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Construction and performance of kinetic schemes for linear systems of conservation laws
No full textWe describe a methodology to build vectorial kinetic schemes, targetting the numerical solution of linear symmetric-hyperbolic systems of conservation laws -a minimal application case for those schemes. Precisely, we fully detail the construction of kinetic schemes that satisfy a discrete equivalent to a convex extension (an additional non-trivial conservation law) of the target system -the (linear) acoustic and elastodynamics systems, specifically -. Then, we evaluate numerically the convergence of various possible kinetic schemes toward smooth solutions, in comparison with standard finite-difference and finite-volume discretizations on Cartesian meshes. Our numerical results confirm the interest of ensuring a discrete equivalent to a convex extension, and show the influence of remaining parameter variations in terms of error magnitude, both for "first-order" and "second-order" kinetic schemes : the parameter choice with largest CFL number (equiv., smallest spurious diffusion in the equivalent equation analysis) has the smallest discretization error
LE TRAITEMENT DES SOLS A LA CHAUX POUR LA CONSTRUCTION D'OUVRAGES HYDRAULIQUES DE FAIBLE HAUTEUR. ÉTAT de L'ART ET DE LA PRATIQUE: Livret 2 DigueELITE
No full textDocument d'information (guide technique)Ce document est un prolongement de référentiels techniques existants concernant le traitement de sol à la chaux pour la réalisation des terrassements lors de la construction d’infrastructures linéaires de transports et de plates-formes. Son champ d’application s’ouvre à la construction d’ouvrages hydrauliques d’une hauteur inférieure à une quinzaine de mètres ou à la réparation d’ouvrages hydrauliques avec l’utilisation de sols traités à la chaux (digues fluviales, petits barrages réservoirs ou écrêteurs de crue). Son rôle est de donner aux donneurs d’ordre, aux bureaux d’études, aux entreprises, aux organismes de contrôle les éléments justifiant l’application de cette technique, les avantages qu’elle procure et la méthodologie à employer.En effet, les performances obtenues permettent d’envisager la pratique du traitement de sol à la chaux de façon plus étendue que la simple réutilisation des sols trop humides. Les résultats obtenus en matière de résistance mécanique, de résistance à l’érosion interne et externe démontrent, qu’en respectant les règles de caractérisation des gisements des sols naturels, de réalisation d’études de laboratoire et de mise en œuvre, la technique de traitement de sol à la chaux présente un intérêt technico-économique et dans une certaine mesure environnementale dans le domaine des ouvrages hydrauliques, hors ouvrages maritimes pour lesquels des études sont en cours.En outre, il a été démontré que la perméabilité du matériau sol-chaux est similaire à celle du sol non traité lorsque des conditions particulières de teneur en eau et de mode de compactage sont appliquées (teneur en eau côté humide et compactage au compacteur vibrant à pieds dameurs).Le document est composé de six parties :Partie A• Rappeler les propriétés de la chaux et son action sur les sols.• Préciser les applications possibles du composant sol-chaux dans un ouvrage hydraulique selon les fonctions d’usage.Partie B• Présenter les propriétés mécaniques et hydrauliques du composant sol-chaux, en particulier les grandeurs utiles pour la conception :o résultats obtenus en laboratoire et in situ,o quantification de la résistance à l’érosion interne et à l’érosion de surface mesurée en vraie grandeur sur les démonstrateurs.Partie C• Décrire l’ingénierie et la réalisation d’un projet d’ouvrage hydraulique ayant recours au traitement des sols à la chaux :o déroulé des études,o caractérisation des gisements et études de laboratoire,o éléments de conception.Partie D• Présenter les méthodologies applicables à la construction d’ouvrages en sol traité à la chaux :o DCE, planches d’essais et épreuve de convenance,o exécution des travaux.Partie E• Identifier les points importants relatifs au contrôle de la construction d’ouvrages en sol traité à la chaux.Partie F• Exposer le suivi d’un ouvrage hydraulique en sol traité à la chaux
Multi-temperature hot carrier solar cell: an issue or an opportunity?
