Modified street canyon flow

The objective of the present work, is to investigate the turbulent airflow in street canyons and in particular, its coupling to the turbulent airflow above roofs. Understanding this coupling is crucial to understanding of how pollution is ventilated out of the street into the layer aloft. A held experiment between two long farm buildings forming a street was performed. Preliminary results are shown here. The distribution of the exit velocity, i.e. vertical velocity at roof-level, across the street showed that the how within the street is greatly affected by the pitched roofs. A channelling flow regime is evident when the wind is parallel to the street axis. The velocity spectra measured at a height 2.26 times the buildings' height approach the inertial sub-range behaviour; a - 2/3 slope at high frequencies is observed and the ratio of the vertical to horizontal spectra is close to the usual 4/3 value. This means that the airflow is not affected by the individual buildings at this level. In addition, the turbulence velocity spectra show that within the street the energy is partitioned in a greater range of eddies than over rural terrain. Above the roofs the spectral peak shifts towards higher frequencies as the roof-level is approached suggesting the confinement of eddies dose to the roofs. Taken together these results suggest to us that roof geometry influences the eddy size distribution. (C) 1998 Elsevier Science Ltd. All rights reserved.