Title: The prediction of toxic gas infiltration for a building in an urban area

We have evaluated a near source ambient air concentrations resulting from hypothetical toxic emissions from a hypothetical source in an urban area. The evaluation used high-resolution Computational Fluid Dynamics (CFD) simulations which take into account the emissions from a gas emission source and the local wind, turbulence and contaminant transport fields in the area to estimate the pedestrian level concentrations as well as concentrations at selected receptor locations on a building. We approach the problem with: (1) identifying a source location and strength (2) identifying a receptor area on a building of interest and incorporate the such area on the geometry and be able to defining as a fluid exit area that locates within a computational domain (3) assuming the prevailing wind as the logarithmic wind profile or using available wind profile such as wind library. (4) predicting gas dispersion using CFD-Urban computational code or ransolver/speciesTransport solver that is recently developed in CFDRC. (5) and calculating the amount of toxic gas infiltration on a specific building. We may demonstrate several important physical phenomena such that (1) the prevailing wind is stronger with the same wind direction; a specific building is exposed to the contaminant less. (2) depending on the wing direction, backward facing intake vent can intake more contaminant (3) and the street channeling flow dominates on ground level in urban area. Ultimately this infiltration prediction may offer useful information about the location and defining sizes of a building HVAC system.