Browsing
     by title


0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

or enter first few letters:   
OK
Full Text
See detailMulti-Months Cycles Observed in Climatic Data
Nicolay, Samuel ULg; Mabille, Georges ULg; Fettweis, Xavier ULg et al

in Simard, Suzanne (Ed.) Climate Change and Variability (2010)

Climatic variations happen at all time scales and since the origins of these variations are usually of very complex nature, climatic signals are indeed chaotic data. The identification of the cycles ... [more ▼]

Climatic variations happen at all time scales and since the origins of these variations are usually of very complex nature, climatic signals are indeed chaotic data. The identification of the cycles induced by the natural climatic variability is therefore a knotty problem, yet the knowing of these cycles is crucial to better understand and explain the climate (with interests for weather forecasting and climate change projections). Due to the non-stationary nature of the climatic time series, the simplest Fourier-based methods are inefficient for such applications (see e.g. Titchmarsh (1948)). This maybe explains why so few systematic spectral studies have been performed on the numerous datasets allowing to describe some aspects of the climate variability (e.g. climatic indices, temperature data). However, some recent studies (e.g. Matyasovszky (2009); Paluš & Novotná (2006)) show the existence of multi-year cycles in some specific climatic data. This shows that the emergence of new tools issued from signal analysis allows to extract sharper information from time series. Here, we use a wavelet-based methodology to detect cycles in air-surface temperatures obtained from worldwide weather stations, NCEP/NCAR reanalysis data, climatic indices and some paleoclimatic data. This technique reveals the existence of universal rhythms associated with the periods of 30 and 43 months. However, these cycles do not affect the temperature of the globe uniformly. The regions under the influence of the AO/NAO indices are influenced by a 30 months period cycle, while the areas related to the ENSO index are affected by a 43 months period cycle; as expected, the corresponding indices display the same cycle. We next show that the observed periods are statistically relevant. Finally, we consider some mechanisms that could induce such cycles. This chapter is based on the results obtained in Mabille & Nicolay (2009); Nicolay et al. (2009; 2010). [less ▲]

Detailed reference viewed: 78 (15 ULg)
See detailMulti-national, Multi-cultural and Multi-levelled Brussels : National and Ethnic Politics in the « Capital of Europe »
Martiniello, Marco ULg; Favell, Adrian

in Fonseca, Maria-Lucinda (Ed.) Cities in Movement : Migrants and Urban Change (2008)

Detailed reference viewed: 50 (2 ULg)
See detailMulti-national, multi-cultural and multi-levelled Brussels: national and ethnic politics in the "Capital of Europe"
Martiniello, Marco ULg; Favell, A.

Book published by The Transnational Communities Programme, University of Oxford (1999)

Detailed reference viewed: 16 (2 ULg)
Full Text
See detailMulti-Objective Optimization of a Fan Blade by Coupling a Genetic Algorithm and a Parametric Flow Solver
Kelner, Vincent ULg; Grondin, Gilles; Léonard, Olivier ULg et al

in Proceedings of EUROGEN'05 (2005, September)

Detailed reference viewed: 39 (0 ULg)
Peer Reviewed
See detailMulti-party Arbitration under the UNCITRAL Rules
Kohl, Benoît ULg

Conference (2008, May 15)

Detailed reference viewed: 55 (9 ULg)
Full Text
See detailMulti-period Stochastic Optimization Problems in Transportation Management
Pironet, Thierry ULg

Doctoral thesis (2014)

The topic of this thesis in management science is to propose a methodology to tackle multi-period decision problems including stochastic forecasts and to apply it to the field of transportation management ... [more ▼]

