This paper presents a wind tunnel study of the pollutant dispersion over an urban area with three different types of thermal stability within the atmospheric boundary layer. The diffusion fields in the boundary layer were examined in three flow obstacle cases: ⅰ) boundary layer without flow obstacles, ⅱ) boundary layer over two-dimensional fence, and ⅲ) boundary layer over three-dimensional cubic building model. The scale of the model experiment is assumed to be at 1:500. In the experiment, gaseous pollutant is discharged in the simulated boundary layer over the flat terrain. Ethylene, C_2H_4, is used as tracer gas and a hydrocarbon analyzer detector (FID) is used to measure its concentration. The effluent velocity of the pollutant is set to be negligible. The density of pollutant gas is the same at the height of the pollutant effluent in the boundary layer. These experiments are performed in the stratified wind tunnel under three atmospheric conditions: stable (Ri_B=0.118), neutral (Ri_B=0.0) and unstable (Ri_B=-0.096). Wind velocity profile of 1/4 power law is simulated for all three cases. Concentration distributions (vertical and horizontal) were measured at four positions in the leeward direction (2, 3 , 6 and 13 H_m from gas source, where H_m is obstacle height). Laser Doppler Velocimetry (LDV) is used to measure the velocity field and the turbulence characteristics are analyzed. The results obtained are as the following: a) a thick internal boundary layer is generated in the case with 2D fence, b) the inner boundary layer is very thick around the wake region due to the turbulence mixing, c) the reattachment length of the separated flows with the 2D fence is longer than that with the cubic model, d) the vertical and horizontal distributions of concentration with the 2D fence are smaller than that with the cubic model. The concentration distributions measured in the experiment may be used for the evaluation of numerical models and expert estimating of air quality in the urban environment.
展开▼
