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Prévision des étiages à l’échelle nationale par la plateforme opérationnelle PREMHYCE
No full textNational audienceMieux prévoir les sécheresses, qu’elles soient météorologiques, agricoles ou hydrologiques, est un enjeu majeur pour la protection des milieux naturels et pour de nombreux secteurs d’activité, comme l’agriculture, la production d’énergie, la production d’eau potable, la navigation, le tourisme, etc.Conscients de la nécessité de doter les gestionnaires de l’eau d’outils performants de prévision des étiages dans un contexte de changement climatique, l’Office français de la biodiversité (OFB) et la Direction de l’eau et de la biodiversité (DEB) ont soutenu depuis 2011 une initiative visant à construire une plateforme opérationnelle de prévision des étiages. Cette plateforme, appelée PREMHYCE, est le fruit d’une collaboration scientifique et technique de long terme entre INRAE, Météo-France, l’Université de Lorraine, le BRGM et EDF (Tilmant et al., 2023).Elle exploite cinq modèles hydrologiques et divers ensembles de scénarios météorologiques, pour produire des prévisions de débit probabilistes, permettant d’estimer les risques de passer sous les seuils d’étiage (typiquement vigilance, alerte, alerte renforcée ou crise). Les échéances de prévision vont de quelques jours à plusieurs semaines, en fonction des objectifs et des bassins concernés. Ces dernières années, cette plateforme a évolué et a été mise à jour dans le cadre d’un projet de recherche (ANR CIPRHES, 2021-2025), avec notamment des travaux sur les prévisions météorologiques, la modélisation hydrologique, la quantification des incertitudes et l’amélioration de l’interface en concertation avec les utilisateurs. La plateforme fournit tous les jours des prévisions de débit sur plus de 1200 points répartis sur le réseau hydrographique français, pour des échéances allant jusqu’à 90 jours. Ces prévisions sont actuellement utilisées par plus d’une quarantaine d’utilisateurs (DREAL, DDT, EPTB, etc.). Le poster vise à présenter le service hydrologique PREMHYCE, ses principales fonctionnalités, son spectre d’application, et ses évolutions récentes
Improving Dantzig-Wolfe relaxation through crosswise decomposition
No full textReformulation of mixed integer programs based on decomposition is a common approach to obtain stronger linear relaxation and thus better bounds in a branch-and-bound type algorithm. We introduce the so-called crosswise decomposition technique as a framework performing double Dantzig-Wolfe decomposition across two orthogonal structures simultaneously. It relies on variable splitting, a technique better known in Lagrangian relaxation. Our theoretical contributions include a dynamic lower bound valid throughout the course of column generation, and two convergence acceleration techniques specific to the double decomposition framework. We illustrate through computational experiments the potential of this framework on a pared-down form of the Unit Commitment Problem which captures the core of its combinatorial complexity
De l'expérience au modèle : modélisation multi-échelle de la carbonatation accélérée d'une pâte de ciment
No full textInternational audienceDe l'expérience au modèle : modélisation multi-échelle de la carbonatation accélérée d'une pâte de cimen
Nonlinear dispersion relationships and dissipative properties of damped metamaterials embedding bistable attachments
No full textInternational audienceDamped metamaterials embedded with numerous micro-bistable attachments hold great potential in wave manipulation and vibration reduction. However, most of the studies are generally devoted to conservative systems, while neglecting a detailed analysis of wave attenuation mechanisms together with complex nonlinear dynamical phenomena. To address these limitations, the nonlinear dispersion relationships of metamaterials composed of internal bistable attachments, are here analysed numerically by combining the harmonic balance method and the extended periodic motion concept. The numerical methods, coupled with an arc-length continuation method, can obtain the dispersion spectrum, internal resonant branches, waveform, and damping ratio under any wave amplitude. Suitable post-processing also allows predicting both the local stability and the wave attenuation properties. To verify the effectiveness and accuracy of this algorithm and provide a comparison for multi-stable metamaterial, a mono-stable metamaterial with cubic-nonlinear resonators is taken as the first example. Then, the undamped dispersion properties and underdamped wave attenuation mechanisms in a nonlinear metamaterial composed of lattices with internal micro-bistable oscillators, are