Ventilation and Pollutant Concentration for the Pedestrian Zone, the Near-Wall Zone, and the Canopy Layer at Urban Intersections

To gain further insight into the ventilation at urban street intersections, this study conducted 3D simulations of the ventilation at right- and oblique-angled intersections under eight wind directions by using the Reynolds-averaged Navier-Stokes (RANS) kappa-epsilon turbulence model. The divergent responses of ventilation and pollution concentration for the pedestrian zone (ped), the near-wall zone (nwz), and the canopy layer to the change in intersection typology and wind direction were investigated. The flow characteristics of the intersections, taken as the air flow hub, were explored by employing indices such as the minimum flow ratio (beta) between horizontal openings. The results show that oblique wind directions lead to a lower total volumetric flow rate (Q(total)) but a higher beta value for right-angled intersections. For T-shaped intersections, a larger cross-sectional area for the outflow helps to increase Q(total). Oblique-angled intersections, for example, the X-shaped intersection, experience a more significant difference in Q(total) but a steady value of beta when the wind direction changes. The vertical air-exchange rate for the intersection was particularly significant when the wind directions were parallel to the street orientation or when there was no opening in the inflow direction. The spatially averaged normalized pollutant concentration and age of air (tau* over bar ) for the pedestrian zone and the canopy layer showed similar changing trends for most of the cases, while in some cases, only the tau ped* over bar or tau nwz* over bar changed obviously. These findings reveal the impact mechanism of intersection configuration on urban local ventilation and pollutant diffusion.