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International audience; This work proposes a semi-analytical method to model the vibroacoustic behavior of submerged cylindrical shells periodically stiffened by axisymmetric frames and excited by a homogeneous and fully developed turbulent boundary layer (TBL). The process requires the computation of the TBL wall-pressure cross spectral density function and the sensitivity functions for stiffened cylindrical shells. The former is deduced from an existent TBL model and the latter are derived from a wavenumber-point reciprocity principle and a spectral formulation of the problem. The stiffeners' dynamic behavior is introduced in the formulation through circumferential admittances that are computed by a standard finite element code using shell elements. Four degrees of freedom are taken into account for the coupling between the shell and the stiffeners: three translation directions and one tangential rotation. To investigate the effect of the stiffeners on the radiated noise, two case studies are considered. The first one examines a fluid-loaded cylindrical shell with regularly spaced simple supports. The influence of Bloch-Floquet waves and the support spacing on the noise radiation are highlighted. The second case study inspects the fluid-loaded cylindrical shell with two different periodic ring stiffeners, namely stiffeners with T-shaped and I-shaped cross-sections. Their influence on the vibroacoustics of the shell is thoroughly analyzed.

International audience; We present a variant of a boundary integral equation proposed by M. Herberthson in 2008-2010 for high-frequency plane wave scattering by perfectly conducting obstacles which is entirely dedicated to one chosen direction of incidence. The solution of this new integral equation is a "pseudo-current" being the physical current distribution multiplied by the complex conjugate of the incident plane wave's phase function. The major advantage of this new integral equation is that the pseudo current has small total variation on large parts of many scattering obstacles and, hence, requires much less degrees of freedom to obtain a correct approximation. Restoring the physical current distribution is a trivial and numerically cheap operation. The price to pay is that, since the operator of this integral equation depends on the propagation direction of the incident plane wave, a new Galerkin matrix must be computed for each direction of incidence. In this paper, we show how this re-computation can be done very efficiently by making an explicit representation of the difference between the operator of the new integral equation and a conventional EFIE. This leads to a performance gain with respect to the EFIE even for multi-incidence problems.; Nous présentons une variante de l'équation intégrale proposée par M. Herberthson pour la diffraction d'une onde plane électromagnétique haute fréquence par un obstacle parfaitement conducteur. Cette équation intégrale est spécifique pour une seule direction d'incidence donnée. La solution de cette équation est un pseudo courant défini par la multiplication du courant physique avec la conjuguée de la fonction de phase de l'onde incidente. L'avantage principal de cette nouvelle équation intégrale est que la variation totale du pseudo courant est plus faible que celle du courant physique pour une grande classe d'objets diffractants. Par conséquent, l'approximation numérique de la solution du problème nécessite un nombre de degrés de liberté réduit. Puisque l'opérateur de la nouvelle équation intégrale dépend de la direction d'incidence, la matrice de Galerkin doit être recalculée pour chaque direction d'incidence. Dans cet article, nous présentons une technique qui permet de rendre ce calcul très efficace en explicitant l'opérateur qui est la différence entre le nouvel opérateur et celui de l'EFIE conventionnelle.Ainsi, on peut obtenir un gain en performance même pour des problèmes multi-incidences.

International audience; This book constitutes the thoroughly refereed post-conference proceedings of the 14th International Conference on Artificial Evolution, EA 2019, held in Mulhouses, France, in October 2019. The revised papers were carefully reviewed and selected from the submissions. The papers cover a wide range of topics in the field of artificial evolution, such as evolutionary computation, evolutionary optimization, co-evolution, artificial life, population dynamics, theory, algorithmics and modeling, implementations, application of evolutionary paradigms to the real world (industry, biosciences, ...), other biologically-inspired paradigms (swarm, artificial ants, artificial immune systems, cultural algorithms...), memetic algorithms, multi-objective optimisation, constraint handling, parallel algorithms,, dynamic optimization, machine learning and hybridization with other soft computing techniques.

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International audience; Acoustic coatings are of primary importance when investigating vibro-acoustic behavior of submerged cylindrical shells. These coatings can be applied on the entire circumference of the shell, or only on a certain stretch or a given angular domain. The latter configuration induces in the models a coupling of the shell circumferential orders, thus resulting in much heavier computation costs. To tackle this issue, it is suggested to simulate the vibroacoustic behavior of the partially coated submerged shell by using subtractive modeling. The principle of the method is to model the partially coated submerged shell, from the model of the fully coated submerged shell to which we remove a given part of the coating. In order to test its validity and its robustness to model errors, the method was at first applied to the academic case of the decoupling of Euler-Bernoulli beams. Several beam models were considered for the subtraction (analytical, FEM 1D, FEM 3D). The numerical results were compared to analytical calculations. Through this study, the method?s sensibility to model errors was investigated and the results allow to giving first validation of the method. Hence, the principle of subtractive modeling is, in a second time, extended to a three-dimensional system through the study of the scattering of a plane wave by a rigid sphere.

