FE - DCA | Dissertações de Mestrado e Teses de Doutoramento
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- Oxygen/Hydrogen Combustion under Supercritical Conditions: A Numerical StudyPublication . Martins, Daniel Ribeiro; Silva, André Resende Rodrigues da; Magalhães, Leandro BarbosaThe pursuit of enhanced propulsion system performance has driven the development of combustion technologies operating at increasingly higher pressures. Such conditions tipically exceed the thermodynamic critical thresholds of the propellants, giving rise to complex transcritical and supercritical flow regimes. The oxygen/hydrogen propellant combination has been extensively and successfully employed in liquid rocket propulsion, and, in the context of global decarbonization objectives, the clean, high-energy characteristics of hydrogen make it a promising candidate for future propulsion applications extending beyond rocketry. The modeling of shear coaxial injection of oxygen and hydrogen under supercritical conditions is addressed through a steady-state axisymmetric Favre-averaged Navier-Stokes formulation, with combustion treated using the Eddy-Dissipation-Concept model in conjunction with a detailed kinetic mechanism consisting of eight species and nineteen elementary reactions. The near-field region of the injector constitutes the primary focus of this work, as it is within this zone that the mixing efficiency is established, ultimately governing the overall combustion and engine performance. The simulations are carried out piecewise to ensure consistency thorugh all the simulation process. Firstly, non-reacting simulations are performed, incorporating both ideal- and real-gas models. After that, reacting simulations are performed in order to validate the developed numerical model against experimental data. Two main innovative contributions distinguish the present study from previous works on O2/H2 coaxial injection. The first concerns computational efficiency, achieved through a substantial reduction of the chamber length to the minimum extent necessary to ensure that the outlet boundary conditions do not affect the near-field region. In addition, the omission of the low-velocity recirculation zone located above the injector head was implemented to further reduce computational cost. The second innovation involves the inclusion of heattransfer effects in both the injector and chamber walls. In particular, the influence of the isothermal walls configuration on the injector is analyzed in light of the thermal disintegration mechanism of the inner jet, previously identified in the literature for cold-flow conditions under supercritical regimes. The numerical results demonstrated the capability of the developed framework to accurately model cryogenic O2/H2 injection and combustion under supercritical conditions. The non-reacting simulations established a robust numerical foundation and captured the key flow features, including the dense oxygen core and the surrounding high-velocity hydrogen stream. The incorporation of real-gas effects proved essential, as the ideal-gas assumption led to significant deviations in core length and mixing behavior. In the reacting regime, the real-gas model reproduced the experimental axial temperature profile and flame structure with good accuracy, outperforming the ideal-gas formulation and yielding results comparable to those reported in the literature, thereby confirming its suitability for high-pressure combustion modeling.
- Modelo de avaliação do processo logístico no desmantelamento de aeronaves em entidades certificadas EASA PART-145Publication . Vale, Francisco Gonçalves Candeias do; Saúde, José Manuel Mota Lourenço da; Ros, Sofia ValériaO aumento do número de aeronaves em fim de vida útil exige processos de desmantelamento eficientes e sustentáveis. Esta dissertação centra-se no desenvolvimento de um modelo de avaliação do processo logístico em entidades certificadas EASA PART-145, utilizando a Dassault Aviation Business Services (DABS) como caso de estudo. A metodologia baseou-se numa análise de capacidade por grupos funcionais (Maintenance, Technical Services e Logística) e no desenvolvimento do Índice de Equilíbrio de Fluxo (IEF), uma ferramenta de gestão orientada para o planeamento estratégico e apoio à decisão. A aplicação do modelo revelou um desequilíbrio estrutural entre a capacidade da Produção e da Logística, com o principal estrangulamento na equipa de Maintenance. O IEF demonstrou ser eficaz na análise do estado do sistema e na simulação de cenários futuros. Conclui-se que o modelo proposto foi validado com sucesso, constituindo uma ferramenta útil para a otimização do desmantelamento de aeronaves. O estudo evidencia que a eficiência global depende do equilíbrio e da sincronização do fluxo de trabalho.
