Publications

2014

• Qualitative and Asymptotic Theory of Detonations
  
  • Faria Luiz
  , 2014. Shock waves in reactive media possess very rich dynamics: from formation of cells in multiple dimensions to oscillating shock fronts in one-dimension. Because of the extreme complexity of the equations of combustion theory, most of the current understanding of unstable detonation waves relies on extensive numerical simulations of the reactive compressible Euler/Navier-Stokes equations. Attempts at a simplified theory have been made in the past, most of which are very successful in describing steady detonation waves. In this work we focus on obtaining simplified theories capable of capturing not only the steady, but also the unsteady behavior of detonation waves. The first part of this thesis is focused on qualitative theories of detonation, where ad hoc models are proposed and analyzed. We show that equations as simple as a forced Burgers equation can capture most of the complex phenomena observed in detonations. In the second part of this thesis we focus on rational theories, and derive a weakly nonlinear model of multi-dimensional detonations. We also show, by analysis and numerical simulations, that the asymptotic equations provide good quantitative predictions.
• Finite Element Heterogeneous Multiscale Method for the Classical Helmholtz Equation
  
  • Ciarlet Patrick
  • Stohrer Christian
  Comptes Rendus. Mathématique, Académie des sciences (Paris), 2014, 352 (9), pp.755–760. We show that the standard Finite Element Heterogeneous Multiscale Method (FE-HMM) can be used to approximate the effective behavior of solutions to the classical Helmholtz equation in highly oscillatory media. Using a novel combination of well-known results about FE-HMM and the notion of T-coercivity, we derive an a priori error bound. Numerical experiments corroborate the analytical findings. (10.1016/j.crma.2014.07.006)
  DOI : 10.1016/j.crma.2014.07.006
• Robin-to-Robin transparent boundary conditions for the computation of guided modes in photonic crystal wave-guides
  
  • Fliss Sonia
  • Klindworth Dirk
  • Schmidt Kersten
  BIT Numerical Mathematics, Springer Verlag, 2014, 55 (1), pp.35. The efficient and reliable computation of guided modes in photonic crystal wave-guides is of great importance for designing optical devices. Transparent boundary conditions based on Dirichlet-to-Neumann operators allow for an exact computation of well-confined modes and modes close to the band edge in the sense that no modelling error is introduced. The well-known super-cell method, on the other hand, introduces a modelling error which may become prohibitively large for guided modes that are not well-confined. The Dirichlet-to-Neumann transparent boundary conditions are, however, not applicable for all frequencies as they are not uniquely defined and their computation is unstable for a countable set of frequencies that correspond to so called Dirichlet eigenvalues. In this work we describe how to overcome this theoretical difficulty introducing Robin-to-Robin transparent boundary conditions whose construction do not exhibit those forbidden frequencies. They seem, hence, well suited for an exact and reliable computation of guided modes in photonic crystal wave-guides. (10.1007/s10543-014-0521-1)
  DOI : 10.1007/s10543-014-0521-1
• On the identification of defects in a periodic waveguide from far field data
  
  • Bourgeois Laurent
  • Fliss Sonia
  Inverse Problems, IOP Publishing, 2014, 30 (9). The aim of this paper is to apply the Linear Sampling Method and the Factorization Method to retrieve some defects in a known periodic 2D waveguide from scattering data. More precisely, some far field approximations of these two sampling methods are derived. They amount to consider the so-called propagating Floquet modes as incident waves. The efficiency of the far field formulation of the LSM is shown with the help of some numerical experiments. (10.1088/0266-5611/30/9/095004)
  DOI : 10.1088/0266-5611/30/9/095004
• Contributions à la simulation numérique en élastodynamique : découplage des ondes P et S, modèles asymptotiques pour la traversée de couches minces
  
