How gable roofs change the mechanisms of turbulent vertical momentum transfer: A LES study on two-dimensional urban canyons

A Large Eddy Simulation (LES) study on the presence of gable roofs in urban canyons is presented: two canyon aspect ratios, ARC = 1, 2, (ARC = W/H corresponding to the ratio of the canyon width, W, to the eaves’ height, H) and variable roof slope, α (α = 0°, 10°, 20°, 30°, 45°) in a periodic configuration are considered. The vertical turbulent momentum flux is analysed by means of the quadrant analysis both in terms of spatial maps and mean vertical profiles in order to assess how the presence of a gable roof and its slope affects the mechanisms of mixing in the roughness sublayer. A gradual modification with increasing roof slope is apparent for the mean flow patterns as well as for the shear layer thickness. On the contrary, the contribution of ejections and sweeps at the rooftop goes through an abrupt change when passing from flat-to gable-roof buildings. This behaviour arises for both the investigated canyon aspect ratios. With gable roofs the imbalance between sweeps and ejections is monotonically increasing with α. However, the case of flat roof differs from the small pitch ones, as it gives much higher values of imbalance, comparable to what was observed for the largest analysed pitches (45°).