Couplages de modèles en hydrodynamique environnementale et hydrologie (sols, nappes, rivières, zones côtières)

Cette conférence présente une revue générale des problèmes de couplages de modèles en hydrodynamique environnementale et en hydrologie. On considère les couplages ou les interactions des écoulements à travers les sols, les nappes souterraines, les zones côtières (oscillations de la mer dans les structures poreuses), et les interactions eaux de surface/souterraines en milieu fluvial (cours d’eau, plaines d’inondation)...

Stochastic PDE's for water flow, solute transport and wave propagation phenomena in heterogeneous geologic media.

This lecture will present stochastic PDE's (Partial Differential Equations) to model various "transport" phenomena like water flow, solute transport and wave propagation, in heterogeneous geologic porous media. The material properties are represented by random functions of space F(x) (random fields). The resulting transport PDE’s contain random field coefficients, and their solutions are stochastic (randomly heterogeneous)

Equivalent permeability tensor in fractured media : an algebraic approach.

This work is part of an extensive investigation on the equivalent permeability of heterogeneous and fractured media. We focus here on the problem of Darcian/Poiseuille flow in an irregular network of fracture segments (in 2D) or conduits (in 2D or 3D). An exact algebraic relation between the mean flux vector(Q) and the mean hydraulic gradient (J) is developed through a mathematical analyzis of the network flow problem, based on concepts from graph theory, leading to a discrete definition of DIV and GRAD operators. The resulting equivalent permeability is a 2nd rank tensor Kij, not necessarily symmetric and not necessarily positive-definite. Its properties are analyzed for given types of boundary conditions and averaging procedures

New insights into the structural and functional involvement of the gate loop in AcrB export activity

AcrB is a major multidrug exporter in Escherichia coli and other Gram-negative bacteria. Its gate loop, located between the proximal and the distal pockets, have been reported to play important role in the export of many antibiotics. This loop location, rigidity and interactions with substrates have led recent reports to suggest that AcrB export mechanism operates in a sequential manner. First the substrate binds the proximal pocket in the access monomer, then it moves to bind the distal pocket in the binding monomer and subsequently it is extruded in the extrusion monomer. Recently, we have demonstrated that the gate loop is not required for the binding of Erythromycin but the integrity of this loop is important for an efficient export of this substrate. However, here we show that the antibiotic susceptibilities of the same AcrB gate loop mutants for Doxorubicin were unaffected, suggesting that this loop is not required for its export, and we demonstrate that this substrate may use principally the tunnel-1, located between transmembranes 8 and 9, more often than previously reported. To further explain our findings, here we address the gate loop mutations effects on AcrB solution energetics (fold, stability, molecular dynamics) and on the in vivo efflux of Erythromycin and Doxorubicin. Finally, we discuss the efflux and the discrepancy between the structural and the functional experiments for Erythromycin in these gate loop mutants

Structures of Gate Loop Variants of the AcrB Drug Efflux Pump Bound by Erythromycin Substrate.

Gram-negative bacteria such as E. coli use tripartite efflux pumps such as AcrAB-TolC to expel antibiotics and noxious compounds. A key feature of the inner membrane transporter component, AcrB, is a short stretch of residues known as the gate/switch loop that divides the proximal and distal substrate binding pockets. Amino acid substitutions of the gate loop are known to decrease antibiotic resistance conferred by AcrB. Here we present two new AcrB gate loop variants, the first stripped of its bulky side chains, and a second in which the gate loop is removed entirely. By determining the crystal structures of the variant AcrB proteins in the presence and absence of erythromycin and assessing their ability to confer erythromycin tolerance, we demonstrate that the gate loop is important for AcrB export activity but is not required for erythromycin binding.Medical Research Council; Wellcome TrustThis is the final version of the article. It first appeared from the Public Library of Science via http://dx.doi.org/10.1371/journal.pone.015915

Effect and propagation of water level fluctuations in a sloping sandy beach -- Unsaturated porous media II: numerical simulation test of single harmonic wave (long run)

In coastal processes, the strong water movements due to short periodic waves (such as sea swell) can induce irregular water level fluctuations in the swash zone and within the sandy beach. The measured water level fluctuations with very complex entry water level fluctuations in a wave canal with a sloping sandy beach were analyzed by using 7 capacitive sensors. Numerical simulations have also been implemented in order to complement the experimental water level signal analyses. In this paper, a numerical test of single harmonic wave (long run) with same computational domain and porous media properties as the experiment is conducted to understand the effect and propagation of the water level fluctuations in the unsaturated porous media. The key coupling parameter of the macro (sea water) porous and micro (sloping sandy beach) porous media in the Richards equation model is further identified by this numerical test, which is the basis to simulate and explain the complex experimental results