  • Burel Aliénor
  , 2014. Cette thèse porte sur la modélisation des ondes élastodynamiques dans deux situations particulières qui pénalisent les méthodes numériques utilisées pour simuler ces phénomènes. Dans la première partie, on se place dans le cas où les ondes de pression (ondes P) se propagent à une vitesse beaucoup plus grande que celle des ondes de cisaillement (ondes S). Les modèles numériques utilisés habituellement pour traiter cette configuration sont pénalisés par la plus petite vitesse qui dicte le choix du pas du schéma. Nous proposons ici un schéma qui découple numériquement, dans le volume, les ondes P et les ondes S, pour deux types de conditions de bord en utilisant la décomposition du déplacement en potentiels de Lamé, en deux dimensions. Les conditions aux limites de Dirichlet homogènes, qui sont des conditions essentielles pour la formulation classique en déplacement, deviennent des conditions naturelles, mais non standard, pour la formulation en potentiels qui se présente comme un système de deux équations d’ondes couplées par les conditions aux limites. Cette formulation préserve une énergie équivalente à l'énergie élastodynamique. Nous construisons un schéma éléments finis en espace et utilisons un thêta-schéma en temps sur les termes de bord afin de ne pas pénaliser la CFL et mener à une condition sur le pas de temps indépendante des termes de couplage au bord. Ce schéma préserve une énergie discrète. Le cas des conditions de surface libre mène à des instabilités. Nous les avons traitées comme des perturbations des conditions de Dirichlet, ce qui permet d'obtenir de bons résultats dans le domaine fréquentiel mais donne naissance à de sévères instabilités après discrétisation en temps. La seconde partie de la thèse est consacrée à la construction, l'analyse et la validation de conditions de transmission effectives (CTE) à travers une couche mince de matériau homogène et isotrope d'épaisseur constante h. Ici, la finesse de la couche affecte les schémas explicites usuels car le maillage de la couche avec des éléments suffisamment petits entraîne une diminution analogue du pas de temps critique via la condition CFL, tandis que l'on espère avec les CTE obtenir un pas de temps indépendant de l'épaisseur de la couche. Une analyse complète du cas de la bande mince rectiligne est donnée en deux et trois dimensions. Les conditions obtenues sont stables via la conservation d'une énergie et l'ordre de l'erreur d'approximation par rapport à l'épaisseur de la couche pour les conditions d'ordre 2 est de O(h^3). Des résultats numériques sont présentés pour les configurations bi et tridimensionnelles, ils valident les résultats de stabilité, d'estimation d'erreur et de conditions de stabilité de schémas en temps proposés, qui sont des modifications du schéma explicite utilisé en l'absence de couche mince. Enfin, le traitement d'une couche curviligne est effectué dans le cas bidimensionnel. Sa stabilité est à nouveau vérifiée par conservation d'énergie et des résultats numériques sont également présentés.
• Optimized Transmission Conditions for Domain Decomposition Methods and Helmholtz Equation. Application to Higher Order Finite Element Methods
  
  • Collino Francis
  • Duruflé Marc
  • Joly Patrick
  • Lecouvez Matthieu
  , 2014. Domain decomposition methods for solving Helmholtz equation are considered. Such methods rely on transmission conditions set on the interfaces between subdomains. The convergence of the iterative algorithm used to solve the associated linear system depends on these transmission conditions. Optimized transmission conditions (such as proposed in [1]) usually rely on transparent boundary conditions or local operators that are an approximation of the exact transparent boundary condition. In this talk, non-local optimized transmission conditions based on Riesz potentials as detailed in [2] are studied. The non-local operators can be replaced by quasi-local operators, and the obtained rate of convergence is independent of the mesh size. These conditions are applied to higher order finite element methods.
• Finite element computation of leaky modes in straight and helical elastic waveguides
  
