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Effect of annealing conditions on the luminescence properties and thermometric performance of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphors
No full textInternational audienceWe report on the synthesis, photoluminescence optimization and thermometric properties of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphor powders. The photoluminescence of Sr2.9Al2O5Cl2:0.1Eu2+ phosphors exhibits a blue-shift with an increasing annealing temperature owing to a decrease in the crystal field strength of the host caused by evaporation of Cl from the material. The quenching of the blue band in favour of the red band observed in the luminescence spectra of Sr2.9Al2O5Cl2:0.1Eu2+ with an increased annealing temperature was explained using the mechanism of the Landau-Zener transitions. The quantum yield and the lifetime of the phosphors depend on the annealing temperature. Phosphor samples annealed at 850 °C, 1000 °C, 1200 °C and 1500 °C were found to be potential luminescence thermometers using the luminescence spectral method. For Sr3Al2O5Cl2:Eu2+ annealed at 1000 °C, the temperature-dependent dual-band intensity ratio demonstrated a high-temperature sensitivity of ∼1.47%/°C in the temperature range of 23 °C to 40 °C which is superior to other reported phosphors with a microsecond decay time, suggesting that the material has potential for sensitive thermometry applications at ambient temperatures
Efficient Exploration of Image Classifier Failures with Bayesian Optimization and Text-to-Image Models
No full textInternational audienceImage classifiers should be used with caution in the real world. Performance evaluated on a validation set may not reflect performance in the real world. In particular, classifiers may perform well for conditions that are frequently encountered during training, but poorly for other infrequent conditions. In this study, we hypothesize that recent advances in text-to-image generative models make them valuable for benchmarking computer vision models such as image classifiers: they can generate images conditioned by textual prompts that cause classifier failures, allowing failure conditions to be described with textual attributes. However, their generation cost becomes an issue when a large number of synthetic images need to be generated, which is the case when many different attribute combinations need to be tested. We propose an image classifier benchmarking method as an iterative process that alternates image generation, classifier evaluation, and attribute selection. This method efficiently explores the attributes that ultimately lead to poor behavior detection
Fast linear solvers for incompressible CFD simulations with compatible discrete operator schemes
No full textInternational audienceFinding a robust and efficient solver for (non-)symmetric systems that arise in incompressible Computational Fluid Dynamics (CFD) is of great interest to both academia and industry. We consider the Compatible Discrete Operator (CDO) discretization that has recently been devised for CFD simulations in the context of incompressible Stokes and Navier–Stokes flows. The discrete problems resulting from CDO schemes yield large saddle-point systems that require relevant numerical methods suitable to deal with large indefinite and poorly conditioned linear systems. In this paper, we focus on two segregated methods: the augmented Lagrangian Uzawa method and the generalized Golub–Kahan bidiagonalization, as well as a monolithic method based on an algebraic transformation by change of variables. We also employ algebraic multigrid (AMG) preconditioned Krylov solvers such as the Flexible Conjugate Gradient (FCG) method, and the Flexible Generalized Minimal Residual (FGMRES) method, to solve the linear systems. Using the CFD software code_saturne, we compare the numerical performance with respect to the choice of linear solvers and numerical strategies for the saddle-point problem. In the numerical experiments, the AMG preconditioned Krylov methods show robustness in test cases of Stokes and Navier–Stokes problems
Surface continentales
No full textSource Agritrop Cirad (https://agritrop.cirad.fr/610951/)International audienc
Simulation of the impact of a high-velocity jet at high-temperature on a liquid surface with a two-temperature diffuse interface model
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Qu'attendre de la géodésie spatiale sur les théories tenseur-scalaire
No full textInternational audienceScalar-tensor theories with screening mechanisms come with non-linearities that make it difficult to study setups of complex geometry without resorting to numerical simulations. In this article, we use the code that we introduced in a previous work in order to compute the fifth force arising in the chameleon model in the Earth orbit. We go beyond published works by introducing a departure from spherical symmetry \unicode{x2014} embodied by a mountain on an otherwise spherical Earth \unicode{x2014} as well as by implementing several atmospheric models, and quantify their combined effect on the chameleon field. Building on the numerical results thus obtained, we address the question of the detectability of a putative chameleon fifth force by means of space geodesy techniques and, for the first time, quantitatively assess the back-reaction created by the screening of a satellite itself. We find that although the fifth force has a supposedly measurable effect on the dynamics of an orbiting spacecraft, the imprecise knowledge