Portail HAL edf
Not a member yet
11029 research outputs found
Sort by
Non-asymptotic confidence intervals for importance sampling estimators of quantiles
No full textInternational audienceBuilding a confidence region (asymptotic or non-asymptotic) is crucial in understanding the quality of point estimators of a distribution. In this presentation, we estimate a quantile of a real random variable in the case where only a sample from another dominating distribution is available. This estimation procedure is known as importance sampling. A CLT is proved for the quantile estimator but the asymptotic variance depends on the true quantile, the unknown, and on its cumulative distribution function. We lift this barrier by building a non-asymptotic confidence interval for the true quantile which can be useful when only a limited sample size is available
Off-the-grid prediction and testing for linear combination of translated features
No full textWe consider a model where a signal (discrete or continuous) is observed with an additive Gaussian noise process. The signal is issued from a linear combination of a finite but increasing number of translated features. The features are continuously parameterized by their location and depend on some scale parameter. First, we extend previous prediction results for off-the-grid estimators by taking into account here that the scale parameter may vary. The prediction bounds are analogous, but we improve the minimal distance between two consecutive features locations in order to achieve these bounds. Next, we propose a goodness-of-fit test for the model and givenon-asymptotic upper bounds of the testing risk and of theminimax separation rate between two distinguishable signals. Inparticular, our test encompasses the signal detectionframework. We deduce upper bounds on the minimal energy,expressed as the -norm of the linear coefficients, tosuccessfully detect a signal in presence of noise. The generalmodel considered in this paper is a non-linear extension of theclassical high-dimensional regression model. It turns out that,in this framework, our upper bound on the minimax separationrate matches (up to a logarithmic factor) the lower bound on theminimax separation rate for signal detection in the highdimensional linear model associated to a fixed dictionary offeatures. We also propose a procedure to test whether thefeatures of the observed signal belong to a given finitecollection under the assumption that the linear coefficients mayvary, but have prescribed signs under the nullhypothesis. A non-asymptotic upper bound on the testing risk isgiven.We illustrate our results on the spikes deconvolution model with Gaussian features on the real line and with the Dirichlet kernel, frequently used in the compressed sensing literature, on the torus
Experimental Evidence Reveals Significant Divergence Between Real and Modeled Mold Growth on plasterboard
No full textInternational audienc
Contrasting effects of urban trees on air quality: From the aerodynamic effects in streets to impacts of biogenic emissions in cities
No full textInternational audienceUrban trees are often not considered in air-quality models although they can significantly impact the concentrations of pollutants. Gas and particles can deposit on leaf surfaces, lowering their concentrations, but the tree crown aerodynamic effect is antagonist, limiting the dispersion of pollutants in streets. Furthermore, trees emit Biogenic Volatile Organic Compounds (BVOCs) that react with other compounds to form ozone and secondary organic aerosols. This study aims to quantify the impacts of these three tree effects (dry deposition, aerodynamic effect and BVOC emissions) on air quality from the regional to the street scale over Paris city. Each tree effect is added in the model chain CHIMERE/MUNICH/SSH-aerosol. The tree location and characteristics are determined using the Paris tree inventory, combined with allometric equations. The air-quality simulations are performed over June and July 2022. The results show that the aerodynamic tree effect increases the concentrations of gas and particles emitted in streets, such as NOx (+4.6 % on average in streets with trees and up to +37 % for NO2). This effect increases with the tree Leaf Area Index and it is more important in streets with high traffic, suggesting to limit the planting of trees with large crowns on high-traffic streets. The effect of dry deposition of gas and particles on leaves is very limited, reducing the concentrations of O3 concentrations by −0.6 % on average and at most −2.5 %. Tree biogenic emissions largely increase the isoprene and monoterpene concentrations, bringing the simulated concentrations closer to observations. Over the two-week sensitivity analysis, biogenic emissions induce an increase of O3, organic particles and PM2.5 street concentrations by respectively +1.1, +2.4 and + 0.5 % on average over all streets. This concentration increase may reach locally +3.5, +12.3 and + 2.9 % respectively for O3, organic particles and PM2.5, suggesting to prefer the plantation of low-emitting VOC species in cities
暴露于极低频电场中的心脏植入物的干扰电压测量与分析