  • Nguyen Khac-Long
  • Treyssede Fabien
  • Hazard Christophe
  • Bonnet-Ben Dhia Anne-Sophie
  , 2014, pp.4p. Elastic guided waves are of interest for inspecting structures due to their ability to propagate over long distances. However, waveguides are often embedded in a large solid domain, considered as unbounded. The waves can attenuate strongly along the guide axis due to the energy leakage into the surrounding medium, which reduces the propagating distance. Searching modes with low attenuation becomes necessary. The goal of this work is to propose a numerical approach to compute modes in embedded elastic waveguides (straight and helical structures), combining the so-called semi-analytical finite element method (SAFE) and a perfectly matched layer (PML) method. The application of this work is the non destructive evaluation of multi-wire strands, which constitute cables in civil structures.
• Étude de deux problèmes de propagation d'ondes transitoires : 1) Focalisation spatio-temporelle en acoustique; 2) Transmission entre un diélectrique et un métamatériau.
  
  • Cassier Maxence
  , 2014. Cette thèse comporte deux parties indépendantes. Dans la première, à l'aide d'un réseau de transducteurs, nous cherchons à émettre dans un milieu contenant des obstacles diffractants dont nous ignorons les positions une onde venant focaliser en espace et en temps au voisinage d'un de ces obstacles. La solution proposée ici est basée sur la méthode DORT (Décomposition de l'Opérateur de Retournement Temporel) qui conduit à des propriétés de focalisation spatiale en régime harmonique. Cette dernière consiste à effectuer une décomposition en valeurs singulières (SVD) de l'opérateur de diffraction qui à une distribution de signaux envoyés aux transducteurs associe la mesure de l'onde diffractée. Dans le cadre du modèle asymptotique petits obstacles de Foldy-Lax (justifié ici dans le cas bidimensionnel), nous montrons comment synchroniser les signaux fournis par la méthode DORT en introduisant une SVD particulière liée au caractère symétrique de l'opérateur de diffraction. Notre méthode est justifiée par des arguments théoriques et une étude numérique. La seconde partie est dédiée à un problème de transmission entre un diélectrique et un métamatériau. La question qui est posée ici consiste à étudier la validité du principe d'amplitude limite (PAL) dans un tel milieu. Ce principe définit le régime périodique établi comme le comportement asymptotique en temps long d'un système soumis à une excitation périodique. Nous proposons ici une réponse dans le cas d'un dioptre plan entre un diélectrique et un métamatériau particulier (modèle de Drude). Dans un tel cadre, les équations de Maxwell sont reformulées en une équation de Schrödinger dont nous réalisons l'analyse spectrale complète. Notre étude permet de voir que le PAL est vérifié sauf à une fréquence particulière, appelée fréquence plasmon, pour laquelle les rapports des valeurs prises par la permittivité et par la perméabilité de part et d'autre l'interface sont égaux à -1. Cette fréquence correspond à une résonance du système et la réponse à une telle excitation croît linéairement en temps.
• Acoustic inverse scattering using topological derivative of far-field measurements-based L2 cost functionals
  
  • Bellis Cédric
  • Bonnet Marc
  • Cakoni Fioralba
  , 2014. Originally formulated in the context of topology optimization, the concept of topological derivative has also proved effective as a qualitative inversion tool for a wave-based identification of finite-sized objects. This approach remains, however, largely based on a heuristic interpretation of the topological derivative, whereas most other qualitative approaches to inverse scattering are backed by a mathematical justification. As an effort toward bridging this gap, this study focuses on a topological derivative approach applied to the L2-norm of the misfit between far-field measurements. Either an inhomogeneous medium or a finite number of point-like scatterers are considered, using either the Born approximation or a full-scattering model. Topological derivative-based imaging functionals are analyzed using a suitable factorization of the far-field operator, for each of the considered cases, in order to characterize their behavior and assess their ability to reconstruct the unknown scatterer(s). Results include the justification of the usual sign heuristic underpinning the method for (i) the Born approximation and (ii) full-scattering models limited to moderately strong scatterers. Semi-analytical and numerical examples are presented. Within the chosen framework, the topological derivative approach is finally discussed and compared to other well-known qualitative methods.
• Le piano rêvé des mathématiciens
  
