simulations. However its applicability for transient simulations is limited by high central processing unit (CPU) demands. The present study proposes an alternative computational method to assess and rank the relative impact of the manifold's thermal properties on its exterior temperature. The results show that stainless steel manifolds potentially minimise heat loss from the exhaust gas when compared with their cast iron counterparts. This may result in an increase in thermal energy being available to heat the catalyst.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010.
AFRIKAANSE OPSOMMING:Die navorsing wat hier uiteengesit word ondersoek die hipotese dat kondensasie en die gevolglike verdamping van water wat teenwoordig is in die uitlaatgas van ‘n binnebrandenjin, plaasvind in die gedeeltes van die uitlaatstelsel tussen die uitlaatklep en die katalitiese-omsetter se uitlaat. Daar word verder veronderstel dat hierdie tweefasevloeieffekte die tydafhanklike temperatuurprofiele in die uitlaatstelsel beïnvloed, wat moontlik kan lei tot ‘n vertraging in die tydsduur vir die katalitiese omsetter om temperature van 200-300 °C te bereik, wat nodig is om noemenswaardige omsetting te bewerkstellig. Om hierdie veronderstelling te evalueer is ‘n tydafhanklike, eendimensionele wiskundige model van die termo-vloei gedrag in die uitlaatstelsel gedurende ‘n koue inwerktreeding, insluitende vogtigheidseffekte, opgestel en opgelos deur van ‘n rekenaaralgoritme gebruik te maak. Warmte- en massaoordragsteorie was gebruik om die ongestadigde massa-, energie- en momentumbehoudsvergeleikings te formuleer. Die tweefasige vogeffekte was gemodelleer deur gebruik te maak van die verhouding tussen warmte- en massaoordrag, wat verdamping en heterogeniese kondensasie (die kondensasie van damp teen die pypwand) voorspel as gevolg van die dampdrukgradient tussen die grootmaat damp en die versadigde damp by die oppervlak van die vloeistoffilm. Homogene kondensasie (die kondensasie van vloeistof in die vorm van druppels in die dampstroom) was ook in aanmerking geneem indien die grootmaatgas temperatuur onder die versadigingstemperatuur van die grootmaatdamp gedaal het. ’n Eksperimentele ondersoek was gedoen deur van twee enjins gebruik te maak, ’n 1.6 L Volkswagen Bora en ’n 1.6 L Ford RoCam, in die toetsselle van Cape Advanced Engineering Pty (Ltd). Om die gastemperature so akkuraat moontlik te meet, was spesiale radiasiegeskermde sensore met vinnige reaksietyd ontwerp en installer in die pypseksies van die uitlaatstelsels van beide enjins. Die geskermde sensore het temperature van tot 50 °C hoër as konvensionele termokoppels in dieselfde areas gemeet. Dit is in koers is met resultate wat deur die foutbeperkingsteorie, geassosieer met die meet van temperature in vloeïende gas in uitlaatstelsels, voorspel word. Vergelyking van die numeriese simulasie met die eksperimenteel gemete temperature het aangedui dat in dele van die uitlaatstelsel voor die katalitieseomsetter, die vog min uitwerking het op die termiese gedrag van die stelsel. In hierdie gedeeltes is die konveksie warmte-oordrag dominant. In die katalitieseomsetter was die vogeffekte invloedryk. Die eksperimentele resultate toon ‘n duidelike vroeë toename in die gastemperature, gevolg deur ‘n tydperk van konstante temperature by nagenoeg die versadigingstemperatuur van die grootmaatdamp (verwys na as die temperatuurplato) by die katalitiese-omsetter se kern en uitlaat. Die numeries gesimuleerde gastemperature het ook hierdie gedrag getoon, maar ‘n baie hoë en skerp piek by die begin van die plato het voorgekom. Hierdie piek was nie te sien by die eksperimentele resultate nie en is toegeskryf aan nie-ewewigstoestande in die verdampingsproses, wat aandui dat die tempo van verdamping wat deur die massa-oordragmodel voorspel word te hoog is vir die model en dat dit verfyn moet word. Verdere ondersoek van die invloed van die individuele massa-oordragprosesse het getoon dat die homogene kondensasie die dominante proses is in die vorming van vloeistof in die katalitiese-omsetter. Heterogeniese kondensasie het plaasgevind, maar ‘n kleiner massa vloeistof is produseer. Die maksimum hoeveelheid vloeistof wat voorspel is om in die katalitiese-omsetter te vorm was 12 g/cm wat gelykstaande is aan ‘n film van 0.05.mm dik indien eweredig versprei oor die binneoppervlak van die monoliet. Daar was in die simulasie gevind dat beide verdamping en kondensasie benodig word om die temperatuurplato te simuleer, vanwaar die gevolgtrekking gemaak kan word dat beide prosesse wel plaasvind en dat die eerste stelling in die oorspronklike hipotese wel geldig is. Daar was egter teen die einde van die toetsperiode gevind dat beide temperature wat met en sonder vogeffekte simuleer was, die eksperimentele temperature nagevolg het, wat aandui dat die invloed van vog beperk is tot die vroeë stadiums van die katalitiese-omsetter se opwarmingstydperk. Die tweede gedeelte van die hipotese wat veronderstel dat die voggedrag ‘n vertraging in die tydsduur om omsetting te bewerkstellig veroorsaak, is dus bevind om ongeldig te wees. Die wiskundige model wat opgestel is tydens die ondersoek is weens noodsaaklikheid ‘n vereenvoudigde simulasie van komplekse termo-vloei prosesse. Dit dien as nuttige grondwerk vir verdere in-diepte ondersoeke en afronding van die teorie met betrekking tot voggedrag en die uitwerking daarvan op die tydsafhanklike temperature in ‘n uitlaatstelsel.
ENGLISH ABSTRACT: The research presented here investigates the hypothesis that condensation and subsequent evaporation of water vapour present in the exhaust gas of an internal combustion engine occur in the sections of the exhaust system between the exhaust port and the catalytic converter exit. It is further hypothesised that these two-phase moisture effects influence the transient temperature profiles in the exhaust system, and potentially cause a delay in the time it takes for the catalytic converter to reach temperatures of 200-300 °C, which are required for light-off to occur. In order to evaluate this hypothesis a transient, one-dimensional mathematical model of the thermo-fluid behaviour in the exhaust system during a cold start, including moisture effects, was created and solved by means of a computer algorithm. Heat and mass transfer theory was used to formulate the unsteady conservation equations for mass, energy and momentum. The two phase moisture effects were modelled using the analogy between heat and mass transfer, which predicts evaporation and heterogeneous condensation (the condensation of vapour against the pipe wall) due to a vapour pressure gradient between the bulk vapour and a saturated vapour at the surface of the liquid film. Homogeneous condensation (the condensation of liquid in the form of droplets in the gas stream) was also accounted for if the bulk gas temperature dropped below the bulk vapour saturation temperature. An experimental investigation was performed using two engines, a 1.6.L Volkswagen Bora and a 1.6.L Ford RoCam, in the test cells of Cape Advanced Engineering Pty (Ltd). In order to measure the gas temperatures as accurately as possible specialised radiation shielded sensors with fast time response were designed and installed in the pipe sections of the exhaust systems of both engines. The shielded sensors measured temperatures up 50 °C higher than the conventional thermocouples installed at the same positions, which is in keeping with the results predicted by the theory governing errors associated with temperature measurement in the flowing gas in the exhaust system. Comparison of the numerically simulated and experimentally measured temperatures indicated that in the sections of the exhaust system leading up to the catalytic converter the moisture has little influence on the temperature behaviour of the exhaust system. In these sections the convective heat transfer is dominant. In the catalytic converter the moisture effects were found to be influential. The experimental results clearly show an early rise in the gas temperatures, followed by a period of constant temperature at approximately the saturation temperature of the bulk vapour (referred to as the temperature plateau) at the catalytic converter mid-bed and exit. The numerically simulated gas temperatures also exhibited this plateau, but an initial very high and sharp peak in the simulated gas temperatures occurred at the start of the plateau. This was not seen in the experimental results and is attributed to non-equilibrium in the evaporation process, indicating that the rate of evaporation predicted by the mass transfer model used is too high for this application and that the model needs to be refined. Further investigation of the influence of the individual mass transfer processes indicated that the homogeneous condensation is the dominant process in the formation of liquid in the catalytic converter. Heterogeneous condensation was found to occur, but produced a smaller mass of liquid. The maximum amount of liquid predicted to form in the catalytic converter was 12 g/cm, which translates to a film 0.05 mm thick if evenly distributed over the inner surface of the monolith. In the simulation it was found that both evaporation and condensation are needed in order to simulate the temperature plateau, from which it was concluded that both these processes do occur and the first statement in the original hypothesis is valid. However, by the end of the test period temperatures simulated both with or without the moisture effects closely approached the final temperatures of the experimental investigation, indicating that the influence of the moisture is limited to the early stages of the catalytic converter warm-up. The second part of the hypothesis, postulating that the moisture behaviour caused a delay in the time taken to reach light-off temperature, is therefore concluded to be invalid. The mathematical model constructed in this research is by necessity a simplified solution to complex thermo-fluid processes. It serves as useful groundwork for further elaboration and refinement of the theory related the moisture behaviour and its influence on the transient temperatures in the exhaust system.

