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From low-frequency oscillations to Markovian bistable stall dynamics
No full textInternational audienceWe study the dynamics of a fixed-wing at stall in a wind tunnel by measuring the aerodynamic forces. We report experimental evidence of a critical Reynolds number from which low frequency oscillations in the force are replaced by random bistable dynamics. In this new regime, the flow explores each state intermittently with long residence times. This stochastic process can be modeled as a continuous Markov chain, and equivalently, it shows a super-exponential scaling for the mean residence times. Furthermore, the probability density function of the lift coefficient exhibits the characteristic heavy tail of extreme events. Extreme minima and maxima are at the origin of the transitions. We analyzed the evolution of these tails using extreme value theory to identify the bifurcation points of the associated dynamical system. The results are in good agreement with a user-defined threshold method, the advantage being the unambiguity in the computation
Combinatoires sur les configurations sociales
No full textInternational audienceIn cooperative game theory, the social configurations of players are modeled by balanced collections. A balanced collection is a set system defined on the set N of players in the game, together with a system of weights such that each player belongs to coalitions whose weights sum to 1. The Bondareva–Shapley theorem, perhaps the most fundamental theorem in cooperative game theory, characterizes the existence of solutions to the game that benefit everyone using balanced collections. Roughly speaking, if the trivial set system {N} is one of the most efficient balanced collections for the game, then the set of solutions from which each coalition benefits, the so-called core, is non-empty.In the following, we discuss some interactions between combinatorics and cooperative game theory that are still relatively unexplored. First, we study the similarities between balanced collections on the one hand and regular or uniform hypergraphs on the other. Second, we present some results leading to the construction of the combinatorial species of structures of uniform hypergraphs, from which we aim to construct the species of regular hypergraphs by duality. Finally, we investigate the possibility of expressing some “minimality” properties of regular or uniform hypergraphs in the language of combinatorial species, hoping to obtain new properties of minimal balanced collections.Dans la théorie des jeux coopératifs, les configurations sociales des joueurs sont modélisées par des collections équilibrées. Une collection équilibrée est un système d'ensembles défini sur l'ensemble N des joueurs du jeu, ainsi qu'un système de poids tel que chaque joueur appartient à des coalitions dont la somme des poids est égale à 1. Le théorème de Bondareva-Shapley, peut-être le théorème le plus important de la théorie des jeux coopératifs, caractérise l'existence de solutions au jeu qui profitent à tout le monde en utilisant des collections équilibrées. En quelques mots, si le système d'ensembles triviaux {N} est l'une des collections équilibrées les plus efficaces pour le jeu, alors l'ensemble des solutions dont bénéficie chaque coalition, le cœur, est non vide.Dans ce qui suit, nous discutons de certaines interactions entre la combinatoire et la théorie des jeux coopératifs qui sont encore relativement inexplorées. Tout d'abord, nous étudions les similitudes entre les collections équilibrées d'une part et les hypergraphes réguliers ou uniformes d'autre part. Ensuite, nous présentons quelques résultats conduisant à la construction des espèces combinatoires de structures d'hypergraphes uniformes, à partir desquelles nous visons à construire les espèces d'hypergraphes réguliers par dualité. Enfin, nous étudions la possibilité d'exprimer certaines propriétés de « minimalité » des hypergraphes réguliers ou uniformes dans le langage des espèces combinatoires, dans l'espoir d'obtenir de nouvelles propriétés de collections minimales équilibrées
The maximal coordination principle in regulatory Boolean networks
No full textInternational audienceWe introduce a coordination index in regulatory Boolean networks and we expose the maximal coordination principle (MCP), according to which a cohesive society reaches the dynamics characterized by the highest coordination index. Based on simple theoretical examples, we show that the MCP can be used to infer the influence graph from opinion dynamics/gene expressions. We provide some algorithms to apply the MCP and we compare the coordination index with existing statistical indexes (likelihood, entropy). The advantage of the coordination approach is its simplicity; in particular, we do not need to impose restrictions on the aggregation functions
New LTE-A Low-Latency Detectors for Cognitive Radio on Low-Cost FPGA SoC
No full textInternational audienceIn this paper, we address the challenge of a real-time solution for the critical detection step in a LTE-advanced (LTE-A) cognitive radio (CR) network implemented using limited computing resources. The detection step is required to identify a free radio channel that can be used to transmit data. We first present two new detectors and their naive implementations on a CR platform running over LTE-A, an orthogonal frequency division multiple access (OFDMA) network. The first detection method is based on the subband’s energy and the second uses both correlation on the cyclic prefix (CP)’s part and the energy of the useful orthogonal frequency division multiplexing (OFDM) signal. We then optimize them to satisfy the low-latency detection constraint on a low-cost embedded board using a ZYNQ XC7Z020 system on a chip (SoC). We use Xilinx Vitis high-level synthesis (HLS) computer-aided design (CAD) tool to design our solutions. Finally we achieve the implementation of a solution that requires less than one OFDM symbol period (70 μs) for a detection system that complies with the worst scenario timing constraint of the LTE-A standard
Sécurisation matérielle de la compilation à la volée des machines virtuelles langage
