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International audience; An interval method based on Pontryagin's Minimum Principle is proposed to enclose the solutions of an optimal control problem with embedded bounded uncertainties. This method is used to compute an enclosure of all optimal trajectories of the problem, as well as open loop and closed loop enclosures meant to validate an optimal guidance algorithm on a concrete system with inaccurate knowledge of the parameters. The differences in geometry of these enclosures are exposed, and showcased on a simple system. These enclosures can guarantee that a given optimal control problem will yield a satisfactory trajectory for any realization of the uncertainties. Contrarily, the probability of failure may not be eliminated and the problem might need to be adjusted.

International audience; Reusable liquid-propellant rocket engines (LPREs) imply more demanding robustness requirements than expendable ones due to their extended capabilities. Therefore, the goal of this work was to develop a control loop adapted to all the operating phases of LPRE, including transients, and robust to internal parametric variations. Firstly, thermo-fluid-dynamic simulators representative of the gas-generator-cycle engines were built. These simulators were subsequently translated into nonlinear state-space models. Based on these models, the continuous subphase of the start-up transient is controlled to track precomputed reference trajectories. Beyond the start-up, throttling scenarios are managed with end-state-tracking algorithm. Model Predictive Control has been applied in a linearised manner with robustness considerations to both scenarios, in which a set of hard state and control constraints must be respected. Tracking of pressure (thrust) and mixture-ratio operating points within the design envelope is achieved in simulation while respecting constraints. Robustness to variations in the predominant parameters, to external state perturbations and to the possible impact of an observer on the loop, is demonstrated.

We present here a method for the quantitative prediction of the spectroscopic specular reflectivity line-shape in anisotropic layered media. The method is based on a 4 &times, 4 matrix formalism and on the simulation from the first principles (through density functional theory&mdash, DFT) of the anisotropic absorption cross-section. The approach was used to simulate the reflectivity at the oxygen K-edge of a 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) thin film on Au(111). The effect of film thickness, orientation of the molecules, and grazing incidence angle were considered. The simulation results were compared to the experiment, permitting us to derive information on the film geometry, thickness, and morphology, as well as the electronic structure.

International audience; Complex-chemistry direct numerical simulation (DNS) data obtained earlier from lean hydrogen-air flames associated with corrugated flame (case A), thin reaction zone (case B), and broken reaction zone (case C) regimes of turbulent burning are analysed to directly assess capabilities of the flamelet approach to predict mean concentrations of species in a premixed turbulent flame. The approach consists in averaging dependencies of mole fractions, reaction rates, temperature, and density on a single combustion progress variable c, which are all obtained from the unperturbed laminar flame. For this purpose, four alternative definitions of c are probed and two probability density functions (PDFs) are adopted, i.e. either an actual PDF extracted directly from the DNS data or a presumed β-function PDF obtained using the DNS data on the first two moments of the c(x, t)-field. Results show that the mean density and mean mole fractions of H2, O2, and H2O are well predicted using both PDFs for each c, although the predictive capabilities are little worse in case C. In cases A and B, the use of the actual PDF and the fuel-based c also offers an opportunity to well predict mean mole fractions of O and H, whereas the mean mole fraction of OH is slightly underestimated. In the highly turbulent case C, the same approach performs worse, but still appears to be acceptable for evaluating the mean radical concentrations. The use of the β-function PDFs or another combustion progress variable yields substantially worse results for these radicals. When compared to the mean mole fractions, the mean rate of product creation, i.e. the source term in the transport equation for the mean combustion progress variable, is worse predicted even for a quantity (species concentration or temperature) adopted to define c and using the actual PDF. Consequently, turbulent burning velocity is not predicted either.

At the nanoscale, materials exhibit unique properties that differ greatly from those of the bulk state. In the case of Ag$_{x}$Pt$_{1-x}$ nanoalloys, we aimed to study the solid-liquid transition of nanoparticles of different sizes and compositions. This system is particularly interesting since Pt has a high melting point (2041 K compare to 1035 K for Ag) which could keep the nanoparticle solid during different catalytic reactions at relatively high temperatures, such as we need in the growth of nanotubes. We performed atomic scale simulations using semi-empirical potential implemented in a Monte Carlo code at constant temperature and chemical composition in canonical ensemble. We observed that the melting temperature decreases with the size (pure systems and alloys) and the composition. We show that the melting systematically passes through an intermediate stage with a crystalline core (pure platinum or mixed PtAg depending on the composition) and a pure silver liquid skin, which strongly questions the idea of having a faceted solid particle in catalytic reactions for carbon nanotubes synthesis.

International audience; This paper presents a new classification framework for both first and second order statistics, i.e. mean/location and covariance matrix. In the last decade, several covariance matrix classification algorithms have been proposed. They often leverage the Riemannian geometry of symmetric positive definite matrices (SPD) with its affine invariant metric and have shown strong performance in many applications. However, this geometry assumes a zero mean. In practice, it is often estimated and then removed in a preprocessing step. This is of course damaging for applications where the mean is a discriminative feature. Unfortunately, the distance associated to the affine invariant metric for both mean and covariance matrix remains unknown. Leveraging previous works on geodesic triangles, we propose two affine invariant divergences that use both statistics. Then, we derive an algorithm to compute the associated Riemannian centers. Finally, a divergence based Nearest centroid, applied on the crop classification dataset Breizhcrops, shows the interest of the proposed framework.

The infrared emission lines observed between 80 and 100 km known as nightglow allow the investigation of dynamic phenomena such as gravity waves. These perturbations act on local temperature and density. However, the observation of the local perturbations in the nightglow layer is mainly performed by spectrally broad cameras. Swenson and Gardner (1998) introduced the cancellation factor linking relative variations of intensity with relative variations of temperature. The cancellation factor is a function of the perturbation vertical wavelength estimated from simulation that do not include spectral variations. In this study, we intend to estimate the spectral variability of the cancellation factor, in particular within the range 0.9-1.7 µm corresponding to infrared InGaAs camera, used during measurement campaigns. We describe briefly the model that resolves the vibrational states of the nightglow main source (OH). Then vertically propagating gravity waves are applied on a 1D scheme and the cancellation factor is computed based on the impact on both temperature and intensity. Spectral variations of the cancellation factor are observed and compared along the variation of the vertical wavelength.

International audience; The adaptive optics systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈90 km above the telescopes. In a Shack–Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor sensor with 1100 × 1600 pixels and 9 μm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the European Southern Observatory’s ELT and the Thirty Meter Telescope.; Les systèmes d'optique adaptative des futurs Extremely Large Telescopes (ELT) seront assistés par des étoiles guides laser (LGS) qui seront créées dans la couche de sodium à une hauteur de ≈90 km au-dessus des télescopes. Dans un capteur de front d'onde de Shack-Hartmann, la longue élongation des taches LGS sur les sous-pupilles éloignées de l'axe du faisceau laser contraint la conception du capteur de front d'onde (WFS) qui doit être capable d'échantillonner entièrement les taches allongées sans sous-échantillonner les taches non allongées. Pour répondre à ces exigences, un grand capteur à semi-conducteurs d'oxyde de métal complémentaire récemment commercialisé avec 1100 × 1600 pixels et un pas de pixel de 9 μm pourrait être employé. Nous présentons ici la caractérisation d'un tel capteur en termes de bruit et de linéarité, et nous évaluons ses performances pour la détection du front d'onde en fonction des variations du centroïde du spot. Nous illustrons ensuite comment ce nouveau détecteur peut être intégré dans un WFS LGS complet pour l'ELT de l'Observatoire européen austral et le télescope de trente mètres.

