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Apprentissage par renforcement avec exploration rapide pour les applications aux réseaux stochastiques
No full textInternational audienceWe introduce FVRL (Fleming–Viot reinforcement learning), a reinforcement learning algorithm for optimisation problems where a long-term objective is largely influenced by states that are very rarely observed under all policies. In this context, usual discovery techniques including importance sampling are inapplicable because no alternative policy exists that increases the observed frequency of the rare states. We instead propose a novel approach that uses Fleming–Viot particle systems, a family of stochastic processes evolving simultaneously under the same law, that exploits prior knowledge of the environment to boost exploration of the rare states. A renewal theory argument allows us to consistently estimate the stationary probability of the rare states from excursions that have considerably lower sample complexity than usual Monte Carlo explorations. We demonstrate how to combine this estimator with policy gradient learning to construct the FVRL algorithm, which is suited to efficiently solve problems where the optimisation function is expressed as a long-run expectation, such as the long-run expected reward. We show that the FVRL algorithm converges to a local optimiser of the parameterised objective function, and illustrate the method on two optimisation problems that aim at minimising the long-run expected cost under admission control policies of threshold type: a simple M/M/1 queue system and a two-job-class loss network. Our experimental results show that, under the same sample complexity, FVRL outperforms a vanilla Monte Carlo reinforcement learning method by converging to the optimum thresholds considerably faster
Estimation of rooftop canopy wind energy with eave deflector panel
No full textInternational audienceUrban wind energy has received increasing attention as a potential clean energy resource. Roofs have been identified as the primary site for urban wind energy development. This study analyzes the impact of different eave deflector panel parameters on rooftop wind energy. The findings indicate that adding an eave deflector panel above or on the eaves facing the wind can enhance the rooftop wind energy. Wind energy density of the selected models with eave panel can raise to 1.88∼2.71 times of the reference model without a eave panel. Among the eave panels of various heights, the lowest wind-facing one is optimal. The optimal angle for a cantilevered eave panel is approximately 36°, with a rooftop wind density 1.1∼4.4 times of the case at 0° Increasing the length of the cantilevered eave panel, raising wind turbine height, and selecting taller buildings all contribute to enhancing the rooftop wind energy. At lower evaluation heights, turbines should be placed in the eave area with a slight forward tilt to utilize upstream wind energy. At higher evaluation heights, it is advisable to place turbines in the upstream areas of wind-facing eaves. This study helps identify the basic patterns of how different eave panel parameters affect rooftop wind energy, aiding in retrofitting practices for improving wind energy utilization of existing buildings and enhancing urban wind energy development
Etude numérique des dommages induits par le phénomène d’arc électrique
No full textInternational audienceDes réseaux électriques haute tension sont intégrés dans une génération d’avions incluant des systèmes propulsifs électriques. Ceci implique la prévention de défauts liés à leur utilisation tels que les arcs électriques. L’énergie dissipée par un arc est à l’origine de différents phénomènes représentant une menace pour la structure de l’avion à proximité de l’arc tels que des effets thermiques ou des effets de forte variation de pression. Afin de garantir l’intégrité de la structure de l’avion, il est essentiel de caractériser l’endommagement généré par ces phénomènes sur les matériaux. Ce travail propose une approche numérique basée sur une méthode par éléments finis. Pour y parvenir, le premier objectif est de définir précisément les données d’entrée du modèle, et en particulier les chargements équivalents des deux phénomènes énoncés précédemment
Solution methods for the VRP with sharing of deliveries between producers in local food logistic
No full textInternational audienceThis study addresses the logistical challenges in Short Food Supply Chains (SFSC), where producers face high logistical costs. A new variant of the Vehicle Routing Problem (VRP) is proposed, enabling producers to share their deliveries with partner producers. With a partner producer, a producer can either drop off goods for a customer (to be served by the partner producer) or pick up goods for a customer they will visit themselves during their routes.Although solving the Vehicle Routing Problem (VRP) is a key area in optimization, with certain variants tailored to the specificities of SFSC, such as Pickup and Delivery or split deliveries, few studies address a delivery sharing aspect like this.Three heuristic methods were tested: GRASP (Greedy Randomized Adaptive Search Procedure), ILS (Iterated Local Search), and VNS (Variable Neighborhood Search), which systematically explores multiple neighborhoods. Initial results show a significant reduction in logistical costs thanks to the sharing approach, while also highlighting an imbalance in the workload distribution between producers.Mots clés -Circuits CourtsAlimentaires de Proximité, VRP, mutualisation, méta-heuristiques Résumé Cette étude traite des défis logistiques dans les Circuits Courts Alimentaires de Proximité (CCAP), où les producteurs