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Optimal physical experimental designs to calibrate functional Weibull models under cost constraints. Application to a structural reliability problem
No full textInternational audienceWe focus on the specific issue of identifying (static) optimal physical experimental designs when the aim is to calibrate functional Weibull models in an informed context and under cost constraints. In this nonlinear parametric context, a major difficulty is that the exact evaluation of the expected utility function to maximize is intractable. Among the proposed approaches to overcome this difficulty, one can distinguish near optimal Bayesian experimental designs defined by maximizing either an analytical approximation or a numerical estimation of the expected utility. These latest approaches are based on quadratic techniques or simulations produced by stochastic algorithms. They remain computationally intensive, which makes them often limited in practice to very simple designs. In this work, we explore the potential of the Bayesian approach to account for legacy data as a prior informative sample when designing physical experiments. We propose generic design criteria based on a weighted combination of utility functions to balance the statistical gain yielded by a given design and the prospective experimental cost, in an understandable way for the engineer. We consider different utility functions to quantify the statistical gain yielded by a given design when fitting Bayesian functional Weibull models. Finally, we derive analytical asymptotic approximations of the associated expected utility functions that we maximize with a simulated annealing algorithm. Our method is applied to a structural reliability problem informed by a real dataset of U.S. ferric steels. The design problem consists in selecting an optimal number of temperature levels, optimal values of temperature levels and optimal numbers of Charpy destructive impact tests to allocate to each temperature level, in order to produce the most precise estimates of the parameters defining the functional Weibull distribution of steel fracture toughness for a French production park under cost constraints
Étude hydrodynamique des courses de natation en eau libre
No full textThis works present an hydrodynamic study of open-water swimming races. It is divided in 2 parts. Firstly, we study the interactions between swimmers (called drafting) in groups of 2 or more athletes. Thanks to experimental measurements and Computational Fluid Dynamics calculations, we are able to determine the impact of the relative position of swimmers on the drag they face. Those results allow us to compute an optimal trajectory to overtake an opponent. Secondly, we determine the impact of the currents, in the Seine river, on the strategy to use during the Paris Olympic Games. Our work proves the existence of an optimal trajectory along the path of the race. By making the most of advantageous currents and by avoiding adverse ones, it minimizes the race duration for an isolated swimmer. We also show the existence of an optimal way to spend energy during the race.Ce travail présente une étude physique des courses de natation en eau libre. Deux grands sujets sont abordés. D'abord, nous étudions les interactions entre nageurs (drafting), dans des configurations de nage à 2 athlètes et en peloton. Grâce à des mesures expérimentales et des calculs numériques, nous sommes capable d'évaluer l'impact de la position relative des nageurs sur la force de traînée qu'ils subissent. Ces résultats nous permettent de proposer une trajectoire optimale de dépassement. Ensuite, nous nous intéressons à l'impact des courants de la Seine sur la stratégie à mener lors de la course des Jeux Olympiques de Paris. Nos travaux montrent qu'il existe une trajectoire optimale le long du parcours, qui permet, en profitant des courants favorables et en évitant les courants les plus défavorables, de minimiser le temps de course d'un nageur. De même, il existe aussi une stratégie optimale de dépense énergétique qui permet à un nageur de minimiser son temps de course
Etude des stocks de carbone organique du sol de prairies en cours de réhabilitation dans le Sud de la France
No full textInternational audienc
Vibrations sous chargement turbulent en géométrie cylindrique avec admissions impactantes : Application à un espace annulaire de cuve de réacteur à eau pressurisée
No full textPressurized water reactor vessels are prone to vibrations, due in particular to the flow in the gap between the vessel and the core barrel: understanding and controlling them is both a scientific and a safety issue.The conventional description of these vibrations is based on a reference fluctuating pressure spectrum, on longitudinal and circumferential correlation lengths, and on a modal forcing based on the joint acceptance integral. It assumes a homogeneous turbulent pressure on the inner cylinder surface, which is a priori not adapted to the industrial configuration, with three or four inlet branches.Using a coupled experimental-numerical approach, the aim is to quantify the consequences of this simplification and improve the representation of forcing, based on an elementary reference case consisting of two co-axial cylinders submitted to two impinging jets perpendicular to the cylinder axis.On a modular water mock-up built-up