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See detailLinear smoothed polygonal and polyhedral finite elements
Francis, Amrita; Ortiz-Bernardin, Alejandro; Bordas, Stéphane UL et al

E-print/Working paper (n.d.)

It was observed in [1, 2] that the strain smoothing technique over higher order elements and arbitrary polytopes yields less accurate solutions than other techniques such as the conventional polygonal ... [more ▼]

It was observed in [1, 2] that the strain smoothing technique over higher order elements and arbitrary polytopes yields less accurate solutions than other techniques such as the conventional polygonal finite element method. In this work, we propose a linear strain smoothing scheme that improves the accuracy of linear and quadratic approximations over convex polytopes. The main idea is to subdivide the polytope into simplicial subcells and use a linear smoothing function in each subcell to compute the strain. This new strain is then used in the computation of the stiffness matrix. The convergence properties and accuracy of the proposed scheme are discussed by solving few benchmark problems. Numerical results show that the proposed linear strain smoothing scheme makes the approximation based on polytopes to deliver improved accuracy and pass the patch test to machine precision. [less ▲]

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See detailA linear smoothed quadratic finite element for the analysis of laminated composite Reissner–Mindlin plates
Wan, Detao; Hu, Dean; Natarajan, Sundararajan et al

in Composite Structures (2017), 180

It is well known that the high-order elements have significantly improved the accuracy of solutions in the traditional finite element analysis, but the performance of high-order elements is restricted by ... [more ▼]

It is well known that the high-order elements have significantly improved the accuracy of solutions in the traditional finite element analysis, but the performance of high-order elements is restricted by the shear-locking and distorted meshes for the plate problems. In this paper, a linear smoothed eight-node Reissner-Mindlin plate element (Q8 plate element) based on the first order shear deformation theory is developed for the static and free vibration analysis of laminated composite plates, the computation of the interior derivatives of shape function and isoparametric mapping can be removed. The strain matrices are modified with a linear smoothing technique by using the divergence theorem between the nodal shape functions and their derivatives in Taylor’s expansion. Moreover, the first order Taylor’s expansion is also employed for the construction of stiffness matrix to satisfy the linear strain distribution. Several numerical examples indicate that the novel Q8 plate element has good performance to alleviate the shear-locking phenomenon and improve the quality of the solutions with distorted meshes. [less ▲]

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See detailLinear smoothing over arbitrary polytopes
Francis, Amrita; Natarajan, Sundararajan; Ortiz-Bernardin, Alejandro et al

Scientific Conference (n.d.)

The conventional constant strain smoothing technique yields less accurate solutions that other techniques such as the conventional polygonal finite element method [1, 2]. In this work, we propose a linear ... [more ▼]

The conventional constant strain smoothing technique yields less accurate solutions that other techniques such as the conventional polygonal finite element method [1, 2]. In this work, we propose a linear strain smoothing scheme that improves the accuracy of linear and quadratic approximations over convex poly- topes. The method relies on sub-division of the polytope into simplical subcells; however instead of using a constant smoothing function, we employ a linear smoothing function over each subcell. This gives a new definition for the strain to compute the stiffness matrix. The convergence properties and accuracy of the proposed scheme are discussed by solving few benchmark problems. Numerical results show that the proposed linear strain smoothing scheme makes the approximation based on polytopes able to deliver the optimal convergence rate as in traditional quadrilateral and hexahedral finite elements. The accuracy is also improved, and all the methods tested pass the patch test to machine precision. [less ▲]

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See detailLinear smoothing over arbitrary polytopes for compressible and nearly incompressible linear elasticity
Natarajan, Sundararajan; Tomar, Satyendra UL; Bordas, Stéphane UL et al

Scientific Conference (2016, June)

We present a displacement based approach over arbitrary polytopes for compressible and nearly incompressible linear elastic solids. In this approach, a volume-averaged nodal projection operator is ... [more ▼]

