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Laser-guided lightning
No full textInternational audienceAbstract Lightning discharges between charged clouds and the Earth’s surface are responsible for considerable damages and casualties. It is therefore important to develop better protection methods in addition to the traditional Franklin rod. Here we present the first demonstration that laser-induced filaments—formed in the sky by short and intense laser pulses—can guide lightning discharges over considerable distances. We believe that this experimental breakthrough will lead to progress in lightning protection and lightning physics. An experimental campaign was conducted on the Säntis mountain in north-eastern Switzerland during the summer of 2021 with a high-repetition-rate terawatt laser. The guiding of an upward negative lightning leader over a distance of 50 m was recorded by two separate high-speed cameras. The guiding of negative lightning leaders by laser filaments was corroborated in three other instances by very-high-frequency interferometric measurements, and the number of X-ray bursts detected during guided lightning events greatly increased. Although this research field has been very active for more than 20 years, this is the first field-result that experimentally demonstrates lightning guided by lasers. This work paves the way for new atmospheric applications of ultrashort lasers and represents an important step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures
Impact of the load level on the creep behaviour of adhesively bonded fastener in tension: experimental investigations, analytical and numerical modelling
No full textInternational audienceThe use of structural adhesive bonding is increasing in structural assembly. It has indeed several advantages compared to other assembly techniques such as welding and riveting: uniform stress distribution, assembly of materials with different natures, no heat production. This last aspect is particularly interesting for the offshore activities. The firm Cold Pad has developed adhesively bonded fastener solutions, whose aim is to replace welding by structural bonding for the reinforcement of steel structures. Yet, some issues remain about the appraisal of the durability for such assembly and particularly creep. To address this issue, experimental creep investigations were carried out at real scale for one geometry of a fastener bonded with a methacrylate adhesive, and at different tension load levels. An inverse identification method has been developed in a previous study to identify the material creep properties from these investigations, independently of the fastener geometry. Considering the industrial applications of this fastener, the aim of this modelling approach is to describe correctly the primary and secondary creep stages. In this article, this identification method was applied to all the experimental results, using the Burger creep model. The dependency of the parameters with the load level is highlighted, which led to a nonlinear model. These results were then compared to finite element investigations to verify the consistency of the method and its ability to correctly identify the adhesive creep behaviour. This offers good perspectives in the transposition of the study to alternative fastener geometries using simple analytical design approaches.Highlights Bonded fastener’s creep behaviour experimentally investigated under different load levels. Creep modelling using a nonlinear and analytical approach with Burger model Identification of the material creep properties from experimental investigations at real scale on the bonded assembly. Numerical model developed to assess the consistency of the analytical approach
Evaluating Kinect, OpenPose and BlazePose for Human Body Movement Analysis on a Low Back Pain Physical Rehabilitation Dataset
No full textInternational audienceAnalyzing human motion is an active research area, with various applications. In this work, we focus on human motion analysis in the context of physical rehabilitation using a robot coach system. Computer-aided assessment of physical rehabilitation entails evaluation of patient performance in completing prescribed rehabilitation exercises, based on processing movement data captured with a sensory system, such as RGB and RGB-D cameras. As 2D and 3D human pose estimation from RGB images had made impressive improvements, we aim to compare the assessment of physical rehabilitation exercises using movement data obtained from both RGB-D camera (Microsoft Kinect) and estimation from RGB videos (OpenPose and BlazePose algorithms). A Gaussian Mixture Model (GMM) is employed from position (and orientation) features, with performance metrics defined based on the log-likelihood values from GMM. The evaluation is performed on a medical database of clinical patients carrying out low back-pain rehabilitation exercises, previously coached by robot Poppy
Une voie polyvalente vers les acylsilanes et les acyl(MIDA)boronates
