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Experimental and Numerical Study of Film Boiling Around a Small Nickel Sphere
No full textInternational audienceThis work—mixing an original experimental approach, as well as numerical simulations—proposes to study film boiling modes around a small nickel sphere. While dealing with a simple looking phenomenon that is found in many industrial processes and has been solved for basic quenching regimes, we focus on describing precisely how vapor formation and film thicknesses, as well as vapor bubble evacuation, affect cooling kinetics. As instrumenting small spheres may lead to experimental inaccuracies, we optically captured, using a high-speed camera, the vapor film thickness at mid height, the vapor bubble volume, and the bubble detachment frequency, along with the heat flux. More precisely, an estimation of the instant sphere temperature, in different conditions, was obtained through cooling time measurement before the end of the film boiling mode, subsequently facilitating heat flux evaluation. We encountered a nearly linear decrease in both the vapor film thickness and vapor bubble volume as the sphere temperature decreased. Notably, the detachment frequency remained constant across the whole temperature range. The estimation of the heat fluxes confirmed the prevalence of conduction as the primary heat transfer mode; a major portion of the energy was spent increasing the liquid temperature. The results were then compared to finite element simulations using an in-house multiphysics solver, including thermic phase changes (liquid to vapor) and their hydrodynamics, and we also captured the interfaces. While presenting a challenge due to the contrast in densities and viscosities between phases, the importance of the small circulations along them, which improve the heat removal in the liquid phase, was highlighted; we also assessed the suitability of the model and the numerical code for the simulation of such quenching cases when subcooling in the vicinity of a saturation temperature
Rockfall detection using lidar point clouds: identification of geometric distortions during acquisition and proposed processing to enable a low detection threshold
No full textInternational audienceThe C2R-IA project (www.c2r-ia.fr) is aimed to better account for the influence of weather conditions on the level of rockfall hazards and to anticipate temporary increases in hazard levels during storms and other specific weather conditions, in order to implement risk mitigation systems (access restrictions, monitoring, mobilization of emergency kits, predictive maintenance). To achieve this, a database of rockfall events is built to train AI predictive models of rockfalls based on weather conditions. One of the monitoring technologies used is a terrestrial laser scanner with a RIEGL VZ-2000i long range 3D laser scanning system. Lidar point clouds are thus used to provide at several time intervals the 3D surface of the study site: the Saint-Eynard cliff, located northeast of Grenoble in the french Alps. From the lidar point cloud series, the goal is to compare the clouds to detect changes and identify rockfall events (Manceau et al, EGU 2025, oral presentaion). For a large and rich database, it is important to achieve very precise alignement between lidar point clouds to detect the smallest possible changes in our point clouds series (small rockfall volumes).In this context, a basic ICP (Iterative Closest Point) alignement reveals artefacts that need to be treated in a special way to achieve high-precision alignement. Geometric distortions are thus observed within the point clouds in the form of vertical strips. This phenomenon occurs at two scales:- Low frequency: observations of decimetric to multi-decimetric jumps with strip widths ranging from 10 to 100 meters during acquisitions from a tripod, a flexible support.- High frequency: observations of centimetric jumps with narrower strip widths (ranging from one to several meters) during acquisitions from a rigid base (reinforced concrete post).Several hypotheses are put forward and tested to explain the existence of these strips: machine-related mechanical issues, independent or dependent on time, interaction between the ground, support, and machine, changes in atmospheric conditions during the acquisition period (lasting 40 minutes), the geometry of the cliff and its local orientation relative to the lidar's line of sight.A processing method is proposed to overcome these geometric distortions during acquisition and maintain a low detection threshold when comparing two point clouds: this involves a new strip-based alignment of the two clouds before change detection. The first step is the extraction of strips from the compared cloud, then an independent alignment of each strip to the reference cloud is performed using the ICP method. Finally, the aligned strips are merged to form the new compared cloud : we reach a detection threshold of less than 10 cm (i.e. 10-4 times the measurement distance) whereas 40 cm has been previously used on the same site in the literature
Adaptive adjoint-based population-control methods for kinetic simulations in TRIPOLI-4
