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A tetrahedron-based discretization for FFT-based computational homogenization with smooth solution fields
No full textInternational audienceWe propose a new discrete FFT-based method for computational homogenization of micromechanics on a regular grid that is simple, fast and robust. The discretization scheme is based on a tetrahedral stencil that generates two independent mechanical problems on distinct face centered cubic subgrids. The resulting equilibrium problems may be merged into a single one that is solved by a gradient-based method equivalent to the original Moulinec-Suquet basic scheme.The tetrahedral stencil displays three crucial properties. First, and most importantly, the Fourier representation of the associated Green operator is defined for any finite q-vector generated by the periodic boundary conditions and that does not belong to the reciprocal lattice of the discrete grids. As shown in the paper, this property guarantees that, for any elastic contrast, even infinite, mechanical equilibrium is always mathematically stable, i.e. free of any unphysical patterns, such as oscillations, ringing or checker-boarding, a property which is not shared by the original Moulinec-Suquet discretization [1, 2] nor by the rotated scheme proposed by Willot [3]. Second, the components of tensorial quantities are all defined on the same location, which permits the use of any elastic anisotropy and any spatial variation of the material fields. Third, convergence to equilibrium using the simplest iterative scheme, i.e. the basic scheme, is fast and the number of iterates stabilizes at high contrast, so that infinite contrast is obtained without additional computational cost. The efficiency and accuracy of the proposed tetrahedral stencil are assessed through several case studies, including microstructures that consist of a single inclusion with various elastic properties (finite eigenstrain, void, hard precipitate), a microstructure that contains simultaneously soft and stiff inclusions and complex 3D microstructures with random spherical voids. Convergence of effective elastic properties with grid refinement is investigated. An implementation of the tetrahedral stencil within the polarization-based fixed point algorithm of Monchiet-Bonnet is also proposed
Optimisation fiabiliste aéroélastique des ailes composites via meta-modélisation
No full textInternational audienceThis paper presents a framework based on surrogate modeling for the reliability-focused optimization of aeroelastic performance in composite wing structures. The optimization process centers on modifying the stacking sequences of composite materials to enhance aeroelastic performance, while explicitly accounting for uncertainties in ply angles resulting from manufacturing variability. To address the high computational cost of direct aeroelastic evaluations, the framework employs a modified high-dimensional Kriging surrogate model combined with an adapted Efficient Global Optimization (EGO) algorithm. This approach enables an efficient exploration of the design space, balancing accuracy with computational feasibility. By integrating reliability analysis within the optimization process, the framework also ensures robust aeroelastic performance under uncertain conditions. Results demonstrate that the proposed method identifies optimal stacking sequences with improved aeroelastic reliability while significantly reducing computational costs. This methodology offers a novel approach to enhancing the reliability of composite aeroelastic structures, with implications for high-performance aerospace applications.Cet article présente un cadre basé sur la modélisation par substitut pour l'optimisation fiabiliste des performances aéroélastiques dans les structures d'ailes en matériaux composites. Le processus d'optimisation se concentre sur la modification des séquences d'empilement des matériaux composites afin d'améliorer les performances aéroélastiques, tout en tenant explicitement compte des incertitudes liées aux angles des plis dues à la variabilité de fabrication. Pour faire face au coût computationnel élevé des évaluations aéroélastiques directes, le cadre utilise un modèle substitut de Krigeage modifié en haute dimension, combiné à un algorithme d'Optimisation Globale Efficiente (EGO) adapté. Cette approche permet une exploration efficace de l'espace de conception, équilibrant précision et faisabilité computationnelle. En intégrant l'analyse de fiabilité dans le processus d'optimisation, le cadre garantit également des performances aéroélastiques robustes dans des conditions incertaines. Les résultats démontrent que la méthode proposée identifie des séquences d'empilement optimales avec une fiabilité aéroélastique améliorée tout en réduisant significativement les coûts computationnels. Cette méthodologie offre une approche novatrice pour renforcer la fiabilité des structures aéroélastiques en matériaux composites, avec des implications pour des applications aéronautiques haute performance
