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Old   April 25, 2009, 07:10
Default How to define the interface between two fluids
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TfG
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Hello,

I am trying to model a tank which is half filled. Due to higher density water is is the fluid at the bottom part of the tank and air is above the water. I am simulating natural convection.

What I want to do is that at the interface the velocities of the air and water are the same in the plane of the interface but cannot penetrate one fluid in the other (i.e. normal velocity is zero). Also I want to set continuity of the heat transfer between the two fluids.

How should I model it? I did the mesh and I asig different fluids to each part, but I do not know how to model the interface. I tried with "interior" but it does not seem to work ( I get divergence and very strange patterns at the interface).

I would appreciate if somebody could help me with this,

thanks in advance
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Old   April 25, 2009, 09:36
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Create two separated fluid volumes (connected), don't set any BC at the interface.
Once your model is set, then you initialize the whole domain, and you patch each one with the right vof-value.
Regarding the non-penetration, I think it should be done in the phase panel (interaction), but I am not sure
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Old   April 25, 2009, 10:13
Default take care, that between both fluids is only one face!!
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If you have two volumes, that are simply attached to each other, and you define (one) interface at interior (or whatever else) Fluent will create a wall for that other interface!!

You can assure that by using in the face command field the "connect faces" button (looks like a plug). Select (all) faces of your domain and connect. That will cause all superposed faces to be joined.

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Old   April 25, 2009, 10:18
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Quote:
Originally Posted by Ralf Schmidt View Post
If you have two volumes, that are simply attached to each other, and you define (one) interface at interior (or whatever else) Fluent will create a wall for that other interface!!
Ralf
If you split one volume into two volumes, the "interface" between the 2 volumes (connected) will be set automatically as interior (or internal).
It will set as wall, if the 2 volumes aren't connected (eg: 2 surfaces superposed)
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Old   April 25, 2009, 11:18
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That is what I did, dont set any boundary at the interface (i.e. the same as to set interior condition). And yes, the faces were connected.

Are you suggesting me that I should use VOF model? Isn't any possibilitie to do it without it?

thanks
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Old   April 25, 2009, 12:01
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I don't see other possibility else than multiphase (but I am not expert).
I did a similar calculation with a tank filled at the 2/3 with oil, and the rest with air, but without convection.
I solved it with multiphase
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Old   April 25, 2009, 14:23
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Tried the VOF model and patch like you said but I get and error at the beginning of the second iteration:
Updating solution at time level N...
Global Courant Number : 560.10

Error: Global courant number is greater than 250.0. The
velocity field is probably diverging. Please check the solution
and reduce the time-step if necessary.
Error Object: ()

The no penetration thing I didn`t find anywhere....
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Old   April 25, 2009, 15:58
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Define/Phases...
Go to the interaction panel
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Old   April 26, 2009, 05:45
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I already did it, and defined my two phases (oil and air)... but, then in interaction there are only available two tabs to edit: Mass and Surface tension.... and nothing about no penetration.
I am pretty lost now... and I need this thing for my final project!
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Old   April 26, 2009, 05:51
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on the 6.3 version there are more options like slip velocity between phases etc...
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Old   April 26, 2009, 06:55
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Yes yes, Im running 6.3.26 and I have those tabs, but it says: this page is not applicable under current settings...
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Old   April 26, 2009, 07:01
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Then check the help, maybe those tabs aren't available with multiphase.
Try mixture model.
But as I said, I am not expert
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Old   April 27, 2009, 03:12
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Dear friend
if u want to see only convection then make that interface to wall type in preprocessor and then import those two seperate volumes to fluen and to that wall & its shadow give 0 shear stress in x y & z direction so that proper convection pattern will form at interface (i.e wall) & model density with boussinesq model

regards Nitin
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Old   April 27, 2009, 04:31
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Hi,

Thanks for the answers, I will try that one.

Only two observations, first, on thermal boundaries I shoul set coupled right?
Second, I see shear stress 0 could be a reasonable assumption for the part of water. But for the air, shouldn't it be more like a non-slip condition (the air will have it difficult to "move" the water (forgetting waves...))?

Just wondering about it, but I am not expert so I may follow your advice and set shear 0 for both sides...
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Old   April 29, 2009, 07:12
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yes u r right but this will give u proper convection velocity pattern for heat transfer near the inerface (avoiding no slip on both sides of interface wall.
have u tried this??

Thank you
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Old   April 29, 2009, 13:29
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Hey,
Yes I tried with slip condition in both side and it seems to work fine, Thanks!

Still, do you think I should mantain lip conditions in the air part or no slip conditions?
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Old   April 30, 2009, 01:02
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with slip on air side will disturb ur convectopn (velocity pattern on air side )
if u want to see this just ceck it out..

Thanks
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Old   May 3, 2009, 10:49
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I tried setting slip for water side and no-slip for air side and the heat transfer in the interface is smaller than if I set slip for both sides (about 20% less).

The question is which result is more "real"... ?
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