Localized effect of building height on street level flows and concentration: water channel modeling

Following field measurements of particulate matter in five Southern Californian cities scaled model of downtown Los Angeles was created and tested in a water channel. Of special interest was the influence of tall buildings on the street level flows and concentration. For concentration measurements dye was released through line sources placed in two major arterials. Detailed velocity and concentration field in regions of interest was measured by simultaneous Particle Image Velocimetry/Planar Laser Induced Fluorescence (PIV/PLIF) system. Experiments were repeated with and without tall buildings.

Results show that the presence of tall buildings in street canyons will enhance dispersion of pollutants to surrounding regions and significantly reduce ground level concentration. However, this dispersion enhancement by tall buildings can be overweighed by other factors such as street canyon aspect ratio and buildings height ratio.

Since Los Angeles model downtown is relatively complex and results may present a challenge for interpretation we decided to build a simple, mock downtown, in which all buildings were presented by regular cubes. Height of the middle building was systematically varied. The ground level flow and concentration is very sensitive to height ratio of surrounding buildings. This ratio is determining vortex strength and coupling of vortices which are the dominant dispersion mechanisms inside street canyons. In the case of large height ratios, strong downdraft flow can be observed, enhancing the lee eddy of the shorter upwind building and hence achieving enhanced dispersion of pollutants.

The effect of different incoming flow regimes, surrounding building geometries, and street canyon ratios on ground level concentration will be presented.