International audience; Le code MANTA a l’ambition de permettre la réalisation de simulations complexes en mécanique sur des supercalculateurs actuels et futurs tout en préservant les fondamentaux des codes développés au CEA : adaptabilité au problème posé, robustesse des algorithmes, pérennité des modèles et du code. On expose les principes de développement de ce code de nouvelle génération, et quelques exemples représentatifs de ses capacités actuelles sont également décrits.

International audience; L'optimisation sous contraintes fiabilistes vise à garantir que les contraintes soient respectées avec une probabilité acceptable dans le cadre de la variabilité de propriétés. Dans cette étude, une approche d'optimisation fiabiliste de stratifiés composites est proposée pour des applications d'instabilité mécanique. Cette méthode est démontrée sur un cas d'optimisation de stratifiés composites avec la prise en compte d'incertitudes des orientations de plis où la rigidité en flexion est maximisée sous une contrainte de flambement. Les résultats montrent une réduction de la probabilité de défaillance jusqu'à 99% et une exploration différente de l'espace de design. Mots clés-optimisation fiabiliste, stratifiés composites, double espace de design, problème inverse, quantification d'incertitudes.

International audience; Nous discutons et comparons deux méthodes d'optimisation structurale pertinentes pour les structures lattices : l'optimisation topologique et le layout optimization. Nous présentons une application originale de ces deux méthodes dans le cadre de la minimisation de la masse structures lattices cellules masse en tenant compte d'un critère de rupture en contrainte. Nous proposons ensuite une comparaison qualitative de ces deux méthodes et discutions de leur pertinence respective pour l'optimisation des structures lattices.

The Lidar Emitter and Multispecies greenhouse gases Observation iNstrument (LEMON) objective is the development and test of a new Differential Absorption Lidar (DIAL) sensor concept for greenhouse gases and water vapor for spaceborne, airborne or ground-based measurements. The innovative instrument is based on a versatile transmitter. The concept of the measurement was recently preliminarily tested for water vapor in a co-dedicated field campaign from 13 to 24 September 2021 over the Aubenas airfield (France, 44° 32' N 4° 22' E). This campaign was also an opportunity to test different approaches for the measurement of the vertical water vapor profile using classical meteorological probes embarked on meteorological balloons and on an airplane, a vibrational Raman lidar WALI (Weather Atmospheric LIdar), a cavity ring-down spectrometer (CRDS) and of course a first version of the LEMON lidar named WaVIL (Water Vapor and Isotope Lidar). The field campaign involved an instrumented van with two lidars and three ULAs carrying various payloads: a backscatter Rayleigh-Mie lidar to identify atmospheric structures from the local to regional scales, a CRDS for water vapor isotope measurements and in situ samplers to characterize cloud-related forcing on atmospheric water vapor concentrations. The measurement strategy adopted made it possible to follow the evaporation of water vapor throughout the course of a thunderstorm and to sample an intrusion of dry air from high altitudes. It also provided initial answers as to the potential of the WaViL instrument for measuring the main isotope of water vapor and its secondary isotope HDO. The measurement campaign will be presented, as well as the first associated results.

International audience; The deformation mechanisms of an Nb–20Si–25Ti–6Al–3Cr–3Mo alloy deformed in compression at 1000 ◦C and at a strain rate of 10−4 s−1 have been investigated, using focused ion beam and transmission electron microscopy techniques. The initial microstructure of the alloy was constituted of a solid solution bcc Nbss matrix and of Nb5Si3 phases of β (tetragonal) and γ (hexagonal) crystallographic structures, with also hcp-Ti precipitates. As expected, the only phase which exhibited plastic deformation at 1000 ◦C was the Nbss matrix. Glide of Burgers vector b = 1/2 dislocations has been observed, in planes compatible with {112} and {123} planes, or with cross-slip in {110} planes, as classically reported in bcc structures. Conversely, climb contribution has not been evidenced, probably because of rearrangement of the climbing dislocations in sub-boundaries. Fine δ-Nb11Si4 precipitates have been detected in the samples compressed at 1000 ◦C. Interactions of the δ-Nb11Si4 precipitates with the dislocations have been observed, which can potentially lead to strengthening of the Nbss phase.; Les mécanismes de déformation d'un alliage Nb-20Si-25Ti-6Al-3Cr-3Mo déformé en compression à 1000 ◦C et à une vitesse de déformation de 10-4 s-1 ont été étudiés, en utilisant des techniques de faisceau d'ions focalisés et de microscopie électronique en transmission. La microstructure initiale de l'alliage était constituée d’une solution solide bcc à base de niobium, Nbss, formant la matrice et de phases Nb5Si3 de structures cristallographiques β (quadratiques) et γ (hexagonales), avec également des précipités hcp-Ti. Comme prévu, la seule phase qui a présenté une déformation plastique à 1000 ◦C était la matrice Nbss. Le glissement des dislocations avec un vecteur de Burgers b = 1/2 a été observé, dans des plans compatibles avec les plans {112} et {123}, ou avec un glissement croisé dans les plans {110}, comme classiquement rapporté dans les structures bcc. Inversement, la contribution à la montée n'a pas été mise en évidence, probablement en raison du réarrangement des dislocations montantes dans les sous-joints. De fins précipités de δ-Nb11Si4 ont été détectés dans les échantillons compressés à 1000 ◦C. Des interactions des précipités δ-Nb11Si4 avec les dislocations ont été observées, ce qui peut potentiellement conduire au renforcement de la phase Nbss.

