Numerical investigation of the influence of street length and building configurations on ventilation and pollutant dispersion in idealized street canyons

The paper investigates the influence of street length on ventilation and dispersion of traffic-induced pollutants in urban street canyons, an aspect often overlooked in previous studies. Employing computational fluid dynamics (CFD), the research explores airflow and pollutant dispersion in symmetric, step-up, and step-down canyons across different aspect ratios (AR = 1 and 3) and street lengths (L), ranging from 2 to 20 times the street width (W). The results indicate that pollutant concentration increases with street length in symmetric and step-up canyons, whereas it decreases in step-down canyons for L > 10W. High-rise (AR = 3) step-down canyons exhibit higher pollutant levels compared to their low-rise (AR = 1) counterparts, while step-up canyons exhibit the opposite trend. In symmetric canyons with L/W ≤ 5, low-rise configurations have nearly double the pollutant concentration of high-rise canyons, but this relationship reverses for L/W > 5. Ventilation is more effective at street ends than at the canyon roof, with turbulent velocity components dominating over mean velocity in driving air exchange. Higher L/W ratios correlate with reduced air exchange rates, signifying decreased ventilation efficiency as street length increases. These findings offer critical insights for enhancing urban air quality through optimized street canyon design.