[EN] The injection system is one of the topics that has been paid most attention to by researchers in the field of direct injection diesel engines, due to its key role on fuel atomization, vaporization and air-fuel mixing process, which directly affect fuel consumption, noise irradiation and pollutant emissions. The increasing injection pressures in modern engines have propitiated the need of studying phenomena such as cavitation, compressible flow or the effect of changes in the fuel properties along the process, whose relative importance was lower in early stages of the reciprocating engines development. The small dimensions of the injector ducts, the high velocities achieved through them and the transient nature of the process hinder the direct observation of these facts. Computational tools have then provided invaluable help in the field. The objective of the present thesis is to analyse the influence of the thermal effects on the performance of a diesel injector. To this end, the fuel temperature variation through the injector restrictions must be estimated. The influence of these changes on the fuel thermophyisical properties relevant for the injection system also needs to be assessed, due to its impact on injector dynamics and the injection rate shape. In order to give answer to the previous objectives, both experimental and computational techniques have been employed. A dimensional and a hydraulic experimental characterization of a solenoid-actuated Bosch CRI 2.20 injector has been carried out, including rate of injection measurements at a wide range of operating conditions, with special attention to the fuel temperature control. A 1D computational model of the injector has been implemented in order to confirm and further extend the findings from the experiments. Local variations of fuel temperature and pressure are considered by the model thanks to the assumption of adiabatic flow, for which the experimental characterization of the fuel properties at high pressure also had to be performed. The limits of the validity of this assumption have been carefully assessed in the study. Results show a significant influence of the fuel temperature at the injector inlet on injection rate and duration, attributed to the effect of the variation of the fuel properties and to the fact that the injector remains in ballistic operation for most of its real operating conditions. Fuel temperature changes along the injector control orifices are able to importantly modify its dynamic behaviour. In addition, if the fuel at the injector inlet is at room temperature or above, the temperature at the nozzle outlet has not been proved to importantly change once steady-state conditions are achieved. However, a significant heating may take place for fuel temperatures at the injector inlet typical of cold-start conditions.
[ES] El sistema de inyección es uno de los elementos que más interés ha despertado en la investigación en el campo de los motores diésel de inyección directa, debido a su papel clave en la atomización y vaporización del combustible así como en el proceso de mezcla, que afectan directamente al consumo y la generación de ruido y emisiones contaminantes. Las crecientes presiones de inyección en motores modernos han propiciado la necesidad de estudiar fenómenos como la cavitación, flujo compresible o el efecto de los cambios de las propiedades del combustible a lo largo del proceso, cuya importancia relativa era menor en etapas tempranas del desarrollo de los motores alternativos. Las pequeñas dimensiones de los conductos del inyector, las altas velocidades a través de los mismos y la naturaleza transitoria del proceso dificultan la observación directa en estas cuestiones. Por ello, las herramientas computacionales han proporcionado una ayuda inestimable en el campo. El objetivo de la presente tesis es analizar la influencia de los efectos térmicos en el funcionamiento de un inyector diésel. Para tal fin, se debe estimar la variación de la temperatura del combustible a lo largo de las restricciones internas del inyector. La influencia de estos cambios en las propiedades termofísicas del combustible más relevantes en el sistema de inyección también debe ser evaluada, debido a su impacto en la dinámica del inyector y en la forma de la tasa de inyección. Para dar respuesta a estos objetivos, se han utilizado técnicas experimentales y computacionales. Se ha llevado a cabo una caracterización dimensional e hidráulica de un inyector Bosch CRI 2.20 actuado mediante solenoide, incluyendo medidas de tasa de inyección en un amplio rango de condiciones de operación, para lo que se ha prestado especial atención al control de la temperatura del combustible. Se ha implementado un modelo 1D del inyector para confirmar y extender las observaciones extra\'idas de los experimentos. El modelo considera variaciones locales de presión y temperatura del combustible gracias a la hipótesis de flujo adiabático, para lo cual también se ha tenido que llevar a cabo una caracterización experimental de las propiedades del combustible a alta presión. Los límites de la validez de esta hipótesis se han analizado cuidadosamente en el estudio. Los resultados muestran una influencia significativa de la temperatura del combustible a la entrada del inyector en la tasa y duración de inyección, atribuida al efecto de la variación de las propiedades del combustible y al hecho de que el inyector permanece en operación balística para la mayoría de sus condiciones de funcionamiento. Los cambios en temperatura del combustible a lo largo de los orificios de control del inyector son capaces de modificar su dinámica considerablemente. Además, si el combustible a la entrada del inyector se encuentra a temperatura ambiente o por encima, se ha observado que la temperatura a la salida de la tobera no varía de manera importante una vez se alcanzan condiciones estacionarias. No obstante, un calentamiento significativo puede tener lugar para temperaturas de entrada típicas de las condiciones de arranque en frío.
[CAT] El sistema d'injecció és un dels elements que més interés ha despertat en la investigació en el camp dels motors dièsel d'injecció directa, degut al seu paper clau en l'atomització i vaporització del combustible, així com en el procés de mescla, que afecten directament el consum i la generació de soroll i emissions contaminants. Les creixents pressions d'injecció en motors moderns han propiciat la necessitat d'estudiar fenòmens com la cavitació, flux compressible o l'efecte dels canvis de les propietats del combustible al llarg del procés, la importància relativa dels quals era menor en les primeres etapes del desenvolupament dels motors alternatius. Les menudes dimensions dels conductes de l'injector, les altes velocitats a través dels mateixos i la natura transitòria del procés dificulten l'observació directa en estes qüestions. Per això, les ferramentes computacionals han proporcionat una ajuda inestimable en el camp. L'objectiu de la present tesi és analitzar la influència dels efectes tèrmics en el funcionament d'un injector dièsel. Per a tal fi, es deu estimar la variació de la temperatura del combustible al llarg de les restriccions internes de l'injector. La influència d'estos canvis en les propietats termofísiques del combustible més relevants en el sistema d'injecció també ha de ser avaluada, degut al seu impacte en la dinàmica de l'injector i en la forma de la tasa d'injecció. Per tal de donar resposta a estos objectius, s'han utilitzat tècniques experimentals i computacionals. S'ha dut a terme una caracterització dimensional i hidràulica d'un injector Bosch CRI 2.20 actuat mitjançant solenoide, incloent mesures de tasa d'injecció en un ampli rang de condicions d'operació, per al que s'ha prestat especial atenció al control de la temperatura del combustible. S'ha implementat un model 1D de l'injector per tal de confirmar i estendre les observacions extretes dels experiments. El model considera variacions locals de pressió i temperatura del combustible gràcies a la hipòtesi de flux adiabàtic, per la qual cosa també s'ha hagut de dur a terme una caracterització experimental de les propietats del combustible a alta pressió. Els límits de la validesa d'esta hipòtesi s'han analitzat acuradament en l'estudi. Els resultats mostren una influència significativa de la temperatura del combustible a l'entrada de l'injector en la tasa i duració d'injecció, atribuïda a l'efecte de la variació de les propietats del combustible i al fet que l'injector roman en operació balística per a la majoria de les seues condicions de funcionament. Els canvis en temperatura del combustible al llarg dels orificis de control de l'injector són capaços de modificar la seua dinàmica considerablement. A més, si el combustible a l'entrada de l'injector es troba a temperatura ambient o per damunt, s'ha observat que la temperatura a l'eixida de la tobera no varia de manera important una vegada s'han assolit condicions estacionàries. No obstant això, un escalfament significatiu pot tenir lloc per a temperatures d'entrada típiques de les condicions d'arrancada en fred.
Carreres Talens, M. (2016). Thermal effects influence on the Diesel injector performance through a combined 1D modelling and experimental approach [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/73066
TESIS

