D1.3 Size of the Computational Domain

In CFD simulations, selecting the flow cross-section is crucial. A cross-section that is too small can significantly distort results because no complete pressure equalization occurs, leading to deviations in absolute pressure coefficients. Unlike wind tunnels, which may have minor leaks, CFD grids are completely sealed and are particularly sensitive to blockage effects. With insufficient flow cross-section, especially suction coefficients, can be greatly exaggerated. To minimize these effects, the blockage ratio for solitary objects should be less than 1%, and under no circumstances should it exceed 2-3%. Complex topologies require special considerations.

For the domain dimensions, the approach distance should generally be twice the displacement width or height of the structure, provided the inflow already exhibits a fully developed profile. The wake area should extend 10-20 structure widths. Vertically, in 3D simulations, an area at least 5 structure heights above the object should be provided, while for 2D calculations, at least 10 structure heights. Laterally, the flow area should extend at least 3 structure heights on each side. The COST guidelines recommend even more generous dimensions, with 5 structure heights in front and on the sides of the object, and 15 structure heights in the wake. Summarized, this table results in:

Table 5: Recommendations for Geometry Modeling of the Computational Domain