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Flow in a pipe with a very large aspect ratio

Kyle Macfarlan
I'm trying to solve what should be a very simple problem as part of a larger project I'm working on. It's just laminar flow in a pipe of length 14,000 feet and radius 0.04 feet. I'm just trying to find the steady-state solution for now. The issue is that the inlet velocity is satisfied on the boundary, but rapidly decays to zero velocity. This is demonstrated in the attached image.

It's a 2-D axisymmetric model. Symmetry on the left boundary, wall on the right, inlet at the top and outlet at the bottom. The outlet is set to a zero pressure, and I have tried many different inlet boundary conditions. I started with a much smaller system, the same radius, but a length of only 8 ft which worked perfectly, but it's not working on the larger system. I feel like I'm missing something obvious.

Thanks!
Kyle


5 Replies Last Post Apr 8, 2013, 2:51 AM EDT
Posted: 5 years ago Apr 4, 2013, 5:01 PM EDT
Hi

I cannot tell why but in my opinion for that pipe size ratio, you should rather consider the Pipe flow module, as that one is made specifically for those kind of pipings

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Good luck
Ivar
Hi I cannot tell why but in my opinion for that pipe size ratio, you should rather consider the Pipe flow module, as that one is made specifically for those kind of pipings -- Good luck Ivar

Nagi Elabbasi Certified Consultant
Posted: 5 years ago Apr 5, 2013, 11:39 AM EDT
Hi Kyle,

I had a similar problem with a high aspect ratio model and I switched to direct fully coupled solver and it worked. Try it out if you are currently using some form of iterative or segregated solver.

Nagi Elabbasi
Veryst Engineering
Hi Kyle, I had a similar problem with a high aspect ratio model and I switched to direct fully coupled solver and it worked. Try it out if you are currently using some form of iterative or segregated solver. Nagi Elabbasi Veryst Engineering

Kyle Macfarlan
Posted: 5 years ago Apr 5, 2013, 3:31 PM EDT
I am using the direct, fully coupled solver. I'm pretty sure it's a mesh issue the more I look at it.
I am using the direct, fully coupled solver. I'm pretty sure it's a mesh issue the more I look at it.

Posted: 5 years ago Apr 5, 2013, 4:06 PM EDT
Hi

what kind of mesh do you use ?
I have notices that if you mesh the entry section correctly to resolve the parabola, then use a sweep mesh with even rather long elements, the results is quite OK in laminar mode, even when the aspect ration or rather poor. This is because once developed, the profile shape changes little e along the tube

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Good luck
Ivar
Hi what kind of mesh do you use ? I have notices that if you mesh the entry section correctly to resolve the parabola, then use a sweep mesh with even rather long elements, the results is quite OK in laminar mode, even when the aspect ration or rather poor. This is because once developed, the profile shape changes little e along the tube -- Good luck Ivar

Niklas Rom COMSOL Employee
Posted: 5 years ago Apr 8, 2013, 2:51 AM EDT
Chances are that there is something something fishy with one of your boundary conditions. Even if the mesh is poor, such a non-mass conserving solution should not be possible. But still guessing, since I have not seen your model.

I also agree with the previous posters: You should have a look at the Pipe Flow module for problems like this.

Kind regards
Niklas

Chances are that there is something something fishy with one of your boundary conditions. Even if the mesh is poor, such a non-mass conserving solution should not be possible. But still guessing, since I have not seen your model. I also agree with the previous posters: You should have a look at the Pipe Flow module for problems like this. Kind regards Niklas

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