Validation du code NS3IFS et comparaison avec des données expérimentales pour des écoulements autour de tabliers de ponts

Dans ce rapport, nous nous intéressons aux effets du vent autour de profils de pont élémentaires. En régime laminaire, plusieurs études ont déjà été effectuées ou sont en cours [19, 1]. Elles ont permis de reproduire quelques caractéristiques globales comme les coefficients aéroélastiques. Cependant, la présence de l'obstacle génère de la turbulence dont l'effet sur le champ de pression qui se développe autour du profil est mal connu. L'objectif est ici de tester et valider les modèles de turbulence mis au point par N. Devaux [6] dans les cas statiques. Les données disponibles fournies par le CSTB de Nantes portent sur les champs de pression autour de trois maquettes : rectangle mince ou épais, viaduc de Millau. L'étude est focalisée sur l'utilisation du modèle k-ε dont on donne une description complète. Des tests de sensibilité sont effectués avant de procéder aux simulations proprement dites. Globalement, les résultats sont assez encourageants. Même si le modèle k-ε présente quelques déficiences dans les zones de recirculation, il reproduit qualitativement et quantitativement les observations expérimentales

Impact of a flood event on the biogeochemical behaviour of a mesotrophic alpine lake: Lake Bourget (Savoy)

International audienceA flood event which occurred during the onset of thermal stratification and of algal growth (March 1996) was studied in mesotrophic Lake Bourget (France). Both physical and biogeochemical processes occurring during this episode were assessed. The dominant effect was a decrease of nutrient concentrations along the river-lake ecotone. This phenomenon seemed mainly linked to biological factors: stimulation of the spring algal and bacterial growth. After the flood, the algae aggregated with allochthonous particles brought by the river and were rapidly eliminated by sedimentation. The effect of this early spring flood, seemed spatially restricted and rather neutral as regards lake water quality. In the lake region crossed by the river plume, the overall effect after the flood was a decrease of P available for the phytoplankton. In the open water, the P concentration was probably not modified by the load supplied by the flood

Impact of a flood event on the biogeochemical behaviour of a mesotrophic alpine lake: Lake Bourget (Savoy)

International audienceA flood event which occurred during the onset of thermal stratification and of algal growth (March 1996) was studied in mesotrophic Lake Bourget (France). Both physical and biogeochemical processes occurring during this episode were assessed. The dominant effect was a decrease of nutrient concentrations along the river-lake ecotone. This phenomenon seemed mainly linked to biological factors: stimulation of the spring algal and bacterial growth. After the flood, the algae aggregated with allochthonous particles brought by the river and were rapidly eliminated by sedimentation. The effect of this early spring flood, seemed spatially restricted and rather neutral as regards lake water quality. In the lake region crossed by the river plume, the overall effect after the flood was a decrease of P available for the phytoplankton. In the open water, the P concentration was probably not modified by the load supplied by the flood

A contribution to the study of thermal and hydrodynamical behaviour of lake Bourget: density currents and internal waves

National audienceUnderstanding the ecological evolution of a lake requires a good knowledge of the main physical processes which occur there. The thesis analyses two mechanisms closely associated with the hydrodynamic and thermal behaviour of lake Bourget (France) the density currents, which are driven by the river inflows, and the internal waves, which are generated by wind events. Two approaches have been carried out : field surveys (wind data, temperatures and currents in the lake have been collected) on the one hand, and mathematical modelling on the other hand. Both for density currents and internal waves, the mathematical approaches have been validated for academic cases simplified geometry, thermal stratification with two layers. At the environmental scale, numerical results are in good agreement with most of the observations. Following this study, a coupling with anecological model will be implemented

Assessing greenhouse climate using CFD: a focus on air humidity issues

International audienceHumidity control is a key factor in greenhouse management as it impacts the growth of most greenhouse crops. Excessive humidity and the presence of droplets may enhance the development of fungal diseases or affect optical properties of the cover material. For that reason, growers have to adapt their climatic strategy in order to warranty an optimal environment to the plants. The mechanisms involving water vapour transfer inside greenhouses include air exchanges processes through vents or leakage, plant transpiration, condensation and evaporation. To analyse and predict these mechanisms, modelling is a promising approach. Among the numerical tools, Computational Fluid Dynamics (CFD) appears as a powerful mean to simulate the climate inside greenhouses. Important developments in CFD have been made in the last decade to assess air humidity patterns. They involve specific requirements: in particular the resolution of the species transport equations both for water vapour and air. Submodels also have to be activated for taking account of sink or source terms of water vapour. Recent developments include the transpiration process at canopy level as well as condensation phenomena or droplets transport associated to fog systems. Water vapour transfers are also generally coupled with heat transfers. CFD studies concerning air humidity inside greenhouses have been conducted at different scales: the leaf, the canopy and the greenhouse itself. Small scale works were carried out essentially to assess singular phenomena and provide validation before extending applications to larger scales. At the scale of the greenhouse itself, the main concern is how to find solutions for dehumidifying air. The ventilation-heating combination is still commonly used. Its main drawback however is its high-energy cost. Research is still on-going on low-energy consumption systems for dehumidification and CFD proved to be an efficient tool to find alternative solutions to the simultaneous ventila¬tion-heating process. Other topics of interest are cooling systems based on evaporative pads or fogging

Numerical simulations of wind effects on flexible civil engineering structures

International audienc

Numerical simulations of wind effects on flexible civil engineering structures

International audienc

Co-simulation of the Microclimate inside a Greenhouse by Coupling Energy and CFD Models

International audienceEnergy Simulation (ES) is an interesting tool to explore and predict the evolution of the mean climatic parameters inside a greenhouse. A limitation of this tool is that it requires the ventilation rate and the heat transfer coefficients along the walls as inputs. This information may be obtained from Computational Fluid Dynamics (CFD). The present study focuses on the coupling of a CFD code with an ES program. The coupling consists in exchanging data between ES and CFD. ES uses the ventilation rate and the convective heat coefficients provided by CFD. Conversely ES provides the wall temperatures to the CFD code as updated boundary conditions. In a preliminary stage, the greenhouse was represented by a 3D geometry and the calcula¬tion domain reduced to the greenhouse itself i.e., without surroundings. The venti¬lation rate was computed by applying the Mean Age of Air (MAA) technique in the CFD code, which implies the resolution of an additional scalar equation. The internal convective heat coefficients were also computed and transmitted to the ES model via a specific routine written under the TRNSYS environment. The obtained CFD results for the MAA were first compared with experimental data available in the literature. The mean temperature and humidity predicted by the co-simulation ES-CFD were then compared with experimental data recorded inside a plastic tunnel greenhouse located in Angers (France) and the effects of ventilation caused by the opening of the door on the greenhouse microclimate were analysed. One limitation of the approach however, is that the restriction of the CFD calculation domain to the greenhouse does not make it possible yet the CFD calculation of external heat transfers coefficients used in ES, which are still estimated from empirical correlations. This drawback should be overcome in the next developments by extending the CFD-grid to the surroundings of the greenhouse