Application of CFD Simulations for Short-Range Atmospheric Dispersion

Computational Fluid Dynamics (CFD) models have been shown to have advantages over traditional dispersion models in simulating the influences of local micrometeorology, and effects of buildings and other surface structures. In order to increase its application in support of air quality studies, comprehensive evaluations of CFD models are necessary to establish best-practice methodologies. Fluent, Inc and the US EPA National Exposure Research Laboratory are working cooperatively to demonstrate CFD simulation as a proven and applied tool in support of environmental assessment studies. Part of the ongoing work has focused on the development of computer models and the evaluation of the performance of the FLUENT code in simulating: (1) the atmospheric boundary layer, (2) plume dispersion over an open field, and (3) dispersion within arrays of buildings.

Examining the ability to simulate atmospheric-like boundary layer flow in absence of buildings is important to establishing best-practice methods before proceeding with more complicated plume dispersion. This work has used atmospheric boundary layer theory as a basis for testing the sensitivity of code parameters, such as grid resolution, boundary conditions, and the turbulence model. The mean profiles were well predicted and were maintained with downwind distance when applied to a finite domain. In addition to a neutrally stable atmospheric boundary layer, steady-state thermal boundary layers have been simulated by adding surface heat flux at the bottom boundary. The results compared well with Monin-Obukhov similarity theory.

Case studies based on the Project Prairie Grass field program were used to develop and evaluate CFD simulations of plume dispersion over an open field under thermally neutral and unstable conditions. The steady-state dispersion solutions were perturbed using the wind direction temporal data to account for variations in wind direction. Results were compared with tracer gas release data, as well as with predictions using Gaussian plume-type models. The CFD model performed well in capturing case specific variability due to unsteady winds.

Simulations of dispersion around buildings are being evaluated with data from the Mock Urban Setting Test (MUST) field experiments. Simulations of pollution dispersion within obstacle arrays over periods of varying wind speed and direction are more challenging than for the same over open fields. Best methods for applying CFD simulations for these complex flow situations are being developed. Updated results will be presented in this paper.

This research is leading to the development of general guidelines to support future broad application of CFD code for simulating short-range atmospheric dispersion of pollution where refined solutions are needed.

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.