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Effect of forward speed on the level-crossing distribution of kinematic variables in multidirectional ocean waves
No full textThe influence of forward speed on stochastic free-surface crossing, in a Gaussian wave field, is investigated. The case of a material point moving with a constant forward speed is considered; the wave field is assumed stationary in time, and homogeneous in space. The focus is on up-crossing events, which are defined as the material point crossing the free surface, into the water domain. The effect of the Doppler shift (induced by the forward speed) on the up-crossing frequency, and the related conditional joint distribution of wave kinematic variables is analytically investigated. Some general trends are illustrated through different examples, where three kinds of wave direction distribution are considered: unidirectional, short-crested anisotropic, and isotropic. The way the developed approach may be used in the context of slamming on marine structures is briefly discussed
Guest Editorial Special Issue on Continual Unsupervised Sensorimotor Learning
No full textInternational audienc
Determinants of employees’ participation in decision-making in developing countries: Does a firm’s formal vs. informal status matter?
No full textPrior studies on the determinants of employees' participation in decision-making have ignored the sector dualism (formal sector vs. informal sector) in developing countries, despite the differences in the characteristics of formal and informal firms. This paper tests the idea that a firm's formal vs. informal status matters, using firm-level data from Côte d’Ivoire. It emerges that a firm's formality status influences positively its probability to involve employees in decision-making. This result holds after controlling for manager-, firm- and sector- level factors. Ultimately, the paper contributes to sending the signal that formalization of informal firms in developing countries should be beneficial to their decision-making process, and thereby to their performance
Assessing the Role of Developmental and Environmental Factors in Chemical Defence Variation in Heliconiini Butterflies
No full textInternational audienceChemical defences in animals are both incredibly widespread and highly diverse. Yet despite the important role they play in mediating interactions between predators and prey, extensive differences in the amounts and types of chemical compounds can exist between individuals, even within species and populations. Here we investigate the potential role of environment and development on the chemical defences of warningly coloured butterfly species from the tribe Heliconiini, which can both synthesize and sequester cyanogenic glycosides (CGs). We reared 5 Heliconiini species in captivity, each on a single species-specific host plant as larvae, and compared them to individuals collected in the wild to ascertain whether the variation in CG content observed in the field might be the result of differences in host plant availability. Three of these species were reared as larvae on the same host plant, Passiflora riparia, to further test how species, sex, and age affected the type and amount of different defensive CGs, and how they affected the ratio of synthesized to sequestered compounds. Then, focusing on the generalist species Heliconius numata, we specifically explored variation in chemical profiles as a result of the host plant consumed by caterpillars and their brood line, using rearing experiments carried out on two naturally co-occurring host plants with differing CG profiles. Our results show significant differences in both the amount of synthesized and sequestered compounds between butterflies reared in captivity and those collected in the field. We also found a significant effect of species and an effect of sex in some, but not all, species. We show that chemical defences in H. numata continue to increase throughout their life, likely because of continued biosynthesis, and we suggest that variation in the amount of synthesized CGs in this species does not appear to stem from larval host plants, although this warrants further study. Interestingly, we detected a significant effect of brood lines, consistent with heritability influencing CG concentrations in H. numata. Altogether, our results point to multiple factors resulting in chemical defence variation in Heliconiini butterflies and highlight the overlooked effect of synthesis capabilities, which may be genetically determined to some extent
Electro-thermal–mechanical performance of a sensor based on PAN carbon fibers and real-time detection of change under thermal and mechanical stimuli
No full textInternational audienceStructural health monitoring (SHM) is a vastly growing field consisting of sensors embedded in or attached with the structure which respond to the strain or other stimuli to monitor the deformation in real-time. In this study, a carbon fiber (CF) sensor was developed using unidirectional Polyacrylonitrile (PAN) carbon filaments aligned straightly together and its sensitivity was calculated experimentally, with the gauge factor (GF) in 10.2-10.8 range. The electro-thermal behavior of this CF sensor showed distinct performance and detected the change in the surrounding temperature. There is a good reproducibility in the results in both piezoresistive and electrothermal behavior of the CF sensor and its electrical performance showed real-time detection of both mechanical and thermal stimuli. The results established that the CF exhibited good potential as a flexible strain sensor for in-situ monitoring of damage or energy release during the failure of composites
Robust Subspace Tracking With Missing Data and Outliers: Novel Algorithm With Convergence Guarantee
No full textInternational audienceIn this paper, we propose a novel algorithm, namely PETRELS-ADMM, to deal with subspace tracking in the presence of outliers and missing data. The proposed approach consistsof two main stages: outlier rejection and subspace estimation. In the first stage, alternating direction method of multipliers (ADMM) is effectively exploited to detect outliers affecting the observed data. In the second stage, we propose an improved version of the parallel estimation and tracking by recursive least squares (PETRELS) algorithm to update the underlying subspace in the missing data context. We then present a theoretical convergence analysis of PETRELS-ADMM which shows that it generates a sequence of subspace solutions converging to the optimum of its batch counterpart. The effectiveness of the proposed algorithm, as compared to state-of-the-art algorithms, is illustrated on both simulated and real data