[ES] The automotive industry represents one of the most important sectors in the world. Given its socio-economic influence, research is aimed at reducing fuel consumption and emissions. Turbochargers provide several benefits including increased power for a given engine size, improved fuel economy and reduced emissions. The turbocharger is an important piece for the new generation of engines that must comply with the Euro 6 or in the U.S. Tier 3 vehicle emissions and fuel standard program. As more effort is made to increase efficiencies and reduce emissions, the complexity of the system increases. The high rotational speeds, pulsating flow conditions and high temperature differences between working fluids (exhaust gases, compressed air, lubricating oil, coolant fluids) make the turbo-charging a challenging task. Numerical simulation opens a range of possibilities to study the performance, efficiency and design of components in the turbocharger, but requires continued accuracy refinements. In this thesis, a great effort has been made to improve the overall understanding of the different physical phenomena that occur inside the turbocharger. Both, experimental and modelling efforts have been made to understand the thermal behaviour of the turbocharger under engine start/stop conditions. After state-of-the-art review of thermal studies and heat transfer simulation codes, this work presents an extensive experimental testing campaign that includes a thermal characterization of the turbocharger in stationary and transient conditions. Subsequently, several turbochargers were measured to assess the consequences that degraded oils can generate in the bearing system during endurance tests of oil-coking. To minimize the possibilities of coke formation, some theoretical studies were done. First, a 1D turbocharger model was used in GT-PowerTM for a detailed study of the temperature rise in the central housing during an engine hot-stop. The simulated cooling strategies aims to find an optimal in terms of minimizing extra energy consumption per K housing temperature reduction. After, a 2D radial model is proposed as improvement of an existing one-dimensional model developed at CMT - Universitat Politècnica de València. Aiming for a low computational cost, the radial model was developed to be compatible with fast one-dimensional engine simulations. Later, a detailed solution of heat fluxes was made by means of CFD using a 3D design of the turbocharger's central housing. The 3D model improved the results when temperature of the bearings/shaft is required. Additionally, thermal properties within the turbocharger can be obtained and therefore a reduction of the experimental tasks in the thermohydraulic test bench. Both 2D and 3D models were validated using experimental data, demonstrating predictive accuracy improvements on the results of previous models.
[CAT] La industria automotriz representa uno de los sectores más importantes del mundo. Dada su influencia socioeconómica, la investigación está destinada a reducir el consumo de combustible y las emisiones. Los turbocompresores ofrecen varios beneficios, entre ellos, mayor potencia para un tamaño de motor determinado, mejor economía de combustible y reducción de emisiones. El turbocompresor es una pieza importante para la nueva generación de motores que deben cumplir con la normativa Euro 6 o en el programa estándar de emisiones y combustible de los EE. UU. Tier 3. A medida que se hacen más esfuerzos para aumentar la eficiencia y reducir las emisiones, la complejidad del sistema aumenta. Las altas velocidades de rotación, las condiciones de flujo pulsante y las altas diferencias de temperatura entre los fluidos de trabajo (gases de escape, aire comprimido, aceite lubricante, fluidos refrigerantes) hacen que la turbocarga sea una tarea desafiante. La simulación numérica abre un rango de posibilidades para estudiar el rendimiento, la eficiencia y el diseño de los componentes en el turbocompresor, pero requiere continuos refinamientos de precisión. En esta tesis, se ha hecho un gran esfuerzo para mejorar la comprensión global de los diferentes fenómenos físicos que ocurren al interior del turbocompresor. Se han hecho esfuerzos experimentales y de modelado para comprender el comportamiento térmico del turbocompresor en condiciones de arranque/parada del motor. Luego de una revisión de los estudios térmicos y de los códigos de simulación de transferencia de calor, éste trabajo presenta una extensa campaña de pruebas experimentales que incluye una caracterización térmica del turbocompresor en condiciones estacionarias y transitorias. Posteriormente, se midieron varios turbocompresores para evaluar las consecuencias que los aceites degradados pueden generar en el sistema de rodamientos durante pruebas de resistencia de coque de aceite. Para minimizar las posibilidades de formación de coque, se realizaron algunos estudios teóricos. En primer lugar, se usó un modelo de turbocompresor 1D en GT-PowerTM para un estudio detallado del aumento de temperatura de la carcasa central del turbocompresor durante un paro en caliente del motor. Las estrategias de enfriamiento simuladas apuntan a encontrar un óptimo en términos de minimizae el consumo de energía extra por reducción de la temperatura de la carcasa en Kelvin. Posteriormente, se propone un modelo radial 2D como mejora de un modelo unidimensional existente desarrollado en la CMT - Universitat Politècnica de València. Con el objetivo de conseguir un bajo costo computacional, el modelo radial 2D se desarrolló para ser compatible con simulaciones unidimensionales rápidas de motor. Después, se realizó una solución detallada de los flujos de calor mediante CFD utilizando un diseño 3D de la carcasa central del turbocompresor. El modelo 3D mejora los resultados cuando se requiere la temperatura de los cojinetes/eje. Además, con ésta campaña de CFD se pueden obtener propiedades térmicas dentro del turbocompresor y, por lo tanto, una reducción de las tareas experimentales en el banco de pruebas termohidráulico. Ambos modelos 2D y 3D fueron validados utilizando datos experimentales, demostrando mejoras de precisión de predicción sobre los resultados de modelos anteriores.
[EN] La indústria automotriu representa un dels sectors més importants del món. Donada la seua influència socioeconòmica, la investigació està destinada a reduir el consum de combustible i les emissions. Els turbocompressors oferixen diversos beneficis, entre ells, major potència per a una grandària de motor determinat, millor economia de combustible i reducció d'emissions. El turbocompressor és una peça important per a la nova generació de motors que han de complir amb la normativa Euro 6 o en el programa estàndard d'emissions i combustible dels EE. UU. Tier 3. A mesura que es fan més esforços per a augmentar l'eficiència i reduir les emissions, la complexitat del sistema augmenta. Les altes velocitats de rotació, les condicions de flux polsen-te i les altes diferències de temperatura entre els fluids de treball (gasos de fuga, aire comprimit, oli lubricant, fluids refrigerants) fan que la turbocarga siga una tasca desafiador. La simulació numèrica obri un rang de possibilitats per a estudiar el rendiment, l'eficiència i el disseny dels components en el turbocompressor, però requerix continus refinaments de precisión. En aquesta tesi, s'ha fet un gran esforç per a millorar la comprensió global dels diferents fenòmens físics que ocorren a l'interior del turbocompressor. S'han fet esforços experimentals i de modelatge per a comprendre el comportament tèrmic del turbocompressor en condicions d'arranque/parada del motor. Després d'una revisió dels estudis tèrmics i dels codis de simulació de transferència de calor, este treball presenta una extensa campanya de proves experimentals que inclou una caracterització tèrmica del turbocompressor en condicions estacionàries i transitòries. Posteriorment, es van mesurar uns quants turbocompressors per a avaluar les conseqüències que els olis degradats poden generar en el sistema de rodaments durant proves de resistència de coc d'aceite. Per a minimitzar les possibilitats de formació de coc, es van realitzar alguns estudis teòrics. En primer lloc, es va usar un model de turbocompressor 1D en GT- Power \textsuperscript{TM} per a un estudi detallat de l'augment de temperatura de la carcassa central del turbocompressor durant una desocupació en calent del motor. Les estratègies de refredament simulades apunten a trobar un òptim en termes de minimizae el consum d'energia extra per reducció de la temperatura de la carcassa en Kelvin. Posteriorment, es proposa un model radial 2D com a millora d'un model unidimensional existent desenrotllat en la CMT - Universitat Politècnica de València. Amb l'objectiu d'aconseguir un baix cost computacional, el model radial 2D es va desenrotllar per a ser compatible amb simulacions unidimensionals ràpides de motor. Después, es va realitzar una solució detallada dels fluxos de calor per mitjà de CFD utilitzant un disseny 3D de la carcassa central del turbocompressor. El model 3D millora els resultats quan es requerix la temperatura dels cojinetes/eje. A més, amb esta campanya de CFD es poden obtindre propietats tèrmiques dins del turbocompressor i, per tant, una reducció de les tasques experimentals en el banc de proves termohidráulico. Ambdós models 2D i 3D van ser validats utilitzant dades experimentals, demostrant millores de precisió de predicció sobre els resultats de models anteriores.
Rodriguez Usaquén, YT. (2019). Experimental study of oil coking problem and contribution to the modelling of heat transfer in turbochargers [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/117314
TESIS

Cette étude sur le démarrage des turbines à gaz (TAG) s’inscrit dans le cadre de l’hybridation d’un turbomoteur d’hélicoptère avec un objectif de réduction de consommation. En effet, un nouveau mode de vol pour hélicoptère bimoteurs est envisagé avec l’extinction d’un moteur en vol, ce qui permettrai tune économie de carburant de l’ordre de 15% d’après Safran Helicopter Engines. La certification d’untel mode de vol nécessite alors de progresser sur la compréhension de la phase de démarrage de la TAG pour garantir un taux d’échec de redémarrage en vol très faible ainsi qu’une capacité de réactivation d’urgence de la TAG éteinte. Le démarrage se décompose en deux grandes phases, la phase d’allumage et la phase d’accélération. A l’issue d’une étude phénoménologique détaillée, les paramètres d’influence ont été identifiés pour chacune de ces phases. Une campagne d’essais de démarrage au sol et à température ambiante, menée chez Safran Helicopter Engines, a été réalisée pour étudie r la capacité de redémarrage du turbomoteur RTM322. L’analyse des résultats a permis d’étayer l’étude phénoménologique effectuée ainsi que de fournir des données pour alimenter les premiers outils numériques 0D développés au court de cette thèse. Le modèle 0D implémenté permet de reproduire une séquence de démarrage complète, avec une prise en compte simple de l’allumage, et ce jusqu’au régime ralenti. Un travail d’extension des caractéristiques des différents composants et systèmes ainsi qu’une étude de différents phénomènes transitoire sont été nécessaire pour y parvenir. Les résultats d’essais étant cependant quasi-inexistants à bas régime,le modèle possède un certain degré d’incertitude dans ces conditions. Enfin, la capacité de réactivation rapide de la TAG a été étudiée avec l’évaluation des gains de temps potentiellement atteignables d’après les essais et à l’aide du modèle développé. Une campagne d’essais complémentaires dédiée aux faibles régimes et pour des conditions environnementales plus sévères en termes de pression et de température permettrait de réduire le degré d’incertitude des projections ainsi obtenues. Une nouvelle campagne d’essai permettrait aussi de tester les différentes améliorations proposées en termes de réactivation
This study on the start-up phase of gas turbine engines takes place in the context of the hybridization of a helicopter turboshaft engine with a decreased fuel consumption target. A new flight mode for a twin engine helicopter with the shutdown of one engine in-flight is being considered. For Safran Helicopter Engines, this mode would allow up to 15% of fuel savings. The certification of this flight mode involves improving the understanding of the start-up phase of gas turbine engines towards a close-to-zero failure rate for in-flight restarts as well as a fast restart capability of the shutdown engine. The start-up phase is composed of two main steps: the light-up and the acceleration. A detailed phenomenological study has highlighted the factors influencing the performance for each of these two steps. A test rig campaign at sea level and ambient temperature, performed at Safran Helicopter Engines, took place in order to study the restart capability of the RTM322 turboshaft engine. Analyzing the results of this campaign provided support to the phenomenological study and fed the first 0D numerical tools developed in this thesis with data. The 0D model implemented can simulate a full start-up sequence towards idle speed, with a basic approach to account for the light-up phase. This achievement required working on the extension of the various component characteristics towards sub-idle speeds and also on the different transient phenomena involved. The lack of test rig data at low speed leads to a level of uncertainty in these conditions. Finally, the fast restart capability was studied, with the assessments of time saved potentially achievable from the test rig data and with the model developed in this thesis. A new test rig campaign dedicated to low speed operations with more severe environmental conditions in pressure and temperature would allow to lower the level of uncertainty of the simulation results. This would also be the opportunity to test the various improvements suggested in terms of fast restart

This thesis presents both a study of the cluster-scale environments in which massive stars form, investigating in particular how the ionized gas in these regions relates to the molecular star-forming material, as well as detailed studies of two luminous forming stars, AFGL 2591 and IRAS 20126+4104, to determine whether they are forming similarly to their low-mass counterparts. The results of this work include the identification of 35 HII regions (20 newly discovered) via a radio continuum survey of ionized gas towards 31 molecular cluster-forming clumps. The observed ionized gas was found to be preferentially associated with the clumps, which were shown to have a range of evolutionary stages. The massive star formation efficiency was determined for the clumps with associated ionized gas, and a relationship was found between the mass of the clumps and the mass of their embedded massive stars. By modelling the SEDs and images of AFGL 2591 and IRAS 20126+4104, it was found that the geometry of their circumstellar material was generally consistent with an envelope plus disk, similar to that expected for low-mass protostars. However, within the central ~1800 AU, the mid-IR images of IRAS 20126+4104 were better described by only a flattened envelope, suggesting that the radiation from IRAS 20126+4104 may be affecting the regions closest to the star. Observations of the ionized and molecular gas towards AFGL 2591 were carried out, and a photoionization code was developed to interpret these observations. The results showed that the observed 3.6 cm emission is likely to be produced by both a shock-ionized jet and a hypercompact HII region that does not appear to have disrupted the jet or the large-scale circumstellar environment. In addition, the C¹⁸O(1-0) emission observed towards AFGL2591 traces the densest parts of the outflow, with the blue-shifted emission exhibiting many of the properties of the outflows from low-mass protostars.

