Modélisation du comportement hydraulique des déversoirs d’orage latéraux en régime transcritique

Le comportement hydraulique des déversoirs d’orage latéraux est le plus souvent marqué par une évolution discontinue de la ligne d’eau caractérisée par le ressaut hydraulique (Torrentiel/fluvial) et, dans certains cas, par une évolution rapidement variée causée par un écoulement également transcritique mais dans le sens fluvial/Torrentiel. Concernant les modèles actuels, ils ne permettent pas de simuler le comportement en transitoire de ce type d’ouvrage. Compte tenu de cela, nous avons modélisé l’ouvrage par les équations de Barré de Saint Venant écrite sous forme conservative en régime transitoire et couplée au modèle de déversoir de Hager. Le caractère conservatif de ces équations permet de transcrire dans un seul système d’équations les écoulements graduellement et rapidement variés. Afin de résoudre ces équations, nous avons utilisé le schéma numérique UPWIND à « capture de choc » du second ordre du type TVD (Total Variation Diminishing) utilisant le solveur de Roe. Dans l’objectif de valider notre démarche, nous avons créé un pilote de déversoir d’orage sur le site d’Obernai (laboratoire d'hydraulique du lycée agricole). Nous avons fait varier : le diamètre de la conduite aval par rapport au diamètre de la conduite amont, la longueur du déversoir, les pentes des conduites amont et aval ainsi que la hauteur de crête. La comparaison des débits déversés entre les modèles physique et numérique a montré que l’erreur observée rapportée au débit amont n’excédait jamais 13 % avec une majorité des cas entre ±5%.In recent years, French and European legislation has introduced regulations about wastewater discharge into natural environments and particularly about combined sewer overflows. As a consequence, it has become essential to control the hydraulic behaviour of these structures and to estimate the pollution loads released at this level. The side weir is the regulation structure that permits the hydraulic regulation of the waste water carried by the sewer system. When the upstream flow intensity exceeds a value referred to as reference flow, the side weir directly rejects part of the waste water to the natural environment. The hydraulic behaviour of the sewer side weir was shown to involve a discontinuous evolution of the water depth, characterized by a hydraulic jump (transition from supercritical to subcritical flow) and also by a rapidly varying transcritical evolution (subcritical to supercritical).Initially, the side weir flow was determined with the use of empirical relations. Using formulae of Engels, Coleman and Smith, Balmaceda and Gonzales or Dominguez, it was possible to calculate the outflow according to the water level at the upstream and/or the downstream region of the weir. These relations were applicable only for certain flow regimes and in certain cases in which the geometry of the side weir was specified. Subsequently, a more physical approach, initiated by Ackers, was based on the assumption of constant energy along the side weir. This approach made it possible to focus not only on an assessment of the side channel flow, but also on the water profile at the crest. Unfortunately, as the study of El Kashab shows, this method falls short in certain cases because the equations are inappropriate. For example, in the case of hydraulics the constant energy approach was not applicable. Finally, the method that is currently used is based on a momentum equation, which makes it possible to establish equations for shallow water. This approach seemed the most appropriate in the case of the side weir. The numerical solution of these equations was always based on an algorithm that describes all the possible cases according to the flow regime and the hydraulic conditions in the side weir. One must know the flow regime a priori. These models don’t properly simulate the transitory behaviour of these kinds of works.In this article we propose hydraulic modelling of a sewer side weir that integrates the geometrical characteristics of the flow (height and length of the crest, variation of width along the crest), and avoids the need for a priori knowledge of flow conditions in the side weir. The model also takes into account hydraulic discontinuities (hydraulic jump, transitions from free surface to pressurised flow) and the transitory character of the flow. The numerical results were compared with measured values obtained from a test bench.For the 1D approach, the solution was found using the 1D shallow water equations written in a conservative form for a transitory situation. The conservative characters of the equations permit us to consider gradually and rapidly varied flows in a single system of equations. In order to account for the lateral overflow, we used the Hager relation, which involves the intensity and direction of the lateral velocity vector and also the influence of width variation of the side weir. The shallow water equations system couldn’t be analytically solved. As a consequence, numerous numerical methods have been developed such as characteristic methods or finite difference methods. Unfortunately, these methods are inadequate when discontinuities such as hydraulic jumps or flow regime transitions (froude number close to 1) appear. To solve these problems, numerical (shock capturing) schemes were developed, based on a finite volume formulation. Godunov was the initiator of this type of finite volume numerical scheme. Eleuterio improved the precision and the ability of these numerical schemes to converge. The result was a combination of total variation diminishing (TVD) interpolation with an appropriate Riemann solver. We used a second order TVD Upwind shock capturing numerical method associated with the Roe Riemann solver.In order to validate the numerical model, we have built a sewer side weir physical test bench at the Obernai site. The variable parameters were the downstream pipe diameter in comparison with the upstream pipe diameter, the side weir length, slopes of the upstream and downstream pipes and the height of the crest. The tested cases permitted us to sweep a slope ranging from 0.5 ‰ to 1 % for the upstream and downstream pipes.Globally, 114 configurations have been tested with the 1D numerical model. The operating curve represents a criterion for characterising the operating of the storm overflow. As long as the upstream flow does not reach the reference value, there is no overflow. As soon as the upstream flow exceeds the value, the downstream flow remains close to the reference value. Because comparisons were made in relation with the upstream flow, the criterion used for judging the modelling performance was the absolute error value in relation to the upstream flow. It is important to note that the results need to be weighted as experimental measurements have a margin of error of approximately 5%.Comparisons between numerical and experimental results permit the following conclusions:1. The distribution of the number of errors was very close to the normal Gaussian distribution curve, with a slight shift towards the positive side. This indicates that the errors were random and therefore very close to experimental values.2. In general, the maximum errors varied from -10 to 13%, with the majority of cases occurring between –5 and +5%. This shows that the performance of the tool was very useful in cases that are as complex as side weirs. However, the model has a slight tendency to overestimate the overflow rate as compared to experimental measurements

Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature

The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc electric field at different temperatures has been studied. In particular, the temperature dependence of the characteristic time constant has been studied both theoretically and experimentally, showing a strongly decreasing time constant with increasing temperature

Effect of total pressure on the formation and size evolution of silicon quantum dots in silicon nitride films

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiN(x)) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell

Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)

International audienceInAs nanostructures were grown on In 0.52 Al 0.48 As alloy lattice matched on InP001 substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (24) to (21) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity either as quantum wires or as quantum dots. Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization

Charge photo-carrier transport from silicon nanocrystals embedded in SiO_2-based multilayer structures

© 2012 American Institute of Physics This work was supported by the French National Agency for Research (ANR) through the DUOSIL project. Financial support has also been partly funded by the RhôneAlpes region in the frame of the PHOSIL project.Experimental investigation of photoconductivity in Si-rich silicon oxide (SRSO)/SiO_2 multilayer (ML) structures prepared by magnetron reactive sputtering is reported. Photocurrent (PC) measurements show that the PC threshold increases with decreasing the thickness of SRSO layer. Photo-conduction processes in our samples are shown to be dominated by carrier transport through quantum-confined silicon nanocrystals embedded in the SiO_2 host. In addition, the observed bias-dependence of photocurrent intensity is consistent with a model in which carrier transport occurs by both tunneling and hopping through defect states in the silicon oxide matrix. A photocurrent density J_(ph) of 1-2mA cm^(-2) is extracted from our results. Although this photocurrent density along the ML absorber film is relatively low, the results presented in this work are believed to be a valuable contribution toward the implementation of all-Si tandem solar cells.French National Agency for Research (ANR)Rhône-AlpesDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu

Dynamic saturation of an intersublevel transition in self-organized InAs/In(x)A(1-x)lAs quantum dots