International audience

International audience; A comparison of the influence of sepiolite and lignin as potential synergists for fire retardant (FR) systems based on ammonium polyphosphate (APP) has been carried out in polyurethane elastomer and polylactide. Different ratios of kraft lignin and sepiolite were tested in combination with APP in both polymers. The thermal stability and the fire behavior of the corresponding composites were evaluated using Thermogravimetric Analysis (TGA), a Pyrolysis Combustion Flow Calorimeter (PCFC) and Cone Calorimeter (CC). The mechanisms of flame retardancy imparted by APP and other components were investigated. Synergistic effects were highlighted but only for specific ratios between APP and sepiolite in polyurethane elastomer (PUE) and polylactide (PLA) on one hand, and between APP and lignin in PLA on the other hand. Sepiolite acts as char reinforcement but through the formation of new phosphorus compounds it is also able to form a protective layer. Conversely, only complementary effects on fire performance were noted for lignin in PUE due to a dramatic influence on thermal stability despite its action on char formation.

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International audience; In a military context, where human capabilities are no longer sufficient to process quickly and reliably an ever-increasing amount of data, the implementation of algorithms based on Artificial Intelligence (AI), through the computing power of modern infrastructure, increases the ability to interpret and correlate massive heterogeneous data. This article will present an original automatic underwater acoustic signature recognition technique. The experiments are carried out from public underwater acoustic dataset. Besides, the performance of three architectures based on Mel-frequency cepstral coefficients (MFCCs), Machine Learning techniques and neural networks are compared.

15 th International Conference on High Speed Machining October 8-9; International audience; The machining of large dimension part presents issues of accuracy and of part handling. Machine-tools of large capacity are classically used, but they are very expensive. The paper investigates original solutions with mobile milling machines. A comparative study of a machining robot and of a portable milling machine is performed. Instrumented experiments of face milling operation were carried out and the resulting quality was analyzed. Finally, the manufacturable precision for the large part is assessed by the tolerance stacking model that is proposed.

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International audience; The thermal design of an industrial shell-and-tube condenser requires the use of heat transfer coefficients, usually obtained from tables or correlations. Willing to develop a numerical model for design purposes, the present authors noticed the surprising diversity of correlations for the shellside heat transfer coefficient in the case of pure vapour condensation outside of horizontal smooth tubes. In order to shed light on this specific topic, a bibliographic study was therefore initiated. This comprehensive review is meant to provide the designers with means to understand how each correlation was obtained, from the assumptions to the resolution method. Thus two main phenomena are well accounted for in this paper: vapour shear stress and condensate inundation. Indeed, the review lists the most important contributions to this field and details their interconnections. Consequently, the present authors conclude this paper with their recommendations.

National audience; L’usinage de pièces de grandes dimensions présente de grandes difficultés quant à la maîtrise de la précision des réalisations et à la manipulation des pièces. Cet article propose une approche originale visant à déplacer des moyens de productions mobiles sur la pièce, afin d’effectuer les usinages. Il est nécessaire de définir un moyen de production permettant de respecter les spécifications locales et dont le repositionnement entre séquences d’usinage assure la précision globale requise. Des essais de faisabilité ont été réalisés avec deux moyens mobiles en suivant un protocole expérimental similaire. Les résultats de ces essais permettent de proposer le moyen de production le plus pertinent et pour chaque moyen sa configuration optimale afin de respecter les spécifications. La modélisation des défauts fabriqués proposée permet d’analyser les défauts générés et de prédire la précision géométrique atteignable en fonction de la précision de repositionnement de la machine mobile.

International audience; To perform long-term and long-range missions, underwater vehicles need reliable navigation algorithms. This paper considers multi-beam Terrain Aided Navigation which can provide a drift-free navigation tool. This leads to an estimation problem with implicit observation equation and unknown likelihood. Indeed, the measurement sensor is considered to be a numerical black box model that introduces some unknown stochastic noise. We introduce a measurement updating procedure based on an adaptive kernel derived from Approximate Bayesian Computational filters. The proposed method is based on two well-known particle filters: Regularized Particle Filter and Rao-Blackwellized Particle Filter. Numerical results are presented and the robustness is demonstrated with respect to the original filters, yielding to twice as less non-convergence cases. The proposed method increases the robustness of particle-like filters while remaining computationally efficient.

