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-   -   Periodic boundary conditions can somebody help? (http://www.cfd-online.com/Forums/fluent/118130-periodic-boundary-conditions-can-somebody-help.html)

 Tarano May 21, 2013 09:58

Periodic boundary conditions can somebody help?

In the picture you can see my geometry. What i want to do is create a periodic boundary condition between those two arrows, (around the geometry there will also be mesh. I have successfully created the periodic boundary conditions. But now i want a time-delay between those two boundaries as those points are connected with a tube with a specific length. I can't seem to figure out how to do that. I can't create the full length of the tube in the geometry due to entailment of the air. If a time-delay is not possible other suggestions would be welcome.

thanks for the help in advance

 Tarano May 21, 2013 10:01

somehow the picture didn't upload with it

 sbaffini May 21, 2013 13:35

If you look at your problem from the programming point of view you will notice that, independently from fluent, you actually need some storage for your profile for all the times between the current one and the lagged one of interest.

Now, to store this data, i can't think of a better option than actually meshing the connecting pipe.

This is because, in time or space, you will just need enough grid points to describe the evolution between the two faces along this connecting tube.

What is the length of the tube over its diameter and how many points you think you will need there? In theory, how many time steps would be required to store the time evolution of the profile during the lag time?

Consider that your periodic faces can be anywhere in the domain so, in theory, the connecting tube can still use the periodic boundary condition to connect the two faces. For example, if your domain is such that A e B are the periodic faces with lag, you can create an additional domain with faces C and D, like sketched below (hope you recognize your domain):

----------|OUT|-------------

A_______ | IN |___________B

------------------------------------------

C__________ tube of length L __________D

So, you can now use the following periodicities:

B with C
D with A

such that the overall geometry is like it is all connected, even if the second domain is totally separate from the main one.

 Tarano May 21, 2013 14:26

he sbaffini, thank you very much for you advice i never though of doing it that way i am going to try it and see if it works but looking at how you explained it this should work very well.

 Tarano May 22, 2013 10:58

He sbaffini,

have you ever done something like this before in fluent. I tried using interface boundary conditions and then having two meshes uploading them into fluent together (this will give me two fluid zones). But i can't seem to set up the interface correctly in fluent.

I have the exact same nodes on both locations but it seems that it doesn't like my offset i set. (i could use the auto because he couldn't find it so i had to do it manually which i did i placed the second zone in front of the first zone this means the offset is both in X and Y directions)

I used periodic boundary conditions within mesh interface in fluent.

This is the error i am getting.

Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Note: zone-surface: cannot create surface from sliding interface zone.
Creating empty surface.
Error: Non-conformal periodic interface with empty intersection.
Primitive Error at Node 0: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 1: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 2: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 3: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 4: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 5: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 6: Couldn't intersect threads 20031 and 14.

Primitive Error at Node 7: Couldn't intersect threads 20031 and 14.

This is likely due to incorrect specification of periodic
offset values. Please check the offset values specified
and recreate the interface.

Error: Couldn't intersect threads 20031 and 14.

If you have any tips for me i would appreciate it

 sbaffini May 22, 2013 11:10

I don't understand why are you using the interface if you have exactly the same nodes on both faces (if i understood correctly).

However, it seems also that you are using a sliding interface, something i never used. Is this still related to the previous case?

 Tarano May 22, 2013 11:30

I used interface because i though that was the way to connect two different flows together. I am not that experienced with CFD or fluent and nobody i know has done something like connecting two flows over a distance from each other.

This is still relevant to the previous case i have the combined mesh in the picture i just uploaded the arrows show where the parts are that i am trying to connect to each other though because its a big mesh in geometry size you can't really see the exact connection points.

These are two meshes 1 of the tube and 1 of the nozzle and the rest

the amount of nodes are exactly the same there 9 nodes in the width and the spacing in exactly the same.

How would you suggest i set the boundary conditions and this fluent solution to link the boundary correctly?

I am using icem to mesh my geometry and set basic boundary conditions.

Btw don't look quality of the the mesh is this is just a test case to get the boundary conditions correct first before i implement it in the real mesh

 Tarano May 22, 2013 11:33

I don't know what a sliding-interface is i didn't set it up knowly

 sbaffini May 22, 2013 12:05

Ok, as i still have some difficulties in understanding, i will assume the following:

1) you are still dealing with the original problem mentioned in this post

2) the last figure you attached is an enlargement of the previous one, where now
the additional connecting tube is also present (actually, it is more like a line but i presume this is just due to the dimension)

3) You are working in 2D, which is the only case where you couldn't actually connect the two original faces

4) For ease of discussion, let's say that the names of the faces are (with respect to the last figure):

A: top right (red one)
B: bottom right (white one)
C: top left (red one)
D: bottom left (white one)

5) I asume you are working with Fluent 12 or higher, otherwise some details might be different (but probably not)

With this premise, what you should actually do is:

a) Set A,B,C and D just as walls in the mesh generator

b) Once in Fluent, you will need to set the periodicities according to the following commands (via TUI):

gr/mo/mp
A_id
D_id
n
y
y
gr/mo/mp
B_id
C_id
n
y
y

where each line is a text to be digited in the Fluent TUI (followed by Enter). A_id, B_id, C_id, D_id are the IDs of the four faces, which you can find in the define->boundary conditions panel.

After this you are done. Consider that connecting red with red and White with White would have been possible but would have required a 180° rotation and a more cumbersome procedure. That's why i suggest to connect the faces in cross.

 Tarano May 22, 2013 18:38

All your assumptions where correct. I think you understood exactly what i wanted. It works aswell thank you, I am using fluent V14, its really strange. I normally only use the GUI and not the TUI. But i notice in the GUI i can't do what you just did but in the TUI you can.

I thank you for you great help its working now, and now i can imply it to the full grid model.

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