No full textInternational audienceA hot-carrier solar cell (HCSC) is a high-efficiency photovoltaic concept where electrons and holes are at a higher temperature than the lattice. If this temperature is converted into voltage through a thermoelectric conversion, these hot carriers can result in higher cell efficiency, possibly allowing to reach the thermodynamic limit of 86% [1]. This gives the two requirements for a HCSC: establishing a hot-carrier population and converting the temperature into extra voltage through energy-selective contacts. On the first aspect, one should consider the generation of the hot carriers, and the design of absorbers that can make this generation easier. In most previous approaches to hot carrier solar cell, e.g. [2], carriers are assumed to form quasi thermal distributions with temperature larger than that of the lattice (Te = Th > Tamb). However, because different carriers may have different effective masses and different coupling with the phonons, the possibility of having Te ≠ Th must be addressed [3]. We present here the how such a situation can be modelled, experimentally studied and we present then a characterization of a such two-temperatures absorber [4] and the impact on the operation of HCSC [5].We propose a purely optical method which allows the direct and distinct estimation of electron and hole temperatures in steady state, published in [4]. This technique, based on photoluminescence, relies on the precise determination of the band-filling signature. We apply this technique to an InGaAsP single quantum well. Electron temperature surpasses 1000 K at largest excitation intensity, while holes remain colder, close to lattice temperature. Nonetheless, the increase in hole temperature is too large to be explained purely by photon absorption, demonstrating an energy transfer from electrons to holes (fig 1). On the second aspect, we address the question of the operation of a HCSC in the regime where electrons are hotter than holes [5]. For that purpose, we develop a two-temperature HCSC model and study its efficiency and sensitivity to the energy-selective contact design. We show that the two-temperature HCSC is always more efficient than the one-temperature one (fig 2). We also present how the theory can be extended to include these cases and discuss what are the relevant key parameters to be considered in such a situation.Actual hot carrier devices would deviate from ideal assumptions of the initial idea of Ross and Nozik [1]. We will finally explore how resilient is the hot carrier device concept to such deviations.[1] R. T. Ross and A. J. Nozik. ‘Efficiency of hot-carrier solar energy converters’. Journal of Applied Physics, vol. 53, no. 5, pp. 3813–3818 (1982)[2] A. Lebris et al. APL 2010[3] F. Gibelli, L. Lombez and J.-F. Guillemoles. ‘Two carrier temperatures non-equilibrium generalized Planck law for semiconductors’. Physica B: Condensed Matter, vol. 498, pp. 7–14 (2016)[4] Thomas Vezin, Nathan Roubinowitz, Laurent Lombez, Jean-François Guillemoles, and Daniel Suchet, ‘Direct determination of electron and hole temperatures from continuous-wave photoluminescence measurement’, Physical Review B (2024)[5] Thomas Vezin, PhD thesis, chap. 6 ‘Operation, design and resilience of a two-temperature hot carrier solar cell’ (2024
Assessing soil health at former thermal power plant sites
No full textInternational audienceSoils inherited from heavy industrial activities are often perceived as highly degraded.However, their health status is more diverse than commonly assumed, and appropriatemethods for evaluating the health of industrial soils remain largely underexplored.Former thermal power plants, in particular, present a wide variety of soil types due to thediversity of activities they once hosted, including: (i) areas with reinforced foundationsfor boilers (plant foundations); (ii) oil storage zones (oil deposits); (iii) coal storage areason concrete slabs (coal deposits); and (iv) ash disposal sites for bottom ash and fly ash(bottom-ash and ash deposits).This study aims to characterize the diversity of soil health states at former thermalpower stations by assessing their physical, chemical, and biological properties. Twodecommissioned thermal power plants with similar geological contexts were selected.Within each site, zones corresponding to different former industrial uses were identified.In total, twelve 100 m² plots of vegetation and soil, considered internally homogeneousbut distinct from one another, were selected. Across these plots, twelve soil profiles and68 soil samples (from surface layers and horizons) were collected and analysed for awide range of physical, chemical and biological parameters, including plant communities,nematodes, and microbial populations.Evaluation of the twelve profiles revealed that each soil exhibited unique characteristics.Even areas with similar historical