The topic of this thesis in management science is to propose a methodology to tackle multi-period decision problems including stochastic forecasts and to apply it to the field of transportation management. This methodology relies mostly on a sequence of numerical experimentations based on a set of algorithms to assess the value of the multi-period setting and the interest to use the stochastic information contained in the forecasts. Moreover, a statistical validation method to compare the performances of algorithms enables us to rank them meaningfully. From theory to practice, the thesis is structured into four parts. Firstly, we define the thesis subject and content. Then, based on a literature review, we present the past and present issues within the research field: "Optimization in Transportation". Mainly, we define the differences between multi-period stochastic models and classical deterministic mono-period ones. We explain how dynamism and stochasticity are taken into account within transportation problems. Secondly, our methodology, which is our main contribution, is exposed from a generic point of view in the theoretical research field: "Multi-period Stochastic Optimization Problems". On the one hand, temporal parameters and computational issues in multi-period optimization are detailed; on the other hand a summary of optimization techniques and algorithms for stochastic optimization problems is provided. Afterwards, the statistical validation of algorithmic performance is discussed. Then, part three contains two applications that lead us to set up the methodology, following an inductive method. The first problem, based on an industrial application at the start of the research, investigates a "multi-period vehicle loading problem with stochastic release dates". The second application, based on a more generic approach for the deployment of the methodology, deals with a "multi-period vehicle assignment problem with stochastic load availability". Finally, in the fourth part, we conclude on the thesis contributions and propose some perspectives. [less ▲]

Detailed reference viewed: 43 (6 ULg)
Full Text
Peer Reviewed
See detailMulti-period stochastic optimization problems in transportation management (Ph. D. Thesis Summary)
Pironet, Thierry ULg

in 4OR : Quarterly Journal of the Belgian, French and Italian Operations Research Societies (2014)

Ph. D Thesis summary see other reference on ORBI

Detailed reference viewed: 8 (1 ULg)
Full Text
See detailMulti-period vehicle assignment problem with stochastic load availability
Crama, Yves ULg; Pironet, Thierry ULg

E-print/Working paper (2014)

In this research, we want to investigate optimization techniques for a generic load assignment problem including a limited fleet of vehicles within a full-truck-load (FTL) multi-period setting including ... [more ▼]

In this research, we want to investigate optimization techniques for a generic load assignment problem including a limited fleet of vehicles within a full-truck-load (FTL) multi-period setting including forecasts on load availability. Several policies are generated from simple heuristics through state of the art approaches such as consensus and restricted expectation algorithms up to the optimization of a subtree of scenarios. Moreover, myopic and a-posteriori deterministic optimizations (including no or fully revealed information) set bounds for policies performance comparisons. Tests are performed for different graphs sizes and sparsity, several distribution laws and number of loads. Performances are compared statistically over paired samples. The robustness of performing policies against a false valuation of the probability distribution is also analyzed. [less ▲]

Detailed reference viewed: 39 (6 ULg)
Full Text
See detailMulti-period vehicle assignment with stochastic load availability
Pironet, Thierry ULg

Conference (2014, January 31)

We investigate the following problem, which is faced by major forwarding companies active in road transportation (see [2]). A company owning a limited fl eet of vehicles wants to maximize its operational ... [more ▼]

