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Anisotropic Goal-oriented mesh adaptation for aerodynamics using functional derivative with respect to nodal coordinates
No full textInternational audienc
Puissance et efficacité d’un propulseur magnétoplasmadynamique modélisé en MHD résistive
No full textInternational audienceExisting numerical work on self-field Magnetoplasmadynamic Thrusters (MPDTs) adopt injection conditions that assume the propellant to be "hot" ( T ∼ 1 eV) and fully ionized. Although such simulations are capable of recovering the experimental values of integral quantities, such as thrust and voltage, it is not clear how the plasma dynamics and thruster' s efficiency are affected by these simplified boundary conditions. Our goal is to build upon and extend a multi-physics computational platform specifically for studying MPDTs under realistic conditions. Here, we focus on understanding the effects of different propellant injection boundary conditions on the energy budget and efficiency. We carry out simulations of argon-fed self-field MPDTs in 2D cylindrical axisymmetric geometry. We use the single-fluid, two-temperature magnetohydrodynamic (MHD) code FLASH with tabulated equation of state calculated with the IONMIX code. A weakly ionized, "cold" propellant ( T ∼ 0.13 eV) injection boundary condition is implemented. The resistivity model is extended to take into account the effects of electron-neutral collisions, which are non-negligible in this case. The simulation results show the importance of injecting a "cold" propellant and its effects on the power balance, Lorentz and thrust efficiencies. Although injecting "cold" propellant boundary results in the similar electromagnetic thrust as the "hot" case, it leads to significant changes in the distribution of MHD parameters and, overall, a more complicated flow pattern. At the same time we are able to eliminate the large powers associated with the pre-heated, fully ionized, and fast inlet, while achieving realistic Lorentz efficiency, consistent with the thrust efficiency. Finally, the work lays the basis for future studies of a more accurate propellant injection model
A predictor-corrector scheme for approximating signed distances using finite element methods
No full textIn this article, we introduce a finite element method designed for the robust computation of approximate signed distance functions to arbitrary boundaries in two and three dimensions. Our method employs a novel prediction-correction approach, involving first the solution of a linear diffusion-based prediction problem, followed by a nonlinear minimization-based correction problem associated with the Eikonal equation. The prediction step efficiently generates a suitable initial guess, significantly facilitating convergence of the nonlinear correction step.A key strength of our approach is its ability to handle complex interfaces and initial level set functions with arbitrary steep or flat regions, a notable challenge for existing techniques. Through several representative examples, including classical geometries and more complex shapes such as star domains and three-dimensional tori, we demonstrate the accuracy, efficiency, and robustness of the method, validating its broad applicability for reinitializing diverse level set functions.</p
Séparation et transition sur un cône-cylindre-jupe : investigations numériques
No full textInternational audienceBaseflow computation and stability analysis were conducted for hypersonic flow over a cone-cylinder-flare (CCF) geometry for conditions that correspond to the experimental runs carried out in three wind tunnels. Owing to the presence of an attached boundary layer, a separation bubble induced by a shock–boundary-layer interaction, and a reattachment region, the chosen flow configuration is physically rich. The complexity of this flowfield encompasses a combination of convective instabilities developing on the cone, global instabilities in the separation bubble, and shear-layer modes and streaks in the reattachment region. Thus, the selected CCF configuration provides the opportunity for a comprehensive comparison of the currently available methodologies for analyzing boundary-layer instabilities. Various tools are used for the analysis, including global stability codes as well as convective instability analyses based on a local theory, a weakly nonparallel analysis, and tools that are applicable to strongly nonparallel flows. The paper presents a comparison of the convective instability characteristics based on different methodologies, such as linear stability theory, the harmonic form of linearized Navier–Stokes equations, and resolvent analysis. The CCF configuration