In the last few decades, the emergence of large-scale infrared surveys has led to a revolution in the study of star formation. In particular, NASA’s Spitzer Space Telescope has recently carried out mid- and far-infrared observations of numerous star formation regions with unprecedented resolution and sensitivity, and has uncovered thousands of forming stars. In combination with present and future large-scale near-infrared and sub-mm surveys, spectral energy distributions from near-infrared to mm wavelengths will be available for these thousands of young stars. Never before has there been such a wealth of multi-wavelength data for so many young stars. Traditional techniques for studying the physical properties of young stars through their spectral energy distributions have usually focused either on the analysis of many sources using simple observational diagnostics such as colours or spectral indices, or on the analysis of a few sources through the detailed modelling of their full spectral energy distributions. The work presented in the first part of this thesis aims to bridge these two techniques through the efficient modelling of the spectral energy distributions of many young stars. In particular, the technique developed for this work makes it straightforward to find out how well different physical parameters are constrained, whether any parameters are degenerate, and whether additional data would resolve the degeneracies. In the second part of this thesis, a census of intrinsically red sources observed by Spitzer in the Galactic plane is presented, including a catalogue of over 11,000 likely young stellar objects. This sample of sources is the largest uniformly selected sample of young stars to date, and effectively provides a map of the sites of star formation in the mid-plane of the Milky-Way. In parallel, this census has uncovered over 7,000 candidate asymptotic giant branch stars, of which over 1,000 are variable at 4.5 or 8.0 microns.

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large Magellanic Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young (similar to 10-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.

Type I X-ray bursts are thermonuclear burning events which occur on the surface of accreting neutron stars. Burning begins in a localised spot in the star’s ocean layer before spreading across the entire surface. By gaining a better understanding of X-ray bursts, tighter limits can be determined for other neutron star properties such as the mass, radius, spin frequency and magnetic field. The ocean environment is very extreme, involving much higher pressure, temperature and magnetic field strength compared to the conditions typically found in terrestrial systems. We shall be looking at the effects of the strong gravitational field, modelling the ocean using general relativistic hydrodynamics. The physics of X-ray bursts acts over a wide range of scales, which introduces a number of challenges when modelling them. In this work, we use the multiscale approach to couple together multiple physical models in order to best capture the physics across these various scales. On the smallest scales, the physics is dominated by turbulent burning. The speed of propagation of the burning front is much slower than the acoustic speed, making it difficult to model this with conventional numerical schemes. We therefore instead use the low Mach number approximation, which we have derived and implemented for the relativistic fluid equations based on the existing approach developed for the Newtonian case. On larger scales, the burning front can be thought of as a discontinuity. To model this, we investigate the reactive Riemann problem for relativistic deflagrations and detonations and develop a numerical solver. The large scale propagation of the burning front is believed to be dominated by the Coriolis force. To capture this behaviour, we have derived and implemented a model for the relativistic form of the shallow water equations. Finally, we construct a hybrid scheme to combine the best features of these approximations, extending existing adaptive mesh refinement techniques to include different physical models at different scales.

Many hot stars exhibit stochastic polarimetric variability, thought to arise from clumping low in the wind. Here we investigate the wind properties required to reproduce this variability using analytic models, with particular emphasis on Luminous Blue Variables. We find that the winds must be highly structured, consisting of a large number of optically-thin clumps; while we find that the overall level of polarization should scale with mass-loss rate – consistent with observations of LBVs. The models also predict variability on very short timescales, which is supported by the results of a recent polarimetric monitoring campaign.

The principal theory concerning the origin of the elements heavier than the Fe-peak, such as Ba, strongly suggest that for old, metal-poor environments, the rapid (r-) process is the most likely path taken in their synthesis, while the slow (s-) process becomes more substantial in younger, more metal-rich stellar populations. In this work I test this theory by evaluating the isotope ratios of Ba. It is understood that Ba consists of seven stable isotopes, five of which are synthesised by the two neutron-capture processes. The two odd isotopes, 135,137Ba, as well as 138Ba are synthesised via both the r- and s-processes while two of the even isotopes, 134,136Ba are synthesised via the s-process only. The relative contribution of the r- and s-process to these isotopes can be understood via nucleosynthesis calculations and is described using the parameter fodd, where fodd = [N (135Ba) + N (137Ba)] /N (Ba). Low values of fodd (~0.11) indicate an s-process regime, while high values of fodd (~0.46) indicate an r-process regime. In the Ba II 4554 A line the even isotopes lie close to the line centre, while the odd isotopes, which are hyperfine split because of their non-zero nuclear spin, lie in the wings of the line. From an analysis of the line profile shape, one can determine whether Ba has been synthesised primarily through the r-process or s-process; a broad, asymmetric line would indicate a high r-process contribution, while a line with a deeper core and shallower wings would indicate a high s-process contribution. Using the radiative transfer code ATLAS, which assumes local thermodynamic equilibrium (LTE) and employs 1-dimensional (1D) KURUCZ06 model atmospheres, I synthesised line profiles for six metal-poor stars: HD140283, HD122563, HD88609, HD84937, BD-04 3208 and BD+26 3578 - for a range of isotope ratios. All six are of sufficiently low metallicity that Ba was expected to have an r-process origin. These were fit to high resolution (R\equiv \lamda/\Delta\lamda = 90 000 - 95 000), high signal-to-noise to the Ba II 4554 A line which has multiple components. In the first test, synthetic spectra were computed using the non local thermodynamic equilibrium (NLTE) radiative transfer code MULTI. The synthetic line profiles were fit to a number of lines in HD140283. Although this technique might have improved the fit in the line core, it was found that such a treatment did not improve upon fitting errors associated with the best fit 1D LTE synthetic profiles. The second test used a 3-dimensional (3D) radiative transfer code (LINFOR3D) that employed 3D, time-dependent atmospheres produced with CO5BOLD. The 3D synthetic pro les were fit to a selection of Fe lines and improvements over the poor fits produced by the 1D LTE synthesis were seen. It was found that the 3D synthesis could almost completely reproduce the line asymmetries seen in the observed stellar spectrum. This result suggests that further work to refine the 3D calculations and synthesis code would be valuable.

We motivate the importance of understanding the kinematics and dynamics of the Milky Way stellar halo both in unravelling the formation history and evolution of our host Galaxy and in the more general context of galaxy dynamics. We present a cleaned picture of the kinematics of the smooth component of the stellar halo: we develop a method to quantify the average distance error on a sample of stars based on the idea of Schoenrich et al. (2012), but adapted so that it uses velocity information only on average. We use this scheme to construct an analytic distance calibration for Blue Horizontal Branch (BHB) field halo stars in Sloan colours and demonstrate that our calibration is a) more accurate than the ones available and b) unbiased w.r.t. metallicity and colour. We measure the rotation of the smooth component of the stellar halo with a tool-set of four estimators that use either only the l.o.s. velocities or the full 3D motion. From two samples of BHB stars from the Sloan Digital Sky Survey, we favour a non-rotating single halo. We critique conflicting results in the literature based on similar samples and trace back the disagreement (either in the sign of rotation or in the morphology of the halo) to sample contaminations and/or neglect account of the halo geometry. We propose a scheme that generalizes any isotropic spherical model to a model where the potential is axisymmetric and the distribution function is a function of the three actions. The idea is to approximate the Hamiltonian as a function of the actions with a library of quadratic fits to surfaces of constant energy in action space and then make explicit the dependence of the energy on the three actions in the ergodic distribution function. The transparency of the physics implied by the model we achieve, should make it possible to combine our spheroidal models to the f(J)-models of Binney (2010) for the disks and of Pontzen & Governato (2013) for the dark-matter halo, and obtain a complete actions-defined dynamical model of the Milky Way Galaxy.

Ce travail étudie sous quelles conditions les molécules survivent dans les vents magnétisés issus de disques d'accrétion, afin d'expliquer les observations de jets moléculaires dans les étoiles jeunes. Un modèle a été développé qui calcule la chimie hors équilibre, l'ionisation, et le profil de température le long de l'écoulement. Le chauffage par collisions entre neutres et particules chargées est calculé en détail. L'effet des rayons X et ultraviolets stellaires est également pris en compte, en incluant l'auto-écrantage du di-hydrogène et du monoxyde de carbone. Nous trouvons que le gaz est assez ionisé pour garder une faible vitesse de dérive ion-neutre sur une large gamme de rayons d'éjection et de taux d'accrétion. Il en résulte une température de 500 à 2000 K le long du jet, assez froide pour que les molécules survivent bien qu'étant en partie dissociées. Les molécules sont plus abondantes dans les lignes d'écoulement externes ou pour les forts taux d'accrétion.

Since Lick indices were introduced in 1994, they have been used as a source of observational data against which computer models of galaxy evolution have been compared. However, as this thesis demonstrates, observed Lick indices lead to mathematical ill-conditioning: small variations in observations can lead to very large differences in population synthesis models attempting to recreate the observed values. As such, limited reliance should be placed on any results currently or historically in the literature purporting to give the star formation history of a galaxy, or group of galaxies, where this is deduced from Lick observations taken from a single instrument, without separate verification from at least one other source. Within these limitations, this thesis also constrains the star formation histories of 21 nearby elliptical galaxies, finding that they formed 13.26 +0.09 -0.06 Gyrs ago, that all mergers are dry, and that galactic winds are formed from AGN activity (rather than being supernovae-driven). This thesis also finds evidence to support the established galaxy-formation theory of “downsizing”. An existing galactic model from the literature is examined and evaluated, and the reasons for it being unable to establish star formation histories of individual galaxies are ascertained. A brand-new model is designed, developed, tested and used with two separate data sets, corroborated for 10 galaxies by data from a third source, and compared to results from a Single Stellar Population model from the literature, to model the star formation histories of nearby elliptical galaxies.

