Committee:
     PRESIDENT: HUETE RUIZ DE LIRA, CESAR
     SECRETARI: FONT MARTÍNEZ, FRANCESC
     VOCAL: FONT GARCÍA, BERNAT
Thesis abstract: This thesis delves into investigating the complexities of heat transfer in supercritical fluids through the application of advanced theoretical and computational methodologies. The research primarily focuses on conducting large-scale direct numerical simulations (DNSs) to capture the intricate thermal behaviors of these fluids. Following the simulations, a comprehensive analysis is performed, leading to the generation of empirical heat transfer correlations tailored for high-pressure transcritical fluids. This work aims to bridge the gap between theoretical predictions and practical applications, offering valuable insights into the thermal dynamics of supercritical systems. In detail, the work introduces methods for compressible turbulence simulations on GPU-accelerated nodes, achieving significant speedups. Next, artificial compressibility method for high-pressure transcritical fluids is extended to enhance computational efficiency. Following, a comprehensive dataset for high-pressure fluid channel flows is presented, facilitating detailed analysis and modeling. Finally, novel heat transfer correlations for transcritical fluids are developed, offering improved accuracy for engineering applications.The work is organized into six chapters. The first chapter introduces the type of flows considered and the methodology used to study them. The next four chapters form the core of the dissertation, containing content published in peer-reviewed journals, thus being largely self-contained with minor changes from the original papers. Consequently, some theoretical and numerical content is repeated. The final chapter provides concluding remarks and suggestions for future work. Two appendices include supplementary material useful for understanding parts of the thesis without disrupting its main flow. Specifically, the second chapter introduces a computational approach for large-scale compressible turbulence simulations on heterogeneous nodes with GPUs. It uses MPI for node distribution and OpenACC for GPU acceleration, discussing GPU porting, data management, solver performance, and scalability. Two OpenACC methods --- automated (managed) and manual (non-managed) --- are compared, with the non-managed strategy outperforming the managed one, achieving up to 6x speedup overall and up to 26x for specific kernels. The approach is validated with high-fidelity simulations of turbulent flow, proving its efficiency for modern GPU-accelerated architectures. The third chapter extends the artificial compressibility method for high-pressure transcritical fluids by decoupling thermodynamic and hydrodynamic pressure fields, allowing the modification of acoustic time scales without affecting flow physics. The method is computationally efficient and includes an automatic speedup factor selection. Numerical tests show it can achieve speedups over 10x while maintaining accuracy in simulating high-pressure transcritical flows. The fourth chapter introduces the first comprehensive, open-source dataset for studying high-pressure transcritical fluid channel flows in microconfined environments. It includes 18 direct numerical simulations of carbon dioxide at various pressures and velocities between differentially-heated walls, covering laminar, transitional, and turbulent regimes. The dataset captures the fluid's transcritical trajectory through the pseudo-boiling region and offers detailed physical quantities for comprehensive parametric analysis, aiming to enhance the study and modeling of high-pressure transcritical fluid dynamics in microfluidics. Finally, the fifth chapter focuses on empirical heat transfer correlations, crucial for optimizing thermal performance in engineering applications like energy conversion and propulsion. This study aims to develop new correlations for high-pressure transcritical fluids at low Reynolds numbers, using data from 18 direct numerical simulations between differentially heated walls.

