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Sessile ⟨100⟩ self-interstitial clusters with non-parallel edge dumbbells in irradiated bcc Fe and other metals
No full textInternational audienceDensity Functional Theory calculations of self-interstitial atom clusters in bcc Fe unexpectedly show that from ∼9–14 self-interstitial atoms, an intriguing new family of sessile ⟨100⟩ clusters, surrounded by ⟨110⟩ dumbbells, are more stable than highly mobile clusters of parallel ⟨111⟩ dumbbells. The ⟨110⟩ edge dumbbells find a favorable location in terms of strain energy on the tensile side around the edges of the ⟨100⟩ center, thus stabilizing the clusters. These sessile clusters might explain resistivity recovery results that suggested an absence of glissile self-interstitial clusters up to large cluster sizes in irradiated Fe, while smaller self-interstitial atom clusters likely would have been present. The mechanism of non-parallel edge interstitials stabilizing an otherwise higher energy interstitial loop is also found in some fcc metals
(Online) Convex Optimization for Demand-Side Management: Application to Thermostatically Controlled Loads
No full textInternational audienceTo counter the challenge of integrating fluctuating renewables into the grid, devices like thermostatically controlled loads (water-heaters, air conditioners, etc) offer flexible demand. However, efficiently controlling a large population of these devices to track desired consumption signals remains a complex challenge. Existing methods lack convergence guarantees and computational efficiency, or resort to regularization techniques instead of tackling the target tracking problem directly. This work addresses these drawbacks. We propose to model the problem as a finite horizon episodic Markov decision process, enabling us to adapt convex optimization algorithms with convergence guarantees and computational efficiency. This framework also extends to online learning scenarios, where daily control decisions are made without prior knowledge of consumer behavior and with daily-changing target profiles due to fluctuations of energy production and inflexible consumption. We introduce a new algorithm, called Online Target Tracker (OTT), the first online learning load control method, for which we prove sub-linear regret. We demonstrate our claims with realistic experiments. This combination of optimization and learning lays the groundwork for more dynamic and efficient load control methods
An innovative method based on CFD to simulate the influence of photovoltaic panels on the microclimate in agrivoltaic conditions
No full textInternational audienceAssessing the impact of photovoltaic panels on solar and infrared radiation, wind speed, and turbulence is essential for understanding how these panels may affect crops or livestock in agrivoltaic (APV) systems, as well as water reservoirs in floating photovoltaic (FPV) installations. However, state-of-the-art numerical methods require huge computing resources and rarely account for many physical phenomena at the same time. This study suggests the implementation of source and sink terms within the Computational Fluid Dynamics (CFD) solver code_saturne, specifically in the Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations and the Discrete Ordinate Radiation Model (DOM). It enables time-efficient simulations of solar and infrared radiation, wind speed, and turbulence in the presence of obstacles. First, this method is compared to wind tunnel measurements of velocity and turbulence fields for a downsized ground-mounted photovoltaic plant, RMSEvel<0.12 m/s, and RMSEturb<0.05 m(2)/s(2 ), for a flow with a wind speed of 2.5 m/s and a turbulent kinetic energy of 0.05 m(2)/s(2 )at the PV panel height. Then, it has been applied to an actual APV power plant to validate solar and infrared radiation simulations, on average RMSEsolar<61.0 W/m(2), and RMSEir<14.0 W/m(2), for a solar radiation reaching 500 W/m2 and an IR radiation of about 350 W/m(2). This innovative method allows for the examination of how obstacles affect the microclimate, and subsequently, key parameters such as evapotranspiration. It paves the way for comprehensive numerical studies of the influence of photovoltaic panels on their environment, with a particular focus on APV and FPV configurations
Implémentation d’un contrôleur non-linéaire dans la méthode Boucle de Phase Asservie (PLL) pour l’identification expérimentale de structures non-linéaires
