Large-Eddy Simulation of pollutant dispersion in downtown Montreal: Evaluation of the convective and turbulent mass fluxes

Large-Eddy Simulation of pollutant dispersion from a stack on the roof of a low-rise building in downtown Montreal is performed. Two wind directions are considered, with different wind flow patterns and plume behaviours. The resulting mean concentration field is observed and analysed with the computed (mean) convective and turbulent mass fluxes. These two concepts allow gaining some insight into the dispersion process and analysing the deficiencies of less sophisticated turbulence models. When the emitting building is located downstream of a high-rise building (case SW), the turbulent mass flux is directed from the high to low levels of mean concentration and the gradient diffusion hypothesis often used with steady models is verified. However, when the influence of the surrounding buildings is smaller (case W), a counter-gradient mechanism is observed in the streamwise direction, confirming the results obtained on isolated buildings. The present study supports the use of generic, simplified cases to investigate environmental processes; the conclusions can subsequently be applied to real and more complex cases.