Committee:
     PRESIDENT: BENITO LÓPEZ, FERNANDO
     SECRETARI: RODRÍGUEZ VILLARREAL, ÁNGELES IVÓN
     VOCAL: BORTOLOTTI, CARLO AUGUSTO
Thesis abstract: Over the last decade, biosensing has been moving towards the miniaturized and cost-effective point-of-care (POC) testing. Even though microfluidics is becoming a key enabling technology for POC testing, the need for robust peripheral equipment has been a notable limiting factor in extending its prevalence. By manipulating microchannels' geometry and surface properties, capillary-driven microfluidics can control fluids spontaneously, reducing the need for external instrumentation. This advantage is becoming more accessible, considering that additive manufacturing technologies are reaching a high level of maturity that allows cost-effective and rapid fabrication of three-dimensional (3D) features down to the sub-millimeter scale. Therefore, capillary-driven microfluidics are achieving higher technological readiness.A key component within the field of capillary-driven microfluidics is the capillary valve. The capillary valve can automatically stop and actuate fluid flows depending on the molecular interactions between the liquid and the microchannel surfaces. However, a major concern for these valves is the presence of unwanted diffusion and mixing during the valve function, leading to cross-contamination between reagents or even with the sample. This thesis studied different methods in the literature to stop and actuate the flow as the two main stages of these valves. Then, passing over the state-of-the-art, a novel 3D diffusion-free capillary valve was developed: the π-valve. This valve incorporates an air gap between solutions to eliminate diffusion between them. Based on the valve's distinctive configuration, the capillarity of the microfluidic circuit displaces the air gap at a predefined time to actuate without producing bubbles into the circuit. The proposed valve's design and study were performed via numerical simulation and experimental assays, while 3D printing (3DP) was employed to fabricate the microfluidic devices.The π-valve's functionality, single and in array, was proven and compared with the conventional capillary valves. Then, it was applied to the precise control of reagents for biosensing, demonstrated by two different competitive immunoassays via (i) the subsequent washing step for the lateral flow assay of cortisol, and (ii) the valve array for the sequential delivery of sample and three reagents to detect benzodiazepine quantitatively. The sensitivity was enhanced by avoiding the reagent diffusive premixing when using the π-valve by approximately 40%. As a result, the proposed capillary valve is a promising capillary component for conducting automated immunoassays at POC.In conclusion, the novel capillary valve addresses a current biosensing sensitivity issue coming from reagent diffusive premixing, and together with other disruptive capillary-driven circuit components, paves the way for truly packing lab-on-a-chip without requiring a lab around the chip. In this regard, the next generation of capillary-driven microfluidic devices will be promising tools offering (i) miniaturized features needing fewer samples and reagents, (ii) non-costly POC testing without peripheral instrumentation, (iii) automated, preprogrammed, and easy-to-use for non-expert users, as well as (iv) sensitive, precise and reliable based on novel 3D designs employing surface properties. This is owing to the recent advancements in microfabrication based on additive manufacturing extending 3D forming freedom.

Committee:
     PRESIDENT: HUESO ALONSO, RICARDO
     SECRETARI: ROVIRA GARCIA, ADRIÀ
     VOCAL: CARDESIN MOINELO, ALEJANDRO
Thesis abstract: Solar System robotic missions explore objects in space to elicit key information that contributes to our understanding of the origins and evolution of the Solar System as well as of life. Noticeably, such an ambitious purpose demands an equally challenging endeavor from the scientific and engineering perspectives—an effort that compels to procure high scientific revenues. In this context, science planning plays a key part to guarantee that the mission obtains as large an information harvest as possible to ultimately fulfill a set of scientific objectives. This list of mission objectives can be very extensive and diverse; from the search for life to the identification of planetary resources or the geomorphological analysis, many disciplines come together in this quest. To meet these objectives, spacecraft are equipped with a varied suite of instruments capable of conducting a wide range of experiments. Thorough planning is required to conciliate the instruments' operations while concurrently guaranteeing the spacecraft performance and safety, ensuring a good management of the available resources, as well as the feasibility to forward this information to Earth. These requirements and constraints often result in trade-offs among spacecraft subsystems, including the payload. All in all, the activity plan of a mission constitutes a complex problem that needs to account for a wide spectrum of agents in order to draw a suitable mission program that effectively fulfills the scientific objectives. Within this context, observation and mission planning software tools have played an important role in space missions—and their relevance is only expected to grow in the near future. From designing the instrumental observational activities to overall management of spacecraft resources and activity scheduling, various software tools have greatly assisted in the planning process of missions like Cassini, Dawn or Rosetta. Drawing upon these, this work presents the development and validation of a software prototype designed to streamline science planning processes for planetary exploration missions. Our study focuses on devising an optimized activity plan for a camera instrument, aiming to maximize its scientific return while ensuring efficient mission operations. This software prototype tool, named Observation Planning Tool for Instrument and Mission Analysis (OPTIMA), consists of two consecutive stages. The first one utilizes spacecraft trajectory and, possibly, attitude data to identify feasible observational opportunities based on geometric constraints derived from instrument measurement requirements. These opportunities, defined as science opportunity windows, are used to constrain the exploration space in the subsequent stage. The second phase applies optimization heuristics to develop potential activity plans that maximize scientific returns while adhering to both geometric and operational constraints. The tool's modularity allows for the application of various optimization heuristics tailored to the specific problem. Additionally, OPTIMA includes a component for simulating the camera's observation through mosaic heuristics that aim to achieve maximum coverage of Regions of Interest (ROIs) with minimal make-span. Finally, the methodology's efficacy is demonstrated through hypothetical case studies set in realistic mission scenarios, including a validation case study on Galileo's flybys over Europa and two others during different phases of the JUICE mission. These cases focus on optimizing coverage and/or resolution across various ROIs, considering additional adjustments such as downlink windows allocation and data compression. The algorithm successfully identifies nearly-optimal activity plans that comply with the specified constraints in all cases, showcasing the tool's efficiency and adaptability across different mission scenarios. This makes OPTIMA a valuable asset for multi-mission science planning in planetary exploration.

