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Reviewing the Vertex-Centered V4 Scheme for More Accurate Mesh Adaptation
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De la robustesse de l’optimisation de formes d’ondes MIMO RadCom
No full textInternational audienceIn this paper, we consider the problem of designing MIMO waveforms for a Dual-Functional Radar-Communication application. The idea is to optimize these waveforms so that they combine in far-field into some predefined desired waveforms in some given directions. The existing methods to tackle these problems however present some lack of robustness. First the quality of the solution to the optimization problem to be solved depends on the predefined energies of the transmitted and desired waveforms. We propose here an alternative solution based on an exterior point convergence over the waveforms energy that proves efficient whatever the initialization. Second the synthesized waveforms depend on the desired directions so that even a slight mismatch can induce a strong degradation of the waveforms. We propose an analysis of the properties of the resulting mismatched signals leading to an improved decoding method.Dans cet article, le problème de la conception de formes d’ondes MIMO pour une application Dual-Functional Radar-Communication est considéré. L’idée est d’optimiser ces formes d’ondes afin qu’en champ lointain, elle se recombinent dans des formes d’ondes prédéfinies, dans des directions données. Les méthodes existantes pour résoudre ce problème présente un manque de robustesse. Tout d’abord, la qualité de la solution du problème d’optimisation dépend des énergies prédéfinies sur les formes d’ondes transmises et souhaitées. Une solution alternative basée sur une méthode de point extérieur sur l’énergie des formes d’ondes est proposée, qui s’avère efficace quelque soit l’initialisation. Ensuite, les formes d’ondes synthétisées dépendant de la direction d’émission, un léger écart aux directions d’intérêt peut introduire une forte dégradation des formes d’ondes. Une analyse des propriétés des signaux ainsi perturbés est proposée, ainsi qu’une méthode permettant d’améliorer le décodage
Design of a fast interstellar medium exploration mission reaching 200 AU in 25 years
No full textInternational audienceTo this date, only five space probes have been sent to an interstellar trajectory, including four of them that have been launched nearly 50 years ago or more. The study of the pre-interstellar medium remains a subject of scientific interest, and could take advantage of the advancement of sensors, in-space propulsion and satellite platform technologies, both in term of performance and weight. Along its path, the probe could achieve various scientific goals, such as measuring the cosmic ray flux outside the solar cavity, collecting interplanetary dust to measure chemical composition gradients, calibrating sensors through Earth observation or performing a Kuiper belt object flyby. However, the time necessary to reach the interstellar medium remains a challenge, and it is important to optimize the mission in order to enable a scientific return in a “reasonable” time. In this paper, we present the successful results of a feasibility study done by a working group of the French aerospace learned society 3AF. The objective of this study is to re-visit the subject of interstellar mission design, with an ambitious goal: reaching 200 Astronomical Units in 25 years, relying on state-of-art technologies only. In addition, it is essential to note that the mission design will be compatible with Europe’s heaviest existing launcher Ariane 64. This is a significant difference with a similar mission previously proposed by John Hopkins University, for which NASA’s Space Launch System (SLS) (with a performance 3 times higher than Ariane 64) was considered. We discuss the Earth-departure strategy, the transfer strategy and the spacecraft’s propulsive architecture. The main results show that the 200 AU/25 years goal is feasible. The mission strategy relies on the use of an efficient combination of cryogenic propulsion, electric propulsion (with HET thrusters) and eventually classic bi and monopropellant propulsion along with a sub-GTO launch and a propulsive boost during Jupiter swing-by. While the paper is focused primarily on mission design and optimization, we also present the spacecraft overall design and discuss the scientific reward according to possible sensors and telecommunication equipment
Microstructure evolution during shear creep loading in Ni-base superalloys