fully addressed using the proposed technique. The results underline that the metamaterial with embedded bistable oscillators has symmetric and asymmetric dispersion branches related to stable and unstable equilibrium points. For small amplitudes, the dynamics is governed by stable asymmetric dispersion solutions and exhibit linear dispersion behaviour with locally resonant bandgap. For medium amplitudes, the bandgap disappears, but only unstable dispersion branches remain over a broader frequency range, which may excite chaotic inter-well motion or targeted energy transfer, enabling a rapid attenuation of wave energy. Two kinds of responses are associated with 1:2/3/4/5 internal resonances from acoustic branches and S-shaped regions in symmetric optic branches. For large amplitudes, waves decay according to symmetric acoustic branches. Notably, regardless of wave amplitude, low-frequency waves always propagate due to extremely lightly damped asymmetric acoustic branches
Reactor performance, system reliability: instrumentation and control
No full textInternational audienceThe safe operation of nuclear power plants relies on Non-destructive Evaluation (NDE) of safety critical components in both the initial manufacturing phase and over the reactor’s lifecycle. The conventional approach consists of in-service inspections scheduled at regular intervals, with a periodicity adapted to expected or observed failure mechanisms and their kinetics. The three projects discussed in this paper address and challenge this approach in different ways. iWeld focuses on the inspection of welds and aims to take information about the microstructure of the material under inspection into account to improve the performance of ultrasound inspections. El-Peacetolero designed a hand-held, low power embedded optoelectronic system for an in-situ real-time assessment of aging polymers. FIND aims to develop in-situ instrumentation adapted to the specific requirements of the nuclear power industry and introduces continuous monitoring of metallic pipes to prevent their failure and optimise maintenance. The three projects discussed are at different stages: El-Peacetolero ends in February 2025, FIND just kicked off, and iWeld is halfway in between. In this review, we try to give a high-level introduction, and discuss particular challenges and achievements
OECD/NEA ETHARINUS Project: A Flagship Project Relevant to Thermal-Hydraulic Safety Analysis Issues
No full textInternational audienceThe Experimental Thermal Hydraulics for Analysis, Research and Innovations in Nuclear Safety(called ETHARINUS) project developed in the frame of the OECD Nuclear Energy Agency (NEA),serves the objectives of investigating complex thermal-hydraulics phenomena. Within this project,issues like the performance of passive heat removal systems and Design Extension Conditions (DEC)scenarios were investigated. The simulation of such events is of high importance to ensure relevantunderstanding of key thermal-hydraulic phenomena, to perform adequate safety analysis, to assessthe efficiency of the adopted accident management procedures and to optimize operator training.ETHARINUS project is highly relevant for the improvement and validation of thermal-hydraulic safetycodes and their use, to maintain a high level of competence and expertise in the field of systemthermal hydraulics. It is also a way to gather the actors within the area (safety authorities, operators,experimental facilities operators, university, R&D institutes, etc.) to develop knowledge and commonculture about key safety issues, for the operating fleet and for innovative designs.The objective of this paper is to describe the opportunities of the thermal-hydraulic research facilitiesemployed for the activities, to briefly outline the tests programme, and finally to highlight the key safetyissues and safety relevance of these tests programme. Such programmes are conducted in aninternational context, to share the costs and the expertise, and to promote quicker and deeperinternational consensus on safety issues. Recommendations are finally proposed regarding how toaddress the loss of critical research infrastructure (i.e. facilities, capabilities and expertise)
Comparing Uniform Price and Discriminatory Multi-Unit Auctions through Regret Minimization