International audience; The deformation mechanisms of an Nb–20Si–25Ti–6Al–3Cr–3Mo alloy deformed in compression at 1000 ◦C and at a strain rate of 10−4 s−1 have been investigated, using focused ion beam and transmission electron microscopy techniques. The initial microstructure of the alloy was constituted of a solid solution bcc Nbss matrix and of Nb5Si3 phases of β (tetragonal) and γ (hexagonal) crystallographic structures, with also hcp-Ti precipitates. As expected, the only phase which exhibited plastic deformation at 1000 ◦C was the Nbss matrix. Glide of Burgers vector b = 1/2 dislocations has been observed, in planes compatible with {112} and {123} planes, or with cross-slip in {110} planes, as classically reported in bcc structures. Conversely, climb contribution has not been evidenced, probably because of rearrangement of the climbing dislocations in sub-boundaries. Fine δ-Nb11Si4 precipitates have been detected in the samples compressed at 1000 ◦C. Interactions of the δ-Nb11Si4 precipitates with the dislocations have been observed, which can potentially lead to strengthening of the Nbss phase.; Les mécanismes de déformation d'un alliage Nb-20Si-25Ti-6Al-3Cr-3Mo déformé en compression à 1000 ◦C et à une vitesse de déformation de 10-4 s-1 ont été étudiés, en utilisant des techniques de faisceau d'ions focalisés et de microscopie électronique en transmission. La microstructure initiale de l'alliage était constituée d’une solution solide bcc à base de niobium, Nbss, formant la matrice et de phases Nb5Si3 de structures cristallographiques β (quadratiques) et γ (hexagonales), avec également des précipités hcp-Ti. Comme prévu, la seule phase qui a présenté une déformation plastique à 1000 ◦C était la matrice Nbss. Le glissement des dislocations avec un vecteur de Burgers b = 1/2 a été observé, dans des plans compatibles avec les plans {112} et {123}, ou avec un glissement croisé dans les plans {110}, comme classiquement rapporté dans les structures bcc. Inversement, la contribution à la montée n'a pas été mise en évidence, probablement en raison du réarrangement des dislocations montantes dans les sous-joints. De fins précipités de δ-Nb11Si4 ont été détectés dans les échantillons compressés à 1000 ◦C. Des interactions des précipités δ-Nb11Si4 avec les dislocations ont été observées, ce qui peut potentiellement conduire au renforcement de la phase Nbss.

International audience; Mesospheric temperature inversions are subject to investigations due to the links with multiscale dynamics such as planetary wave and gravity waves. Knowing the impact on climatological inversions also requires understanding the phenomena occurring before, through, and after a mesospheric inversion. We use data obtained during a measurement campaign over Maïdo observatory in La Réunion Island and focus on a specific event occurring in the night between the 9th and the 10th of October 2017. Among the several observations available, LIDAR measurements provided vertical profiles of temperature and gravity waves potential energy completed by high vertical resolution radiosoundings. The airglow layer observed by an InGaAs camera shows the evolution of gravity wave structures at about 87 km between 0.9 and 1.7 km. Gravity wave parameters such as horizontal wavelengths or intensity emission variations are extracted, along with potential energy compared with LIDAR data. We use atmospheric models (ERA5, WACCM, WRF) and specific tools (NEMO, GROGRAT) to add supplementary information about the night selected. We present here the first results related to the gravity waves and energy exchanges in the frame of the temperature inversion.

International audience; Chalcogenide glasses have attracted growing interest for their potential to meet the demands of photonic applications in the Mid-Wavelength InfraRed (MWIR) and Long-Wavelength InfraRed (LWIR) transmission windows. In this work, we investigated the photosensitivity to femtosecond laser irradiation of a dedicated chalcogenide glass, along with its possible applications in micro-optics. In order to address the SWaP problem (Size, Weight and Power), this work took advantage of recent techniques in femtosecond laser direct writing to imprint flat and integrated optical systems. Here, we wanted to simplify an infrared multispectral imaging system which combines a lens array and a filter array. Each channel has a focal length of 7 mm and an f-number of 4. We show in this paper that the chosen GeS2-based chalcogenide glass is very promising for the fabrication of graded index optics by fs-laser writing, and particularly for the fabrication of Fresnel lenses. We note a very important phase variation capacity in this infrared material corresponding to refractive index variations up to +0.055. A prototype of Fresnel GRIN lens with a refractive index gradient was fabricated and optically characterized in the Vis range.; Les verres de chalcogénure ont suscité un intérêt croissant pour leur potentiel à répondre aux exigences des applications photoniques dans les fenêtres de transmission de l'infrarouge moyen (MWIR) et de l'infrarouge lontain (LWIR). Dans ce travail, nous avons étudié la photosensibilité à l'irradiation laser femtoseconde d'un verre chalcogénure dédié, ainsi que ses applications possibles en micro-optique. Afin de résoudre le problème SWaP (Size, Weight and Power), ce travail a profité des techniques récentes d'écriture directe par laser femtoseconde pour imprimer des systèmes optiques plats et intégrés. Ici, nous avons voulu simplifier un système d'imagerie multispectrale infrarouge qui combine un réseau de lentilles et un réseau de filtres. Chaque canal a une distance focale de 7 mm et un nombre d'ouverture de 4. Nous montrons dans cet article que le verre chalcogénure à base de GeS2 choisi est très prometteur pour la fabrication d'optiques à gradient d'indice par écriture laser fs, et en particulier pour la fabrication de lentilles de Fresnel. Nous constatons une capacité de variation de phase très importante dans ce matériau infrarouge correspondant à des variations d'indice de réfraction jusqu'à +0.055. Un prototype de lentille de Fresnel à gradient d'indice de réfraction a été fabriqué et caractérisé optiquement dans le domaine Vis.

International audience; A cooperation on the aerodynamic optimization of helicopter rotors has been formed by JAXA, ONERA and DLR. This work represents the conclusions drawn from the first phase: optimization of a hovering rotor. The HART-II blade is first investigated with low-fidelity tools and compared against state-of-the art CFD simulations. Following up on this, the HART-II blade chord distribution and twist are optimized with the low-fidelity tools as well as CFD. Due to observed differences in the outcome of the CFD simulations for the low-fidelity optimized blades by the partners, a deeper investigation of the effects of the turbulence modelling approach, elasticity and grid topology is undertaken. The outcome is that the chosen flight condition is close to thrust of the maximum Figure of Merit and the vortex triggered separation on the outboard sections of the blade has to be modelled correctly. In this study, the blade grids had the most noticeable effect on the results, followed by the turbulence model and elasticity. With respect to the optimization, care must be taken when optimizing with lowfidelity methods, whereas CFD optimized blades were found to lead to more robust designs even though they have only been optimized for a single point. Reason is the more accurate modelling of the stall phenomenon with respect to geometrical changes.; Une coopération sur l'optimisation aérodynamique des rotors d'hélicoptères a été formée entre le JAXA, l'ONERA et le DLR. Ce travail présente les conclusions obtenues lors de la première phase: l'optimisation d'un rotor en vol stationnaire. La pale HART-II est d'abord étudiée avec des outils basse fidélité, et comparée aux simulations CFD les plus récentes. Suite à cela, les distributions de corde et de vrillage de la pale HART-II sont optimisées avec des outils numériques basse fidélité ainsi que par CFD. En raison des différences observées dans les résultats de simulations CFD pour les pales optimisées à basse fidélité par les partenaires, une étude plus approfondie des effets de la modélisation de la turbulence, de l'élasticité de la pale, et de la topologie des maillages est entreprise. Le résultat est que la condition de vol choisie est proche de la poussée de la figure de mérite maximale et que la séparation déclenchée par les tourbillons sur les sections extérieures de la pale doit être modélisée correctement.Dans cette étude, les maillages des pales ont eu l'effet le plus notable sur les résultats, suivis par le modèle de turbulence et l'élasticité. En ce qui concerne l'optimisation, il faut être prudent lors de l'optimisation avec des méthodes de basse fidélité, alors que les pales optimisées par CFD se sont avérées conduire à des conceptions plus robustes, même si elles n'ont été optimisées que pour un seul point. La raison en est la modélisation plus précise du phénomène de décrochage par rapport aux modifications géométriques.

