Quantification of pollutant removal mechanism within a two-dimensional street canyon equipped with eaves under various roof configurations

Abstract

Terraced houses, often described as two-dimensional (2D) street canyons, typically feature eaves and different roof designs. The present study investigates the combined effects of eaves with various roof designs on ventilation and pollutant removal in a street canyon under a wake interference flow regime. Steady Reynolds-Averaged Navier-Stokes (RANS) simulations are performed over flat, pitched, and wedge roofs. To resemble typical residential layouts, the buildings are arranged into two aspect ratios (the ratio of street width S to building height H) based on the main street and setback distance, which are fixed at 3 and 1, respectively. Ventilation performance is evaluated using air exchange rate (ACH), age of air (AoA), and purging flow rate (PFR). The results indicate that varying the roof shape in the absence of eaves increases the size of the primary vortex in a vertical direction, which facilitates in improving in-canyon ventilation. Incorporation of eaves at mid-building height drastically alters the flow structures, since multiple vortices develop beneath both eaves. The ACH has significantly improved with roof modifications solely, increasing by 1.5 to 4 times, and in combination with eaves, it demonstrates a positive impact on the ACH across all roof configurations. However, the effects are very limited, with increments ranging from 1.5% to 27%. The presence of eaves increases the accumulation of pollutants, resulting in higher AoA (up to 1.5 ∼ 2) and contributing to reduced PFR values (approximately 32 ∼ 55%). Thus, it is advisable to assess all three indices concurrently to enhance their synergy.