Contamination of fresh air intakes due to downwash from a rooftop structure.

The effect of rooftop structures on the dispersion of building exhaust was evaluated using wind tunnel experiments with an isolated building for four different full-scale equivalent heights varying from 15 to 70 m. The stack was located on the roof of the building downwind of the rooftop structure. In some cases, downwash produced by the rooftop structure caused a significant reduction in dilution (i.e., increased contaminant concentration) compared with results obtained with a flat-roofed model. For the wind normal to the building, the effect of the rooftop structure depended on building height and the crosswind width of the structure. Maximum reduction in dilution occurred for the 15-m building with a wide penthouse. The influence of the rooftop structure on plume behavior for the normal wind diminished as building height increased because of an increase in size of the separated flow region above the roof. On the other hand, for an oblique wind at 45 degrees, the rooftop structure produced low dilution for all building heights. The paper shows that the American Society of Heating, Refrigerating and Air-Conditioning Engineers geometric stack design method and dilution model can lead to an unconservative design.