International audience; The instruments developed for the upcoming Extremely Large Telescopes (ELTs) will need efficient Adaptive Optics (AO) systems to correct the effects of the atmospheric turbulence and allow imaging at the highest angular resolution. One of the most important requirement for ELT AO-assisted instruments will be to deliver diffractionlimited images in a significant part of the sky. For that, the instruments will be equipped with Laser Guide Stars (LGSs) providing most of the information required by AO instruments. But even with LGSs, AO systems still require the use of Natural Guide Stars (NGSs) to compensate for image motion (jitter) and some low order aberrations. These NGSs are eventually limiting the fraction of the sky that can be achieved by AO systems, the so-called Sky Coverage. In this paper, we first present the sky coverage assessment methods used for HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph) and MAORY/MICADO (Multi-conjugate Adaptive Optics RelaY/Multi-AO Imaging Camera for Deep Observations), that are both instruments for the Extremely Large Telescope of the European Southern Observatory (ESO). They are based on a semi-analytical description of the main contributors in the AO error budget, allowing for a fast estimation of the residual jitter. As such, these methods are well suited for statistical estimation of the sky coverage on multiple science fields, and/or to efficiently explore the system parameter space. We then compute the sky coverage of the two instruments in cosmological fields from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) catalog. The goal is to provide an insight on the possibilities given by two different types of tomographic AO systems, i. e. Laser Tomography AO (LTAO) with HARMONI and Multi-Conjugate AO (MCAO) with MAORY, on the same telescope. In particular, we show that HARMONI and MAORY/MICADO are complementary, meaning that the overall sky coverage of ESO's ELT is much improved for applications common to both systems.

International audience; Laser guide star (LGS) wave-front sensing (LGSWFS) is a key element of tomographic adaptive optics system. However, when considering Extremely Large Telescope (ELT) scales, the LGS spot elongation becomes so large that it challenges the standard recipes to design LGSWFS. For classical Shack–Hartmann wave-front sensor (SHWFS), which is the current baseline for all ELT LGS-assisted instruments, a trade-off between the pupil spatial sampling [number of sub-apertures (SAs)], the SA field-of-view (FoV) and the pixel sampling within each SA is required. For ELT scales, this trade-off is also driven by strong technical constraints, especially concerning the available detectors and in particular their number of pixels. For SHWFS, a larger field of view per SA allows mitigating the LGS spot truncation, which represents a severe loss of performance due to measurement biases. For a given number of available detectors pixels, the SA FoV is competing with the proper sampling of the LGS spots, and/or the total number of SAs. We proposed a sensitivity analysis, and we explore how these parameters impacts the final performance. In particular, we introduce the concept of super resolution, which allows one to reduce the pupil sampling per WFS and opens an opportunity to propose potential LGSWFS designs providing the best performance for ELT scales.; La détection du front d'onde en utilisant une étoile laser (LGS) est un élément clé des systèmes optique adaptative tomographiques. Cependant, si l'on considère la taille des très grands télescopes (ELT), l'élongation du spot laser devient si importante qu'elle remet en question les recettes classiques de conception du LGSWFS. Pour le senseur classique de type Shack-Hartmann (SHWFS), qui constitue la référence actuelle pour tous les instruments d'optique adaptative assisté par laser de l'ELT, un compromis entre l'échantillonnage spatial de la pupille (nombre de sous-ouvertures), le champ de vue de ces sous-ouvertures et l'échantillonnage des pixels dans chaque sous-ouverture est nécessaire. Pour les ELT, ce compromis est également dicté par de fortes contraintes techniques, notamment concernant les détecteurs disponibles et en particulier leur nombre de pixels. Pour les SHWFS, un champ de vue plus large par sous-ouverture permet d'atténuer la troncature du spot LGS, qui représente une perte sévère de performance due aux biais de mesure. Pour un nombre donné de pixels de détecteurs disponibles, le champ de vision [FoV] de la sous-ouverture est en concurrence avec l'échantillonnage correct des taches LGS, et/ou le nombre total de sous-ouvertures. Dans cet article, nous proposons une analyse de sensibilité, et nous explorons l'impact de ces paramètres sur la performance finale. En particulier, nous introduisons le concept de super résolution, qui permet de réduire l'échantillonnage de la pupille par WFS, et ouvre une opportunité de proposer des conceptions potentielles de LGSWFS offrant les meilleures performances pour les ELT.

International audience; Reduction of the specific consumption of aircraft would lead to substantial economic and environmental benefits. A promising technique consists in reducing skin friction drag by delaying the laminar-turbulent transition point as far aft as possible, particularly on the wings, in order to obtain lower friction coefficient in the presence of laminar flow. This can be achieved by Hybrid Laminar Flow Control (HLFC) technology. This paper presents the aerodynamic design of a largescale HLFC wing model for experiments that will be conducted in ONERA's S1MA wind-tunnel. The wing model is designed to investigate the major aerodynamic challenges that must be tackled to enable the development of HLFC airfoils: assessment in transonic flow conditions of the wing drag reduction due to HLFC suction, surface tolerance requirements on a HLFC wing and investigation of Attachment Line Contamination.

The lower troposphere is where the surface evapotranspiration flux has a strong impact on the atmospheric water vapor isotopic composition, enabling the investigation of the hydro-ecological features of a specific study area. Even though several studies investigated in the last decade the spatial and temporal variability of tropospheric water vapor isotopic composition with ships, aircrafts, satellites and at fixed locations at ground level, vertical profiles and spatial observations acquired within the same time window in the lower troposphere (2 and in less than 0.12 hours. We hypothesize this large horizontal variability to be related to development of thermals within the boundary layer. Our next step will be to summarize the spatial and temporal variability of water vapor isotopic composition for allowing a fair comparison between high-resolution isotope-enabled general circulation models, remote sensing and water vapor observations in the boundary layer.

International audience; To provide for an ever-increasing need for camouflage, experts in stealth have been envisioning for a few years to use metasurfaces and their unconventional electromagnetic properties [S. B. Glybovski et al., "Metasurfaces: From microwaves to visible," 2016] to serve as efficient cloaking devices. To prepare for the use of surface-mounted electronic components, useful for frequency-selecting purposes, we took interest in an equivalent circuit analysis of metasurfaces. Indeed, the works of [R. Dubrovka and R. Donnan, "Equivalent circuit of FSS loaded with lumped elements using modal decomposition equivalent circuit method," 2011] on metasurfaces using electronic components invite one to combine the passive behavior of the host structure with that of the lumped elements. The equivalent circuit analysis of the hosting surface deprived of electronic components can be done using the works of [F. Mesa et al., "Unlocking complexity using the ECA: The equivalent circuit model as an efficient and physically insightful tool for microwave engineering," 2018] on passive metasurfaces. This project therefore aims at translating the results of [F. Mesa et al., "Unlocking complexity using the ECA: The equivalent circuit model as an efficient and physically insightful tool for microwave engineering," 2018] onto the coaxial geometry of interest here. The validity of such a method is then assessed by comparison with the simulation results obtained using a finite-element method.