Modélisation mathématique et méthode numérique pour la simulation du contrôle santé intégré par ultrasons de plaques composites stratifiées
No full textThis thesis is embedded in the context of « Structural Health Monitoring ». This method of non-destructive testing aiming at monitoring in-situ an engineered structure is increasingly used in numerous industrial fields, e.g. the aeronautics industry. It is based upon elastic guided waves propagating over large distances. The interactions between incident wave fields and structural defects are gathered through a network of receiving sensors. The dispersive nature of guided waves, combined with the inherent anisotropy of some industrial materials, such as composites, makes the interpretation of the output signals difficult. The goal of this thesis is to provide meaningful numerical tools enhancing the understanding and analysis of propagation and interaction phenomena, appearing during the control experiment. The thesis lies between the physical and mathematical modelling of elastic waves and the construction of relevant numerical schemes, altogether in an innovating industrial context involving complex geometries and materials.Ce sujet de thèse s’inscrit dans le contexte du contrôle intégré des structures, ou « Structural Health Monitoring » (SHM). Cette technique de contrôle non-destructif vise à utiliser un ou plusieurs capteurs, installés dans ou sur la structure d’intérêt. Le contrôle se fait in-situ et de façon périodique, afin d’obtenir des informations sur l’éventuelle apparition de défauts, tels que les défauts de corrosion pour les matériaux métalliques ou les défauts de délaminage pour les matériaux composites. Les données recueillies par les capteurs alimentent une analyse statistique dont le but est d’évaluer la santé de la structure au moment du contrôle, d’estimer son temps de vie restant et de faciliter la prise de décision quant à sa maintenance. Le SHM est de plus en plus présent dans de nombreux domaines industriels, en particulier dans le secteur aéronautique. Aussi le développement de modèles numériques pertinents comme performants constitue un atout majeur dans la conception de ces systèmes. Grâce à leur capacité à se propager sur de très grande distance, l’utilisation de capteurs ultrasonores générant des ondes guidées élastiques est une solution attirante. En pratique, des capteurs piézo-électriques fins, disposés à la surface de la structure, ou éventuellement enfouis pendant le procédé de fabrication, sont utilisés. Ils permettent l’émission et la réception des perturbations ultrasonores. Cependant, la nature dispersive des ondes guidées, combinée avec l’anisotropie inhérente aux matériaux composites rend difficile l’analyse des signaux obtenus lors du contrôle. De plus, proposer une modélisation fine de la propagation de ce type d’onde dans des configurations industrielles faisant intervenir des géométries complexes est une tâche difficile. Ces deux points constituent des obstacles non négligeables au développement de la méthodologie SHM, et l’objectif de cette thèse est de constituer l’ensemble des outils numériques qui permettront de proposer des solutions concrètes à ces problèmes
Numerical bifurcation and stability analysis of variational gradient-damage models for phase-field fracture
No full textInternational audienceGradient damage models used in phase-field approaches to brittle fracture are characterised by material softening and instabilities. We present novel numerical techniques for the bifurcation and stability analysis along quasi-static evolution paths as well as practical tools to select stable evolutions. Our approach stems from the variational approach to fracture and the theory of rate-independent irreversible processes whereby a quasi-static evolution is formulated in terms of incremental energy minimisation under unilateral constraints. Focussing on the discrete setting obtained with finite elements techniques, we discuss the links between bifurcation criteria for an evolution and stability of equilibrium states. Key concepts are presented through the analytical solution of a two-degreesof-freedom model featuring a continuum family of bifurcation branches. We introduce numerical methods to i) assess (second-order) stability conditions for time-discrete evolutions subject to damage irreversibility, and ii) to select possible stable evolutions based on an energetic criterion. Our approach is based on the solution of a coupled eigenvalue problem which accounts for the time-discrete irreversibility constraint on damage. Several numerical examples illustrate that this approach allows us to filter out unstable solutions provided by standard (first-order) minimisation algorithms as well as to effectively compute stable evolution paths. We demonstrate our purpose on a multifissuration problem featuring complex fracture patterns, to show how the eigenvalue analysis enables to compute and retrieve morphological properties of emerging cracks
Limiting Amplitude Principle for a Hyperbolic Metamaterial in Free Space
No full textHarmonic wave propagation in hyperbolic metamaterials is described by the Maxwell equations with a frequency-dependent tensor of dielectric permittivity. For a range of frequencies, this tensor has eigenvalues of opposite signs, and thus, in two dimensions, the harmonic Maxwell equations can be written as a Klein-Gordon equation. This technical report is mainly dedicated to the proof of the limiting amplitude principle for the simplest case of such a problem, and is a companion to the manuscript
Scattering of acoustic waves by a nonlinear resonant bubbly screen
No full textInternational audienceSome of the authors of this publication are also working on these related projects: PARIS code View project Homogenization of thin and thick microstructured materials View projec