Macro-permeability distribution and anisotropy in a 3D fissured and fractured clay rock: ‘Excavation Damaged Zone’ around a cylindrical drift in Callovo-Oxfordian Argilite (Bure)

The Underground Research Laboratory at Bure (CMHM), operated by ANDRA, the French National Radioactive Waste Management Agency, was developed for studying the disposal of radioactive waste in a deep clayey geologic repository. It comprises a network of underground galleries in a 130 m thick layer of Callovo Oxfordian clay rock (depths 400–600 m). This work focuses on hydraulic homogenization (permeability upscaling) of the Excavation Damaged Zone (EDZ) around a cylindrical drift, taking into account: (1) the permeability of the intact porous rock matrix; (2) the geometric structure of micro-fissures and small fractures synthesized as a statistical set of planar discs; (3) the curved shapes of large ‘chevron’ fractures induced by excavation (periodically distributed). The method used for hydraulic homogenization (upscaling) of the 3D porous and fractured rock is based on a ‘frozen gradient’ superposition of individual fluxes pertaining to each fracture/matrix block, or ‘unit block’. Each unit block comprises a prismatic block of permeable matrix (intact rock) obeying Darcy’s law, crossed by a single piece of planar fracture obeying either Darcy or Poiseuille law. Polygonal as well as disc shaped fractures are accommodated. The result of upscaling is a tensorial Darcy law, with macro-permeability Kij(x) distributed over a grid of upscaling sub-domains, or ‘voxels’. Alternatively, Kij(x) can be calculated point-wise using a moving window, e.g., for obtaining permeability profiles along ‘numerical’ boreholes. Because the permeable matrix is taken into account, the upscaling procedure can be implemented sequentially, as we do here: first, we embed the statistical fissures in the matrix, and secondly, we embed the large curved chevron fractures. The results of hydraulic upscaling are expressed first in terms of ‘equivalent’ macro-permeability tensors, Kij(x,y,z) distributed around the drift. The statistically isotropic fissures are considered, first, without chevron fractures. There are 10,000 randomly isotropic fissures distributed over a 20 m stretch of drift. The resulting spatially distributed K ij tensor is nearly isotropic (as expected). At the scale of the whole EDZ, the global K FISSURES is roughly 5000 times larger than permeability matrix KM. The detailed distribution of the equivalent K FISSURES (x, y, z) defined on a grid of voxels is radially inhomogeneous, like the statistics of the disc fissures. In addition, a moving window procedure is used to compute detailed radial profiles of K FISSURES versus distance (r) to drift wall, and the results compare favorably with in situ permeability profiles (numerical vs. experimental boreholes at Bure’s GMR drift). Finally, including the large curved chevron fractures in addition to the random fissures, the resulting K ij (x, y, z) appears strongly anisotropic locally. Its principal directions are spatially variable, and they tend to be aligned with the tangent planes of the chevron fracture surfaces. The global equivalent Kij of the whole EDZ is also obtained: it is only weakly anisotropic, much less so than the local Kij’s. However, because of the radially divergent structure of the ‘chevrons’ (although not quite cylindrical in geometry), it is recognized that the global Kij due to chevrons lacks physical meaning as a tensor. Considering only the magnitude, it is found that the permeability due to ‘chevrons’ (K CHEVRONS ) is about 4 orders of magnitude larger than that due to statistical fissures (K FISSURES ), assuming a hydraulic aperture a CHEVRON = 100 microns. By a simple argument, K CHEVRONS would be only one order of magnitude larger than K FISSURES with the choice a CHEVRON = 10 microns instead of 100 microns. This significant sensitivity is due to several factors: the large extent of chevron fractures, the assumption of constant hydraulic aperture, and the cubic law behavior based on the assumption of Poiseuille flow. The equivalent macro-permeabilities obtained in this work can be used for large scale flow modeling using any simulation code that accommodates Darcy’s law with a full, spatially variable permeability tensor Kij(x)