We present a displacement based approach over arbitrary polytopes for compressible and nearly incompressible linear elastic solids. In this approach, a volume-averaged nodal projection operator is constructed to project the dilatational strain into an approximation space of equal or lower-order than the approximation space for the displacement field, resulting in a locking-free method. The formulation uses the usual Wachspress interpolants over arbitrary polytopes and the stability of the method is ensured by the addition of bubble like functions. The smoothed strains are evaluated based on the linear smoothing procedure. This further softens the bilinear form allowing the procedure to search for a solution satisfying the divergence- free condition. The divergence-free condition of the proposed approach is verified through systematic numerical study. The formulation delivers optimal convergence rates in the energy and L2-norms. Inf-sup tests are presented to demonstrated the stability of the formulation. [less ▲]

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See detailLink Optimization in Future Generation Satellite Systems
Mengali, Alberto UL

Doctoral thesis (2018)

In recent years, communication networks have seen a huge growth in the amount of requested throughput, pushed from the combination of two main drivers: the introduction of new services and the improvement ... [more ▼]

In recent years, communication networks have seen a huge growth in the amount of requested throughput, pushed from the combination of two main drivers: the introduction of new services and the improvement of existing ones, requiring increased amount of traffic (e.g. higher quality of video content). These effects mandate the constant evolution of current systems in order to cope with the growing user demand and should be tackled from multiple angles. On the one hand, better utilization of available resources might help in the short term to keep up with the market and has always been an important priority for operators of terrestrial and satellite networks alike. On the other hand, acquisition and exploitation of currently unused resources might fuel the growth for a significantly longer period of time, ensuring longevity and thus enabling future-proofing of current systems. Both these topics are addressed in this thesis with specific applications relevant to satellite communication networks. In the first part, this thesis focuses on maximization of the user capacity by better exploiting the available radio resources. Motivated by the substantial capacity gains enabled by a higher bandwidth allocation, we investigate the optimization of satellite systems employing full-frequency reuse on the user downlink. Unlike most of the literature on the subject, usually resorting to precoding techniques to mitigate the interference, we propose a combination of predistortion and precoding to jointly counteract on-board non-linear distortions and multi-user interference. First, a flexible framework for the optimization of transmit processing schemes in communication chains is presented. This framework expands on the application of the well known gradient descent technique by applying it to the maximization of the received Signal to Noise plus Interference ratio in complex communication systems. To do so, it identifies a suitable mathematical representation of various key blocks of the system and exploits the chain rule of the derivative to compute the overall gradient as a cascade of the single components. Afterwards, this framework is validated by optimizating the coefficients of the proposed predistortion architecture for the satellite system in analysis. The obtained results highlight the flexibility of the developed optimization framework and the benefits of the suggested predistortion strategy compared to existing state of the art solutions. In the second part of the thesis, the focus is shifted towards investigating the exploitation of novel resources by looking at the use of optical frequencies for ground-to-space feeder links. The topic is introduced by a survey of existing benefits and limitations of free space optical communications. Subsequently, the implications of employing optical frequencies in long distance ground-to-space feeder links with transparent satellites are addressed. Furthermore, a powerful and flexible simulation tool was developed and exploited during the course of this thesis to model and assess the Physical (PHY) layer performance of hybrid optical/Radio Frequencies (RF) satellite networks. This tool is presented together with the scenarios and results obtained as part of the project ONSET (Optical Feeder Links Study for Satellite Networks - ESA Contract No. 40000113462/15/NL/NDe). Finally, the thesis investigates a scenario that combines the transmit processing techniques analyzed in the first part and the context of optical feeder links evaluated in the second part. A hybrid optical/RF system is considered with an electrical predistorter in place to counteract the impairments induced by the combined effects of electrical and optical non-linearities encountered along the end-to-end chain. The developed mathematical framework is exploited to jointly optimize the predistortion coefficients and the working point for the electro-optical modulator. The performance results obtained after the optimization procedure demonstrate the efficacy of the proposed approach for hybrid optical/RF systems with analog modulations. [less ▲]