Electrohydrodynamic characteristics of a needle-to-ring positive corona discharges: self-consistent modeling and turbulence effects
No full textInternational audienceAbstract We report on the voltage–current characteristics as well as the voltage–velocity characteristics of a needle-to-ring configuration by self-consistent, 2D-axisymmetric, multi-physics plasma-fluid simulations. Parametric studies are performed to investigate the effects of background ionization level on the plasma electrical characteristics as well as turbulence parameters on the resulting flow. Our results show that the discharge and the flow exhibit an annular structure around the needle (anode) tip, with a maximum positive ion density from 1.5 × 10 18 –2.7 × 10 18 m − 3 and a maximum flow velocity from 10–15 m s − 1 , for overvoltages between 4−12 kV and an anode–cathode distance of 20 mm . Good overall agreement with experimental findings is achieved, noting that background ionization level can be used as a tuning parameter to better match experimentally measured current values even with a simplified plasma-chemistry. A better agreement between simulation and experiments is achieved concerning the voltage–velocity curves. Therefore, the latter are likely to be better indicators for assessing and validating electrohydrodynamic effects of corona actuators through numerical modeling. The flow dynamics indicate that positive-corona-induced ionic winds are most likely laminar to turbulent transitional flows, with a ratio between eddy viscosity and air viscosity varying between 0 − 200
Évaluation critique des fermetures quadratiques pour la prédiction du décollement en coin
No full textInternational audienceThe work proposed in this paper aims at improving the physical understanding and modeling of junction flows by conducting an analysis of several quadratic constitutive relations (QCRs) applied to corner vortices. Reynolds-averaged Navier–Stokes computations are performed on two academic configurations for verification purposes and on a technical application of wing–body juncture featuring corner flows to achieve a better comprehension of the QCR variants’ ability to reproduce turbulent stress combinations relevant to secondary flows generation. This addresses the more theoretical interest of analyzing the onset of corner separation (at an angle of attack [Formula: see text]) and the role of corner flow vortices. Linear eddy viscosity model and the extended QCR failed to detect these vortical structures; the latter encountered numerical stability difficulties. On the other hand, QCR2000, QCR2020, and QCR(r) showed behavior consistent with a simulation using the Reynolds Stress Model, experiments, and literature results. This better prediction was found to be related to the ability of these models to accurately predict the stress-induced vortex that acts in delaying and reducing the size of the corner separation. Notably, the more recent QCR(r) and QCR2020 have enhanced the estimation of the normal stress combination [Formula: see text] compared to the more widespread QCR2000, demonstrating their validity as alternative models
Integrated Decision-Making Approach for Automated Diversion Runway Selection and Flight Path Generation
No full textInternational audienceThe automated in-flight airport diversion trajectory generation in case of emergency is, to this day, an open problem and a real need for general and civil aviation. This problem is often addressed by solving three subproblems independently in cascade: i) selection of diversion airport, runway, and approach mode; ii) obstacle-free path planning; and iii) flyable trajectory generation under given aircraft performance. This paper proposes an integrated decision-making approach for subproblems (i) and (ii) by performing multitarget minimum-risk path planning. The [Formula: see text]-best runway candidates are first identified based on runway-risk criteria. Then the Monte Carlo tree search algorithm is applied to simultaneously search for obstacle-free paths to these [Formula: see text] candidates while minimizing a global risk by aggregating the en route and runway risks. Unlike the cascaded approaches, the proposed integrated approach can evaluate the en route risk more precisely and efficiently and use it in deciding the most suitable diversion runway. Moreover, the planning difficulty is also reflected in the runway selection. The proposed approach is tested in simulation for 1000 randomly generated missions. The results are compared with those of cascaded approaches, demonstrating a higher success rate of feasible path generation in difficult mission configurations