- Piezoresistive Sensing for Structural Health Monitoring in Liquid Hydrogen Tanks: A Study on Composite Cryotank TechnologiesPublication . Figueiredo, Matilde Rola; Pereira, João Pedro Nunes; Silva, Abílio Manuel Pereira daHydrogen has been emerging as a potential key energy carrier for sustainable aviation, yet its adoption is constrained by distinct engineering challenges, particularly its safe and lightweight storage. The present study investigates the feasibility of using the intrinsic piezoresistive properties of carbon-fibre reinforced prepreg composites, commonly employed in Type V cryogenic hydrogen tanks, for structural health monitoring. Given that matrix microcracking constitutes the predominant damage mechanism in these tanks and typically initiates at strain levels of 0.3%-0.5%, a preventive strain sensing approach was adopted based on these critical deformation thresholds. To achieve a proof of concept, 17-ply multidirectional carbon fibre/epoxy laminates based on Boeing’s cryotank from the Composite Cryotank Technologies and Demonstration program were produced and tested in their as-fabricated state and after controlled cryogenic cycling through five and ten cycles of immersion in liquid nitrogen (-196 ?) followed by reheating (52 ?). Unconditioned specimens showed a clear and repeatable piezoresistive response from surface measurements, with gauge factors ranging from -11.02 to -12.70, confirming their intrinsic self-sensing capability. Cryogenically cycled specimens exhibited moderate mechanical degradation and increased baseline electrical resistance, yet retained self-sensing performance, with gauge factor increases exceeding 85%. While volume (edge-to-edge) measurements were inconclusive, the consistent surface response demonstrates that unmodified carbon fibre/epoxy prepreg composites employed in Type V cryotanks can retain effective strain sensing functionality after cryogenic exposure and cycling, providing insights for the development of integrated structural health monitoring strategies in hydrogen storage applications.
- Efeito da temperatura no comportamento de compósitos de resina epóxi nano reforçados com nano fibras de carbonoPublication . Tavares, Rimaldini da Veiga; Silva, Abílio Manuel Pereira da; Santos, Paulo Sérgio Pina dosA indústria aeronáutica tem se destacado como um fator crucial no desenvolvimento tecnológico, impulsionada pela inovação em compósitos. Os compósitos, especialmente os reforçados com fibras de carbono e nano materiais, têm revolucionado o design e a fabricação de aeronaves, oferecendo melhorias significativas em termos de leveza, resistência e durabilidade. Neste contexto, realizou-se esta dissertação com o prepósito de investigar o efeito da temperatura no comportamento de compósitos de resina epóxi reforçada com nano fibras de carbono (CNF’s), por meio de um estudo numérico e experimental. Visa entender como as variações térmicas afetam as propriedades mecânicas e térmicas desses compósitos avançados, essenciais para aplicações na indústria aeroespacial e aeronáutica. A metodologia utilizada foi a agitação mecânica e a moldação manual de fácil aplicação, custos reduzidos e facilmente escalável para a indústria. Os ensaios de flexão realizados evidenciaram que tanto as matrizes quanto os laminados aditivados com 0,75% em peso de CNF’s apresentaram desempenho superior em relação às respetivas amostras de controlo. A 50 ?, a incorporação de CNF’s na matriz promoveu um aumento de 42,34% na tensão de flexão, um incremento de 44,33% na rigidez e uma redução de 5,77% na deformação, quando comparada à matriz de controlo. Para os laminados submetidos à mesma condição térmica, a adição de 0,75% em peso de CNF’s resultou em um acréscimo de 17,75% na tensão de flexão, um ganho de 5,35% na rigidez e uma diminuição de 7,09% na deformação, em relação ao laminado de controlo. Nos testes de relaxação de tensões, ocorre a redução da tensão ao longo do tempo, e em termos de fluência, o deslocamento aumenta ao longo do tempo e os dados experimentais foram ajustados com sucesso pelo modelo de Kohlrausch-Williams-Watts (KWW).
