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Différentiation algorithmique d'un graph d'opérateurs et ses applications
No full textInternational audienc
Abel inversion from central-projection background oriented schlieren observations for reconstruction of axisymmetric refractive media
No full textInternational audienceThe background oriented schlieren (BOS) technique provides quantitative measurements of integrated light deflection caused by gradients of refractive index in an optical medium. For an axisymmetric medium and parallel rays orthogonal to the axis of symmetry, Abel inversion enables fast, direct, and non-iterative reconstruction of the refractive index distribution. In this work, we relax the parallel ray hypothesis, demonstrating that Abel inversion can be effectively applied to observations obtained by central projection with a standard entocentric lens, with a simple correction for measured deflections. This correction derives from an original second-order approximation of ray deflection under the paraxial approximation. The accuracy of the method is demonstrated using a synthetic low-density jet and a cone of vision aperture of approximately 40°. Its practical relevance is further illustrated by reconstructing the temporal mean density fields of two experimental cases: a heated high-subsonic dual-stream jet and an under-expanded supersonic jet characterized by significant shock-induced discontinuities
SACRED EYE: Secure Communication and Decentralized Monitoring for Swarm UAV Mission Completion
No full textInternational audienceThe flexibility, scalability, and robustness of Unmanned Aerial Vehicle (UAV) swarms make them highly adaptable to dynamic environments. However, these advantages also introduce a range of optimization challenges, including constraints on size, weight, and energy consumption, as well as less explored topics such as lightweight, secure communication and reliable mission completion. Inspired by the ant identification methods, we propose a decentralized solution to enhance communication security and ensure mission completion within UAV swarms. Unlike existing methods that rely on Physical Unclonable Functions (PUFs), our solution introduces a PUF-less framework. Specifically, we define private IDs and group keys for individual UAVs, enabling secure communication while minimizing the risk of intrusion. The effectiveness of our method is validated through simulations, which highlight the importance of periodically updating the group key. Additionally, we address targeted attacks on private IDs by incorporating behavioral analysis and shared monitoring among swarm members. Our proposed approach paves the way for realizing the full potential of UAV swarms
Développement de stratégies d’adaptation de maillage pour les problèmes à paramètres variables
No full textIn numerical simulations of physical models, Adaptive Mesh Refinement (AMR) iteratively adjusts local mesh resolution to solution features, achieving meshes that meet prescribed accuracy. Beyond computing the final solution, AMR introduces overhead from repeated solution evaluations and mesh updates, which becomes significant when multiple simulations under varying operating conditions are needed, such as in parametric or sensitivity analyses. Strategies that avoid repeated full AMR are thus of practical interest.This work addresses simultaneous control of cost and accuracy under variable operating conditions through three mesh adaptation paradigms:Mean Mesh Adaptation (MMA): Builds a single adapted mesh minimizing the average error over a range of conditions, using Monte Carlo estimates of local error at each iteration. Applied to an industrial supersonic scramjet case, MMA outperforms state-of-the-art methods, but becomes costly for low mean errors or highly condition-dependent features.Error-Based Mesh Selection (EMS): For solutions with highly sensitive topologies, EMS selects the mesh from a precomputed library that minimizes error for the chosen condition. A fast approximate selection method is proposed, and tests on simplified shock flows demonstrate near-optimal mesh selection and error control.Transport-Based Mesh Interpolation (TMI): EMS relies on meshes adapted to specific conditions, which limits its robustness. TMI constructs meshes tailored to new conditions via interpolation between adapted meshes represented as point clouds, using Optimal Transport Wasserstein barycenters. A key contribution of TMI is the reconstruction of the interpolated point clouds into meshes. With similar libraries, this approach outperforms EMS error levels with minimal overhead.Pour les simulations numériques de modèles physiques, le raffinement adaptatif de maillage (AMR) ajuste de manière itérative la résolution locale des maillages en fonction des caractéristiques des solutions, produisant ainsi des maillages répondant aux exigences de précision prescrites. Au-delà du calcul de la solution finale, l’AMR introduit des surcoûts liés aux évaluations répétées de la solution et aux mises à jour du maillage, qui deviennent significatifs lorsque plusieurs simulations avec des conditions de fonctionnement variables sont nécessaires, comme dans les analyses paramétriques ou de sensibilité. Des stratégies évitant le recours à un AMR complet répété sont donc d’un intérêt pratique.Ce travail aborde le contrôle simultané du coût et de la précision avec conditions