References of "Delhez, Eric"
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See detailThe age as a diagnostic of the dynamics of marine ecosystem models
Delhez, Eric ULg; Lacroix, Geneviève; Deleersnijder, Eric

in Ocean Dynamics (2004), 54(2), 221-231

The constituent-oriented age theory (CAT) worked out by Delhez et al. (1999) is a flexible tool that can be applied to diagnose complex models. It is shown here how this can be used to quantify the pace ... [more ▼]

The constituent-oriented age theory (CAT) worked out by Delhez et al. (1999) is a flexible tool that can be applied to diagnose complex models. It is shown here how this can be used to quantify the pace at which an ecosystem model works. At the cost of the introduction of one additional evolution equation for each compartment of the ecosystem model, the mean age of the biological material forming these compartments can be computed. The information obtained in this way complements the information provided by the concentration data; while the latter measures the standing stocks, the former provides an integrated assessment of the interaction rates and matter fluxes. The benefits of the method are demonstrated with a simple Lotka-Volterra system and a one-dimensional vertical model of the nitrogen cycle in the Ligurian Sea. The theory can be used to study the biological compartments individually or the ecosystem as a whole. In particular, the age is a valuable tool to quantify the overall cycling rate of nitrogen in the food web. [less ▲]

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See detailResidence time in a semi-enclosed domain from the solution of an adjoint problem
Delhez, Eric ULg; Heemink, Arnold W.; Deleersnijder, Eric

in Estuarine Coastal & Shelf Science (2004), 61(4), 691-702

The residence time measures the time spent by a water parcel or a pollutant in a given water body and is therefore a widely used concept in environmental studies. While many previous studies rely on ... [more ▼]

The residence time measures the time spent by a water parcel or a pollutant in a given water body and is therefore a widely used concept in environmental studies. While many previous studies rely on severe hypotheses (assuming stationarity of the flow and/or neglecting diffusion) to evaluate the residence time, the paper introduces a general method for computing the residence time and/or the mean residence time without such simplifying hypotheses. The method is based on the resolution of an adjoint advection-diffusion problem and is therefore primarily meant to be used with numerical models. The method and its implications are first introduced using a simplified one-dimensional analytical model. The approach is then applied to the diagnostic of the three-dimensional circulation on the Northwest European Continental Shelf. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲]

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See detailSymmetry and asymmetry of water ages in a one-dimensional flow
Deleersnijder, Eric; Delhez, Eric ULg

in Journal of Marine Systems (2004), 48(1-4), 61-66

Hall and Haine [J. Mar. Syst., in press] briefly addressed the problem of estimating the age of irreducible fluid elements or water parcels in a one-dimensional flow with constant velocity and diffusivity ... [more ▼]

Hall and Haine [J. Mar. Syst., in press] briefly addressed the problem of estimating the age of irreducible fluid elements or water parcels in a one-dimensional flow with constant velocity and diffusivity. Herein further developments are achieved on this subject. The age of every water parcel is set to zero at the moment it passes through the point x = 0, where x is an appropriate space coordinate. As time progresses, the age of the water is seen to increase unboundedly upstream of the point x = 0, and tend to the steady-state value \x/u\ downstream of the point x = 0, where a is the water velocity. By contrast, the age of the water parcels that have touched at least once the point x = 0 is symmetric with respect to the point x = 0 is smaller than the water age at any tune and position, and tends to \x/u\ as time progresses. Asymptotic expansions are derived for large times. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

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See detailA trust region algorithm for automatic identification of elasto-viscoplastic model parameters in metallurgical finite element model
Jeunechamps, Pierre-Paul ULg; Walmag, Jérôme; Duysinx, Pierre ULg et al

in Steel Grips 2 (2004), Supplement Metal Forming 2004

Simulation of manufacturing processes, here metal forming, has made major progresses during the last years. The virtual simulation of the manufacturing process being now quite well established, a natural ... [more ▼]