provided an effective framework for conducting a detailed cross-validation of this type, which had not yet been addressed in existing literature. This document is accompanied by a companion paper that is focused on the experimental aspects of the CCF configuration. Both papers highlight the research activities of the NATO STO Research Task Group AVT-346.Des calculs de l'écoulement de base et des analyse de stabilité sont effectués pour l'écoulement hypersonique autour d'une géométrie cône-cylindre-jupe (CCF) pour des conditions qui correspondent aux essais expérimentaux effectués dans les souffleries françaises et américaines. en raison de la présence d'une couche limite attachée, d'une bulle de séparation due à l'interaction choc-couche limite et d'une région de rattachement, la configuration d'écoulement choisie est physiquement riche. cela implique une combinaison d'instabilités convectives se développant sur le cône, d'instabilités globales dans la bulle de séparation, de stries, et de modes de la couche de cisaillement. ainsi, la configuration CCF permet une comparaison complète des méthodologies actuellement disponibles pour analyser la stabilité de la couche limite. des analyses sont effectuées à l'aide de différents outils tels qu'analyse de résolvent, analyse de stabilité locale et résolution des équations de stabilité parabolisées. les solutions calculées basées sur différentes méthodologies pour les mécanismes d'instabilité convective, sont comparées les unes aux autres. la configuration du CCF offre un cadre efficace pour une validation croisée détaillée de ce type de méthodes, qui fait encore défaut dans la littérature. cet article est accompagné d'un article complémentaire qui portera sur les travaux expérimentaux relatifs à la configuration CCF. ces deux articles sont destinés à la session spéciale consacrée aux activités de recherche du groupe de travail STO AVT-346 de l'OTAN
Contributions aux méthodes de filtrage particulaire pour la navigation des véhicules autonomes
No full textCe mémoire présente une synthèse des travaux de recherche sur les méthodes séquentielles de Monte-Carlo, aussi appelées filtrage particulaire, que j’ai menés au sein du département Traitement de l’Information et Systèmes de l’ONERA. Ces travaux ont contribué à apporter des solutions innovantes aux problèmes de la navigation des systèmes aérospatiaux (missiles, satellites, drones). Une première partie de mes travaux porte sur le recalage de la navigation inertielle par corrélation de terrain, en utilisant les méthodes de filtrage particulaire. Les domaines d’application traités sont la navigation aéroportée, la navigation terrestre et plus récemment la navigation sous-marine. La seconde partie de mes travaux se concentre sur des problèmes de planification de trajectoires en environnement fortement contraint pour lesquels les techniques de commande optimale ne tenant pas compte de l’estimation de l’état sont peu performantes, voire inopérantes. Enfin, je me suis intéressé aussi à l’estimation d’attitude par filtrage particulaire en utilisant les approches basées sur les groupes de Lie, qui permettent d'améliorer la précision et la robustesse des filtres particulaires par rapport aux approches classiques. Ces résultats ont permis d’améliorer l’expertise de l’ONERA sur ce sujet et ont abouti à une exploitation dans un cadre contractuel au profit de la DGA et d’industriels
Study of the computational complexity of the repair problem on simple temporal networks with uncertainty
No full textInternational audienceLes réseaux temporels simples avec incertitude (STNU) sont un modèle basé sur les contraintes conçu pour vérifier la contrôlabilité temporelle d'un plan sous incertitude (lorsque certaines durées contingentes ne peuvent être déterminées par l'agent de planification). Quoi qu'il en soit, dans les cas de planification multi-agents ou dépendante des ressources, il peut être possible de négocier ou de reconsidérer la flexibilité exogène et d'ajuster les limites des durées incontrôlables. Plusieurs travaux proposent une solution optimale à ce problème de réparation selon les trois niveaux de contrôlabilité (faible, dynamique, forte). Cependant, aucun d'entre eux ne caractérise la complexité théorique de ce problème. Cet article propose une évaluation approfondie de la complexité du problème de réparation pour les STNU
Peripheral transcutaneous electrical stimulation to improve cognition: a review of the main effects in healthy humans and in mildly cognitively impaired patient populations
