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See detailA warmer world
van Dam, Tonie UL; Weigelt, Matthias UL; Jäggi, Adrian

in Pan European Networks: Science & Technology (2015), (14), 58-59

Detailed reference viewed: 236 (21 UL)
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See detailWeakening the tight coupling between geometry and simulation in isogeometric analysis
Bordas, Stéphane UL; Tomar, Satyendra UL; Atroshchenko, Elena et al

Scientific Conference (2016, June 05)

In the standard paradigm of isogeometric analysis, the geometry and the simulation spaces are tightly integrated, i.e. the same non-uniform rational B-splines (NURBS) space, which is used for the geometry ... [more ▼]

In the standard paradigm of isogeometric analysis, the geometry and the simulation spaces are tightly integrated, i.e. the same non-uniform rational B-splines (NURBS) space, which is used for the geometry representation of the domain, is employed for the numerical solution of the problem over the domain. However, there are situations where this tight integration is a bane rather than a boon. Such situations arise where, e.g., (1) the geometry of the domain is simple enough to be represented by low order NURBS, whereas the unknown (exact) solution of the problem is sufficiently regular, and thus, the numerical solution can be obtained with improved accuracy by using NURBS of order higher than that required for the geometry, (2) the constraint of using the same space for the geometry and the numerical solution is particularly undesirable, such as in the shape and topology optimization, and (3) the solution of the problem has low regularity but for the curved boundary of the domain one can employ higher order NURBS. Therefore, we propose to weaken this constraint. An extensive study of patch tests on various combinations of polynomial degree, geometry type, and various cases of varying degrees and control variables between the geometry and the numerical solution will be discussed. It will be shown, with concrete reasoning, that why patch test fails in certain cases, and that those cases should be avoided in practice. Thereafter, selective numerical examples will be presented to address some of the above-mentioned situations, and it will be shown that weakening the tight coupling between geometry and simulation offers more flexibility in choosing the numerical solution spaces, and thus, improved accuracy of the numerical solution. Powered by [less ▲]

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See detailWeakening the tight coupling between geometry and simulation in isogeometric analysis
Tomar, Satyendra UL; Atroshchenko, Elena; Xu, Gang et al

Presentation (2016, June 07)

In the standard paradigm of isogeometric analysis, the geometry and the simulation spaces are tightly integrated, i.e. the same non-uniform rational B-splines (NURBS) space, which is used for the geometry ... [more ▼]

In the standard paradigm of isogeometric analysis, the geometry and the simulation spaces are tightly integrated, i.e. the same non-uniform rational B-splines (NURBS) space, which is used for the geometry representation of the domain, is employed for the numerical solution of the problem over the domain. However, there are situations where this tight integration is a bane rather than a boon. Such situations arise where, e.g., (1) the geometry of the domain is simple enough to be represented by low order NURBS, whereas the unknown (exact) solution of the problem is sufficiently regular, and thus, the numerical solution can be obtained with improved accuracy by using NURBS of order higher than that required for the geometry, (2) the constraint of using the same space for the geometry and the numerical solution is particularly undesirable, such as in the shape and topology optimization, and (3) the solution of the problem has low regularity but for the curved boundary of the domain one can employ higher order NURBS. Therefore, we propose to weaken this constraint. An extensive study of patch tests on various combinations of polynomial degree, geometry type, and various cases of varying degrees and control variables between the geometry and the numerical solution will be discussed. It will be shown, with concrete reasoning, that why patch test fails in certain cases, and that those cases should be avoided in practice. Thereafter, selective numerical examples will be presented to address some of the above-mentioned situations, and it will be shown that weakening the tight coupling between geometry and simulation offers more flexibility in choosing the numerical solution spaces, and thus, improved accuracy of the numerical solution. [less ▲]

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See detailWeakening the tight coupling between geometry and simulation in isogeometric analysis: from sub- and super- geometric analysis to Geometry Independent Field approximaTion (GIFT)
Atroshchenko, Elena; Tomar, Satyendra UL; Xu, Gang et al

in International Journal for Numerical Methods in Engineering (2018)