Le travail présenté dans cette thèse s’inscrit dans le domaine de l'astrophysique de laboratoire, qui consiste à étudier en laboratoire des processus physiques qui se produisent dans des objets astrophysiques. Les principaux avantages ici sont que les processus peuvent être étudiés de manière contrôlée et que leur dynamique complète peut être étudiée. Présentement, nous avons profité des installations laser à haute intensité pour effectuer nos études.Pour cela, dans ce manuscrit, seront traitées les questions liées à l'astrophysique de laboratoire qui impliquent l'interaction d’un plasma en détente dans le vide en présence d’un champ magnétique ambiant. La présence d'un champ magnétique dans une variété de phénomènes astrophysiques rend l’introduction de cette composante magnétique dans le laboratoire nécessaire afin que ces études soient pertinentes. Pour ce faire, en collaboration avec Laboratoire National des Champs Magnétiques Intenses -LNCMI, une bobine Helmholtz, spécialement conçue pour travailler dans un environnement laser a été développée, permettant d'atteindre une force de champ magnétique jusqu'à 30 T.Les objets astrophysiques sur lesquels cette étude est centrée sont les étoiles jeunes ou « Young Stellar Objects » (YSOs). Plusieurs étapes du processus de formation de ces étoiles seront ici étudiées : (i) la génération de jets collimatés à très grande échelle, (ii) la dynamique d'accrétion impliquant, dans la représentation standard, des flux de matière tombant sur la surface de l’étoile sous forme de colonnes magnétiquement confinées, et (iii) des canaux d'accrétion plus exotiques, comme l'accrétion équatoriale qui implique la propagation du plasma perpendiculairement aux lignes de champ magnétique.Plus précisément, dans un premier chapitre, la dynamique de formation des jets sera discutée. Une première partie est dédiée au mécanisme de formation de jet dans un champ magnétique poloïdal (aligné par rapport à l'axe principal d’expansion du plasma). Une seconde partie traite de la distorsion d'une telle formation de jet par l'interaction du même plasma en expansion avec un champ magnétique désaligné (c'est-à-dire présentant un angle par rapport à l'axe d’expansion du plasma). Enfin, une troisième partie détaille la propagation du plasma dans un champ magnétique perpendiculaire. Cette dernière partie nous permet d'étudier des canaux exotiques d'accumulation de matière sur les étoiles, consistant en une accrétion du disque d’accrétion directement vers l'étoile, c’est-à-dire sur le plan équatorial, impliquant une propagation orthogonale aux lignes de champ magnétiques. Le deuxième chapitre aborde le thème de la dynamique d'accrétion par l'intermédiaire de colonnes de matière magnétiquement confinées, tombant sur la surface stellaire. En utilisant la même configuration expérimentale que dans le premier chapitre, le jet formé (dans le cas du champ magnétique parfaitement aligné) est utilisé pour imiter la colonne d'accrétion et est lancé sur une cible secondaire qui agit comme la surface stellaire. La dynamique de choc à l'emplacement de l'obstacle est soigneusement étudiée et des liens avec les observations de phénomènes d’accrétion astrophysique sont construits. Un cocon de plasma, formé autour de la région d'impact via l'interaction avec le champ magnétique, est observé être similaire à celui trouvé dans les simulations astrophysiques. Ce cocon est un élément important en tant que milieu potentiel d'absorption des émissions X. Ce milieu permettrait en effet d'expliquer les écarts observés entre les émissions UV / optiques et les émissions X provenant des étoiles lors des phases d’accrétion
The work that is presented here has been performed in the frame of laboratory astrophysics, which consists in studying in the laboratory physical processes occurring in astrophysical objects. The main advantages in doing so are that the processes can be studied in a controlled way and that their full dynamics can be investigated. Here, we have been taking advantage of high-intensity laser facilities to perform our studies.In this manuscript, will be treated issues that include the interaction of a plasma expanding into vacuum with an ambient magnetic field. The presence of a magnetic field in a variety of astrophysical phenomena makes the inclusion of this component in the laboratory of great interest. We have used for our study a split Helmholtz coil, specifically designed in order to work in a laser environment, that allows for reaching a magnetic field strength up to 30 T.The astrophysical objects on which this study is focused are Young Stellar Objects (YSOs). Several steps of the star formation process are here investigated: (i) the generation of very long range, bright jets, (ii) the accretion dynamic involving, in the standard representation, matter falling down on the star in the shape of magnetically confined columns, and (iii) more exotic accretion channels, as the equatorial accretion that implies propagation of plasma perpendicularly to magnetic field lines.More precisely, in a first chapter, the jet formation dynamic will be discussed. A first part is dedicated to the jet formation mechanism in a poloidal magnetic field (aligned with respect to the main plasma expansion axis). A second part is dealing with the distortion of such jet formation via the interaction of the same expanding plasma with a misaligned magnetic field (i.e. presenting an angle with respect to the plasma expansion axis). Finally, a third part details the propagation of the plasma within a perpendicular magnetic field. This last part allows us to investigate exotic channels of matter accretion onto the stars, consisting of equatorial accretion from the disk to the star, through orthogonal magnetic field lines. The second chapter addresses the topic of the standard accretion dynamic via magnetically confined columns of matter, falling down onto the stellar surface. Using the same experimental setup as in the first chapter, the formed jet (in the case of the perfectly aligned magnetic field) is used to mimic the accretion column, and is launched onto a secondary target that acts as the stellar surface. The shock dynamic at the obstacle location is carefully studied and links with astrophysical accretion observations are built. A plasma cocoon, shaped around the impact region via the interaction with the magnetic field, is found to be similar to the one found in astrophysical simulations. This cocoon is an important element as a potential X-ray absorptive medium in order to explain observed discrepancies, between observed UV/Optical and X-ray emissions emitted from accreting stars

This thesis contains a study of molecules within circumstellar discs around young stars. Firstly, the chemistry of a disc around a young, Class 0 protostar is modelled. Such discs are thought to be massive, and thus experience gravitational instabilities, which produce spiral density waves. These affect the chemistry in three ways; by desorbing molecules from dust grains, by providing extra energy for new reactions to take place, and by mixing the internal structure of the disc to provide a rich chemistry near the midplane. Secondly, high resolution near-infrared spectra of 20 massive young stellar objects are presented. The objects display CO first overtone bandhead emission, which is excited in the conditions expected within circumstellar discs. The emission is modelled using a simple analytic model of a Keplerian disc, and good fits are found to all spectra. On average, the discs correspond to being geometrically thin, spread across a wide range of inclinations. The discs are located within the dust sublimation radius, providing strong evidence that the CO emission originates in small gaseous discs, supporting the scenario in which massive stars form via disc accretion. Finally, medium resolution near-infrared spectra of 5 Herbig Ae & Be stars are presented. The spectra cover both CO bandhead and Br γ emission. Accretion rates are derived from the measuring the Br γ emission and through modelling the CO emission, however these accretion rates are found to be inconsistent. High resolution archival data of one of the targets is presented, and it is shown that this CO disc model is unable to fit the high resolution data. Therefore, it is concluded that to properly fit CO spectra, high resolution data are needed, and that previously published information determined from low resolution spectra should be treated with caution.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-05-25T20:49:09Z No. of bitstreams: 1 brunocortesdesouza.pdf: 4659510 bytes, checksum: 73ac58d8200d2432ce64c9ebbd6d8c94 (MD5)
Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-05-26T13:11:08Z (GMT) No. of bitstreams: 1 brunocortesdesouza.pdf: 4659510 bytes, checksum: 73ac58d8200d2432ce64c9ebbd6d8c94 (MD5)
Made available in DSpace on 2017-05-26T13:11:08Z (GMT). No. of bitstreams: 1 brunocortesdesouza.pdf: 4659510 bytes, checksum: 73ac58d8200d2432ce64c9ebbd6d8c94 (MD5) Previous issue date: 2017-02-20
Esse trabalho apresenta estudos, modelagem e análises de um sistema de bombeio centrífugo submerso (BCS), considerando as características dos principais componentes presentes neste sistema. As contribuições do trabalho estão na partida do BCS, como no desenvolvimento de soluções para mitigar alguns dos problemas observados, como por exemplo, os relacionados à qualidade de energia elétrica nos terminais do BCS. A modelagem de cada equipamento elétrico que compõe o BCS é apresentada no trabalho, destacando-se alguns dos aspectos operacionais relevantes que são considerados na operação. Foi feito um projeto para dimensionamento dos filtros passivos utilizados na atenuação das componentes harmônicas das tensões e correntes produzidas pelo sistema, além de auxiliar na regulação da tensão no ponto em que o BCS está conectado. Nas simulações realizadas, considerou-se um sistema de média tensão com um motor de indução trifásico em gaiola de esquilo de 3835HP que acoplado a uma bomba centrífuga possui um alto coeficiente de inércia. Com a inserção de modelos de atritos rotacionais para o torque do conjunto motor-bomba, pôde-se representar os efeitos que acontecem nos instantes iniciais da partida do sistema e avaliar o desempenho da estratégia de controle utilizada para o acionamento. Assim como ocorre nos sistemas reais que são alimentados por cabos submarinos de longo comprimento, utilizou-se um sistema de controle do sistema em malha aberta com uma metodologia para estimar e compensar a queda de tensão que ocorre entre os terminais do conversor e o terminal de entrada do motor elétrico. O trabalho apresenta ainda uma breve comparação entre o conversor fonte de tensão (VSC) de 2 níveis e o conversor de 3 níveis com neutro grampeado (NPC). Por fim, é analisada a situação onde a alimentação do BCS é desequilibrada devido à diferença de valores entre os parâmetros do cabo submarino.
This work shows studies, modelling and analyses of an electrical submersible pump system (ESP), considering the characteristics of key elements in this system. The contributions are in its start-up, as also in the development of solutions for mitigation of some observed problems, e.g., those related to power quality at the equipment terminals. The model for each electrical equipment inside the ESP is presented in this work and highlighted some of the pertinent operational aspects that is commonly considered on practical operation. A project is settled for sizing the passive filters to attenuate the harmonic components of the voltages and currents supplied by the grid, in addition to helping voltage regulation at ESP connection point. In the study cases, it was considered a medium voltage power system with a 3835HP three phase squirrel cage induction motor which is connected with a centrifugal pump that results in a high inertia coefficient. Rotational friction model is considered in the studies to characterize the effects that occur at the initial seconds of the start-up of the system and also to evaluate the performance of the implemented control strategy. In the same way that occurs in real systems that are supplied by downhole electric cables, it was considered an open-loop V/f control with a methodology to estimate and compensate the voltage drop that exists between the terminals of the converter and the motor input. This work also establishes a short comparison between a 2-level voltage source converter (VSC) and a 3-level neutral-point clamped (NPC) VSC. Finally, the system is analyzed through the occurrence of an unbalance of downhole electric cable parameters.