No full textInternational audienceExperimental continuation methods are used to retrieve and identify nonlinear characteristics of vibrating structures. Among the available methods, Phase-Locked Loop (PLL) allows for an easy-to-implement yet efficient method to continue nonlinear solutions such as backbone curves or frequency response functions. The PLL automatically locks onto the prescribed phase and thanks to a linear (proportional-integral) controller, can stabilize unstable periodic orbits. However, the tuning of the different parameters to be used in such a loop are seldomly documented in the literature, which in turn might lead to long duration tests. To ease the tuning effort and reduce the experimenting time, a nonlinear controller is here proposed as a way to improve the efficacy of Phase-Locked Loop testing. Thanks to the proposed design, named NCPLL (Nonlinear Controller PLL), most of the parameters are tuned easily, while a rapid locking to the prescribed state is at hand. The nonlinear gain can be easily adapted to reach a locked state rapidly. The efficacy of the NCPLL is first demonstrated on simple numerical examples including nonlinear oscillators with smooth restoring forces and Coulomb friction, and a finite element beam model with localized nonlinearities. Then the method is deployed on two different experimental test rigs. First, the case of smooth nonlinearity is tackled thanks to a cantilever beam vibrating in the magnetic field created by two magnets. Finally, the case of friction is addressed by considering an assembled beam with friction joints. In all the tested cases, the NCPLL shows excellent performance, requiring minimal tuning efforts whilst leading to fast measurements.Les méthodes de continuation expérimentales sont utilisées pour extraire et identifier les caractéristiques non linéaires des structures vibrantes. Parmi les méthodes disponibles, la méthode de Boucle de Phase Asservie (Phase-Locked Loop - PLL) constitue une approche simple à mettre en œuvre et efficace pour poursuivre des solutions non linéaires telles que des "backbone curves" ou des fonctions de réponse en fréquence, étendant ainsi le champ d’application des méthodes de résonance de phase (PRM). La PLL se verrouille automatiquement sur la phase prescrite et, grâce à un contrôleur linéaire (proportionnel-intégral), elle peut stabiliser des orbites périodiques instables. Cependant, le réglage des différents paramètres à utiliser dans une telle boucle est rarement documenté dans la littérature, ce qui peut entraîner des essais de longue durée. Afin de faciliter le réglage et de réduire le temps d’expérimentation, un contrôleur non linéaire est proposé ici pour améliorer l’efficacité des tests basés sur la PLL. Grâce à ce design, nommée NCPLL (Nonlinear Controller PLL), le nombre de paramètres à ajuster est réduit, tout en assurant un verrouillage rapide sur l’état prescrit. Une borne supérieure pour le gain du contrôleur non linéaire est déduite à partir de développements théoriques. L’efficacité du NCPLL est d’abord démontrée sur des exemples numériques simples, incluant des oscillateurs non linéaires avec des forces de rappel régulières et des forces de frottement de type Coulomb, ainsi qu’un modèle éléments finis de poutre avec des non-linéarités localisées. Ensuite, la méthode est appliquée à deux bancs d’essais expérimentaux distincts. Dans un premier temps, le cas d’une non-linéarité régulière est étudié avec une poutre cantilever vibrant dans le champ magnétique créé par deux aimants. Enfin, le cas du frottement est abordé à travers une poutre assemblée comportant des joints frottants. Dans tous les cas testés, le NCPLL démontre d’excellentes performances, nécessitant un réglage minimal tout en permettant des mesures rapides
Open Review of "How to introduce an initial crack in phase field simulations to accurately predict the linear elastic fracture propagation threshold?"
No full textOpen Review of "How to introduce an initial crack in phase field simulations to accurately predict the linear elastic fracture propagation threshold?" published in JTCAM.We would like to thank both reviewers for their thoughtful and constructive feedback on our manuscript, "How to introduce an initial crack in phase-field simulations to accurately predict the linear elastic fracture propagation threshold?".We appreciate the time and effort invested in evaluating</div
Non-destructive measurement of fuel rods inner pressure and composition : numerical consideration of different spring dimensions
No full textInternational audienc
Approaches and limitations of Non-Destructive Testing for evaluating bituminous concrete using wave propagation: A review from the laboratory test to in situ case study
No full textInternational audienceThis work addresses the challenges of using non-destructive testing (NDT) methods based on mechanical wave propagation as alternatives to conventional destructive tests for evaluating the mechanical behavior of bituminous concrete materials and structures. Given the viscoelastic nature of these materials, the paper first outlines the fundamental principles of their mechanical behavior and the key parameters to be identified. The study then focuses on two categories of NDT techniques involving wave propagation: laboratory methods, such as impact resonance testing and ultrasonic excitation; and in-situ methods, referring to geophysical techniques. Finally, the paper presents a comprehensive literature review of these NDT applied to bituminous concrete and discusses their respective advantages and limitations
Comment les centrales photovoltaïques flottantes et agrivoltaïques réduisent la température de panneaux PV par rapport aux centrales au sol ?
