Analysis of overhang location and its influence on indoor pollution within an urban street canyon context

Abstract

Urban street canyons are particularly susceptible to pollutant build-up due to limited natural ventilation and proximity to vehicular emissions. This study investigates how projecting overhangs on building façades affect NO2 ingress into a naturally ventilated ground-floor room. The research addresses the gap in understanding how façade-level architectural features influence indoor air quality in dense urban environments. A series of transient Computational Fluid Dynamics (CFD) simulations were conducted using ANSYS® Fluent 2025 R1 to analyse airflow and pollutant transport in a simplified street canyon under varying overhang configurations and wind directions. Four scenarios were tested: a baseline case with no overhang, and location scenarios with overhangs on the windward façade, the leeward façade, and both façades. Each location scenario was assessed under opposing wind conditions perpendicular to the canyon in order to capture directional effects on flow regimes and pollutant ingress. The results revealed a clear contrast between wind directions. Under left wind conditions, which facilitated more direct transmission of pollutants toward the façade, most scenarios resulted in indoor nitrogen dioxide (NO2) concentrations that exceeded recognised health thresholds. Conversely, all right wind simulations maintained pollutant levels below these limits, indicating a reduced exposure risk under these flow regimes. Among the left wind cases, which presented more critical ventilation challenges, the scenario with overhangs on both façades demonstrated the most balanced performance, effectively limiting pollutant ingress during the active phase while supporting more efficient pollutant clearance during the decay phase. These findings suggest that while overhangs can help modulate pollutant transport, their effectiveness is context-dependent, varying with factors such as wind direction and canyon geometry. Therefore, passive design interventions must be carefully tailored to avoid intensifying indoor exposure in naturally ventilated spaces.