TRISHNA: an Indo-French Space Mission to Study the Thermography of the Earth at Fine Spatio-Temporal Resolution
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Mediterranean forest traits retrieval from hybrid inversion: a multi-sensor and radiative transfer modelling comparison
No full textInternational audienceCharacterizing the condition of Mediterranean forests is crucial for fire risk prevention, and for assessing the impact of increasing droughts and changes in land use. Optical remote sensing provides access to various biophysical and biochemical vegetation properties, including essential biodiversity variables. This capacity enhances our understanding of the structure and functioning of these forests. However, the accuracy these properties can be estimated depends on several factors: the scale of observation (i.e. the spatial resolution), the spectral richness (i.e. the number of spectral bands, sampling and resolution) and the revisit time (for monitoring purposes) of the remote sensing sensors. Hybrid inversion methods are increasingly used as they require fewer field measurements compared to empirical methods. These methods adopt a more generalizable and physically-based approach by relying on the combination of radiative transfer model (RTM) simulations and machine learning regression methods (MLRA). Key challenges include the adequate choice of the RTM (e.g. 1D, 2D or 3D) and the accurate input parameterization of the RTM, in order to account for the particularities of the observed landscape and to simulate realistic remote sensing images (e.g. differences between processing dense or open forests). The objective of this study is to compare the accuracy of vegetation trait estimations for the tree overstory layer based on a hybrid inversion using either 1D or 3D RTM and a multi-sensor dataset encompassing multi-/hyperspectral and airborne/satellite data. Two Mediterranean forests located in the South of France are studied: Pic Saint Loup and Puéchabon. We focused on two oak species: Quercus ilex (evergreen) and Quercus pubescens (deciduous). The MEDOAK campaign [1] occurred in June 2021 and resulted in the collection of a variety of field and lab measurements, including overstory and understory inventory, plant area index, spectroscopic data, leaf-clip optical data, trait measurements and inversions with the leaf model PROSPECT [2][3]. This ground data collection was carried out simultaneously to the Hypersense campaign, a joint airborne campaign organized by NASA and ESA, and managed by the University of Zurich in cooperation with NASA/JPL to support the upcoming Copernicus Hyperspectral Imaging Mission for the Environment (CHIME) and Surface Biology and Geology (SBG) satellite missions. Airborne imaging spectroscopy data were acquired with AVIRIS-Next Generation sensor at 1m and 3m spatial resolutions. These airborne acquisitions were complemented with Sentinel-2, PRISMA and DESIS acquisitions over the same period. In addition, UAV LiDAR scans were acquired on some forest plots. Past work aimed at designing detailed 3D forest mockups from these LiDAR point clouds, assessing the accuracy of the simulations performed with the DART 3D RTM [4], and studying the contribution of factors including understory reflectance, woody element and abstract modeling of leaf optical properties in total canopy reflectance [5]. The current work focuses on identifying the optimal geometrical parameterization for vegetation trait estimations by comparing: (1) a homogeneous and simple representation with PROSAIL 1D RTM [6], (2) a simplified forest representation using geometric objects with DART and (3) a detailed 3D representation obtained from LiDAR mockups with DART. Targeted vegetation traits are plant area index, leaf pigments, water and dry matter content, and live fuel moisture content. Inversion methods are based on existing tools [7] and based on [8]. For comparison purposes, several MLRAs are compared, including PLSR, SVMR and RFR. The results are explored and discussed between the three different geometrical modelling strategies, the remote sensing sensor characteristics (spatial and spectral), and the studied forest plot characteristics (canopy cover, species composition). This work also aims at preparing the French hyperspectral satellite mission BIODIVERSITY, featuring 10 m spatial resolution [9][10]. Therefore, 10 m images were simulated from 1m airborne data and comparisons were performed to assess the potential of improved spatial resolution compared to current operational hyperspectral missions at 30 m. To conclude, these results will contribute to assess the complementarity of remote sensing data (multi- vs hyperspectral, airborne vs satellite) to map vegetation traits useful to determine forest water stress and risk prevention through their monitoring. [1] K. Adeline, J.