sont confrontés à des coûts logistiques élevés. Une nouvelle variante du problème de tournées de véhicules (VRP) est proposée, permettant aux producteurs de mutualiser leurs livraisons avec d'autres producteurs partenaires. Avec un producteur partenaire, un producteur peut soit lui déposer des marchandises pour un client (que ce producteur servira), soit prendre des marchandises pour un client qu'il visitera lui-même lors de ses tournées. Bien que la résolution du Problème de Tournées de Véhicules (VRP) soit un domaine clé en optimisation, avec certaines variantes adaptées aux spécificités des CCAP, comme le Pickup and Delivery ou les livraisons fractionnées, peu d'études abordent un aspect de mutualisation des tournées comme celui-ci. Trois méthodes de résolutions approchées ont été testées : GRASP (Greedy Randomized Adaptative Search Procedure), ILS (Iterated Local Search), et VNS (Variable Neighborhood Search), qui explore plusieurs voisinages de manière systématique. Les premiers résultats montrent une réduction significative des coûts logistiques grâce à la mutualisation, et mettent en évidence un déséquilibre dans la répartition des charges entre producteurs.</div
CO2 as an alternative carbon source for ethanol production: A simulation-based technical and environmental analysis
No full textInternational audienceThree ethanol production scenarios from CO2 are analyzed: (S1) syngas catalytic conversion; (S2) syngas fermentation; and (S3) bio-conversion of CO₂ with H₂. These three scenarios were simulated in ProSimPlus® software, using flue gas from cement production and biogas as CO2 sources. Technological performance and environmental impacts evaluated through the Life Cycle Assessment (LCA) method were analyzed. Chemical absorption (CA) and membrane separation (MS) are utilized for carbon capture, while hydrogen is generated by electrolysis. When considering flue gas and CA, scenario S1_CA yielded the lowest ethanol production (0.110 kg/kg-CO2) but the highest energy consumption: 130 MJ electricity/kg-ethanol and 73 MJ heat/kg ethanol. Contrarily, S3_CA achieved the highest ethanol production (0.345 kg/kg-CO2) with energy consumption comparable to S2 (86 MJ electricity/kg-ethanol and 20 MJ heat/kg-ethanol). S1 demonstrated the lowest global warming impact (GWP100) with values of –8.60 kg CO₂-eq per kg ethanol for CA (S1_CA) and –10.13 kg CO₂-eq for MS (S1_MS). S1 outperformed both S2 (CA: 0.13 kg CO₂-eq, MS: –0.83 kg CO₂-eq) and S3 (CA: –0.43 kg CO₂-eq, MS: –1.00 kg CO₂-eq). When biogas was used, S3_CA exhibited the lowest impact: 3.00 kg CO₂-eq, compared to S2_CA: 6.36 kg CO₂-eq and S1_CA: 4.10 kg CO₂-eq. S1_MS and S3_MS have close results: 2.34 and 2.44 kg CO₂-eq, respectively, much lower than S2_MS (5.22 kg CO₂-eq). When considering the end-of-life with ethanol combustion, GWP100 results of flue gas-based scenarios become similar (S1) or worse (S2, S3) than current crop/vegetable fermentation production routes, while biogas-based scenarios strongly depend on fugitive emissions control
Room-Temperature H<sub>2</sub> Splitting and N<sub>2</sub>-Hydrogenation Induced by a Neutral Lu(II) Complex
No full textInternational audienceThe direct splitting of the H2 Splitting and N2 molecules are challenging reactions that are closely related to the Haber-Bosch ammonia synthesis process. Until now, such reactivity has never been observed in the case of molecular lanthanide species. Here, we show that careful selection of the ligand scaffold allows the isolation and characterization of a kinetically stable but highly reactive Lu(II) complex. This divalent lanthanide species enables direct H2 splitting at room temperature, an unknown reactivity in lanthanide chemistry, which has been fully corroborated by DFT calculations. In addition, the Lu(II) complex readily binds N2, leading to an end-on coordinated diazenido (N2)2- lanthanide complex. The latter can be hydrogenated under very smooth conditions (ca. 1.2 bar H2, ambient temperature) to form a unique Lu(III)-NH2 complex. Direct N2 hydrogenation and cleavage are thus accessible using low-valent molecular rare-earth metal complexes
Transport non dissipatif des etats topologiques issus de lignes nodales dans des semi-metaux de Weyl
No full textInternational audienceTopological materials, such as topological insulators or semimetals, usually not only reveal the non-trivial properties of their electronic wavefunctions through the appearance of stable boundary modes, but also through very specific electromagnetic responses. The anisotropic longitudinal magnetoresistance of Weyl semimetals, for instance, carries the signature of the chiral anomaly of Weyl fermions. However for topological nodal line semimetals—materials where the valence and conduction bands cross each other on one-dimensional curves in the three-dimensional Brillouin zone—such a characteristic has been lacking. Here we report the discovery of a peculiar charge transport effect generated by topological nodal lines in trigonal crystals: a dissipationless transverse signal in the presence of coplanar electric and magnetic fields, which we attribute to a Zeeman-induced conversion of topological nodal lines into Weyl nodes under infinitesimally small magnetic fields. We evidence this dissipationless topological response in trigonal PtBi2 persisting up to room temperature, consistent with the presence of extensive topological nodal lines in the band structure of this non-magnetic material. These findings provide a pathway to engineer Weyl nodes by arbitrary small magnetic fields and reveal that bulk topological nodal lines can exhibit non-dissipative transport properties
A tri-dimensional model for the numerical treatment of ductile fracture in dynamics using the Extended Finite Element Method
