Computational analysis of pollutant dispersion mechanism in an urban street canyon

Abstract

In the present study, computational modelling and simulations of pollutant dispersion mechanism in urban street canyon have been carried out using Computational Fluid Dynamics (CFD) approach. The effect of presence of elevated metro rail track corridor (EMRTC) in an urban street canyon on pollutant dispersion has been studied. In addition, effect of varying aspect ratio of the street canyon and that of wind orientation on pollutant dispersion have also been investigated. The simulations were carried out using both two-dimensional and three-dimensional geometries. In this study, suitability of CFD approach (with k-and#949; turbulence and species transport model) for predicting pollution in an urban street canyon has been investigated. This study reports its findings based on comparison of pollutant dispersion for street canyons without EMRTC and for street canyons with EMRTC. It was found that, comparatively speaking, the street canyons with EMRTC have higher pollutant concentration in the lower regions of the street canyon. Further, the pollutant concentration contours indicate that, for street canyons with EMRTC, the pollutant concentrations are relatively higher in the vicinity of the metro rail pillars (as compared to other portions of street canyon). For regular street canyon (i.e. aspect ratio of 1), introduction of EMRTC lead to significant increase (24 to 104% depending on wind orientation) in normalised average volumetric pollutant concentration. This increase was about 17 to 144% for wide street canyon (i.e. aspect ratio 0.5) similarly, with the introduction of EMRTC, the increase in average normalised pollutant concentration at pedestrian level was 28 to 175% and 60 to 182%, respectively for aspect ratios 1 and 0.5. When comparing normalised average volumetric pollutant concentration amongst perpendicular wind flow (i.e. 90o wind flow orientation), oblique wind flow (i.e. 60o, 45o and 30o wind flow orientations) and parallel wind flow (i.e. 0o wind flow orientation), while going from perpendicular to other wind flow orientations (oblique and parallel), the value of normalised average volumetric pollutant concentration decreased by 44 to 76% and 55 to 80%, respectively for aspect ratios 1 and 0.5. Depending on the case under consideration, the decrease in aspect ratio from 1 to 0.5, decreased the average volumetric normalised pollutant concentration by 29 to 75%. The above important findings and their details will help us in effectively designing elevated metro rail tracks and related infrastructure for urban street canyons, so that health of inhabitants is not compromised due to vehicular pollution under such situations. Inputs from this study will help in optimising the geometry of structures like EMRTC, flyovers, bridges and residential buildings of street canyon newline