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See detailEfficient Computation of the Extrema of Algebraic Quality Measures for Curvilinear Finite Elements
Johnen, Amaury ULg; Geuzaine, Christophe ULg; remacle, Jean-François

Conference (2016, June)

The development of high-order computational methods for solving partial differential equations on unstructured grids has been underway for many years. Such methods critically depend on the availability of ... [more ▼]

The development of high-order computational methods for solving partial differential equations on unstructured grids has been underway for many years. Such methods critically depend on the availability of high-quality curvilinear meshes, as one bad element can degrade the solution in the whole domain. The usual way of generating curved meshes is to first generate a (high-quality) straight-sided mesh. Then, mesh entities that are classified on the boundaries of the domain are curved. This operation introduces a "shape-distortion" that should be controlled. Quality measures allow to quantify to which point an element is well-shaped. They also provide tools to improve the quality of meshes through optimization. In this work we propose an efficient method to compute several quality measures for curved elements, based on the Jacobian of the mapping between the straight-sided elements and the curved ones. Contrary to the approach presented in "A. Gargallo-Peiró, X. Roca, J. Peraire, and J. Sarrate. Distortion and quality measures for validating and generating high-order tetrahedral meshes. Engineering with Computers, pages 1–15, 2014.", which relies on an L2-norm over the elements, we compute the actual minimum and maximum of the local quality measure for each element. The method is an extension of previous works on the validity of those elements (A. Johnen et al., 2013). The key feature is that we can adaptively expand functions based on the Jacobian matrix and its determinant in terms of Bézier functions. Bézier functions have both properties of boundedness and positivity, which allow sharp computation of minimum or maximum of the interpolated functions. [less ▲]

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See detailEfficient Continuous Flow Benzotriazole Activation and Coupling of Amino Acids
Seghers, Sofie; Van Waes, Frederik E. A.; Cukalovic, Ana et al

in Journal of Flow Chemistry (2015), 5(4), 220-227

Despite extensive research into peptide synthesis, coupling of amino acids with weakly nucleophilic heterocyclic amines remains a challenge. The need for microwave technology to promote this coupling ... [more ▼]

Despite extensive research into peptide synthesis, coupling of amino acids with weakly nucleophilic heterocyclic amines remains a challenge. The need for microwave technology to promote this coupling interferes with the scalability of the process. By applying the microwave-to-flow paradigm, a library of (α-aminoacyl)amino-substituted heterocycles was continuously produced at near quantitative conversions and the reaction was scaled up successfully. Various N-Cbz-protected amino acids were activated using BtH/SOCl2 under continuous-flow conditions with excellent yields. Their coupling with heterocyclic amines was accomplished in MeCN—NMP on a preparative scale. However, performing both steps in-line resulted in an inconvenient work-up. Therefore, a two-step approach was taken, isolating the intermediate Bt-activated amino acid via simple filtration. This allows for a solvent switch to DMSO for the coupling reaction which led to excellent conversions for a broad range of substrates. [less ▲]

Detailed reference viewed: 16 (4 ULg)
See detailEfficient controlled radical polymerization of vinyl acetate based on cobast complexes
Debuigne, Antoine ULg; Detrembleur, Christophe ULg; Bryaskova, Rayna et al

Conference (2006, May 18)

Detailed reference viewed: 17 (3 ULg)
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See detailEfficient database pruning for large-scale cover song recognition
Osmalsky, Julien ULg; Pierard, Sébastien ULg; Van Droogenbroeck, Marc ULg et al

in International Conference on Acoustics, Speech, and Signal Processing (ICASSP) (2013, May)

This paper focuses on cover song recognition over a large dataset, potentially containing millions of songs. At this time, the problem of cover song recognition is still challenging and only few methods ... [more ▼]