No full textInternational audienceTime-dependent Monte Carlo simulations for reactor kinetics applications require special variance-reduction and population-control techniques in order to efficiently cope with the typically huge imbalance between the respective time scales and population sizes of neutrons and precursors. Building upon the legacy implementation of the algorithms devoted to kinetics in the Monte Carlo code TRIPOLI-4, in this work we propose an adaptive adjoint-based population-control method that considerably improves the behaviour of time-dependent simulations. Thanks to a time-dependent importance-sampling scheme, based on the solution of the adjoint point-kinetics equations, neutron and precursor weights are continuously adjusted, which paves the way towards the simulation of previously unattainable reactor transients involving long times and large reactivity excursions. The computational effectiveness of the newly developed method is evaluated in terms of Figure of Merit (FoM) over a set of time-dependent scenarios encompassing the Flattop-Pu, SPERT III E-core and CROCUS benchmarks
Satellite-based mapping of annual canopy height and aboveground biomass in African dense forests
No full textInternational audienceAccurate maps of canopy height (CH) and aboveground biomass (AGB) are needed for monitoring forests over large regions. Producing such data is particularly challenging over the complex, diverse and dense humid tropical forests of Africa where signal saturation observed from optical and radar satellites and complex responses in LiDAR data require advanced mapping techniques to capture high biomass and tall height values. Here, we trained a deep learning (U-Net) model to generate the first annual maps (2019–2022) of top CH at 10 m resolution over the African dense forest region, using Sentinel-1/-2 images trained on LiDAR-derived height data from the Global Ecosystem Dynamics Investigation mission (GEDI). To predict AGB from CH on a 30-m grid, we calibrated allometric models combining AGB data from field inventories, CH from our map, and wood density from a new high-resolution (1 km) map. The CH map has a mean absolute error (MAE) of 4.54 m and an underestimation bias of 1.54 m compared to independent airborne LiDAR data (5.93 m and 1.40 m compared to independent GEDI data). Evaluation of the AGB map against independent measurements from field sites suggests an improved accuracy (MAE = 79.65 Mg/ha, bias = 6.47 Mg/ha) compared to recent datasets such as ESA-CCI, NCEO, and GEDI L4B. Our map also captures the large-scale spatial gradients of AGB across African dense forests, as observed in a comprehensive dataset of forest concession measurements aggregated at a 1-km scale. Interpretable machine learning was used to assess the contribution of ancillary variables (e.g., climate, soil, forest type) to biomass prediction. While some variables were relevant, their inclusion failed to improve AGB estimates in high and low biomass extremes and introduced spatial artifacts, limiting their utility for consistent annual mapping. Together, our annual CH and AGB maps offer an open, scalable tool for monitoring forest disturbances and interannual biomass dynamics. Future work will focus on refining biomass–height relationships to further improve AGB estimation
Feature Calibration for Computer Models
No full textInternational audienceComputer model calibration involves using partial and imperfect observations of the real world to learn which values of a model’s input parameters lead to outputs that are consistent with real-world observations. When trying to calibrate to high-dimensional output (e.g., a spatial field), what is important to the credibility of the model is that key emergent physical phenomena are represented, even if not faithfully or in the right place. Commonly used approaches, which represent the output as a linear combination of a small set of basis vectors, often fail to appropriately compare model output and data when the position of key emergent phenomena shifts, consequently leading to poor model calibration. To overcome this, we present kernel-based history matching (KHM), generalizing the meaning of the technique sufficiently to be able to project model outputs and observations into a higher-dimensional feature space, where patterns can be compared without their location necessarily being fixed. We develop the technical methodology, present an expert-driven kernel selection algorithm, and then apply the techniques to the calibration of boundary layer clouds for the French climate model IPSL-CM
Modélisation numérique du chaînage d’agrafe des murs gouttereaux de Notre-Dame de Paris : Vers une nouvelle considération technique des fers de construction à l’époque gothique
No full textNational audienceLes travaux de restauration de Notre-Dame de Paris, suite à l’incendie du 15 avril 2019, ont conduit les différentes équipes de chercheurs du Chantier scientifique CNRS/MC Notre-Dame à explorer l’histoire du monument, notamment en remettant au jour l’usage des fers de construction à l’époque gothique. Les travaux de modélisation menés de concert avec les membres du Groupe de Travail « Structure » du Chantier scientifique et MiMeTICS engineering ont permis une reconsidération technique de ce métal au sein des maçonneries de la cathédrale. Ces fers architecturaux, utilisés principalement dans la perspective de liaisonner ou fixer les éléments lithiques entre eux, représentent des composants essentiels à prendre en compte dans la structure des édifices. Cette démarche, qui vient compléter le travail des bureaux d’études techniques à la demande de la maîtrise d’ouvrage, offre un regard supplémentaire sur des