- Análise de propagação de dano e da resposta piezoresistiva em laminados de CFRPPublication . Pereira, Duarte Amaro; Silva, Abílio Manuel Pereira da; Pereira, João Pedro NunesOs laminados de polímero reforçado com fibra de carbono (CFRP) são amplamente utilizados em estruturas aeronáuticas devido à sua elevada resistência específica, rigidez e durabilidade. Contudo, a sua suscetibilidade a defeitos internos e externos, como a rutura frágil da matriz, delaminação e rutura de fibras, pode comprometer significativamente o desempenho estrutural e a segurança em serviço. Neste trabalho estudou-se experimentalmente a propagação de fissuras em laminados de CFRP através de ensaios de flexão em três pontos e a monitorização piezoresistiva em condições de carga cíclicas. Foram fabricados laminados em fibra de carbono em autoclave com diferentes configurações de empilhamento: unidirecional [0]10, [0]16 e [0]22, cruzada [0/90]5 e simétrica [0/45/90/-45/0]S e realizaram-se ensaios mecânicos de flexão em três pontos e ensaios cíclicos com monitorização em tempo real da resposta elétrica e observação microscópica da evolução dos danos. Os ensaios de flexão em três pontos evidenciaram comportamentos distintos à fratura para cada tipologia de empilhamento. O laminado unidirecional de 10 camadas apresentou a maior resistência máxima à flexão (1469 MPa) e o módulo de elasticidade mais elevado (136 GPa), enquanto os laminados de 16 e 22 camadas exibiram resistências 26 % e 29 % inferiores, respetivamente. O laminado simétrico revelou a maior deformação máxima (1,35 %) e flecha a meio do vão (8,16 mm), evidenciando maior capacidade de absorção de energia, ao passo que o laminado cruzado apresentou resistência de 588 MPa e deformação de 1,27 %, associadas à redistribuição de tensões entre camadas ortogonais. Nos ensaios cíclicos com monitorização piezoresistiva, todos os laminados mostraram correlação direta entre o carregamento e a variação relativa de resistência (?R/R0). O laminado simétrico apresentou a maior sensibilidade eletromecânica, com Gauge Factor (GF) médio de 247,9, aproximadamente 1,56 vezes superior ao do laminado unidirecional de 16 camadas (GF= 158,8) e 2,63 vezes superior ao do laminado unidirecional de 10 camadas (GF= 94,4). O laminado cruzado apresentou o valor mais baixo (GF= 36,4), correspondendo a uma redução de cerca de 85 % face ao laminado simétrico. Estes resultados confirmam que os laminados unidirecionais apresentam o melhor desempenho mecânico, enquanto os laminados simétricos evidenciam maior sensibilidade piezoresistiva, demonstrando o potencial dos CFRP como materiais multifuncionais self-sensing aplicáveis em sistemas de Structural Health Monitoring (SHM) aeronáuticos.
- Piezoresistive Sensing for Structural Health Monitoring: A Temperature Dependence CharacterisationPublication . Pinto, Daniel Fernando Moreira; Pereira, João Pedro Nunes; Silva, Abílio Manuel Pereira da; Parente, João Miguel NunesThe aeronautical industry holds safety as its core and its main drive for innovation, with sensors, materials science and self-governance to ensure safe travel through the skies. However, operational costs and maintenance requirements increase proportionally with the increase of number of sensors, which is a growing concern and makes cost-effective solutions desirable. This dissertation aims to advance the knowledge and viability of a promising candidate to resolve this issue, studying a type of what are called self-sensing materials as an alternative to the conventional structural health monitoring (SHM) sensors. This material is a type of piezoresistive nanocomposite (PNC) that, in theory, could replace an airplanes’ complicated arrays of sensors while providing even better performance. However, PNCs have a glaring fault that is hindering their commercial availability, namely their perceptual conditioning of operational temperature. Current state of the art has mostly neglected this correlation, especially in the SHM sector, an oversight that this dissertation aims to rectify. For this reason, electromechanical properties of a PNC specimen made of glass-fibre epoxy resin with 0.5%𝑤𝑡 multiwalled carbon nanotubes (MWCNT) as filler were thoroughly tested at three different test temperatures: 30°𝐶, 50°𝐶 and 70°𝐶. The experimental analysis, static and cyclical tests, revealed that temperature has a profound effect on sensing performance, outlining key factors of response change with temperature variation. Three gauge factors (GF) were calculated, one for each temperature, with values ranging from a maximum of 2.42 to a minimum of 0.83. For a broader scope on this effect, multiple variations of the basic three-point bending (3PB) tests were performed to determine which factors were application-specific and which were constant.