We investigate the following problem, which is faced by major forwarding companies active in road transportation (see [2]). A company owning a limited fl eet of vehicles wants to maximize its operational pro fit over an infi nite horizon divided into equal periods (days). The pro fit stems from revenues for transporting full truckloads and from costs derived from waiting idle and moving empty. A decision leading to a set of actions is made at every period and is based on the dispatcher's information over a restricted horizon, called rolling horizon, which subsequently rolls over period per period. The data provided by the customers concern their prospective loads, or requirements for transportation: locations of departure and destination cities, and a unique pick-up period for each load. Moreover, the dispatcher has data regarding travel times between cities, current location and status (empty or loaded) of trucks. These data are known with certainty and represent the deterministic component of the problem. The stochastic component of the problem arises from the uncertainty on the eff ective materialization of each transportation order. More precisely, the availability of each order can be either con rmed, or denied, a few periods ahead of the loading period (meaning that clients con firm their order, which the transporter may still decide to ful ll, or not). For prospective orders in the remote part of the rolling horizon, the dispatcher only knows the order con firmation probability which represents the stochastic load availability. In this setting, trucking orders are provided by the dispatching center to the drivers and to the customers on the eve of the pick-up period at the latest. Typically, the loading decisions are made when all orders are con firmed for the next day. The decision problem faced by the dispatcher is to select or to reject loads, and to assign the selected loads to trucks, taking into account con firmed and expected loads as well as the availability and current location of trucks. The main objective of this research is to provide e fficient algorithmic strategies to tackle this multi-period vehicle-load assignment problem over a rolling horizon including prospective transportation orders. This problem is computationally di fficult owing to the large number of possible realizations of the random variables, and to the combinatorial nature of the decision space. The methodology is based on optimizing decisions for deterministic scenarios. By solving the assignment problem for a sample of scenarios, by mixing solutions and by evaluating them at each period, we aim at finding actions per decision period leading to pro table policies in the long run. Several policies are generated in this way, from simple myopic heuristics to more complex approaches, such as consensus and restricted expectation algorithms [3], up to policies derived from network flow models formulated over subtrees of scenarios. Similar approaches have proved eff ective for other problems; see, e.g., [1]. Myopic and a-posteriori deterministic optimization models are used to compute bounds allowing for performance evaluation. Test are performed on various instances featuring di fferent numbers of loads, graph sizes, sparsity, and probability distributions. Performances are compared statistically over paired samples to assess the signi ficance of the observed differences among algorithmic policies. The robustness of various policies with respect to erroneous evaluations of the probability distributions is also analyzed. Numerical experiments show that the best algorithms close a signi ficant fraction of the gap between the worst (myopic) and best (a posteriori) bounds for a broad range of datasets and for several probability distributions. Furthermore, the subtree algorithm remains quite robust against a variety of probability distributions when it is calibrated with a distribution re flecting maximum uncertainty . Acknowledgements. The project leading to these results was partially funded by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy O ffice (grant P7/36). References [1] Arda, Y., Crama, Y., Kronus, D., Pironet, Th., and Van Hentenryck, P. (2013), Multi-period vehicle loading with stochastic release dates, EURO Journal on Transportation and Logistics, pp. 1-27, available on-line http://dx.doi.org/10.1007/s13676-013-0035-z. [2] Powell, W. B. (1996), A stochastic formulation of the dynamic assignment problem, with an application to truckload motor carriers, Transportation Science, Vol. 30, pp. 195-219. [3] Van Hentenryck, P., and Bent,R. W. (2006), Online Stochastic Combinatorial Optimization, MIT Press, Cambridge, Massachussetts. [less ▲]

Detailed reference viewed: 10 (3 ULg)
Full Text
See detailMulti-period vehicle assignment with stochastic load availability
Pironet, Thierry ULg

Conference (2014, February 27)

This work investigates optimization techniques for a vehicle-load assignment problem. A company owning a limited fleet of vehicles wants to maximize its operational profit over an infinite horizon divided ... [more ▼]

This work investigates optimization techniques for a vehicle-load assignment problem. A company owning a limited fleet of vehicles wants to maximize its operational profit over an infinite horizon divided into periods. The profit stems from revenues for transporting full truckloads and costs derived from waiting idle and moving unladen. The stochastic component of the problem arises from projections on the realization of each transportation order, i.e. load. The methodology is based on optimizing decisions for deterministic scenarios. Several policies are generated in this way, from simple heuristics to more complex approaches, such as consensus and restricted expectation algorithms, up to policies derived from network flow models formulated over subtrees of scenarios. Myopic and a-posteriori deterministic optimizations models are used to compute bounds allowing for performance evaluation. Tests are performed on various instances featuring different number of loads, graph sizes, sparsity, and probability distributions. Performances are compared statistically over paired samples. The robustness of various policies with respect to erroneous evaluations of the probability distributions is also analyzed. [less ▲]

Detailed reference viewed: 8 (0 ULg)
Full Text
See detailMulti-period vehicle assignment with stochastic load availability
Pironet, Thierry ULg

Conference (2014, June 23)

This work investigates optimization techniques for a vehicle-load assignment problem. A company owning a limited fleet of vehicles wants to maximize its operational profit over an infinite horizon divided ... [more ▼]

This work investigates optimization techniques for a vehicle-load assignment problem. A company owning a limited fleet of vehicles wants to maximize its operational profit over an infinite horizon divided into periods. The profit stems from revenues for transporting full truckloads and costs derived from waiting idle and moving unladen. The stochastic component of the problem arises from projections on the realization of each transportation order, i.e. load. The methodology is based on optimizing decisions for deterministic scenarios. Several policies are generated in this way, from simple heuristics to more complex approaches, such as consensus and restricted expectation algorithms, up to policies derived from network flow models formulated over subtrees of scenarios. Myopic and a-posteriori deterministic optimizations models are used to compute bounds allowing for performance evaluation. Tests are performed on various instances featuring different number of loads, graph sizes, sparsity, and probability distributions. Performances are compared statistically over paired samples. The robustness of various policies with respect to erroneous evaluations of the probability distributions is also analyzed. [less ▲]