Simulation of manufacturing processes, here metal forming, has made major progresses during the last years. The virtual simulation of the manufacturing process being now quite well established, a natural step ahead consists in trying to find automatic procedures to optimize it. Another perspective is also to take care of the influence of the manufacturing constraints upon the design, which is the long term objective of the present research project. It aims at optimizing the design subject to both service constraints (stiffness and strength) and manufacturing constraints. A preliminary stage is to be able to build high fidelity digital models. Complex models require more and more material parameters for behavior laws (e.g. material constitutive and friction laws), which have to be identified numerically from experimental data. In this material parameter identification process, three steps can be identified. At first, an experimental testing is carried out. The second step consists in building a simulation model of the experiment. Finally, the unknown model parameters are determined to match the experimental data. A standard identification procedure consists in minimizing a given norm (here the Euclidean norm) of the error between the model predictions and the experimental results. Even if the identification problem is generally quasi-unconstrained, it has the same complexity as structural optimization problems because of the highly nonlinear and implicit character of the functions, which is especially amplified by the large deformation simulation analysis. In this paper, we use an approach of elastoplastic calculation by finite elements combined with two optimization algorithms: a Levenberg-Marquardt algorithm, which is rather classical in the literature for solving identification problems (see [2]) and a trust-region one (see [1]), which is a rather novel approach at least for structural problems. The results obtained with these two methods are then compared and discussed on two test cases. The first application is an academic test case to validate the identification method. The second one, the compression of a cylinder, takes into account an actual experiment. In this application, the material is assumed to be elasto-viscoplastic and described by a Norton-Hoff behavior law and an isotropic strain hardening law. The parameters to be identified are the coefficients of the Norton-Hoff law. [less ▲]

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See detailIdentification of elastoplastic model in large deformation problems
Jeunechamps, Pierre-Paul ULg; Walmag, Jérôme ULg; Delhez, Eric ULg et al

in Cinquini, Carlo (Ed.) Proceedings of the 5th World Congress of Structural and Multidisciplinary Optimization WCSMO5 (2003, May)

This paper reports on preliminary results of parameter identification problems in finite element non-linear analyses such as metal forming simulations. Two approaches are compared. The first one is the ... [more ▼]

This paper reports on preliminary results of parameter identification problems in finite element non-linear analyses such as metal forming simulations. Two approaches are compared. The first one is the classic Levenberg-Marquardt algorithm. The second one is a trust-region algorithm based on a quadratic model. The two algorithms are compared on two cases, one of them being an actual experiment. [less ▲]

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See detailGlobal water and salt budget of the Aral Sea from 1960 to 1991.
Sirjacobs, Damien ULg; Grégoire, Marilaure ULg; Delhez, Eric ULg et al

Conference (2003, May)

Focusing on the global water and salt budget of the Aral sea from 1960 till 1991, a compilation of data available in the litterature is analysed with simple models and compared to the measured evolution ... [more ▼]

Focusing on the global water and salt budget of the Aral sea from 1960 till 1991, a compilation of data available in the litterature is analysed with simple models and compared to the measured evolution of the sea. [less ▲]

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See detailInfluence of the Aral Sea negative water balance on its seasonal circulation and ventilation patterns : use of a 3D hydrodynamic model
Sirjacobs, Damien ULg; Grégoire, Marilaure ULg; Delhez, Eric ULg et al

in Geophysical Research Abstracts (2003, April), 5(09052), 2003509052

Within the context of the EU INCO-COPERNICUS program "Desertification in the Aral Sea Region: A study of the Natural and Anthropogenic Impacts" (Contract IAC2-CT-2000-10023), a large-scale 3D hydrodynamic ... [more ▼]