No full textInternational audienceA 3D phase-field model coupled to a crystal plasticity framework based on dislocation densities is used to investigate microstructural evolution in Ni-base superalloys during shear creep loading. Under (100)[010] shear loading, the microstructure largely retains its initial cuboidal morphology, exhibiting only minor rotations of precipitate/matrix interfaces and precipitate alignments. In contrast, under (011)[01 1] shear loading, the model predicts the formation of rafts oriented such that the remaining channels correspond to those in which the primary slip systems are most active. These results are consistent with available experimental observations. We also analyze the contribution of elastic inhomogeneity to these microstructural changes, using both the phase field model and analytical methods. In addition, this work demonstrates the relevance of calculating 3D spatial autocorrelations for statistical analysis of simulated microstructures in Ni-base superalloys
Exciton Self-Trapping in Twisted Hexagonal Boron Nitride homostructures
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Mapping of CH4 atmospheric plumes from point sources by open-path TDLAS and AI-assisted tomographic reconstruction: numerical proof of concept
No full textInternational audienceOptical diagnostics, such as Tunable Diode Laser Absorption Spectroscopy (TDLAS), are commonly used to measure concentration in a non-intrusive way. One possible application is the detection and quantification of gas into the atmosphere, such as CH 4 leaks from industrial sites and facilities. However, the TDLAS technique does not allow the retrieval of the spatial distribution of the corresponding atmospheric plume. With the aim to recover an atmospheric concentration map of the corresponding atmospheric plume, we study the feasibility and relevance of developing a tomography reconstruction algorithm. The principle is to use multiple laser beam paths in a plane, in order to probe the gas absorption. We encode the prior information about the plume shape into a neural network. Tomography is studied on synthetic data generated from Gaussian plume models and Large Eddy Simulations (LES)
Inertia-induced power law scaling in martensites
No full textInternational audienceWhile martensites subjected to quasi-static deformation are known to exhibit power law distributed acoustic emission in a broad range of scales the origin of the observed scaling behavior and the mechanism for self-organization towards criticality remains obscure. Here, we argue that the power-law structure of intermittent fluctuations can be at least partially attributed to inertia. We build on the insight that inertial dynamics, evidenced by acoustic emission, can become an important factor if the underlying mechanical system is only marginally stable. We first illustrate the possibility of inertiainduced heavy-tailed avalanche size distributions using a prototypical example of a discrete chain with bi-stable springs. We then explore the effects of inertia in fully realistic two-and three-dimensional continuum models of elastic phase transitions. In particular, we demonstrate that a three-dimensional model of this type can produce not only qualitative but also quantitative agreement with experiment
Numerical investigation of acoustic perturbations radiated from a transitional and turbulent boundary layer in hypersonic wind tunnel flow conditions
No full textInternational audienceDirect numerical simulations of transitional and turbulent hypersonic boundary layers are performed on a flat plate geometry to reproduce the free stream acoustic perturbations observed in conventional hypersonic wind tunnels. Transition of the boundary layer is achieved through volume forcing computed either from optimal perturbation modes obtained with a resolvent-based linear stability analysis or following 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. For all three cases, the 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
When Quality Matters: Constraint Programming for Automated Temporal and Numeric Planning
No full textInternational audienceAutomated planning is a field of Artificial Intelligence interested in finding a set of actions that drives the evolution of the environment from an initial state to a desired goal state. Much of the work of the community has been on socalled domain-independent planning where a solver is expected to produce a plan from an abstract problem, specified in a common description language. This has led the community to produce a number of highly-efficient solvers that can be expected to work on a large variety of domains without any fine-tuning.Where most of the work has focused on sequential plans over purely symbolic states, we instead propose a constraint-based planner whose focus is on more expressive variants, namely numeric and temporal planning, essential in many practical applications. We extend an existing CP encoding of temporal planning with support for optimization and numeric states and leverage an existing lazy clause generation CP solver to find and optimize plans. Where the most successful automated planners rely on some form of forward-search, we show that constraintprogramming can be just as effective in finding satisfiable solutions while substantially improving the quality of the produced plan