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See detailA local dynamic route and green time swapping control algorithm maximizing total network capacity
Viti, Francesco UL; Rinaldi, Marco UL

in Proceedings of the 25th Mediterranean Conference on Control and Automation, MED 2017 (2017)

This paper deals with the traffic signal control problem. More specifically it investigates the impact at a network level of simple dynamic local traffic control policies. A dynamic route swapping rule is ... [more ▼]

This paper deals with the traffic signal control problem. More specifically it investigates the impact at a network level of simple dynamic local traffic control policies. A dynamic route swapping rule is adopted to model the behavioral response of the travellers to signal changes, while a dynamic signal control swapping rule based on an equi-pressure policy is used to implicitly consider the flow response within the control updating process. Results on a simple network show that the flow responsive control policy outperforms pre-timed control, as well as a more conventional local control policy based on signal equi-saturation. Numerical results show also that the swapping rule based on equi-pressure is less susceptible to local optima, to systematically improve total network throughput, and to increase its effectiveness with when demand increases. © 2017 IEEE. [less ▲]

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See detailLocal Nusselt number enhancements in liquid-liquid Taylor flows
Mac Giolla Eain, Marc UL

in International Journal of Heat and Mass Transfer (2015), 80

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See detailLocal vs. Global Search Strategies in Evolutionary GRID-based Conformational Sampling & Docking
Horvath, Dragos; Brillet, Lorraine; Roy, Sylvaine et al

in 2009 IEEE CONGRESS ON EVOLUTIONARY COMPUTATION, VOLS 1-5 (2009)

Conformational sampling, the computational prediction of the experimental geometries of small proteins (folding) or of protein-ligand complexes (docking), is often cited as one of the most challenging ... [more ▼]

Conformational sampling, the computational prediction of the experimental geometries of small proteins (folding) or of protein-ligand complexes (docking), is often cited as one of the most challenging multimodal optimization problems. Due to the extreme ruggedness of the energy landscape as a function of geometry, sampling heuristics must rely on an appropriate trade-off between global and local searching efforts. A previously reported "planetary strategy", a generalization of the classical island model used to deploy a hybrid genetic algorithm on computer grids, has shown a good ability to quickly discover low-energy geometries of small proteins and sugars, and sometimes even pinpoint their native structures - although not reproducibly. The procedure focused on broad exploration and used a tabu strategy to avoid revisiting the neighborhood of known solutions, at the risk of "burying" important minima in overhastily set tabu areas. The strategy reported here, termed "divide-and-conquer planetary model" couples this global search procedure to a local search tool. Grid nodes are now shared between global and local exploration tasks. The phase space is cut into "cells" corresponding to a specified sampling width for each of the N degrees of freedom. Global search locates cells containing low-energy geometries. Local searches pinpoint even deeper minima within a cell. Sampling width controls the important trade-off between the number of cells and the local search effort needed to reproducibly sample each cell. The probability to submit a cell to local search depends on the energy of the most stable geometry found within. Local searches are allotted limited resources and are not expected to converge. However, as long as they manage to discover some deeper local minima, the explored cell remains eligible for further local search, now relying on the improved energy level to enhance chances to be picked again. This competition prevents the system to waste too much effort in fruitless local searches. Eventually, after a limited number of local searches, a cell will be "closed" and used - first as "seed", later as tabu zone - to bias future global searches. Technical details and some folding and docking results will be discussed. [less ▲]

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See detailLocal/global model order reduction strategy for the simulation of quasi-brittle fracture
Kerfriden, P.; Passieux, J. C.; Bordas, Stéphane UL

in International Journal for Numerical Methods in Engineering (2012), 89(2), 154-179

This paper proposes a novel technique to reduce the computational burden associated with the simulation of localized failure. The proposed methodology affords the simulation of damage initiation and ... [more ▼]

