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 ofa low-rise building in downtown Montreal is performed. Two wind directions areconsidered, with different wind flow patterns and plume behaviours. The resultingmean concentration field is observed and analysed with the computed (mean)convective and turbulent mass fluxes. These two concepts allow gaining someinsight into the dispersion process and analysing the deficiencies of lesssophisticated turbulence models. When the emitting building is locateddownstream of a high-rise building (case SW), the turbulent mass flux is directedfrom the high to low levels of mean concentration and the gradient diffusionhypothesis often used with steady models is verified. However, when the influenceof the surrounding buildings is smaller (case W), a counter-gradient mechanism isobserved in the streamwise direction, confirming the results obtained on isolatedbuildings. The present study supports the use of generic, simplified cases toinvestigate environmental processes; the conclusions can subsequently be appliedto real and more complex cases.