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Développement d'une modélisation thermomécanique du meulage pour l'estimation de l'impact sur les CR de soudage
No full textNational audienceCette étude aborde la modélisation numérique du meulage manuel, en se concentrant sur les contraintes résiduelles. Elle propose une méthode macroscopique basée sur des simulations thermiques et thermomécaniques sur Code_Aster en 3D pour simuler le meulage. L’approche a été validée à l’aide de données expérimentales recueillies via des thermocouples placés sur une maquette en inconel 600. Bien que le modèle initial offre une approximation prometteuse des températures, une expansion du modèle pour inclure le cisaillement et des analyses de contraintes résiduelles expérimentales pourrait améliorer sa précision
Jean-Pierre Olivier de Sardan, La revanche des contextes. Des mésaventures de l’ingénierie sociale, en Afrique et au-delà
No full textInternational audienc
The Navier-Stokes system with temperature and salinity for free surface flows. Numerical scheme and validation
No full textInternational audienceIn this paper, we propose and study a numerical scheme for the Navier-Stokes-Fourier system derived and studied by the authors in Boittin at al. (2023). This system models hydrostatic free surface flows with density variations depending on temperature or salinity. We show that the finite volume/finite element scheme presented -- based on a layer averaged formulation of the model -- is well-balanced with regards to the steady state of the lake at rest and preserves the nonnegativity of the water height. A maximum principle on the density is also proved as well as a discrete entropy inequality (when the thermal and viscous effects are neglected). Some numerical validations are finally shown with comparisons to 3D analytical solutions and experiments
Population exposure to outdoor NO2 , black carbon, particle mass, and number concentrations over Paris with multi-scale modelling down to the street scale
No full textInternational audienceThis study focuses on mapping the concentrations of pollutants of health interest (NO2, black carbon (BC), PM2.5, number of particles (PN)) down to the street scale to represent as accurately as possible the population exposure. Simulations are performed over the Greater Paris area with the WRF-CHIMERE/MUNICH/SSH-aerosol chain, using either the top-down inventory EMEP or the bottom-up inventory Airparif with correction of the traffic flow. The concentrations of the pollutants are higher in streets than in the regional-scale urban background, due to the strong influence of road-traffic emissions locally. Model-to-data comparisons were performed at urban background and traffic stations, and evaluated using two performance criteria from the literature. For BC, harmonized equivalent BC (eBC) concentrations were estimated from concomitant mea-surements of eBC and elemental carbon. Using the bottom-up inventory with corrected road-traffic flow, the strictest criteria are met for NO2, eBC, PM2.5, and PN. Using the EMEP top-down inventory, the strictest criteria are also met for NO2, eBC and PM2.5, but errors tend to be larger than with the bottom-up inventory for NO2, eBC and PN. Using the top-down inventory, the concentrations tend to be lower along the streets than those simulated using the bottom-up inventory, especially for NO2 con-centrations, resulting in less urban heterogeneities. The impact of the size-distribution of non-exhaust emissions was analyzed at both regional and local scales, and it is higher in heavy-traffic streets. To assess exposure, a french database detailing the number of inhabitants in each building was used. The population-weighted concentration (PWC) was calculated by weighting populations by the outdoor concentrations to which they are exposed at the precise location of their home. An exposure scaling factor (ESF) was determined for each pollutant to estimate the ratio needed to correct urban background concentrations in order to assess exposure. The average ESF in Paris and Paris Ring Road is higher than 1 for NO2, eBC, PM2.5, PN, because the concentrations simulated at the local scale in streets are higher than those modelled at the regional scale. It indicates that the Parisian population exposure is under-estimated using regional-scale concentrations. Although this underestimation is low for PM2.5, with an ESF of 1.04, it is very high for NO2 (1.26), eBC (between 1.22 and 1.24), and PN (1.12). This shows that urban heterogeneities are important to be considered in order to represent the population exposure to NO2, eBC, and PN, but less so for PM2.5
Experimental study of particle lift initiation on roller-compacted sand-clay mixtures
No full textInternational audienceCivil engineering works are sources of dust emissions, which can cause severe security, health and environmental damages to workers and neighbourhoods. This is particularly significant for implementation of earthwork sites. The present paper reports a study conducted to characterise the soil–atmosphere interaction above compacted soils where particle lift is initiated. Mixtures of kaolin clay and sand have been compacted using a laboratory roller compactor that reproduces near-field compaction conditions. Shear testing conducted at the interface confirms that the sand content affects the friction angle between the soil and the compaction roller. The experimental velocity profiles above the compacted samples have been obtained in a wind tunnel using a non-intrusive measurement technique (laser Doppler velocimetry). Results show that the sand fraction affects velocity profiles. Compaction, therefore, may not fully reduce the roughness of the soil surface. The airflow friction velocities at the sample surfaces have been determined from the boundary layer profiles. The results achieved demonstrate that all tested soils reach the threshold friction velocity required to initiate particle lift, and the higher the sand content of the soil, the more likely it is that particle lift occurs