Committee:
     PRESIDENT: HIDALGO DÍAZ, VICTOR HUGO
     SECRETARI: CASTILLA LOPEZ, ROBERTO
     VOCAL: ZHANG, DESHENG
Thesis abstract: The recent developments in hydraulic machinery will increase the possibility of damage resulting from cavitation and/or vortex-induced vibrations. Yet, many researchers have shed light on the vortex street flow and the associated vortex-induced vibration in cavitating-free regimes, but less attention has been paid to the presence of cavitation. In the present study, the effects of cavitation on the vortex street flow behind a blunt trailing edge hydrofoil and its interaction with the vortex-induced vibration have been investigated. This will help to understand the mechanisms involved and to predict the dynamic of vortex-induced vibration with the presence of cavitation, allowing further control of this complex phenomenon in hydraulic machinery and systems.Thus, numerical solvers have been developed that are capable of accounting for the cavitating vortex street flow behind bluff bodies which have been validated and verified using some benchmark cases. Then, the effects of the fluid compressibility in the vortex street flow behind the wedge have been determined. Moreover, a series of numerical studies on cavitating vortex street flow behind a blunt trailing edge hydrofoil NACA 0009 have been conducted by predicting the boundary layer transition. As a result, the effects of cavitation on flow dynamics and its interaction with vortex-induced vibration have been examined.The impacts of fluid compressibility on the dynamics of the cavitating wake flow have been found to depend on the frequency range. At low frequencies, the effects of fluid compressibility are minimal and can be disregarded. On the other hand, fluid compressibility has been observed to amplify spectral energy at high frequencies. Interestingly, it has been found that almost identical numerical results with and without fluid compressibility are obtained in terms of predicted mean pressure profiles, dominant vortex shedding frequencies, and instantaneous and mean void fraction fields, which suggests that the compressibility effects on cavitating vortex shedding can be neglected. For the cavitating vortex street flow behind a blunt trailing edge hydrofoil NACA 0009, high-fidelity numerical simulations have been established in the current thesis. Based on the results, it has been observed that cavitation has a significant impact on the wake flow dynamics, notably increasing the shedding frequency of the primary vortices. Additionally, cavitation growth leads to increased hydrodynamic loads on the hydrofoil surface. Furthermore, cavitation development enhances the advected velocity of the vortices while decreasing the streamwise inter-vortex spacing. Both factors are believed to contribute to the increase of the vortex shedding frequency while the reduction of streamwise inter-vortex spacing tends to dominate this increase.The numerical results related to cavitating vortex street flow behind a blunt trailing edge hydrofoil NACA 0009 subjected to forced oscillation have shown that the induced dynamic response due to the oscillation is influenced by the presence of cavitation. In the lock-in regime, cavitation appears to widen the upper bound of this regime and cause a decrease in the added moment of inertia and an increase in the added damping. Moreover, the timing of the vortex shedding changes with the presence of cavitation. On the other hand, the oscillation of the hydrofoil affects the dynamics of cavitation in terms of cavitation inception and cavity length. Generally, the cavitation inception number gradually increases with the forced oscillation frequency. Meanwhile, approaching the bounds of the lock-in regime will cause local decreases in cavitation inception numbers. Besides, the length of the cavitating wake is influenced by the variation of the oscillation frequency, and the maximum cavitating wake length occurs under stationary conditions.

