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Analysis of untrusted-node quantum key distribution from a geostationary satellite
No full textIn pursuit of a global quantum key distribution (QKD) network, a service based on untrusted nodes on geostationary satellites could offer wide coverage, continuous operation, and enhanced security compared to the trusted node alternative. Although this scenario has been studied for entanglement-based protocols, such an approach would require large-area telescopes both on the ground and in space. In this work, we analyze the performance of two QKD protocols well adapted to this scenario, namely twin-field (TF) and mode-pairing (MP) QKD, which exhibit high resilience to high-loss channels. Leveraging an in-depth simulation of communication channels corrected with adaptive optics, we assess the expected secret key rates for both protocols in a configuration involving two 50 cm telescopes on board the satellite and ground-based telescopes ranging from 20 cm to 1 m in aperture. Our results show that, in the best case and considering realistic detectors, it is possible to achieve secret key rates on the order of a few hundred bit/s for both TF and MP-QKD. We show, notably, that secret key generation is potentially feasible even with 20 cm ground telescopes, highlighting the high scalability potential of such a configuration
Data Assimilation and Uncertainty Quantification for the RANS prediction of separated flows
No full textInternational audienceThis work presents a formalism for enhancing RANS models by data assimilation, that incorporates an assessment of the quality of the assimilated results through uncertainty quantification. The framework is applied to the two-dimensional separated flow in a converging-diverging channel with a Reynolds number of 12,600. The Spalart-Allmaras (SA) model is selected for its relative simplicity compared to other RANS models. The methodology is evaluated using sparse mean velocity measurements that are used to infer among two types of corrective terms in the RANS-SA equations: one acting on the momentum equations, and the other on the modified eddy-viscosity equation, via the turbulent production term. Results show that, in the case of sparse data, correcting the turbulent production term yields more accurate posterior mean flow reconstructions. In this case, the UQ strategy, based on a low-rank approximation of the Hessian operator, successfully predicts posterior standard deviations that align with the true residual errors, highlighting regions of inaccurate flow reconstruction
Fluctuations de phase de type omega induites par la lacune dans Ti et Zr cubiques centrés
No full textInternational audienceLa précipitation de phase ω dans les alliages de Ti et Zr cubiques centrés (cc) peut se produire de façon athermique, et s'accompagne typiquement d'une distribution dense et fine de particules ω. Dans ce travail, nous montrons que la lacune se comporte comme une source de fluctuations de type ω dans les matrices Ti et Zr cc, qui sont mécaniquement instables à température nulle, mais stabilisées à température finie. Via des calculs ab initio, nous mettons en évidence la similitude existant entre le champ de relaxation statique de la lacune dans une matrice cc instable et le mode de phonon LA 2/3[111], grâce à une analyse de la structure atomique et électronique autour du défaut lacunaire. Ensuite, grâce à des calculs en potentiels empiriques, nous montrons qu'à température finie, la structure de la lacune est complexe, alternant au cours du temps entre une configuration symétrique et une configuration linéaire entourée de 'germes' de phase ω. Cette configuration linéaire, de symétrie brisée, met en jeu un grand nombre d'atomes, et sa géométrie est en accord avec la morphologie de la distribution d'intensité diffuse observée expérimentalement pour des alliages de Ti et Zr cc. Nous proposons ensuite que la germination de la phase ω athermique se fasse de façon hétérogène sur les lacunes. Ce mécanisme est en accord avec l'homogénéité spatiale des précipités ω athermiques observés en MET, ainsi qu'avec l'estimation de densité de particules, et suggère une façon de modérer cette précipitation
Magnetism in iron alloys: thermodynamics, kinetics and defects
No full textInternational audienceSteels are among the technologically and economically most relevant materials. Key innovations in important sectors of human society such as mobility, energy and safety, are currently based on alloying of Fe with other transition-metal elements such as Mn, Cr, or Co. Due to strong impacts and conceptual challenges related to magnetism, however, the fundamental understanding and the ability to computationally design these steels in high-throughput approaches lags behind other classes of alloys. In this article, we will provide a substantial review of the role of magnetism, magnetic excitations and transformations for alloy thermodynamics, point defects, interfaces and kinetics. This will be achieved by combining insights from different methods: Ab initio simulations have the advantage that the magnetic ground state is intrinsic part of the electronic minimization. Due to the coarsening of the many-electron structures and therewith magnetic interactions, tight-binding methods can handle larger system sizes. Effective interaction models provide the freedom to exploit more sophisticated magnetic interactions. The performance of these methods in terms of magnetic properties of Fe alloys will be evaluated by providing state-of-the-art results for their sensitivity to magnetism. Furthermore, dedicated experiments will be discussed to complete the understanding of magnetic effects in Fe alloys and to validate the modeling strategy