This paper proposes a novel technique to reduce the computational burden associated with the simulation of localized failure. The proposed methodology affords the simulation of damage initiation and propagation while concentrating the computational effort where it is most needed, that is, in the localization zones. To do so, a local/global technique is devised where the global (slave) problem (far from the zones undergoing severe damage and cracking) is solved for in a reduced space computed by the classical proper orthogonal decomposition while the local (master) degrees of freedom (associated with the part of the structure where most of the damage is taking place) are fully resolved. Both domains are coupled through a local/global technique. This method circumvents the difficulties associated with model order reduction for the simulation of highly nonlinear mechanical failure and offers an alternative or complementary approach to the development of multiscale fracture simulators. © 2011 John Wiley & Sons, Ltd. [less ▲]

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See detailLocalized meshless point collocation method for time-dependent magnetohydrodynamics flow through pipes under a variety of wall conductivity conditions
Loukopoulos, Vasilis; Bourantas, Georgios UL; Skouras, Eugene

in Computational Mechanics (2011), 47(2), 137-159

In this article a numerical solution of the time dependent, coupled system equations of magnetohydrody- namics (MHD) flow is obtained, using the strong-form local meshless point collocation (LMPC) method ... [more ▼]

In this article a numerical solution of the time dependent, coupled system equations of magnetohydrody- namics (MHD) flow is obtained, using the strong-form local meshless point collocation (LMPC) method. The approxima- tion of the field variables is obtained with the moving least squares (MLS) approximation. Regular and irregular nodal distributions are used. Thus, a numerical solver is developed for the unsteady coupled MHD problems, using the collo- cation formulation, for regular and irregular cross sections, as are the rectangular, triangular and circular. Arbitrary wall conductivity conditions are applied when a uniform mag- netic field is imposed at characteristic directions relative to the flow one. Velocity and induced magnetic field across the section have been evaluated at various time intervals for sev- eral Hartmann numbers (up to 105) and wall conductivities. The numerical results of the strong-form MPC method are compared with those obtained using two weak-form mesh- less methods, that is, the local boundary integral equation (LBIE) meshless method and the meshless local Petrov– Galerkin (MLPG) method, and with the analytical solutions, where they are available. Furthermore, the accuracy of the method is assessed in terms of the error norms L 2 and L ∞ , the number of nodes in the domain of influence and the time step length depicting the convergence rate of the method. Run time results are also presented demonstrating the efficiency and the applicability of the method for real world problems. [less ▲]

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See detailA localized mixed-hybrid method for imposing interfacial constraints in the extended finite element method (XFEM)
Zilian, Andreas UL; Fries, T.-P.

in International Journal for Numerical Methods in Engineering (2009), 79(6), 733-752

The paper proposes an approach for the imposition of constraints along moving or fixed immersed interfaces in the context of the extended finite element method. An enriched approximation space enables ... [more ▼]

The paper proposes an approach for the imposition of constraints along moving or fixed immersed interfaces in the context of the extended finite element method. An enriched approximation space enables consistent representation of strong and weak discontinuities in the solution fields along arbitrarily-shaped material interfaces using an unfitted background mesh. The use of Lagrange multipliers or penalty methods is circumvented by a localized mixed hybrid formulation of the model equations. In a defined region in the vicinity of the interface, the original problem is re-stated in its auxiliary formulation. The availability of the auxiliary variable enables the consideration of a variety of interface constraints in the weak form. The contribution discusses the weak imposition of Dirichlet- and Neumann-type interface conditions as well as continuity requirements not fulfilled a priori by the enriched approximation. The properties of the proposed approach applied to two-dimensional linear scalar- and vector-valued elliptic problems are investigated by studying the convergence behavior. © 2009 John Wiley & Sons,Ltd. [less ▲]

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See detailLocally equilibrated stress recovery for goal oriented error estimation in the extended finite element method
Bordas, Stéphane UL; gonzález-estrada, octavio andrés; ródenas, Juan josé et al

in Computers & Structures (2015)

Goal oriented error estimation and adaptive procedures are essential for the accurate and efficient evaluation of finite element numerical simulations that involve complex domains. By locally improving ... [more ▼]