Committee:
     PRESIDENT: TALBOT, JAMES PETER
     SECRETARI: PÀMIES GÓMEZ, TERESA
     VOCAL: PADRON HERNANDEZ, LUIS ALBERTO
Thesis abstract: The development of more accurate, computationally efficient and practical methodologies for a better understanding of dynamic soil-structure interaction and prediction of ground-borne vibration levels in the vicinity of man-made induced vibration sources is greatly sought-after by practising engineers, as these vibration levels might cause harmful effects on buildings and/or on their occupants. This dissertation aims to develop an approach that requires low computational resources to simulate the response of pile-soil systems accurately and is intended to be simplistic in formulation terms towards a more practical methodology. Thus, an efficient, fully coupled three-dimensional approach for solving pile-soil interaction problems is presented in the current dissertation, as well as its numerical and experimental validation. The proposed approach employs classic rod and Euler-Bernoulli theories to model the pile. The soil, in the presence of the pile's corresponding cavity, is modelled as an elastic half-space using an emerging meshless methodology that approximates the dynamic unknown states of the soil to linear combinations of the fundamental solution of the medium. Thus, this methodology, referred to as the singular boundary method, is adapted in this dissertation to deal with three-dimensional elastodynamic problems. Furthermore, the proposed piled foundation model allows the rotational motions and reaction torques associated with the pile to be accounted for in the pile-soil interaction, and their contribution to the accuracy of the scheme is assessed.To achieve an acceptable trade-off between the accuracy and numerical performance of the methodology, a comprehensive criterion to define the discretization scheme is also proposed. The robustness of the approach is studied in the context of single-pile and pile-group systems, the latter with and without attaching its corresponding pile cap, where the relevance of including torsional motions in the piles-cap coupling is discussed. Comparisons with existing approaches show that the formulation provides strong computational advantages to detailed modelling approaches, such as the ones based on the three-dimensional finite element-boundary element method, as well as overcoming the fundamental limitations of plainstrain and axisymmetric methods. Finally, a set of experimental tests on a full-scale single-pile foundation and 2x2 pile-group system embedded in layered soil were conducted to validate the modelling strategy for the simulated responses.

Committee:
     PRESIDENT: GÓMEZ GRAS, GIOVANNI
     SECRETARI: NESCOLARDE SELVA, LEXA DIGNA
     VOCAL: ABAD ROLDAN, MANUEL DAVID
Thesis abstract: In this thesis, the mechanical behavior of parts manufactured by Material Extrusion Modeling (MEX) has been evaluated, using a copolimer of polylactic acid (PLA) and polyhydroxybutyrate (PHB), commercially known as NonOilen. The objective was to analyze how the addition of PHB affects the mechanical properties of pure PLA and how the printing parameters influence these properties. To do so, compression and rotational fatigue tests were performed, using the design of experiments methodology to optimize these parameters.In addition, the performance of NonOilen was compared with pure PLA and with Timberfill, a PLA-wood composite material. The viscoelastic behavior of NonOilen was also studied, and the experimental data were adjusted to a viscoelastic model based on a three-term Prony series, in order to evaluate its influence on the apparent Young's modulus.The results showed that, although the addition of PHB reduces the stiffness and strength of the material compared to pure PLA, it improves its impact resistance and decreases fragility. Optimizing printing parameters such as extruder diameter and infill density was critical to maximizing the mechanical performance of the material. The design of experiments allowed identifying optimal printing configurations for different applications. For example, to achieve high compressive strength, a small layer height and a large extruder diameter were the best configuration, while for higher fatigue strength, higher layer heights and lower printing speeds were recommended.The study of the viscoelastic behavior of NonOilen underlined the importance of selecting the appropriate printing orientation to maximize the durability of the parts. As for the comparison with pure PLA, it demonstrated greater strength in compression and fatigue tests, with a 35% higher Young's modulus and a 40% higher yield strength than NonOilen. However, the addition of PHB made NonOilen more impact resistant and less brittle, being more appropriate for applications requiring energy absorption. In the rotary fatigue tests, the Wöhler curves of both materials showed differences due to the copolymer structure of NonOilen, whose fatigue resistance was lower than that of pure PLA. Still, its higher resistance to deformation makes it competitive in applications where toughness is a priority.When comparing NonOilen with Timberfill, it was observed that NonOilen surpassed Timberfill in stiffness and strength, presenting a 20% higher Young's modulus and a 35% higher yield strength. However, Timberfill resisted more load cycles in the rotary fatigue tests, suggesting that this material is more suitable for situations that demand greater durability under cyclic loads.In summary, although pure PLA presented better results in stiffness and strength, the combination of PHB and PLA in NonOilen offers an interesting balance, improving impact resistance and reducing brittleness. Optimizing printing parameters is essential to adjust the material properties to the specific needs of each application.Finally, the work carried out in this thesis has allowed not only to characterize the mechanical properties of NonOilen in comparison with other commercial materials, but also to highlight its viability as an ecological and competitive alternative, especially in applications where biodegradability and impact resistance are prioritized. The improvements introduced in the methods of viscoelastic analysis and in the 3D printing optimization process have contributed to a better understanding of the behavior of the material under different conditions of use.

