Dependence of city ventilation by thermal buoyancy on thermal stratification

Abstract

Atmospheric stability is quite important on the heat removal and dispersion of air pollutants in urban area. But the existing numerical studies on this subject are limited. The effects of thermal stratification on air flows in city street canyons were studied using stratified inlet airflow. Two simple Hong Kong city models with complex terrain were considered here for modelling the city ventilation of a whole city under different atmospheric conditions. A 3D RNG k-ε turbulent model was used for the numerical simulation. The results showed that the influence of thermal stratification can be significant on the city ventilation by thermal buoyancy. For a simple model I, when the wind speed is relatively large, the impact of thermal stratification on the airflow in the city street canyons is minor. When the wind speed is smaller relative to the buoyancy force, the airflow in the street canyons is depended on the thermal stratification. The greater the instability, the stronger the vertical mix and bigger the normalized flow rate by turbulent fluctuation. For the model II, the influence of thermal stratification is strong under relative strong background wind. The study can provide a good understanding of urban ventilation by thermal buoyancy and useful information for urban planning.