International audience; Composites represent approximately 50% of the weight of structural parts in new aircraft as Airbus 350 or Boeing 787. Damages could occur on these parts and their monitoring is required for the safety of users. A Structural Health Monitoring system composed by a Lamb’s waves generator and a sensor is a privileged candidate to detect such damages. The flexibility of composite manufacturing allows the integration of such a system. The topic of this study is to optimize the integration of a Structural Health Monitoring system in aircraft structural parts. The present article focuses on the optimization of the integration method of a piezoelectric Lead Zirconate Titanate transducer into laminated composite (Carbon/Epoxy), cured in autoclave according to aircraft manufacturing requirements. The health state of the integrated transducer with three connecting methods is evaluated using X-ray scanning. Punctual stress is responsible for the crack of the transducer occurred during composite curing. The connecting method with aluminum sheet and silver joint is selected because it minimizes local stresses and keeps the transducer integrity. The cohesion between the integrated transducer and the host material is observed by optical microscopy, it shows the presence of void zones located in the PZT edges. A Laser Doppler Vibrometer scanning shows the ability of the integrated piezoelectric transducer to generate Lamb waves.

International audience; We present our activities on the development of narrow linewidth tunable optical parametric sources and their integration in lidar systems. In particular, we present different implementations of the nested cavity optical parametric oscillator (NesCOPO) that enables tunable single-frequency emission from the SWIR to the LWIR, when pumped by a fixed or a tunable wavelength laser beam. We show how to amplify the output energy and while preserving the spectral linewidth to perform standoff detection of greenhouse gases and toxic chemicals with direct detection lidars.

Stall and surge are strong limitations in the operating range of compressors and thus one of the major limits of jet engine performance. A promising way to push the stability limit of compression machines is to inject a small amount of flow at the blade tip to alter the physical mechanism responsible for stall onset. This study focuses on the experimental performance of such a system. To do so, an axial compressor test bench was equipped with 40 actuators connected to an auxiliary pressurised air supply system. They were able to generate high-speed jet blowing just at the tip of the rotor blades. The opening of each actuator was controlled by an electromagnetic valve. This allowed generating continuous or pulsed jets with frequencies up to 500 Hz at different duty cycles. The performance of the control system was investigated for various control strategies, where the injected flow rate, the injection angle, the number of injectors, the jet frequency and the duty cycle were systematically varied. This paper is concluded by a study of the energy balance of the system for various configurations. To the best of the authors’ knowledge, this constitutes a rarely seen analysis in the literature. This project (ACONIT) has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 886352.

Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed to perform a priori assessment of predictive capabilities of the flamelet approach for evaluating mean species concentrations. For this purpose, dependencies of mole fractions and rates of production of various species on a combustion progress variable c , obtained from the laminar flame, are averaged adopting either the actual Probability Density Function (PDF) P ( c ) extracted from the DNS data or a common presumed β -function PDF. On the one hand, the results quantitatively validate the flamelet approach for the mean mole fractions of all species, including radicals, but only if the actual PDF P ( c ) is adopted. The use of the β -function PDF yields substantially worse results for the radicals’ concentrations. These findings put modeling the PDF P ( c ) on the forefront of the research agenda. On the other hand, the mean rate of product creation and turbulent burning velocity are poorly predicted even adopting the actual PDF. These results imply that, in order to evaluate the mean species concentrations, the flamelet approach could be coupled with another model that predicts the mean rate and turbulent burning velocity better. Accordingly, the flamelet approach could be implemented as post-processing of numerical data yielded by that model. Based on the aforementioned findings and implications, a new approach to building a presumed PDF is developed. The key features of the approach consist in (i) adopting a re-normalized flamelet PDF for intermediate values of c and (ii) directly using the mean rate of product creation to calibrate the presumed PDF. Capabilities of the newly developed PDF for predicting mean species concentrations are quantitively validated for all species, including radicals.

The effects of ageing at 800 $^\circ$C on the properties of the IRIS alloy (Ti$_{49.9}$Al$_{48}$W$_2$B$_{0.1}$) are studied. The initial microstructure of this alloy densified by Spark Plasma Sintering (SPS) is mainly composed of lamellar colonies which are surrounded by $\gamma$ grains. The evolutions of the alloy strength and creep resistance resulting from this ageing treatment are measured by the related mechanical tests. The microstructural changes are investigated by scanning and transmission electron microscopies and by X-ray diffraction. The main structural evolutions consist in a shrinkage of the lamellar areas and in a precipitation of $\beta_0$ phase, which is accompanied by a moderate segregation of tungsten and a decrease of the $\alpha_2$ lamellar width. However, these evolutions are relatively limited and the microstructural stability is found to result mainly from the low diffusivity of tungsten. Conversely, a moderate effect of this ageing treatment on mechanical properties, at room and high temperatures, is measured. Such experimental results are interpreted and discussed in terms of the microstructural evolutions and of the deformation mechanisms which are activated at different temperatures under various solicitations.

In this paper we present two applications of a new Belief Function-based Inter-Criteria Analysis (BF-ICrA) approach for the assessment of redundancy of criteria involved in Multi-Criteria Decision-Making (MCDM) problems. This BF-ICrA method allows to simplify the original MCDM problem by suppressing redundant criteria (if any) and thus diminish the complexity of MCDM problem. This approach is appealing for solving large MCDM problems whose solution requires the fusion of many belief functions. We show how this approach can be used in two distinct fields of applications: The GPS surveying problem, and the car selection problem.

International audience; Continuum crystal plasticity models based on the kinematics of plastic slip are widely developed and used in recent years to describe the anisotropic elasto-viscoplastic behaviour of metallic material. Various microstructural physical-based models have been proposed to reproduce experimental evolutions. This work aims to analyse and compare different crystal plasticity finite element models, especially those based on dislocation densities, from the simulations of stress–strain curves of a nickel base super alloy CMSX-4. Special attention is paid on hardening matrix. The adequacy between the simulated results and experimental data is commented, and it is found that a strain-rate- dependent double hardening matrix model can give a better prediction of the CMSX-4 behaviours.

A benchmark dedicated to RANS-informed analytical methods for the prediction of turbofan rotor&ndash, stator interaction broadband noise was organised within the framework of the European project TurboNoiseBB. The second part of this benchmark focuses on the impact of the acoustic models. Twelve different approaches implemented in seven different acoustic solvers are compared. Some of the methods resort to the acoustic analogy, while some use a direct approach bypassing the calculation of a source term. Due to differing application objectives, the studied methods vary in terms of complexity to represent the turbulence, to calculate the acoustic response of the stator and to model the boundary and flow conditions for the generation and propagation of the acoustic waves. This diversity of approaches constitutes the unique quality of this work. The overall agreement of the predicted sound power spectra is satisfactory. While the comparison between the models show significant deviations at low frequency, the power levels vary within an interval of &plusmn, 3 dB at mid and high frequencies. The trends predicted by increasing the rotor speed are similar for almost all models. However, most predicted levels are some decibels lower than the experimental results. This comparison is not completely fair&mdash, particularly at low frequency&mdash, because of the presence of noise sources in the experimental results, which were not considered in the simulations.