Les supernovæ à effondrement de cœur sont l’un des phénomènes connus les plus puissants de l’univers. Elles résultent de l’explosion d’étoiles très massives, ayant brûlé tout leur combustible. Le résidu chaud et compact, appelée proto-étoile à neutrons, se refroidit pour devenir une étoile à neutrons, objet inerte. La dynamique et la structure des étoiles compactes, c’est-à-dire les supernovæ à effondrement de cœur, les proto-étoiles à neutrons et les étoiles à neutrons, ne sont pas encore complètement connues, et sont aujourd’hui au cœur d’intenses recherches, en association avec les observations astrophysiques et les expériences nucléaires. L’un des ingrédients clés de la modélisation d’étoile compacte concerne l’équation d’état. La difficulté de l’obtention d’une équation d’état réaliste et consistante pour tous ces objets stellaires réside dans le fait que l’on doit considérer une large variété de conditions thermodynamiques, c’est-à-dire des valeurs de densités, de fractions de protons et de températures, très différentes. Le travail de cette thèse consiste à modéliser, à partir des degrés de libertés nucléoniques, la structure microscopique ainsi que la composition interne de la matière baryonique des étoiles compactes, afin d’obtenir une équation d’état réaliste et unifiée. En particulier, on est intéressé à utiliser un formalisme qui peut s’appliquer à des densités aussi bien sous-saturées que sur-saturées, et qui, à la limite thermodynamique de température nulle, est compatible avec les interactions effectives modernes et réalistes données par la théorie microscopique d’Hartree-Fock-Bogoliubov et contraintes par les expériences nucléaires. Pour atteindre cet objectif, on présente, pour la matière sous-saturée, un modèle en équilibre statistique nucléaire, qui correspond à une superposition statistique de configurations finies, appelées cellules de Wigner-Seitz. Chaque cellule contient un noyau, ou agrégat, baignant dans un gaz homogène d’électrons ainsi que dans un gaz homogène de neutrons et de protons. Au sein de chaque cellule, on étudie les différentes composantes de l’énergie nucléaire des agrégats en interaction avec les gaz. L’utilisation de la théorie nucléaire de champ moyen pour la description des agrégats ainsi que du gaz de nucléons permet de traiter de façon consistante la matière sous-saturée et la matière sur-saturée. À des densités de plus de deux-trois fois la densité de saturation, l’apparition de degrés de liberté supplémentaires pose de nouveau des problèmes de consitance théorique qui ne sont pas traités dans cette thèse. La thèse est organisée selon trois parties. Dans la partie I, on présente le modèle en équilibre statistique nucléaire, basé sur l’ensemble grand canonique et sur les interactions non relativistes de Skyrme. Des résultats en équilibre β sont présentés, et l’importance de la distribution en masse d’agrégats d’une part, et d’un traitement réaliste de l’énergie libre d’autre part, est discutée. Dans la partie II, on étudie le comportement fonctionnel de l’énergie baryonique des cellules de Wigner-Seitz, en utilisant l’approximation de Thomas-Fermi étendue. En particulier, les effets de volume et de surface dus au milieu stellaire sont étudiés, et leur dépendance en termes de taille et d’asymétrie du noyau, ainsi que de densité et d’asymétrie du gaz de nucléons est analysée. Des résultats préliminaires de l’effet de l’interaction de surface du milieu sont présentés, sous hypothèse de certaines approximations et dans le cas de l’équilibre β. Dans la partie III, on développe des approximations afin d’obtenir une expression analytique fiable de formule de masse, directement reliée à la forme fonctionnelle et aux paramètres de l’interaction de Skyrme. Dans cette partie, on se concentre principalement sur les noyaux dans le vide, et l’on analyse les différentes composantes de l’énergie de liaison en termes de propriétés de volume et de surface, ainsi que de propriétés isoscalaire et isovecteur
The core collapse supernova is one of the most powerful known phenomena in the universe. It results from the explosion of very massive stars after they have burnt all their fuel. The hot compact remnant, the so-called proto-neutron star, cools down to become an inert catalyzed neutron star. The dynamics and structure of compact stars, that is core collapse supernovae, proto-neutron stars and neutron stars, are still not fully understood and are currently under active research, in association with astrophysical observations and nuclear experiments. One of the key components for modelling compact stars concerns the Equation of State. The task of computing a complete realistic consistent Equation of State for all such stars is challenging because a wide range of densities, proton fractions and temperatures is spanned. This thesis deals with the icroscopic modelling of the structure and internal composition of baryonic matter with nucleonic degrees of freedom in compact stars, in order to obtain a realistic unified Equation of State. In particular, we are interesting in a formalism which can be applied both at sub-saturation and super-saturation densities, and which gives in the zero temperature limit results compatible with the microscopic Hartree-Fock-Bogoliubov theory with modern realistic effective in- teractions constrained on experimental nuclear data. For this purpose, we present, for sub-saturated matter, a Nuclear Statistical Equilibrium model which corresponds to a statistical superposition of finite configurations, the so-called Wigner-Seitz cells. Each cell contains a nucleus, or cluster, embedded in a homogeneous electron gas as well as a homogeneous neutron and proton gas. Within each cell, we investigate the different components of the nuclear energy of clusters in interaction with gases. The use of the nuclear mean-field theory for the description of both the clusters and the nucleon gas allows a theoretical consistency with the treatment at saturation and beyond. At densities above two-three times saturation, other degrees of freedom are expected to appear, which potentially lead to other consistency problems but this issue will not be treated in this thesis. The thesis is divided into three parts. In part I, we present the Nuclear Statistical Equilibrium model based on the grand canonical statistics and non-relativistic Skyrme interactions. Results at β-equilibrium are shown and the importance of the clusters distribution as well as a realistic treatment for the free energy model is discussed. Part II investigates the functional behavior of the baryonic energy in the Wigner-Seitz cell within the Extended-Thomas-Fermi approximation. In particular, both bulk and surface in-medium effects are studied, and their dependence on cluster size and asymmetry as well as gas densities and asymmetry is investigated. A preliminary result of in-medium surface effects is presented within some approximations in the case of β-equilibrated matter

The Balmer emission lines in the spectra of symbiotic stars are often seen to have a double-peaked morphology when observed at high resolution. In this respect, they bear striking similarities to the Balmer line profiles in the spectra of cataclysmic variables, (Cv's) where their origin is ascribed to emission from an accretion disc. The role which accretion discs play within the class of symbiotic stars however, is much more uncertain and modelling the lines on this basis could, if successful, help to strengthen the evidence for their presence in symbiotic systems. This is the main motivation for the work described in this thesis, which is a direct offshoot of the Manchester Echelle/European Southern Observatory High Resolution Spectroscopic Survey of symbiotic stars (see van Winckel et al., 1993; Ivison, Bode & Meaburn., 1994) We first provide a critique of previously developed, alternative Balmer line profile models and offer our reasons for opting in favour of the accretion disc hypothesis. We then describe two models for computing synthetic line profiles from accretion discs, which were originally developed to simulate line profiles in CV spectra. One of these models (Home & Marsh, 1986) is adapted by us for the purpose of modelling line profiles in the Manchester survey spectra. In its basic form, this model produces line profiles with emission peaks of equal height, whereas virtually all of the survey spectra showed unequal peak heights. We ascribe this to the effects of absorption, possibly due to the outflowing wind of the cool giant, which is universally accepted as an integral part of all symbiotic systems. The model is used to produce fits to a total of 21 spectra; i.e. all of those spectra in the Manchester survey which were considered suitable for modelling in this way. Only the higher emission peak is modelled, it being assumed that this is the least affected by the aforementioned absorption. Some of the fits are very good indeed while others are not so convincing. This was in part due to the poorer quality of some of the spectra but also possibly to the effects of absorption. We then investigate the effect on line profiles from an accretion disc, of absorption by a surrounding wind. This is done by constructing a simple wind model which is then incorporated into the Home & Marsh model. Model fits are produced for the best quality spectra from the Manchester survey. Finally, future developments to the model are discussed as well as further observations which would help to guide this promising line of investigation.

This thesis deals with application of optimisation methods based on linear and mixed-integer linear programming to various problems in the power sector related to electricity production. The thesis goal is to test the applicability of such methods to formulating and solving various instances from the class of real-world electricity production problems, and to find the advantages and disadvantages associated with using these methods. Introductory chapters describe the main characteristics of power markets, including the historical and regulatory context. Fundamental properties of power markets on both demand and supply side are also described, both from a real-world and a modelling point of view. Benefits of optimisation and modelling are discussed, in particular the solution feasibility and optimality as well as insights gained from sensitivity analysis which is often difficult to replicate with the original system. In the core of the thesis, optimisation techniques are applied to three case studies, each of which deals with a specific problem arising during electricity production. In the first problem, the profit of gas-fired power plant in Slovakia from selling power on the day-ahead market is maximised. The model is set up using both technical and commercial constraints. The second problem deals with the problem of representing a two-dimensional production function which primarily arises for a hydro generator with large variations in the level of its reservoir. Several representations of the original function using piecewise linear subsets are presented, compared, and characterised by their computational intensity both theoretically and practically. In the third problem, the prices on the German day-ahead market in 2011 are modelled. Contrary to the previous two models, the model does not capture an optimisation problem faced by a single producer, but incorporates a large subset of the whole market instead. Consequently the model is formed out of generic constraints relevant to all power plants whose parameters are estimated. By combining information about the aggregate availability of power plants with the estimated efficiencies a full supply curve for each day is created. Different scenarios are analysed to test the impact of uncertain inputs such as unknown or estimated constraints. The choice of the investigated problems stems from the attempt to cover electricity production problems from the point of view of multiple criteria. The three investigated electricity production problems span a broad range from the decisions of a single power plant to the modelling a power market as a whole. Formulations of the production function with different level of detail are presented ranging from a simple linear relationship to several bivariate function formulations. While each problem answers a specific question, they all illustrate the ease with which various electricity production problems can solved using optimisation methods based on linear and mixed-integer linear programming. This is mainly due to the ability of these methods to approximate even non-linear functions and constraints over non-convex domains and find global solutions in reasonable time. Moreover, models formulated with these methods allow sensitivity and scenario analyses to be carried out easily as is illustrated in each of the case studies.

