As noted, .stl files are not for simulation purposes.
In any case, perhaps, the following documentation page will help:
“A self intersection in the mesh means that there are at least two mesh faces that are going through each other.”
For successful meshes, the CAD must be clean. Perhaps, performing the Boolean operation may help clean the CAD. See documentation page below:
It is also probably better to fix the CAD before sending it to SimScale. Perhaps, it may be helpful to first consider this past discussion. It may also help in understanding/resolving the “face to face inconsistency” error received after importing the model to SimScale.
Perhaps, other notes to keep in mind:
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Try the hex-dominant parametric algorithm for meshing
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“Small entities can be a problem when it comes to meshing. If there are very small faces with sharp angles the surface meshing might fail.” See CAD Preparation & Upload | Simulation Setup | SimScale
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Make sure geometry is watertight.
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Here is a tutorial on the Formula Student Car Tutorial: Incompressible Flow around a Formula Student Car