No full textInternational audienceObjective. The possibility of interference by electromagnetic fields in the workplaces with cardiac implants is a concern for both individuals and employers. This article presents an analysis of the interference to which cardiac implants are subjected under high-intensity electric field at the power frequency. Approach. Evaluations of interference were conducted by studying the induced voltages at the device input in the real case study and the substitute study, and establishing an association between them with the equivalence factor F. A funnel-shaped phantom, designed for in vitro testing and representing the electrical characteristics of the locations where cardiac implants are installed, was used in the substitute study. A measuring system was implemented to measure the induced voltage at the device input under high intensity electric fields. Main results. The induced voltages obtained in the experimental measurements align with the findings of the numerical study in the phantom. By applying the equivalence factors derived between the real case study and the substitute study (2.39 for unipolar sensing; 3.64 for bipolar sensing), the induced voltages on the cardiac implants can be determined for the real case using the substitute experimental set-up. Significance. The interference voltages on the cardiac implants under EF exposures at low frequency were experimentally measured with detailed description. The findings provide evidence for an analysis method to systematically study the electromagnetic interference on the cardiac implants at low frequency.Objectif. La possibilité d'interférence par des champs électromagnétiques sur les lieux de travail avec des implants cardiaques est une préoccupation pour les individus et les employeurs. Cet article présente une analyse de l'interférence à laquelle les implants cardiaques sont soumis sous un champ électrique de haute intensité à la fréquence de puissance. Approche. Les évaluations des interférences ont été réalisées en étudiant les tensions induites à l'entrée de l'appareil dans l'étude de cas réelle et l'étude de substitution, et en établissant une association entre elles avec le facteur d'équivalence F. Un fantôme en forme d'entonnoir, conçu pour les tests in vitro et représentant les caractéristiques électriques des endroits où les implants cardiaques sont installés, a été utilisé dans l'étude de substitution. Un système de mesure a été mis en place pour mesurer la tension induite à l'entrée de l'appareil sous des champs électriques de haute intensité. Principaux résultats. Les tensions induites obtenues lors des mesures expérimentales sont en accord avec les résultats de l'étude numérique dans le fantôme. En appliquant les facteurs d'équivalence dérivés entre l'étude de cas réelle et l'étude de substitution (2,39 pour la détection unipolaire; 3,64 pour la détection bipolaire), les tensions induites sur les implants cardiaques peuvent être déterminées pour le cas réel en utilisant le dispositif expérimental de substitution. Signification. Les tensions d'interférence sur les implants cardiaques exposés aux champs électriques à basse fréquence ont été mesurées expérimentalement avec une description détaillée. Les résultats fournissent des preuves pour une méthode d'analyse permettant d'étudier systématiquement l'interférence électromagnétique sur les implants cardiaques à basse fréquence.摘要目的。工作场所的电磁场对心脏植入物的干扰可能性是个人和雇主关心的问题。本文分析了心脏植入物在高强度电场下受到的干扰情况。方法。通过研究实际案例研究和替代研究中设备输入端的感应电压，并通过等效因子F将它们关联起来，进行干扰评估。在替代研究中使用了一个漏斗形模型，该模型设计用于体外测试，代表了心脏植入物安装位置的电气特性。我们建立了一个测量系统，用于测量高强度电场下设备输入端的感应电压。主要结果。实验测量中获得的感应电压与模型仿真中的数值研究结果一致。通过应用实际案例研究和替代研究之间得出的等效因子（单极导线为2.39；双极导线为3.64），可以使用替代实验装置确定实际案例中心脏植入物的感应电压。意义。详细描述了低频电场曝光下心脏植入物的干扰电压的实验测量。研究结果为系统分析低频电磁干扰对心脏植入物的影响提供了证据
A note on the relaxation process in a class of non-equilibrium two-phase flow models
No full textInternational audienceWe focus here on the relaxation process in a class of two-phase flow models, considering first gas-liquid flows, and then liquid-vapour mixtures. The whole analysis enables to exhibit a few conditions on the flow in order to guarantee the time decay of some variables. The former may depend on initial conditions but also on equations of state within each phase. The present analysis aims at providing some better understanding of inner processes, and it is also useful for numerical purposes, as emphasized in appendix B. It is a sequel of paper [J.M. Hérard and G. Jomée, ESAIM Proc. Surv., 72:19–40, 2023] where the sole pressure relaxation process in some multiphase flow models is investigated
Experimental assessment of high-energy high nickel-content NMC lithium-ion battery cycle life at cold temperatures