This thesis investigates the modelling of massive protostellar envelopes in dust and molecular emission. A detailed study is undertaken for a sample of 30 targets believed to contain young high-mass protostars. SCUBA observations of the dust emission are used to help distinguish between the genuine high-mass protostars and clusters of lower mass protostars. Using the single power-law density structure, models are found that fit 16 sources, that are well-separated from other sources, in half of the 30 target regions. Indices of the power-law profiles are compared to values obtained from the literature. The distributions of the indices are similar but there are differences in the underlying distributions which show that observations at longer wavelengths give generally steeper profiles. Investigating the underlying nature of the sources between the fitted and non-fitted sources shows some interesting differences between the groups. These differences indicate that the modelled sources are more likely to be massive protostars while the other sources are probably lower mass protostars. Single power-law modelling is also used to investigate molecular emission in a second group of targets from the RMS survey. Dust models are obtained for six of the sources which show similar results to the sources described above. Molecular line emission from the targets is mapped and compared to the dust emission which shows they trace the same regions. The dust models are used as a basis to obtain the molecular abundances for the sources. Abundances from the modelling the lines are broadly similar values to those observed in other high-mass protostars and association with radio and maser emission indicates that high-mass star formation is definitely occurring in some of these regions.

Circumstellar discs of pre-main sequence stars undergo different processes depending on the nature of the circumstellar environment, which is governed by stellar mass. I have performed numerical simulations of the circumstellar regions of classical T~Tauri stars (CTTs) and Herbig~Ae (HAe) stars using the radiative transfer code TORUS in order to test the paradigm of magnetospheric accretion in CTTs, and to ascertain the nature of the material in the inner regions of HAe discs. The process of magnetospheric accretion (MA) involves disc material attaching to stellar magnetic field lines and impacting the photosphere, producing accretion shocks. When the magnetic field is inclined to the star, disc warps form which periodically occult the photosphere. With specific reference to the CTTs AA Tau I perform three-dimensional MA models to study this variability. By comparing synthetic photometry with observational data I show that the geometry of the system can be constrained. I go on to study Balmer line profiles in the context of MA and disc wind outflows. I present three-dimensional models of a system comprising the star, magnetosphere, disc, and disc wind, producing synthetic line profiles and images. Using these profiles I perform time-series fitting to observational data and demonstrate that the mass accretion rate, mass loss rate, and magnetosphere temperature can be constrained. I show that there is a degeneracy between wind temperature and wind acceleration which require alternative methods to constrain further. While an outflow model alternative to a sole disc wind may produce better fits to observations, MA models reproduce various observational features well. Finally I test the hypothesis that refractory grains produce the innermost emission in HAe discs. Focussing on the HAe stars MWC 275 and AB Aur, I perform radiative equilibrium modelling to create synthetic images of these objects from which interferometric visibility profiles are produced. I show that the temperatures at which these refractory grains are required to survive are too high to be physically plausible. I also find that silicate dust is shielded when sufficiently high mass fractions of refractory grains are used, enabling the silicates to survive interior to the classical sublimation radius. While refractory dust may provide a significant contribution to the emission observed in these inner regions, this alone is not sufficient.

Understanding the molecular composition of astrophysical atmospheres depends on the interpretation of their spectra, which is possible through a combination of observational data analysis and computational modelling. However, this process is difficult to perform as the observed spectra have an extremely rich structure and their opacity is dominated by multiple and complex molecules. These issues can only be addressed through a sophisticated understanding of molecular behaviour, together with fundamental data for all the species that contribute significantly to an atmosphere’s opacity. The present work describes the motivation and requirements for the creation of molecular line lists, and their applications. It specifically focuses on phosphine, PH3, due to its relevance in both earthly and astronomical studies, in particular cool stars and exoplanets. A complete line list for phosphine is presented, having been generated by employing ab initio quantum mechanical methods, performing an empirical refinement based on experimental spectroscopic data and harnessing high performance computing. A full account of how the spectral data were obtained, from the creation of the requisite surfaces to the production of cross-sections, can be found in these pages. An analysis of the partition function and associated thermodynamic properties of both phosphine and ammonia, NH3, is also done. Additionally, ongoing work from a preliminary investigation into the use of selected symmetries to produce fast and inexpensive low resolution spectra is described, and results from an introductory study of the tunnelling motion of phosphine are presented. The connections between past, present and future work are discussed, as well as their applications to the fields of astronomy and molecular physics.

Dans l’Univers, on observe des galaxies lointaines ne formant plus d’étoiles, mais les astrophysiciens n’ont pas encore identifié avec certitude les phénomènes physiques à l’origine de leur “mort”. Pour apporter des éléments de réponse, je me suis penchée sur l’étude de phénomènes qui pourraient y jouer un rôle : les processus de rétroaction des étoiles et des trous noirs supermassifs actifs, la formation stellaire, et les vents galactiques. Le Chapitre 1 présente toutes les notions nécessaires à la compréhension du problème : les caractéristiques des galaxies typiques de l’Univers proche et lointain ; les vents galactiques ; la mort des galaxies; les trous noirs supermassifs actifs (noyaux actifs de galaxies, AGN) et les étoiles ; et leur rétroaction. Dans le Chapitre 2, je présente les techniques numériques utilisées : le code de simulations astrophysiques RAMSES et le code de transfert radiatif Cloudy, que j’ai utilisé pour développer une méthode de calcul de l’état d’ionisation d’une galaxie, détaillée au Chapitre 3. Le Chapitre 4 étudie le couplage entre les trous noirs actifs et les étoiles, avec le projet POGO, Origines Physiques des Vents Galactiques. Durant cette thèse, j’ai montré que les trous noirs actifs n’étaient pas en mesure de tuer subitement leur hôte, même en prenant en compte la rétroaction des étoiles, et que leur couplage peut réduire ou renforcer les vents dans les galaxies en fonction de leur masse. Le Chapitre 5 fait un état de l’art du domaine avant et pendant mon doctorat, reprend les conclusions de cette thèse et donne quelques perspectives, notamment en ce qui concerne le rôle additionnel des rayons cosmiques dans la mort des galaxies
In the Universe, we observe galaxies forming no, or almost no, stars anymore, but astrophysicists do not know yet what physical mechanisms cause their “death”. To give clues to solve the problem, I studied feedback processes from stars and active supermassive black holes, star formation and galactic outflows. Chapter 1 presents all the notions to understand the problem: the characteristics of typical galaxies in the local and distant Universe, galactic outflows, galaxy death, active supermassive black holes, stars, and their feedback processes. In Chapter 2, I describe the numerical techniques I used: the simulation code RAMSES, and the radiative transfer code Cloudy, which I used to develop a computation method to get the ionization state of an entire galaxy. This method is presented in Chapter 3. Chapter 4 studies the coupling between the feedback processes of active supermassive black holes and stars, with the POGO project, Physical Origins of Galactic Outflows. During this thesis, I showed that typical active supermassive black hole cannot suddenly kill their host, even when stellar feedback processes are accounted for, and that their coupling either reduces or enhances the mass outflow rate depending on the mass of the host. In Chapter 5, I give a state-of-the-art about active supermassive black holes before and during my thesis, sum up the conclusions of the work, and give perspectives to enlarge the scope of the study, especially regarding the additional role of cosmic rays in the death of galaxies

Remarkable progress has been made over the last few decades in furthering our understanding of the growth of cosmic structure. Nonetheless, there remains a great deal of uncertainty regarding the precise details of the complex baryonic physics that regulate galaxy formation. Any theory of star formation in galaxies must encompass the radiative cooling of gas into dark matter haloes, the formation of a turbulent, multiphase interstellar medium (ISM), the efficiency with which molecular gas is able to collapse into cores and ultimately stars, and the subsequent interaction of those stars with the gas through ionizing radiation, winds and supernova (SN) explosions. Given the highly non-linear nature of the problem, numerical simulations provide an invaluable tool with which to study galaxy formation. Yet, even with contemporary computational resources, the inherently large dynamical range of spatial scales that must be tackled makes the development of such models extremely challenging, inevitably leading to the adoption of `subgrid' approximations at some scale. In this thesis, I explore new methods of incorporating the physics of star formation and stellar feedback into high resolution hydrodynamic simulations of galaxies. I first describe a new implementation of star formation and SN feedback that I have developed for the state-of-the-art moving mesh code Arepo. I carry out a detailed study into various classes of subgrid SN feedback schemes commonly adopted in the literature, including injections of thermal and/or kinetic energy, two parametrizations of delayed cooling feedback and a 'mechanical' feedback scheme that injects the appropriate amount of momentum depending on the relevant scale of the SN remnant (SNR) resolved. All schemes make use of individually time-resolved SN events. Adopting isolated disk galaxy setups at different resolutions, with the highest resolution runs reasonably resolving the Sedov-Taylor phase of the SNR, I demonstrate that the mechanical scheme is the only physically well-posed method of those examined, is efficient at suppressing star formation, agrees well with the Kennicutt-Schmidt relation and leads to converged star formation rates and galaxy morphologies with increasing resolution without fine tuning any parameters. However, I find that it is difficult to produce outflows with high enough mass loading factors at all but the highest resolution. I discuss the various possible solutions to this effect, including improved modelling of star formation. Moving on to a more self-consistent setup, I carry out a suite of cosmological zoom-in simulations of low mass haloes at very high resolution, performed to z = 4, to investigate the ability of SN feedback models to produce realistic galaxies. The haloes are selected in a variety of environments, ranging from voids to crowded locations. In the majority of cases, SN feedback alone has little impact at early times even in low mass haloes ($\sim10^{10}\,\mathrm{M_\odot}$ at z = 0). This appears to be due largely to the build up of very dense gas prior to SN events, suggesting that other mechanisms (such as other stellar feedback processes) are required to regulate ISM properties before SNe occur. The effectiveness of the feedback also appears to be strongly dependent on the merger history of the halo. Finally, I describe a new scheme to drive turbulence in isolated galaxy setups. The turbulent structure of the ISM very likely regulates star formation efficiencies on small scales, as well as affecting the clustering of SNe. The large range of potential drivers of ISM turbulence are not fully understood and are, in any case, unlikely to arise ab initio in a whole galaxy simulation. I therefore neglect these details and adopt a highly idealised approach, artificially driving turbulence to produce an ISM structure of my choice. This enables me to study the effects of a given level of ISM turbulence on global galaxy properties, such as the fragmentation scale of the disk and the impact on SN feedback efficiencies. I demonstrate this technique in the context of simulations of isolated dwarfs, finding that moderate levels of turbulent driving in combination with SN feedback can produce a steady-state of star formation rates and global galaxy properties, rather than the extremely violent SN feedback that is produced by a rapidly fragmenting disk.