No full textThis research work focuses on the synthesis and applications of acylsilanes and acyl(MIDA)boronates by utilizing the radical chemistry of dithiocarbonates (xanthates) in which xanthates act as the radical source. The degenerative reversible transfer of xanthate is emerging as one of the most effective tools for the generation of C-C bonds in recent years, allowing the formation of numerous complex molecules. The purpose of this work is to develop new-fashioned radicals derived from xanthates and related to acylsilanes and acyl(MIDA)boronates. Moreover, the aim is also to deeply understand their reactivity, and to employ them in various useful transformations.This thesis consists of three chapters and contains two main projects. In Chapter I, the basic information on radicals and radical reactions is summarized and the generation of radicals from xanthates, and particularly the xanthate addition-transfer process, is mainly introduced.In Chapter II, a modular, straightforward, and flexible approach to functional acylsilanes is described. The use of acylsilanes in organic synthesis has increased significantly over the last few years due to their unique properties and the discovery of valuable new reactions. In this work, a wide variety of functional acylsilanes were efficiently synthesized, implemented by the electrophilic feature of the intermediate acylsilane radical derived from the xanthate and the nucleophilic nature of the olefin partner. In addition, the formed acylsilanes were engaged in the formation of 2-pyrrolyl silanes, even the multi-substituted pyrrolyl silanes that are difficult to access by other means.Acylboranes and acylboronates are an intriguing class of compounds, and their remarkable properties and reactivities for synthesis have only recently been recognized. In Chapter III, a novel and reactive xanthate as an acylboron transfer reagent for synthesizing various acyl(MIDA)boronates is presented. Our synthesis relies on the addition of radical derived from the corresponding xanthate and functional alkenes, leading to the formation of valuable acyl(MIDA)boronates. The mildness of the reaction conditions and broad tolerance for a variety of functional groups allow the construction of more complex scaffolds, including but not limited to 2-pyrrolyl (MIDA)boronates and furyl (MIDA)boronates.Ce travail de recherche se concentre sur la synthèse et les applications des acylsilanes et des acyl(MIDA)boronates en utilisant la chimie radicalaire des dithiocarbonates (xanthates), dans laquelle les xanthates agissent en tant que source de radicaux. Le transfert dégénératif réversible du xanthate apparaît comme l'un des outils les plus efficaces pour la génération de liaisons C-C au cours des dernières années, permettant la formation de nombreuses molécules complexes. L'objectif de ce travail est de développer des nouveaux radicaux dérivés des xanthates et liés aux acylsilanes et aux acyl(MIDA)boronates. En outre, l'objectif est également de comprendre en profondeur leur réactivité et de les utiliser dans diverses transformations utiles.Cette thèse se compose de trois chapitres et contient deux projets principaux. Dans le chapitre I, les informations de base sur les radicaux et les réactions radicalaires sont résumées et la génération de radicaux à partir de xanthates, et en particulier le processus d'addition-transfert de xanthate, est principalement introduite.Le chapitre II décrit une approche modulaire, directe et flexible des acylsilanes fonctionnels. L'utilisation des acylsilanes en synthèse organique a augmenté de manière significative au cours des dernières années en raison de leurs propriétés uniques et de la découverte de nouvelles réactions intéressantes. Dans ce travail, une grande variété d'acylsilanes fonctionnels a été synthétisée efficacement, grâce à la caractéristique électrophile du radical acylsilane intermédiaire dérivé du xanthate et à la nature nucléophile de l'oléfine partenaire. En outre, les acylsilanes formés ont été engagés dans la formation de 2-pyrrolylsilanes, ainsi que des pyrrolylsilanes multi-substitués qui sont difficiles d'accès par d'autres moyens.Les acylboranes et les acylboronates constituent une classe intrigante de composés, dont les propriétés remarquables et la réactivité pour la synthèse n'ont été reconnues que récemment. Le chapitre III présente un xanthate nouveau et réactif comme réactif de transfert d'acylboronates pour synthétiser divers acyl(MIDA)boronates. Notre synthèse repose sur l'addition de radicaux dérivés du xanthate correspondant et d'alcènes fonctionnels, ce qui conduit à la formation de précieux acyl(MIDA)boronates. La douceur des conditions de réaction et la grande tolérance pour une variété de groupes fonctionnels permettent la construction d'échafaudages plus complexes, y compris, mais sans s'y limiter, les (MIDA)boronates de 2-pyrrolyle et les (MIDA)boronates de furyle
Evaluation of a two-step clustering method for radar pulse deinterleaving
