Physical modeling of the downwash effect of rooftop structures on plume dispersion

One of the major causes of poor indoor air quality in buildings is exhaust re-ingestion at fresh air intakes. The downwash effect of an RTS on plume may lead to significantly increased levels of re-ingestion. The present study aims to quantify the downwash effect of an RTS located upwind of a stack.;The minimum dilution models recommended by ASHRAE (2003, 2007) as well as the ASHRAE Geometric Stack Design Method (AGM) were evaluated. It was found that the ASHRAE models do not account for the downwash effect of the RTS on plumes and result in either overly conservative or un-conservative predictions. Therefore, a new empirical model, which takes into consideration the downwash effect of RTS was proposed. A validation study of the proposed model using results from previous experimental studies showed that the model is more accurate than the ASHRAE dilution models. Thus the proposed model will be very useful to practicing engineers. In addition, some modifications to the ASHRAE (2007) dispersion model and the AGM are also proposed.;Wind-tunnel modeling is used to model the flow and dispersion associated with various building/RTS arrangements with a focus on "micro-scale" urban dispersion; that is, modeling dispersion within a range of 100 m of the building exhaust. Wind-tunnel techniques include flow visualization and tracer gas experiments. The influence of various key parameters, such as building height, RTS crosswind width, stack height, exhaust momentum ratio, stack location, and wind direction is considered. Concentration measurements were obtained at sampling locations downwind of the RTS on the building roof. It was found that the downwash effect of RTS on plumes could increase roof -level concentrations by a factor of 2 to 100. The RTS downwash effect was intensified with an increase in RTS crosswind width and was generally stronger for an oblique wind than for a normal wind. The results also showed that an RTS engulfed inside the building recirculation zone may not have a significant effect on plume dispersion for winds approximately normal to the building face.