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Old   June 17, 2014, 12:15
Default Comsol CFD with porous media
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Carreira
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Hi everyone,

I would like to ask your for help in COMSOL.

I'm trying to join three consecutive tubes, the first with turbulent flow,
the second with a porous medium, and the last turbulent again.

I'd like to solve all the parameters continuously, in particular the speed and pressure.
I discovered that it is enough to put the same name of parameters in both physicals ( u,v,w,p), but the speed is still not continuous.
That is, at the end of the first tube velocity falls to zero, the second starts at zero and so forth.

Does anybody know which is the better way to solve this problem? Any suggestion?

Thanks,
Carreira
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Old   June 17, 2014, 14:19
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Иван Сташко
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Carreira,

I haven't seen this happen.

Do you have a sample .mph file that you could post? I would look at it for you.
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Old   June 18, 2014, 07:34
Default mph file
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Hi Ivan, thank you for your interest!!

Here is the sample file:
https://www.dropbox.com/s/kax6kztmdc...ous_3tubes.mph

you can clearly see (in the second chart 1D plot) that the velocity drops to zero.

I may have to make a merge of the mesh, but I don't know how to do this.

Thank you!
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Old   June 18, 2014, 16:19
Default Inlet and outlet defined
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Иван Сташко
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Carreira,

At least in the model file which I received, there is no inlet and outlet boundary condition. The only boundary conditions are the default ones, Fluid Properties, Wall, and Initial Values. You need to define an Inlet and an Outlet. Without these, there will be no flow. Do you have these defined in your model on your system?

-Ivan
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Old   June 18, 2014, 16:45
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Yes i have,
but...
I think you didn't open the correct file because I did't put any materials and in this image I can see two materials and....no porous media.

https://www.dropbox.com/s/kax6kztmdc...ous_3tubes.mph

Ty Ivan
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Old   June 18, 2014, 19:30
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OK, now I see the behaviour.

It looks to me like it is set up correctly.

Normally, however, I do assign a material to every region. But I see you have the viscosity and density specified directly. I don't know if it makes a difference, maybe you should assign a material to all regions just to check.

Also it appears the continuity equation is violated. If V is high going into the porous section and low coming out of it, then there must be mass build-up in the porous section.

One other interesting thing, you have the pressure as p in the porous section and 0 in the turbulent section. Normally, you can open the Model Inputs section of the Fluid Properties tab, and there will be a reference pressure. But this model does not offer this ability. I have seen this before, as well, and I don't know how to consistently get to and set the reference pressure.
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Old   June 19, 2014, 06:01
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Do you have any ideia how to solve the problem with continuity equation?

I added one material to my geometry and this made no difference...in fact i can't converge the solution.

Last edited by Carreira; June 19, 2014 at 08:32.
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Old   June 19, 2014, 16:21
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Quote:
Originally Posted by Carreira View Post
Do you have any ideia how to solve the problem with continuity equation?

I added one material to my geometry and this made no difference...in fact i can't converge the solution.
Yeah, this is really touchy stuff... I'm surprised the solution now doesn't converge. Is it a new material, or one from their library?

The continuity equation, which is commonly "\div J = 0" appears to be part of that equation set, written as "\rho \div u = 0". It just *looks* like it's violated, you would have to integrate across one of the cross-sectional planes to actually check. They should all have the same amount af flux. (Expressed in your system, it would be the integral at one cross-sectional plane minus the integral at another cross-sectional plane should be zero.)

And that brings up another point. Make sure you integrate "\rho |u|" across the cross-sections. Since "\rho" is involved, this means looking at a map of |u| may be misleading. On the other hand, the density shouldn't change that much.

By the way, I'm still curious how to get to the reference pressure, which is under Fluid Properties > Model Inputs. As I have said, I've seen it in some models, but not it all, even when they employ the exact same physics, and I don't know how to make it just appear. Let me know if you find out.

Your pressure results look correct, but I do not see them for the outlet. Could you generate them?

Why are you using the k-omega model instead of k-epsilon? Have you tried to run it using k-epsilon?
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Old   June 20, 2014, 11:45
Default k-e v k-omega
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Carreira,

I have been using k-e with great success in models without curl, such as long pipe sections similar to your model.

But you have selected k-omega. I ran across this thread:
Use of k-epsilon and k-omega Models
and I recommend you read it, particularly near the bottom, vivek**'s comment about the applicability of k-e. It seems, after reading this thread, I would chose k-e as well for your model.

I'm still curious how this turns out.
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