Goal oriented error estimation and adaptive procedures are essential for the accurate and efficient evaluation of finite element numerical simulations that involve complex domains. By locally improving the approximation qual- ity, for example, by using the extended finite element method (XFEM), we can solve expensive problems which could result intractable otherwise. Here, we present an error estimation technique for enriched finite element approxi- mations that is based on an equilibrated recovery technique, which considers the stress intensity factor as the quantity of interest. The locally equilibrated superconvergent patch recovery is used to obtain enhanced stress fields for the primal and dual problems defined to evaluate the error estimate. [less ▲]

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See detailA locking-free meshfree method for the simulation of shear-deformable plates based on a mixed variational formulation
Hale, Jack UL; Baiz, P. M.

in Computer Methods in Applied Mechanics & Engineering (2012), 241-244

The problem of shear-locking in the thin-plate limit is a well known issue that must be overcome when discretising the Reissner-Mindlin plate equations. In this paper we present a shear-locking-free ... [more ▼]

The problem of shear-locking in the thin-plate limit is a well known issue that must be overcome when discretising the Reissner-Mindlin plate equations. In this paper we present a shear-locking-free method utilising meshfree maximum-entropy basis functions and rotated Raviart-Thomas-Nédélec elements within a mixed variational formulation. The formulation draws upon well known techniques in the finite element literature. Due to the inherent properties of the maximum-entropy basis functions our method allows for the direct imposition of Dirichlet (essential) boundary conditions, in contrast to methods based on moving least squares basis functions. We present benchmark problems that demonstrate the accuracy and performance of the proposed method. © 2012. [less ▲]

Detailed reference viewed: 155 (18 UL)
See detailA Logic of Privacy
Barker, Steve; Genovese, Valerio UL

in Data and Applications Security and Privacy XXIV (2010)

We consider the problem of developing an abstract meta-model of access control in terms of which policies for protecting a principal’s private information may be specified. Our concern is with developing ... [more ▼]

We consider the problem of developing an abstract meta-model of access control in terms of which policies for protecting a principal’s private information may be specified. Our concern is with developing the formal foundations of our conceptual model. For both the specific access control models and privacy policies, which may be defined in terms of the meta-model, we adopt a combining approach: we combine access control concepts to form the meta-model and we use a fibred logic for the formal foundations. Our approach enables data subjects to specify flexibly what access controls they wish to apply on their personal data and it provides a formal foundation for policies that are defined in terms of the meta-model. [less ▲]

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See detailLow-energy Secondary Ion Mass Spectrometry: Investigation of buried interfaces in multi-layer organic thin films
Ngo, Khanh Quyen UL

Doctoral thesis (2012)

Recently, the application of conjugated organic compounds increases significantly in optoelectronic devices. The main performances of these devices such as charge and energy transport, operational ... [more ▼]

Recently, the application of conjugated organic compounds increases significantly in optoelectronic devices. The main performances of these devices such as charge and energy transport, operational lifetime and energy conversion efficiency depend strongly on the doping and the interface structure. Therefore, in this project we develop dynamic SIMS analysis conditions using low energy primary ion beam to analyse multi-layered samples used in organic optoelectronic devices. The objective of this thesis is to study the different artefacts and mechanisms, which may arise in the low-energy depth profiling of organic materials and to optimize the SIMS conditions for interface resolution. For the objectives, in chapter I and II, a general introduction about the organic molecules, the fabrication techniques as well as fundamental aspects of the SIMS technique and the analysis conditions are presented. In chapter III, a study of the fragmentation of different organic films during sub-keV bombardment is performed. The typical secondary ions of different organic molecule are identified and chosen for depth profiling. Chapter IV is a study about air-contact induced topography change on Cs+ sputtered surface. This is useful to avoid artefacts when characterizing the Cs+ sputtered surfaces by AFM or by other techniques. The ability of low-energy SIMS to characterize the metal/organics interfaces is investigated in chapter V and VI. Chapter V is about the SIMS depth resolution and sputter-induced surface roughness studied on a series of metal – organic layered samples. In chapter VI, the diffusion of metal into organic layer seen in SIMS depth profiles is characterized. [less ▲]