This paper focuses on cover song recognition over a large dataset, potentially containing millions of songs. At this time, the problem of cover song recognition is still challenging and only few methods have been proposed on large scale databases. We present an efficient method for quickly extracting a small subset from a large database in which a correspondence to an audio query should be found. We make use of fast rejectors based on independent audio features. Our method mixes independent rejectors together to build composite ones. We evaluate our system with the Million Song Dataset and we present composite rejectors offering a good trade-off between the percentage of pruning and the percentage of loss. [less ▲]

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See detailAn efficient decentralized on-line traffic engineering algorithm for MPLS networks
Blanchy, François; Mélon, Laurent; Leduc, Guy ULg

in Providing Quality of Service in Heterogeneous Environments (2003, August)

Multi-Protocol Label Switching (MPLS) provides ways to control the Label Switched Paths (LSPs) followed by traffic trunks in a network and thereby to better traffic engineer it. In this context, we look ... [more ▼]

Multi-Protocol Label Switching (MPLS) provides ways to control the Label Switched Paths (LSPs) followed by traffic trunks in a network and thereby to better traffic engineer it. In this context, we look at the problem of organizing the mapping of LSPs in an optimal way throughout the network on the basis of a given objective function. This problem is highly combinatorial and makes dynamic and real-time features a difficult issue for any LSP routing scheme. For this reason, we propose a computationally efficient, though approximate, on-line scheme adapted to an incremental optimization of the network state. It is then applied to a seldom mentioned traffic engineering problem: the compromise between load-balancing and traffic minimization. It is expected that clever routing strategies to balance the network load will sometimes favor longer paths in order to avoid congestion, leading to an increase of the overall network utilization. This reasoning is confirmed by our study, and we show that an improvement in network management can be made by appropriately tuning this compromise. [less ▲]

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See detailEfficient Delta-Parametrization of 2D Surface-Impedance Solutions
Krähenbühl, Laurent; Dular, Patrick ULg; Péron, Victor et al

in Proceedings of COMPUMAG 2015 (2015, June)

Impedance boundary condition methods (IBCs) are among the most efficient methods for solving time-harmonic eddy-current problems with a small skin depth (delta). However for a wide range of frequencies ... [more ▼]

Impedance boundary condition methods (IBCs) are among the most efficient methods for solving time-harmonic eddy-current problems with a small skin depth (delta). However for a wide range of frequencies (or material conductivities) the standard approach is no more efficient, since it requires for each frequency (or conductivity) the computation of a finite element (FE) complex-valued problem. Moreover, the accuracy of IBC decreases dramatically for large delta. As an extension of our previous work, we propose here a more detailed method of parametrization in delta of the 2D small-delta eddy-currents problem. This numerically efficient method gives a very good precision for all the frequencies difficult to address, i.e. from the frequency corresponding to the last good solution obtainable by meshing the conductor up to infinity (perfect conductor solution). [less ▲]

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See detailEfficient determination of diffusion coefficients by monitoring transport during recovery delays in NMR
Augustyniak, Rafal; Ferrage, Fabien; Damblon, Christian ULg et al

in Chemical Communications (2012), 48

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See detailAn efficient DNA isolation protocol for filamentous cyanobacteria of the genus Arthrospira
Morin, Nicolas ULg; Vallaeys, Tatiana; Hendrickx, Larissa et al

in Journal of Microbiological Methods (2010), 80

Thanks to their photosynthetic and nutritive properties, cyanobacteria of the Arthrospira genus are of interest as food supplements, as efficient oxygen producing life support system organisms for manned ... [more ▼]

Thanks to their photosynthetic and nutritive properties, cyanobacteria of the Arthrospira genus are of interest as food supplements, as efficient oxygen producing life support system organisms for manned space flight, and for the production of biofuels. Despite these potential valuable applications, full genome sequences and genetic information in general on Arthrospira remain scarce. This is mainly due to the difficulty to extract sufficient high molecular weight nucleic acids from these filamentous cyanobacteria. In this article, an efficient and reproducible DNA extraction procedure for cyanobacteria of the genus Arthrospira was developed. The method is based on the combination of a soft mechanical lysis with enzymatic disruption of the cell wall. The comparison with other extraction protocols clearly indicates that this optimised method allows the recovery of a larger amount of DNA. Furthermore, the extracted DNA presents a high molecular weight, a reduced degradation and an excellent overall quality. It can be directly used for molecular biology purposes such as PCR, and clone library construction. [less ▲]