International audience; These last decades, Earth Observation brought a number of new perspectives from geosciences to human activity monitoring. As more data became available, Artificial Intelligence (AI) techniques led to very successful results for understanding remote sensing data. Moreover, various acquisition techniques such as Synthetic Aperture Radar (SAR) can also be used for problems that could not be tackled only through optical images. This is the case for weather-related disasters such as floods or hurricanes, which are generally associated with large clouds cover. Yet, machine learning on SAR data is still considered challenging due to the lack of available labeled data. To help the community go forward, we introduce a new dataset composed of co-registered optical and SAR images time series for the detection of flood events and new neural network approaches to leverage these two modalities. Figure 1. The SEN12-FLOOD dataset is composed of SAR and optical time series in which a flood event may occur.

International audience; In the spirit of the core-shell approach that made it possible to greatly enhance the photostability of semiconductor nanoparticles, we study the emission stability of single carbon nanotubes emitting at telecommunication wavelengths protected by a double shell of polymer. Two types of structures with different polymer layers have been studied at cryogenic temperatures, whereas nanotubes wrapped with poly(9,9-di-n-octylfluorenyl-2,7-diyl) and embedded in a bulk polystyrene matrix were used as a reference. A Mandel parameter description applied to the emission intensity and an Allan deviation analysis of the spectral diffusion show quantitatively that the polymer shells stabilize the emission of single carbon nanotubes by more than one order of magnitude in comparison with micelle wrapped nanotubes deposited on a SiO2 surface. This represent an additional step towards the use of single-walled carbon nanotubes as quantum emitters at telecommunication wavelengths.

International audience; We developed an automated Random Telegraph Signal detection and characterization method, as well 7 as a blinking noise and slow drift separation method adapted to focal plane arrays. Utilizing these methods, a 8 study of the evolution of the number of RTS pixels and the amplitudes of the blinking signal as a function of the 9 reverse bias voltage and temperature is conducted. It is shown that physical characteristics of RTS follow 10 Arrhenius laws and increase with bias. Finally, the origin of the increase of the amplitude as a function of the 11 reverse bias is discussed.; Nous avons mis au point une méthode automatisée de détection et de caractérisation du signal télégraphique, ainsi qu'une méthode pour séparer le bruit RTS de la dérive temporelle. Grâce à ces méthodes, nous étudions l'évolution du nombre de pixels RTS et des amplitudes du signal clignotant en fonction de la tension de polarisation inverse et de la température. Il est montré que les caractéristiques physiques du RTS suivent les lois d'Arrhenius et augmentent avec la polarisation. Enfin, l'origine de l'augmentation de l'amplitude en fonction de la polarisation inverse est discutée.

Bayesian Optimization using Gaussian Processes is a popular approach to deal with optimization involving expensive black-box functions. However, because of the assumption on the stationarity of the covariance function defined in classic Gaussian Processes, this method may not be adapted for non-stationary functions involved in the optimization problem. To overcome this issue, Deep Gaussian Processes can be used as surrogate models instead of classic Gaussian Processes. This modeling technique increases the power of representation to capture the non-stationarity by considering a functional composition of stationary Gaussian Processes, providing a multiple layer structure. This paper investigates the application of Deep Gaussian Processes within Bayesian Optimization context. The specificities of this optimization method are discussed and highlighted with academic test cases. The performance of Bayesian Optimization with Deep Gaussian Processes is assessed on analytical test cases and aerospace design optimization problems and compared to the state-of-the-art stationary and non-stationary Bayesian Optimization approaches.

International audience; In the aeronautical field, materials are used in severe environmental conditions (temperature, atmosphere, exposure time ...), particularly for engine applications. In order to characterize the use of these materials in the evaluation of their properties, it is necessary to carry out tests in conditions close to their operating environment. Hot hardness is a simple method which can be applied on many different materials such as oxidized layers, coatings, composite materials, brazing cords, additive manufacturing materials. ONERA is developing micromechanical characterization means to carry out Vickers microhardness tests from room temperature up to 600 °C. In principle, a pyramidal punch is applied on the surface of a material and the applied load is continuously measured during indenter’s moving in the material. The material is tested locally under conditions close to the actual conditions of employment. The goal of this research is to improve microindentation in order to achieve temperature test campaigns up to 600 °C under a controlled atmosphere of argon and to validate a method to produce a series of results during the first hour of using up to 600 °C. Stainless material is studied to compare the evolution of its hot hardness properties versus different parameters such as load, holding time at the maximum load, atmosphere, and thermal duration. A discussion about these measurements and the technical limits of hot hardness technology is presented.

Point clouds are unstructured and unordered data, as opposed to images. Thus, most machine learning approach developed for image cannot be directly transferred to point clouds. In this paper, we propose a generalization of discrete convolutional neural networks (CNNs) in order to deal with point clouds by replacing discrete kernels by continuous ones. This formulation is simple, allows arbitrary point cloud sizes and can easily be used for designing neural networks similarly to 2D CNNs. We present experimental results with various architectures, highlighting the flexibility of the proposed approach. We obtain competitive results compared to the state-of-the-art on shape classification, part segmentation and semantic segmentation for large-scale point clouds., 12 pages

International audience; On 30 September 2017, an Air France Airbus A380-800 suffered a failure of its fourth engine while over Greenland. This failure resulted in the loss of the engine fan hub, fan blades and surrounding structure. An initial search recovered 30 pieces of light debris, but the primary part of interest, a ~220 kg titanium fan hub, was not recovered because it had a different fall trajectory than the light debris, impacted into the ice-sheet's snow surface, and was quickly covered by drifting snow. Here we describe the methods used for the detection of the fan hub and details of the field campaigns. The search area included two crevasse fields of at least 50 snow-covered crevasses 1 to ~30 m wide with similar snow bridge thicknesses. After 21 months and six campaigns, using airborne synthetic aperture radar, ground-penetrating radar, transient electromagnetics and an autonomous vehicle to survey the crevasse fields, the fan hub was found within ~1 m of a crevasse at a depth of ~3.3 to 4 m and was excavated with shovels, chain saws, an electric winch, sleds and a gasoline heater, by workers using fall-arrest systems.

Nickel-base superalloys display cuboidal precipitates aligned along the cubic directions, which are the elastic soft directions. At high precipitate volume fraction, the microstructure is often described as a regular array of precipitates organized on a simple cubic macro-lattice. In the present work, we use a stability analysis and 3D phase field simulations to show that such a regular array is in fact unstable whatever the volume fraction of precipitates. The two main instability modes are the longitudinal [100] mode and the transverse [110] mode along the [ 1 1 ¯ 0 ] eigenvector. We argue that these instabilities lead to formation of configurational defects closely related to experimentally observed branches and herringbone patterns. The roles of elastic anisotropy and elastic homogeneity are also discussed.