Identifying and fixing ambiguities in, and semantically accurate formalisation of, behavioural requirements
No full textInternational audienceTo correctly formalise requirements expressed in natural language, ambiguities must first be identified and then fixed. This paper focuses on behavioural requirements (i.e. requirements related to dynamic aspects and phenomena). Its first objective is to show, based on a practical, public case study, that the disambiguation process cannot be fully automated: even though natural language processing (NLP) tools and machine learning might help in the identification of ambiguities, fixing them often requires a deep, application-specific understanding of the reasons of being of the system of interest, of the characteristics of its environment, of which trade-offs between conflicting objectives are acceptable, and of what is achievable and what is not; it may also require arduous negotiations between stakeholders. Such an understanding and consensus-making ability is not in the reach of current tools and technologies, and will likely remain so for a long while. Beyond ambiguity, requirements are often marred by various other types of defects that could lead to wholly unacceptable consequences. In particular, operational experience shows that requirements inadequacy (whereby, in some of the situations the system could face, what is required is woefully inappropriate or what is necessary is left unspecified) is a significant cause for systems failing to meet expectations. The second objective of this paper is to propose a semantically accurate behavioural requirements formalisation format enabling tool-supported requirements verification, notably with simulation. Such support is necessary for the engineering of large and complex cyber-physical and socio-technical systems to ensure, first, that the specified requirements indeed reflect the true intentions of their authors and second, that they are adequate for all the situations the system could face. To that end, the paper presents an overview of the BASAALT (Behaviour Analysis and Simulation All Along systems Life Time) systems engineering method, and of FORM-L (FOrmal Requirements Modelling Language), its supporting language, which aims at representing as accurately and completely as possible the semantics expressed in the original, natural language behavioural requirements, and is markedly different from languages intended for software code generation. The paper shows that generally, semantically accurate formalisation is not a simple paraphrasing of the original natural language requirements: additional elements are often needed to fully and explicitly reflect all that is implied in natural language. To provide such complements for the case study presented in the paper, we had to follow different formalisation patterns, i.e. sequences of formalisation steps. For this paper, to avoid being skewed by what a particular automatic tool can and cannot do, BASAALT and FORM-L were applied manually. Still, the lessons learned could be used to specify and develop NLP tools that could assist the disambiguation and formalisation processes. However, more studies are needed to determine whether an exhaustive set of formalisation patterns can be identified to fully automate the formalisation process
New penalized criteria for smooth non-negative tensor factorization with missing entries
No full textInternational audienceTensor factorization models are widely used in many applied fields such as chemometrics, psychometrics, computer vision or communication networks. Real life data collection is often subject to errors, resulting in missing data. Here we focus in understanding how this issue should be dealt with for nonnegative tensor factorization. We investigate several criteria used for non-negative tensor factorization in the case where some entries are missing. In particular we show how smoothness penalties can compensate the presence of missing values in order to ensure the existence of an optimum. This lead us to propose new criteria with efficient numerical optimization algorithms. Numerical experiments are conducted to support our claims
Construction et analyse des signatures spectrales pour des défauts dans des milieux complexes