No full textInternational audienceIn electronic warfare, deinterleaving is the task which sorts radar pulses in order to separate radar waveforms, without prior knowledge. This paper intends to evaluate two versions of a two-step deinterleaving method. It starts with a RF-PW clustering, then tries to merge some clusters by defining a distance related to their set of pulse times of arrival. After a parameter optimization, the method is evaluated with five metrics on a dataset consisting of simulated frequency-agile and PRI-modulated signals, and on a real data observation
Student Reflective Practice As Part of Engineering Programmes
No full textInternational audienceReflective learning can be defined as “practice which involves the development of learning and understanding through self-review to help determine progress against goals and future learning needs”. In a CDIO context, the use of reflective learning has found its way into the most recent iteration of the syllabus while it can also be argued that self-review is, in particular, a part of Standard 8 - Active Learning. This work looks at a survey (n=38) carried out among academic staff involved in CDIO and in the wider engineering education community to establish the extent to which reflective learning is embedded in engineering degrees and how, at the highest level it is taught, implemented and assessed. The survey also looks at motivations, barriers and best practice in the field. Among the findings, respondents to the survey were enthusiastic about the topic as might be expected in a voluntary survey, however there was more skepticism as to whether students would see the value of the approach and so may not engage. Reflective journals and/or end of module reports and reviews were common tools used to embody reflective practice into activity, though these might be part of a more general activity or assessment and not be entirely focused around reflective practice. Key barriers to adoption of reflective practice included the pressure on an already overcrowded syllabus and students struggling to engage in the process, staff reporting that structure and frameworks need to be used to develop true reflections as opposed to simple records of events. The work concludes by highlighting some routes forward for the approach both in terms of implementation and possible development of the methodology
Méthode de décomposition de domaine pour les problèmes couplés acoustique-élastique, dans le domaine temporel. Application aux explosions sous-marines.
No full textThis work addresses global-in-time domain decomposition approaches for the numerical solution of transient fluid-structure interaction problems. To determine an optimal algorithm, we first study the solvability for the transient acoustic and elastodynamic problems with Robin and Neumann boundary conditions. We state solvability results along with the different space-time regularities of the solutions. We also study the solvability for the transient coupled elastodynamic-acoustic problem. Using on these mathematical results we then propose a global-in-time iterative algorithm based on Robin boundary conditions for the coupled elastodynamicacoustic problem and we prove its convergence.These results are leveraged to design a computational methodology by coupling two efficient numerical methods. The fluid response is formulated in the discrete-time domain, using a Z-BEM approach that combines (i) a boundary element method (BEM) accelerated with hierarchical matrix implemented in the Laplace domain and (ii) a convolution quadrature method. The structure response is modelled using the finite elements method. We thus propose a global-in-time iterative coupling with guaranteed convergence, which enables the use of two distinct numerical methods in a non-intrusive manner.Several improvements are then explored: an acceleration method is implemented and a high-frequency approximation is proposed to improved the Z-BEM efficiency. A second iterative global-in-time coupling based on an acoustic-acoustic interface is then proposed and its convergence is also proved. This coupling enables the addition of non linear effects due to the cavitation phenomenon to derive a more realistic fluid model. The Z-BEM is lastly adapted using the method of images to take a free surface into account.This method is applied on fast-time problems of acoustic shock wave scattering by submerged elastic structures and enables to simulate realistic configurations from naval industry.Ce travail étudie les approches globales en temps de décomposition de domaine pour résoudre des problèmes transitoires d'interaction fluide-structure. Afin de déterminer un algorithme optimal, nous étudions dans un premier temps la solvabilité des problèmes élastodynamiques et acoustiques transitoires avec des conditions aux frontières de type Robin et de Neumann. Nous énonçons des résultats de solvabilité, en soulignant les différentes régularités espace-temps des solutions. Nous étudions également la solvabilité du problème couplé élastodynamique-acoustique transitoire. Puis en nous basant sur ces résultats mathématiques, nous proposons ensuite un algorithme itératif global en temps basé sur les conditions aux limites de type Robin pour le problème couplé et prouvons sa convergence.Ces résultats sont ensuite mis en oeuvre pour coupler deux méthodes numériques efficaces. La réponse du fluide