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Maximization of Lift-to-Drag Ratio for VLEO Platforms using Free-Form Deformation Techniques
No full textInternational audienceThe present work introduces a novel approach to optimize the shape of platforms operating in Very Low Earth Orbit (VLEO) by maximizing their Lift-to-Drag (L/D) ratio. VLEO platforms encounter significant atmospheric drag due to the residual atmosphere at these altitudes, posing challenges for long-duration missions However, this residual atmosphere can also be harnessed to generate beneficial aerodynamic forces, offering a potential strategy to reduce the costs of orbital maneuvers. To this end, the presented method leverages Free-Form Deformation (FFD) techniques to simultaneously minimize drag and maximize lift through the use of an aerodynamic evaluation tool tailored for free molecular flow. Central to this approach is a custom-developed shape generator, which converts parametric geometries into 3D meshes, enabling efficient exploration of various configurations. A first test case is conducted on a sphere under volume and bounding box constraints to validate the optimization framework, allowing the algorithm to explore optimal shapes within defined physical limits. The results are analyzed and benchmarked against previous studies, demonstrating notable potential for improving aerodynamic performance. Subsequently, we extend the study to a more complex shape and apply the same FFDdriven optimization. This second test case explores the potential of exploiting non-conventional geometries in VLEO environments, where the trade-offs between lift generation and drag reduction are crucial for mission efficiency. Preliminary results demonstrate the effectiveness of the proposed shape optimization strategy in refining vehicle configurations for enhanced aerodynamic performance. The findings are anticipated to offer valuable insights into the design of future VLEO platforms, potentially increasing mission lifetimes and reducing fuel requirements through more efficient aerodynamic designs
Building-Aware Path Loss Modeling in ns-3
No full textInternational audienceWe propose a novel propagation model named First Order Buildings Aware Propagation Loss Model (FOBA) to address the lack of reactive interaction between signal propagation and urban building topology. The model incorporates signal loss based on three components: the direct path of the signal through buildings, diffraction, and reflection. The proposed model enhances signal path loss coherence in short-distance, object-obstructed environments. These contributions improve ns-3 representation of urban network connectivity
Validation expérimentale d’une loi de contrôle prédictive basée sur la LQG VAR2 pour les télécommunications optiques LEO-sol
No full textInternational audienceOptical communication links between space satellites and ground stations are impaired by atmospheric turbulence, that is inducing optical phase fluctuations and limiting the global link performance due to fadings and loss of coupling efficiency. Adaptive optics (AO) is the most advanced solution to tackle this issue. It consists in measuring the turbulence effects typically with a Shack-Hartmann wavefront sensor (SHWFS). Then, a real-time controller (RTC) computes a correction to counteract these effects, and the correction is sent to a deformable mirror (DM).The quality of the correction depends on the control scheme applied in the AO loop. Usually an integrator control law is used in the RTC, as it is robust and easy to implement. However, it does not allow for a reduction of the intrinsic temporal error associated to the AO loop delay. A way to overcome this issue is to use predictive controllers, such as a Linear Quadratic Gaussian (LQG) controller. They use evolution models of the turbulence as well as SHWFS measurements to compute DM corrections.In the case of optical downlinks with scrolling LEO satellites, improvements in the evolution models have been done in the recent years. P. Robles [2] proposed a Vectorial Auto-Regressive model of second order (VAR2), particularly suited for LEO tracking conditions. The simulations proved a better performance in terms of coupling efficiency and fadings compared to integrator control. However, no experimental validation of this improved control law has yet been carried out at the moment. In this presentation, we propose to test the VAR2 LQG on the PICOLO+LISA bench, which is composed of PICOLO, a turbulence emulator [3], and LISA, an AO system, as a first step towards implementation in the ONERA’s FEELINGS optical ground station [4]. The experimental results are compared to simulations of this control law, and to traditional integrator control.[1] Williams, W. D. et al., (2007, March). RF and optical communications: A comparison of high data rate returns from deep space in the 2020 timeframe. In 12th Ka and Broadband Communications Conference (No. E-15723).[2] Pablo Rodríguez Robles. Adaptive Optics for LEO-to-Ground Optical Communication: Modeling, Optimization, and Experiment. Engineering Sciences [physics]. Aix-Marseille Université (AMU), 2023. English. NNT: 2020AIXM0001. tel04746416[3] Pablo Robles et al., "Emulating and characterizing strong turbulence conditions for space-to-ground optical links: the PICOLO bench", J. Astron. Telesc. Instrum. Syst. 9(4) 049002 (13 December 2023). doi: 10.1117/1.JATIS.9.4.049002[4] Cyril Petit et al., “FEELINGS : the ONERA’s optical ground station for Geo Feeder links demonstration“, IEEE ICSOS 2023, p. 255‑260. doi:10.1109/ICSOS53063.2022.9749705
