there are parts inside the "tube" used for the computational domain but Solidworks says they are out of the computational domain, and they are not taken into account for the flow simulation
Actually, the problem was with the mesh : the local mesh around the flat plates has to be small enough so that the flow doesn't go through.
Usually this happens when your domain isn't actually including what you think it is. It's possible that you have leaks in your model and so Flow Sim is only considering some small cavity somewhere in your model to be the entire domain. Use the 'leak tracing' and 'check geometry' tools to see what your computational domain actually is. Personally I find the "show fluid" feature inside the 'Check Geometry' tool to be very useful for this purpose.
I made the model again from the beginning and this time I made every part directly in the assembly instead of making them separately, and now every part is taken into account.
I made the model again because there were some errors, and now I have the same problem as before, and I don't know why : the streamlines are going through the solid parts
they don't seem to go though solid part...
There are inclined flat plates in the middle and the streamlines are going through them
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