Atmospheric pollution became a big issue in densified urban areas where the ventilation in streets is not sufficient. It is particularly the case for street surrounded by high buildings socalled street canyons. The ventilation and, thus, the concentrations in this kind of street are highly relying on geometric properties of the street (width of the street, heights of the buildings, etc.). Reynolds-averaged Navier-Stokes equations are used to investigate the impact of two geometric street ratios on pollutant dispersion: the ratio of the leeward to the windward building height (H1/H2) and the ratio of the street width to the windward building height (W/H2). The aim is to quantitatively assess the evolution of mean pollutant concentrations in the case of stepdown street canyons with H1/H2 ranging from 1.0 to 2.0 and street width ratios W/H2 ranging from 0.6 to 1.4. Three types of recirculation regimes could be established, depending on the number and the direction of the vortices occurring inside and outside the canyon. Evolution of pollutant concentrations as a function of both ratios is provided as well as the recommended regimes in the perspective of reducing pollutant concentration in step-down street canyons at pedestrian level and near building faces.
People around the world increasingly live in urban areas where traffic-related emissions can reach high levels, especially near heavy-traffic roads. It is therefore necessary to find short-term measures to limit the exposure of this population and noise barriers have shown great potential for achieving this. Nevertheless, further work is needed to better understand how they can act on pollution reduction. To do this, a Reynolds-Averaged Navier-Stokes model that takes into account thermal effects is used to study the effects of wind speed and atmospheric stability on the concentration reduction rates (CRR) induced by noise barriers. This study shows that the CRR behind the barriers may depend on both wind and thermal conditions. Although only the wind direction, and not the wind speed, has an impact on CRR in a neutral atmosphere, this parameter can be changed by both wind speed and thermal variations in non-neutral atmospheres. Stable cases lead to a higher CRR compared to unstable cases, while the neutral case gives intermediate results. Finally, it is shown that the variation of CRR is negligible for Richardson numbers ranging from -0.50 to 0.17.
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