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National audience; Dans cet article, nous démontrons qu'il est possible de remplacer les couches diélectriques d'un absorbant à métamatériaux initialement conçu avec des matériaux RF par des matériaux composites structuraux, i.e. renforcés de fibres. Ces matériaux, compatibles avec les procédés de fabrication du domaine naval militaire ont été réalisés et mesurés. L'effet des différentes couches de l'absorbant sur la bande passante est détaillé. Après optimisation de l'épaisseur de chaque couche grâce à un modèle analytique, l'absorbant présente un coefficient de réflexion inférieur à-14 dB dans la bande 4.6 GHz-17.2 GHz en incidence normale, pour une épaisseur totale de 8.9 mm. Le comportement de l'absorbant en incidence oblique est aussi discuté.

International audience; Using structural composite materials for the fabrication of Radar Absorbing Materials (RAM) allows combining the strong load bearing capability with the radar absorbing functionality in a unique structure. This article shows the possibility to transpose an already existing metamaterial absorber to the domain of composite materials. The dielectric layers of the absorber initially designed with radio-frequency (RF) materials are replaced with fiber-reinforced composite materials and their thickness is optimized again. The working principle of the absorber is explained layer by layer on the Smith chart. The performances of the composite absorber are compared against the initial RF design and against other classical absorbers. The composite design has a total thickness of 8.9 mm and achieves a reflection coefficient below 14 dB within the band 4.6 GHz - 17.2 GHz at normal incidence. The reflection coefficient remains under -10 dB at oblique incidence up to 45°.; L'utilisation de matériaux composites structurels pour la fabrication de matériaux absorbants radar permet de combiner la capacité de charge élevée avec la fonctionnalité d'absorption radar dans une structure unique. Cet article montre la possibilité de transposer un méta matériaux absorbant déjà existant, dans le domaine des matériaux composites. Les couches diélectriques de l'absorbant initialement conçues avec des matériaux radiofréquences (RF) sont remplacées par des matériaux composites renforcés de fibres et leur épaisseur est à nouveau optimisée. Le principe de fonctionnement de l'absorbant est expliqué couche par couche sur la carte de Smith. Les performances de l'absorbant composite sont comparées à la conception RF initiale et à d'autres absorbants classiques. La conception composite a une épaisseur totale de 8,9 mm et atteint un coefficient de réflexion inférieur à -14 dB dans la bande 4,6 GHz - 17,2 GHz à incidence normale. Le coefficient de réflexion reste inférieur à -10 dB en incidence oblique jusqu'à 45 °.

Identification of regions on a vibrating structure which radiate energy to the far field is critical in many areas of engineering. Non-negative intensity is a means to visualize contributions of local surface regions to sound power from vibrating structures. Whilst the non-negative intensity has been used for structures under deterministic excitation due to structural forces or harmonic incident acoustic pressure excitation, it has not been considered for analyzing a structure under stochastic excitation. This work analytically formulates non-negative intensity in the wavenumber domain to investigate the surface areas on a vibrating planar structure that are contributing to the radiated sound power in the far field. The non-negative intensity is derived in terms of the cross spectrum density function of the stochastic field and the sensitivity functions of either the acoustic pressure or normal fluid particle velocity. The proposed formulation can be used for both infinite planar structure and finite plate in an infinite baffle. To demonstrate the technique, a simply supported baffled panel excited by a turbulent boundary layer as well as an acoustic diffuse field is considered and those regions contributing to the radiated sound power are identified. It is demonstrated that the non-negative intensity distribution is dependent on the stochastic excitation. It is also found that for a panel under stochastic excitation the more the non-negative intensity distribution is concentrated within the panel surface, the more efficient the panel radiates sound to the far field.