Committee:
     PRESIDENT: CORTESÃO GODINHO, LUIS MANUEL
     SECRETARI: CLOT RAZQUIN, ARNAU
     VOCAL: DENIA GUZMÁN, FRANCISCO DAVID
Thesis abstract: Meshless methodologies have emerged as a valuable tool in the field of computational acoustics, offering an efficientapproach to model complex acoustic phenomena. These innovative numerical techniques offer a promising alternative totraditional mesh-based methods to deal with scattering and radiation acoustic wave propagation problems. Unlikeconventional mesh-based approaches, meshless methods do not rely on structured grids of the domain or its boundary,enabling more flexible and adaptive discretisation. The absence of a mesh eliminates the need for time-consuming gridgeneration and refinement, simplifying the simulation process and reducing the computational effort. This efficiency isespecially valuable in addressing large-scale acoustic simulations, such as those encountered in environmental noiseassessments and underwater acoustics.This dissertation is particularly centred on the study and development of a novel group of numerical meshless methodsrelated to boundary collocation approaches. These methods are employed to address problems involving the propagation ofacoustic waves in unbounded domains. The novel approaches presented in this research offer several benefits with respectto existing methodologies, in terms of robustness, accuracy and computational efficiency. Furthermore, in contrast to a fullythree-dimensional analysis, the approaches presented in this dissertation are formulated in the two-and-a-half-dimensionaldomain. This domain is particularly suited for scenarios where the system is subjected to longitudinally moving loads orsources and where the geometry of the system remains longitudinally invariant.The meshless methodologies developed in this thesis mainly rely on two of the most well-established meshless methods inthe field: the singular boundary method and the method of fundamental solutions. In the first instance, an approach based ona two-and-a-half-dimensional version of the singular boundary method is proposed and studied to address acousticradiation and scattering problems. Subsequently, its applicability for real case acoustic scenarios is evaluated throughsimulations involving point source diffraction in the presence of thin noise barriers. As probably representing the mostsignificant novelty of this dissertation, a hybrid method that combines the singular boundary method and the method offundamental solutions is introduced. It is specifically devised to tackle acoustic wave propagation problems featuringcomplex boundary geometries with corners and sharp edges. Finally, two modification techniques are proposed to enhancethe previously mentioned approach based on the two-and-a-half-dimensional sin- gular boundary method. The Burton–Millerformulation in a first instance, and a dual surface scheme in the second. These modifications aim to overcome the issue ofspurious eigensolutions, which arises from the non-uniqueness solution problem associated with boundary collocationmethods. To comprehensively assess the capabilities and performance of the proposed meshless methods, the availableanalytical solutions and alternative numerical strategies such as the well-known boundary element method are also utilisedin various designed benchmark problems.

