Integrated influence of wind direction and street length on pollutant dispersion and pedestrian exposure in idealized street canyons: A CFD approach

This study numerically examines the combined effects of wind direction and street length on pollutant dispersion, ventilation, and pedestrian-level exposure in urban street canyons. Idealized symmetric, step-up, and step-down canyon configurations with aspect ratios (AR = 1, 3) and street lengths (L/W = 2–20) were analyzed under four wind directions (α = 0°, 30°, 60°, 90°). Ventilation performance was evaluated using dimensionless air exchange rate (ACH∗) and pedestrian net escape velocity (NEV_ped∗), while exposure was quantified through intake fraction (<P_IF>). Results show that symmetric canyons accumulate the highest pollutant concentrations under parallel winds (α = 0°), while step-up and step-down canyons show peak levels under oblique and perpendicular winds, respectively. Ventilation efficiency is highest in symmetric and step-down canyons under oblique winds (α = 30°–60°), and in step-up canyons under perpendicular winds (α = 90°). The lowest <P_IF> occurs under oblique, parallel, and perpendicular winds for symmetric, step-down, and step-up canyons, respectively. Longer streets generally reduce ventilation and increase <P_IF>, particularly under α = 0°. However, step-down canyons with AR = 3 demonstrate improved ventilation for L/W > 10 under α = 30°–90°. These findings offer practical insights for optimizing canyon geometries to enhance urban air quality.