Mass and momentum transfer in the urban boundary layer

The dispersion of pollutants in the atmosphere in urban areas is a complex process which depends on several physical phenomena. The present work analyzes the mechanisms of the mass and momentum exchange in the urban atmospheric boundary layer. In particular we focused our attention on the lower part of the atmospheric boundary layer, where the °ow dynamics are typically determined by the size and the density of the buildings and by the street geometry. In order to analyze these processes we have performed a wind tunnel investigation of the °ow dynamics and scalar dispersion in the near-ground region of a neutral atmospheric boundary layer. An idealized street geometry was simulated by an array of 2D parallel canyons, made of a set of square section bars placed normal to the wind; the spacing of the bars (i. E. The ratio H=W between building height and canyon width) could be varied. The velocity measurements have been performed by means of hot-wire anemometry and Particle Image velocimetry (PIV) whilst passive scalar concentration measurements have been performed with a Flame Ionization Detector (FID). In the ¯rst part of the work we studied the in°uence of small scale roughness (roof shape, chimney. . . . ) at the top of the buildings on the °ow and the dispersion in the turbulent stream above buildings roofs. The in°uence of the roof roughness was studied by adding small scale 2D roughness elements to the tops of the bars. In order to evaluate the mass and momentum exchange in the boundary layer above the obstacles, di®erent experiments were conducted for each geometrical con¯guration: the pro¯les of mean and °uctuating velocities were measured above the obstacle roof; a passive scalar was released from a an elevated line source and from a ground level source, and concentration pro¯les were measured downstream of the source. We veri¯ed that the presence of a smaller scale roughness is felt by the overlying °ow only if the larger scale obstacles are su±ciently packed together. The smaller scale structures produced by the small scale roughness in°uence the °ow dynamics if their size is the same order of that of the eddies shed by the shear layer developing at the canopy top: that happens if the canyon width is not too large (i. E. For street aspect ratio H=W » 1). In the second part of the work we focused on the processes that determine the mass exchange between the recirculating region within the street canyons and the external °ow. The goal of the study was to evaluate how di®erent conditions within and outside the cavity determine the velocity and concentration ¯elds within the cavity itself; the aim was to ¯nd the appropriate reference velocity and length scales that characterize the mass exchange between the recirculating region and the external °ow. We veri¯ed that the exchange processes are dependent on the canyon geometry as well as on the intensity of the external turbulence, but are not sensitive to the external integral length scale. As a general conclusion we may say that the mass and momentum exchange between a recirculating region and the external °ow is a process which is driven by the °ow instabilities, arising within the shear layer which develops at the interface between the two region, and it is in°uenced even by the turbulent kinetic energy °uxes from the external °ow toward the cavity.