Experimental study on flow and gaseous diffusion behind an isolated building.

To assist validation of numerical models of urban pollution dispersion, the effect of obstacles building on the gaseous diffusion in the wake region have been investigated experimentally in the boundary layer wind tunnel under neutral atmospheric conditions using a tracer gas technique from a point source without buoyancy. The flow and diffusion fields in the boundary layer in an urban environment were investigated in the downwind distance of the obstacle building using an isolated high-rise building model. The scale of the model experiment was assumed to be at 1:500. In the experiment, gaseous pollutant was discharged in the simulated boundary layer over the flat terrain. The effluent velocity of the pollutant was set to be negligible. The velocity field and the turbulence characteristics were analyzed and measured using a hot wire anemometer with a split-fibre probe. The experimental technique was involved the continuous release of tracer gas from a ground level source which was located in the downwind distance of the obstacle model and measured using a fast flame ionization detector (FID). Diffusion characteristics were studied and included both the vertical and lateral mean concentrations and concentration fluctuation intensity at various downwind distances. The results of study were demonstrated that the vertical profiles of the longitudinal mean velocity are very thick around the obstacle wake region due to the turbulence mixing and the smoothing of concentration differences was increased with downwind distance from the obstacle model. Furthermore, the experimental results can help to improve the understanding of mechanisms of pollutant dispersion in an urban environment and also use to validate the corresponding computational fluid dynamics (CFD) prediction.