On the effect of gable roof on natural ventilation in two-dimensional urban canyons

Flow regimes occurring in urban canyons are strongly influenced by the geometrical shape of the buildings; however, fluid dynamic investigations are typically carried out using parallelepiped obstacles. The present study is focused on assessing the effect of gable roofs on the flow regimes characterizing urban canyons (skimming flow, wake interference, isolated roughness) and the implications in terms of integral parameters (air exchange rate and friction factor), which are useful in practical applications. Numerical simulations are performed by means of RANS modeling of idealized two dimensional urban canyons between series of identical gable roof buildings with pitch ranging from 0° up to 40°, and wind direction perpendicular to the canyon axis. Simulations performed for different canyon aspect ratios show the key role played by the roof pitch in enhancing turbulence and in increasing ventilation, in particular for narrow canyons. Furthermore, turbulence-driven ventilation is observed to be related to the square root of the friction coefficient by a single linear relation, despite of the roof pitch. These results may have an impact on design and planning strategies aimed at enhancing natural ventilation and promoting efficient pollutant and heat dispersion in urban areas.