-B. Féret, H. Clenet, J.-M. Limousin, J.-M. Ourcival, F. Mouillot, S. Alleaume, A. Jolivot, X. Briottet, L. Bidel, E. Aria, A. Defossez, T. Gaubert, J. Giffard-Carlet, J. Kempf, D. Longepierre, F. Lopez, T. Miraglio, J. Vigouroux and M. Debue (2024). Multi-scale datasets for monitoring Mediterranean oak forests from optical remote sensing during the SENTHYMED/MEDOAK experiment in the north of Montpellier (France). Data in Brief, 53, 110185. [2] Féret J-B, Gitelson AA, Noble SD and Jacquemoud S (2017). PROSPECT-D: Towards modeling leaf optical properties through a complete lifecycle. Remote Sensing of Environment, 193, 204–215. [3] J.-B. Féret, J. Giffard-Carlet, S. Alleaume, X. Briottet, V. Chéret, H. Clénet, J.-P. Denux, J.-P. Gastellu-Etchegorry, A. Jolivot, J.-M. Limousin, F. Mouillot, J.-M. Ourcival and K. Adeline (2022). Estimating functional traits in Mediterranean ecosystems using spectroscopy from leaf to canopy scale. 2nd Workshop on International Cooperation in Spaceborne Imaging Spectroscopy, 19-21 october 2022, Frascati, Italy, oral. [4] Yingjie Wang, Abdelaziz Kallel, Xuebo Yang, Omar Regaieg, Nicolas Lauret, Jordan Guilleux, Eric Chavanon, Jean-Philippe Gastellu-Etchegorry (2022). DART-Lux: An unbiased and rapid Monte Carlo radiative transfer method for simulating remote sensing images. Remote Sensing of Environment, Volume 274, 2022, 112973. [5] M. Debue, G. Vincent, S. Alleaume, F. de Boissieu, X. Briottet, J.-B. Féret, J.-P. Gastellu-Etchegorry, J.-M. Limousin, D. Longepierre and K. Adeline. Adequacy of Mediterranean forest simulations from DART radiative transfer model and UAV laser scanning data to multi- and hyperspectral images. SPIE Remote Sensing, 3-6 September 2023, Amsterdam, Netherlands, oral and proceeding. [6] Jacquemoud S, Verhoef W, Baret F, Bacour C, Zarco-Tejada PJ, Asner GP, François C and Ustin SL (2009). PROSPECT+ SAIL models: A review of use for vegetation characterization. Remote Sensing of Environment, 113:S56–S66. [7] Féret, J.-B. & de Boissieu, F. An R package for the simulation of canopy reflectance using the model PROSAIL (PROSPECT+SAIL). https://gitlab.com/jbferet/prosail/. [8] Miraglio, T., Adeline, K., Huesca, M., Ustin, S. and Briottet, X. (2022). Assessing vegetation traits estimates accuracies from the future SBG and biodiversity hyperspectral missions over two Mediterranean Forests. International Journal of Remote Sensing, 43(10), 3537-3562. [9] K. Adeline. APR CNES TOSCA SENTHYMED, REMOTE sensing and in situ observations to study TREEs in natural and manmade landscapes (2023). https://remotetree.sedoo.fr/senthymed/. [10] X. Briottet, K. Adeline, T. Bajjouk, V. Carrère, M. Chami, Y. Constans, Y. Derimian, A. Dupiau, M. Dumont, S. Doz, S. Fabre, P.Y. Foucher, H. Herbin, S. Jacquemoud, M. Lang, A. Le Bris, P. Litvinov, S. Loyer, R. Marion, A. Minghelli, T. Miraglio, D. Sheeren, B. Szymanski, F. Romand, C. Desjardins, D. Rodat, B. Cheul (2024). End-to-end simulations to optimize imaging spectroscopy mission requirements for seven scientific applications. ISPRS Open Journal of Photogrammetry and Remote Sensing, 12, 100060
Implémentation d’un contrôleur non-linéaire dans la méthode Boucle de Phase Asservie (PLL) pour l’identification expérimentale de structures non-linéaires
No full textInternational audienceExperimental continuation methods are used to retrieve and identify nonlinear characteristics of vibrating structures. Among the available methods, Phase-Locked Loop (PLL) allows for an easy-to-implement yet efficient method to continue nonlinear solutions such as backbone curves or frequency response functions. The PLL automatically locks onto the prescribed phase and thanks to a linear (proportional-integral) controller, can stabilize unstable periodic orbits. However, the tuning of the different parameters to be used in such a loop are seldomly documented in the literature, which in turn might lead to long duration tests. To ease the tuning effort and reduce the experimenting time, a nonlinear controller is here proposed as a way to improve the efficacy of Phase-Locked Loop testing. Thanks to the proposed design, named NCPLL (Nonlinear Controller PLL), most of the parameters are tuned easily, while a rapid locking to the prescribed state is at hand. The nonlinear gain can be easily adapted to reach a locked state rapidly. The efficacy of the NCPLL is first demonstrated on simple numerical