structures confrontées à d’importants enjeux patrimoniaux et à de réelles difficultés techniques pour appréhender les phases de confortation et de restauration.L’exemple présenté lors de cette communication porte sur la modélisation numérique des agrafes de la partie sommitale des murs gouttereaux, invisibles avant l'incendie de Notre-Dame de Paris. Ces éléments métalliques viennent constituer une ceinture de fer sur la partie haute de la cathédrale, où la pierre laisse place au bois. La démarche de modélisation a pour but de comprendre le rôle structurel de ces agrafes et s’articule en deux temps. D’abord une approche par éléments finis multiphasiques sans contact sur un Volume Élémentaire Représentatif (VER) est mise en œuvre pour définir les lois de comportement, ainsi que les conditions aux limites et sollicitations. Par la suite, les approches développées sur le VER permettent un passage à un modèle par la méthode des éléments discrets intégrant une loi de contact entre blocs pour traiter les interactions aux interfaces des différents matériaux du modèle (agrafe en fer, scellement en plomb, pierre et joints de mortier).Les travaux sur Notre-Dame de Paris ont servi de précurseur, démontrant que le modèle développé pouvait être réutilisé. Cette approche s’est notamment révélée pertinente pour la restauration de la flèche de la basilique Saint-Michel à Bordeaux, où des éléments en fer sont aussi présents dans la structure de cette partie de l’édifice. Dans cette optique, il est essentiel de poursuivre cette démarche collaborative lors de la restauration de tel monuments historiques
On the techniques for primary calibration of electronic radon detectors
No full textInternational audienceSuccessful calibration of RadonEye +2 electronic radon detectors was performed at typical indoor activity concentrations using the facilities at LNHB and PTB. The calibration uncertainties using primary radon activity standards were: below 1.5 % at 300 Bq/m3, below 1.7 % at 130 Bq/m3 and below 2.5 % at 55 Bq/m3 (k = 1). When using the secondary standard AlphaGUARD, the uncertainty at 55 Bq/m3 was below 3.5 %. Maintaining stable activity concentrations proved crucial and appears to be the only feasible approach for calibrations below 100 Bq/m3. While calibration under exponentially decaying radon activity concentration remains useful for evaluating the devices’ linearity across a broad range, it proved unsuitable for calibration of user-grade monitors at low activities due to the high statistical variation in their signal.The linearity of RadonEye +2 was demonstrated in the range 50 Bq/m3 - 300 Bq/m3 and they will be utilized for the sensor networks developed within the RadonNET project. Dynamic background correction, applicable to non-spectrometric detectors, was applied based on the monitor's exposure history. Furthermore, it was observed that the pulse-processing algorithm of RadonEyes +2 distorts the Poisson distribution of the signal, thereby increasing its variation. Potentially, lower measurement uncertainty could be achieved with electronic radon detectors that report the registered pulses and allow access to their processing algorithms
Oral supplementation with Algae extract enhances the humoral response to attenuated PRRSV-1 vaccine
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The Composite Spectrum of QSO Absorption Line Systems in DESI DR2
No full textInternational audienceWe present details regarding the construction of a composite spectrum of quasar (QSO) absorption line systems. In this composite spectrum we identify more than 70 absorption lines, and observe oxygen and hydrogen emission features at a higher signal-to-noise ratio than in any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium environment of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 238,838 quasar spectra from the second data release of the Dark Energy Spectroscopic Instrument (DESI), each identified to have absorption lines resulting from such an interaction. By stacking these spectra in the restframe of the absorption, and calculating a median composite spectrum, we are able to isolate and enhance these absorption lines. We provide a full atlas of all detected absorption and emission lines as well as their fit centroids and equivalent width values. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers
Search for squarks and gluinos in collisions at TeV and TeV in events with -leptons, jets and missing transverse momentum using the ATLAS detector
No full textInternational audienceA search for R-parity-conserving supersymmetry in events with large missing transverse momentum, jets and at least one hadronically decaying -lepton is presented. Both gluino and squark pair production are considered, with the cascade decay of each gluino or squark producing either a -slepton or a -sneutrino. Three channels are examined, requiring either exactly one hadronically decaying -lepton and no other leptons, exactly one hadronically decaying -lepton and at least one other lepton, or two or more hadronically decaying -leptons. Analyses in the three channels are optimised independently and combined statistically. Two separate analysis strategies, either a cut-and-count or machine-learning approach, are used. The search uses 140 and 51.8 of collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015-2018 at TeV and 2022-2023 at TeV, respectively. Gluino masses below TeV and squark masses up to TeV are excluded