This paper presents an approach to generalize the concept of isogeometric analysis (IGA) by allowing different spaces for parameterization of the computational domain and for approximation of the solution ... [more ▼]

This paper presents an approach to generalize the concept of isogeometric analysis (IGA) by allowing different spaces for parameterization of the computational domain and for approximation of the solution field. The method inherits the main advantage of isogeometric analysis, i.e. preserves the original, exact CAD geometry (for example, given by NURBS), but allows pairing it with an approximation space which is more suitable/flexible for analysis, for example, T-splines, LR-splines, (truncated) hierarchical B-splines, and PHT-splines. This generalization offers the advantage of adaptive local refinement without the need to re-parameterize the domain, and therefore without weakening the link with the CAD model. We demonstrate the use of the method with different choices of the geometry and field splines, and show that, despite the failure of the standard patch test, the optimum convergence rate is achieved for non-nested spaces. [less ▲]

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See detailWell Conditioned and Optimally Convergent Extended Finite Elements and Vector Level Sets for Three-Dimensional Crack Propagation
Agathos, Konstantinos UL; Ventura, Giulio; Chatzi, Eleni et al

Scientific Conference (2016, June)

A three-dimensional (3D) version of the vector level set method [1] is combined to a well conditioned and optimally convergent XFEM variant in order to deal with non-planar three dimensional crack ... [more ▼]

A three-dimensional (3D) version of the vector level set method [1] is combined to a well conditioned and optimally convergent XFEM variant in order to deal with non-planar three dimensional crack propagation problems. The proposed computational fracture method achieves optimal convergence rates by using tip enriched elements in a fixed volume around the crack front (geometrical enrichment) while keeping conditioning of the resulting system matrices in acceptable levels. Conditioning is controlled by using a three dimensional extension of the degree of freedom gathering technique [2]. Moreover, blending errors are minimized and conditioning is further improved by employing weight function blending and enrichment function shifting [3,4]. As far as crack representation is concerned, crack surfaces are represented by linear quadrilateral elements and the corresponding crack fronts by ordered series of linear segments. Level set values are obtained by projecting points at the crack surface and front respectively. Different criteria are employed in order to assess the quality of the crack representation. References [1] Ventura G., Budyn E. and Belytschko T. Vector level sets for description of propagating cracks in finite elements. Int. J. Numer. Meth. Engng. 58:1571-1592 (2003). [2] Laborde P., Pommier J., Renard Y. and Salaün M. High-order extended finite element method for cracked domains. Int. J. Numer. Meth. Engng. 64:354-381 (2005). [3] Fries T.P. A corrected XFEM approximation without problems in blending elements. Int. J. Numer. Meth. Engng. 75:503-532 (2008). [4] Ventura G., Gracie R. and Belytschko T. Fast integration and weight function blending in the extended finite element method. Int. J. Numer. Meth. Engng. 77:1-29 (2009). [less ▲]

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See detailA well-conditioned and optimally convergent XFEM for 3D linear elastic fracture
Agathos, Konstantinos; Chatzi, Eleni; Bordas, Stéphane UL et al

in International Journal for Numerical Methods in Engineering (n.d.)

A variation of the extended finite element method for 3D fracture mechanics is proposed. It utilizes global enrichment and point-wise as well as integral matching of displacements of the standard and ... [more ▼]

A variation of the extended finite element method for 3D fracture mechanics is proposed. It utilizes global enrichment and point-wise as well as integral matching of displacements of the standard and enriched elements in order to achieve higher accuracy, optimal convergence rates and improved conditioning for two and three dimensional crack problems. A bespoke benchmark problem is introduced to determine the method's accuracy in the general 3D case where it is demonstrated that the proposed approach improves the accuracy and reduces the number of iterations required for the iterative solution of the resulting system of equations by 40% for moderately refined meshes and topological enrichment. Moreover, when a fixed enrichment volume is used, the number of iterations required grows at a rate which is reduced by a factor of 2 compared to standard XFEM, diminishing the number of iterations by almost one order of magnitude. [less ▲]

