The Effect of Wind Direction and Speed on Ventilation and Pollutant Concentrations in Street Canyons

This study aims to investigate the role of wind speed and wind direction in ventilation condition and pollutants concentration in street canyons with different aspect ratios through using ENVI-met software in a quantitative and experimental way. The results demonstrate how the combination of different wind variables (direction & speed) with different aspect ratios of canyons, can alter the ventilation condition, flow pattern and also pollutant concentration inside the urban street canyons. The result of simulations in some sections of Valiasr and Taleqani streets considering three different ratio of height to width proportion (0.3, 0.65 and 1.5) for the canyons, and two minimum and maximum threshold values for wind speed (1 m/s and 2.5 m/s) in west and south west direction, showed that increasing the wind speed to all directions (in this case west and south west) in a canyon with different aspect ratios (in this study 0.3, 0.65 and 1.5) can improve ventilation condition inside the canyon and reduce pollutants concentration within it. When the wind direction is perpendicular to the canyon, by increase of aspect ratio, the pollutant concentration will significantly increase inside the canyon, whereas the parallel wind direction in the same test shows opposite results. The findings of this study as well as subsequent findings resulted from combination of different concerned variables, as a general framework can help urban designers and planners to decide on the geometry and direction of street canyons.