A few things:
- You should only include the interior wall of the geometry in the STL file. Here you have both sides.
- You should orient the normal vectors so they point towards the fluid region. Here they point towards the solid region, so you are meshing the wall region, not the fluid region.
- I like to have the STL geometry stick out from the inlet and outlet planes to get a better mesh along these planes.
- You should always inspect the geometry/mesh before running the solver. This will most likely help you catch the issues above.
- If you use incompressible gas, you may wan to increase the number of iterations for the Gas pressure equation (say 100, in the Numerics pane, Linear solver tab). You can also switch to ideal gas law, which may help with initial convergence.