# CFD Prefers NURBS Over STL

Let me explain why, given the option, you should prefer analytic geometry (e.g., NURBS) over faceted geometry (e.g., STL) for Computational Fluid Dynamics (CFD).

Note spurious flat spots

While many CFD packages accept faceted geometry, it is usually inferior to analytic geometry because faceted geometry has:

- Limited discrete resolution of curvature (faceted assumes planar definition between points), whereas analytic definitions are continuous.
- Limited, if any, topology.

Representing curvature as a series of linear (planar) elements means that simulation tools downstream do not see the original continuous geometry and so produce compromised results on the wrong geometry.

Note smooth variation around the wheel

Note flat spots and artificial corners around the wheel

Note smooth Cp variation around the wheel

Note spurious Cp peaks around the artificial corners on the wheel

It is often frustrating converting a faceted definition into a watertight solid/volume, because the original connectivity (topology) is lost in the conversion process from analytic to faceted. Resorting to splitting and grouping facets according to included angles and tolerance-based point comparisons to recover the original topology is less than ideal.

Note missing edges compared to analytic topology

Sometimes there is no option but to use faceted geometry. For instance many scanned organic objects never had an analytic definition, so facets are your only recourse. Other times your choice of a faceted modeler (e.g., SketchUp) will dictate that your geometry is faceted. However, if you have control over your modeler software then it will pay dividends if you choose a CAD system that uses analytic geometry - which most do.

Faceted representation sourced from SketchUp

Exporting faceted geometry from a CAD system is nearly always a bad idea, given you could just as easily export the analytic geometry in a transfer format such as STEP or IGES. Exporting faceted geometry leaves you at the mercy of the CAD facet conversion process, which is typically optimized for visualization, i.e., a surface mesh in CFD speak, without much control over the quality of the facets.

Note individual facets are visible around curvature regions

Some CFD tools either only accept faceted geometry or perform an internal conversion process to facets prior to meshing. Here again, like in the CAD facet conversion process, you are at the mercy of a hidden surface meshing stage where the resolution of curvature is reduced to planar facets, thus losing the original continuous definition. The best CFD tools retain the analytic geometry, if supplied, and ensure that the final generated mesh points are located on the true analytic definition.

If you have taken great care to generate accurate geometry in your CAD system, then it is worth ensuring that the same analytic definition is the basis for your CFD simulations, rather than a potentially inferior faceted definition.

- Richard Smith's blog
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