- Effects of Wing-Fuselage Interface Position on Lift and DragPublication . Bombas, Rodrigo Manuel Cação do Carmo; Gamboa, Pedro VieiraThe interference effect between two aircraft components has become increasingly relevant in the design process, in parallel with the advances in aerodynamic efficiency observed over the last decades. In order to understand how this phenomenon manifests itself in small aircraft operating at low Reynolds numbers, a study was carried out on the influence of wing position relative to the fuselage, as well as the effect of fairings at the interface between the two components and the fuselage cross-section on interference drag. To determine the most favourable wing–fuselage interface positions in terms of drag and lift—considering also two types of fairings and fuselage cross-sections—the aircraft provided by the UBIAT team was modelled in CATIAV5. A simplified model was developed that nonetheless preserved the essential features of the real aircraft. The geometry was then prepared in Ansys Discovery to define the body of influence, the simulation volume, and the boundary conditions. This setup was subsequently imported into Ansys Fluent, where the computational mesh was generated according to a previously conducted mesh-independence study. The CFD simulations were performed using the k − ω SST turbulence model with standard parameters, including corrections for transitional flow and low Reynolds number regimes. Four alternative wing–fuselage interface configurations, in addition to the baseline case, were analysed: two vertical and two horizontal. The vertical configurations were further assessed with fairings and with a circular fuselage cross-section. The results regarding wing–fuselage positioning for both fuselage geometries were consistent with expectations and supported by literature. As for the introduction of fairings, both were found to improve aerodynamic efficiency, although the results did not fully align with published references. The main conclusions drawn from this work are the advantage of a high-wing configuration when interfaced with a square fuselage cross-section, and of a mid-wing configuration when interfaced with a circular fuselage cross-section. The addition of fairings yielded improvements of up to 4.7% in aerodynamic efficiency for the high-wing configuration with a square-section fuselage, while their effect was negligible for the other vertical positions.
- Development and Experimental Study of a Series-Parallel Hybrid Powertrain Prototype Using a Planetary GearsetPublication . Pereira, Simão Ribeiro; Brojo, Francisco Miguel Ribeiro ProençaIn recent years, unmanned aerial vehicles (UAVs) have emerged as a versatile solution across a wide range of applications. Their adaptability, safety, and cost-efficiency make them particularly valuable in complex or hazardous environments. At the same time, growing environmental concerns and the global push toward reducing greenhouse gas emissions have sparked interest in more sustainable propulsion technologies. While electric UAVs represent a promising convergence of innovation and ecological responsibility, these still present several limitations such as payload capacity and endurance. In this regard, fuel-electric hybrid UAVs present an auspicious alternative, covering the disadvantages of electric UAVs and being more environmental-friendly than UAVs powered exclusively by fossil fuels. This dissertation presents the design, development, and experimental validation of a prototype series-parallel hybrid powertrain integrating a planetary gearset. The system combines an internal combustion engine (ICE) with two electric motors (MG1 and MG2), enabling multiple operating modes: series hybrid, parallel hybrid, electric-only, and engine start; without altering the mechanical configuration. Inspired by the Toyota Hybrid System, the planetary gearset serves as a power-split device, allowing dynamic energy distribution among the powerplants. The prototype was constructed using a combination of 3D-printed components and custom-machined parts, with control implemented via manual throttles and a microcontroller-based data acquisition system. Despite adverse events that restricted the system’s idealized operation, there were obtained results that validate the core functionality of the hybrid architecture and highlight key areas for improvement, including material durability, sensor reliability, and control automation. This work lays the foundation for future iterations aimed at UAV integration and enhanced energy efficiency.