Detailed reference viewed: 279 (10 UL)
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See detailWhat do we accept after an announcement?
de Boer, Mathijs UL; Herzig, Andreas; de Lima, Tiago et al

Scientific Conference (2008)

Detailed reference viewed: 27 (2 UL)
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See detailWhat makes Data Science different? A discussion involving Statistics2.0 and Computational Sciences
Ley, Christophe; Bordas, Stéphane UL

E-print/Working paper (2017)

Data Science is today one of the main buzzwords be it in business, industrial or academic settings. Machine learning, experimental design, data-driven modelling are all, undoubtedly, rising disciplines if ... [more ▼]

Data Science is today one of the main buzzwords be it in business, industrial or academic settings. Machine learning, experimental design, data-driven modelling are all, undoubtedly, rising disciplines if one goes by the soaring number of research papers and patents appearing each year. The prospect of becoming a ``Data Scientist'' appeals to many. A discussion panel organised as part of the European Data Science Conference (European Association for Data Science (EuADS)) asked the question: ``What makes Data Science different?'' In this paper we give our own, personal and multi-facetted view on this question, from an engineering and a statistics perspective. In particular, we compare Data Science to Statistics and discuss the connection between Data Science and Computational Science. [less ▲]

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See detailWhite Manipulation in Judgment Aggregation
Grossi, Davide; Pigozzi, Gabriella UL; Slavkovik, Marija UL

Scientific Conference (2009)

Distributive systems consisting of autonomous and intelligent components need to be able to reason and make decisions based on the information these components share. Judgment aggregation investigates how ... [more ▼]

Distributive systems consisting of autonomous and intelligent components need to be able to reason and make decisions based on the information these components share. Judgment aggregation investigates how individual judgments on logically connected propositions can be aggregated into a collective judgment on the same propositions. It is the case that seemingly reasonable aggregation procedures may force the group to hold an inconsistent judgment set. What happens when the agents realize that the group outcome will be inconsistent? We claim that, in order to avoid an untenable collective outcome, individuals may prefer to declare a non-truthful, less preferred judgment set. Thus, the prospect of an individual trying to manipulate the social outcome by submitting an insincere judgment set is turned from being an undesirable to a “virtuous” (or white) manipulation. We define white manipulation and present the initial study of it as a coordinated action of the whole group. [less ▲]

Detailed reference viewed: 58 (1 UL)
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Peer Reviewed
See detailWorkplace Relocation and Mobility Changes in a Transnational Metropolitan Area: The Case of the University of Luxembourg
Sprumont, François UL; Viti, Francesco UL; Caruso, Geoffrey UL et al

in Transportation Research Procedia (2014, December), 4

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was ... [more ▼]

The aim of this paper is to study the utility variation related to the commuting mobility of University staff members due to their future workplace relocation. During the year 2012, a travel survey was completed by a total of 397 staff members, representing 36.4% of the university employees, who filled in a questionnaire which revealed complex decision making patterns due to the special traveling scenario involving four countries at once. A Multinomial Logit model has been used to anticipate the impact of university relocation from the capital city to a developing area in the south of the country which will happen between 2015 and 2018 and that will affect most of the employees. The effects of several Travel Demand Management measures are discussed based on the analysis of alternative scenarios [less ▲]

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See detailXDEM for Tuning Lumped Models of Thermochemical Processes Involving Materials in the Powder State
Copertaro, Edoardo UL; Chiariotti, Paolo; Estupinan Donoso, Alvaro Antonio UL et al

in Engineering Journal (2016), 20(5), 187-201

Processes involving materials in gaseous and powder states cannot be modelled without coupling interactions between the two states. XDEM (Extended Discrete Element Method) is a valid tool for tackling ... [more ▼]