Autoregressive regularized score-based diffusion models for multi-scenarios fluid flow prediction
No full text34 pages, 17 figuresBuilding on recent advances in scientific machine learning and generative modeling for computational fluid dynamics, we propose a conditional score-based diffusion model designed for multi-scenarios fluid flow prediction. Our model integrates an energy constraint rooted in the statistical properties of turbulent flows, improving prediction quality with minimal training, while enabling efficient sampling at low cost. The method features a simple and general architecture that requires no problem-specific design, supports plug-and-play enhancements, and enables fast and flexible solution generation. It also demonstrates an efficient conditioning mechanism that simplifies training across different scenarios without demanding a redesign of existing models. We further explore various stochastic differential equation formulations to demonstrate how thoughtful design choices enhance performance. We validate the proposed methodology through extensive experiments on complex fluid dynamics datasets encompassing a variety of flow regimes and configurations. Results demonstrate that our model consistently achieves stable, robust, and physically faithful predictions, even under challenging turbulent conditions. With properly tuned parameters, it achieves accurate results across multiple scenarios while preserving key physical and statistical properties. We present a comprehensive analysis of stochastic differential equation impact and discuss our approach across diverse fluid mechanics tasks
Score-Based Diffusion Models with Autocorrelation Regularization for Fluid Flow Prediction
No full textInternational audienceThe simulation of turbulent flows is an essential field of study with widespread applications. While traditional solvers struggle to provide rapid estimations of solutions due to the intrinsic chaotic nature of turbulence, recent breakthroughs in generative modeling, particularly diffusion models, have demonstrated promising performance for such complex problems. Our method, based on score-matching diffusion models, aims to sample a probability density function representing snapshots of a fluid simulation by learning a score function through reverse-time diffusion processes. This score function is conditioned on previous fluid flow states, following an autoregressive framework. By leveraging denoising score-matching (DSM) techniques, we further regularize the traditional loss function to improve prediction quality (total error on the autocorrelation matrices of the velocity field fluctuations). The conditioning is implemented using a denoising U-Net architecture with input concatenation (known for effectively learning conditional distributions). The model is trained on two-dimensional turbulent compressible flow data (Karman vortex street), and inference is conducted on an extrapolation task for velocities within the transonic regime (frequent shock waves apparition). Early results indicate that incorporating an energy loss function is essential for preserving temporal stability and coherence in intermediate predictions. However, achieving accurate predictions over extended time horizons remains difficult and resource-intensive
Développement d'une méthode numérique originale pour la simulation du cycle de vie d'une traînée de condensation
No full textCondensation trails, or contrails, are long ice clouds visible in aircraft wakes on clear days. Their radiative forcing impact is estimated to be twice that of carbon dioxide emissions. However, this estimate is currently debated due to significant uncertainties surrounding the value. These uncertainties highlight the need for further studies to better understand the formation and evolution of contrails, from the creation of initial ice crystals to their possible transition into induced cirrus clouds that can persist for several hours in the troposphere. Such studies are challenging due to the wide range of spatial and temporal scales involved in the life cycle of a contrail.This thesis proposes a numerical methodology for simulating contrails, from the formation of the initial