Detailed reference viewed: 5 (1 ULg)
Full Text
Peer Reviewed
See detailMulti-period vehicle loading with stochastic release dates
Arda, Yasemin ULg; Crama, Yves ULg; Kronus, David ULg et al

in EURO Journal on Transportation and Logistics (2014), 3(2), 93-119

This paper investigates a multi-period vehicle loading problem with stochastic information regarding the release dates of items to be transported. The deterministic version of the problem can be ... [more ▼]

This paper investigates a multi-period vehicle loading problem with stochastic information regarding the release dates of items to be transported. The deterministic version of the problem can be formulated as a large-scale set covering problem. Several heuristic algorithms are proposed to generate decision policies for the stochastic optimization model over a long rolling horizon. The resulting policies have been extensively tested on instances which display the main characteristics of the industrial case-study that motivated the research. The tests demonstrate the benefits of the multi-period stochastic model over simple myopic strategies. A simple and efficient heuristic is shown to deliver good policies and to be robust against errors in the estimation of the probability distribution of the release dates. [less ▲]

Detailed reference viewed: 157 (35 ULg)
Full Text
Peer Reviewed
See detailMulti-physical processes in geomechanics – an introduction to constitutive modelling and coupling aspects
Charlier, Robert ULg; Dizier, Arnaud ULg; Laloui, Lyesse et al

in European Journal of Environmental and Civil Engineering [=EJECE] (2009), 7-8

This paper is the basis for a course dedicated to the geomechanics modelling, taking into account multiphysics couplings. A number of different coupling are discussed, with respectively the fluid flow ... [more ▼]

This paper is the basis for a course dedicated to the geomechanics modelling, taking into account multiphysics couplings. A number of different coupling are discussed, with respectively the fluid flow (saturated and unsaturated) and the thermal transfers in deformable porous media. Eventually some aspects on the numerical modelling with the finite element method are discussed. [less ▲]

Detailed reference viewed: 90 (20 ULg)
Full Text
See detailMulti-physics modeling
Terrapon, Vincent ULg

Scientific conference (2010, February)

Detailed reference viewed: 13 (3 ULg)
Peer Reviewed
See detailMulti-Residue Screening and Confirmatory Analysis of Anabolic Steroids in Urine by Gas Chromatography Coupled with Tandem Mass Spectrometry
Van Vyncht, G.; Gaspar, P.; De Pauw, Edwin ULg et al

in Journal of Chromatography. A (1994), 683(1), 67-74

The diversity of substances used illegally as growth promoters in meat production requires the development of multi-analyte methods of analysis involving a sample pretreatment step that is as rapid and as ... [more ▼]

The diversity of substances used illegally as growth promoters in meat production requires the development of multi-analyte methods of analysis involving a sample pretreatment step that is as rapid and as easy as possible, followed by a specific and sensitive determination of several residues within the same run. A general strategy for the screening and confirmatory analysis of fifteen artificial anabolic compounds in urine samples is described. It is based on solid-phase extraction on C18 Empore discs and amino-bonded columns followed, after derivatization (trimethylsilyl or methyloxime-trimethylsilyl derivatives), by gas chromatography coupled with collisionally activated dissociation tandem mass spectrometry. [less ▲]

Detailed reference viewed: 48 (2 ULg)
Full Text
See detailMulti-row approaches to cutting plane generation
Poirrier, Laurent ULg

Doctoral thesis (2012)

This thesis focuses on the use of cutting-plane techniques to improve general-purpose mixed-integer linear programming solvers. The first topic covered here is a fast separation method for two-row cuts ... [more ▼]