for that purpose, after characterizing the turbulent forcing with a fixed inner cylinder using fluctuating wall pressure sensors, the vibrations obtained by arranging the inner cylinder on flexible plates are analyzed: the strong inhomogeneity of convection velocities (between 0 and 4 m/s), as well as of fluctuating pressure spectra (several orders of magnitude depending on frequency) and of their correlation lengths (different evolutions with frequency and variations up to a factor two) numerically predicted, is actually observed, with a particularly remarkable calculation-test agreement.These LES simulation and test results thus constitute a well-documented reference test case for flow-induced vibrations in annular gap configurations with impinging inlets. It will be a basis for future numerical modelling and experimental mock-ups representing more complex configurations.Les cuves de réacteurs à eau pressurisée subissent des vibrations, dues notamment à l’écoulement dans l'espace séparant la cuve de l’enveloppe de cœur : leur compréhension et leur maîtrise constituent un enjeu tant scientifique que de sûreté.La description conventionnelle de ces vibrations repose sur un spectre de pression fluctuante de référence, des longueurs de corrélation longitudinales et circonférentielles, et l’élaboration d’un chargement modal sur la base de l’intégrale de joint acceptance. Elle suppose une pression turbulente homogène sur la surface du cylindre interne, hypothèse a priori non adaptée à la configuration industrielle, avec la présence de trois ou quatre branches d'admission.Via une démarche couplée expérimentale – numérique, l’objectif est de quantifier les conséquences de cette simplification et d’améliorer la représentation du chargement, en s’appuyant sur un cas de référence élémentaire, constitué de deux cylindres co-axiaux soumis à deux jets impactants perpendiculaires à l’axe des cylindres.Sur une maquette en eau modulable conçue à cet effet, après la caractérisation, au moyen de capteurs de pression fluctuante en paroi, du chargement turbulent avec cylindre interne fixe, les vibrations obtenues en disposant le cylindre interne sur des languettes flexibles sont analysées : la forte inhomogénéité des vitesses de convection (entre 0 et 4 m/s), ainsi que des spectres de pression fluctuante (plusieurs ordres de grandeur selon la fréquence) et de leurs longueurs de corrélation (évolutions différentes selon la fréquence et variations du simple au double) numériquement prédite, est effectivement observée, selon un accord calculs-essais particulièrement remarquable.L’ensemble des résultats d’essais et de simulations LES constitue ainsi un cas-test de référence bien documenté de vibrations sous écoulement turbulent en espace annulaire avec admissions impactantes. Il servira de base pour des modélisations numériques et un moyen d’essai représentant des configurations plus complexes
Fissuration par champ de phase : Vers une méthode générique pour suivre le chemin d'équilibre de la structure
No full textInternational audienc
Nothing has changed, everything has changed. The surge in public opinion in its support for nuclear energy in France and Europe (2020-2024)
No full textInternational audienceFrance may be the most nuclearized country in the world, support from French public opinion has gradually but largely eroded over the last decades. However, since 2020, the support has considerably risen to reach a rate 60% agreeing to use nuclear energy in the future. How can one explain such a shift in such a short time?Using longitudinal European and French opinions polls from 2020 to 2024 as well as web-data taken from the French-speaking X (ex-Twitter), we mobilize the RAS model (Zaller, 1992 - RAS standing for Receive-Accept-Sample, a model which assumes that the opinions expressed reflect the messages received and accepted that are most salient at the time of the survey, and that there are no real structured political belief systems) as well as notions of ‘spiral of silence’ (Noelle-Neumann, 1984), ‘focusing event’ (Birkland, 1997; Kingdon, 1995) or ‘latent public opinion’ (Warshaw, 2017). The French surveys mobilized notably have an experimental aspect, trying to seek how strongly individuals hold to their opinions when presented with contradictory arguments.Part of those changes on a European scale can be explained by the geopolitical context opened by the war in Ukraine – understood as a focusing event whose ramifications weigh equally on all countries. Hence a contextual effect that can be measured in the French and European polls on the general public opinion. But some aspects of this evolution clearly depend on national events such as a reframing of energy policies intertwined with economical or employment during political elections. Data from X here shows the impact of local elections and then the presidential one on the French energy debate, the arguments used and the reconfigurations of interested communities. Initially restricted to communities of experts and politicized people, an argumentative effect can be measured in our surveys for the general public since 2023 – the same arguments being more performative in 2024 than in 2020. Hence, those moments constitute successful tests for nuclear energy in a general context more favourable than ever. In the end, we aim at explaining the chaining of sequences that overlap on local, national and European scales, with impacts the general public, and at understanding how public opinions may change so sharply and quickly even though technologies and infrastructures have not changed over this temporal sequence
