Abstract
CFD simulations of near-ground gas dispersion depend significantly on the accuracy of the wind field. When simulating wind fields with conventional RANS turbulence models, the velocity and turbulence profiles specified as inlet boundary conditions change rapidly in the approach flow region. As a result, when hazardous materials are released, the extent of hazardous areas is calculated based on an approach flow that differs significantly from the boundary conditions defined. To solve this problem, a turbulence model with consistent boundary conditions was developed to ensure a horizontally homogeneous approach flow. Instead of the logarithmic vertical velocity profile, a power law is used to overcome the problem that with the logarithmic profile, negative velocities would be calculated for heights within the roughness length. With this, the problem that the distance of the wall-adjacent cell midpoint has to be higher than the roughness length is solved, so that a high grid resolution can be ensured even in the near-ground region which is required to simulate gas dispersion. The evaluation of the developed CFD model using the German guideline VDI 3783/9 and wind tunnel experiments with realistic obstacle configurations showed a good agreement between the calculated and the measured values and the ability to achieve a horizontally homogenous approach flow.
Highlights
When assessing hazards from industrial plant applications, calculating the gas dispersion is one of the main tasks for determining safety distances
By using a power law to describe the vertical velocity profile and introducing the additional source term Sε, it should be possible to simulate a horizontally homogenous boundary layer flow with a grid resolution not depending on the roughness length
The transient, compressible flow solver rhoReactingBuoyantFoam of OpenFOAM 5.0 in combination with the standard k-ε turbulence model is promising for simulating the dispersion process of pollutants in the atmosphere
Summary
When assessing hazards from industrial plant applications, calculating the gas dispersion is one of the main tasks for determining safety distances. Simple models for calculating the gas dispersion are generally not able to account for obstacles such as buildings or the topography of the dispersion area. To simulate dispersion scenarios in complex areas, computational fluid dynamics (CFD) codes can be used. By solving the full Navier–Stokes equations, CFD codes can simultaneously model the wind field and the dispersion of pollutants, taking into account obstacles, topography, and thermal stratification, as well as the type of release (with or without momentum). The main problem when using CFD codes is the cost–benefit ratio, as the computational time is exceedingly high compared to simpler models. To reduce the computational time, relying on RANS (Reynolds-averaged Navier–Stokes) equations is a common practice. To close the RANS equations, the k-ε turbulence model is used in a wide number of publications for calculating the atmospheric wind field
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