Development and Application of CFD Simulations Supporting Urban Air Quality and Homeland Security

Prior to September 11, 2001 developments of CFD were begun to support air quality applications. This presentation reviews the original intent and present priorities to support analyses understanding the dust cloud on September 11, 2001, transport from the fires over the following weeks, and continued general development now supporting the Department of Homeland Security New York City Urban Dispersion Program. These developments and applications will be summarized to illustrate what has been done and to characterize remaining challenges.

Material transport within these urban centers is dominated by the influences of buildings. Computational Fluid Dynamics (CFD) simulations are emerging as a promising technology for supporting such assessments, in part due to the advancing power of computational hardware and software. CFD simulations have the potential to yield more accurate solutions than other modeling methodologies because they are a solution of the fundamental physics equations and include the effects of detailed three-dimensional geometry and local environmental conditions. However, the tools are not well validated for environmental modeling and best-practice methodologies have not been established. The results of CFD simulations can both be directly used to better understand specific case studies as well as be used to support the development of more simplified algorithms that may be generally applied. Boundary layer turbulence is being simulated as characterized by surface roughness (characterized by u*) and surface heat flux (characterized by Obukhov length L) in the absence of urban buildings.

This presentation summarizes ongoing developments and applications of CFD simulations through case studies using CFD software (FLUENT) for simulating air pollutant concentrations from sources near buildings. Present results are very promising. Building geometry has been to date based on photogrammetry applied by Vexcel Corporation. Photogrammetry derived buildings are based on analyses of a series of photographs. While starting with a good ArcView file of basic building geometry there is work required to clean up and refine the geometry before it is ready for CFD applications. Improved methods for creating working building geometry and mesh are key elements for improvement in order to minimize the overall time required to develop a CFD application in real urban areas. The process is much smoother for idealized building shapes. Tests are conducted to determine what level of building details is necessary. Once good building geometry is applicable, then the set up turns to defining mesh refinement and defining the surface boundary conditions, the turbulence model, and the options for selecting code parameters. The amount of required computational resources varies greatly as one moves for simpler turbulence models up to fine-scale “large eddy simulation” models. Some of this selection is critically determined by the software being used and the available hardware or supporting computations. Many of the parameters used within CFD code for the turbulence models and other settings have been developed based on experiences gained from a history of development and application in the aerospace and automotive industries. New tuning of these parameters may be necessary for appropriate application to simulating flows in urban environments. Future wind tunnel and field studies should play a very critical role in supporting these developments.

DISCLAIMER

The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.