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Modélisation de la propagation de fissure et formation d'écaille dans une piste de roulement sous chargement de contact Hertzien
No full textUne thématique beaucoup plus sur la fissuration : la ruptureInternational audienceCette étude propose de modéliser l’écaillage comme la propagation multi-branchée de fissures en fatigue, à partird’une fissure principale initiée en surface. La méthode des éléments finis, couplée avec la mécanique linéaireélastique de la rupture, est utilisée pour calculer les états de contraintes locaux sur un modèle 2D de piste deroulement muni d’une fissure inclinée, soumis à un chargement de contact hertzien. Pour prendre en comptele contact frottant au niveau des lèvres de fissures, une modélisation originale utilisant des éléments de zonescohésives a été mise en place. Les facteurs d’intensité de contraintes sont calculés pour différentes tailles defissures sur ce modèle. L’insertion d’une branche secondaire, dérivée de la fissure principale dans le modèle 2Dpermet d’affiner la compréhension du mécanisme d’écaillage et ses implications sur la durée de vie des roulementshybrides céramiques
Metasurface Doppler Cloak for Broadband Radar Stealth
No full textInternational audienceAbstract Concealing an aircraft is a multi‐faceted endeavor, notably involving radio and infrared frequencies. In a radar stealth context, it often translates to the reduction of the radar cross‐section (RCS). However, other routes that take advantage of radar signal processing exist. For instance, a solution has recently been developed, which consists in compensating the motion‐induced Doppler shift with a time‐modulated metasurface since Doppler radars filter out static targets to avoid being swamped by radar clutter (buildings, trees, etc.). Such a coating, referred to as a Doppler cloak, is able to compensate any frequency shift. However, frequency‐modulated radar signals require a broadening of the frequency conversion bandwidth of existing Doppler cloaks, which are all designed for harmonic signals. In this work, the focus is thus placed on a broadband Doppler cloak able to suppress the Doppler information over a wide frequency range. To achieve this, the reflection coefficient of a varactor diode‐loaded metasurface is linearized in time to obtain a linear phase ramp necessary to shift the frequency of impinging waves. Numerical and experimental validations are performed using frequency‐modulated continuous wave (FMCW) broadband radar signals over the VHF‐UHF range
Monolithic Dual-Polarized Leaky-Wave Array with Off-Axis Pointing, 36 dBi Gain and Unbalanced Beamforming Networks for Radar Applications
No full textInternational audienceThis paper proposes a compact, lightweight radiating panel for polarimetric radar, consisting of 32 dual-mode leaky waveguides fed by orthomode transducers and unbalanced beamforming networks. The array generates a high-gain pencil beam (46 λ long) with low sidelobes, achieved through equi-phase non-uniform excitation and modulation of the waveguide perforations. The three-layer monolithic panel, 3D printed using selective laser melting, integrates two beamforming networks and the radiating elements. The prototype is well matched for both polarization, it presents high isolation and a high-gain beam with off-axis pointing. This work demonstrates the first monolithic large radiating aperture with such a performance, providing a solution for balancing compactness and complexity in high-gain antenna arrays
Compact Geodesic Lens Antenna with 220° Field-of-View: from the Concept to Additive Manufacturing
No full textInternational audienceThis paper presents a geodesic lens antenna that provides a wide-angle coverage. First a lens was designed to tradeoff scan loss, gain and compactness for airborne radars. The field-of-view is then obtained by stacking this initial lens including a rotation between each element. The final geodesic lens antenna includes 18 pointing directions with a total coverage of 219.4 deg in the azimuth plane at 13.5 GHz. For the first time, the concept has been optimized specifically for full-metal additive manufacturing. The paper presents the final prototype with its early measurements showing good matching with simulations
Classification of Mental Workload Spatial Effects using Riemannian Manifold
No full textInternational audienceThis study investigates the use of Riemannian geometry to classify mental workload from an EEG dataset collected in an aeronautical context. The analysis, based on EEG data recorded from 16 participants performing a Simon task, aimed to differentiate low and high workload conditions. Using covariance matrices and a Minimum Distance to Mean (MDM) classifier, the results demonstrate spatial effects of mental workload irrespective of the investigated spectral domain. This demonstrates that spatial information is distributed evenly across all explored frequency bands
Chaining the DART and SOLENE microclimat models to support the use of TIR satellite data in urban climate studies
No full textInternational audienceLand surface temperature (LST) derived from thermal infrared (TIR) satellite data is widely used in urban climate research due to the repetitive availability of data over large areas worldwide. LST directly reflects the interactions between urban surfaces, the atmosphere and human activities, supporting hotspots identification, comfort indices estimation or mitigation strategies planning. However, its use is limited by the spatial and temporal resolutions of current spaceborne sensors. The upcoming TIR satellite missions (LSTM, TRISHNA, SBG), with spatial resolution between 37 and 60 m and up to 3-days revisit, open up new opportunities to study urban climate at the neighborhood scale. At this scale, retrieving accurate and comparable LST over cities remains a challenge. Urban heterogeneity and 3D structure greatly impact satellite measurements, requiring a good understanding of 3D radiative processes for reliable LST estimates. Another challenge is the transition to air temperature, which