examples including nonlinear oscillators with smooth restoring forces and Coulomb friction, and a finite element beam model with localized nonlinearities. Then the method is deployed on two different experimental test rigs. First, the case of smooth nonlinearity is tackled thanks to a cantilever beam vibrating in the magnetic field created by two magnets. Finally, the case of friction is addressed by considering an assembled beam with friction joints. In all the tested cases, the NCPLL shows excellent performance, requiring minimal tuning efforts whilst leading to fast measurements.Les méthodes de continuation expérimentales sont utilisées pour extraire et identifier les caractéristiques non linéaires des structures vibrantes. Parmi les méthodes disponibles, la méthode de Boucle de Phase Asservie (Phase-Locked Loop - PLL) constitue une approche simple à mettre en œuvre et efficace pour poursuivre des solutions non linéaires telles que des "backbone curves" ou des fonctions de réponse en fréquence, étendant ainsi le champ d’application des méthodes de résonance de phase (PRM). La PLL se verrouille automatiquement sur la phase prescrite et, grâce à un contrôleur linéaire (proportionnel-intégral), elle peut stabiliser des orbites périodiques instables. Cependant, le réglage des différents paramètres à utiliser dans une telle boucle est rarement documenté dans la littérature, ce qui peut entraîner des essais de longue durée. Afin de faciliter le réglage et de réduire le temps d’expérimentation, un contrôleur non linéaire est proposé ici pour améliorer l’efficacité des tests basés sur la PLL. Grâce à ce design, nommée NCPLL (Nonlinear Controller PLL), le nombre de paramètres à ajuster est réduit, tout en assurant un verrouillage rapide sur l’état prescrit. Une borne supérieure pour le gain du contrôleur non linéaire est déduite à partir de développements théoriques. L’efficacité du NCPLL est d’abord démontrée sur des exemples numériques simples, incluant des oscillateurs non linéaires avec des forces de rappel régulières et des forces de frottement de type Coulomb, ainsi qu’un modèle éléments finis de poutre avec des non-linéarités localisées. Ensuite, la méthode est appliquée à deux bancs d’essais expérimentaux distincts. Dans un premier temps, le cas d’une non-linéarité régulière est étudié avec une poutre cantilever vibrant dans le champ magnétique créé par deux aimants. Enfin, le cas du frottement est abordé à travers une poutre assemblée comportant des joints frottants. Dans tous les cas testés, le NCPLL démontre d’excellentes performances, nécessitant un réglage minimal tout en permettant des mesures rapides
Imagerie de température dans les écoulements gazeux et liquides en utilisant des particules luminescentes
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Simulations détaillées de la veine vide de la soufflerie transsonique S3Ch
No full textInternational audienceIn order to be able to reach a numerical digital twin for a wind tunnel, precise validation must be achieved. Due to the complexity of the wind tunnels (corners with vanes, engine, honey combs and filters. . . ) and the numeric complexity (meshes, turbulence models, spatial schemes, implementation. . . ) the validation of the full circuit as a whole is challenging. Thus, in this paper, we focus on the validation of the high speed leg of the S3Ch tunnel (ONERA S3Ch return circuit wind tunnel located in Meudon). The paper features several RANS simulations from various partners with comparisons to experimental data. The empty test section is investigated at transonic speed (M = 0.700). Most of the detailed comparisons are carried out while operating the tunnel fully subsonic. Some insights are given on the operation of the tunnel with the second sonic throat active. This mode is challenging for the numerical simulations.Afin d'obtenir un jumeau numérique pour une soufflerie, une validation précise doit être réalisée. La complexité des souffleries (aubes dans les coudes, moteur, nids d'abeille, filtres…) combinée à la complexité numérique (maillages, modèles de turbulence, schémas en espace, implémentation…) rendent la validation sur le circuit complet très difficile. C'est pourquoi cette communication porte sur la validation de la portion allant du convergent au diffuseur de la soufflerie S3Ch (soufflerie ONERA à retour située à Meudon). La communication présente des simulations de divers partenaires comparées aux résultats expérimentaux. La veine vide est étudiée en régime transsonique (M = 0,700). La plupart des comparaisons portent sur les conditions d'opération en veine subsonique. Quelques résultats sont présentés en fonctionnement avec second col. Ce fonctionnement se révèle exigeant pour les méthodes numériques