A THREE-DIMENSIONAL CFD-BASED APPROACH FOR THE DISPERSION OF RADIOACTIVE CLOUD IN URBAN ENVIRONMENT

The presence of buildings and obstacles in urban environment can modify the velocity and spatial concentration fields of gaseous pollutants emitted by a source, therefore affecting the dispersion of the plume. The dispersion of the pollutants can be analytically approximated by using Briggs’ coefficients limited by the lack of detailed geometrical information of the obstacles. In the work, similarity Theory (MOST) for the entire vertical Atmospheric Boundary Layer profile under non-neutral stability conditions has been included as an accurate inlet boundary condition in the framework of the Reynolds-Averaged Navier-Stokes (RANS) approach. The effectiveness of the Shear Stress Transport (SST) variant of the k-ω model has been highlighted as the appropriate turbulence closure model to be used for the dispersion of cloud in urban environment. The geometry used in the numerical simulation was inspired by an urban agglomeration and refinement regions were strategically set to accurately capture the flow field and plume transport near obstacle surfaces, close to the ground, and around the chimney. Comparisons between Gaussian plume and CFD-based models are reported showing differences and asymmetries especially at shorter distance. Numerical results have been obtained by considering different stability atmospheric conditions and comparisons and differences with Huber approximation are presented and discussed.