EXP 1-2 Introduction
Pollutant transport between a street canyon and a 3D urban array as a function of wind direction and roof height non-uniformity
Introduction
In urban environments, the exchange of air and pollutants between the street canyon and the urban canopy is a critical factor in air quality and the dispersion of pollutants. The street canyon refers to the restricted airflow within narrow streets flanked by tall buildings, while the urban canopy represents the overall structure of buildings and vegetation that form the urban landscape. This interaction is complex and influenced by various meteorological, topographical and anthropogenic factors.
From studies on very simplified street canyon models (2D or 3D, but sufficiently long and with a uniform roof height), it is known that the most unfavourable conditions for air quality in a street canyon occur when the wind blows perpendicular to its longitudinal axis. In this case, there is usually a strong vertical recirculation of the flow within the street canyon, resulting in a higher residence time of pollutants at street level, and the pollutants are transported out of the street canyon through the opening mainly by turbulence. In contrast, if the wind blows along the street canyon, the flow in the street canyons is channelled and the pollutants are most effectively transported out of the street canyon by advection through the lateral openings.
However, the architecture of urban areas is quite complex and does not usually consist of street canyons of uniform height. Nor does the wind blow only perpendicularly or parallel to the along- canyon axis. The flow and pollutant transport processes in such more complex situations are rather limited and it is not yet clear which street canyon geometry improves urban air quality.
The aim of the present wind tunnel experiment was therefore to investigate the transport of pollutants from a ground-level line source through the openings of a street canyon by means of turbulence and advection, with the main influencing variables being the roof height along the street canyon walls and the wind direction. The experiment was conducted at a scale of 1:400 and with a flow regime independent of the Reynolds number in order to be transferable to the full scale.
Because of the non-uniform roof heights, the flow and pollutant transport were measured in two horizontal planes: at the height of the city model with uniform roof heights (corresponding to the mean height of the model with the non-uniform roof heights) and at the height corresponding to the lowest roof height of the non-uniform case. Due to the three-dimensionality of the problem, the flow and pollutant transport were also measured at the two lateral openings of the studied street canyons.
The measurement of the turbulent pollutant fluxes through the respective opening also provides the mean (advective) pollutant fluxes and velocity vector statistics as mean momentum fluxes (Reynolds stresses) and mean velocity values. The pollutant concentration statistics are also provided.
Contributed by: Štěpán Nosek — Institute of Thermomechanics of the CAS, v. v. i.
© copyright ERCOFTAC 2024