Computational Fluid Dynamics Modelling of the Diurnal Variation of Flow in a Street Canyon

Urban surface and radiation processes are incorporated into a computational fluid dynamics (CFD) model to investigate the diurnal variation of flow in a street canyon with an aspect ratio of 1. The developed CFD model predicts surface and substrate temperatures of the roof, walls, and road. One-day simulations are performed with various ambient wind speeds of 2, 3, 4, 5, and 6 ms^-1, with the ambient wind perpendicular to the north-south oriented canyon. During the day, the largest maximum surface temperature for all surfaces is found at the road surface for an ambient wind speed of 3 ms^-1 (56.0°C). Two flow regimes are identified by the vortex configuration in the street canyon. Flow regime I is characterized by a primary vortex. Flow regime II is characterized by two counter-rotating vortices, which appears in the presence of strong downwind building-wall heating. Air temperature is relatively low near the downwind building wall in flow regime I and inside the upper vortex in flow regime II. In flow regime II, the upper vortex expands with increasing ambient wind speed, thus enlarging the extent of cool air within the canyon. The canyon wind speed in flow regime II is proportional to the ambient wind speed, but that in flow regime I is not. For weak ambient winds, the dependency of surface sensible heat flux on the ambient wind speed is found to play an essential role in determining the relationship between canyon wind speed and ambient wind speed.