# Discussion Forum

## Flow in a pipe with a very large aspect ratio

 Topics: 4.3, Fluid Flow
RSS feed   |   Turn on email notifications   |   5 Replies   Last post: April 8, 2013 6:51am UTC

Kyle Macfarlan

April 4, 2013 7:47pm UTC

Flow in a pipe with a very large aspect ratio

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

Ivar Kjelberg

April 4, 2013 9:01pm UTC in response to Kyle Macfarlan

Re: Flow in a pipe with a very large aspect ratio

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

Nagi Elabbasi
Certified Consultant
Veryst Engineering

April 5, 2013 3:39pm UTC in response to Ivar Kjelberg

Re: Flow in a pipe with a very large aspect ratio

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

April 5, 2013 7:31pm UTC in response to Nagi Elabbasi

Re: Flow in a pipe with a very large aspect ratio

I am using the direct, fully coupled solver. I'm pretty sure it's a mesh issue the more I look at it.

Ivar Kjelberg

April 5, 2013 8:06pm UTC in response to Kyle Macfarlan

Re: Flow in a pipe with a very large aspect ratio

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

Niklas Rom
COMSOL Employee

Moderator

April 8, 2013 6:51am UTC in response to Ivar Kjelberg

Re: Flow in a pipe with a very large aspect ratio

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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