A Study on the Effects of Density and FAR (Floor Area Ratio) on Urban Ventilation

The characteristics of building masses, their settlement and density play an important role in the pattern of urban airflow, air stagnation and accumulation of pollutants in the lower atmosphere layer. Aiming at finding the optimised lodgement pattern in a given density, this research studies the effects of changes in height and site coverage (FAR, Floor Area Ratio), on a local scale in Velenjak area of Tehran. Given its interdisciplinary nature, the methodology of this research is hybrid, with information collected by library resources, CFD used for simulating wind regime, and modelling urban area carried out based on the Municipality GIS data. A net, constant density of 270%, has been distributed in three building models: seven-storey buildings with the site coverage area of 38.5%, 12-storey buildings with the site coverage area of 22.5% and 20-storey buildings with 13.5% site coverage area. A 4.5 m/s reference air flow above these models from north, west, northwest and northeast has been considered, while the studied urban block surrounded with similar blocks, regardless of the complex effect of atmosphere thermal stratification. The wind was simulated by Ansys Fluent software and k-ε turbulence model. The results indicate that natural ventilation occurs in 38% of time-places in seven-storey buildings, 44¬% in 12-storey buildings and 65% in the high-rise buildings. Thus, it can be concluded that increasing height can only improve airflow with significant reduction of site coverage. This, in turn, depends on several factors, including urban fabric orientation, geometric features of the structure and wind characteristics of area. Therefore Individual numerical and experimental modelling and simulation are necessary for each region in order to obtain an optimal model.