No full textInternational audienceAn essential element for the diffusion and environmental impact assessment of electrochemical storage systems is their lifetime. This lifetime also impacts the overall cost of the equipment in which the storage system is used. At the same time, a number of applications, e.g. electric vehicles, do not allow a temperature control of the batteries, although the performances of the latter are very strongly linked to them. The phenomena investigated, such as aging, behavior at low temperature and rapid charge for low temperature, are complex and difficult to model. Only the establishment of databases representative of the conditions of use can lead to elements of response. The exploitation of these databases by artificial intelligence tools is certainly relevant but requires a learning on data representative of the conditions of use and of the phenomena to be evaluated.The work presented in this paper is in this context and is intended to model the influence of the rapid charge at low temperature (0 °C, −10 °C, −20 °C) on the lifetime (initial capacity loss) of lithium-Ion NMC elements. The problem with data-based approaches is the large number of tests required and the development in relevant experimental designs. Faced to this challenge, a consortium of French laboratories and manufacturers has been working together since the 2000s to carry out this type of work. The paper proposes to present a test campaign and summarizes the main results to assess the service life (loss of capacity) for a fast-charging application at low temperature and position them against the manufacturer data established at 25 °C. The impact of two partial discharge processes (50 %) will also be assessed. In a last part, we show that for low temperatures, even for the nominal charge current the life time is reduced to few dozens of cycles and a mechanical destruction of the cell with no activation of the current interrupt device is achieved. This study demonstrates and quantifies the very important impact of low-temperature charging processes on the lifetime of high energy lithium-ion NMC batteries
Multivariate State And Parameter Estimation With Data Assimilation Applied To Sea-Ice Models Using A Maxwell Elasto-Brittle Rheology
No full textInternational audienceIn this study, we investigate the fully multivariate state and parameter estimation through idealised simulations of a dynamics-only model that uses the novel Maxwell elasto-brittle (MEB) sea-ice rheology and in which we estimate not only the sea-ice concentration, thickness and velocity, but also its level of damage, internal stress and cohesion. Specifically, we estimate the air drag coefficient and the so-called damage parameter of the MEB model. Mimicking the realistic observation network with different combinations of observations, we demonstrate that various issues can potentially arise in a complex sea-ice model, especially in instances for which the external forcing dominates the model forecast error growth. Even though further investigation will be needed using an operational (a coupled dynamics–thermodynamics) sea-ice model, we show that, with the current observation network, it is possible to improve both the observed and the unobserved model state forecast and parameter accuracy
Le projet Overcome : comprendre et modéliser les processus d’érosion par surverse des digues et barrages en remblai constitués de matériaux grossiers à granulométries étalées
No full textInternational audiencePredicting overflowing erosion of levees and embankment dams is one of the major difficulties in assessing the safety of these structures. Although it has been identified as the main failure mechanism in almost half of all levee failures, the methods and tools available to engineers to characterize it deterministically are still very limited. In particular, no numerical modelling tool validated to the required level is yet available to characterize the erosion process in geometric and temporal terms, taking into account the behavior of the soil material constituting the embankment and its foundation and the hydraulic loadings specific to levees. For the time being, therefore, engineers have to make do with empirical approaches: so-called “expert judgement” criteria or empirical formulae. These highly simplified approaches are known for their high discrepancies and their inability to guarantee systematically conservative results. It was with the ultimate aim of significantly improving the robustness and accuracy of overflowing erosion prediction tools for engineers that EDF and CNR decided to launch the Overcome research project. The aim of this project is to include, within the TELEMAC-2D open-source digital platform, modules representing different types of physical processes associated with levee overflowing, based on a highly advanced multi-scale experimental approach. This paper presents the very first results of one of the first subjects investigated by this project: the description of the physical mechanisms of overflowing erosion of embankments constituted with coarse gap-graded soils. These soils are found in a large proportion of levees located in mountain areas or in valleys just downstream. An initial series of small-scale overflow tests was carried out in the laboratory of the Polytechnic University of Madrid, using homogeneous embankments 55cm high constituted with three different materials: fairly uniform sand, granite gravel and alluvial coarse gap-graded soil from the CNR site at Montfaucon. While the tests with the sand embankments confirmed the expected surface erosion mechanism, it was