Adaptation of the lagrangian module of a cfd code for atmospheric dispersion of pollutants in complex urban geometries and comparison with existing eulerian results

In the general context of atmospheric dispersion in urban neighborhood or around an industrial site, Lagrangian models consist in calculating and tracking the trajectories of particles of pollutant emitted into the turbulent atmosphere. These models are particularly suitable for the study of complex, unsteady or inhomogeneous flows, which is precisely the case of atmospheric flows in urban areas and complex industrial sites. They are also recommended to deal with dispersion near the sources. Usually, these models use wind and turbulence fields computed by an external code. In this work, the objective is to compare the Lagrangian and the Eulerian atmospheric dispersion modules in the same Computational Fluid Dynamics (CFD) open source code (Code_Saturne), therefore using the same wind and turbulence field for both . For each simulation we want to compare the turbulent dispersion of pollutants obtained with the Lagrangian approach to the existing results previously obtained with the Eulerian methods. The stochastic Lagrangian model used in this work is the Simple Langevin Model (SLM) of Pope (2001) and pertains to the approaches referred to as PDF (Probability Density Function) methods. To our knowledge, this formulation of model has not previously been used in the context of atmospheric dispersion. In this paper, we first show tha t our model respects the well-mixed criterion. Then, we validate our model in the case of a continuous punctual release with uniform mean wind speed and turbulent diffusivity, by checking with the existing analytical solution. Finally, we validate the model with several experimental campaigns, considering atmospheric stratification and buildings. The first field experiment program considered in this paper has been conducted on the ‘SIRTA’ site (Site Instrumental de Recherche par Télédétection Atmosphérique), in the southern suburb of Paris, and involves a stably stratified surface layer.