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See detailAn Efficient Dual and Triple Frequency Preprocessing Method for GALILEO and GPS Signals
Lonchay, Matthieu ULg; Bidaine, Benoît ULg; Warnant, René ULg

in 3rd International Colloquium – Scientific and Fundamentals Aspects of the GALILEO Programme (2011, September 02)

Data preprocessing is a mandatory stage for most of GNSS applications. In the frame of space weather and precise point positioning applications, the Geomatics Unit of the University of Liège has purchased ... [more ▼]

Data preprocessing is a mandatory stage for most of GNSS applications. In the frame of space weather and precise point positioning applications, the Geomatics Unit of the University of Liège has purchased two Septentrio PolaRx3eG receivers which allow tracking GPS L1/L5 and Galileo E1/E5a signals. In order to fully exploit these new data, we developed a preprocessing method extending existing techniques. Our preprocessing method consists of three consecutive steps. The first step is devoted to the compensation of receiver clock slips affecting code pseudorange and carrier-phase measurements. The second step covers cycle slips detection and the third step assesses data quality in terms of noise essentially affecting code pseudorange measurements. This preprocessing method was initially developed for GPS L1/L5 and Galileo E1/E5a dual frequency data but finally enhanced to also preprocess triple frequency data from first operational Galileo satellites as soon as data are available. The developed method already showed promising results. [less ▲]

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See detailAn efficient dynamic state estimator for large-scale electric power systems
Rousseaux, Patricia ULg

in Proceedings of the IFAC Symposium on Power Systems. Modelling and Control applications (1988)

Detailed reference viewed: 18 (1 ULg)
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See detailEfficient estimation of the high-order response statistics of a wind-excited oscillator with nonlinear velocity feedback
Denoël, Vincent ULg; Carassale, Luigi

in Proceedings of the XIII Conference of the Italian Association for Wind engineering (2014)

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See detailEfficient evaluation of the geometrical validity of curvilinear finite elements
Johnen, Amaury ULg; Remacle, Jean-François; Geuzaine, Christophe ULg

Conference (2011, November 14)

The development of high-order numerical techniques on unstructured grids has been underway for many years. The accuracy of these methods strongly depends of the accuracy of the geometrical discretization ... [more ▼]

The development of high-order numerical techniques on unstructured grids has been underway for many years. The accuracy of these methods strongly depends of the accuracy of the geometrical discretization, and thus depends on the availability of quality curvilinear meshes. The usual way of building such curvilinear meshes is to first generate a straight sided mesh. Then, mesh entities that are classified on the curved boundaries of the domain are curved accordingly. Some internal mesh entities may be curved as well. If we assume that the straight sided mesh is composed of well shaped elements, curving elements introduces a kind of "shape distortion" that should be controlled so that the final curvilinear mesh is also composed of well shaped elements. In this work we propose a method to analyze curvilinear meshes in terms of their elementary jacobians. The method does not deal with the actual generation of the high order mesh. Instead, it provides an efficient way to guarantee that a curvilinear element is geometrically valid, i.e., that its jacobian is strictly positive in all its reference domain. It also provides a way to measure the distortion of the curvilinear element. The key feature of the method is to adaptively expand the elementary jacobians in a polynomial basis, built using Bézier functions, that has both properties of boundedness and positivity. The algorithm has been implemented in the open-source mesh generator Gmsh, and allows to control the geometrical validity of curvilinear meshes made of triangles, quadrangles, tetrahedra, hexahedra and prisms of any order. [less ▲]

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See detailEfficient finite element assembly of high order Whitney forms
Marsic, Nicolas ULg; Geuzaine, Christophe ULg

in IET Conference Proceedings (2014)

