Coupling of external winds and recirculations with interior contaminant release modeling

Today's world faces threats from unintended environmental disasters such as a railroad tank car derailment or a gas main break as well as the ever-present specter of terrorist action. Detailed contaminant transport modeling that includes the effects of realistic complex geometry is central to any technology aimed at providing a timely, effective response to a chemical or biological threat or to assess the effects of an obscurant cloud. A key requirement is the ability to compute the flow and deposition of contaminant particles and gases throughout geometrically-complex urban environments under a variety of meteorological conditions. The FAST3D-CT code, an extension of the FAST3D Flux-Corrected Transport model to contaminant transport problems, represents a current hazard assessment capability for urban environments and steep terrain. This scalable Computational Fluid Dynamics model was used to perform detailed simulations of the Bacillus gobigii and propylene release trials conducted in 1997 at the Michael Army Airfield (MAA) hangar and the German Village complex, Dugway Proving Ground, and recently released. "Reach back" simulations were performed using internal building measurements, heating, ventilation, and cooling (HVAC) system configuration, and external meteorological conditions at the time and location of the trials.

This paper summarizes FAST3D-CT sensitivity studies performed using contaminant release scenarios within the convention center mockup (i.e., the MAA hangar). The numerical studies focus on the influence of external winds on the interior contaminant transport and distribution. For the scenarios investigated, direct connectivity between internal and external flow environments is a prerequisite to modeling accuracy. Leaks present in the hangar have a significant effect on interior flow patterns due to the strong external winds flowing over the building. The effect depends on the strength of the winds relative to the properties of the HVAC system and the building leakage characteristics. Simulations with and without imposing prevailing atmospheric conditions are contrasted to highlight the importance of direct coupling to the external environment. The studies also identify areas where better models and better initial measurements of the configuration are required.