Committee:
     PRESIDENT: DEL COZ DÍAZ, JUAN JOSÉ
     SECRETARI: VELAZQUEZ AMEIJIDE, JUAN
     VOCAL: CITO, SALVATORE
Thesis abstract: Steel concentrically braced frames (CBF) are considered efficient in resisting lateral forces because they provide high lateral strength and stiffness, which is particularly beneficial in seismic zones. However, this type of structure has a significant overstrength that is not considered during foundation design, which can lead to uplift of the footings under the braced members and to rocking mechanisms. On the other hand, soil-structure interaction (SSI) phenomena and foundation flexibility are generally not considered in performance-based analyses of new or existing structures.In this research, the influence of SSI phenomena on the seismic response of CBF is evaluated by considering the nonlinear response of embedded footings using the BNWF (Beam on Nonlinear Winkler Foundation) approach. To model the vertical stress-displacement behaviour of the footings, it is possible to define a backbone curve by combining the response of the compression zone in series with that of the tension zone.When uplift of the footing is allowed, it is shown that it is necessary to consider the horizontal stresses of the native soil in situ and the degree of compaction of the soil above the footing to avoid significant deviations between the analysis results and the real response. The tension zone in the response curves has been calibrated to take these parameters into account and, given the difficulty associated with their calibration, an estimate is reported which could also be used for practical applications.The implementation has been validated by various pushover analyses on a steel CBF archetype originally tested in the fixed base condition, and predictions for the flexible base condition have been made considering different soil types and footing embedment depths (Df). The results show a direct relationship between the nonlinear response of steel CBF and the uplift mechanism of embedded footings, with the embedment depth (Df) and the shape of the tension-displacement curve being the most important variables that influence the response.The model has been incorporated in various nonlinear time-history analyses considering a set of seismic records on a well-known steel CBF archetype. The results obtained suggest that, when analysing the response of steel CBF on a nonlinear flexible base condition, it is possible to report variations in the yielding sequence of the superstructure with respect to the fixed base condition. It is reported that the embedment depth (Df) of the footing, and hence its uplift stiffness, is able to modify the overall response of the superstructure. On the other hand, the cyclic response of the foundation determines a process of energy dissipation which is reflected in a reduction of the seismic force on the bracing system of the structure. The energy dissipation mechanism due to vertical response is more efficient for the supports located under the braced frames compared to those footings located under unbraced columns, and the effect is much more pronounced in soft or loose soils. The proposed model can be implemented as part of both static and dynamic analyses to have a more realistic prediction of the seismic response during performance-based designs of structures supported on embedded footings, particularly for the case of steel CBF.

Committee:
     PRESIDENT: SAN JOSÉ LOMBERA, JOSÉ TOMÁS
     SECRETARI: VALLS VIDAL, CRISTINA
     VOCAL: BARRIS PEÑA, CRISTINA
Thesis abstract: The primary goal of this thesis is to unlock the potential of hemp stalk, an agricultural byproduct that currently lacks practical applications. The overarching aim is to promote the use of eco-friendly materials, reduce waste, emphasize recyclability, and embrace the principles of a circular economy, all while combating climate change by adopting more sustainable materials in lieu of synthetic ones.To achieve these objectives, an extensive review of existing literature served as an initial step to explore the untapped potential of hemp stalk. This thorough exploration identified five possible applications: acoustic insulation materials, thermal insulation materials, particleboard, coffered ceiling, and parquet flooring.Subsequent research delved into identifying suitable plant-based binders and environmentally responsible manufacturing techniques.Following this material-focused phase, an initial investigation was carried out by experimenting with various materials and manufacturing methods to ascertain which of the five applications held the most promise.The results were particularly encouraging for insulation materials, although coffered ceiling also displayed significant potential. The insulation materials utilize recycled cardboard as a binder and employ manufacturing methods with minimal energy consumption during mixing and drying, resulting in low carbon emissions and sustainable materials.Once the material was selected, efforts were made to enhance its properties. This involved incorporating a plant-based coating (comprising arabic gum and colophony) to provide protection and introducing a crosslinking agent (citric acid) to bolster durability and material properties. These additions were meticulously assessed through a battery of tests related to thermal and acoustic insulation, mechanical characteristics, water resistance, fire resistance, and chemical analysis, such as FTIR spectroscopy.To validate the material’s performance, durability tests were conducted, exposing samples to humidity cycles over 0, 3, 6, and 9 months, alongside a scaled-down life cycle analysis. The end result was a material boasting superior thermal insulation properties compared to commercial alternatives, albeit with slightly lower acoustic insulation performance. Notably, citric acid was found to marginally enhance the material’s durability.Furthermore, it was determined that different coatings could be selected depending on the application environment, thereby optimizing specific properties.

