Buildings are always found to be in the vicinity of other buildings, especially in urban areas. This causes effluents released from stacks located on one of the buildings to re-enter the same or an adjacent building, generating potential health problems to the occupants of the building. Earlier, Computational Fluid Dynamics (CFD) has been used in simulating pollutant transport for isolated buildings, with only few studies examining the effects of adjacent buildings. In this paper three cases that include an isolated low-rise building (source), a taller building placed upwind of the source and a case with taller buildings placed upwind and downwind of the source were considered.CFD simulations using the Realisable k-ε model for different turbulent Schmidt numbers (Sct) and wind tunnel experiments were performed for these cases. ASHRAE 2007 was also used to assess plume dispersion for the isolated building. It was found that a strong dependence of Sct on CFD simulations of pollutant transport exists for the isolated building configuration. However, variations of Sct have less impact on assessing pollutant dispersion in the presence of adjacent buildings. The ASHRAE 2007 model predicted very low dilutions for the isolated building, making it necessary to re-visit its formulations.

This paper examines the effects of near-field pollutant dispersion characteristics of upstream buildings in the built environment and compares them to the ASHRAE 2007 model. Wind tunnel simulations were performed for nine different building configurations for three exhaust momentum ratios (M) and three stack heights (hs). The effect of spacing (S) between the buildings and stack location from the upwind edge of the emitting building (X) were also investigated. Measurements of gas concentrations were performed on the roof and leeward wall of the emitting and upstream buildings.Data show that within the recirculation zone a change in along wind dimension of the upstream building has a negligible effect on the dilution of emissions from the downwind building. However, spacing between buildings and the height of the upstream building were found to be critical parameters in assessing plume dilutions. The plume geometry is largely governed by the upwind dimensions of the upstream building. ASHRAE 2007 predicts lower dilutions for all cases examined, leading to conservative or very conservative design. Additionally, ASHRAE 2007 cannot model the effect of upstream buildings requiring further investigation of its formulations. Guidelines for placement of intake and stack on the roof of the building to avoid problems of re-ingestion are discussed.

The effect of near-field pollutant dispersion characteristics for the case of downstream buildings in the urban environment has been presented in this paper. Wind tunnel data were obtained for nine different building configurations, three exhaust momentum ratios (M) and three stack heights (hs), for wind azimuth of 0 o . Tracer gas concentrations were measured on the roof, windward and leeward walls of each building.When a tall downstream building was located within the recirculation length of the emitting building, higher rooftop concentration was measured on the emitting building than for the isolated building case. Results also show that the height and across-wind dimension of the downstream building, as well as the spacing between buildings are critical parameters in assessing plume dilution. ASHRAE 2007 and ASHRAE 2011, which apply Gaussian-based models for the evaluation of dilution, are unable to model the effect of adjacent buildings; the former yielded lower dilution for all cases examined whilst the latter was found to be suitable only for specific limited cases. Design guidelines for the placement of stack and intakes to avoid or minimize plume re-ingestion are proposed.

a b s t r a c tThe prediction of pollutant dispersion in urban environment is an extremely complex phenomenon, particularly in the vicinity of a cluster of buildings. Dispersion of effluents released from stacks located on building roofs are severely affected by adjacent surroundings. This paper investigates the impact of an upstream building on the near field of a pollutant source in terms of dilution distribution on the roof of an emitting building. The study was carried out using Computational Fluid Dynamics (CFD) approach with Realizable k-e for turbulent flow modeling. A limited number of cases were also modeled in a wind tunnel for validation purposes. The study shows that when the source is located within the recirculation zone, dilution is highly sensitive to the height of the upstream building and much less sensitive to the width and length of the upstream building. It is also shown that dilution value has an asymptotic behavior which defines the particular point where dilution becomes independent of the upstream building configuration. Some discrepancies between CFD and wind tunnel data were found, specifically for extreme configurations e.g. significantly taller upstream building. These differences are mainly due to the inherent unsteady fluctuations in the wake of buildings which are not detectable by RANS.

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