This Ph.D is sponsored by the French company Naval Group in collaboration with LHEEA Laboratory from Ecole Centrale de Nantes and GeM Institute from ICAM de Nantes, and deals with the development of new composite marine propellers with improved efficiency, improved acoustic discretion and more environment-friendly. One of the key solutions lies in the application of composite materials to marine structures, in order to benefit from their reduced weight, increased flexibility and bend-twist coupling capacity. Indeed, the latter enables the shape-adaptability of the structure to passively adapt to the incoming flow. To meet this challenge, we first set-up a tightly coupled numerical fluid-structure method using two commercial CFD (Starccm+) and CSD (Abaqus) solvers on two flexible hydrofoils and we validated this method against experimental results available in the literature. Second, we specifically developed a composite hydrofoil to behave closely like a marine propeller and tested it in the hydrodynamic tunnel of the Ecole Navale. Thanks to the combined experimental and numerical analysis of this composite hydrofoil we reached the following conclusions: i) we helped demonstrate the industrial application of a state-of-the-art strain measurement technique using optical fibers directly embedded within the composite plies, ii) we provided some insights into the physics of the fluid-structure interaction occurring on composite hydrofoils and iii) we presented the current limitations of this coupled numerical fluid-structure method relatively to its industrial application.; Les travaux de cette thèse s’inscrivent dans le cadre d’une collaboration CIFRE entre Naval Group, le Laboratoire d’Hydrodynamique, d’Energétique et d’Environnement Atmosphérique (LHEEA) de l’Ecole Centrale de Nantes et de l’Institut de Génie des Matériaux (GeM) de l’ICAM de Nantes, sur la problématique de développement d’hélices marines plus efficaces, plus discrètes, et plus respectueuses de l’environnement. Une des solutions passe par le développement de structures composites plus légères et plus flexibles, capables de se déformer passivement pour s’auto-adapter à l’écoulement incident grâce à leurs propriétés spécifiques de couplage flexion-torsion. En lien direct avec cette problématique, nous avons réalisé les travaux de thèse en deux temps. Nous avons dans un premier temps mis en place, montré la faisabilité et validé une méthode de couplage numérique fluide- structure implicite fort entre les codes commerciaux de CFD Starccm+ et de CSD Abaqus pour un cas d'application issu de résultats expérimentaux disponibles dans la littérature sur deux hydrofoils flexibles déformables. Puis dans un second temps nous avons développé, réalisé et testé, dans le tunnel hydrodynamique de l’Ecole Navale, un profil portant composite spécifiquement conçu pour s’approcher du comportement d’une hélice. L’étude expérimentale et numérique de cette configuration nous a permis i) d’éprouver l'utilisation en milieu académique et industriel de nouvelles méthodes expérimentales d'instrumentation d'une pièce composite par insertion directe de fibres optiques dans les plis de composite, et d'une méthode mixte numérique - expérimentale de calibration fine d'un modèle numérique structure ; ii) d’apporter un éclairage sur la physique de l'interaction fluide- structure se produisant sur une surface portante composite ; et iii) de préciser les limitations actuelles concernant la diffusion en milieu industriel de cette méthode de couplage numérique fluide-structure.

This Ph.D is sponsored by the French company Naval Group in collaboration with LHEEA Laboratory from Ecole Centrale de Nantes and GeM Institute from ICAM de Nantes, and deals with the development of new composite marine propellers with improved efficiency, improved acoustic discretion and more environment-friendly. One of the key solutions lies in the application of composite materials to marine structures, in order to benefit from their reduced weight, increased flexibility and bend-twist coupling capacity. Indeed, the latter enables the shape-adaptability of the structure to passively adapt to the incoming flow. To meet this challenge, we first set-up a tightly coupled numerical fluid-structure method using two commercial CFD (Starccm+) and CSD (Abaqus) solvers on two flexible hydrofoils and we validated this method against experimental results available in the literature. Second, we specifically developed a composite hydrofoil to behave closely like a marine propeller and tested it in the hydrodynamic tunnel of the Ecole Navale. Thanks to the combined experimental and numerical analysis of this composite hydrofoil we reached the following conclusions: i) we helped demonstrate the industrial application of a state-of-the-art strain measurement technique using optical fibers directly embedded within the composite plies, ii) we provided some insights into the physics of the fluid-structure interaction occurring on composite hydrofoils and iii) we presented the current limitations of this coupled numerical fluid-structure method relatively to its industrial application.; Les travaux de cette thèse s’inscrivent dans le cadre d’une collaboration CIFRE entre Naval Group, le Laboratoire d’Hydrodynamique, d’Energétique et d’Environnement Atmosphérique (LHEEA) de l’Ecole Centrale de Nantes et de l’Institut de Génie des Matériaux (GeM) de l’ICAM de Nantes, sur la problématique de développement d’hélices marines plus efficaces, plus discrètes, et plus respectueuses de l’environnement. Une des solutions passe par le développement de structures composites plus légères et plus flexibles, capables de se déformer passivement pour s’auto-adapter à l’écoulement incident grâce à leurs propriétés spécifiques de couplage flexion-torsion. En lien direct avec cette problématique, nous avons réalisé les travaux de thèse en deux temps. Nous avons dans un premier temps mis en place, montré la faisabilité et validé une méthode de couplage numérique fluide- structure implicite fort entre les codes commerciaux de CFD Starccm+ et de CSD Abaqus pour un cas d'application issu de résultats expérimentaux disponibles dans la littérature sur deux hydrofoils flexibles déformables. Puis dans un second temps nous avons développé, réalisé et testé, dans le tunnel hydrodynamique de l’Ecole Navale, un profil portant composite spécifiquement conçu pour s’approcher du comportement d’une hélice. L’étude expérimentale et numérique de cette configuration nous a permis i) d’éprouver l'utilisation en milieu académique et industriel de nouvelles méthodes expérimentales d'instrumentation d'une pièce composite par insertion directe de fibres optiques dans les plis de composite, et d'une méthode mixte numérique - expérimentale de calibration fine d'un modèle numérique structure ; ii) d’apporter un éclairage sur la physique de l'interaction fluide- structure se produisant sur une surface portante composite ; et iii) de préciser les limitations actuelles concernant la diffusion en milieu industriel de cette méthode de couplage numérique fluide-structure.