Flash flood modeling with the MARINE hydrological distributed model

International audienceFlash floods are characterized by their violence and the rapidity of their occurrence. Because these events are rare and unpredictable, but also fast and intense, their anticipation with sufficient lead time for warning and broadcasting is a primary subject of research. Because of the heterogeneities of the rain and of the behavior of the surface, spatially distributed hydrological models can lead to a better understanding of the processes and so on they can contribute to a better forecasting of flash flood. Our main goal here is to develop an operational and robust methodology for flash flood forecasting. This methodology should provide relevant data (information) about flood evolution on short time scales, and should be applicable even in locations where direct observations are sparse (e.g. absence of historical and modern rainfalls and streamflows in small mountainous watersheds). The flash flood forecast is obtained by the physically based, space-time distributed hydrological model "MARINE'' (Model of Anticipation of Runoff and INondations for Extreme events). This model is presented and tested in this paper for a real flash flood event. The model consists in two steps, or two components: the first component is a "basin'' flood module which generates flood runoff in the upstream part of the watershed, and the second component is the "stream network'' module, which propagates the flood in the main river and its subsidiaries. The basin flash flood generation model is a rainfall-runoff model that can integrate remotely sensed data. Surface hydraulics equations are solved with enough simplifying hypotheses to allow real time exploitation. The minimum data required by the model are: (i) the Digital Elevation Model, used to calculate slopes that generate runoff, it can be issued from satellite imagery (SPOT) or from French Geographical Institute (IGN); (ii) the rainfall data from meteorological radar, observed or anticipated by the French Meteorological Service (Météo France); and (iii) the spatially distributed soil and other surface properties viewed from space (land cover map from SPOT or LANDSAT, main rivers, ...). The stream flood propagation model simulates flood propagation in main rivers by solving 1-D Saint Venant equations. The data required for this part of the model are the river morphology, topography and roughness. The MARINE model has already been used previously for real time flash floods forecasting in the frame of the PACTES project on "forecasting and anticipation of floods with spatial techniques'' (funded by the CNES and the French Ministry of Research) concerning the catastrophic 1999 flash flood that occurred in the South of France. The main advantages of MARINE are its ability to run on insufficiently gauged basins (with the help of satellite information) and to run in an operational mode for real-time flood forecasting

Equivalent Upscaled Hydro Mechanical Properties of a Damaged and Fractured Claystone Around a Gallery (Meuse / Haute Marne Underground Research Laboratory).

In this work, we present calculations and analyses of equivalent continuum (upscaled) coefficients describing the damaged, fissured and fractured claystone around an underground gallery. We focus here on mechanical and coupled hydro-mechanical properties of the damaged claystone (the upscaled Darcy permeability of the same claystone was studied in a previous paper focused on hydraulics without mechanical deformations). Concerning the geometric structure of the damaged clay stone around the cylindrical excavation, we use a hybrid 3D geometric model of fissuring and fracturing, comprising (a) a set of 10 000 statistical fissures with radially inhomogeneous statistics (size, thickness and density increasing near the wall), and (b) a deterministic set of large curved ‘chevrons’ fractures, periodically spaced along the axis of the drift according to a 3D chevron pattern. The hydro-mechanical coefficients calculated here are second- and fourth-rank tensors, which are displayed using ellipsoids. For simplicity, we also calculate equivalent isotropic coefficients extracted from these tensors: Young’s modulus (E), bulk modulus (K), Lame' shear modulus (m), Poisson’s ratio (Nu), Biot coefficient (B, stress–pressure coupling) and Biot modulus (M, pressure–fluid production coupling). All of these coefficients are affected by the degree of damage and fracturing, which increases near the wall of the gallery. Both 3D and ‘2D transverse’ distributions are analysed, on grids of 3D cubic voxels and 2D pixels, respectively. Global coefficients upscaled over the entire damaged and fractured zone are also analysed. Other types of averages are presented, for example, upscaled values over a cylindrical annular shell at various radial distances from the gallery wall. The relation to the degree of fracturing is discussed, including for instance the effect of fracturing on bulk and shear stiffnesses, and on the hydro-mechanical coupling coefficients of the damaged claystone

Hydrogen ion passivation of multicrystalline silicon solar cells

It has been recognized that hydrogen can be chosen to passivate the defects present in polycrystalline materials. Technically, the best approach is to use hydrogen ion implantation at low energy (0.5 to 5 keV) by means of a Kaufman or similar type ion source in order to reduce the processing time. For our multiple beam ion source, we have determined the effective concentration profile of the introduced hydrogen, the modification of the optical properties of the implanted wafers and the conditions under which two multicrystalline materials (POLYX and SILSO) will give the greatest improvement in solar cell performance