This thesis focuses on the use of cutting-plane techniques to improve general-purpose mixed-integer linear programming solvers. The first topic covered here is a fast separation method for two-row cuts. Two-row cuts are intersection cuts from two rows of a simplex tableau describing the LP relaxation of the problem. This type of cuts recently gathered a lot of attention from the scientific community following a paper by Andersen, Louveaux, Weismantel and Wolsey describing the facets of the underlying two-row model and providing an intuitive geometric classification the the derived cuts. The specificity of the approach adopted here is that it does not rely on an "infinite relaxation" point of view and generate intersection cuts from fixed lattice-free sets. Instead, given a fractional point, it aims at always finding a most violated facet-defining inequality for the two-row model. This can be achieved by optimizing over the polar set of the integer hull of the model. A fast way of performing this is provided, by means of a polyhedron that is equivalent to the polar for that purpose, but has a more compact representation. Moreover, a row-generation algorithm is developed in order to avoid the costly computations of integer hulls of two-dimensional cones. An implementation of the resulting algorithm performs separation of two-row cuts in a few milliseconds on average, on the standard MIPLIB 3 and 2003 testsets. While this two-row separator is quick, the measurements of the computational usefulness of the cuts do not yield satisfactory results. Since all the cuts generated are facet-defining, this might suggest that the underlying two-row models are too weak. This observation prompted the second part of this thesis, an attempt to evaluate the strength of various multi-row relaxations, on small instances, using a generic separator. To that end, a separator is developed, which is able to compute facet-defining inequalities from arbitrary (yet reasonably small) mixed-integer sets. A row-generation approach is again adopted, but this time the slave part consists in the resolution of a mixed-integer problem instead of a closed-form oracle. Some interesting computational tricks are developed, in order to speedup the inherently hard computations. [less ▲]

Detailed reference viewed: 69 (15 ULg)
See detailMulti-scale analysis of carbon stocks and deforestation monitoring. Case of the Eastern tropical humid forest of Madagascar.
Ratsimba, H; Rajoulison, L G; Rabenilalana, F M et al

in Azevedo, J C; Feliciano, M; Castro, J (Eds.) et al Forest landscapes and Global Change (2010)

Detailed reference viewed: 15 (4 ULg)
Full Text
Peer Reviewed
See detailA multi-scale approach to facultative paedomorphosis of European newts (Salamandridae) in the Montenegrin karst: Distribution pattern, environmental variables, and conservation
Denoël, Mathieu ULg; Ficetola, Gentile Francesco; Cirovic, Ruza et al

in Biological Conservation (2009), 142(3), 509-517

Facultative paedomorphosis, a process in which newt larvae can opt for reproduction before or after metamorphosis, is geographically heterogeneous. Despite numerous ecological studies and recent evidence ... [more ▼]

Facultative paedomorphosis, a process in which newt larvae can opt for reproduction before or after metamorphosis, is geographically heterogeneous. Despite numerous ecological studies and recent evidence of declines in paedomorphic populations, however, no attempt to model environmental variables that explain the presence of paedomorphs has been made at a multi-scale level. Our aim was to fill this gap in studying three newt species (Lissotriton vulgaris, Mesotriton alpestris, and Triturus macedonicus) of the Montenegrin karst as model species. To this end, we used multivariate analysis on three scales of habitat: the breeding pond, the land use and the climatologic features. Results show that the study area is both an important hotspot for paedomorphosis and where intraspecific diversity is quickly disappearing (20-47% extirpation) because of fish introductions. Other habitat variables (water permanency, PH or the habitat origin) were shown to act on paedomorphosis but not consistently across species, confirming complexity of the evolutionary and ecological processes. This study appeals for more long-term and detailed landscape studies of polyphenisms, a neglected but promising topic, to better understand and protect alternative modes of development. Particularly, measures should be taken to identify hotspots of intraspecific diversity at a global scale and stop fish introductions before we reach a point of no-return. (C) 2008 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 319 (27 ULg)
Full Text
Peer Reviewed
See detailMulti-scale aquifer characterization and groundwater flow model parameterization using direct push technologies
Rogiers, Bart; Vienken, Thomas; Gedeon, Matej et al

in Environmental Earth Sciences (2014)

Direct push (DP) technologies are typically used for cost-effective geotechnical characterization of unconsolidated soils and sediments. In more recent developments, DP technologies have been used for ... [more ▼]