uses displayed differences in the number of soilhorizons, rooting depth, and organic carbon stocks—some soils even holding nearlytwice the carbon stock compared to their control counterparts. Our results highlightthat soil degradation cannot be inferred solely from the type of past industrial activity.A functional, multi-parametric analysis is essential to accurately assess the potentialof these soils for future uses. Furthermore, the integration of biological indicatorsalongside physico-chemical properties proved crucial, especially for evaluating dynamicfunctions such as soil fertility and greenhouse gas storage and sequestration. Ultimately,considering entire soil profiles rather than focusing solely on the surface layer revealsthat the inherited vertical heterogeneity of highly anthropized soils has a major impacton their health status
Conformal Approach to Gaussian Process Surrogate Evaluation with Coverage Guarantees
No full textInternational audienceGaussian processes (GPs) are a Bayesian machine learning (ML) approach widely used to construct surrogate models for the uncertainty quantification (UQ) of computer simulation codes in industrial applications. It provides both a mean predictor and an estimate of the posterior prediction variance, the latter being used to produce Bayesian credibility intervals. Interpreting these intervals relies on the Gaussianity of the simulation model and the well-specification of the priors, which may not be appropriate. We propose to address this issue with the help of conformal prediction (CP), which is a finite-sample and distribution-free technique for estimating prediction intervals with marginal coverage guarantees. In the present work, a method for building adaptive cross-conformal prediction intervals is proposed by weighting the nonconformity score with the posterior standard deviation of the GP. The resulting CP intervals exhibit a level of adaptivity akin to Bayesian credibility sets and display a significant correlation with surrogate model local approximation error while being free from the underlying model assumptions and having marginal frequentist coverage guarantees. These estimators can be used to evaluate the quality of a GP surrogate model and can assist a decisionmaker in choosing the best prior to the specific application of the GP. We illustrate the proposed method's performance through a panel of numerical examples based on various computer experiments, including the GP metamodeling of analytical functions and an expensive-to-evaluate simulator of the clogging phenomenon in steam generators of nuclear reactors
Optimized Schwarz Methods in Time for Discrete Transport Control
No full textWe investigate optimized Schwarz domain decomposition methods in time for the control of the 1D transport equation. In the case of an internal control over the whole domain, the optimization problem can be transformed into a system of two coupled PDEs. We then apply the time-domain decomposition (without overlap) strategy on this PDE system as well as on its discretized counterpart. Under Fourier analysis, we analyse three different iterations: the fixed point iteration, the relaxed iteration and the preconditioned GMRES method. For each case, we propose parameters for the transmission conditions that lead to fast convergence of the method. We illustrate our results by numerical examples
Velocity-pressure coupling using an algebraic fractional step algorithm in low-order hybrid face-based discretizations
No full textWe investigate an evolution of the fractional step algorithm for the velocity/pressure coupling occuring when solving the Navier-Stokes equations. This work takes place in the context of face-based Compatible Discrete Operator (CDO) schemes which are low-order hybrid face-based discretizations and are well-suited to handle distorted and/or polyhedral meshes. A straightforward derivation of the fractional step algorithm is not applicable to these schemes since there is no mass matrix naturally associated to the velocity face unknowns. Therefore, we adopt an algebraic viewpoint relying on an approximated LU factorization of the original saddle-point system. We also devise an approximation of the Schur complement which naturally arises in this context. This strategy exhibits good performances. Several numerical tests in 2D and 3D are conducted to evaluate the accuracy, efficiency of the proposed algebraic fractional step (AFS) algorithm compared to the fully coupled approach at different Reynolds numbers. Finally, the algorithm is applied to simulate unsteady laminar flows past a cylinder. The results obtained fits well with existing literature data
Investigations méthodologiques et compréhension des dynamiques de transport des microplastiques dans la SeineRencontres 2025 du GDR Plastiques Environnement Santé, Juin 2025.
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