INFLUENCE OF BUILDING SIZE ON ROOFTOP DISPERSION OF EXHAUST-GAS

An exhaust gas dilution model that accounts for initial near-source di lution and the influence of turbulence scale and intensity at roof lev el is presented. The performance of the model is compared with tracer gas measurements of rooftop concentrations from vertically directed hu sh vents on 11 flat-roofed building configurations in an urban boundar y layer simulated in a wind tunnel. A power-law approximation for disp ersion in homogeneous turbulence predicts that distance-dependent dilu tion should be proportional to the product of the square root of build ing frontal area, and the source-to-receptor distance to the 1.5 power . Comparison with existing wind-tunnel data shows that this building s ize-dependent relation is a better estimate for dilution than the usua l assumption of dilution proportional to the square of downwind distan ce, independent of building size. The data supports the use of a linea rly additive initial plume radius to account for near-source dilution, and shows that this initial dilution affects total dilution far downw ind.