Committee:
     PRESIDENT: HOUDE, SEBASTIEN
     SECRETARI: VALENTIN RUIZ, DAVID
     VOCAL: ALVAREZ ALVAREZ, EDUARDO
Thesis abstract: Current environmental challenges have driven the importance of renewable energies worldwide. Consequently, solar and wind generation plants, in particular, have become increasingly popular and integrated into the electrical grid. Although these plants have increased the worldwide generation capacity, their intermittent nature has generated the need for power regulation mechanisms that allow maintaining a constant energy supply to the grid. In this line, hydroelectric plants have played a crucial role not only in the generation of renewable energies but also in the regulation of the total energy supplied to the electrical grid. Thanks to their quick response and their ability to work in an extended operating range, hydraulic turbines have been used to generate and store available energy (as in the case of pump turbines) based on energy demand.To regulate the energy needed by the grid, units operate under off-design conditions and experience frequent start and stop cycles. With this, machines undergo intense pressure pulsations due to hydraulic instabilities, transient conditions and intense periodic excitations such as rotor-stator interactions. This can lead to potential structural problems and resonances in critical turbine structures, such as the runner, which can result in mechanical failures caused by structural fatigue. To avoid these problems, it is of paramount importance to have an accurate knowledge of the modal behaviour of these structures. Nevertheless, this implies to analyze very complex phenomena such as the influence of heavy and viscous fluids, boundary conditions and structural characteristics on the modal parameters of the structure, and overcoming all of the challenges that can arise during the analysis. In this thesis, the structural modal behavior of reaction hydraulic turbines is studied through the analysis of modal parameters of simple structures and real prototype runners. First, the influence of vibration amplitude and confinement on the damping of a runner in resonance is examined. This study includes an experimental and theoretical analysis of the influence of vibration amplitude on the fluid added damping in a submerged disc vibrating in resonance. The effect of a nearby rigid wall on the correlation between the vibration amplitude and the fluid damping is also analyzed. Analytical explanation and formulation for the complex energy dissipation mechanism are given.Secondly, the mode shapes of bladed runners similar to axial flow machines, including Kaplan turbines, are characterized. This study includes a numerical and experimental analysis of the mode shapes of simple disks and bladed disks. Later, the relationship between the characteristics of the global mode shapes and the number of blades of the structure is examined. Finally, the numerical results of the bladed disks are contrasted with experimental results of a real runner. Both the global and local mode shapes are discussed in this section.Lastly, a real case of a real prototype turbine is analyzed. In this case, strong vibrations and noise were detected in the machine during its commissioning. Through this study, the natural frequencies and mode shapes of the machine are analyzed experimentally and numerically. To determine the origin of the vibrations, a transmissibility analysis is also carried out. Finally, operation tests are conducted to identify the point at which the levels of vibration and noise start to increase. The effectiveness of the proposed measurement strategy, along with the importance of the measurement techniques used, such as the transmissibility test, is demonstrated through a real case. The cause of the problem is found and a solution is proposed and implemented.This thesis is based on three papers published during its development period, in scientific journals indexed in first and second quartiles of the JCR. The details of these articles are explained throughout the thesis and are attached at the end of it.

Committee:
     PRESIDENT: KAUSHIK, AJEET
     SECRETARI: CUSOLA AUMEDES, ORIOL
     VOCAL: WESTERBERG, LARS-GÖRAN
Thesis abstract: This thesis arose from the need to design new tools that enable a better control of indoor air quality in vehicles. Even though indoor air quality has been widely explored in buildings, this topic has often been overlooked by most automotive manufacturers. In recent years, especially after the COVID-19 pandemic, a better control of in-vehicle air quality has become a prominent need for most customers and operators of vehicle fleets. Traditionally, on-demand ventilation systems have used carbon dioxide (CO2) concentration to regulate indoor air quality. However, within the confined space of vehicles, the concentration of volatile organic compounds (VOCs) can also pose a real risk to the health and comfort of passengers. Therefore, in order to ensure optimal air quality conditions, it seems necessary to complement CO2 detectors with sensors that can effectively monitor VOCs. In the new era of e-mobility and connectivity, vehicles incorporate dozens of sensors in their architecture. Thus, reducing the fabrication and operational costs of new monitoring devices has become a critical requirement to justify their implementation. Recent advancements in micro-fabrication techniques and new materials have enabled the creation of VOCs sensors that are increasingly sensitive, compact, and cost-effective. However, the performance of some of these devices is still conditioned to high energy requirements during operation. In addition, the effectiveness of current gas sensors needs further