de fonctionnement variables à travers trois paradigmes d’adaptation de maillage :Adaptation de Maillage Moyenne (MMA) : Construit un maillage adapté unique minimisant l’erreur moyenne sur une gamme de conditions, en utilisant des estimations de l’erreur locale moyenne, par Monte Carlo, à chaque itération. Appliquée à un cas industriel de scramjet supersonique, la MMA surpasse les approches existantes, mais devient coûteuse pour de faibles erreurs moyennes ou lorsque les caractéristiques de la solution dépendent fortement des conditions.Sélection de Maillage Basée sur l’Erreur (EMS) : Pour les solutions dont la topologie est très sensible, l’EMS sélectionne, dans une bibliothèque pré-calculée, le maillage minimisant l’erreur pour la condition choisie. Une méthode de sélection approximative rapide est proposée, et les tests sur des écoulements de choc simplifiés montrent une sélection de maillage quasi optimale et un contrôle efficace de l’erreur.Interpolation de Maillage basée sur le Transport (TMI) : L’EMS repose sur des maillages adaptés à des conditions spécifiques fixées, ce qui limite sa robustesse. La TMI construit des maillages pour de nouvelles conditions via l’interpolation entre des maillages adaptés représentés par des nuages de points, en utilisant leur barycentre de Wasserstein. Une contribution clé de la TMI est la reconstruction des nuages de points interpolés en maillages. A bibliothèque équivalente, cette approche surpasse EMS en terme d'erreur ateinte avec un surcoût minimal
Particle Pairing through Vector Field Consensus for 3D Particle Tracking at High Seeding Densities
No full textInternational audienceA robust pairing algorithm with outlier removal is introduced in the context of two-pulse 3D Particle Tracking Velocimetry at high seeding densities, with high concentrations of ghost particles. Integrating the Vector Field Consensus approach from (Ma et al., 2014), the algorithm, its underlying hypotheses and its relevant input parameters are investigated in the context of turbulent flow measurements. 2D synthetic tests are first carried out to quantify the algorithm's performance and derive simple guidelines for optimal parameter tuning strategies based on experimental quantities. 3D synthetic tests are then implemented to test the tracking strategy robustness to increasing image densities and ghost particle concentrations. We show that our algorithm can be used for particle pairing in particle clouds with up to 50% of ghost particles
Aile Haubanée vs Aile cantilever dans le cadre d'une propulsion LH2 : Compromis aéro-structure et Challenges
No full textInternational audienc
Simulation of a contrail formation and early life cycle for a realistic airliner geometry
No full textInternational audienceContrails—ice clouds that form in aircraft wakes—are thought to have a radiative impact up to twice that of CO2 emissions, although this estimate remains debated due to significant uncertainties. These uncertainties underline the need for further research into the entire life cycle of contrails, from the formation of initial ice crystals to their potential evolution into persistent cirrus clouds. The challenge lies in the wide range of spatial and temporal scales involved in contrail development. This work presents a novel numerical methodology for simulating contrails from the onset of ice crystal formation to the dissipation of wingtip vortices. Unlike conventional methods that rely on analytical initialization, our approach couples Reynolds-averaged Navier–Stokes (RANS) with Large Eddy Simulation (LES) and synthetic turbulence techniques. This allows for a more accurate capture of near-field effects and a detailed consideration of how aircraft geometry influences the aerodynamic wake and subsequent contrail evolution. Applied to a realistic aircraft under standard atmospheric conditions, our methodology revealed that horizontal tailplane vortices can trigger short-wavelength instabilities in wingtip vortices, significantly modifying the structure of the secondary wake. Comparisons with classical approaches show that the contrails generated by this work methodology are wider and more opaque, indicating a potentially greater warming effect
Caractérisation de l'aérodynamique des sillages tourbillonnaires et des ondes de choc des avions de transport
No full textThis HDR thesis presents my research in two fundamental areas of aerodynamics applied to ransport aircraft: the dynamics of wake vortices and shock/boundary-layer interactions. The research conducted on the dynamics of wingtip vortices aims to understand the physical mechanisms governing the evolution of aircraft wake vortices, from the near field to their dissipation. These wakes, intrinsically linked to lift generation, are structured around a pair of counter-rotating vortices that can persist over several hundred wingspans, posing risks to flight safety and contributing to climatic effects through interaction with condensation trails. The work relies on wind tunnel experiments, numerical simulations (DNS),and theoretical approaches (linear stability models, optimal perturbations, resolvent analysis). Several phenomena have been investigated : vortex meandering, long-wavelength cooperative instabilities of the Crow type, short-wavelength elliptic instabilities, transitions toward non-axisymmetric states, and ground effect. The objective is to identify the triggering