International audience; Ultra-High Temperature Ceramics (UHTC) are good candidates to fulfil the harsh requirements of hypersonic applications. For more than a decade, ONERA has been actively involved in several programs to develop such materials for different applications (hypersonic flights, propulsion systems ...). In our laboratories, monolithic and composite materials have been investigated as well as several processing methods. In this paper, we present for example the ZrB2-SiC and HfB2-SiC compositions with TaSi2 or Y2O3 additions which have been especially studied in the European Projects ATLLAS and ATLLAS 2. Assessments of several prototypes in realistic environment are also described. Furthermore, based on these material developments, a specific study on the oxidation behaviour of such monoliths from 1200°C to 2400°C with a dedicated test bench using a 2 kW CO2 laser has been carried out (oxidation under air and water vapour atmospheres). Recently, some work on the manufacturing of Ultra-High Temperature Ceramic Matrix Composites (UHTCMC) has been initiated using slurry infiltration and pyrolysis. The behaviour and properties of these materials are encouraging.; Les céramiques ultraréfractaires (UHTC) sont de bons candidats pour répondre aux besoins sévères des applications liées à l'hypersonique. Depuis plus de dix ans, l'ONERA est impliquée activement dans plusieurs programme de recherche pour développer ce type de matériaux pour différentes applications (vols hypersonique, systèmes de propulsion ...). Au sein de nos laboratoires, des matériaux monolithiques et composites ont été étudiés ainsi que plusieurs méthodes d'élaboration. Dans cet article, nous présentons par exemple les compositions de type ZrB2-SiC et HfB2-SiC contenant des ajouts de TaSi2 ou Y2O3 qui ont été spécifiquement étudiées dans les projets européens ATLLAS et ATLLAS 2. L'évaluation de plusieurs prototypes dans des conditions réalistes est également présentée. Par ailleurs, en se basant sur ces développements matériaux, une étude spécifique sur le comportement à l'oxydation de tels monolithes entre 1200°C et 2300°C a été effectuée avec un moyen d'essai utilisant un laser CO2 de 2 kW (oxydation sous air et sous vapeur d'eau). Récemment, des travaux sur l'élaboration de composites à matrice ultraréfractaire (UHTCMC) ont été engagés en employant l'infiltration de suspension et leur pyrolyse. Le comportement et les propriétés de ces matériaux sont encourageants.

International audience; Reusability of the first stage of launch vehicles may offer new perspectives to lower the cost of payload injection into orbit if sufficient reliability and efficient refurbishment can be achieved. One possible option that may be explored is to design the vehicle first stage for both reusable and expendable uses, in order to increase the flexibility and adaptability to different target missions. This paper proposes a multilevel multidisciplinary design optimization (MDO) approach to design aerospace vehicles addressing multimission problems. The proposed approach is focused on the design of a family of launchers for different missions sharing commonalities using multi-objective MDO to account for the computational cost associated with the discipline simulations. The multimission problem addressed considers two missions: 1) a reusable configuration for a sun synchronous orbit with a medium payload range and recovery of the first stage using a gliding-back strategy; 2) an expendable configuration for a medium payload injected into a geostationary transfer orbit. A dedicated MDO formulation introducing couplings between the missions is proposed in order to efficiently solve such a coupled problem while limiting the number of calls to the exact multidisciplinary analysis thanks to the use of Gaussian processes and multi-objective efficient global optimization.

International audience; This paper presents aeroacoustic investigations on a full-scale Ultra High Bypass Ratio (UHBR) engine with inflow distortion at transonic conditions. Computational Fluid Dynamics (CFD) simulations are first realized to compute the shocks in the vicinity of the fan. These shocks are then radiated in the near-field thanks to Computational AeroAcoustics (CAA) simulations. The chaining between CFD and CAA simulations is done by injecting the shocks in terms of usual conservative variables using a non-reflecting boundary condition. The CAA solver is based on the non-linearized Euler equations which allows to define the CFD/CAA interface close to the fan where the propagation of shocks is highly non-linear. Both shock generation and propagation mechanisms are investigated and the effects of inflow distortion are highlighted by comparison with a baseline case without distortion. The flow distortion is shown to be responsible for a modification of shock amplitudes along the circumferential direction. Thus, azimuthal modes appear in addition to the rotor-locked mode present without distortion. A particular feature of the studied configuration is the presence of a supersonic flow region in the bottom part of the nacelle. This supersonic pocket blocks the shocks in the lower half of the nacelle and also modifies the angle at which they leave the nacelle in its upper half. Consequently, the far-field radiation (obtained using Kirchhoff’s integral method) is mainly oriented skywards and towards the side of descending blades. Acoustic power estimates are provided to quantify these effects.

International audience; In the present paper, a surrogate-based methodology is developed and applied to the design of a control surface placed on an artillery projectile. An artificial neural network is used to estimate the aerodynamic contribution of the control device from numerical simulations. These estimations are coupled to a flight mechanics code to evaluate the trajectories of controlled projectiles. From these six-/seven-degree-of-freedom computations, the trajectories modifications are modeled using kriging. A correction device should minimize the distance to a 3σ ellipse (with σ as the projectile standard deviation of the projectile dispersion) around the mean impact point while ensuring problem-specific constraint fulfillment. The kriging databases are sequentially enriched with the impact point position of the projectile configurations maximizing the expected improvement criterion until a convergence state, assessed by leave-one-out cross validation, is reached. Computational-fluid-dynamics-based evaluations of the optimum configuration coefficients provide a refinement of the neural network databases in the relevant area of the design space. A course correction corresponding to a decrease of two standard deviations in the lateral direction and an increase of two standard deviations in range is achieved, respectively, by reducing the pitching moment of the projectile and increasing its lift-to-drag ratio.

International audience; A methodology is presented that exploits both data-assimilation techniques and resolvent analysis for reconstructing turbulent flows, containing organized structures,with an efficient set of measurements. The mean (time-averaged) flow is obtained using variational data-assimilation that minimizes the discrepancy between a limited set of flowmeasurements, generally from an experiment, and a numerical simulation of the Navier–Stokes equations. The fluctuations are educed from resolvent analysis and time-resolved data at a single point in the flow. Resolvent analysis also guides where measurements of the mean and fluctuating quantities are needed for efficient reconstruction of a simple example case study: flow around a circular cylinder at a Reynolds number ofRe = 100.For this flow, resolvent analysis reveals that the leading singular value,most amplifiedmodes, and themean profile for 47 < Re < 320 scalewith the shedding frequency and length of the recirculation bubble. A relationship between these two parameters reinforces the notion that a wave maker, forwhich the length scaleswith the recirculation bubble, determines the frequency and regionwhere an instability mechanism is active. The procedure offers away to choose sensor locations that capture the main coherent structures of a flow and a method for computing mean pressure by using correctly weighted resolvent modes.