We present a novel backward Itô-Ventzell formula and an extension of the Aleeksev-Gr\"obner interpolating formula to stochastic flows. We also present some natural spectral conditions that yield direct and simple proofs of time uniform estimates of the difference between the two stochastic flows when their drift and diffusion functions are not the same, yielding what seems to be the first results of this type for this class of anticipative models. We illustrate the impact of these results in the context of diffusion perturbation theory, interacting diffusions and discrete time approximations

WOS:000468268700001; International audience; This article deals with the use of the Volume Integral Method (VIM) to compute the magnetic anomaly created by a non-conductive ferromagnetic thin shell placed in a static inductor field. An original facet integral formulation considering the magnetic induction as unknowns is presented. The use of thin shell element assumption leads to a surface mesh decreasing highly the number of elements so the computation cost. The method is very performant in terms of speed and accuracy.

International audience; Acoustic stealth is one of the main operational capabilities for a submarine vehicle. To meet always stricter requirements in terms of radiated noise, it is important for the shipbuilder to be able to predict the noise at the early design stages. One of the key elements is the transfer function between a mechanical excitation on the pressure hull of the submarine vehicle and the far-field pressure radiated into water. This transfer function is commonly called p/F. A dedicated numerical method, called the Condensed Transfer Function (CTF) method, has been developed to calculate the vibro-acoustic behavior of a submarine hull. The method is based on a substructuring approach, where a model of an infinite fluid-loaded cylindrical shell is coupled to models of internal structures such as stiffeners and bulkheads. The fluid-loaded cylindrical shell is described analytically, while the internal structures are modeled by the finite element method (FEM). For each uncoupled subsystem, mechanical admittances are calculated at the junctions between the subsystems, and approximated with a set of functions called condensation functions. The interactions between the subsystems are estimated using the superposition principle, the force equilibrium and the displacements continuity. Compared to fully analytical models, the CTF method has more flexibility on the geometry of the internal frames. Compared to models completely described by the FEM, the present approach has lower calculation cost. Consequently, transfer functions can be calculated on a wide frequency range (from some Hz to several kHz) with limited calculation costs. Besides, an experimental campaign has been conducted to measure the transfer function p/F of a scaled model of the pressure hull. Based on laser vibrometer measurements on a model in air, the radial acceleration are measured at the surface of the shell. The far-field radiated pressure is calculated through the stationary phase theorem to deduce the transfer function p/F. This approach has two main advantages. First, no microphones are needed to estimate the radiated noise from the shell and second, the directivity of the radiated pressure can be studied. In this paper, the principle of both the numerical and the experimental approaches are described. A test case is presented and the results using both approaches are compared. The directivity pattern and some physical features of the vibro-acoustic behavior of the hull are discussed.

International audience; This research investigates the fluid-structure interaction and hydroelastic response of a composite hydrofoil using an innovative joint experimental and numerical method. The main novelties are, first, the use of a state-of-the-art strain measurement technique, via a fully-distributed-optical fiber sensor directly 20 embedded within the composite plies. This method allows for a finer representation of the structural deformations under hydrodynamic loading. Second, a tightly-coupled high-fidelity fluid-structure interaction numerical model taking into account the turbulent effects in the flow and the ply-by-ply modelling of the composite, is compared to the experimental results. A composite profile is specifically designed as a trapezoidal hydrofoil and is tested for moderate Reynolds number and pre-stall and post-stall incidences. High-speed imaging of the hydrofoil tip and vibrometer measurements are carried out to determine the experimental tip displacements and hydrofoil's vibrations. The numerical and experimental results show a very strong hydroeleastic response, with a structural resonance even for low Reynolds numbers due to the high flexibility of the structure. Strong coupling of the fluid and the structure, with lock-in of the von Kármán vortex-shedding to the structure for small incidences, and an excitation of the 30 structure by leading-edge vortex-shedding for higher incidences, are also observed.

