Relating street canyon vertical mass-exchange to upstream flow regime and canyon geometry

Understanding and management of air quality is important to the sustainability of the urban environment and pedestrian level air quality is strongly influenced by the vertical airflow and consequent pollutant mass transfer that takes place at the roof level of street canyons. Using data from a scaled wind tunnel street canyon flow, the present work shows how a simple, first order “dead-zone” model may be successfully applied to provide a link between the vertical velocities at roof level and the magnitude of the mass-exchange. In addition, it is shown how the model may be modified to provide a prediction that takes into account both the geometry of the canyon as well as the canyon flow characteristics and those of the upstream roughness. The mass-exchange is also shown to be linked to the largest scales in the boundary layer passing over the canyon. Finally, it has also been demonstrated that, for the six configurations investigated here (two canyon geometries immersed in three different types of upstream roughness), the probability distribution function of the exchange velocity agrees very well with a log-normal distribution, thus allowing derivation of a simplified model of the instantaneous exchange velocity using a random number generator.