location: Cherbourg-Octeville Radioecology Laboratory (LRC) - Cherbourg (50)
Masters degree in Atmospheric Physico-chemistry or Environmental Sciences.
Age limit: 26 years old unless otherwise stated.
Studies of the impact of atmospheric releases must take into account the meteorological conditions representative of the considered site. Situations of low wind tend to cause the stagnation of pollutants close to their source of emission and are penalizing for dispersion. However, these situations are generally ignored or taken into account in an approximate way because they are very difficult to model. Improving the understanding and modelling of low wind situations is hampered by the difficulties of experimental characterisation (these situations cannot be reproduced in wind tunnels). Moreover, at the international level, few on-site measurement campaigns have been carried out for this type of conditions, especially in built environments. The existing data sets only partially cover situations of low wind and associated thermal stratifications. The built environment induces profound changes in flow, turbulence and therefore dispersion. CFD (Computational Fluids Dynamics) models are generally considered to be the most suitable for realistic modelling of these modifications. However, the application of this type of model in low wind conditions comes up against specific problems (in particular: more difficult numerical convergence, turbulence modelling, taking into account the "meandering", validity of the wall law). The thesis will focus on the study of dispersion in the near field, in a built environment (representative of an industrial site or an urban environment), in low wind conditions including cases of calm winds. The objectives of the thesis will be : - to acquire experimental data of tracer gas dispersion on site and to analyse them; - to improve the knowledge and understanding of the mechanisms that control atmospheric dispersion in low wind conditions; - to define and validate a modelling methodology (Eulerian and Lagrangian approach) allowing to take into account the specificities of these low wind situations. The scientific approach adopted within the framework of the thesis combines the experimental approach (use of a tracer gas) and CFD modelling (Code_Saturne, developed by EDF-R&D and CEREA) of dispersion in low wind conditions. The thesis will be supervised by CEREA, EDF-R&D and IRSN. The thesis will take place partly at IRSN (at the LRC in Cherbourg-en-Cotentin) and partly at CEREA (at the EDF-R&D premises in Chatou).