The fact that changing your domain size alleviates this problem leads me to believe it may have something to do with your mesh. How many cells do you have along the diameter of the pipe in the problematic calculation?
One other thing I would try is setting an outlet mass flow rate or volumetric flow rate. By setting a constant velocity you are forcing a linear profile at the outlet which may be messing with your simulation.
I have tried to use a volume flow rate instead of velocity, but no changes in the results.
Regarding the mesh, I confess that I'm not familiar with all parameters, but I set the Level of initial mesh to 5, The minimum gap size to 0.05m, and the Minimum wall thickness to 0.00095m (which is the thickness of the pipe). With these configurations, the mesh in a section of the pipe has 16x16 cells. But also the same problem occur, without any apparent improvement.
Any other idea of what can be wrong?
Thank you very much!
Your pipe diameter is 250mm, so I would set that as the minimum gap. You don't need to worry about the wall thickness in this case, since unless I'm mistaken this is a simple internal flow situation with no ribs or small features inside the pipe, and there is no heat transfer involved. Try performing the simulation with a level 3 mesh, 250mm minimum gap, make sure that "narrow channel refinement" is enabled and let's see what results you get back.
Also if you upload your project I could take a look at it sometime when I have a free moment.
A couple of other things I'd recommend taking a look at, in addition to what Amit mentioned:
- You mention defining a "real wall" on the pipe walls. What conditions do you have attached to this? By default, the wall will be recognized as a no-slip wall, with the roughness specified in the general settings (default - 0 roughness), generally you don't need a secondary condition unless you want to control the roughness of specific faces.
- If you monitor the average static pressure at the outlet goal, rather than the bulk average pressure, do you still see the same oscillations?
As Amit said, the project file will help if you need a deeper review, but I would recommend taking a look at the Flow Simulation tutorials - the very first chapter should replicate exactly what you need to do, so if you reproduce that process and its settings on your model, I'd be interested to see if the issue still persists.