crystals to the dissipation of wingtip vortices. The originality of this method lies in its ability to simulate the formation and evolution of contrails without relying on analytical formulations to initialize the calculations, as is commonly done in the literature. This approach allows for a comprehensive consideration of near-field effects on contrail evolution in the far field, particularly by accounting for the influence of aircraft geometry on the aerodynamic wake.The application of this methodology to a realistic aircraft geometry under standard atmospheric conditions highlighted the significant impact of horizontal tailplane vortices, through the development of short-wavelength instabilities on the wingtip vortices, profoundly altering the structure of the secondary wake. Comparisons with classical approaches showed that contrails obtained using the method developed in this thesis are opaque and wider, potentially having a greater warming effect.Les traînées de condensation sont les longs nuages de glace visibles dans les sillages d'avion les jours de temps clair. Leur impact en termes de forçage radiatif est estimé à deux fois celui des émissions de dioxyde de carbone. Néanmoins, cette estimation est aujourd'hui controversée en raison des fortes incertitudes entourant cette valeur. Ces incertitudes traduisent en réalité le besoin de mener d'avantage d'études pour mieux comprendre la formation et l'évolution des traînées de condensation, de la création des premiers cristaux jusqu'à la transition possible de la traînée en cirrus induit pouvant persister plusieurs heures dans la troposphère. Ces études sont rendues difficiles en raison de la grande diversité d'échelles spatiales et temporelles relative au cycle de vie d'une traînée de condensation. Cette thèse propose alors une méthodologie numérique pour la simulation des traînées de condensation de la formation des premiers cristaux jusqu'à la destruction des tourbillons marginaux. L'originalité de cette méthode tient dans sa capacité à simuler la formation et l'évolution d'une traînée de condensation sans avoir recours à une formulation analytique pour initialiser les différents calculs, comme cela est réalisé classiquement dans la littérature. Une telle approche permet alors de pleinement considérer les effets du champ proche sur l'évolution de la traînée dans le champ lointain, notamment via l'influence de la géométrie de l'avion sur le sillage aérodynamique. L'application de la méthodologie à une géométrie réaliste d'avion et pour des conditions atmosphériques standards a permis de mettre en évidence l'impact significatif des tourbillons d'empennage horizontal via le développement d'instabilités de courte longueur d'onde sur les tourbillons marginaux, modifiant alors profondément la structure du sillage secondaire. La comparaison avec les approches classiques a montré que les traînées de condensation obtenues via la méthode développée dans cette thèse sont plus opaques et plus larges, avec potentiellement un effet réchauffant plus important
Dielectric and Mechanical Relaxations Contribution to the Microstructure Understanding of PE / EVOH / PE Multilayer Thin Films for Stratospheric Balloons Applications
No full textInternational audienceTo use polyethylene thin film as the envelope of pressurized stratospheric balloons with long flight duration, one has to improve its helium permeability. A multilayer film composed of outside layers of polyethylene (PE) and an inside layer of ethylene vinyl alcohol (EVOH) has been selected. The contribution of each layer to the general microstructure behavior of such material is always complex to understand. The aim of this work was to measure the improvement in helium permeability of two multilayer PE/EVOH/PE (A and B) in comparison with pure PE. The multilayer B has a higher EVOH weight percentage. In addition, a detailed analysis of the contribution of each component to the molecular mobility of the multilayer is addressed. The helium permeance of PE/EVOH/PE B has been divided by 4 thanks to EVOH compared with pure PE. Its crystalline phase as well as melting enthalpy has not changed with the addition of EVOH apart from the presence of a small melting peak associated with EVOH melting. The tensile mechanical properties of PE/EVOH/PE have been improved compared to pure PE due to the highly cohesive structure of the EVOH layer. The contribution of each component to the molecular mobility of PE/EVOH/PE has been observed and analyzed by Dynamic Mechanical Analysis (DMA) and Dielectric Dynamic Spectroscopy (DDS). γ, β and α c modes have been observed by both techniques for PE. Due to its polar nature, β and α relaxations of EVOH have been followed by DDS. Experimental correlation of molecular mobility techniques with different stimuli (mechanic or dielectric) leads to the best results regarding multicomponent materials