International audience; The problem of magnetic detumbling, also known as magnetic momentum unloading, is considered. The objective is to stabilize a rigid spacecraft angular velocity to zero only through magnetic actuation, i.e. magneto-torquers. We propose a generalized control law that is uniformly, globally exponentially stabilizing (UGES) and show that the celebrated b-dot law is one particular case of this law. Furthermore, we provide an explicit (time-varying) strict Lyapunov function to establish the stability claim.

International audience; The presented adaptive modelling approach aims to jointly control the level of refinement for each of the building-blocks employed in a typical chain of finite element approximations for stochas-tically parametrized systems, namely: (i) finite error approximation of the spatial fields (ii) surro-gate modelling to interpolate quantities of interest(s) in the parameter domain and (iii) Monte-Carlo sampling of associated probability distribution(s). The control strategy seeks accurate calculation of any statistical measure of the distributions at minimum cost, given an acceptable margin of error as only tunable parameter. At each stage of the greedy-based algorithm for spatial discreti-sation, the mesh is selectively refined in the subdomains with highest contribution to the error in the desired measure. The strictly incremental complexity of the surrogate model is controlled by enforcing preponderant discretisation error integrated across the parameter domain. Finally, the number of Monte-Carlo samples is chosen such that either (a) the overall precision of the chain of approximations can be ascertained with sufficient confidence, or (b) the fact that the computational model requires further mesh refinement is statistically established. The efficiency of the proposed approach is discussed for a frequency-domain vibration structural dynamics problem with forward uncertainty propagation. Results show that locally adapted finite element solutions converge faster than those obtained using uniformly refined grids.

Modelocked lasers constitute the fundamental source of optically-coherent ultrashort-pulsed radiation, with huge impact in science and technology. Their modeling largely rests on the master equation (ME) approach introduced in 1975 by Hermann A. Haus. However, that description fails when the medium dynamics is fast and, ultimately, when light-matter quantum coherence is relevant. Here we set a rigorous and general ME framework, the coherent ME (CME), that overcomes both limitations. The CME predicts strong deviations from Haus ME, which we substantiate through an amplitude-modulated semiconductor laser experiment. Accounting for coherent effects, like the Risken-Nummedal-Graham-Haken multimode instability, we envisage the usefulness of the CME for describing self-modelocking and spontaneous frequency comb formation in quantum-cascade and quantum-dot lasers. Furthermore, the CME paves the way for exploiting the rich phenomenology of coherent effects in laser design, which has been hampered so far by the lack of a coherent ME formalism., Modelocked lasers are typically modeled using the Haus master equation which, albeit powerful, provides only a limited description in certain parameter regimes. The authors introduce another master equation formalism to describe laser modelocking that accounts for coherent effects and overcomes these limitations.

International audience; As nanocrystals (NCs) gain maturity, they become central building blocks for optoelectronics in devices such as solar cells and, more recently, infrared focal plane arrays. Now that proof of concept of these devices has been established, their optimization requires a deeper understanding of their electronic and optical features to engineering their optoelectronic properties accurately. Though PbS NCs have been extensively investigated, the complex optical index of PbS NC thin films remains mostly unknown. Some previous works have unveiled the optical index for this type of material optimized for solar cells (excitonic peak at 940 nm), but longer wavelengths remain scarce and surface chemistry effects, which are known to be of central importance for the layer doping, are simply unexplored. Here, we conduct a systematic investigation of the complex optical index of PbS NC thin films using broadband spectrally resolved ellipsometry. The obtained results are then compared with simulations combining Tight-Binding (TB) modeling at the NC level and Bruggeman model to expand the results to the film scale. While TB calculation gives the NC optical indices, we extract keys NC film parameters as the NC volume fraction and ligand indices by fitting Bruggeman formula to ellipsometry measurement. We also bring evidence that this joint modeling method can be conducted without the need for ellipsometry data while preserving the main feature of the experimental result. Finally, the unveiled optical indices are used to model the absorption of short-wave infrared diode stack based on PbS NCs and are relevant for state-of-the-art devices. Our electromagnetic modeling shows that the absorption within the contact is now a major limitation of the current device operated at telecom wavelength

International audience; We carry out a theoretical investigation of the backward interaction optical parametric oscillator (OPO), in which one of the generated waves is counterpropagating with respect to the pump. We derive simple, self-consistent analytical formulas for the continuous-wave and pulsed regimes. While consistent with previous works on the continuous-wave regime, our study enables us to extend the analysis to the pulsed regime. In particular, we derive simple expressions of the oscillation build-up time, pulsed threshold, and efficiency, for the first time, to our knowledge. We also investigate the peculiar spectral features of the backward OPO when pumped with a narrow-linewidth pulsed radiation, in particular, the absence of tolerance to phase mismatch and the natural ability to emit two waves with a Fourier-transform-limited spectrum. A comparison with the conventional forward OPO is also carried out to emphasize the unique properties of the backward OPO.

Context: The detection and characterization of Earth-like exoplanets (exoEarths) from space requires exquisite wavefront stability at contrast levels of $10^{-10}$. On segmented telescopes in particular, aberrations induced by cophasing errors lead to a light leakage through the coronagraph, deteriorating the imaging performance. These need to be limited in order to facilitate the direct imaging of exoEarths. Aims: We perform a laboratory validation of an analytical tolerancing model that allows us to determine wavefront error requirements in the $10^{-6} - 10^{-8}$ contrast regime, for a segmented pupil with a classical Lyot coronagraph. We intend to compare the results to simulations, and we aim to establish an error budget for the segmented mirror on the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed. Methods: We use the Pair-based Analytical model for Segmented Telescope Imaging from Space (PASTIS) to measure a contrast influence matrix of a real high contrast instrument, and use an analytical model inversion to calculate per-segment wavefront error tolerances. We validate these tolerances on the HiCAT testbed by measuring the contrast response of segmented mirror states that follow these requirements. Results: The experimentally measured optical influence matrix is successfully measured on the HiCAT testbed, and we derive individual segment tolerances from it that correctly yield the targeted contrast levels. Further, the analytical expressions that predict a contrast mean and variance from a given segment covariance matrix are confirmed experimentally., Comment: 16 pages, 16 figures, accepted for publication in A&A

International audience; Transitional Shockwave/Boundary Layer Interactions (SBLI) is studied experimentally. Experiments are conducted on a hollow-cylinder flare model in the R2Ch blowdown facility at a Mach number of 5 for three different Reynolds numbers in the transitional regime. Unsteady wall pressure measurements are conducted along with mean and unsteady heat flux measurements and high-speed Schlieren imaging. The images are then processed using Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD) to extract relevant information. Two main phenomena are identified and documented: the oblique modes traveling in the shear layer above the recirculation region and the streaks appearing in the reattachment region. New results illustrating the multiple physical origins of the streaks, either linked with globally unstable modes and convectively unstable mechanisms, are discussed.