Application of the LIF/Mie technique to an aeronautic kerosene Jet A-1 spray injected under relight conditions
No full textInternational audienceCertification process of an aircraft engine requires the engine to be able to relight under high-altitude conditions. This capability is strongly affected by the characteristics of the spray injected into the combustion chamber, by means of an aeronautical injection system. The understanding of the atomization process is a key factor in order to forecast the engine's relight capability. Among the parameters of interest (gas-phase velocity, concentration and liquid volume fraction), the droplet size is essential. In the literature, this parameter is commonly characterized under ambient temperature and pressure conditions. The present study focuses on the application of the LIF/Mie technique for the characterization of an aeronautic kerosene Jet A-1 spray injected under relight conditions. A qualitative study of the effect of liquid temperature and injection pressure on the Sauter Mean Diameter is first presented. The technique is then applied under critical high-altitude conditions and provide quantitative SMD mappings. Comparisons with PDA measurements exhibit a very good agreement, which validates the LIF/Mie technique as a reliable tool for studying sprays in these harsh conditions
Modélisation RANS multi-échelles de la turbulence et application aux couches limites turbulentes soumises à des forts taux de turbulence extérieure
No full textInternational audienceThe present paper aims at describing the development rationale behind a two-scale kω RANS (Reynolds-Averaged Navier-Stokes) turbulence model. This model is designed to cope with the difficulties of classical single-scale models on the prediction of turbulent boundary layers under the influence of high free-stream turbulence (FST). From a detailed literature review, two physical phenomena are spotted and careful modelling is proposed by using the possibilities offered by the multi-scale RANS framework. The numerical results obtained with the new model on a flat plate under strong FST with a wide range of turbulence levels demonstrate the improved skin friction predictions attained when compared to a kω single-scale model.Cet article vise à décrire la logique de développement d'un modèle de turbulence k − ω RANS (Reynolds-averaged Navier-Stokes) à deux échelles. Ce modèle est conçu pour traiter les apories des modèles classiques à une seule échelle pour la prévision de couches limites turbulentes sous l'influence d'une forte turbulence extérieure. Une revue de la littérature détaillée a permis d'identifier deux phénomènes physiques et de proposer des modélisations précises exploitant les possibilités offertes par le cadre RANS multi-échelles. Les résultats numériques obtenus avec ce nouveau modèle sur une plaque plane soumise à une forte turbulence extérieure démontrent l'amélioration des prévisions de frottement pariétal par rapport à un modèle à une seule échelle
Assessment of mesh adaptation methodologies for the automatic generation of three-dimensional hypersonic boundary layer solutions
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Vacancy-induced omega phase fluctuations in body-centered cubic Ti and Zr
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A Quasi-Trefftz Domain Decomposition Method for the Iterative Solution of Time-Harmonic Wave Problems
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Comparison and optimisation of hybridization algorithms for onboard classical and quantum accelerometers
No full textInternational audienceWe study two hybridization algorithms used for the combination of a quantum inertial sensor based on atom interferometry with a classical inertial sensor for onboard acceleration measurements. The first is based on the direct extraction of the interferometer phase, and was previously used in seaborne and airborne gravity measurement campaigns. The second is based on the combination of three consecutive measurements and was originally developed to increase the measurement range of the quantum sensor beyond its linear range. After comparing their performances using synthetic data, we implement them on acceleration data collected in a recent airborne campaign and evaluate the bias and the scale factor error of the classical sensor. We then extend their scope to the dynamical evaluation of other key measurement parameters (e.g. alignment errors). We demonstrate an improvement in the correlation between the two accelerometers' measurements and a significant reduction of the error in the estimation of the bias of the classical sensor