Within the context of the EU INCO-COPERNICUS program "Desertification in the Aral Sea Region: A study of the Natural and Anthropogenic Impacts" (Contract IAC2-CT-2000-10023), a large-scale 3D hydrodynamic model was adapted to address specifically the macroscale processes affecting the Aral Sea water circulation and ventilation. The particular goal of this research is to simulate the effect of lasting negative water balance on the 3D seasonal circulation, temperature, salinity and water-mixing fields of the Aral Sea. The original Aral Sea seasonal hydrodynamism is simulated with the average seasonal forcings corresponding to the period from 1956 to 1960. This first investigation concerns a period of relative stability of the water balance, before the beginning of the drying process. The consequences of the drying process on the hydrodynamic of the Sea will be studied by comparing this first results with the simulation representing the average situation for the years 1981 to 1985, a very low river flow period. For both simulation periods, the forcing considered are the seasonal fluctuations of wind fields, precipitation, evaporation, river discharge and salinity, cloud cover, air temperature and humidity. The meteorological forcings were adapted to the common optimum one-month temporal resolution of the available data sets. Monthly mean kinetic energy flux and surface tensions were calculated from daily ECMWF wind data. Monthly in situ precipitation, surface air temperature and humidity fields were interpolated from data obtained from the Russian Hydrological and Meteorological Institute. Monthly water discharge and average salinity of the river water were considered for both Amu Darya and Syr Darya river over each simulation periods. The water mass conservation routines allowed the simulation of a changing coastline by taking into account local drying and flooding events of particular grid points. Preliminary barotropic runs were realised (for the 1951-1960 situation, before drying up began) in order to get a first experience of the behaviour of the hydrodynamic model. These first runs provide results about the evolution of the following state variables: elevation of the sea surface, 3D fields of vertical and horizontal flows, 2D fields of average horizontal flows and finally the 3D fields of turbulent kinetic energy. The mean seasonal salinity and temperature fields (in-situ data gathered by the Russian Hydrological and Meteorological Institute) are available for the two simulated periods and will allow a first validation of the hydrodynamic model. Various satellites products were identified, collected and processed in the frame of this research project and will be used for the validation of the model outputs. Seasonal level changes measurements derived from water table change will serve for water balance validation and sea surface temperature for hydrodynamics validation. [less ▲]

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See detailIdentification of elastoplastic model parameters in large deformation problems using gamma methods
Jeunechamps, Pierre-Paul ULg; Walmag, J.; Mathonet, V. et al

in Proceedings of the 5th World Congress on Structural and Multidisciplinary Optimization (2003)

This paper reports on preliminary results of parameter identification problems in finite element non-linear analyses such as metal forming simulations. Two approaches are compared. The first one is the ... [more ▼]

This paper reports on preliminary results of parameter identification problems in finite element non-linear analyses such as metal forming simulations. Two approaches are compared. The first one is the classic Levenberg-Marquardt algorithm. The second one is a trust-region algorithm based on a quadratic model. The two algorithms are compared on two cases, one of them being an actuel experiment. [less ▲]

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See detailMécanique Rationnelle - Théorie et Applications
Delhez, Eric ULg

Book published by Etienne Riga (2003)

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See detailA spline interpolation technique that preserves mass budgets
Delhez, Eric ULg

in Applied Mathematics Letters (2003), 16(1), 17-26

The usual practice of forcing budget models by linear interpolations of mean data does not produce a forcing whose mean is the data value required. The usual third-order spline is modified into a fourth ... [more ▼]

The usual practice of forcing budget models by linear interpolations of mean data does not produce a forcing whose mean is the data value required. The usual third-order spline is modified into a fourth-order spline, called mc-spline, to cope with this issue. The technique provides a smooth and faithful continuous interpolation of the original data that is well suited for its graphical representations or for the forcing of numerical models. (C) 2002 Elsevier Science Ltd. All rights reserved. [less ▲]

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See detailA note on the age of radioactive tracers
Delhez, Eric ULg; Deleersnijder, Eric; Mouchet, Anne ULg et al

in Journal of Marine Systems (2003), 38(3-4), 277-286

wThe age of a water mass is often estimated experimentally using the radio-age computed from the distribution of a radioactive tracer (radiocarbon, helium-tritium). Deleersnijder et al. [J. Mar. Syst. 28 ... [more ▼]

wThe age of a water mass is often estimated experimentally using the radio-age computed from the distribution of a radioactive tracer (radiocarbon, helium-tritium). Deleersnijder et al. [J. Mar. Syst. 28 (2001) 229.] have shown that the radio-age underestimates the age of the water and is larger than the age of the radioactive tracer used for its evaluation. This result is generalized here to radio-ages computed from the ratio of two radioactive tracers. The differences between the different ages are also studied analytically and numerically as functions of the decay rate of the radioactive tracers. For small decay rates, the difference between the age of the water mass and the radio-age is shown to be proportional to the decay rate. It depends also on the level of mixing in the system; even radioactive tracers with small decay rates can provide poor estimates of the age of the water mass in a strongly diffusive flow. For small half lives, both the radio-age and the age of radioactive tracers decrease as the inverse of the square root of the decay rate. The same analysis applies to some extent to the estimates of the age of a water mass from stable tracers with known time dependent sources (e.g. chloroflurocarbons). (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailCAT, The Constituent-oriented Age Theory, and its application to marine flows
Deleersnijder, Eric; Delhez, Eric ULg; Mouchet, Anne ULg et al

