Abstract
Ventilation in cities is crucial for the well being of their inhabitants. Therefore, local governments require air ventilation assessments (AVAs) prior to the construction of new buildings. In a standard AVA, however, only neutral stratification is considered, although diabatic and particularly unstable conditions may be observed more frequently in nature. The results presented here indicate significant changes in ventilation within most of the area of Kowloon City, Hong Kong, included in the study. A new definition for calculating ventilation was introduced, and used to compare the influence of buildings on ventilation under conditions of neutral and unstable stratification. The overall ventilation increased due to enhanced vertical mixing. In the vicinity of exposed buildings, however, ventilation was weaker for unstable stratification than for neutral stratification. The influence on ventilation by building parameters, such as the plan area index, was altered when unstable stratification was considered. Consequently, differences in stratification were shown to have marked effects on ventilation estimates, which should be taken into consideration in future AVAs.
Highlights
Air ventilation is a crucial factor of city climate and has a major impact on the well being of the urban population
The analysis focused on the pedestrian height level 2 m above ground
The hexagonal structures visible in these figures had a size of about 2 km, which is within the typical range of
Summary
Air ventilation is a crucial factor of city climate and has a major impact on the well being of the urban population. LES was used to analyze and compare ventilation in a large real metropolitan area (Kowloon City) for neutral and unstable stratification, whereas previous studies only focused on idealized building setups (e.g., [13]). There are a number of additional challenges for unstable stratification, including that a large model domain is required to catch all relevant turbulent structures, which are considerably larger than for neutral stratification, while the grid size must be kept small to sufficiently resolve the street-canyon flows (e.g., [10]). This substantially increases the computational expense of the simulations.
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