is essential for improving comfort and quality of life in cities.To address these challenges, a model chaining approach is implemented to generate physically coherent datasets linking remote sensing measurements to microclimate variables over any urban configuration in order to investigate how they relate to each other. On the one hand, the DART radiative transfer model simulates radiative exchanges in the urban canopy and the corresponding remotely sensed images, provided that the surface temperature distribution in the 3D urban scene is known. On the other hand, the thermo-radiative model SOLENE-microclimat simulates the surface temperature distribution in the 3D scene required by DART, as well as the air temperature in the canopy but does not allow the simulation of multispectral satellite data. Chaining the two models bridges the gap between remotely sensed TIR parameters and microclimate variables. This presentation gives an overview of the modelling chain and presents some concrete examples of its application to urban climate studies
Classification Algorithms for Fast Retrieval of Atmospheric Vertical Columns of CO in the Interferogram Domain
No full textInternational audienceOnboard the MetOp satellite series, Infrared Atmospheric Sounding Interferometer (IASI) is a Fourier Transform spectrometer based on the Michelson interferometer. IASI acquires interferograms, which are processed to provide high-resolution atmospheric emission spectra. These spectra enable the derivation of temperature and humidity profiles, among other parameters, with exceptional spectral resolution. In this study, we evaluate a novel, rapid retrieval approach in the interferogram domain, aiming for near-real-time (NRT) analysis of large spectral datasets anticipated from next-generation tropospheric sounders, such as MTG-IRS. The Partially Sampled Interferogram (PSI) method, applied to trace gas retrievals from IASI, has been sparsely explored. However, previous studies suggest its potential for high-accuracy retrievals of specific gases, including CO, CO2, CH4, and N2O at the resolution of a single IASI footprint. This article presents the results of a study based on retrieval in the interferogram domain. Furthermore, the optical pathway differences sensitive to the parameters of interest are studied. Interferograms are generated using a fast Fourier transform on synthetic IASI spectra. Finally, the relationship to the total column of carbon monoxide is explored using three different algorithms—from the most intuitive to a complex neural network approach. These algorithms serve as a proof of concept for interferogram classification and rapid predictions of surface temperature, as well as the abundances of H2O and CO. IASI spectra simulations were performed using the LATMOS Atmospheric Retrieval Algorithm (LARA), a robust and validated radiative transfer model based on least squares estimation. The climatological library TIGR was employed to generate IASI interferograms from LARA spectra. TIGR includes 2311 atmospheric scenarios, each characterized by temperature, water vapor, and ozone concentration profiles across a pressure grid from the surface to the top of the atmosphere. Our study focuses on CO, a critical trace gas for understanding air quality and climate forcing, which displays a characteristic absorption pattern in the 2050–2350 cm−1 wavenumber range. Additionally, the study explores the potential of correlating interferogram characteristics with surface temperature and H2O content, aiming to enhance the accuracy of CO column retrievals. Starting with intuitive retrieval algorithms, we progressively increased complexity, culminating in a neural network-based algorithm. The results of the NN study demonstrate the feasibility of fast interferogram-domain retrievals, paving the way for operational applications
Méthodologie numérique avancée pour la prévision de l'aérodynamique instationnaire autour des gouvernes à grille de lanceurs spatiaux réutilisables
No full textInternational audienceOver the past decade, mastering launch vehicle reusability has emerged as a pivotal strategic stake for access to space. Such a capability enables reduced launch costs as well as increased frequency. In this context, numerous current configurations of reusable space launchers incorporate grid fins to facilitate controlled first-stage recovery. However, the flow around these devices exhibits significant variability depending on geometric features (e.g. angles of attack and sideslip, Mach number, stagnation pressure and temperature). Traditionally, preliminary design phases focus on rapid prediction of average aerodynamic performances across a wide range of configurations. Yet, given the complexity inherent to reusable launch vehicle (RLV) flight timelines, it appears crucial to also anticipate fluctuating quantities driving dynamic loads, ensuring structural integrity from early development stages rather than relying solely on downstream risk mitigation studies as for expendable launch vehicles (ELV). To address this, the design of a highly-resolved reference numerical test case to assess the accuracy of faster approaches facilitating the set-up and execution of CFD calculations should be prioritized. Among them, one can mention the Zonal Immersed Boundary Conditions (ZIBC) 11-13, 20 which permit to approximate complex geometry mimicking the presence of obstacles with source terms in the discretized compressible Navier-Stokes equations or to orient automatic mesh adaptation goals as used in the ONERA's SoNICS computational framework. 8 In this context, a numerical workflow based on the ZIBC strategy using a RANS approach is applied to several installed grid fin configurations in order to assess the compatibility of load reconstruction methods developed by Manueco et al. 11, 12 on these devices containing thin walls. This mandatory step paves the way to use unsteady high-fidelity methods such as Zonal Detached Eddy Simulation (ZDES) mode 2 (2020) 5 to predict rapidly and accurately the effect of grid fins on the fluctuating field of RLV