This paper presents an efficient method for the finite element assembly of high order Whitney elements. We start by reviewing the classical assembly technique and by highlighting the most time consuming ... [more ▼]

This paper presents an efficient method for the finite element assembly of high order Whitney elements. We start by reviewing the classical assembly technique and by highlighting the most time consuming part. Then, we show how this classical approach can be reformulated into a computationally efficient matrix - matrix product. We also address the problem of the basis orientation by considering more than one reference space. We conclude by presenting numerical results on a wave guide problem. [less ▲]

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See detailEfficient finite element assembly of high order Whitney forms
Marsic, Nicolas ULg; Geuzaine, Christophe ULg

in IET Science, Measurement & Technology (2015), 9(2), 204-210

This study presents an efficient method for the finite element assembly of high order Whitney elements. The authors start by reviewing the classical assembly technique and by highlighting the most time ... [more ▼]

This study presents an efficient method for the finite element assembly of high order Whitney elements. The authors start by reviewing the classical assembly technique and by highlighting the most time consuming part. Then, they show how this classical approach can be reformulated into a computationally efficient matrix-matrix product. They also address the global orientation problem of the vector valued basis functions. They conclude by presenting numerical results for a three-dimensional wave propagation problem. [less ▲]

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See detailAn efficient flamelet-based combustion model for compressible flows
Saghafian, Amirreza; Terrapon, Vincent ULg; Pitsch, Heinz

in Combustion and Flame (2015), 162(3), 652-667

A combustion model based on a flamelet/progress variable approach for high-speed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and ... [more ▼]

A combustion model based on a flamelet/progress variable approach for high-speed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and tabulated species mass fractions. Only three additional scalar equations need to be solved for the combustion model. Additionally, a flamelet library is used that is computed in a pre-processing step. This approach is very efficient and allows for the use of complex chemical mechanisms. An approximation is also introduced to eliminate costly iterative steps during the temperature calculation. To better account for compressibility effects, the chemical source term of the progress variable is rescaled with the density and temperature. The compressibility corrections are analyzed in an a priori study. The model is also tested in both Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) computations of a hydrogen jet in a supersonic transverse flow. Comparison with experimental measurements shows good agreement, particularly for the LES case. It is found that the disagreement between RANS results and experimental data is mostly due to the mixing model deficiencies and the presumed probability density functions used in the RANS formulation. A sensitivity study of the proposed model shows the importance of the compressibility corrections especially for the source term of the progress variable. [less ▲]

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See detailAn Efficient Flamelet-based Combustion Model for Supersonic Flows
Saghafian, Amirreza; Terrapon, Vincent ULg; Ham, Frank et al

in 17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference 2011, Vol. 2; No AIAA 2011-2267 (2011, April)

A combustion model based on a Flamelet/Progress Variable approach for high-speed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and ... [more ▼]

A combustion model based on a Flamelet/Progress Variable approach for high-speed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and tabulated species mass fractions. The combustion is thus modeled by 3 additional scalar equations and a chemistry table that is computed in a pre-processing step. This approach is very efficient and allows the use of complex chemical mechanisms. An approximation is also introduced to eliminate costly iteration steps during the temperature calculation. To better account for compressibility e ects, the source term for the progress variable is rescaled with the pressure. The model is tested in both RANS and LES computations of a hydrogen jet in a supersonic transverse flow. Comparison with experimental measurements shows good agreement, particularly in the LES case. It is also found that the disagreement between RANS results and experimental data is mostly due to the mixing model de ciencies used in RANS. [less ▲]

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See detailAn efficient formulation for the simulation of elastic wave propagation in 1-dimensional collinding bodies
Depouhon, Alexandre ULg; Detournay, Emmanuel; Denoël, Vincent ULg

in Proceedings of the 11th International Conference on Vibration Problems (2013, September 09)