- LSTM Based Trajectory Prediction for Aerospace Vehicle ReentryPublication . Coelho, João Pedro Monteiro; Bousson, KouamanaDue to the rapid development of the machine learning field, its applications in trajectory prediction are now a matter of focus in academia and industry. In order to further broaden previous research, focusing more on a lower-atmosphere approach, this thesis aims to develop a model more focused on the latter steps of the reentry phase. The main objective is to create a deep learning model to predict the next position of the reentry aerospace vehicle based on a set of three previous positions. In order to test the robustness of this model, a Kullback-Leibler (Jensen-Shannon) divergence test was applied to each of the prediction models. The results were 6.07 and 0.492 for the UARS and KOSMOS 482 satellites, respectively. The results show a good degree of correspondence between the training and test sets, implying that the learning process was well done. However, should a similar study be conducted, efforts should be taken to obtain more realistic data, whether through more complex simulation, or using real sensor input. This thesis made use of the Dryden turbulence model, as well as many atmospheric models such as the International Civil Aviation Organization and the United States of America 1976. Long-Short Term Memory was the chosen recurrent neural network to have the evolution through time into account, having a dedicated section to its creation and analysis. The optimization and regularization processes are also presented, making use of the Tree-Structured Parzen Estimation to tune hyperparameters present in the model such as loss functions and ratio, number of hidden layers and their inputs, batch size and optimization models. Due to the absence of a sufficiently broad data pool to train this model, a chapter dedicated to the simulation procedures was also written, describing the algorithm used for it. The satellites chosen to test this model were UARS and KOSMOS 482, choosing initial conditions based on their latest recorded orbit epochs, varying the true anomaly and therefore obtaining about 1500000 inputs divided between training and testing the model. For the LSTM architecture, a MISO approach was taken due to dimensionality, dividing the trained model into three separate position predictions (x, y and z) in a time step of 0.1 seconds.
- Fisiologia de Voo: Efeitos da Altitude na Pressão Intraocular e nas Estruturas Peripapilares - Estudo com Tomografia de Coerência Ótica e Técnicas de Análise DigitalPublication . Marques, Miguel Santiago; Almeida, Vasco Miguel Nina de; Ribeiro, João Manuel da Costa; Silva, Jorge Miguel dos Reis; Soares, Ivo Miguel da Fonseca GravitoNa aviação, os voos comerciais atingem habitualmente tetos máximos próximos dos 12km de altitude. A estas altitudes, a atmosfera terrestre apresenta condições extremamente adversas e incompatíveis com a vida humana, motivo pelo qual os aviões estão equipados com sistemas de pressurização que mantêm a altitude de cabine a níveis inferiores. Ainda assim, o ambiente de pressão sentido pela tripulação e passageiros encontra-se normalmente entre os limites de 1800 e 2100m de altitude equivalente. Neste contexto, o presente trabalho procurou responder a uma questão central: verificar se a subida em altitude pode induzir alterações fisiológicas ao nível ocular, com especial enfoque nas camadas retinianas da região peripapilar junto à cabeça do nervo ótico. Pretendeu-se obter resultados em ambiente de montanha que pudessem ser extrapolados para a aviação. Para tal, foi delineado e seguido um plano experimental com 30 participantes voluntários, alunos da Universidade da Beira Interior, com idades compreendidas entre os 18 e os 25 anos. Estes foram submetidos a medições oculares em duas condições ambientais distintas, ao longo de três semanas consecutivas: em ambiente basal, nas instalações da universidade a 700m de altitude (1.ª e 3.ª semana), e em altitude, na Torre da Serra da Estrela a 1993m (2.ª semana), valor que se encontra dentro da faixa de altitude de pressão de cabine referida. As medições incluíram pressão intraocular (PIO), saturação periférica de oxigénio, frequência cardíaca, densidade vascular e volume das camadas retinianas na região peripapilar. Para tal, foram utilizados um aparelho de tomografia de coerência ótica, um tonómetro de sopro e um oxímetro digital, sendo todas as medições realizadas por um especialista. Os dados obtidos foram posteriormente analisados com recurso a algoritmos desenvolvidos especificamente para este estudo, garantindo maior rigor na comparação entre os diferentes dias experimentais. A análise estatística foi conduzida em linguagem Python, aplicando métodos paramétricos ou não paramétricos conforme a normalidade dos dados. Os resultados obtidos confirmaram a hipótese de que a subida em altitude induz alterações fisiológicas oculares. Observaram-se diferenças significativas em todos os parâmetros avaliados. A pressão intraocular (PIO) diminuiu após quatro horas de permanência em altitude, tendo sido registadas de igual forma diminuições na frequência cardíaca e na saturação periférica de oxigénio. A densidade vascular apresentou variações significativas na zona superior e na máscara total da camada profunda da retina, bem como na máscara total considerando a totalidade das camadas em estudo. Por fim, o volume retiniano mostrou alterações nas regiões nasais tanto na camada superficial como total.