Spatial variability of mean flow and turbulence fields in street canyons

Air pollution episodes in cities are often related to emissions from traffic. The pollutants are released near the ground and the dispersion is highly affected by the complex flow phenomena in the near surrounding of buildings. Dispersion models dealing with this kind of problems must be able to predict the flow patterns observed inside the streets and in the zone above roof level. During the last few years, the flow and dispersion characteristics in street canyons were investigated in several wind-tunnel studies. There are two major questions left with respect to these studies: how strong is the influence of the particular experimental setup on the observed flow characteristics in street canyons, and to what extent these characteristics can be parameterized?

In the present paper a comparison of flow characteristics obtained in three different wind-tunnel studies of street canyons with idealized geometry as well as in a study with a detailed model of a real street canyon surrounded by an urban canopy is undertaken. In all experiments considered, mean values and turbulent statistics of three velocity components were obtained from high-resolution flow measurements. The idealized canyon studies included systematic variations of building and roof geometry. Canyons formed by isolated buildings as well as urban-type configurations with sequences of canyons have been investigated. The experiments were conducted in two neutral boundary layer wind tunnels with differences in tunnel dimensions and approach flow characteristics.

We consider the spatial variability of the flow and turbulence fields inside and above the canyons, and discuss the influence of urban canopy irregularities on the properties of spatially averaged profiles. We have found good agreement between the results for idealized street canyons of similar geometry studied in the two different tunnels. A rotating vortex associated with significant reverse flow in the lower part of the canyon is observed in isolated as well as in urban-type canyons with flat roofs. In canyons formed by buildings with slanted roofs, this vortex is much weaker and less stable. This phenomenon is also observed at street-canyon locations in the detailed urban - canopy model. The absolute values of the along-wind velocity component close to the canyon bottom in the slanted-roof cases are found to be significantly smaller than in the situations with flat roofs. A pronounced maximum in the turbulent kinetic energy profiles in the region closely above the buildings appears to be associated with situations in which the canyon is close to a change in surface structure. The largest energy maxima are observed for isolated two-dimensional canyons.