  • Chabassier Juliette
  • Chaigne Antoine
  • Duruflé Marc
  • Joly Patrick
  Interstices, INRIA, 2014.
• Modélisation numérique des guides d’onde ouverts : cas des structures élastiques courbes
  
  • Nguyen Khac-Long
  • Treyssede Fabien
  • Bonnet-Ben Dhia Anne-Sophie
  • Hazard Christophe
  , 2014, pp.7p. Pour le contrôle non destructif des matériaux et structures, les ondes élastiques guidées sont intéressantes en raison de leur longue distance de propagation. Cependant pour les guides enfouis dans un milieu infini, les modes peuvent devenir à fuite. Ces modes s'atténuent selon l'axe du guide en raison du rayonnement de l'énergie, ce qui réduit leur distance de propagation. La recherche des modes les moins atténués est donc nécessaire. Le calcul numérique des modes dans les guides enfouis présente une difficulté: les amplitudes des modes à fuite croissent dans la direction transversale. Une technique consiste à combiner la méthode des éléments finis semi-analytique (SAFE) et la méthode des couches parfaitement adaptées (PML). Cette méthode a été implémentée récemment par les auteurs pour simuler des plaques multi-couches enfouies et des guides enrobés de section arbitraire. Néanmoins, ces travaux ne considèrent que des guides droits, sans courbure. Nous nous intéressons ici à la modélisation par méthode SAFE-PML des structures courbes, plus particulièrement hélicoïdales, enrobées dans un solide. La motivation de ce travail est le contrôle non destructif des torons, structures multi-brins hélicoïdales, constituant les câbles de génie civil. Dans le vide, on peut simuler les guides hélicoïdaux par méthode SAFE grâce à des coordonnées hélicoïdales. Notre travail consiste à appliquer des PML selon des directions transversales twistées pour calculer les modes à fuite. Deux cas tests sont étudiés. Le premier cas test correspond à un cylindre "twisté" dans une matrice solide et permet de valider la méthode SAFE-PML twistée. L'effet du twist sur le spectre des modes propres est présenté. Le deuxième cas consiste à simuler un brin hélicoïdal enrobé. L'effet de la courbure du guide sur l'atténuation axiale des modes est étudié. Enfin, la simulation d’un toron plongé dans un coulis de ciment sera présentée.
• T-coercivity for the Maxwell problem with sign-changing coefficients
  
  • Bonnet-Ben Dhia Anne-Sophie
  • Chesnel Lucas
  • Ciarlet Patrick
  Communications in Partial Differential Equations, Taylor & Francis, 2014. In this paper, we study the time-harmonic Maxwell problem with sign-changing permittivity and/or permeability, set in a domain of R^3. We prove, using the T-coercivity approach, that the well-posedness of the two canonically associated scalar problems, with Dirichlet and Neumann boundary conditions, implies the well-posedness of the Maxwell problem. This allows us to give simple and sharp criteria, obtained in the study of the scalar cases, to ensure that the Maxwell transmission problem between a classical dielectric material and a negative metamaterial is well-posed.
• Numerical modeling of nonlinear acoustic waves in a tube connected with an array of Helmholtz resonators
  
  • Lombard Bruno
  • Mercier Jean-François
  Journal of Computational Physics, Elsevier, 2014, 259 (15). (10.1016/j.jcp.2013.11.036)
  DOI : 10.1016/j.jcp.2013.11.036
• Surface integral equations for electromagnetic testing: the low-frequency and high-contrast case
  
  • Vigneron Audrey
  • Demaldent Édouard
  • Bonnet Marc
  IEEE Transactions on Magnetics, Institute of Electrical and Electronics Engineers, 2014, 50, pp.7002704. This study concerns boundary element methods applied to electromagnetic testing, for a wide range of frequencies and conductivities. The eddy currents approximation cannot handle all configurations, while the common Maxwell formulation suffers from numerical instabilities at low frequency or in presence of highly contrasted media. We draw on studies that overcome these problems for dielectric configurations to treat conductive bodies, and show how to link them to eddy current formulations under suitable assumptions. This is intended as a first step towards a generic formulation that can be modified in each sub-domain according to the corresponding medium. (10.1109/TMAG.2013.2283297)
  DOI : 10.1109/TMAG.2013.2283297
• On the use of perfectly matched layers in the presence of long or backward propagating guided elastic waves
  
