### 6.3.2. Geometry Generation

Before the grid generation can start, the geometry has to be created or imported from CAD-data. In both cases, attention should be given to:

- The use of a correct coordinate system

- The use of the correct units

- The use of geometrical simplification, for example, symmetry planes

- Local details. In general, geometrical features with dimensions below the local mesh size (for example, wall roughness or porous elements) are not included in the geometrical model. These should be incorporated through a suitable model.

In the case that the geometry is imported from CAD-data, the data should be checked beforehand. Frequently, after the import of CAD-data, the CAD-data has to be adapted (cleaned) before it can be used for mesh generation. It is essential for mesh generation to have closed volumes. The various CAD-data formats do not always contain these closed volumes. Therefore, the CAD-data has to be altered in order to create the closed volumes. It has to be ensured that these changes do not influence the flow to be computed.

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