Detailed reference viewed: 79 (4 UL)
See detailLuxembourg’s free public transport sounds great. It isn't
Carr, Constance UL

Article for general public (2019)

This is a reproduction of Carr/Hesse's 2019 "Luxembourg’s free public transport sounds great, but it won’t help people get from A to B" published Jan 16, 2019, in The Conversation. Available at:https ... [more ▼]

This is a reproduction of Carr/Hesse's 2019 "Luxembourg’s free public transport sounds great, but it won’t help people get from A to B" published Jan 16, 2019, in The Conversation. Available at:https://theconversation.com/luxembourgs-free-public-transport-sounds-great-but-it-wont-help-people-get-from-a-to-b-109695 [less ▲]

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See detailMachine learning techniques for atmospheric pollutant monitoring
Sainlez, Matthieu UL; Heyen, Georges

Poster (2012, January 27)

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See detailMachine Learning to Geographically Enrich Understudied Sources: A Conceptual Approach
Viola, Lorella UL; Verheul, Jaap

in Rocha, Ana; Steels, Luc; van den Herik, Jaap (Eds.) Proceedings of the 12th International Conference on Agents and Artificial Intelligence - Volume 1: ARTIDIGH (2020)

This paper discusses the added value of applying machine learning (ML) to contextually enrich digital collections. In this study, we employed ML as a method to geographically enrich historical datasets ... [more ▼]

This paper discusses the added value of applying machine learning (ML) to contextually enrich digital collections. In this study, we employed ML as a method to geographically enrich historical datasets. Specifically, we used a sequence tagging tool (Riedl and Padó 2018) which implements TensorFlow to perform NER on a corpus of historical immigrant newspapers. Afterwards, the entities were extracted and geocoded. The aim was to prepare large quantities of unstructured data for a conceptual historical analysis of geographical references. The intention was to develop a method that would assist researchers working in spatial humanities, a recently emerged interdisciplinary field focused on geographic and conceptual space. Here we describe the ML methodology and the geocoding phase of the project, focussing on the advantages and challenges of this approach, particularly for humanities scholars. We also argue that, by choosing to use largely neglected sources such as immigrant newspapers (a lso known as ethnic newspapers), this study contributes to the debate about diversity representation and archival biases in digital practices. [less ▲]

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See detailMaintenance location routing for rolling stock under line and fleet planning uncertainty
Tönissen, Denise; Arts, Joachim UL; Shen, Zuo-Jun

in Transportation Science (2019), 53(5), 1252-1270

Detailed reference viewed: 275 (25 UL)
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See detailMaking the Case for Evidence-based Standardization of Data Privacy and Data Protection Visual Indicators
Rossi, Arianna UL; Lenzini, Gabriele UL

in Journal of Open Access to Law (JOAL) (2020), 8(1),

Lately, icons have witnessed a growing wave of interest in the view of enhancing transparency and clarity of data processing practices in mandated disclosures. Although benefits in terms of ... [more ▼]

Lately, icons have witnessed a growing wave of interest in the view of enhancing transparency and clarity of data processing practices in mandated disclosures. Although benefits in terms of comprehensibility, noticeability, navigability of the information and user’s attention and memorization can be expected, they should also be supported by decisive empirical evidence about the efficacy of the icons in specific contexts. Misrepresentation, oversimplification, and improper salience of certain aspects over others are omnipresent risks that can drive data subjects to wrong conclusions. Cross-domain and international standardization of visual means also poses a serious challenge: if on the one hand developing standards is necessary to ensure widespread recognition and comprehension, each domain and application presents unique features that can be hardly established, and imposed, in a top-down manner. This article critically discusses the above issues and identifies relevant open questions for scientific research. It also provides concrete examples and practical suggestions for researchers and practitioners that aim to implement transparency-enhancing icons in the spirit of the General Data Protection Regulation (GDPR). [less ▲]

Detailed reference viewed: 78 (7 UL)