No full textInternational audienceRepeated multi-unit auctions, where a seller allocates multiple identical items over many rounds, are common mechanisms in electricity markets and treasury auctions. We compare the two predominant formats: uniform-price and discriminatory auctions, focusing on the perspective of a single bidder learning to bid against stochastic adversaries. We characterize the learning difficulty in each format, showing that the regret scales similarly for both auction formats under both full-information and bandit feedback, as and , respectively. However, analysis beyond worst-case regret reveals structural differences: uniform-price auctions may admit faster learning rates, with regret scaling as in settings where discriminatory auctions remain at . Finally, we provide a specific analysis for auctions in which the other participants are symmetric and have unit-demand, and show that in these instances, a similar regret rate separation appears
Harmonised boundary layer wind profile dataset from six ground-based Doppler wind lidars in a transect across Paris, France
No full textInternational audienceDoppler wind lidars (DWL) offer high-resolution wind profile measurements that are valuable for understanding atmospheric boundary layer (ABL) dynamics. Here six ground-based DWL, deployed in a multi-institutional effort along a 40 km transect through the centre of Paris (France), are used to retrieve horizontal wind speed and direction through the ABL at 18–25 m vertical and 1–60 min temporal resolution. Data are available for June 2022–March 2024 (three DWL) and two Intensive Observation Periods (six DWL) across 9 weeks in September 2023–December 2023. Data from all sensors are harmonised in terms of quality control, file format, as well as temporal and vertical resolutions. The quality of this DWL dataset is evaluated against in-situ measurements at the Eiffel Tower and radiosonde profiles. This unique, spatially dense, open dataset will allow urban boundary layer dynamics to be explored in process-studies, and is further valuable for the evaluation of high-resolution weather, climate, inverse and air pollution models that resolve city-scale processes. The dataset is available at https://doi.org/10.5281/zenodo.14761503 (Morrison et al., 2025)
Évaluer les modèles d’atmosphère en écoutant la houle océanique
No full textBasses fréquences et infrasons : émission, propagation, instrumentation et perception; GABE - Acoustique du Bâtiment et de l'Environnement: GPS - Perception Sonore: GAHA - Aéro et Hydro-AcoustiqueNational audienceLa houle océanique est une source globale et continue d’ondes acoustiques basse fréquence appelées microbaroms, avec un pic d’émission à 0.2 Hz. Les stations infrason du système de surveillance international en détectent les signaux au gré de guides, présents dans la moyenne atmosphère (MA, stratosphère-mésosphère) ou la haute atmosphère (HA, mésosphère-thermosphère), qui en assurent la propagation sur plusieurs centaines à plusieurs milliers de kilomètres. Les modèles météorologiques utilisés en opérationnel pour la simulation de propagation infrason souffrent de biais à ces hautes altitudes du fait de l’absence de mesures (notamment de vent) pour les systèmes d’assimilation de données. Nous expliquons ici comment il est possible d’évaluer la performance de ces modèles dans la MA et la HA en utilisant un modèle global opérationnel de source acoustique océanique en entrée d’une chaîne de simulation des détections de microbaroms en station. Ces simulations sont comparées aux observations au travers d’une métrique pour apporter un diagnostic sur la qualité de simulation du milieu de propagation. Cette chaîne de traitement doit permettre de démontrer l’intérêt de l’assimilation des microbaroms dans les modèles d’atmosphère, et ce afin de pallier le manque de données météorologiques conventionnelles à ces altitudes. En particulier, un objectif est d’apporter des spécifications atmosphériques plus réalistes dans la MA et la HA pour la simulation de propagation, dans le cadre de la surveillance par infrason
Graphene assisted III-V epitaxy towards substrate recycling
No full textInternational audienceRe-using the substrate is identified as a method for reducing the cost of high efficiency III-V solar cells. The approach investigated here consists in inserting a graphene layer onto a (001)GaAs substrate prior to the epitaxial growth of GaAs. To obtain a monocrystalline GaAs grown layer, the graphene layer is patterned, followed by a two-step epitaxial growth, here performed by molecular beam epitaxy (MBE). The first step is a selective area growth of GaAs in graphene openings, followed by a lateral overgrowth, under a modulated Ga flux. The second step, after reaching coalescence, consists in a regular growth under continuous Ga supply. It is observed that the pattern orientations relative to the crystallographic direction of the GaAs substrate below the graphene have an influence on GaAs morphology and quality. The best result was obtained for patterns oriented along [1̅10]+22.5° with a graphene coverage of 50%, with a significantly reduced roughness down to 3.3 nm