Conference (2002, May)

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See detailThe concept of age in marine modelling II. Concentration distribution function in the English Channel and the North Sea
Delhez, Eric ULg; Deleersnijder, Eric

in Journal of Marine Systems (2002), 31(4), 279-297

The age of seawater and the age of real or idealized tracers are often used as diagnostic tools to better understand complex hydrodynamic flows. In most studies, the focus is on some averages of the ages ... [more ▼]

The age of seawater and the age of real or idealized tracers are often used as diagnostic tools to better understand complex hydrodynamic flows. In most studies, the focus is on some averages of the ages of the different particles making up a water parcel. The theory developed in Delhez et al. [Ocean Modell. I (1999) 17] and Deleersnijder et al. [J. Mar. Syst. 28 (2001) 229] provides, however, a more detailed description of the distribution of the ages of these particles through the so-called concentration distribution function. In this paper, the numerical aspects of the resolution of the evolution equation for the concentration distribution function in a five-dimensional space (time x 3D space x age dimension) are developed. Evolution equations for the moments of the concentration distribution function up to any order are also derived. A real case application of this theory to the simulation of the advection-dispersion of tracers (technetium-99) discharged at the nuclear fuel reprocessing plant of Cap de La Hague is described. The comparison of the results with those from previous studies demonstrates the advantages of the new method for the computation of the mean age. In particular, the method provides a detailed description of the temporal variations of the mean age only. The analysis of the full concentration distribution function and of its basic statistics shows that the standard deviation of the age of the different particles is far from negligible and should never be overlooked when analyzing age fields. A simplified analytical example suggests that the standard deviation of the age distribution is a measure of the integrated diffusion undergone by the tracer along its path from the source to the observation point. (C) 2002 Elsevier Science B.V All rights reserved. [less ▲]

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See detailTransient behaviour of water ages in the World Ocean
Deleersnijder, Eric; Mouchet, Anne ULg; Delhez, Eric ULg et al

in Mathematical & Computer Modelling (2002), 36(1-2), 121-127

The transient behaviour of the age of the water and that of the surface water is studied in the World Ocean. At any time and position, the age of the water, the age and the concentration of the surface ... [more ▼]

The transient behaviour of the age of the water and that of the surface water is studied in the World Ocean. At any time and position, the age of the water, the age and the concentration of the surface water are seen to obey a simple algebraic relation. The latter is illustrated by means of results of a three-dimensional World Ocean model and the analytical solution of an idealised, purely-diffusive, one-dimensional problem. (C) 2002 Elsevier Science Ltd. All rights reserved. [less ▲]

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See detailResidual circulation and thermohaline distribution of the Ria de Vigo: a 3-D hydrodynamical model
Lopez, S. T.; Varela, R. A.; Delhez, Eric ULg

in Scientia Marina (2001), 65(Suppl. 1), 277-289

A three-dimensional, non-linear, baroclinic model is described and tested for the first time to study the residual circulation and the thermohaline distribution of the Ria de Vigo (NW Spain) at short time ... [more ▼]

A three-dimensional, non-linear, baroclinic model is described and tested for the first time to study the residual circulation and the thermohaline distribution of the Ria de Vigo (NW Spain) at short time scales and under different wind stress regimes. Two markedly different realistic scenarios were chosen: northerly upwelling-favourable winds and southerly downwelling-favourable winds. The numerical experiments carried out indicate that the hydrodynamic regime of the Ria de Vigo is mostly a consequence of wind events. As could be expected, moderately strong North winds reinforce the normal (positive) Ria circulation, while winds blowing from the South, when sufficiently strong, reverse the typical circulation pattern and reduce the characteristic outgoing velocities and the flushing time inside the Ria. The temperature and salinity fields generated by the model in both situations were compared with observations and found to be in qualitatively good agreement, supporting the 3D velocity field distribution. [less ▲]

