Portail HAL ONERA
Not a member yet
12842 research outputs found
Sort by
X-ray to visible light scattering by fractal agglomerates: A model comparison applied to soot
No full textInternational audienceCharacterizing nanoparticles in terms of size and morphology is essential to understand their physical and optical properties. Small or Ultra Small-Angle X-ray Scattering (SAXS/USAXS) is a powerful in situ technique that allows the analysis of such nanostructures exhibiting size and morphology through the Guinier and Porod regimes, which can be repeated at different scales (e.g., primary spheres and agglomerates for mass fractal agglomerates, as focused on in this study). This is the principle of the widely used Beaucage’s unified model, built in reciprocal -space (the momentum transfer or scattering wave vector) in order to interpret experimental results. An alternative method based on pair correlation modeling can be used. Indeed, by definition, the scattered signal corresponds to the Fourier transform of the particles’ pair correlation function. The present study compares the two approaches for fractal agglomerates of point-contact spheres. It is shown that the Beaucage’s model is limited to agglomerates consisting of a large number of primary spheres and that certain destructive interferences, not taken into account in this model, can affect data inversion, potentially affecting the extraction of the primary sphere size distribution. In this work, both models are compared in the direct way applied on synthetic data, and used to invert experimental SAXS data from soot particles in a laminar ethylene diffusion flame measured at the European Synchrotron Radiation Facility (ESRF) and processed via Abel inversion. This study highlights the fundamental differences between the two models and emphasizes their respective limitations. Nevertheless, both models provide similar results except for the mass fractal dimension and primary sphere geometric standard deviation. The present study demonstrates thus the potential of the pair correlation modeling and encourages its further development
Closed Form Solution of Skip Re-Entry Trajectories with Velocity-Dependent Aerodynamic Coefficients using Matched Asymptotic Expansions
No full textReal-time trajectory optimization is a crucial aspect of aerospace applications, particularly for atmospheric re-entry missions. Since in this phase space vehicles are subject to disturbances or mission changes during flight, a trajectory replanning module is generally used to compute corrections within computational time and resources. Analytical guidance laws meet these requirements without resorting to solve large numerical optimization problems associated with direct methods or indirect methods, which are highly sensitive to initialization. One method of deriving such analytical guidance laws is the application of the Pontryagin Maximum Principle, which provides the necessary conditions for a control to be optimal. However, for nonlinear dynamics, the resulting equations are difficult to solve analytically and require numerically solving a two-point boundary value problem in most cases. Approximation methods, in particular asymptotic expansions, have been used to obtain analytical solutions for this application. A preliminary step to obtain solutions of the optimal control problem is to compute the analytical solutions of the equations of motion for the uncontrolled system. This work then focuses on solving the equations of motion using a more complex aerodynamic model than those reported in the literature
Technical report on direct geolocalization from remote sensing S1 images
No full textThis small technical report highlights that coarse direct position prediction from image is somehow possible given current deep network efficiency. 80% localisation bin accuracy is achieved accross a 400km x 300km corridor quantified on a 10km x 10km grid
Analyse spatio-temporelle expérimentale et numérique de la dynamique de l'effet Portevin-Le Chatelier dans un superalliage à base de nickel
No full textInternational audienceThis study examines the Portevin-Le Chatelier (PLC) effect in the nickel-based superalloy Inconel 718 through a combined experimental-computational approach using statistical indicators from nonlinear dynamical systems theory. We develop a finite element model incorporating the Kubin-Estrin-McCormick constitutive law to capture Dynamic Strain Ageing effects, accounting for machine stiffness influence, and reproduce various dynamics under both hard and soft loading conditions. Statistical analysis reveals that machine stiffness significantly affects serration morphology and dynamics, influencing mean amplitudes, stress drop periods, and dynamical indicators such as the correlation dimension and Lyapunov exponents. Comparison of simulated and experimental stress time series demonstrates chaotic behaviour for type B and C serrations across all strain rates, with no evidence of self-organised criticality in experimental data. However, simulations predict self-organised criticality at high strain rates corresponding to type A bands, consistent with literature references. Statistical indicators reveal power-law behaviour for stress drop amplitudes as a function of strain rate, with critical exponents dependent on band type. Multifractal analysis shows that simulations overestimate complexity relative to experimental observations, suggesting the need for additional internal variables and finer-scale dynamics in modelling. Additionally, multifractal analysis of spatio-temporal diagrams reveals power-law distributions of plastic strain rates with consistent critical exponents across all strain rates, demonstrating its potential for characterising PLC spatio-temporal dynamics. Statistical, dynamical, and multifractal indicators show consistent correlations, collectively capturing transitions between serration regimes and serving as reliable quantitative metrics for characterising PLC dynamics. The analysis is finally applied to the spatio-temporal strain fields measured by digital image correlation. The results demonstrate the value of multi-indicator analysis for assessing the agreement between experiment and simulation and subsequently improving constitutive model parameter identification
Émissivité bidimensionnelle infrarouge d’une surface de mer perturbée par un périscope de forme cylindrique