  • Bonnet-Ben Dhia Anne-Sophie
  • Chambeyron Colin
  • Legendre Guillaume
  Wave Motion, Elsevier, 2014, 51 (2), pp.266-283. An efficient method to compute the scattering of a guided wave by a localized defect, in an elastic waveguide of infinite extent and bounded cross section, is considered. It relies on the use of perfectly matched layers (PML) to reduce the problem to a bounded portion of the guide, allowing for a classical finite element discretization. The difficulty here comes from the existence of backward propagating modes, which are not correctly handled by the PML. We propose a simple strategy, based on finite-dimensional linear algebra arguments and using the knowledge of the modes, to recover a correct approximation to the solution with a low additional cost compared to the standard PML approach. Numerical experiments are presented in the two-dimensional case involving Rayleigh--Lamb modes. (10.1016/j.wavemoti.2013.08.001)
  DOI : 10.1016/j.wavemoti.2013.08.001
• XLiFE++, an eXtended Library of Finite Elements in C++
  
  • Lunéville Éric
  • Kielbasiewicz Nicolas
  , 2014. XLiFE++ is an FEM-BEM C++ library that can solve 1D / 2D / 3D, scalar / vector, transient / stationnary / harmonic problems. It is autonomous, providing everything required for solving PDE problems : mesh tools, a wide range of finite elements on every mesh cell (nodal at any order, edge at any order and H_2 elements), a wide range of essential conditions, including periodic and quasi-periodic conditions, absorbing conditions (DtN, PML), direct / iterative / eigen solvers.
• Inverse material identification in coupled acoustic-structure interaction using a modified error in constitutive equation functional
  
  • Warner James E.
  • Diaz Manuel I.
  • Aquino Wilkins
  • Bonnet Marc
  Computational Mechanics, Springer Verlag, 2014, 54, pp.645-659. This work focuses on the identification of heterogeneous linear elastic moduli in the context of frequency-domain, coupled acoustic-structure interaction (ASI), using either solid displacement or fluid pressure measurement data. The approach postulates the inverse problem as an optimization problem where the solution is obtained by minimizing a modified error in constitutive equation (MECE) functional. The latter measures the discrepancy in the constitutive equations that connect kinematically admissible strains and dynamically admissible stresses, while incorporating the measurement data as additional quadratic error terms. We demonstrate two strategies for selecting the MECE weighting coefficient to produce regularized solutions to the ill-posed identification problem: 1) the discrepancy principle of Morozov, and 2) an error-balance approach that selects the weight parameter as the minimizer of another functional involving the ECE and the data misfit. Numerical results demonstrate that the proposed methodology can successfully recover elastic parameters in 2D and 3D ASI systems from response measurements taken in either the solid or fluid subdomains. Furthermore, both regularization strategies are shown to produce accurate reconstructions when the measurement data is polluted with noise. The discrepancy principle is shown to produce nearly optimal solutions, while the error-balance approach, although not optimal, remains effective and does not need a priori information on the noise level. (10.1007/s00466-014-1018-0)
  DOI : 10.1007/s00466-014-1018-0
• Propagation in waveguides with varying cross-section and curvature: A new light on the role of supplementary modes in multimodal methods
  
