Geometric Effect on the Dispersion of Carbon Dioxide Plume in Urban Street

The dispersion of heavy gas cloud in urban areas due to hazardous gas leakage accidents has been attracting the attention of researchers. Geometric conditions play an important role in determining both flow field and dispersion of cloud within urban street canyons. Geometric effect on the flow field and the dispersion of carbon dioxide plume in the three-dimension urban street canyon with building height variation, aspect ratio variation and roof shape variation were studied respectively approaching wind and neglecting thermal effect by wind tunnel experiment and computational fluid dynamics (CFD) method in this paper. A qualitative analysis for the flow behavior was done by conducting the flow visualization experiment and the concentration at typical measuring points was obtained in CO2 dispersion experiment. The flow field in street canyon was computed before releasing by a steady simulation and the steady-state atmospheric flow field obtained was then used as the initial conditions to study the cloud dispersion process in transient conditions. Statistical performance indicators indicated that the numerical simulations agreed reasonably with wind tunnel data. The flow field in the street canyon was highly inhomogeneous and three-dimensional spatially, and the concentration distribution of carbon dioxide followed the flow structure. Increasing the leeward-side building height would cause the wind speed in the canyon decreasing and the main vertical vortex disappearing. However, the presence of a higher windward-side building could greatly enhance wind speed and the vertical recirculation. So carbon dioxide concentration and the uniformity of horizontal distribution in street canyon increased as the leeward-side building height increased but decreased as the windward-side building height increased. In W/H=2/3 case, two counter-rotating vortices appeared in the street canyon, and most of plume is transported according to the counterclockwise vortex in the lower region. When increasing aspect ratio to 3/2, a lager vertical vortex whose center located in the lower layer was created and the channeling flow enhanced the removal of plume across lateral end. The slanted-shaped roof also had a significant influence on the concentration distribution. The shape of the vortex in the upper region was influenced by the roof form and carbon dioxide in the upper region moved towards the leeward roof according to the flow pattern.