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International audience; Modelling the vibro-acoustic behaviour of an underwater vehicle is important to predict the impact on the marine environment, to ensure that the noise and vibration levels do not pose discomfort for crew, and to assist in identifying mitigation strategies to reduce underwater radiated noise. The motivation of this paper is to examine the vibro-acoustic responses of a simplified physical model of an underwater vehicle corresponding to a finite cylindrical shell submerged in a heavy fluid of infinite extent, representing a classic fluid-structure interaction problem. The cylindrical shell is excited by a point force in the radial direction. The structural and acoustic responses of the submerged hull in the low to medium frequencies are predicted using a range of techniques to cover and bridge the frequency ranges. The advantages and limitations of each technique are discussed.

This short note provides an explicit description of the Frechet derivatives of the principal square root matrix function at any order. We present an original formulation that allows to compute sequentially the Frechet derivatives of the matrix square root at any order starting from the first order derivative. A Taylor expansion at any order with an integral remainder term is also provided, yielding the first result of this type for this class of matrix function.

National audience

In this article, we consider importance sampling (IS) and sequential Monte Carlo (SMC) methods in the context of one-dimensional random walks with absorbing barriers. In particular, we develop a very precise variance analysis for several IS and SMC procedures. We take advantage of some explicit spectral formulae available for these models to derive sharp and explicit estimates; this provides stability properties of the associated normalized Feynman–Kac semigroups. Our analysis allows one to compare the variance of SMC and IS techniques for these models. The work in this article is one of the few to consider an in-depth analysis of an SMC method for a particular model-type as well as variance comparison of SMC algorithms.

This paper provides a general framework for optimizing the trajectory of an observer by means of bearings-only tracking (BOT) in the case where the source is maneuvering. The basic problem of target motion analysis (TMA) is to estimate the trajectory of an object from noise corrupted sensor data. In the BOT context, the source state is only partially observed through nonlinear measurements, and the quality of the estimation strongly depends on the observer trajectory. We assume that the source has maneuvering capabilities, which is a case of practical interest though the TMA performance may be dramatically degraded. To characterize more precisely observer-optimal maneuvers, we have to be able to determine the optimal course. To this purpose, the common approach consists in maximizing the determinant of the Fisher information matrix (FIM) by an iterative algorithm to obtain an optimal observer trajectory. In our framework, a cumulative sum of bearing rates and relative distance between the target and observer is used as reward function, and the chosen maneuver will be the one that maximizes the mathematical expectation of the reward function. The target's motion is quantized, and an extended Kalman filter is used to estimate the position and velocity of the target and to provide the initial distribution for each optimization period. The proposed algorithm is a closed-loop control, and by maximizing the reward function, it allows to automatically compute the suboptimal maneuver. The applicability of our approach is demonstrated on realistic scenarios.

The present paper deals with Heat Exchanger sizing methods and offers a comparison between two of them: 1D global method and CFD porous media method. Following Prithiviraj et al. work [1], new developments are based on recent knowledge acquired on porous media, using a coupling strategy of a three-dimensional commercial code with an in-house code library. The distributed hydraulic resistance concept and the numerical model are briefly described and confronted with pressure drop measurements from an experimental E-type STHE setup (shell-and-tube heat exchanger) from the literature. The present paper will put into perspective capabilities and limits of each method with needs for heat exchanger rating. Flow rate repartition is calculated with CFD-porous media using Tinker’s current approach. This new analysis provides a complete comparison with 1D global method. It also reveals the major impact of leakage flow rate between baffle and tubes. The numerical estimation of pressure losses, consistent with experimental measurements of Halle et al. [2], implies that our future work will include thermal performance characterization and geometrical optimization.

Corrosion product layers formed on mild steel coupons after 6 years of permanent immersion at shallow depth in marine sediments were characterized by X-ray diffraction and μ-Raman spectroscopy. They proved mainly composed of FeS which testifies for an intense sulfate-reducing bacteria activity. Other identified compounds were siderite FeCO3 and four varieties of green rust associated with local variations of composition of the environment. The corrosion rates determined by residual thickness measurements were higher on the lower half of the coupons (average 40 μm/yr, local maximum 140 μm/yr) and smaller on the upper half, suggesting that differential aeration effects took place.