Detailed reference viewed: 31 (4 ULg)
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See detailAn efficient global methodology for hazard analysis of dams complexes and cascades
Dewals, Benjamin ULg; Archambeau, Pierre ULg; Erpicum, Sébastien ULg et al

in Brebbia, C. A.; Popov, V.; Beriatos, E. (Eds.) Risk analysis Vol. 6 (2008)

Hazard analysis of dams arranged in complex or in cascade may involve the assessment of a large number of different scenarios of combined failures. However, a tremendous computation time would be needed ... [more ▼]

Hazard analysis of dams arranged in complex or in cascade may involve the assessment of a large number of different scenarios of combined failures. However, a tremendous computation time would be needed to perform a detailed modelling of the flows generated by all possible scenarios. Therefore, the present paper describes a methodology for analysing the flows induced by the most relevant incidents occurring on a complex or in a cascade of dams and reservoirs. The methodology combines several hydrodynamic models, including the two-dimensional flow solver WOLF 2D and a simplified lumped hydrodynamic model, to simulate the flows induced on the dams complex, the potential breaching in cascade of other dams, as well as the propagation of the flood wave in the whole downstream valley. The application of the methodology to a practical case study involving a complex of five dams is also described. [less ▲]

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See detailEfficient hydraulic numerical modeling with multiblock grids and linked models
Erpicum, Sébastien ULg; Archambeau, Pierre ULg; Dewals, Benjamin ULg et al

in Houille Blanche (2011), 3

In the field of numerical hydraulic engineering, the size of the simulations is one of the current challenges to consider with sufficient precision the large areas involved by practical applications. In ... [more ▼]

In the field of numerical hydraulic engineering, the size of the simulations is one of the current challenges to consider with sufficient precision the large areas involved by practical applications. In this scope, the linking in the same computational domain of different solvers, each one used in the part of the simulation where its characteristics are most suitable, opens the door to huge modeling possibilities. Such optimized hydraulic numerical modeling is presented in this paper with a multiblock multimodel 2D solver and its linking to a 1D model. After a description of the solvers, with emphasis on the linking procedures, theoretical test cases and real applications are presented. [less ▲]

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See detailEfficient hydrogen-bond donor activators for the synthesis of bio-based cyclic carbonates from CO2 and vegetable oils: a combined in-situ FT-IR and DFT study
Alves, Margot ULg; Méreau, Raphaël; Grignard, Bruno ULg et al

Conference (2015, July 06)

The present research aims at developing new very efficient organocatalysts for the chemical fixation of carbon dioxide onto epoxides that are precursors of non-isocyanate polyurethanes (NIPUs). Although ... [more ▼]

The present research aims at developing new very efficient organocatalysts for the chemical fixation of carbon dioxide onto epoxides that are precursors of non-isocyanate polyurethanes (NIPUs). Although this area of research is the subject of many works, the catalytic performance must be further enhanced in particular for the carbonatation of vegetable-based precursors while respecting environmental standards. In this context, we developed a new organocatalytic platform based on the combination of ammonium salts with single or double hydrogen bond donor activators that showed unexpected catalytic activity for the fast addition of CO2 onto epoxidized oils under mild conditions. First of all, in situ kinetic studies of the cycloaddition of CO2 onto model epoxidized oils were monitored by FT-IR spectroscopy in order to evaluate the influence of the hydrogen bond structure and various parameters such as the pressure, the temperature, the catalyst loading, and the nature of the epoxide on the reaction kinetics. Thanks to this catalyst screening, we found that ammonium salt/fluorinated hydrogen bond donors bicomponent organocatalysts were by far more efficient than that proposed in the literature under mild conditions (60°C, 2MPa). Then, the reaction mechanism of the organocatalyzed cycloaddition of propylene oxide onto CO2 was elucidated by performing Density Functional Theory (DFT). Our theoretical results highlighted the key role of the hydrogen bond interaction between the epoxide and the activators for the enhancement of the catalytic platform’s efficiency. [less ▲]

Detailed reference viewed: 76 (5 ULg)