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Étude de modèles de diffusion pour la modification d'images de danse sportive
No full textNational audienceGenerating images and videos using generative and diffusion models is a very active area of research, notably through popularizing models such as Chat-GPT or LAMA. In recent years, increased computational resources have enabled the development of models to generate new images and videos from original sources, or modify them according to constraints provided by prompts. However, these models still have difficulty in preserving human anatomy when applied to images displaying human bodies performing complex movements, such as those found in gymnastics or dance. In this article, we provide experiments relying on the latest reference methods for image modification in the context of artistic dance, as well as a new framework based on Poisson Blending allowing better respect of dancers' morphology.La génération d'images et de vidéos à travers des modèles génératifs et de diffusions est un domaine très actif de la recherche, notamment à travers la popularisation de modèles tels que Chat-GPT ou LAMA. L'augmentation des ressources de calcul a permis ces dernières années le développement de modèles capables de générer de nouvelles images et vidéos à partir de sources originales, ou encore de modifier ces dernières en suivant des contraintes fournies sous forme de prompts. Pourtant, ces modèles ont encore des difficultés à préserver l'anatomie humaine lorsqu'on les applique à des images présentant des corps humains effectuant des mouvements complexes, tels que ceux présents en gymnastique ou en danse. Dans cet article, nous proposons un retour d'expérience sur l'utilisation des dernières méthodes de référence pour de la modification d'images de danse artistique, ainsi qu'un nouveau cadre d'étude basé sur du Poisson Blending permettant un meilleur respect de la morphologie des danseurs
Directional light scattering in Mie-resonant Si particles with ultra-thin plasmonic shells
No full textInternational audienceWe present the synthesis and characterization of Au-decorated Si core-shells as candidate meta-atoms. We found a damped magnetic dipole (MD) for smaller Si cores (100 – 130 nm) and an enhanced MD for larger cores (150 – 200 nm). Continuous plasmonic shells of ~12 nm are needed to significantly improve forward scattering.Subwavelength-sized Si particles interact strongly with visible (vis) and near-infrared (NIR) light to produce strong electric and magnetic resonances. These can combine to produce interesting optical effects, such as pure forward scattering. A requirement for this to occur efficiently is that the electric dipole (ED) and magnetic dipole (MD) modes must be of similar amplitude and phase. At wavelengths where this occurs, the particles act analogously to the forward-propagating point sources of light used in Huygens’ constructions. This directional scattered light has a range of potential applications in the creation of metamaterials.We have investigated dielectric@metal core-shell architectures comprised of both resonant cores and resonant shells as candidate particles in which the spectral overlap of the electric and magnetic dipoles might be controlled to create strong directional scattering. There are currently two reports of the synthesis and characterization of Au shells around Si cores, both thicker than desirable. [1,2] Chaâbani et al. presented Si@Au particles, with non-uniform shells composed of Au particle diameters between 10 and 25 nm, which presented enhanced electric field and Fano-resonances due to the coupling of the Mie modes of the Si core and the localized surface plasmon resonance (LSPR) of the Au shell. [1] Sugimoto et al. similarly presented Si@Au particles with a rough ~25 nm Au shell. [2] In both of these reports of Si@Au core-shell particles, the experimental data could not be accurately fit by simulations due to non-spherical cores and inhomogeneous shells. Ultrathin and homogeneous coatings have not yet been achieved around spherical Si particles.In this study, we present a two-step aqueous approach to prepare Si@Au core-shell particles with controllable shell thickness below 10 nm. The Au nanoparticle (AuNP) density around the Si particles can be increased by performing a second functionalization/deposition step. We studied the electromagnetic response of the particles using single-particle scatter spectroscopy and electron energy loss spectroscopy (EELS). Our results were compared with reference Si spheres and SiO2@Au core-shell particles, to allow us to establish the contribution from the Au decoration to the optical response of the hybrid particles. To further elucidate the nature of the electromagnetic response of the particles, these observations were supported by T-matrix simulations which replicated our experimental findings, and showed the importance of controlling the shell/core dimensions and the need for a continuous shell to maximize forward scattering. We found that continuous plasmonic shells of ~12 nm thickness are needed to significantly improve forward scattering intensity.References1.Chaâbani, W, J Proust, S Ouellet, A Movsesyan, J Béal, R Bachelot, T Xu, A L Baudrion, PA Adam, D Boudreau, A Chehaidar, and J Plain, “Si@Au core–shell nanostructures: Toward a new platform for controlling optical properties at the nanoscale.” J Phys Chem C, Vol. 125, 20606. 2021. DOI: 10.1021/acs.jpcc.1c061822.Sugimoto, H., T Hinamoto, Y Kazuoka, A Assadillayev, S Raza, and M Fujii. “Mode hybridization in silicon core–gold shell nanosphere.” Small Vol. 18, 2204890, 2022. DOI: 10.1002/smll.20220489