This article provides a comprehensive, rigorous, and self-contained introduction to the analysis of Wishart matrix moments. This article may act as an introduction to some aspects of random matrix theory, or as a self-contained exposition of Wishart matrix moments. Random matrix theory plays a central role in nuclear and statistical physics, computational mathematics and engineering sciences, including data assimilation, signal processing, combinatorial optimization, compressed sensing, econometrics and mathematical finance, among numerous others. The mathematical foundations of the theory of random matrices lies at the intersection of combinatorics, non-commutative algebra, geometry, multivariate functional and spectral analysis, and of course statistics and probability theory. As a result, most of the classical topics in random matrix theory are technical, and mathematically difficult to penetrate for non-experts and regular users and practitioners. The technical aim of this article is to review and extend some important results in random matrix theory in the specific context of real random Wishart matrices. This special class of Gaussian-type sample covariance matrix plays an important role in multivariate analysis and in statistical theory. We derive non-asymptotic formulae for the full matrix moments of real valued Wishart random matrices. As a corollary, we derive and extend a number of spectral and trace-type results for the case of non-isotropic Wishart random matrices. We also derive the full matrix moment analogues of some classic spectral and trace-type moment results. For example, we derive semi-circle and Marchencko-Pastur-type laws in the non-isotropic and full matrix cases. Laplace matrix transforms and matrix moment estimates are also studied, along with new spectral and trace concentration-type inequalities.

International audience; The authors present in this study a numerical method which computes the optimal trajectory of a underwater vehicle subject to some mission objectives. The method is applied to a submarine whose goal is to best detect one or several targets, or/and to minimise its own detection range perceived by the other targets. The signal considered is acoustic propagation attenuation. This approach is based on dynamic programming of a finite horizon Markov decision process. A quantisation method is applied to fully discretise the problem and allows a numerically tractable solution. Different scenarios are considered. The authors suppose at first that the position and the velocity of the targets are known and in the second they suppose that they are unknown and estimated by a Kalman type filter in a context of passive tracking.

International audience; Shallow waters acoustic propagation has been extensively studied in the last few years. Indeed, its understanding is of strong interest in a wide range of applications, from the impact on marine fauna to the modelling of harbour surveillance systems or sonar performance assessment. The issue is also a hot topic in standardization for the measurement of radiated sound levels from ships, at relatively short ranges. The study of shallow water environments is complex because of the interferences due to sea-floor and sea surface reflections. On the one hand, different methods have already been developed to model the acoustic propagation, the most widespread being rays, normal modes, parabolic equations or wavenumber integration. These methods, based on different assumptions, can be applied to shallow water environments in some cases. On the other hand, an experimental campaign was done off the coasts of Brittany, in France, to measure the propagation between a source and a hydrophone placed at short distance in a shallow water environment. The transfer functions have been measured for a frequency range from 100 Hz to 10 kHz. The aim of this paper is to compare some of the numerical approaches to experimental data in the case of short range propagation. So far, such comparisons cannot easily be found in the literature. The comparison between the simulations and the experimental results shows good agreement in most of cases. A study to check the sensitivity of a range of input parameters such as water depth, water density or sound velocity in the sediment, is proposed using the Monte-Carlo method. Results for that case study show that shallow water acoustic propagation is more sensitive to geometrical parameters than material properties.

International audience; This study aims to propose a methodology to rapidly obtain low-cycle fatigue life predictions. The main idea is to use a recently developed simplified method to estimate the stabilized stress-strain curve in confined plasticity in order to apply a fatigue criterion. This original simplified method is based on the use of an adjustable scale transition rule which has been demonstrated to be more robust to multiaxial loadings than classic plasticity correction rules. Different procedures are used to calibrate the localization operator and are compared to each other. The whole method is validated on a double notched specimen subjected either to compressive or tensile repeated loading. Kinematics and isotropic hardenings with threshold are used to model the partial mean stress relaxation that is observed experimentally. A modification of the Morrow fatigue criterion is proposed to enhance the prediction of the influence of the load ratio, represented by the local stabilized mean stress. The full design chain validation on notched specimens has shown a constant conservatism on the fatigue life predictions whatever the nominal load ratio and level. The results obtained with the simplified method are very similar to those obtained with finite elements.

Corrosion product layers formed on carbon steel coupons after 6–8 years of permanent immersion in natural seawater were characterized by X-ray diffraction and μ-Raman spectroscopy. Some of the coupons showed blisters of corrosion products on a surface covered elsewhere by a much thinner layer. In anodic zones, the formation of the sulfate green rust proved to be favored at the expense of that of magnetite. In contrast, the formation of magnetite was favored in cathodic zones, together with that of aragonite and carbonated corrosion products. The galvanic effects are then self-sustaining via their influence on the corrosion products.

International audience; Terrain Aided Navigation (TAN) provides a driftfree navigation approach for Unmanned Underwater Vehicles. This paper focuses on an improved version of the Weighted Ensemble Kalman Filter (WEnKF) to solve the TAN problem. We analyze some theoretical limitations of the WEnKF and derive an improved version which ensures that the asymptotic variance of weights remains bounded. This improvement results in an enhanced robustness to nonlinearities in practice. Numerical results are presented and the robustness is demonstrated with respect to conventional WEnKF, yielding twice as less nonconvergence cases.