Processes involving materials in gaseous and powder states cannot be modelled without coupling interactions between the two states. XDEM (Extended Discrete Element Method) is a valid tool for tackling this issue, since it allows a coupled CFD- DEM simulation to be run. Such strength, however, mainly finds in long computational times its main drawback. This aspect is indeed critical in several applications, since a long computational time is in contrast with the increasing demand for predictive tools that can provide fast and accurate results in order to be used in new monitoring and control strategies. This paper focuses on the use of the XDEM framework as a tool for fine tuning a lumped representation of the non-isothermal decarbonation of a CaCO3 sample in powder state. The tuning of the lumped model is performed exploiting the multi-objective optimization capability of genetic algorithms. Results demonstrate that such approach makes it possible to estimate fast and accurate models to be used, for instance, in the fields of virtual sensing and predictive control. [less ▲]

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See detailXDEM: from HPC to the Cloud
Besseron, Xavier UL

Scientific Conference (2017, January)

Detailed reference viewed: 23 (2 UL)
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Peer Reviewed
See detailXFEM coupling of granular flows interacting with surrounding fluids
Pasenow, F.; Zilian, Andreas UL; Dinkler, D.

in ECCOMAS 2012 - European Congress on Computational Methods in Applied Sciences and Engineering, e-Book Full Papers (2012)

In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled ... [more ▼]

In this paper, ideas for the simulation of sliding dry granular materials interacting with surrounding fluids are presented and first results are presented. The compressible granular material is modeled as a medium which can show solid-like and fluid-like characteristics. Therefore a weighted decomposition of stress tensors of a solid-like and a fluid-like phase is applied. The surrounding incompressible fluids are described with a Newtonian constitutive model. Interface dynamics are handled with the level-set method. The model equations are discretized with the space-time finite element method. Discontinuous solution characteristics across interfaces are captured numerically by the extended finite element method (XFEM). For all discontinuities the space of ansatz functions is enriched with Heaviside functions. [less ▲]

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See detailXLME interpolants, a seamless bridge between XFEM and enriched meshless methods
Amiri, F.; Anitescu, C.; Arroyo, M. et al

in Computational Mechanics (2013)

In this paper, we develop a method based on local maximum entropy shape functions together with enrichment functions used in partition of unity methods to discretize problems in linear elastic fracture ... [more ▼]

In this paper, we develop a method based on local maximum entropy shape functions together with enrichment functions used in partition of unity methods to discretize problems in linear elastic fracture mechanics. We obtain improved accuracy relative to the standard extended finite element method at a comparable computational cost. In addition, we keep the advantages of the LME shape functions, such as smoothness and non-negativity. We show numerically that optimal convergence (same as in FEM) for energy norm and stress intensity factors can be obtained through the use of geometric (fixed area) enrichment with no special treatment of the nodes near the crack such as blending or shifting. © 2013 Springer-Verlag Berlin Heidelberg. [less ▲]

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See detailZur Rotationskapazität von Verbundanschlüssen bei der Bemessung nach dem Fließgelenkverfahren
Hahn, Christoph UL

Doctoral thesis (2009)

The present work deals with the load bearing and rotational behaviour of composite connections in steel and concrete constructions. For typical buildings, composite connections are assumed to be rigid or ... [more ▼]

The present work deals with the load bearing and rotational behaviour of composite connections in steel and concrete constructions. For typical buildings, composite connections are assumed to be rigid or pinned. This assumption can lead to uneconomic structures. A design concept for the estimation of the available rotation capacity, the initial stiffness and the moment bearing capacity of semi-rigid connections is presented. On the basis of the plastic hinge theory diagrams were developed to prove the load carrying capacity and the serviceability of composite beams with semi-rigid connections. The presented design model is based on the evaluation of 9 self conducted tests and on the evaluation of 74 tests on composite connections from all over Europe. The availability of the model was proved by intensive nonlinear finite element simulations. [less ▲]

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See detailZwei IT-Sicherheitsmethoden im Vergleich: Mehari versus BSI IT-Grundschutz
Dagorn, Nathalie; Schiltz, Jang UL

E-print/Working paper (2008)

Detailed reference viewed: 76 (3 UL)