quite surprising to see significant differences in the erosion mechanism between the Montfaucon and granite gravels materials. In the Montfaucon material, the mechanism resembles surface erosion, whereas with the granite gravel, the erosion mechanism begins with “headcut migration” and then evolves into surface erosion. In addition, the Montfaucon soil proved to be four times less erodible than the granite gravel. These initial results need to be confirmed by additional small-scale repeatability tests and larger-scale tests.La prédiction de l’érosion par surverse des digues de protection contre les crues et des barrages en remblai constitue l’une des difficultés majeures dans l’analyse de la sûreté de ces ouvrages. Bien qu’identifié comme étant le mécanisme de rupture dominant de près de la moitié des ruptures de digues fluviales, les méthodes et outils dont disposent les ingénieurs pour le caractériser, de manière déterministe, restent encore très limités. Notamment, la caractérisation du processus d’érosion, en termes géométriques et temporels, prenant en compte le comportement des matériaux constitutifs du remblai et de sa fondation et les spécificités des sollicitations hydrauliques sur les digues, n’est encore à la portée d’aucun outil de modélisation numérique validé au niveau requis. Les ingénieurs doivent donc, pour l’instant, se contenter d’approches recourant largement à l’empirisme : critères dits « de jugement d’expert » ou formules empiriques. Ces approches très simplifiées sont connues pour présenter de fortes incertitudes et ne pas garantir des résultats systématiquement du côté de la sécurité. C’est dans le but final d’améliorer significativement la robustesse et la précision des outils de prédiction de l’érosion par surverse pour les ingénieurs, qu’EDF et CNR ont décidé de lancer le projet de recherche Overcome. L’objectif de ce projet est d’inclure au sein de la plateforme numérique open source TELEMAC-2D des modules représentant différents types de processus physiques en lien avec les ruptures par surverse de digues, basés sur une approche expérimentale multi-échelles très poussée. Cette communication présente les tous premiers résultats de l’un des premiers sujets investigués par ce projet : la description des mécanismes physiques d’érosion par surverse des remblais constitués de sol grossiers à granulométrie étalés. Ces sols sont présents dans une grande partie des digues situées en zones montagneuses ou dans les vallées situées juste en aval. Une première série d’essais de surverse à petite échelle a été menée au laboratoire de l’Université Polytechnique de Madrid, en réalisant des remblais homogènes de 55 cm de hauteur à partir de trois matériaux différents : un sable assez uniforme, un gravier de granite et un sol alluvionnaire à granulométrie étalée, issu du site CNR de Montfaucon. Si les essais avec le remblai en sable ont confirmé le mécanisme attendu d’érosion de surface, il a été assez surprenant de voir des différences significatives de mécanisme d’érosion entre les matériaux de Montfaucon et graviers de granite. Dans le matériau de Montfaucon, le mécanisme s’apparente à de l’érosion de surface, tandis qu’avec le gravier de granite, le mécanisme d’érosion débute avec du « Headcut migration » puis évolue à la fin en érosion de surface. Par ailleurs, le sol de Montfaucon s’est avéré quatre fois moins érodable que le gravier de granite. Ces premiers résultats appellent à être confirmés par des essais de répétabilité à petite échelle et des essais à plus grande échelle
Multi-scale analysis of the effects of hysteresis on the hydrothermal behaviour of bio-based materials: Application to hemp concrete
No full textInternational audienceExperimental measurements of sorption isotherms show a difference between the water contents in theadsorption phase and the desorption one. This is mainly due to the hysteresis phenomenon. Not considering thisphenomenon in hygrothermal behaviour models impacts the reliability of the latter. In this paper, we havestudied the effects of hysteresis on the hygrothermal behaviour at the building scale. For this purpose, a newimplementation method of a hysteresis model was proposed and validated at the material and wall scales using abi-climatic chamber which controlled the indoor and outdoor climate conditions. Then, the effect of the hysteresisphenomenon at the building scale was evaluated following a COMSOL/TRNSYS co-simulation approach toconsider the gains and losses in the hydric fluxes going through walls. The results showed significant differencesin terms of the relative humidity of the living environment. However, no difference was noticed in the temperaturevariations. On the other hand, the total heat fluxes were also affected by the hysteresis phenomenon dueto latent heat parts, that were driven by variations in relative humidity. The maximum gain and loss discrepancies(with and without hysteresis) in total heat fluxes of 1121 kJ/h and 1086 kJ/h, respectively, were obtainedduring the first week of February (winter period). The results also revealed that these discrepancies are sensitiveto the number of persons in the living environment