International audienceWe have observed a dynamic saturation of an intersublevel transition in InAs/InxAl1-xAs quantum dots related to the discrete nature of electron states using midinfrared femtosecond spectroscopy. This dynamic saturation is a consequence of the gradual filling of the discrete quantum-dot electron states due to the capture of electrons injected in the barrier. Our interpretation of the differential transmission experiments is confirmed by a comparison with a rate-equation model with the capture and intersublevel relaxation time as fit parameters yielding 10 ps and 1 ps, respectively. We discuss the mechanism responsible for these relaxation times

WEST Physics Basis

With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 907-12), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 075020) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L-H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas.EURATOM 63305

Optical anisotropy and photoluminescence temperature dependence for self-assembled InAs quantum islands grown on vicinal (001) InP substrates

International audienceIn this paper, we report on a detailed investigation of the effect of misorientated InP(OOl) substrates on the optical properties of InAs quantum islands grown by molecular beam epitaxy in the Stranski-Krastanow regime. Temperature-dependent photoluminescence and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization, near 40%, for the sample grown on the substrate with 2°off miscut angle towards [110] direction (2°F) and only 16% for the sample grown on the substrate with 2°off miscut angle towards [010] direction (2°B). This result pointing out the growth ofInAs quantum wires (QWr) on 2°F substrate and of quasi-isotropic InAs quantum dots (QD) on 2°B substrate. The luminescence remains strong at 300 K as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(OOl)

Toward understanding the dynamics of land change in Latin America : potential utility of a resilience approach for building archetypes of landsystems change

Rocha, Juan C. Stockholm University. Stockholm Resilience Centre. Stockholm, Suecia.Baraibar, Matilda M. Stockholm University. Department of Economic History and International Relations. Stockholm, Suecia.Deutsch, Lisa. Stockholm University. Stockholm Resilience Centre. Stockholm, Suecia.Bremond, Ariane de. University of Bern. Centre for Development and Environment. Bern, Suiza.Oestreicher, Jordan S. Universidade de Brasília. Centro de Desenvolvimento Sustentável. Distrito Federal, Brasil.Rositano, Florencia. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura. Buenos Aires, Argentina.Gelabert, Cecilia Corina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Economía, Desarrollo y Planeamiento Agrícola. Cátedra de Sistemas Agroalimentarios. Buenos Aires, Argentina.e17, 82 p.Climate change, financial shocks, and fluctuations in international trade are some of the reasons why resilience is increasingly invoked in discussions about land-use policy. However, resilience assessments come with the challenge of operationalization, upscaling their conclusions while considering the context-specific nature of land-use dynamics and the common lack of long-term data. We revisit the approach of system archetypes for identifying resilience surrogates and apply it to land-use systems using seven case studies spread across Latin America. The approach relies on expert knowledge and literature-based characterizations of key processes and patterns of land-use change synthesized in a data template. These narrative accounts are then used to guide development of causal networks, from which potential surrogates for resilience are identified. This initial test of the method shows that deforestation, international trade, technological improvements, and conservation initiatives are key drivers of land-use change, and that rural migration, leasing and land pricing, conflicts in property rights, and international spillovers are common causal pathways that underlie land-use transitions. Our study demonstrates how archetypes can help to differentiate what is generic from context dependant. They help identify common causal pathways and leverage points across cases to further elucidate how policies work and where, as well as what policy lessons might transfer across heterogeneous settings

A multidating approach applied to historical slackwater flood deposits of the Gardon River, SE France

