1,721,016 research outputs found
A Trefftz Discontinuous Galerkin method with Absorbing Boundary Conditions for the numerical simulation of the Helmholtz problem
Full text linkLAUREA MAGISTRALELa simulazione numerica dei problemi di Helmholtz nel contesto di onde acustiche tempo-periodiche in domini illimitati richiede un approccio in domini numerici limitati dotati di mesh.
Perciò, presentiamo un metodo Trefftz, che è un particolare metodo di Galerkin Discontinuo, applicato al problema complementato da Condizioni al Bordo di Assorbimento.
I metodi Trefftz sono stati ampiamenti studiati nel contesto dell'equazione di Helmholtz e sono risultati essere adatti. In particolare, l'uso di onde piane è efficace nell'approssimare la soluzione. Tuttavia, il loro uso è ancora limitato da problemi di condizionamento che sorgono ogni volta in cui lo spazio approssimante diventa troppo grande.
La combinazione del metodo Trefftz per le onde e la relativa Formulazione Variazionale Ultra Debole con Condizioni al Bordo di Assorbimento rappresenta una novità che puntiamo a studiare e investigare nel presente lavoro. Porta a una nuova formulazione del problema i cui risultati numerici sono utilizzati per valutare l'efficienza dell'approccio.The numerical simulation of Helmholtz problems in the time-harmonic acoustic framework set in unbounded domains requires a mesh based approach in bounded numerical domains.
Therefore, we present a Trefftz method, which is a particular Discontinuous Galerkin method, applied to the problem complemented by Absorbing Boundary Conditions.
Trefftz methods have been largely studied in the framework of Helmholtz equation and have turned out to be suitable. In particular, the use of propagative plane-waves is effective in approximating the solution. Nevertheless, their use is still limited by conditioning issues that arise whenever the approximate space becomes too large.
The combination of the wave-based Trefftz method and the relative Ultra Weak Variational Formulation with Absorbing Boundary Conditions represents a novelty that we aim at studying and investigating in the present work. It leads to a new formulation of the problem whose numerical results are exploited to evaluate the efficiency of the approach
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Optimizing the PML in the discrete context
No full textPerfectly Matched Layers (PML) are widely used for the simulation of unbounded wave-like problems. However their performance depends critically on both an absorption coefficient and the spatial discretization. Tuning the coefficient requires a costly (and case-dependent) optimization procedures... so that in most applications it is set empirically.
In this talk we discuss the influence of the absorption coefficient on the PML performance for time-domain simulations, in both continuous and discrete contexts. We present efficient coefficients that allow to avoid any tuning in discrete contexts, and compare those with other frequent choices by means of benchmarks solved with both finite differences and finite elements (continuous and discontinuous). A realistic 3D discontinuous Galerkin benchmark for Maxwell's equations will highlight the advantages of our approach
Absorbing Layers for Wave-Like Time-Dependent Problems - Design, Discretization and Optimization
Full text linkThe numerical simulation of wave-like phenomena occurring in large or infinite domains is a great challenge for a wide range of technological and scientifical problems. A classical way consists in considering only a limited computational domain with an artificial boundary that requires a specific treatment. In this thesis, \textit{absorbing layers} are developed and studied for time-dependent problems in order to deal with such artificial boundary.
A large part of this thesis is dedicated to the \textit{perfectly matched layers} (PMLs), which exhibit appealing properties. They are first studied in a fundamental case with non-dispersive linear scalar waves. A procedure for building PMLs is proposed for convex domains with regular boundary. It permits a great flexibility when choosing the shape of the computational domain. After, the issue of choosing PML parameters is addressed with the aim of optimizing the PML effectiveness in discrete contexts. The role of each parameter, including the so-called \textit{absorption function}, is highlighted by means of analytical and numerical results. A systematic comparison of different kinds of absorption functions is performed for several classical numerical schemes (based on finite differences, finite volumes or finite elements). Then, while the PMLs do not a priori account for incoming signals generated outside the computational domain, different problem formulations that account for such forcing are detailed and compared. The interest of the whole approach is finally illustrated with two- and three-dimensional numerical examples in electromagnetism and acoustics, using a discontinuous finite element scheme.
In regional oceanic models, modeling open-sea boundaries brings new difficulties. Indeed, additional linear/nonlinear dynamics are involved and the external forcing is generally poorly known. In this context, different absorbing layers and the widely used Flather boundary condition are compared by means of classical benchmarks. The choice of the absorption function and the way of prescribing the external forcing are discussed in specific marine cases
Accelerated iterative DG finite element solvers for large-scale time-harmonic acoustic problems
No full textInternational audienceFinite element methods are widely used to solve time-harmonic wave propagation problems, but solving large cases can be extremely difficult even with the computational power of parallel computers. In this work, the linear system resulting from the finite element discretization is solved with iterative solution methods, which are efficient in parallel but can require a large number of iterations. In standard discontinuous Galerkin (DG) methods, the numerical solution is discontinuous at the interfaces between the elements. In hybridizable DG methods, additional unknowns are introduced at the interfaces between the finite elements, and the physical unknowns are eliminated from the global system, resulting in a hybridized system. We have recently proposed a new strategy, called CHDG, where the additional unknowns correspond to transmission variables, whereas in the standard approach they are numerical fluxes. This strategy improves the properties of the hybridized system for faster iterative solution procedures. In this talk, we present and study a 3D CHDG implementation with nodal finite element basis functions. The resulting scheme has properties amenable to efficient parallel computing. Numerical results are presented to validate the method, and preliminary 3D computational results are proposed
Étude de modèles de frontière ouverte pour des problèmes de propagation d'ondes
No full textCe travail est consacré à l'étude expérimentale de conditions aux limites et de couches absorbantes qui modélisent des frontières ouvertes dans des problèmes de propagation d'ondes. Nous considérons ici la condition de Flather et les versions de la condition de radiation proposées par Orlanski et Raymond & Kuo. Du côté des couches absorbantes, nous étudions la zone FRS de Martinsen & Engedahl, la "Pretty good sponge" de Lavelle et une version complète de PML.
L'étude commence par une analyse caractéristique de l'équation d'onde et une présentation détaillée des différents modèles de frontière ouverte. Après, deux problèmes de base sont proposés pour permettre de quantifier la qualité de ces modèles. L'influence des différents paramètres des couches absorbantes est étudiée afin de déterminer les valeurs optimales. Ensuite, le comportement des différents champs est analysé au sein des couches optimales. Enfin, les différents modèles de frontière ouverte sont comparés sur base des deux problèmes. Les couches absorbantes le sont également sur un cas d'ajustement géostrophique, déjà traité par Lavelle dans un article récent
Contributions to Efficient Finite Element Solvers for Time-Harmonic Wave Propagation Problems
No full textThe numerical simulation of wave propagation phenomena is of paramount importance in many scientific and engineering disciplines. Many time-harmonic problems can be solved with finite elements in theory, but the computational cost is a strong constraint that limits the size of the problems and the accuracy of the solutions in practice. Ideally, solution techniques should provide the best accuracy at minimal computational cost for real-world problems. They should take advantage of the power of modern parallel computers, and they should be as easy as possible to use for the end user. In this HDR thesis, contributions are presented on three topics: the improvement of domain truncation techniques (i.e. high-order absorbing boundary conditions and perfectly matched layers), the acceleration of substructuring and preconditioning techniques based on domain decomposition methods (i.e. non-overlapping domain decomposition methods with interface conditions based on domain truncation techniques), and the design of a new hybridization approach for efficient discontinuous finite element solvers
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