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Proton therapy is an advanced cancer treatment modality that demands precise dosimetry for effective and safe patient outcomes. This thesis investigates the ionization chamber (IC) within the IBA proton therapy machine, which is crucial for measuring the proton dosage administered to patients. However, vibration disturbances can cause significant measurement variations and saturation problems in the IC, potentially compromising treatment accuracy. The study is structured into three parts. The first part involves the numerical modeling the IC, focusing on the geometry and tension of the electrodes, which are configured as sheets. This model aims to understand the baseline behavior of the IC electrodes under pre-tension. The second part investigates the actual dynamic behavior of the IC through Experimental Modal Analysis (EMA). The third part correlates the numerical and experimental results by optimizing the numerical model using a surrogate-assisted genetic algorithm. Results indicate that the tension applied to the sheets significantly influences the modal analysis outcomes. Optimization of the numerical model determined optimal tensions of 77.2 N vertically and 40.1 N horizontally, achieving a global variation of 3.7% between the numerical and experimental natural frequencies. The dynamic analysis also reveals a significant vibratory influence of the IC casing on the measurement sheets, leading to perturbations in electrometer readings. This research contributes to medical physics by providing insights into the behavior of proton therapy dosimetry devices, with the aim of improving the performance and accuracy of proton therapy in clinical settings.
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In the new high speed booster currently developed at Safran Aero Booster, the use of variable stator vanes seems necessary. This structure is made of several pieces which makes the prediction of vibration frequencies difficult. Indeed, the methodology currently in use at Safran Aero Boosters leads to frequency differences up to 50 % between experimental and numerical results. This thesis aims at studying the different methodologies and models available at Safran Aero Booster in order to find a method enabling a better estimation of the resonance frequencies than the one currently in use. This study covers different aspects of the correlation procedure from the post-processing of the signal measured during the tests to the introduction of nonlinearities in the model. The methodology used at Safran Aero Booster is first reviewed to establish its limitations and identify points that could be improved. Then, several sets of post-processing parameters are used to determine the modes relevant for the rest of the study as well as the frequency precision that will be expected from the models. Afterwards the resonance frequencies obtained in Ansys Mechanical with a vane alone and a complete model are compared. These models are studied in various configurations therefore enabling to determine the importance of the different boundary conditions considered. Since the complete model is more representative of an actual variable stator vane assembly, it is also determined if adding complexity in the model enable to obtain an improved prediction of the frequencies. Based on the analysis of the boundary conditions and of the pieces that compose the assembly, a model taking into account some nonlinearities is built in Ansys Motion in order to represent the influence of contacts on the resonance frequencies. This work finally provides a suggested methodology for vibration prediction as well as areas for improvement in the modeling of variable stator vanes.
Vibration analysis --- Booster --- Stator --- Nonlinear simulation --- Ingénierie, informatique & technologie > Ingénierie aérospatiale
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Nonlinearities are an important aspect to consider in the study of the vibrations of a structure, as they can influence the dynamics of the analysed system, resulting in unexpected behaviours. Nonlinear system identification aims at finding a mathematical model of the structure taking into account the nonlinear forces. One method used in the nonlinear system identification is the acceleration surface method, which helps at determining the functional form of these nonlinear forces by creating stiffness and damping curves. While the method has proven to work well in real-life applications, some artefacts sometimes appear in the stiffness and damping curves, making it more difficult to correctly interpret these curves. The aim of this work is thus to understand their origin and determine how the curves can be improved. For this purpose, the acceleration surface method is applied to systems composed of 1, 2 and 10 degrees of freedom, whose response to sine sweep excitations is obtained through numerical integration. The nonlinearities consist of a cubic stiffness, a piecewise linear stiffness and Coulomb friction, which are all common types of nonlinearities encountered in real-life structures. The analysis of the stiffness and damping curves obtained for the different systems reveals that the presence of harmonic components in the response of the system to the excitation can impact the curves, such that the curves can be composed of different lines, which can complicate the determination of the functional form of the nonlinear forces. Such phenomena particularly occur at superharmonic resonances and modal interactions. An effective way to improve the curves in such cases is the use of filters to remove the harmonic components from the response of the system to the excitation. Furthermore, in multiple degree of freedom systems, the linear forces that have been ignored in the equation of the acceleration surface method are responsible for the quality of the stiffness and damping curves. Applying the method to modes for which these forces are small thus yields better results. In particular, modes for which the neighbouring degrees of freedom of the linear connections oscillate in phase with the degree of freedom to which the acceleration surface method is applied have shown to produce better results. The position of the excitation and the extremity of the nonlinear connection to consider in the acceleration surface method also influence the quality of the stiffness and damping curves. While the identification of the stiffness force can be done successfully in most cases, the functional form of the damping force remains difficult to determine with the acceleration surface method and it can only be found in some specific cases. The artefacts in the stiffness and damping curves can be explained by several phenomena, but are always due to the terms neglected in the equation of the acceleration surface method.
