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

Abstract

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

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

“…While the velocity magnitude for AR 1.33 is similar to that seen for AR 1, it drastically drops for AR 2 and 4, indicating that the multivortex ow is characterized by very poor ventilation, particularly at the street level. e results presented above conform to those presented in [9,58,61,[65][66][67]71], which simulated canopy ows through CFD, employing di erent turbulence models.…”

“…e latter is a measure of the rate of air removal from the street canyon [66]: 8 Advances in Meteorology where the subscript "+" indicates positive (upward) vertical velocity, while W is the canyon length. Since RANS models do not calculate the instantaneous velocity components, according to [67], ACH has been estimated as the sum of its average and uctuating parts:…”

Air exchange in urban canyons with variable building width: a numerical LES approach

“…It is important to remember that, especially in the skimming flow regime, the air exchange at the roof level is mainly controlled by the turbulent mixing with a notable effect of the building aspect ratio, ARB, (i.e Soulhac, 2000), while the increasing canyon width promotes the turbulent mixing also below the roof top. Accordingly, the air exchange is less dependent on the flow at the roof level, as demonstrated in Badas et al, (2017). Turbulent quantities play a fundamental role in promoting the air exchange between the canyon and the overlaying flow, as stated by Ng and Liu (2014).…”

Natural ventilation in cities: the implications of fluid mechanics

“…Further, traffic induced flow and turbulence may be important, particularly in low wind speed conditions (e.g. Brown, Lawson, De Croix and Lee, 2000;Yassin, 2011;Badas, Ferrari, Garau and Querzoli, 2017).…”