Two pseudo-time-dependent chemical models have been utilised - one of a dark cloud,and one of a hot core - in order to model these clouds in low metallicity environments, such as other galaxies. The dark cloud model uses gas-phase chemistry, whereas the hot core model includes both gas-phase and surface chemistry. The simulations have been calculated with varying initial elemental abundances of C, O, N, S and the heavy metals Fe, Mg and Na (henceforth, M). These initial abundances are taken from observations of HII regions in the Galaxy, the Large Magellanic Cloud and the Small Magellanic Cloud. The results have been used to identify species which potentially trace the underlying metallicity in dark clouds and hot cores. In the dark cloud models, the most useful tracers are ratios of two species, notably CO/OH and HCO+/CO, which trace the underlying C and M abundances respectively. In the hot core models, the most useful metallicity tracer species are HNC and NH3. The HNC abundance traces an underlying change in metallicity, independent of any changes to the dust/gas ratio. The NH3 abundance traces the underlying N abundance. The hot core model output abundances were used with RATRAN, a non-LTE radiative transfer code, to predict the integrated intensity as a function of hot core radius for various species. The RATRAN results are more directly comparable with observations than the results from the chemical models. Less common isotopes have been used to limit the optical depth of the species modelled. The results show that the extent of the emission may not reflect the size of the hot core. HN13C and NH3 are confirmed as the most useful metallicity tracer species in hot cores.

Data are the lifeblood of an organisation, being employed by virtually all business functions within a firm. Data management, therefore, is a critical process in prolonging the life of a company and determining the success of each of an organisation’s business functions. The last decade and a half has seen data warehousing rising in priority within corporate data management as it provides an effective supporting platform for decision support tools. A cross-sectional survey conducted by this research showed that data warehousing is starting to be used within organisations for their engineering asset management, however the industry uptake is slow and has much room for development and improvement. This conclusion is also evidenced by the lack of systematic scholarly research within asset management data warehousing as compared to data warehousing for other business areas. This research is motivated by the lack of dedicated research into asset management data warehousing and attempts to provide original contributions to the area, focussing on data modelling. Integration is a fundamental characteristic of a data warehouse and facilitates the analysis of data from multiple sources. While several integration models exist for asset management, these only cover select areas of asset management. This research presents a novel conceptual data warehousing data model that integrates the numerous asset management data areas. The comprehensive ethnographic modelling methodology involved a diverse set of inputs (including data model patterns, standards, information system data models, and business process models) that described asset management data. Used as an integrated data source, the conceptual data model was verified by more than 20 experts in asset management and validated against four case studies. A large section of asset management data are stored in a relational format due to the maturity and pervasiveness of relational database management systems. Data warehousing offers the alternative approach of structuring data in a dimensional format, which suggests increased data retrieval speeds in addition to reducing analysis complexity for end users. To investigate the benefits of moving asset management data from a relational to multidimensional format, this research presents an innovative relational vs. multidimensional model evaluation procedure. To undertake an equitable comparison, the compared multidimensional are derived from an asset management relational model and as such, this research presents an original multidimensional modelling derivation methodology for asset management relational models. Multidimensional models were derived from the relational models in the asset management data exchange standard, MIMOSA OSA-EAI. The multidimensional and relational models were compared through a series of queries. It was discovered that multidimensional schemas reduced the data size and subsequently data insertion time, decreased the complexity of query conceptualisation, and improved the query execution performance across a range of query types. To facilitate the quicker uptake of these data warehouse multidimensional models within organisations, an alternate modelling methodology was investigated. This research presents an innovative approach of using a case-based reasoning methodology for data warehouse schema design. Using unique case representation and indexing techniques, the system also uses a business vocabulary repository to augment case searching and adaptation. The system was validated through a case-study where multidimensional schema design speed and accuracy was measured. It was found that the case-based reasoning system provided a marginal benefit, with a greater benefits gained when confronted with more difficult scenarios.

This thesis researches the problems concerning the specification and implementation of the temporal requirements in data warehouses. The thesis focuses on two areas, firstly, the methods for identifying and capturing the business information needs and associated temporal requirements at the conceptual level and; secondly, methods for classifying and implementing the requirements at the logical level using the relational model. At the conceptual level, eight candidate methodologies were investigated to examine their suitability for the creation of data models that are appropriate for a data warehouse. The methods were evaluated to assess their representation of time, their ability to reflect the dimensional nature of data warehouse models and their simplicity of use. The research found that none of the methods under review fully satisfied the criteria. At the logical level, the research concluded that the methods widely used in current practice result in data structures that are either incapable of answering some very basic questions involving history or that return inaccurate results. Specific proposals are made in three areas. Firstly, a new conceptual model is described that is designed to capture the information requirements for dimensional models and has full support for time. Secondly, a new approach at the logical level is proposed. It provides the data structures that enable the requirements captured in the conceptual model to be implemented, thus enabling the historical questions to be answered simply and accurately. Thirdly, a set of rules is developed to help minimise the inaccuracy caused by time. A guide has been produced that provides practitioners with the tools and instructions on how to implement data warehouses using the methods developed in the thesis.

La formation des étoiles s’accompagne généralement de la formation d'undisque où peuvent se former les planètes. Ces disques protoplanétaires contiennent un mélange de 99 % de gaz et de 1 % de particules solides appelées grains de poussière. Initialement de taille sub-micrométrique, ces grains vont progressivement s’agglomérer, grossir, et potentiellement permettre la formation de planètes autour de l’étoile.L’étude de la composition en molécules et en grains des disques jeunes est fondamentale pour contraindre les conditions physico-chimiques initiales de la formation planétaire et l’origine de la composition chimique des planètes.L’objectif de la thèse a été de construire des modèles sophistiqués de disques jeunes typiques constitués de gaz et d’une population de grains de différentes tailles puis, de manière inédite, de tester par simulations numériques l’implication de cette distribution en taille et en température sur l’évolution chimique.Pour ce faire, nous avons couplé le code de transfert radiatif 3D Monte-Carlo POLARIS au code de simulation gaz-grain dépendant du temps NAUTILUS. Le code de transfert radiatif nous a permis de calculer finement la température des grains en fonction de leur taille et de leur position ainsi que le flux UV au sein du disque. Le code gaz-grain, quant à lui, a ensuite pu simuler l’évolution des abondances chimiques dans nos modèles de disques. De plus, le calcul du flux UV effectué par POLARIS couplé à l’utilisation de section efficaces moléculaires provenant de bases de données a permis le calcul en fonction de la fréquence des taux de photoabsorption, de photodissociation et de photoionisation des molécules
The star formation process usually proceeds with protoplanetary disks. These disks contain a mixture of gas, accounting for 99 % of the disk mass, and of solid particles called dust grains (1 % of the disk mass). These grains, initially at sub-micro metric sizes, gradually coagulate, grow, and potentially allow for the formation of planets about the star.The study of the dust and molecular composition of young disks is fundamental to constraint the physical and chemical initial conditions of planetary formation and the origins of the chemical composition of the planetary cores.The goal of this thesis was to build state-of-the-art models of typical young disks consisting of gas and of a population of grains of multiple sizes, then, in a new approach, to test with the use of numerical simulations the implication of the size and temperature distributions on the chemical evolution of disks.To achieve this, we have coupled the 3D Monte-Carlo radiative transfer code POLARIS to the time-dependent gas-grain code NAUTILUS. The radiative transfer code allowed us to finely compute the grain temperature as a function of the size and location as well as the UV flux within the disk. The gas-grain code was able to simulate the evolution of the chemical abundances in our disk models. Moreover, the computation of the UV flux by POLARIS coupled to a set of molecular cross-sections extracted from a comprehensive database allowed us to compute as a function of the frequency the rates of molecular photoabsorption, photodissociation, and photoionization

Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmosphere of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. The work descried in this thesis details the development of a new 1D ionospheric model to describe the upper atmospheres of Extrasolar Giant Plants (EGPs). The model is time-dependent and includes photo-chemistry and diffusive transport. Electron-impact ionisation processes are taken into account through coupling with a suprathermal electron transport code. Neutral composition and temperature profiles are obtained by using a thermospheric model that incorporates atmospheric escape. Atmospheres composed of H, H2, He, and their associated ions are considered. Efforts have been made to obtain accurate X-ray and Extreme Ultraviolet (EUV) spectral irradiance of the stars studied. To this effect, synthetic spectra are used originating from a detailed coronal model for three different low-mass stars of different activity levels: epsilon Eridani, AD Leonis and AU Microscopii. This work is the first study of the ionosphere of EGPs that takes into account the different spectral energy distribution of low-mass stars. In planets subjected to radiation from active stars, the transition from slow, Jeans escape to a regime of rapid hydrodynamic escape at the top of the atmosphere is found to occur at larger orbital distances than for planets around low activity stars (such as the Sun). To correctly estimate the critical orbital distance of this transition, the spectral shape of stellar XUV radiation is important. A novel method to scale the EUV region of the solar spectrum based upon stellar X-ray emission is developed in this work. This new method produces an outcome in terms of the planet's upper atmosphere and escape regime that is very similar to that obtained using a detailed coronal model of the host star. EGP ionospheres at all orbital distances and around all stars studied are dominated by the long-lived H+ ion. In addition, planets in the Jeans escape regime also have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, H3+ densities undergo significant diurnal variations, their peak value being determined by the stellar X-ray flux. In contrast, H+ densities show very little day/night variability and their value is determined by the level of stellar EUV flux. The H3+ peak in EGPs in the rapid hydrodynamic escape regime under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (C, O, etc.).