of the mass distribution inside the Earth greatly curtails the constraining power of such space missions. Finally, we show how this degeneracy can be lifted when several measurements are performed at different altitudes.Les théories du tenseur-scalaire avec mécanismes d'écrantage présentent des non-linéarités qui rendent difficile l'étude de configurations à géométrie complexe sans recourir à des simulations numériques. Dans cet article, nous utilisons le code femtoscope que nous avons introduit dans un travail précédent afin de calculer la cinquième force apparaissant dans le modèle du caméléon en orbite terrestre. Nous allons au-delà des travaux publiés en introduisant un écart par rapport à la symétrie sphérique — incarné par une montagne sur une Terre par ailleurs sphérique — ainsi qu'en mettant en œuvre plusieurs modèles atmosphériques, et en quantifiant leur effet combiné sur le champ du caméléon. Sur la base des résultats numériques ainsi obtenus, nous abordons la question de la détectabilité d'une cinquième force caméléon supposée au moyen de techniques de géodésie spatiale et, pour la première fois, nous évaluons quantitativement la rétroaction créée par l'écrantage d'un satellite lui-même. Nous constatons que, bien que la cinquième force ait un effet supposément mesurable sur la dynamique d'un engin spatial en orbite, la connaissance imprécise de la distribution de la masse à l'intérieur de la Terre réduit considérablement le pouvoir contraignant de ces missions spatiales. Enfin, nous montrons comment cette dégénérescence peut être levée lorsque plusieurs mesures sont effectuées à différentes altitudes
A micro-hot-wire anemometry probe with elongated stubs for turbulent boundary layer measurements
No full textInternational audienceThe development of micro-hot-wire anemometry probes for turbulence studies requires mitigating spatial filtering, end-conduction effects and probe intrusivity. Keeping these factors in mind, this work analytically and experimentally investigates the relevance of a micro-hot-wire probe design featuring elongated stubs, inspired by Wollaston-wire probes but fabricated using modern micro-fabrication techniques. The resulting probes are shown to be relatively easy to manufacture and capable of providing satisfactory velocity measurements in a zero-pressure-gradient turbulent boundary layer at Reτ≈1150. Different probes were tested, all featuring a micro-wire length sufficiently small to alleviate spatial filtering of near-wall small-scale turbulent structures. The investigation focuses on assessing end-conduction effects and probe intrusivity, with the latter still observable close to the wall for such micro-probes
Deep-learning Based Wall Mitigation Method for Through-the-wall Radar Imaging
No full textInternational audienceThrough-the-wall radar imaging is used for imaging targets hidden behind walls. However, the presence of a wall acting as a spatial filter impacts the scattering quality of the target’s field, influencing the resultant image’s clarity. Various f iltering strategies have been developed to separate the target’s and the wall’s responses. While effectively distinguishing the target and differentiating between multiple targets, these techniques also affect the imaging capability and the discrimination process. Here, we propose a deep-learning-based wall mitigation method to avoid this
POST-FLIGHT SYSTEM IDENTIFICATION AND AEROSERVOELASTIC MODEL UPDATING FOR PREDICTION AND VALIDATION OF THE ONSET OF FLUTTER
No full textInternational audienceA validated aeroservoelastic (ASE) model allows, among other things, the extensive study of system performance and characteristics, the verification of analytical predictions, the support of flight envelope expansion during prototype testing, and the design of flight control laws. The ASE stability analysis is another crucial component of the configuration optimization and certification process over the intended operational envelope. In this context, the flutter phenomenon is a well-known example of a self-excited aeroelastic instability resulting from the interaction between unsteady aerodynamic forces and structural vibrations. The investigation of flutter through flight flutter testing is an essential part of aircraft certification. Significant amplitudes of vibration can be induced, eventually resulting in the structure's catastrophic failure. Instabilities have been derived that exceed well beyond the basic bending-torsion flutter into complex mechanisms involving ASE dynamics. With the guidance of accurate ASE models, a reliable prediction of an aircraft's susceptibility to flutter across its intended flight envelope is possible.Using data from flight tests of the fixed-wing P-FLEX UAV with a 6m wing span, this paper will demonstrate post-flight system identification results and, by extension, ASE model updating using modal parameters identified from Ground Vibration Test. Predictions provided by the updated model regarding flutter boundary will be thoroughly assessed. An additional significant topic is the post-flight verification of the open-loop flutter speed obtained through system identification using flight test data. This is achieved through the monitoring of aeroelastic damping and qualitative comparison of the stability diagrams of the system's poles at different flight speeds. Finally, flutter boundary expansion enabled by the Active Flutter Suppression (AFS) controller of the closed-loop system will be verified via post-flight analysis of the critical flight flutter test data