CFD study of PM10 dispersion in a sports stadium using a mesh based on geometry obtained from a 3-D cloud of laser points
No full textInternational audienceThis work presents a dispersion study of a multi-sport stadium using local scale simulation (CFD) using the open-source software code_saturne. A recently developed time scheme for indoor airflow is used. A high-fidelity numerical mesh is built from a cloud of points and used. Besides providing the local dynamics in the stadium, simulations results are compared to experimental PM10 concentration data from a handball game, where firework were lighted, and a 0-D model. CFD results were shown to correctly reproduce the PM10 variation
A simplified ω-ALDF rank-correlated full-spectrum k-distribution model for combustion applications
No full textInternational audienceThe objective of this paper is to present a method that allows simplifying the use of ω-absorption line distribution functions (ω-ALDF) inside rank correlated full-spectrum k-distribution (RCFSK) models for application in combustion problems. In this simplified version, the ω-ALDF is constructed without any a priori information on the problem treated. It can be used directly but, in order to simplify further the concept for possible users, we suggest here approximating this ω-ALDF using an Absorption Line Blackbody Distribution Function (ALBDF) at a temperature defined in terms of the ω-ALDF. The method is validated in some combustion scenarios. The model is assessed by comparison with a narrow band correlated-k (NBCK) model through decoupled radiative simulations of eight turbulent axisymmetric non-premixed jet flames covering a wide range of optical-thicknesses and contributions of soot to radiation. The predictions are within 4 % of the reference solution. A consequence of the proposed approach is that the FSCK parameters, namely the absorption coefficient and the stretching function, depends only on local variables. This allows one to forgo the specification of an arbitrary blackbody source temperature, and to develop a specific storage strategy to provide an efficient model for Computational Fluid Dynamics (CFD) simulations of combustion problems
An introduction to Spent Nuclear Fuel decay heat for Light Water Reactors: a review from the NEA WPNCS
No full textInternational audienceThis paper summarized the efforts performed to understand decay heat estimation from existing spent nuclear fuel (SNF), under the auspices of the Working Party on Nuclear Criticality Safety (WPNCS) of the OECD Nuclear Energy Agency. Needs for precise estimations are related to safety, cost, and optimization of SNF handling, storage, and repository. The physical origins of decay heat (a more correct denomination would be decay power) are then introduced, to identify its main contributors (fission products and actinides) and time-dependent evolution. Due to limited absolute prediction capabilities, experimental information is crucial; measurement facilities and methods are then presented, highlighting both their relevance and our need for maintaining the unique current full-scale facility and developing new ones. The third part of this report is dedicated to the computational aspect of the decay heat estimation: calculation methods, codes, and validation. Different approaches and implementations currently exist for these three aspects, directly impacting our capabilities to predict decay heat and to inform decision-makers. Finally, recommendations from the expert community are proposed, potentially guiding future experimental and computational developments. One of the most important outcomes of this work is the consensus among participants on the need to reduce biases and uncertainties for the estimated SNF decay heat. If it is agreed that uncertainties (being one standard deviation) are on average small (less than a few percent), they still substantially impact various applications when one needs to consider up to three standard deviations, thus covering more than 95% of cases. The second main finding is the need of new decay heat measurements and validation for cases corresponding to more modern fuel characteristics: higher initial enrichment, higher average burnup, as well as shorter and longer cooling time. Similar needs exist for fuel types without public experimental data, such as MOX, VVER, or CANDU fuels. A third outcome is related to SNF assemblies for which no direct validation can be performed, representing the vast majority of cases (due to the large number of SNF assemblies currently stored, or too short or too long cooling periods of interest). A few solutions are possible, depending on the application. For the final repository, systematic measurements of quantities related to decay heat can be performed, such as neutron or gamma emission. This would provide indications of the SNF decay heat at the time of encapsulation. For other applications (short- or long-term cooling), the community would benefit from applying consistent and accepted recommendations on calculation methods, for both decay heat and uncertainties. This would improve the understanding of the results and make comparisons easier