No full textLanguage Virtual Machines (VMs) are the run-time environment of popular high level managed languages. They offer portability and memory handling for the developer and are deployed on most computing devices. Their widespread distribution, handling of untrusted user inputs, and low-level task execution make them interesting to attackers. Software-only solutions that isolate their different components often incur a high performance overhead incompatible with just-in-time (JIT) compilation. Hardware-accelerated run time protections are pushed in vendor processors as a solution to conciliate strong security guarantees with performance. To allow experimentation in the design and comparison of such solutions, this thesis is interested in the RISC-V instruction set and its extension capabilities. We present Gigue, a workload generator that outputs binaries similar to JIT code directly executable on RISC-V softcores. It provides an interface for custom instructions and guarantees their execution. We present an instruction-level domain isolation solution added to Gigue binaries and implemented in an application-class processor with processor modifications. The solution adds negligible performance overhead while enforcing strong properties on domains. As an effort to motivate deployment in real use cases, we extend the Pharo JIT compiler to the RISC-V instruction set along with its testing infrastructure.Les machines virtuelles langage (VM) sont l’environnement d’exécution des langages de haut niveau les plus répandus. Elles permettent une portabilité du code applicatif et la gestion automatique de la mémoire. Leur large diffusion couplée à l’exécution de tâches de bas niveau les rendent intéressantes pour les attaquants. Les solutions purement logicielles entraînent souvent une perte de performance incompatible avec la compilation just-in-time (JIT). Des solutions accélérées matériellement sont ajoutées dans des processeurs commerciaux pour concilier des garanties de sécurité fortes avec la performance. Pour comparer ces solutions, cette thèse s’intéresse au jeu d’instructions RISC-V et à ses capacités d’extension. Nous présentons Gigue, un générateur de binaires similaires au code JIT directement exécutables sur les softcores RISC-V. Il fournit une interface pour des instructions personnalisées et garantit leur exécution. Nous présentons une solution d’isolation de domaine au niveau des instructions ajoutée aux binaires de Gigue et déployée dans un processeur avec des modifications minimales. La solution ajoute un surcoût de performance négligeable tout en garantissant des propriétés fortes sur les domaines. Afin de motiver le déploiement dans des cas d’utilisation réels, nous étendons le compilateur JIT Pharo au jeu d’instructions RISC-V, ainsi que son infrastructure de test
On Environmental Justice and Outer Space
No full textInternational audienceThe talk explores the intersections between thinking about outer space and thinking about environmental justice. It makes a plea for a multi-dimensional approach to thinking outer space, arguing that the belief in the possibility of aliens, and the cultivation of a respect for the need for planetary protection, can play a general role in the cultivation of ethical consciousness
Physics-embedded machine learning method for the prediction of long-range electromagnetic field propagation
No full textInternational audienceWe propose a sequential physics-embedded machine learning method to predict propagation over rural terrain. The training process uses artificial data computed by solving the parabolic wave equation. A fine-tuning strategy is also proposed, rendering the method versatile. Numerical simulations are provided to validate the proposed approach
On the performance of high power pulsed repeated laser irradiation for spatial structures propulsion
No full textInternational audienceChanging debris orbits using High Power Pulsed Laser Interaction finds interest in the Low-Earth Orbit (LEO) due to space debris congestion. Laser facilities allowing both high energy and repetition rate of a short pulsed irradiation become available as provided by the high power HILASE beamline facility (Prague, CZ) with BIVOJ (100J, 10ns, 10Hz, 1030nm). In order to illustrate such an application, originally Crookes radiometer concept was adapted to quantify the efficiency of repeated laser shots in increasing rotating speed according to laser matter interaction. Various materials, from model to space applicative materials, allowed to estimate the material response with various irradiation characteristics (single shots vs. repeated shots). Matter behaviors (ablation, cratering, spallation, perforation) bracketed the laser conditions suitable in the perspective of laser propulsion with limited creation of extra debris or irradiated structures damages. Next is to strengthen the robustness of the simulation/experiment dialog to use simulation as a pre- design tool for laser space propulsion
Multiscale modeling for a class of high-contrast heterogeneous sign-changing problems
No full textThe mathematical formulation of sign-changing problems involves a linear second-order partial differential equation in the divergence form, where the coefficient can assume positive and negative values in different subdomains. These problems find their physical background in negative-index metamaterials, either as inclusions embedded into common materials as the matrix or vice versa. In this paper, we propose a numerical method based on the constraint energy minimizing generalized multiscale finite element method (CEM-GMsFEM) specifically designed for sign-changing problems. The construction of auxiliary spaces in the original CEM-GMsFEM is tailored to accommodate the sign-changing setting. The numerical results demonstrate the effectiveness of the proposed method in handling sophisticated coefficient profiles and the robustness of coefficient contrast ratios. Under several technical assumptions and by applying the T-coercivity theory, we establish the inf-sup stability and provide an a priori error estimate for the proposed method
RIR-in-a-Box: Estimating Room Acoustics from 3D Mesh Data through Shoebox Approximation
No full textInternational audienceThis paper describes a method for estimating the room impulseresponse (RIR) for a microphone and a sound source located atarbitrary positions from the 3D mesh data of the room. Simulat-ing realistic RIRs with pure physics-driven methods often failsthe balance between physical consistency and computational ef-ficiency, hindering application to real-time speech processing.Alternatively, one can use MESH2IR, a fast black-box estima-tor that consists of an encoder extracting latent code from meshdata with a graph convolutional network (GCN) and a decodergenerating the RIR from the latent code. Combining these twoapproaches, we propose a fast yet physically coherent estimatorwith interpretable latent code based on differentiable digital sig-nal processing (DDSP). Specifically, the encoder estimates a vir-tual shoebox room scene that acoustically approximates the realscene, accelerating physical simulation with the differentiableimage-source model in the decoder. Our experiments showedthat our method outperformed MESH2IR for real mesh data ob-tained with the depth scanner of Microsoft HoloLens 2, and canprovide correct spatial consistency for binaural RIRs