International audience; Terrain-aided navigation provides a drift-free navigation approach for autonomous underwater vehicles. However, velocity is often tricky to estimate with conventional bathymetry (mono or multi-beam telemetry) sensors. Cold atom gravimetry is a promising absolute and autonomous additional sensor that is seldom considered for this kind of application. We investigate a multi-beam telemeter and gravimeter centralized fusion scenario and the resulting observability gain on velocity. To do so, an Adaptive Approximate Bayesian Computation Regularized Particle Filter is implemented and compared to conventional Regularized Particle Filter. Numerical results are presented and the robustness of the bathymetry and gravimetry fusion strategy is demonstrated, yielding less non-convergence cases and more accurate position and velocity estimation.

International audience; Nowadays, many types of materials are elaborated for microwave absorption applications. Carbon-based nanoparticles belong to these types of materials. Among these, graphene presents some distinctive features for electromagnetic radiation absorption and thus microwave isolation applications. In this paper, the dielectric characteristics and microwave absorption properties of epoxy resin loaded with graphene particles are presented from 2 GHz to 18 GHz. The influence of various parameters such as particle size (3 µm, 6–8 µm, and 15 µm) and weight ratio (from 5% to 25%) are presented, studied, and discussed. The sample loaded with the smallest graphene size (3 µm) and the highest weight ratio (25%) exhibits high loss tangent (tanδ = 0.36) and a middle dielectric constant ε′ = 12–14 in the 8–10 GHz frequency range. As expected, this sample also provides the highest absorption level: from 5 dB/cm at 4 GHz to 16 dB/cm at 18 GHz.

This paper considers the classic problem of bearings-only target motion analysis. This has extensively been studied in the past and the novelty here is the use of the unscented Kalman filter (UKF) with the modified polar coordinates state representation. To take advantage of this filter, we keep the exact non linear plant equation of the model in the prediction stage with the coupling between the different state vector components. In this representation, the prediction equation is not straightforward, but the counterpart is a trivial measurement equation. We compare the performance, in terms of estimation rms (root mean square) error and bias, of this filter with the widely used Cartesian representation in which the plant equation is trivial and the non linearity is reported in the measurement equation. The Cramer Rao Lower Bound is finally computed and helps as a reference.

We consider the classical Yaglom limit theorem for a branching Markov process $X = (X_t, t \ge 0)$, with non-local branching mechanism in the setting that the mean semigroup is critical, i.e. its leading eigenvalue is zero. In particular, we show that there exists a constant $c(f)$ such that \[ {\rm Law}\left(\frac{\langle f, X_t\rangle}{t} \bigg| \langle 1, X_t\rangle > 0 \right) \to {\mathbf e}_{c(f)}, \qquad t \to \infty, \] where ${\mathbf e}_{c(f)}$ is an exponential random variable with rate $c(f)$ and the convergence is in distribution. As part of the proof, we also show that the probability of survival decays inversely proportionally to time. Although Yaglom limit theorems have recently been handled in the setting of branching Brownian motion in a bounded domain and superprocesses, \cite{Ellen, Yanxia}, these results do not allow for non-local branching, which complicates the analysis. Our approach and the main novelty of this work is based around a precise result for the scaled asymptotics for the $k$-th martingale moments of $X$ (rather than the Yaglom limit itself). We then illustrate our results in the setting of neutron transport, for which the non-locality is essential, complementing recent developments in this domain \cite{SNTE, SNTEII, SNTEIII, MCNTE, MultiNTE}., 2 figures

International audience

International audience; Discrete algebraic Riccati equations and their fixed points are well understood and arise in a variety of applications, however, the time-varying equations have not yet been fully explored in the literature. In this article we provide a self-contained study of discrete time Riccati matrix difference equations. In particular, we provide a novel Riccati semigroup duality formula and a new Floquet-type representation for these equations. Due to the aperiodicity of the underlying flow of the solution matrix, conventional Floquet theory does not apply in this setting and thus further analysis is required. We illustrate the impact of these formulae with an explicit description of the solution of time-varying Riccati difference equations and its fundamental-type solution in terms of the fixed point of the equation and an invertible linear matrix map, as well as uniform upper and lower bounds on the Riccati maps. These are the first results of this type for time varying Riccati matrix difference equations.

Growth-fragmentation processes model systems of cells that grow continuously over time and then fragment into smaller pieces. Typically, on average, the number of cells in the system exhibits asynchronous exponential growth and, upon compensating for this, the distribution of cell sizes converges to an asymptotic profile. However, the long-term stochastic behaviour of the system is more delicate, and its almost sure asymptotics have been so far largely unexplored. In this article, we study a growth-fragmentation process whose cell sizes are bounded above, and prove the existence of regimes with differing almost sure long-term behaviour., 34 pages