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The effects on turbulent street canyon flow of the morphological model and the existence of an upstream tall building are investigated in a wind tunnel, using Stereoscopic Particle Image Velocimetry (S-PIV) measurements on two orthogonal vertical planes. The cathedral in the "Rue de Strasbourg, (Nantes, France)" morphological model is regarded as a local tall building and is tested and then replaced by generic, square planform buildings of 4 different heights. The cathedral induces an asymmetrical wake while no obvious different influence on the street canyon flow can be identified, in comparison with a generic building of the same height. A strong along-canyon flow occurs which may be due to the intersections, while the along-canyon ventilation decreases with increasing building height. The higher the building, the stronger its sheltering effect, leading to a reduction of the vertical ventilation and mass transport across the canyon opening, at least in the canyon portion directly downstream of the building. • A strong along-canyon flow is observed in the morphological model studied. • Turbulence statistics in the street canyon are larger than in an idealized canyon. • The mass flux across the canyon roof reduces with a higher upstream building. • Tall building shape does not impact turbulence statistics within a street canyon. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]