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See detailThe concept of age in marine modelling I. Theory and preliminary model results
Deleersnijder, Eric; Campin, Jean-Michel; Delhez, Eric ULg

in Journal of Marine Systems (2001), 28(3-4), 229-267

The age of a particle of a seawater constituent is defined to be the time elapsed since the particle under consideration left the region, in which its age is prescribed to be zero. An Eulerian theory of ... [more ▼]

The age of a particle of a seawater constituent is defined to be the time elapsed since the particle under consideration left the region, in which its age is prescribed to be zero. An Eulerian theory of the age is presented, in which advection, diffusion, production and destruction phenomena are properly accounted for. The key hypothesis is that the mean age of a set of particles is to be evaluated as the mass-weighted average of the ages of the particles under study. The basic variable is the concentration distribution function, representing, at a given time and location, the distribution over the age of the concentration of the constituent being considered. This function satisfies a partial differential equation, which, upon appropriate integration over the age, yields the equations, in flux form, governing the evolution of the concentration and the age concentration. The ratio of the latter variable to the former is the mean age. Further theoretical developments are presented, including a thought experiment showing that mixing processes cause the ages of various constituents to be different from each other. The potential of the age as a tool for understanding complex marine flows is briefly demonstrated by analysing the results of two numerical models. The ages of a passive tracer, a radioactive tracer and the water are computed, along with a suitably defined radio-age. First, the fate of tracers released into the English Channel at La Hague is simulated. Then, ages are computed in the World Ocean as a measure of the time that has elapsed since leaving the surface layers. A theorem is demonstrated, which specifies that the age of the radioactive tracer must be smaller than the relevant radio-age, the latter being smaller than the age of the passive tracer, which, under appropriate hypotheses, can be seen to be equivalent to the age of the water. These inequalities seem to be remarkably robust, since they are found to hold valid in most of the numerical and analytical results examined in the present study. On the other hand, a dimensionless number is highlighted, which is believed to play an important role in the scaling of the differences between ages. (C) 2001 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailIntegrated modelling of the Belgian Coastal Zone
Delhez, Eric ULg; Carabin, G.

in Estuarine Coastal & Shelf Science (2001), 53(4), 477-491

The management of the water resources in coastal or delta plains asks for an integrated modelling of the water system at a regional scale. In the SALMON project, detailed descriptions of the groundwater ... [more ▼]

The management of the water resources in coastal or delta plains asks for an integrated modelling of the water system at a regional scale. In the SALMON project, detailed descriptions of the groundwater, river and marine domains are provided by coupling appropriate numerical models of these different sub-systems. The application of this three-fold model to the Scheldt and Belgian Coastal Zone reveals a marked river plume extending along the Belgian Coast with strong offshore gradients. This plume cannot be explained by the Eulerian residual transport velocity field but requires Lagrangian aspects to be taken into account. The computation of the age of this water mass shows that the different parts of the coastal zone are influenced by a discharge in the Grote Schijn River after a delay that varies between a few days (for the marine part of the estuary) to about 100 days (for the centre of the residual gyre). The computation shows also that the groundwater around Antwerp can also be contaminated by a pollution in the river but the resulting mass loss from the river does not lead to a significant reduction of the flux to the coastal zone. (C) 2001 Academic Press. [less ▲]

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See detailSome properties of generalized age-distribution equations in fluid dynamics
Beckers, Jean-Marie ULg; Delhez, Eric ULg; Deleersnijder, Eric

in SIAM Journal on Applied Mathematics (2001), 61(5), 1526-1544

The concept of age in fluid dynamics is analyzed in the case of a tracer advection-diffusion equation. From the general solution in a uniform velocity field, it is shown that unexpected symmetry ... [more ▼]

The concept of age in fluid dynamics is analyzed in the case of a tracer advection-diffusion equation. From the general solution in a uniform velocity field, it is shown that unexpected symmetry properties arise for the age field. In particular, for a point release, the age field is isotropic, regardless of the direction of the ow and the value of the diffusion coefficient. The analysis is then extended to situations with time-varying currents, where the symmetry can be broken under some circumstances. Finally, we show a method by which a time-dependent problem can be used to assess a stationary concentration distribution function, providing details about the propagation of younger and older material at a given location. [less ▲]

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