No full textNational audienceLe photovoltaı̈que flottant (FPV) (Sahu, et al., 2016), ainsi que l’agrivoltaı̈sme (APV) (Dupraz, et al., 2010), tout d’abord développés pour répondre à un manque de surfaces sur lesquelles installer des panneaux photovoltaı̈que (PV), permettraient également d’améliorer le rendement électrique. Effectivement, les centrales FPV et APV diffèrent d’une centrale au sol classique de part leur géométrie et la surface au sol sur laquelle elles sont disposées. Alors que les panneaux d’une centrale FPV, dont l’objectif est de recouvrir un plan d’eau, sont installés très proches de la surfacede l’eau (≈ 30cm de hauteur), les panneaux d’une centrale APV recouvrent moins de 40% d’un champ agricole et sont généralement surélevés (≈ 4.5m de hauteur) pour laisser les engins agricoles circuler. Les spécificités géométriques, ainsi que le type d’environnement influencent les niveaux de refroidissement des panneaux et donc leurs performances électriques : d’après (Kaldellis, et al., 2014) une baisse de 1 ◦ C entraı̂ne un gain de 0.4 % sur le rendement électrique. L’objectif de cette étude est de déterminer quels sont les paramètres clés influençant la réduction de la température des panneaux photovoltaı̈ques.Afin d’étudier l’impact des changements de géométrie et de surface (plan d’eau, ou culture agricole) un modèle d’estimation de la température de panneaux PV (T pv ) a été développé dans le logiciel de mécanique des fluides code saturne. Les modifications du microclimat (vent, rayonnement, température de l’air) causées par la centrale PV et le type de surface au sol sont simulées par code saturne. En fonction des conditions météorologiques et du microclimat simulé, un bilan énergétique est effectué pour chaque panneau photovoltaı̈que, permettant de calculer satempérature. L’étude débute par l’estimation de la température des panneaux d’une centrale au sol classique. Plusieurs itérations sont ensuite réalisées en faisant varier les paramètres géométriques (espacement, hauteur, longueur et inclinaison des panneaux) ainsi que le type de sol. Pour chaque configuration, l’impact des paramètres sur la température des panneaux est analysé. La géométrie influe principalement sur les échanges convectifs. Par exemple, l’espacement et l’élévation accrus des rangées des centrales APV favorisent la convection. Le type de sol agit davantage sur le microclimat. Typiquement, la proximité de l’eau permet aux centrales FPV d’expérimenter une température ambiante plus fraı̂che. Par conséquent, le modèle numérique prédit une diminution globale de la température des panneaux de plusieurs degrés par rapport à des centrales au sol
Island biogeography through the lens of multiscale metapopulation dynamics: insights into species-area relationships
No full textWhile island biogeography focuses on species richness equilibrium driven by immigration and extinction, and metapopulation theory examines single-species dynamics across fragmented habitats, their interplay remains poorly understood. In particular, the species-area relationship remains a subject of ongoing debate, yet there are limited theoretical foundations to explain it. To address this, we developed a multiscale stochastic metapopulation model to investigate diversity patterns on islands, bridging the gap between island biogeography and metapopulation theory. Our model integrates regional colonization from a mainland with local colonization-extinction processes within islands at the single-species level, then extends this to multiple, independent species. By analyzing the stationary properties of this model, we generate novel predictions of Species Area Relationship (SAR) based on local extinction rates, within-island colonization rates, and mainland immigration rates. We demonstrate how the interplay of these parameters influences the relationship, predicting patterns that can resemble either the power-law of Arrhenius or the semi-logarithmic relationship of Gleason, depending on the relative importance of mainland immigration versus within-island dynamics, and on the nature of the species abundance distribution in the mainland. This unified framework offers new insights into the mechanisms driving species richness and distribution across spatial scales, providing a more holistic understanding of biodiversity patterns in fragmented landscapes
A few techniques to achieve invisibility in waveguides
No full textDoctoralThe aim of this lecture is to consider a concrete problem, namely the identification of situations of invisibility in waveguides, to present techniques and tools that may be useful in various fields of applied mathematics. To be more specific, we will be interested in the propagation of acoustic waves in guides which are unbounded in one direction. In general, the diffraction of an incident field in such a structure in presence of an obstacle generates a reflection and a transmission characterized by some scattering coefficients. Our goal will be to play with the geometry, the frequency and/or the index material to control these scattering coefficients. We will explain how to:- develop a continuation method based on the use of shape derivatives to construct invisible defects;- exploit complex resonances located closed to the real axis to hid obstacles;- construct a non self-adjoint operator whose eigenvalues coincide with frequencies such that there are incident fields whose energy is completely transmitted.Our approaches will mainly rely on techniques of asymptotic analysis as well as spectral theory for self-adjoint and non self-adjoint operators. Most of the results will be illustrated by numerical experiments