Influence of wind direction and tree plantings on flow structure and pollutant dispersion within urban street canyon

Street canyon is one of the basic units for urban microscale region. The pollutant transportation inside is jointly governed by multiple factors. The current study systematically investigated the coupling impact of inflow wind direction and vegetation on the mechanism of pollutant dispersion inside the canyon. Steady RANS simulations were conducted for a canyon with an aspect ratio of two. Seven inflow wind angles (θ) from 0° to 90° at 15° intervals, two tree arrangements, and four different leaf area density (S_LAD) values were considered. Results demonstrate that three flow regimes characterized by complex vortex (θ = 0°), spiral flow (θ = 15°–75°) and channeling flow (θ = 90°), respectively, appear inside the canyon as the inflow wind direction varies. The presence of trees exacerbates pollutant accumulation near building walls and at pedestrian level inside the canyon, with the negative influence peaking at θ = 75°. Pollutant removal efficiency inside the whole canyon exhibits a non-monotonic trend with θ—an initial increase, followed by a decrease, and a subsequent increase—for canyons with and without trees. While θ = 0° constitutes the least favorable condition for pollutant removal in tree-free (TF) canyons, the 75° angle proves most detrimental for canyons with both central single line (CSL) and two-side lines (TSL) trees. At S_LAD = 2.0 m²/m³, the dimensionless net escape velocity for CSL and TSL decrease by 76.1% and 78.5%, respectively, relative to the TF scenario. In addition, pollutant removal efficiency inside the canyon decreases with the increasing of S_LAD across all investigated wind directions.