Geostationary electron dynamics: ICARE_NG2 observations and new analytical model of daily electron fluxes driven by solar wind conditions
No full textInternational audienceThe effects of the space radiation on spacecraft materials and devices are significant design considerations for space missions. In order to meet these challenges, the radiation environment must be understood. Measuring energetic particles in the radiation belts is therefore crucial, and this is why ICARE (Influence sur les Composants Avancés des Radiations de l’Espace) radiation monitors have been developed over several decades. Two ICARE_NG2 (for the second version of the New Generation of the instrument) radiation monitors have recently been launched on HotBird 13F and 13G satellites at the end of 2022, reaching the geostationary orbit in 2023 and providing measurements of electrons of the outer belt. The methods used to derive these electron fluxes are detailed and the results compared with NGRM (Next Generation Radiation Monitor) and specification models. The observed electron dynamics are strongly correlated with solar wind data, and a fully analytical model is developed. This model provides an accurate representation of the measurements, using a limited number of parameters
exoALMA XVI : Prévoir les signatures de la turbulence à grande échelle dans les disques protoplanétaires
No full textInternational audienceTurbulent gas motions drive planet formation and protoplanetary disk evolution. However, empirical constraints on turbulence are scarce, halting our understanding of its nature. Resolving signatures of the large-scale perturbations driven by disk instabilities may reveal clues on the origin of turbulence in the outer regions of planet-forming disks. We aim to predict the observational signatures of such large-scale flows, as they would appear in high-resolution Atacama Large Millimeter/submillimeter Array observations of CO rotational lines, such as those conducted by the exoALMA Large Program. Post-processing 3D numerical simulations, we explored the observational signatures produced by three candidate (magneto)hydrodynamical instabilities to operate in the outer regions of protoplanetary disks: the vertical shear instability (VSI), the magnetorotational instability (MRI), and the gravitational instability (GI). We found that exoALMA-quality observations should capture signatures of the large-scale motions induced by these instabilities. Mainly, flows with ring, arc, and spiral morphologies are apparent in the residuals of synthetic velocity centroid maps. A qualitative comparison between our predictions and the perturbations recovered from exoALMA data suggests the presence of two laminar disks and a scarcity of ring- and arc-like VSI signatures within the sample. Spiral features produced by the MRI or the GI are still plausible in explaining observed disk perturbations. Supporting these scenarios requires further methodically comparing the predicted perturbations and the observed disks' complex dynamic structure.Les mouvements turbulents du gaz sont des mécanismes cruciaux pour la formation des planètes et l'évolution des disques protoplanétaires. Cependant, les contraintes empiriques sur la turbulence sont rares, ce qui nous empêche de comprendre sa nature. La résolution des signatures des perturbations à grande échelle induites par les instabilités du disque peut révéler des indices sur l'origine de la turbulence dans les régions extérieures des disques de formation de planètes. Nous visons à prédire les signatures observationnelles de ces flux à grande échelle, telles qu'elles apparaîtraient dans les observations ALMA à haute résolution des lignes de rotation du CO, telles que celles menées par le programme exoALMA Large. Après avoir traité des simulations numériques 3D, nous avons exploré les signatures observationnelles produites par trois instabilités (magnéto-)hydrodynamiques candidates à opérer dans les régions extérieures des disques protoplanétaires : L'instabilité de cisaillement vertical (VSI), l'instabilité magnéto-rotationnelle (MRI), et l'instabilité gravitationnelle (GI). Nous avons trouvé que les observations de qualité exoALMA devraient capturer les signatures des mouvements à grande échelle induits par ces instabilités. Principalement, les flux avec des morphologies en anneau, en arc et en spirale sont apparents dans les résidus des cartes de centroïdes de vitesse. Une comparaison qualitative entre nos prédictions et les perturbations récupérées à partir des données exoALMA suggère la présence de deux disques laminaires et une rareté de signatures VSI de type anneau et arc au sein de l'échantillon. Les spirales produites par l'IRM ou l'IG sont toujours plausibles pour expliquer les perturbations observées sur le disque. Pour étayer ces scénarios, il faut comparer méthodiquement les perturbations prédites et la structure dynamique complexe des disques observés