improvement, so that they not only have high sensitivity, but also a certain degree of selectivity towards multiple VOCs in the environment.This thesis focuses on the design, fabrication, and implementation of a new device for the monitoring of VOCs in vehicles. The results demonstrate the suitability of integrating a gas sensor and a microfluidic channel, which exploit the unique properties of polymers for the sensitive and selective detection of VOCs at ambient temperature. First, this work evaluates the response of a gas sensor coated with single and hybrid polymer films. The addition of carbon nanoparticles within the polymer not only contributes to an increase in the sensitivity and response times of the sensor, but also helps to improve the selectivity of the polymer films. Secondly, this work evaluates the responses of a microfluidic channel for the discretization of VOCs in a mixture. The empirical results obtained in this thesis indicate that polymer thickness is critical in both the sensitivity and selectivity of the different fabricated devices. In general, thicker polymer films help to increase or optimize the performance of both, the gas sensor and the microfluidic channel. Moreover, the chemical compatibility between VOCs and the polymer also proves to be very important in the performance of both devices. For this reason, this work proposes a new methodology, based on the Hansen solubility parameters, which can help assess and predict the response of polymers for VOCs monitoring. Finally, this thesis describes the implementation of an air quality monitoring system that effectively measures CO2 and VOCs levels inside the vehicle cabin. A series of pilot tests validate that VOCs monitoring is relevant to the customer experience and the proper control of indoor air quality in vehicles. Furthermore, these activities contribute to defining how this system should be integrated with the overall vehicle and environment. To the authors knowledge, it is the first time that the devices presented in this thesis have been investigated for their implementation in vehicles, which showcase the originality and potential of this work.

Committee:
     PRESIDENT: GUSTAFSSON, ROLF
     SECRETARI: GAMEZ MONTERO, PEDRO JAVIER
     VOCAL: HUANG, XINGXING
Thesis abstract: The introduction of the intermittent and unpredictable solar and wind power into the electrical grid is pushing hydraulic turbines to work for longer periods of time in off-design conditions. Under these circumstances, it is necessary to investigate the prediction of structural damage induced on hydraulic turbines by flow instabilities. For this reason, the present work aims firstly to characterize the rotating vortex rope and vaneless space vortex structures, then to understand the fluid-added modal coefficients induced by the water surrounding runners and finally to quantify the fatigue damage inflicted on runner blades. In this sense, experimental onboard and off-board measurements of strain, force, torque, pressure, displacement and acceleration on different components of a Kaplan turbine model were performed when operated at part load, speed-no-load and during ramping of load. The data was analyzed in the time and frequency domains as well as using a novel experimental methodology to infer fatigue damage on runner blades from blade strains. Additionally, two new methods, which only use shaft strains and off-board measurements, were developed to compute fatigue damage. On the other hand, new numerical and experimental methodologies, combined with proposed analytical models, were employed to investigate the fluid-added modal coefficients of confined rotors partially submerged in water. In summary, the present work has permitted to conclude that speed-no-load, and specially with cavitation, is the most damaging operating condition, that the fatigue damage on runner blades can be predicted using shaft strains and that rotor dynamic responses depend on the water surrounding the runner, the rotating speed and the ramp acceleration and direction.

Committee:
     PRESIDENT: GRILLO DOLSET, MARIA JESUS
     SECRETARI: ESCALER PUIGORIOL, FRANCESC XAVIER
     VOCAL: VERNET PEÑA, ANTON
Thesis abstract: The current methods used in microbiological quality analysis rely on the use of traditional and manual methods, such as the pour plate method. The need to perform more analyses of different matrices makes it necessary to improve these methods. Thus, this thesis focuses on the design of systems in which the manual work of laboratory technicians is reduced. In order to meet the main objective, this thesis has been separated into two subjects, i.e., microbiology and mechanical and fluid engineering. The union of these two fields is essential for the achievement of this need.The starting point for microbiology focused on the separation of culture media into their different phases. This separation would allow both easier maintenance of the media and a simpler dosage of the same. In addition, studies and the establishment of a protocol for sterilizing the culture medium in a microwave oven were also carried out.By means of Computational Fluid dynamics (CFD), studies were carried out to design a static homogenizer so that the culture medium would be completely mixed when pouring onto the plates. In addition, a device was designed with which the dosing of the culture medium, once separated into its components, was possible. The favorable results obtained in both microbiological and engineering studies demonstrate that a pour plate method automation system is possible. Hence, the development of this new line of research will continue, as this study is only the starting point for the development of such a system.