High resolution unsteady RANS simulation of wind, thermal effects and pollution dispersion for studying urban renewal scenarios in a neighborhood of Toulouse

Gao, Zhenlan ; Bresson, Raphaël ; Qu, Yongfeng ; Milliez, Maya ; De Munck, Cecile ; Carissimo, Bertrand

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Detailed, high resolution, unsteady RANS simulations are used to study short episodes of local pollution dispersion in the neighborhood of Bordelongue in Toulouse in the framework of the French ANR project EUREQUA (Haoues-Jouve et al. 2015). These urban areas consist of various types of buildings and obstacles: small houses, tower blocks, highway, local streets, vegetation areas, etc. The 3D geometry of this urban area was constructed with an in house tool developed around the open-source geometry and mesh generator SALOME, based on the available geophysical data from the French geographical institute (IGN). The open-source computational fluid dynamics (CFD) code Code_Saturne, with the atmospheric option developed at CEREA, was used to carry out the simulations. The vegetation composed of tall trees is considered as a porous volume which induces a drag force to the air flowing through it. The pollutants of the local traffic emissions are considered as passive scalars (no chemical reaction). The global meteorology, including stratification conditions, is taken into account using boundary conditions obtained from mesoscale simulations performed over the region, with a zoom over the city by the Meso-NH code and the TEB urban parameterization. The simulation results of the air flow and pollution dispersion are compared with measurements obtained with fixed stations especially set up in the area during the campaigns. A good agreement is found between the measurements and simulations in terms of wind velocity and air temperature. For the wind direction the agreement is only fair with a Mean Bias of nearly 25° but nevertheless we find a good agreement with the NOx concentration time series at a local measuring station inside the neighborhood. This good agreement is explained partly by the adjustment of the unknown local emission factor but also by the configuration of the ring road surrounding in part the neighborhood, making it less sensitive to wind direction errors. Two urban renewal scenarios proposed by architects and local inhabitants are simulated under the same meteorological conditions. Increasing the height of anti-noise walls (from 3 to 6 m) does not improve the neighborhood air quality (except very locally) and the suppression of a big building block next to the ring road has a mixed effect, displacing the pollution (concentration increased in some area, decreased in some others).
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