CHARACTERISTICS OF WIND-FIELD IN A STREET CANYON

Characteristics of the wind field in a North-South oriented street canyon were studied by a numerical model to couple the heat and mass transfer processes. The heat fluxes were traced in and out of the street canyon, and the feedback of the heating processes inside the street canyon to the atmosphere was simulated by simultaneous solutions of the two-dimensional Navier-Stokes equations and heat transfer equation inside the street canyon, using a large eddy simulation (LES) model, coupling with the solutions of the heat conduction equations for roof, walls and road, It was found that with the existence of a wind blowing over the roof of the buildings on both sides of the street, large eddies were formed in the street canyon and transported the sensible heat out of the street canyon. The number of large eddies formed inside the street canyon depends on the aspect ratio, that is the ratio between the height of the buildings and the width of the street canyon. With an aspect ratio smaller than unity, only one large eddy was formed, while for aspect ratios larger than 2, two or more large eddies were formed. The time from the beginning of the computation until the statistically steady solutions of the flow and air temperature fields were achieved, was different for different street canyons, increasing with the street canyon heights, but was usually less than 100 s. With small aspect ratio and strong outside wind, the thermal convection due to heating of the road and wall surfaces was much less than the mechanically generated turbulence. However, with a weak outside wind or large aspect ratio, the thermal convection patterns could be clearly seen.