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In this study, Enoval, a high-speed, low-pressure compressor developed by Safran Aero Boosters, is introduced. Despite its initial commendable performance in efficiency and fuel-to-rate ratio, Enoval encounters a concerning vibratory instability under conditions nearing stall. Through meticulous spectral analysis, this instability is identified as a NSV Tip Vortex phenomenon, triggered by the interaction of circumferentially propagating vortices with blade tip flow oscillations, consequently inducing substantial vibration levels. Understanding the potential detrimental effects of NSV Tip Vortex on the blades, the study underscores the importance of pre-emptive prediction during the design phase rather than reliance on post-production testing. As a result, a predictive model is meticulously developed based on Brandstetter and Stapelfeldt's model, capable of anticipating NSV Tip Vortex occurrences based on intricate geometric and aerodynamic inputs. Application of this predictive model to Enoval demonstrates promising efficacy in accurately forecasting NSV Tip Vortex occurrences. Moreover, a comprehensive parametric investigation delves into the nuanced influences of aerodynamic and geometric factors on NSV Tip Vortex, broadening the researches horizon. Insights gleaned from this investigation pave the path for devising preventive strategies geared towards shielding the rotor from NSV Tip Vortex-related risks. These strategies encompass a spectrum of interventions, including deliberate adjustments to aerodynamic parameters by innovative geometric treatments on the casing or the adoption of structural detuning techniques, which involve altering blade profiles to strategically modify natural frequencies along the rotor. While applied on Enoval, both preventive strategies appeared to effectively prevent NSV Tip Vortex. By shedding light on the underlying mechanisms of NSV Tip Vortex and proposing proactive measures, this study contributes to fortifying the operational resilience and longevity of advanced compressors like Enoval, or any other booster.
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Ce rapport présente une méthodologie stochastique pour la prédiction des jeux radiaux dans les compresseurs basse pression, visant à améliorer la fiabilité et les performances des moteurs aéronautiques. En utilisant une approche statistique basée sur la technique de Monte-Carlo, le modèle final permet de mieux estimer l'incertitude des jeux radiaux. L'analyse des facteurs influençant ces jeux, tels que les tolérances, l'ovalisation et les adders, aide à comprendre les variations possibles et propose des pistes pour l'optimisation des processus de conception et de maintenance des moteurs. Cette étude fournit une base pour la gestion des phénomènes aléatoires liés aux jeux radiaux et ouvre des perspectives pour des améliorations futures.
Jeu --- Prédiction --- Monte-Carlo --- Safran --- booster --- compresseur basse pression --- stochastique --- Ingénierie, informatique & technologie > Ingénierie mécanique
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The mechanical strengths of scroll compressors are often analyzed by finite element analyses. They requires lots of simulations to cover the main working points of a compressor and a finite element analysis takes time. This work allows to reduce the time spent to perform finite element analyses on scroll compressor. It automates the creation of the numerical model of a scroll and the post-processing of the results. To reduce the number of simulations, a model that interpolates the maximum stress in a scroll as a function of the working points of the compressor is created.