  • Maurel Agnes
  • Mercier Jean-François
  • Félix Simon
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2014, 470 (2166), pp.20130743. We present an efficient multi-modal method to describe the acoustic propagation in waveguides with varying curvature and cross section. A key feature is the use of a flexible geometrical transformation to a virtual space in which the waveguide is straight and has unitary cross section. In this new space, the pressure field has to satisfy a modified wave equation and associated modified boundary conditions. These boundary conditions are in general not satisfied by the Neumann modes, used for the series representation of the field. Following previous work, an improved modal method (MM) is presented, by means of the use of two supplementary modes. Resulting increased convergences are exemplified by comparison with the classical MM. Next, the following question is addressed: when the boundary conditions are verified by the Neumann modes, does the use of supplementary modes improve or degrade the convergence of the computed solution? Surprisingly, although the supplementary modes degrade the behaviour of the solution at the walls, they improve the convergence of the wavefield and of the scattering coefficients. This sheds a new light on the role of the supplementary modes and opens the way for their use in a wide range of scattering problems. (10.1098/rspa.2014.0008)
  DOI : 10.1098/rspa.2014.0008
• Finite element computation of trapped and leaky elastic waves in open stratified waveguides
  
  • Treyssede Fabien
  • Nguyen Khac-Long
  • Bonnet-Ben Dhia Anne-Sophie
  • Hazard Christophe
  Wave Motion, Elsevier, 2014, 51 (7), pp.pp.1093-1107. Elastic guided waves are of interest for inspecting structures due to their ability to propagate over long distances. In numerous applications, the guiding structure is surrounded by a solid matrix that can be considered as unbounded in the transverse directions. The physics of waves in such an open waveguide significantly differs from a closed waveguide, i.e. for a bounded cross-section. Except for trapped modes, part of the energy is radiated in the surrounding medium, yielding attenuated modes along the axis called leaky modes. These leaky modes have often been considered in non destructive testing applications, which require waves of low attenuation in order to maximize the inspection distance. The main difficulty with numerical modeling of open waveguides lies in the unbounded nature of the geometry in the transverse direction. This difficulty is particularly severe due to the unusual behavior of leaky modes: while attenuating along the axis, such modes exponentially grow along the transverse direction. A simple numerical procedure consists in using absorbing layers of artificially growing viscoelasticity, but large layers may be required. The goal of this paper is to explore another approach for the computation of trapped and leaky modes in open waveguides. The approach combines the so-called semi-analytical finite element method and a perfectly matched layer technique. Such an approach has already been successfully applied in scalar acoustics and electromagnetism. It is extended here to open elastic waveguides, which raises specific difficulties. In this paper, two-dimensional stratified waveguides are considered. As it reveals a rich structure, the numerical eigenvalue spectrum is analyzed in a first step. This allows to clarify the spectral objects calculated with the method, including radiation modes, and their dependency on the perfectly matched layer parameters. In a second step, numerical dispersion curves of trapped and leaky modes are compared to analytical results. (10.1016/j.wavemoti.2014.05.003)
  DOI : 10.1016/j.wavemoti.2014.05.003
• Improved multimodal method in varying cross section waveguides
  
  • Maurel Agnes
  • Mercier Jean-François
  • Pagneux Vincent
  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2014, 470, pp.20130448. An improved version of the multimodal admittance method in acoustic waveguides with varying cross sections is presented. This method aims at a better convergence with respect to the number of transverse modes that are taken into account. It is based on an enriched modal expansion of the pressure: the N first modes are the local transverse modes and a supplementary (N+1)th mode, called boundary mode, is a well-chosen transverse function orthogonal to the N first modes. This expansion leads to the classical form of the coupled mode equations where the component of the boundary mode is of evanescent character. Under this form, the multimodal admittance method based on the Riccati equation on the admittance matrix (the Dirichlet-to-Neumann operator) is straightforwardly implemented. With this supplementary mode, in addition to the improvement of the convergence of the pressure field, results show a superconvergence of the scattered field outside of the varying cross sections region. (10.1098/rspa.2013.0448)
  DOI : 10.1098/rspa.2013.0448
• Extraordinary transmission through subwavelength dielectric gratings in the microwave range
  
