Evaluation of numerical flow and dispersion simulations for street canyons with avenue-like tree planting by comparison with wind tunnel data

Flow and traffic-originated pollutant dispersion in an urban street canyon with avenue-like tree planting have been studied by means of wind tunnel and CFD investigations. The study comprises tree planting of different crown porosity, planted in two rows within a canyon of street width to building height ratio W/H = 2 and street length to building height ratio L/H= 10 exposed to a perpendicular approaching boundary layer flow. Numerical simulations have been performed with the commercial CFD code FLUENT™ by employing the RSM turbulence model. In the presence of tree planting, both measurements and simulations show considerable larger pollutant concentrations in proximity of the leeward wall and slightly lower concentrations in proximity of the windward wall in comparison to the tree-less street canyon. In particular, FLUENT slightly underestimated pollutant concentrations in proximity of the leeward wall in all cases studied, while near the windward wall there is no general tendency towards underestimation or overestimation. Overall, numerical computations compare qualitatively well with experimental data. Results from commonly used statistical tests also suggest the CFD predictions to he satisfactory. Results obtained in this work by combining wind tunnel experiments and CFD based simulations in a novel aspect of research suggest ways to obtain quantitative information for planning and implementation of exposure mitigation using trees in urban street canyons.