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EDAPHOS - Utilisation de l’imagerie hyperspectrale pour la caractérisation et le suivi de solscontaminés phytostabilisés en Europe
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Robust stability and performance analysis in the presence of LTI and LTV uncertainties with bounded rates of variation
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Évaluation des effets de la sécheresse et de l'urbanisation sur les arbres urbains à l'aide de la série temporelle Sentinel-2
No full textInternational audienceUrban trees provide essential ecosystem services, including temperature regulation, carbon storage, and biodiversity conservation, which are crucial for enhancing urban living conditions. However, they are exposed to various stress factors in urban environments, such as limited light, restricted growth space, and increased exposure to pollutants. Moreover, climate forecasts predict an increase in the frequency and severity of droughts and drought events which can lead to tree defoliation, increased sensitivity to pests and pathogens and therefore increase tree mortality. Quantifying and monitoring the response of urban trees to stress factors can be used as a proxy for tree health assessment.The effects of stress factors (drought, urban conditions) on urban trees dynamics have already been observed in situ with dendrochronology and biochemical analysis. These effects have been shown on a large scale using remote sensing data, but the coarse spatial resolution of the data used (MODIS, 250m) make it impossible to discriminate species that suffer from stress from those that are more resilient. We first investigated tree dynamics in Rennes (France) at fine spatial scale using Sentinel-2 time series to analyze the impact of drought events and urbanization on tree health. Specifically, we analyzed five deciduous tree species (Platanus Acerifolia, Acer Platanoides, Fraxinus Excelsior, Quercus Rubra and Quercus Robur) during two years characterized by very contrasting climatic conditions (2021 and 2022) and along an urban-rural gradient. Vegetation dynamics were monitored using a vegetation index (ARVI). Phenological, productivity and disturbance metrics which characterize respectively key dates and growth-cycle duration, tree growth and productivity, and intra-annual anomalies in tree-dynamics, were derived from the ARVI filtered time-series. Drought events were characterized using a standardized precipitation index derived from climate data and urban intensity was determined at pixel-scale using the Copernicus’ Imperviousness Density data. Temporal metrics values were aggregated at patch-scale (group of contiguous trees of the same species). The results showed a notable extension in the length of the growing season and maturity periods for four species in 2022 (the driest year), driven primarily by delayed senescence and end-of-season dates. Productivity metrics displayed mixed responses, with some species showing reduced growth under drought, while others, like Platanus Acerifolia, exhibited increased productivity, suggesting potential resilience mechanisms. Elevated disturbance levels were observed in 2022, indicating higher stress conditions compared to the previous year. Furthermore, the results outlined that urban intensity was most often correlated with extended growing seasons and altered productivity dynamics. Based on these results, we then developed an exploratory approach at the city scale to identify trees with unusual temporal profiles that correspond to exacerbated trees responses to stress factors. We determined a reference profile for each tree species, corresponding to the average profile of trees growing in similar and contrasting urban conditions, and compared tree profiles to this reference profile based on statistical analysis. This analysis was conducted for period 2016-2024 to monitor tree disturbance levels. This tool can be used by urban tree managers to locate, guide and schedule in situ investigations to monitor tree health and potentially identify trees in decline. This study underscores the value of Sentinel-2 time series in supporting urban tree management and policy decisions amidst changing environmental and climate conditions
Extension de l'expérience de la Phase-Locked Loop pour l'application à l'essai de vibration au sol
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Analyse résolvante de champs moyens turbulents : une application à l’aéroacoustique
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Approche adjoint body-force pour l'optimisation des systèmes moteurs fortement couplés à l'aérodynamique de la cellule avion
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Révéler le couplage neurovasculaire avec une haute résolution spatiale et temporelle dans la rétine humaine in-vivo
No full textInternational audienceNeurovascular coupling (NVC) regulates local blood flow in response to neuronal activity, yet its precise characterization at the capillary level has been hindered by the lack of a noninvasive, high-resolution imaging method. Here, we introduce the adaptive optics rolling slit ophthalmoscope, a unique noninvasive, label-free, high-speed, cellular-resolution clinical imaging system for assessing retinal NVC in the living human eye. Using an off-axis phase contrast approach combined with camera-based confocal slit gating, our method provides large field-ofview imaging of arterial and venular walls, enabling the study of vascular dynamics with unprecedented spatiotemporal precision. Our findings highlight that this level of precision is essential for accurately distinguishing NVC-driven vasodilation from spontaneous fluctuations, such as vasomotion and the cardiac cycle. By bridging the gap between fundamental neurovascular research and clinical applications, this approach offers a powerful tool for neuroscience research and early disease detection and monitoring of neurodegenerative and vascular disorders.Le couplage neurovasculaire (NVC) régule le flux sanguin local en réponse à l'activité neuronale, mais l'absence de