Direct push (DP) technologies are typically used for cost-effective geotechnical characterization of unconsolidated soils and sediments. In more recent developments, DP technologies have been used for efficient hydraulic conductivity (K) characterization along vertical profiles with sampling resolutions of up to a few centimetres. Until date, however, only a limited number of studies document high-resolution in situ DP data for three-dimensional conceptual hydrogeological model development and groundwater flow model parameterization. This study demonstrates how DP technologies improve building of a conceptual hydrogeological model. We further evaluate the degree to which the DP-derived hydrogeological parameter K, measured across different spatial scales, improves performance of a regional groundwater flow model. The study area covers an area of ~60 km² with two overlying, mainly unconsolidated sand, aquifers separated by a 5-7 m thick highly heterogeneous clay layer (in north-eastern Belgium). The hydrostratigraphy was obtained from an analysis of cored boreholes and about 265 cone penetration tests (CPTs). The hydrogeological parameter K was derived from a combined analysis of core and CPT data and also from hydraulic direct push tests. A total of 50 three-dimensional realizations of K were generated using a non-stationary multivariate geostatistical approach. To preserve the measured K values in the stochastic realizations, the groundwater model Krealizations were conditioned on the borehole and direct push data. Optimization was performed to select the best performing model parameterization out of the 50 realizations. This model outperformed a previously developed reference model with homogeneous K fields for all hydrogeological layers. Comparison of particle tracking simulations, based either on the optimal heterogeneous or reference homogeneous groundwater model flow fields, demonstrate the impact DP-derived subsurface heterogeneity in K can have on groundwater flow and solute transport. We demonstrated that DP technologies, especially when calibrated with site-specific data, provide high-resolution 3D subsurface data for building more reliable conceptual models and increasing groundwater flow model performance. [less ▲]

Detailed reference viewed: 11 (3 ULg)
Full Text
Peer Reviewed
See detailMulti-scale aquifer characterization and groundwater flow model parameterization using direct push technologies
Rogiers, Bart; Vienken, T; Batelaan, Okke et al

in Novel Methods for Subsurface Characterization and Monitoring: From Theory to Practice (2013, May)

Groundwater flow and contaminant transport models are used to support decision making regarding waste disposal options, sites contaminated by surface or subsurface sources, or to develop and test cost ... [more ▼]

Groundwater flow and contaminant transport models are used to support decision making regarding waste disposal options, sites contaminated by surface or subsurface sources, or to develop and test cost-effective groundwater remediation schemes. Such models are influenced by different sources of uncertainty, including those due to spatial variability in aquifer and aquitard properties including hydraulic conductivity (K). However, quantifying spatial variability in K remains challenging. Classical drilling techniques for shallow heterogeneous unconsolidated sedimentary deposits involving continuous coring are expensive and time-consuming, especially when the area of interest exceeds several tens of km². Alternative techniques such as direct push technologies use hydraulic rams, supplemented with vehicle weight, or high-frequency hammering, to advance small-diameter tools into the subsurface. These tools are typically used for cost-effective geotechnical characterization of unconsolidated deposits; recent developments also allow for hydraulic characterization. The depth of investigation is up to ~40 m, depending on the tools used (i.e applied load) and sediment properties (friction). Up to now, only a limited number of studies document using this type of data to parameterize regional groundwater flow models. To fill this gap, this study aims at parameterizing a regional groundwater flow model using data from various types of direct push technologies. We discuss the characterization of an area (~60 km²) near the nuclear zone of Mol/Dessel (Belgium), using various direct push technologies. Most of the measurements are concentrated in an area of 200×400 m². The data include 265 cone penetration tests (CPTs), 113 pore pressure dissipation tests (PPDTs), 17 direct push injection logs (DPIL), 6 hydraulic profiling tool (HPT) logs and 19 direct push slug tests (DPST). Resulting K values, either calculated or estimated, and the corresponding spatial variability are compared with that of borehole and outcrop studies. The benefit of using standard CPT data for the parameterization of an aquitard at the study site has previously been shown. The approach is now applied to the aquifer units and incorporates new direct push data for the entire upper ~40 m of the hydrogeological domain. The effect of the 3D heterogeneous hydraulic conductivity field on the performance of the groundwater flow model is discussed; the value of the different direct push technologies is equally addressed. [less ▲]

Detailed reference viewed: 31 (2 ULg)