The surface is visible in my stl-file. Should it work?
“This is done inside OpenLB and not with the STL-file. You need to assign different material numbers to the different regions ”by hand” (see cylinder3d).”
What do you mean by “the surface is visible in my stl-file”? Can you link the file?
The general approach is to import the STL file (or rather the surface described by it) as a material number and then rename indicator-described subsets to represent e.g. inlets. See e.g. examples/turbulence/aorta3d.