mechanisms and explore possible control strategies. In addition, studies on vortex detection using radar are also described. The second research axis focuses on shock/boundary-layer interactions, aiming to better understand and model low-frequency unsteady phenomena associated with transonic and supersonic regimes, in particular shock buffet on airfoils in transonic flow. The work is based on experiments conducted in transonic wind tunnels (notably the S3Ch facility at ONERA Meudon), on both rigid airfoils and airfoils mounted on elastic systems, using non-intrusive optical measurement techniques (schlieren imaging, PIV) and onboard instrumentation. The research has revealed the peculiar role of laminar-to-turbulent transition in the nature and variability of shock buffet on airfoils. A strong interaction between the dynamics of the laminar separation bubble and the shock position has been identified. In the turbulent regime, studies on fluid-structure coupling—by testing an airfoil mounted on pivots with pitch and plunge degrees of freedom—have highlighted a combined dynamics between wing bending modes and shock motion, revealing a self-sustained coupling that can lead to significant oscillations. Finally, a modal analysis using a linearized resolvent approach around RANS flows has enabled the identification of dominant modes, with an amplifier-type and non-resonant behavior similar to shock buffet, responsible for the flow fluctuations in a shock/boundary-layer interaction at Mach 1.6 in the case of an oblique shock.Ce mémoire d’HDR retrace mes travaux dans deux domaines fondamentaux de l’aérodynamique appliquée aux avions de transport : la dynamique des sillages tourbillonnaires et les interactions onde de choc / couche limite.Les recherches menées sur la dynamique des tourbillons marginaux visent à comprendre les mécanismes physiques qui gouvernent l’évolution des sillages tourbillonnaires d’avions, du champ proche jusqu’à leur dissipation. Ces sillages, intrinsèquement liés à la génération de portance, sont structurés autour de deux tourbillons contra-rotatifs qui peuvent persister sur plusieurs centaines d’envergures, induisant des risques pour la sécurité aérienne et des effets climatiques à travers l’interaction avec les traînées de condensation. Les travaux s’appuient sur des approches expérimentales en soufflerie, numériques (DNS) et théoriques (modèles de stabilité linéaire, perturbations optimales, résolvantes). Plusieurs phénomènes ont été investigués : le flottement tourbillonnaire, les instabilités coopératives de type Crow (longues longueurs d’onde), les instabilités elliptiques (courtes longueurs d’onde), la transition vers des états non axisymétriques, et l’effet de sol. L’objectif est d’identifier les mécanismes déclencheurs et d’envisager des stratégies de contrôle. Par ailleurs, des travaux sur la détection des tourbillons par Radar sont également décris. Le second axe concernant les interactions onde de choc / couche limite vise à mieux comprendre et modéliser les phénomènes instationnaires à basse fréquence associés aux régimes transsoniques et supersonique, en particulier le tremblement de choc (shock buffet en anglais) sur les profils d’aile en transsonique. Les travaux s’appuient sur des expérimentations en soufflerie transsonique (notamment la soufflerie S3Ch de l’ONERA Meudon), sur des profils d’aile rigides ou montés sur systèmes élastiques, avec des techniques de mesure optique non-intrusive (strioscopie, PIV) et une instrumentation embarquée. Les recherches ont mis en évidence le rôle majeur de la transition laminaire dans le type de l’oscillation de choc sur les profils. Une interaction forte entre la dynamique de la bulle de séparation laminaire et la position de l’onde de choc a été identifiée. Dans le régime turbulent, les travaux sur le couplage fluide-structure, en testant un profil monté sur pivots avec degrés de liberté en tangage et pompage ont mis en évidence une dynamique combinée entre les modes de flexion de l’aile et le déplacement du choc, révélant un couplage auto-entretenu pouvant mener à des oscillations importantes. Enfin, une analyse modale par approche résolvante linéarisée autour d’écoulements RANS a permis d’identifier les modes dominants, au comportement amplificateur et non résonateur comme le tremblement de choc, responsables des fluctuations de L4écoulement d’interaction choc / couche limite dans une interaction de choc oblique à Mach 1.6
Wide angle scanning KU-BAND lenses with additive manufacturing processes
No full textInternational audienceThis paper presents the ONERA work related to the design of Luneburg lens antennas and their fabrication through several additive manufacturing processes. The first configuration is a standard geodesic lens with a 17 dBi gain fabricated with aluminium selective laser melting. The device requires the use of internal supporting structures due to its native sharp profile. The second topology aims to reach a 220° scan beam coverage. For the first time the authors demonstrate a prototype of two geodesic lenses that are superimposed and rotated to reach the targeted objective. The antenna shape is strategically modified to enable a monolithic selfsupportive optimal fabrication. The last design focuses on the reduction of the lens profile and weight considering a planar 20 dBi geometry that is printed with plastic stereolithography and copper plated