Effects of building aspect ratio and wind speed on air temperatures in urban-like street canyons

The objective of this study is to simulate the characteristic role of building aspect ratio (AR) and wind speed on air temperatures during different street canyon heating situations. A two-dimensional Renormalization Group (RNG) k-epsilon turbulence model is employed to solve the Reynolds-averaged Navier-Stokes (RANS) and energy transport equations. A comparison of the results from the adopted model with those reported by similar experimental and numerical works demonstrated that the model is quite reliable when simulating temperature and wind profiles. The model is employed to predict air temperatures in idealized street canyons of aspect ratios (building-height-to-street-width ratio) of 0.5-8 with ambient wind speeds of 0.5-4 m/s. Three situations were identified for simulating diurnal heating of street canyon. It is noted that air temperatures are positively correlated with the bulk Richardson number (R-b) in most of the cases. The results show that the air temperature difference between high and low AR street canyon (Delta theta(AR)) was the highest during the nighttime (i.e., around 7.5 K between AR8 and AR0.5), but low or even negative during the daytime. it is also found that air temperatures rose as high as 1.3 K when ambient wind speed decreased from 4 m/s to 0.5 m/s. It is also revealed that the Delta theta(AR) during different diurnal situations and the nighttime and daytime air temperature difference between urban and rural areas (Urban Heat Island, UHI) closely resemble one another. Conclusively, the results of this study have highlighted the importance of street canyon AR and wind speed on urban heating. (C) 2009 Elsevier Ltd. All rights reserved.