No full textNational audienceLe rayonnement intrinsèque infrarouge d’unesurface de mer est caractérisé par son émissivité. Elle peutêtre calculée selon deux approches (i) Soit analytiquement(AN), pour laquelle la moyenne statistique de l’émissivitélocale est calculée analytiquement en supposant que ladistribution des pentes suit une loi normale centrée (ii)Soit par un processus numérique (NUM) de Monte Carlo,qui signifie que l’émissivité moyenne est obtenue à partirde plusieurs réalisations statistiques des hauteurs de lasurface. L’objectif de cette communication est d’appliquerl’approche NUM à une surface de mer 2D perturbée parun mât de sous-marin (périscope) simulée par un logicielde mécanique des fluides. Ainsi, la comparaison de cetteémissivité avec celle d’une mer libre permettra par mesureinfrarouge de détecter la présence du périscope
Implementation and certification of ML-based surrogate models in avionic systems
No full textInternational audienceThis paper argues that surrogate models can be used in legacy systems because they are provable, implementable and certifiable with respect to the future development assurance guidance for ML applications anticipated by the European union Aviation Safety Agency (EASA)
Thermal effects on polyimide electrical conductivity: experimental and numerical analysis
No full textInternational audienceKapton ® is a widely used material on spacecraft due to its excellent thermal and mechanical properties. Despite its widespread usage, the thermal dependence of its electrical properties remains a significant concern. An experimental investigation was conducted using the SIRENE facility (ONERA, Toulouse, France) in order to characterize the effect of temperature on the electrical behavior of Kapton ® . Through experimental evidence using a thermally stimulated current method, the trap distribution along the forbidden gap was highlighted. In addition, the existence of dark conductivity, unrelated to radiation exposure, was clearly identified. These physical mechanisms and characteristics were integrated into a numerical and physical conductivity model called THEMIS, which has been developed to assess charge transport in dielectrics in one dimension. The modified version of the THEMIS model was validated through experimental characterization. This work extends our understanding of charge transport in dielectric materials used in space, and suggests a validated method to model their behavior
On the Posterior Gap in Plug & Play Diffusion Methods for Sparse-View Computed Tomography
No full textInternational audienceRecent advances in conditional diffusion models have demonstrated impressive performance on inverse problems such as Sparse-View Computed Tomography (SVCT), typically evaluated using estimation error metrics like PSNR or SSIM. However, in ultra-sparse regimes, where the number of projections is extremely limited, the sinogram is weakly informative resulting in high variability in posterior samples. This is especially true in applications involving dynamic phenomena, such as imaging the exhaust gas from a nozzle, where only a handful of projections can be captured simultaneously. In this context, evaluation based solely on estimation error (e.g. PSNR/SSIM) is no longer adequate. To address this limitation and complement the PSNR/SSIM evaluation, we propose to evaluate the Posterior Gap (PG), which measures the discrepancy between the true posterior distribution and the approximation induced by a given generative model. We introduce a practical framework for PG estimation using only prior samples, the observation model, and posterior samples. We consider three state-of-theart Plug & Play diffusion models DPS, ∏G, and FPS. Since DPS includes a sensitive hyperparameter, we present a simple calibration strategy to reduce the PG. Our study benchmarks DPS, ∏G and FPS across three SVCT datasets. Results show that when carefully calibrated, DPS outperforms FPS and ∏G. By complementing traditional metrics, our evaluation protocol offers a more comprehensive understanding of reconstruction performance, particularly in severely ill-posed regimes such as dynamic gas imaging with six or fewer projections
Benchmark for two-dimensional large scale coherent structures in partially magnetized E×B plasmas -Community collaboration & lessons learned
No full textLow-temperature plasmas are essential to both fundamental scientific research and critical industrial applications. As in many areas of science, numerical simulations have become a vital tool for uncovering new physical phenomena and guiding technological development. Code benchmarking remains crucial for verifying implementations and evaluating performance. This work continues the Landmark benchmark initiative, a series specifically designed to support the verification of low-temperature plasma codes. In this study, seventeen simulation codes from a collaborative community of nineteen international institutions modeled a partially magnetized E×B Penning discharge. The emergence of large scale coherent structures, or rotating plasma spokes, endows this configuration with an enormous range of time scales, making it particularly challenging to simulate. The codes showed excellent agreement on the rotation frequency of the spoke as well as key plasma properties, including time-averaged ion density, plasma potential, and electron temperature profiles. Achieving this level of agreement came with challenges, and we share lessons learned on how to conduct future benchmarking campaigns. Comparing code implementations, computational hardware, and simulation runtimes also revealed interesting trends, which are summarized with the aim of guiding future plasma simulation software development.</div
Influence of tow-preg composition on microstructure and mechanical behaviour of oxide/oxide CMCs
No full textInternational audienceCeramic matrix composites could soon be used in an ever-increasing number of high-temperature applications. This study explores the potential of a new tow-preg process for the manufacturing of oxide-based ceramic matrix composites. With these techniques, the choice of impregnation slurries is crucial for the quality of the materials. This study focuses on the development of several grades of alumina matrices CMCs. The impact of the slurry composition on the CMCs microstructure and mechanical behaviour is evaluated. The benefits of a minimum amount of boehmite, a gel-forming agent which is a precursor of alumina, on inter-ply cohesion have been demonstrated