  • Ahmed Akarid
  • Ourir Abdelwaheb
  • Maurel Agnes
  • Félix Simon
  • Mercier Jean-François
  Optics Letters, Optical Society of America - OSA Publishing, 2014, 39 (13), pp.3752-3755. We address the problem of the transmission through subwavelength dielectric gratings. Following Maurel et al. [Phys. Rev. B 88, 115416 (2013)], the problem is reduced to the transmission by an homogeneous slab, either anisotropic (for transverse magnetic waves, TM) or isotropic (for transverse electric waves, TE), and an explicit expression of the transmission coefficient is derived. The optimum angle realizing perfect impedance matching (Brewster angle) is shown to depend on the contrasts of the dielectric layers with respect to the air. Besides, we show that the Fabry–Perot resonances may be dependent on the incident angle, in addition to the dependence on the frequency. These facts depart from the case of metallic gratings usually considered; they are confirmed experimentally both for TE and TM waves in the microwave regime. (10.1364/OL.39.003752)
  DOI : 10.1364/OL.39.003752
• On the absence of trapped modes in locally perturbed open waveguides
  
  • Hazard Christophe
  IMA Journal of Applied Mathematics, Oxford University Press (OUP), 2014, pp.14. This paper presents a new approach for proving that the presence of a bounded defect in a uniform open waveguide cannot produce trapped modes, contrary to the case of a closed waveguide. The originality of the proof lies in the fact that it relies on a modal decomposition. It shows in particular that the absence of trapped modes results from a strong connection between the various modal components of the field. The case of the three-dimensional scalar wave equation is considered. (10.1093/imamat/hxu046)
  DOI : 10.1093/imamat/hxu046
• Finite Element Heterogeneous Multiscale Method for the Wave Equation: Long-Time Effects
  
  • Abdulle Assyr
  • Grote Marcus J.
  • Stohrer Christian
  Multiscale Modeling and Simulation: A SIAM Interdisciplinary Journal, Society for Industrial and Applied Mathematics, 2014, 12 (3), pp.1230–1257. A new finite element heterogeneous multiscale method (FE-HMM) is proposed for the numerical solution of the wave equation over long times in a rapidly varying medium. Our new FE-HMM-L method captures not only the short-time behavior of the wave field, well described by classical homogenization theory, but also more subtle long-time dispersive effects, both at a computational cost independent of the microscale. Optimal error estimates in the energy norm and the $L^2$-norm are proved over finite time intervals, which imply convergence to the solution from classical homogenization theory when both the macro- and the microscale are refined simultaneously. Numerical experiments illustrate the usefulness of the FE-HMM-L method and corroborate the theory. (10.1137/13094195X)
  DOI : 10.1137/13094195X
• Mathematical modeling of a discontinuous Myers condition
  
  • Lunéville Éric
  • Mercier Jean-François
  ESAIM: Mathematical Modelling and Numerical Analysis, Société de Mathématiques Appliquées et Industrielles (SMAI) / EDP, 2014, 48 (5), pp.1529-1555. (10.1051/m2an/2014008)
  DOI : 10.1051/m2an/2014008
• Generalized method for retrieving effective parameters of anisotropic metamaterials
  
  • Mercier Jean-François
  • Castanié Aurore
  • Félix Simon
  • Maurel Agnes
  Optics Express, Optical Society of America - OSA Publishing, 2014, 22 (24), pp.29977-29953. Electromagnetic or acoustic metamaterials can be described in terms of equivalent effective, in general anisotropic, media and several techniques exist to determine the effective permeability and permittivity (or effective mass density and bulk modulus in the context of acoustics). Among these techniques, retrieval methods use the measured reflection and transmission coefficients (or scattering coefficients) for waves incident on a metamaterial slab containing few unit cells. Until now, anisotropic effective slabs have been considered in the literature but they are limited to the case where one of the axes of anisotropy is aligned with the slab interface. We propose an extension to arbitrary orientations of the principal axes of anisotropy and oblique incidence. The retrieval method is illustrated in the electromagnetic case for layered media, and in the acoustic case for array of tilted elliptical particles. (10.1364/OE.22.029937)
  DOI : 10.1364/OE.22.029937