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Pretension, the intentional application of preload to bolted joints before they are subjected to external loads, is vital for the functionality and reliability of bolts in aeronautical assemblies. This preload is achieved by tightening the bolt to a specified torque or stretching it to a predetermined length, ensuring the joint remains secure and stable under operational loads. Pretension creates a compressive load within the joint, eliminating gaps and enhancing load transfer, reducing the likelihood of fretting or fatigue failure. It also prevents bolt self-loosening during flight-induced vibrations and dynamic loads, maintaining bolt clamp load and resisting relaxation. In the aerospace industry, the significance of pretension on bolts is often overlooked, leading to designs without preload, which is considered conservative. This thesis investigates the influence of prestressing in bolted assemblies under shear conditions. It compares cases with and without preload, examining loosely assembled configurations and those with only enough torque to bring parts into contact. The study evaluates the physical behavior of a slat track assembly under different preload levels, focusing on the axial load in the bolt, as well as the shear load and bending moment. The research involves evaluating an analytical method developed by Sonaca, which analyzes the behavior of bolted joints under shear forces without considering prestressing, providing a foundational understanding of bolted joint principles. To incorporate prestressing effects, a finite element model (FEM) is developed and compared with the analytical method to validate or challenge initial hypotheses. Additionally, the study explores the influence of various materials and geometric configurations on bolted joint performance through a parametric study, aiming to identify key factors that affect behavior and performance. Finally, the results are discussed in detail, highlighting their implications, limitations, and future research directions, offering valuable insights into the significance of prestressing in bolted assemblies. Overall, pretension optimizes the performance and reliability of bolts within aeronautical assemblies, ensuring secure fastening, resistance to self-loosening, and enhanced load-bearing capacity, crucial for maintaining aircraft safety and the longevity of critical components.
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This project aims to test the combination of existing equipment to obtain efficient hybrid-electric propulsion for a microlight aircraft which feats the ULM description and has a MTOW of 530 kg. Some of the additional objectives would be to dimension a battery, get data for the possible emissions of the system, study efficiencies of various components and an estimation of the costs.
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Ce travail de fin d'étude fut réalisé au sein de l'entreprise Safran Aero Boosters. L'objectif de ce stage était de développer une méthode permettant de mesurer les contraintes dans les filets des écrous de palier employés par Safran Aero Booster. De fait, ces écrous diffèrent de la visserie standardisée et nécessitent donc une méthodologie customisée. Ce document permis de mettre en évidence un comportement de l'écrou s'éloignant la littérature, car il subit une flexion modifiant la distribution des contraintes dans le filet.
Ecrou --- Filet --- Contraintes --- Méthodologie --- Ingénierie, informatique & technologie > Ingénierie mécanique
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Friction is the main mechanical loss taking place inside scroll compressors. It can lead to wear and reduces the reliability and service life of the compressors. The friction phenomenon needs to be understood and modelled so that the friction can ultimately be reduced and correctly taken into account during the design of the scroll compressors. This thesis researches and develops a friction model for the tip-base interface contact of the scroll, in order to obtain a friction coefficient depending on the operating conditions of the compressor. This model is implemented in Python. It corresponds to a mixed lubrication model able to model friction for all operating conditions and lubrication regimes. The model is discrete, and the tip-base interface is thus discretized using a regular mesh composed of quadrangle elements. The model is fed with different inputs. MBD simulations are used to generate the tip-base gap geometry. The boundary and axial load are derived from analytical computations, while the fluid and material parameters are sourced from the literature. It is composed of a fluid and a dry model that are combined to give the most general model possible. The fluid model is based on the Reynolds equation that is averaged in order to take into account the influence of the rough surface on the lubricant flow when the lubricant film is very small. The Reynolds equation is solved by an iterative process using finite difference method. The dry model is based on the stochastic model of Greenwood-Tripp that makes assumptions on the statistical distribution of the surface roughness. The two models are then assembled, and the tip-base gap is adjusted through the offset film thickness determined using a root-finding algorithm on the load balance condition. The fluid model is verified using known academic results, and the numerical solution is optimized through a convergence study. The complete implemented model is then used to understand the frictional behaviour of the tip-base contact, first, for operating conditions -5 to 60°C at 8000 RPM. The average friction coefficient has been found to be 0.023 with a fluid friction significantly lower than the dry one. The influence of the rotational speed is then explored by studying the results at 2900 and 6000 RPM. It is shown that the friction coefficient decreases when the speed increases, as the speed increase tends to separate the surfaces. The influence of operating conditions is finally studied by changing the inlet and outlet temperature to 8 to 45°C and keeping the speed at 2900 RPM. It is shown that the friction coefficient decreases when the temperature ratio decreases as the inlet boundary pressure is higher for these conditions. Overall, it is shown that the average friction coefficient for a complete revolution of the scroll varies between 0.02 and 0.03, suggesting that the 0.04 value used in Copeland computations is slightly overestimated. Stribeck curve for this contact is finally derived. This curve shows that at low speeds, the coefficient decreases when speed increases, as the surfaces are separated. At high speeds, the coefficient increases with the rotational speed, as the viscous shear stresses are increased.
Compressor --- Friction --- Tribology --- Lubrication --- Scroll Compressors --- Ingénierie, informatique & technologie > Ingénierie mécanique
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