No full textInternational audiencePeripheral nerve stimulation (PNS) is an ancient technique, up to now mainly used for pain management. The least invasive approach for PNS is transcutaneous electrical stimulation (TENS), which is performed by delivering mild electric currents through the skin and, depending on the stimulation pattern, activates the somatosensory Aβ-, Aδ- and C-fibers. In addition to its use for pain relief, accumulating data indicates that TENS can have broad-spectrum cognitive effects through the activation of neuromodulatory brain pathways. This review aims to summarize the current evidence on the cognitive effects of TENS, from healthy participants and mildly cognitively affected patients. Most studies on this topic have investigated the effects of TENS on memory, while fewer studies have explored attention, executive functions, and verbal fluency. Overall, promising evidence suggests that TENS may exert positive effects on specific cognitive functions. Further research is needed to build consensus on the most effective stimulation protocols, for both neurorehabilitation and enhancement, and to better understand the neurobiological mechanisms underlying the cognitive effects of TENS
Simultaneous multi-class detection of interplanetary space weather events
No full textInternational audienceCataloging past space weather events, such as Interplanetary Coronal Mass Ejections (ICMEs) and Stream Interaction Regions (SIRs), is essential for both scientific research and operational applications. Firstly, it enables a comprehensive statistical analysis of their intrinsic physical and climatological properties. This, in turn, helps improve our understanding of their impact on the near-Earth space environment, including effects on human activities. It also enables the definition of event-driven space weather scenarii. Such studies benefit directly from the rapid and reproducible expansion of these catalogs, made possible by automatic event detection from in-situ time series data. Previous studies revealed the efficiency of deep-learning based methods for this task over traditional threshold-based techniques. Nevertheless, these methods have never been designed to simultaneously identify multiple event types. In this paper, we present a novel method for the multi-class automatic detection of ICMEs and SIRs. Our approach is inspired by the You Only Look Once (YOLO) family of algorithms, widely used in object detection. It works by directly identifying candidate time intervals, providing quick visual indicators of event occurrences that can be readily used by human observers in an operational space weather context. Thanks to its simple architecture, our method is easily implementable in data visualization tools and could be easily extended to additional event types. Tested on OMNI data between 1995 and 2024, the method detects at its best 644 out of 840 existing ICMEs and 1110 out of 1237 existing SIRs. 174 out of the 876 identified ICMEs and 192 out of the 1358 identified SIRs are actual False Positives for a maximal F1-score of 0.784 for ICMEs and 0.878 for SIRs. Our model performs slightly better at detecting events than other existing deep-learning based methods while achieving comparable results when estimating the events beginning and ending times. A detailed analysis of our method output shows that the great majority of detection errors are short events with a weak in-situ signature, which are expected to be among the least geoeffective event or misclassified events. We also show that the confusion made between ICMEs and SIRs is also shown to be comparable to the one actually made by human observers when manually establishing their catalogs
Influence of core codopants and components on laser properties of high-power holmium-doped silica all-fiber lasers
No full textInternational audienceWe demonstrate and analyze the performance of a continuous all-fiber laser delivering up to 180W at 2.12μm in a single spatial mode, with an optimum optical efficiency of 60%, based on a triple clad holmium doped fiber. For this laser we realized several holmium doped fibers, with a core either based on alumina-silica and surrounded with a pedestal, or on alumino-phospho-silica. We thus compare the results obtained with these fibers, and analyze them with our numerical simulation. We also present our efforts to realize a truly all-fiber architecture, comprising an all-fiber pump laser, a pump combiner, fiber Bragg gratings (for an oscillator configuration) and an end-cap, so we will present numerically and experimentally how these components affect the performance of a high power laser, in terms of optical efficiency or spatial beam quality
Impact acoustique des drones en milieu urbain : simulation de la propagation du son et calculs d'indicateurs acoustiques
No full textAcoustique urbaineInternational audienc