méthode d'imagerie non invasive et à haute résolution empêche sa caractérisation précise à l’échelle des capillaires. Ici, nous présentons l'ophtalmoscope rolling-slit (à fente roulante), un système d'imagerie clinique unique, non invasif, sans marquage, qui acquière des images de la rétine à haute cadence et à résolution cellulaire, pour évaluer le NVC rétinien dans l'œil humain in-vivo. En utilisant une approche de détection hors axe générant du contraste de phase combinée au filtrage confocal par la fente roulante de caméra rétinienne, notre méthode permet une imagerie sans distorsion des parois artérielles et veineuses, rendant possible l'étude de la dynamique vasculaire avec une précision spatio-temporelle sans précédent. Nos résultats montrent que ce niveau de précision est essentiel pour distinguer avec exactitude la vasodilatation induite par le NVC des fluctuations spontanées, telles que la vasomotion et le cycle cardiaque. En faisant le lien entre la recherche fondamentale sur le réseau neurovasculaire et les applications cliniques, cette approche offre un nouvel outil puissant pour la recherche en neurosciences, ainsi que pour la détection précoce et le suivi des maladies neurodégénératives et vasculaires
Nouveau dispositif Pistil dédié à la métrologie de surfaces d'onde segmentées pour les lasers à combinaison cohérente
No full textTo develop lasers capable of delivering high peak and average powers, new strategies have emerged over the past few decades. These approaches are based on the combined use of numerous pre-amplified laser fibers to generate a single, ultra-intense recombined beam. To ensure optimal efficiency, it is essential to precisely control the phase of each individual fiber. In this context, the Pistil interferometry technique was developed at ONERA, providing a metrology, control, and diagnostic tool for Coherent Beam Combining (CBC) laser sources. The technique is based on the use of a hole mask and a diffraction grating to analyze, through interferograms, the phase differences between all neighboring fibers in the laser system. During this PhD work, a new version of the interferometer was developed, abandoning the hole mask in favor of a microlens array. This innovation allows the full incident wavefront to be taken into account and results in a more compact and easier-to-implement device. This manuscript presents a theoretical description and a detailed modeling of this new instrument, named Full- Pistil. It was then tested on an experimental bench developed to simulate a segmented wavefront surface using a segmented mirror. Initial tests have verified the reconstruction algorithm and shown a metrology accuracy better than λ/160. Finally, the prototype was integrated into a CBC laser chain to evaluate its performance under real operating conditions and identify possible improvements. The interferometer is intended for multiple uses on a CBC laser chain, in both closed and open loops, from alignment assistance to laser diagnostics, including fast measurement for control-command applications.Afin de réaliser des lasers pouvant délivrer des puissances crête et moyenne élevées, de nouvelles stratégies ont émergé depuis quelques dizaines d’années, reposant sur l’utilisation combinée de nombreuses fibres laser pré-amplifiées pour générer un faisceau recombiné ultra intense. Pour garantir son efficacité, il est nécessaire de contrôler parfaitement la phase de chacune des fibres. Dans ce contexte, l’interférométrie Pistil a été développée à l’ONERA, offrant un outil de métrologie, contrôle et diagnostic des sources laser CBC. La technique repose sur l’utilisation d’un masque à trous et d’un réseau de diffraction afin d’analyser, à travers des interférogrammes, les différences de phase entre toutes les fibres voisines du système laser. Durant cette thèse, une nouvelle version de l’interféromètre a été développée, délaissant le masque à trous au profit d’une matrice de microlentilles. Cette innovation permet de prendre en compte l’intégralité de la surface d'onde incidente et rend le dispositif plus compact et facile à mettre en œuvre. Ce manuscrit propose une description théorique et une modélisation détaillée de ce nouvel instrument, nommé FullPistil. Il a ensuite été testé sur un banc expérimental développé pour simuler une surface d’onde segmentée grâce à un miroir segmenté, ce qui a permis de vérifier l’algorithme de reconstruction et révélé une fidélité de métrologie en dessous de λ/160. Enfin, le prototype a été intégré sur une chaîne laser CBC afin d’évaluer ses performances en conditions réelles et identifier les améliorations possibles. L’interféromètre est promis à une utilisation multiple sur une chaîne laser CBC, en boucle fermée comme en boucle ouverte, de l’aide à l’alignement au diagnostic laser, en passant par la mesure rapide pour le contrôle-commande
Extraction de grandeurs de couche limite à partir de calculs CFD sur des géométries complexes: méthodes et application à la conception de véhicules
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Étude numérique des perturbations acoustiques en soufflerie hypersonique conventionnelle
No full textInternational audienceIn order to reproduce the free stream acoustic perturbations found in conventional hypersonic wind tunnels, direct numerical simulations of transitional and turbulent boundary layers are performed for a flat plate geometry in representative flow conditions. The transition of the boundary layer is triggered either through volume forcing of optimal perturbation modes obtained with a resolvent-based linear stability analysis, or with a random forcing approach. This numerical setup allows to simulate the noise radiation not only from the turbulent region but also from the transition region. Characteristics of the noise generated by the turbulent region compare well with existing data. When using a harmonic forcing to trigger the transition, the spectral content of the transition region is reflected into the acoustic perturbations originating from this region, whereas the radiation from the turbulent region shows broadband spectra