Abstract
It is a known problem that CFD models using the standard k - e turbulence model do not maintain the correct atmospheric boundary layer (ABL) profiles along a flat, unobstructed domain. The present work examines the impact of these errors in the ABL profiles on dispersion model predictions for three field-scale experiments from the Prairie Grass and Thorney Island datasets. The modified ABL profiles produced by the CFD model in the Prairie Grass experiments result in differences in the predicted concentrations of up to a factor of two, as compared to a reference model. For the Thorney Island experiment, the results for the standard k - e turbulence model are sensitive to the ground surface roughness and problems are identified in relation to the grid resolution near the ground. Industrial risk assessments involving atmospheric dispersion of toxic or flammable substances using CFD models should take into account these limitations of the k - e turbulence model.
Highlights
There is a growing interest in the use of CFD to assess the risks posed by atmospheric releases of toxic and flammable gases from industrial sites, such as chemical plants and refineries (e.g., Pontiggia et al, 2009; Hansen et al, 2010)
The CFD model with a z0-based wall function may capture the overall effect of the rough wall on the atmospheric boundary layer (ABL), but it will still not resolve the localised variations in velocity or concentration through the unresolved roughness elements, which may be important in the context of dense-gas dispersion of flammable or toxic substances
For the stably-stratified Prairie Grass PG36 case, Figure 6(b) shows that concentrations from the scalar released at source B and source D are practically identical, but that both concentrations are around a factor of two lower than the concentrations obtained with the fixed ABL profiles
Summary
There is a growing interest in the use of CFD to assess the risks posed by atmospheric releases of toxic and flammable gases from industrial sites, such as chemical plants and refineries (e.g., Pontiggia et al, 2009; Hansen et al, 2010). There were limited options available to modify these boundary conditions in ANSYS-CFX Another important consideration is that the modifications to the standard k – ε model aimed at improving predictions of ABLs may adversely affect the prediction of more complex cases, for example involving flow around obstacles. The paper proceeds by briefly reviewing the boundary conditions used by CFD models to simulate atmospheric dispersion This is followed by a description of the CFD model configuration for the Prairie Grass and Thorney Island experiments, the results in terms of the ABL profiles and concentrations for the three test cases and a summary of conclusions
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