en temps discret est obtenue à l'aide d'une approche Z-BEM qui combine (i) une méthode d'éléments de frontière (BEM) accélérée par la méthode des matrices hiérarchiques dans le domaine de Laplace et (ii) une quadrature de convolution. La réponse de la structure est modélisée à l'aide de la méthode des éléments finis. Nous développons de cette manière une méthode numérique de couplage itérative globale en temps à convergence garantie, permettant en outre d'utiliser deux méthodes numériques distinctes de manière non intrusive.Plusieurs améliorations sont ensuite proposées: une méthode d'accélération de convergence est mise en œuvre et une approximation à haute fréquence est proposée pour améliorer l'efficacité de la Z-BEM. On propose ensuite un deuxième couplage itératif global-en-temps basé sur une interface acoustique-acoustique, dont la convergence est également démontrée. Ce couplage permet ensuite d'introduire des effets non linéaires dus au phénomène de cavitation pour préciser le modèle fluide. La Z-BEM est enfin adaptée en utilisant la méthode des images pour permettre la prise en compte d'une surface libre.Cette méthode est appliquées à des problèmes à dynamique rapide de dispersion d'ondes de choc acoustiques par des structures élastiques immergées et permet de simuler des configurations réalistes rencontrées dans l'industrie navale
Analysis of Heterogeneous Combustion Processes in Application to AP/HTPB Composite Propellants with Detailed Kinetics
No full textInternational audienceHeterogeneous combustion of an AP/HTPB composite propellant is modeled employing detailed kinetics. A revised kinetic model is assembled to represent the gas-phase combustion processes. The combustion of an isolated AP particle surrounded by a layer of homogenized binder is simulated, highlighting the effect of pressure and AP particle diameter on the flame. The main reaction zones are characterized leveraging clustering methods
Fatigue de composites thermoplastiques à fibres courtes pour applications aéronautiques sous sollicitations de compression : loi de comportement et approche de dimensionnement
No full textThe interest in thermoplastics reinforced with short carbon fibers is growing in the aeronautical field. The prediction of the fatigue behavior of structures made of these materials raises many issues, due to the complex heterogeneous orientation distribution of the fibers, inducing a strong anisotropy. Many works have been carried out on this type of material but few concern compressive loadings. In order to predict the high cycle life of PEEK CF30(PolyEtherEtherKetone reinforced with 30% in weight of short carbon fibers ) for compressive loading, a fatigue design approach is proposed. A protocol to perform well-controlled compression tests is developped. Fatigue tests and self-heating tests are performed at room temperature, for different load ratios (in compression in particular) and loading directions. Fatigue criteria allowing to predict the fatigue life in a unified way for different loading directions and load ratios are identified. The possibility of accelerating fatigue characterization using the self-heating method is evaluated. The mechanical behavior in compression of the material is investigated to identify the parameters of an anisotropic visco-elasto-plastic behavior law, aimed at predicting the quantities involved in the selected fatigue criteria and refining the interpretation of self-heating curves and the link with fatigue.L’intérêt envers les thermoplastiques renforcés par des fibres courtes de carbone est grandissant dans le domaine de l’aéronautique. La prévision du comportement en fatigue de structures constituées de ces matériaux soulève de nombreuses problématiques, du fait notamment de la distribution d'orientation hétérogène complexe des fibres, induisant une forte anisotropie. De nombreux travaux ont été menés sur ce type de matériau mais peu concernent des sollicitations en compression. Dans l’objectif de prévoir la durée de vie à grand nombre de cycles du PEEK CF30 (PolyEtherEtherKetone renforcé de 30% en masse de fibres courtes en carbone) pour des chargements en compression, une démarche de dimensionnement en fatigue est proposée. Un protocole permettant de réaliser des essais de compression cyclique bien contrôlés est développé. Des essais de fatigue et des essais d’autoéchauffement sont réalisés à température ambiante, pour différents rapports de charges (en compression notamment) et pour différentes directions de sollicitation. Des critères de fatigue permettant de prévoir la durée de vie de manière unifiée, pour différentes directions de sollicitation et différents rapports de charge, sont identifiés. La possibilité d’accélérer la caractérisation de la fatigue à l’aide de la méthode d’autoéchauffement est évaluée. Le comportement mécanique en compression du matériau est investigué pour identifier les paramètres d’une loi de comportement visco-élasto-plastique anisotrope, visant à prévoir les grandeurs intervenant dans les critères de fatigue retenus et à affiner l’interprétation des courbes d’auto-échauffement et du lien avec la fatigue