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Illustrative CFD workflow
MODEL SYSTEM
The first step is to gather information necessary to create a virtual representation of a physical system. This includes the dimensions and locations of major structures that affect wind flow.
In this example, a three-dimensional model of an industrial stockyard was developed from site drawings. The model includes oxide pellet and HBI stockpiles and a 25-meter perimeter wind fence.
MESH
The final mesh contains over 13 million elements, with the highest mesh density concentrated over the piles and near the wind fence. Three successive mesh refinements were conducted to resolve regions of high velocity gradients and turbulent structures.
A near-wall boundary layer mesh consisting of 10 graded layers was applied to solid surfaces to resolve shear stress at air-solid interfaces.
TURBULENCE MODEL
To compute the airflow, Fluent solves the Reynolds averaged Navier-Stokes (RANS) equations using the realizable k-epsilon turbulence model.
Atmospheric wind enters the domain as a fully developed boundary layer. Inlet velocity, turbulent kinetic energy, and dissipation rate profiles are specified. The profiles shown represent wind speeds of 5 and 45 m/s at 10 m elevation.
RESULTS (VERIFICATION)
The system of equations is solved iteratively until residuals fall below 0.001. In this example, 700 to 2,000 iterations were required for each simulation case.
Localized flow separation and recirculation downstream of the piles can slow convergence due to persistent velocity fluctuations. The streamlines shown illustrate a recirculation zone between piles for the 45 m/s wind-fence scenario.
RESULTS (VALIDATION)
A CFD model is validated when it is shown to accurately represent experimental measurements. For large-scale atmospheric flows over industrial stockyards, site-specific measurements are typically limited.
Here, pressure drops across the wind fence panels were compared with empirical fabric-resistance coefficients. These checks support the modeled fence behavior; they do not constitute field validation of dust capture. The velocity contours show the deceleration of airflow downstream of the fence.
FUGITIVE DUST FATE AND TRANSPORT
Once the flow field is established, particles are released from the tops of the piles. Fluent’s discrete-phase modeling is used to compute particle dispersion due to airflow, turbulence, gravity, and interaction with the ground and wind fence.
Particles with aerodynamic diameters of 2.5, 10, 20, and 50 microns were evaluated. The trajectories shown are for 10-micron particles released from one stockpile at 45 m/s with the perimeter fence in place.
WIND SHEAR AT EMISSION SURFACES
The entrainment of dust particles into the air is correlated with the shear stress at the air-ground interface. Dust is entrained when this wind shear reaches a critical value.
Contour plots of surface stresses and near-ground velocities can be used to examine probable locations that exceed this threshold. The contours shown illustrate surface shear stress on the piles at 45 m/s with the perimeter fence in place.