No full textConcrete is widely used in construction, particularly in the reactor building of nuclear power plants. Monitoring its evolution and identifying any defects that could compromise its proper functioning can be achieved through non-destructive testing. Concrete is composed of aggregates, but their high concentration and proximity pose challenges for classical methods, such as the Linear Sampling Method, which fail to produce quantitative results or exploitable images.The objective of this thesis is to build an imaging algorithm that can estimatethe density of the aggregates and recover the local distribution of those smallinhomogeneities.To address this challenge, inspiration was drawn from a monotonicity property of the Transmissioneigenvalues. They correspond to the frequencies for which an incident wave exists such that the scattered field is trivial outside the scatterers.Instead of comparing the scattered field to the vacuum, this thesis introduces a new approach: comparing it, at a fixed wavenumber, to a numerical scattering problem, referred to as the background. This led to the introduction of a new class of eigenvalues known as the f-averaged Steklov eigenvalues. Each of these eigenvalues is associatedwith an artificial background problem that contains a resonator and is solution to a simple spectral problem inside the resonator. In addition, each f-averaged Steklov eigenvalue is monotonically increasing with respect to the number of inhomogeneities inside that resonator. These spectral signatures can be recovered from the data. By computing theseeigenvalues for various positions of the resonator, we can estimate the variation ofthe local density of the inhomogeneities in an unknown medium.Le béton est largement employé dans le secteur de la construction, notamment dans les bâtiments réacteurs des centrales nucléaires. La surveillance de son évolution et la détection des défauts susceptibles de compromettre son bon fonctionnement peuvent être effectuées grâce à des tests non destructifs. Le béton est constitué d'agrégats. Ces derniers présentent des défis en raison de leur concentration élevée et de leur proximité. Ces caractéristiques rendent les méthodes classiques, telles que la Linear Sampling Method, inefficaces pour fournir des résultats quantitatifs ou des images exploitables.Cette thèse a pour objectif de développer un algorithme d'imagerie capable d'estimer la densité des agrégats et de retrouver la distribution locale de ces petites hétérogénéités.Pour résoudre ce problème, l'inspiration provient d'une propriété de monotonie des valeurs propres de transmission. Ces valeurs correspondent aux fréquences pour lesquelles il existe une onde incidente générant un champ diffracté trivial à l'extérieur des obstacles.Plutôt que de comparer le champ diffracté à celui du vide, cette thèse propose une nouvelle approche consistant à le comparer, à un nombre d'onde fixé, à un problème de diffraction numérique. Cela a conduit à l'introduction d'une nouvelle classe de valeurs propres, appelées f-averaged Steklov eigenvalues. Chacune de ces valeurs est associée à un problème artificiel contenant un résonateur et est la solution d'un problème spectral simple à l'intérieur de ce résonateur. En outre, chaque f-averaged Steklov eigenvalue croît de manière monotone en fonction du nombre d'hétérogénéités présentes dans le résonateur. Ces signatures spectrales peuvent être extraites des données. En calculant ces valeurs propres pour différentes positions du résonateur, il devient possible d'estimer les variations de densité locale des hétérogénéités dans un milieu inconnu
Cybersécurité et FOH : quelques éléments de retour d'expérience issus du cas Stuxnet
No full textInternational audienceThis paper aims at analysing one of the most spectacular cyberattack of these recent years, which impressed by its technical complexity and its ability to integer a technical system, and how the Stuxnet worm was able to directly attack and industrial risky system, namely nuclear machines in a uranium Iranian factory. We suggest here to highlight the Human and Organisational Factors (HOF) of this out-of-norms attack, to produce a useful Return of Experience for the whole vital infrastructures industry.Cette communication propose de revenir sur l'une des plus graves cyberattaques de ces dernières année, qui impressionna par sa complexité technique et sa capacité à s'immiscer dans un système technique, mais également par la manière dont le vers Stuxnet parvint à attaquer directement des outils industriels à risques, en l'occurrence des centrifugeuses nucléaires dans une usine iranienne. Nous proposons ici une approche mettant en lumière les Facteurs Organisationnels et Humains (FOH) de cette attaque hors-normes, dans le but d'en tirer un Retour d'Expérience utile à l'ensemble industries à risques et Opérateurs d'Importance Vitale (OIV)
Spatial and temporal analysis of the sand dynamics in alpine rivers for 3D numerical modelling
No full textInternational audienceHydraulic structures can impact the sediment continuity of the river and can create a sediment imbalance downstream of dams according to the different sediment classes (silt, sand, gravel). For operators, one important issue is the presence of sand (d>63 μm), which can lead to exploitation difficulties, issues for flushing or emptying operations as well as to ecological, in-dustrial or safety issues. It was recognized that the percentage of sand particles that is transported by suspension compared to bedload transport can often prevail. However, today it is still a real challenge to evaluate the spatial and temporal variability of these sand concentrations in gravel-bed rivers since sand dynamics are generally supply limited. Using 3D hydro-sedimentary modelling could help to better understand processes of sand transport in rivers. In this article, we will present results from experimental monitoring that will then be used for 3D numerical modelling (TELEMAC 3D + Gaia) on engineered rivers in the Northern French Alps