A multidating approach was carried out on slackwater flood deposits, preserved in valley side rock cave and terrace, of the Gardon River in Languedoc, southeast France. Lead-210, caesium-137, and geochemical analysis of mining-contaminated slackwater flood sediments have been used to reconstruct the history of these flood deposits. These age controls were combined with the continuous record of Gardon flow since 1890, and the combined records were then used to assign ages to slackwater deposits. The stratigraphic records of terrace GE and cave GG were excellent examples to illustrate the effects of erosion/preservation in a context of a progressively self-censoring, vertically accreting sequence. The sedimentary flood record of the terrace GE located at 10 m above the channel bed is complete for years post-1958 but incomplete before. During the 78-year period 1880-1958, 25 floods of a sufficient magnitude (> 1450 m3/s) have covered the terrace. Since 1958, however, the frequency of inundation of the deposits has been lower:only 5 or 6 floods in 52 years have been large enough to exceed the necessary threshold discharge (> 1700 m3/s). The progressive increase of threshold discharge and the reduced frequency of inundation at the terrace could allow stabilisation of the vegetation cover and improved protection against erosion from subsequent large magnitude flood events. The sedimentary flood record seems complete for cave GG located at 15 m above the channel bed. Here, the low frequency of events would have enabled a high degree of stabilisation of the sedimentary flood record, rendering the deposits less susceptible to erosion. Radiocarbon dating are used in this study and compared to the other dating techniques.Eighty percent of radiocarbon dates on charcoals were considerably older than those obtained by the other techniques in the terrace. On the other hand, radiocarbon dating on seeds provided better results. This discrepancy between radiocarbon dates on charcoal and seeds is explained by the nature of the dated material (permanent wood vs. annual production and resistance to degradation process). Finally, we showed in this study that although the most common dating technique used in paleoflood hydrology is radiocarbon dating, usually on charcoal preserved within slackwater flood sediments, this method did not permitus to define a coherent age model. Only the combined use of lead-210, caesium-137, and geochemical analysis of mining-contaminated sediments with the instrumental flood record can be applied to discriminate and date the recent slackwater deposits of the terrace GE and cave GG.Une approche par datations multiples a été menée sur les dépôts de sédiments de crue retrouvés dans les gorges du Gardon (Languedoc). Le plomb-210, le césium-137 et l’analyse géochimique des sédiments contaminés par les mines ont été utilisés pour reconstruire l’histoire des dépôts de crue. Cette approche a été combinée avec l’historique des crues du Gardon depuis 1890 pour affiner l’approche géochronologique. Au niveau de la cavité GG et de la terrasse GE les analyses ont permis d’illustrer les mécanismes d’érosion-redéposition pouvant perturber les séquences de sédiments. Les enregistrements des dépôts de sédiments de crue sont complets sur la terrasse GE située à 10 m au-dessus du lit mineur à partir de 1958, et incomplets auparavant. Sur la période 1880-1958, 25 crues ont eu un débit suffisant (>1450 m3/s) pour recouvrir la terrasse, alors que l’on recense seulement 5 à 6 crues depuis 1958 (pour un débit de submersion supérieur à 1700 m3/s). L’augmentation progressive du débit de submersion et la fréquence moindre d’inondation de la terrasse ont pu permettre sur la période récente la stabilisation de la couche de surface par la végétation et limiter l’érosion lors de crues importantes. Les enregistrements semblent complets dans la cavité GG située à 15 m au-dessus du lit mineur. La fréquence moindre de submersion à ce niveau plus élevé peut expliquer une meilleure stabilisation des dépôts de crue, rendus moins vulnérables à l’érosion. La datation au radiocarbone a été testée même si elle ne permet pas de dater avec précision les dépôts de crue des derniers siècles. Sur la terrasse GE, 80% des dates au carbone 14 sur charbon de bois sont beaucoup trop vieilles par rapport aux autres techniques de datation, ce qui peut s’expliquer par une remobilisation/déstockage de charbons de bois anciens. Les datations radiocarbone sur graines donnent de meilleurs résultats, car issues d’une production annuelle, plus facilement décomposable et moins sujette à des dépôts intermédiaires. Au final, il apparaît que la technique de datation par radiocarbone sur des charbons de bois retrouvés dans les séquences sédimentaires n’est pas fiable, alors qu’elle est la plus pratiquée en paléo-hydrologie. L’utilisation combinée du plomb-210, du césium-137, de l’analyse géochimique des sédiments et des informations sur les crues historiques a permis de discriminer et de dater les différents dépôts de crue de ce dernier siècle dans la cavité GG et la terrasse GE