Turbulent flow and dispersion inside and around elevated walkways

The influence of an elevated walkway on turbulent flow and dispersion inside a street canyon is investigated numerically with large-eddy simulation (LES). Different walkway designs and elevations are examined for a unit-aspect-ratio street canyon and an external wind perpendicular to the canyon axis...

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Bibliographic Details
Published in:Building and environment 2020-04, Vol.173, p.106711, Article 106711
Main Authors: Duan, G., Brimblecombe, P., Chu, Y.L., Ngan, K.
Format: Article
Language:English
Subjects:
Aspect ratio
Computational fluid dynamics
Computer simulation
Deceleration
Decks
Dependence
Dispersion
Fluid flow
Kinetic energy
Large eddy simulation
Obstacle
Pollutants
Street canyon
Street canyons
Turbulence
Turbulent flow
Urban microenvironment
Ventilation
Walkways
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Summary:The influence of an elevated walkway on turbulent flow and dispersion inside a street canyon is investigated numerically with large-eddy simulation (LES). Different walkway designs and elevations are examined for a unit-aspect-ratio street canyon and an external wind perpendicular to the canyon axis. In place of a principal vortex extending through the canyon, smaller primary vortices appear above the walkway lid or below the walkway deck, in accord with the effective aspect ratio of the upper and lower regions. The mean flow decelerates below the deck and the turbulence kinetic energy (TKE) increases around the walkway structure for all configurations. Ventilation mechanisms for a pollutant emitted by a ground-level area source are qualitatively altered. Consequently mean concentrations at the pedestrian and walkway levels may increase by up to 20% and 30%, respectively. The velocity and pollutant statistics show a complicated dependence on the walkway design; this dependence is determined in part by interactions between the mean flow and turbulence. •A distinct canyon microenvironment is established by the presence of an elevated walkway.•The existence of coherent vortices above and below the walkway structure is in accord with the dynamics of 2D street canyons.•Dispersion routes are qualitatively altered due to the diversion of streamlines and the increase of TKE around the walkway.•Displacing pedestrians from the street level to an elevated walkway lowers pollutant exposure.
ISSN:0360-1323
1873-684X
DOI:10.1016/j.buildenv.2020.106711