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-   -   roto-translational Periodic BC in CFX (http://www.cfd-online.com/Forums/cfx/26759-roto-translational-periodic-bc-cfx.html)

 Ivandp December 3, 2008 11:26

roto-translational Periodic BC in CFX

Hello,

I would like to simulate fully developed conditions in helical tubes with CFX10. Describing an entire pitch (2*pi), I can impose periodic boundary conditions (INLET-OUTLET) with a 'Periodicity Type'='Translational'. This works well. But I would like to simulate only small portion of the tube; in this case the correct periodicity should be 'rotational' and 'translational', i.e. roto-translational. Does anybody know how to impose this condition in CFX10?

 CycLone December 4, 2008 00:07

Re: roto-translational Periodic BC in CFX

Apply two periodic interfaces; one in the translational direction and the other in the circumferential. That's it.

-CycLone

 Ivandp December 4, 2008 08:38

Re: roto-translational Periodic BC in CFX

If I apply two periodic interfaces on the same 2D regions, i have an error, and it is not possible. Should I duplicate the 2D regions?

 Ivandp December 5, 2008 05:40

Re: roto-translational Periodic BC in CFX

It does not seem to work, because CFX does not accept two domain interfaces on the the same 2D primitive regions. Somebody has another suggestion to solve the problem? How is it possible to impose rotational AND translational periodicity on the same 2D regions?

 Timon December 5, 2008 06:50

Re: roto-translational Periodic BC in CFX

Why would you impose two interfaces on the same region? The interface is either translationally or rotationally periodic, not both. In your case, your inlet/outlet surfaces are translationally periodic and your side surfaces rotationally. So there's no need whatsoever to impose two interfaces on the same regions. Just make sure you have created separate surfaces for your inlet/outlet/side regions.

 Ivandp December 5, 2008 07:23

Re: roto-translational Periodic BC in CFX

I am sorry, but I don't understand. Translational periodicity works properly when an entire pitch of the coil is modelled. In fact in this case the is effectively translational periodicity. For a small fraction of a pitch, the translational periodic condition is wrong, because velocity components must be rotated according to the surface orientation. Can somebody help me?

Thankyou, Ivan

 Timon December 5, 2008 09:01

Re: roto-translational Periodic BC in CFX

Okay, I understand your question now. That's not possible, because your interface region is simply not periodic. You need to use a full period of revolution as the domain.

 Ivandp December 5, 2008 11:02

Re: roto-translational Periodic BC in CFX

With previous versions of CFX (es. CFX4.1), it was possible to apply periodic conditions in such cases with the command line option 'GLUE PATCHES'. Does anybody know if it's possible now with the latest versions?

 CycLone December 5, 2008 21:44

Re: roto-translational Periodic BC in CFX

How can you possibly have two regions that are both translationally and rotationally periodic? It is simply not possible.

If you have two regions that are periodic in one direction and another two that are translationally periodic in another direction, you can define them as such. Any thing else is not possible, nor is it physically sensible.

-CycLone

 Ivandp December 6, 2008 11:31

Re: roto-translational Periodic BC in CFX

It's an helical coil. That's all. I would like to simulate only a portion instead of an entire pitch. I have already explained this in previous messages.

 CycLone December 8, 2008 10:42

Re: roto-translational Periodic BC in CFX

Right. So you have translational periodicity in the axial direction and rotational periodicity in the circumferential.

Create a translational periodic pair with the faces on each end of the helix, including your pressure drop or mass flow rate, then create another rotational periodic pair between the faces on the circumferentially periodic sides.

If you have create your model properly, you should be able to copy and translate the mesh in the axial direction and have the two faces line up. Similarly, you should be able to copy and rotate the mesh around the axis of symmetry and have the two rotationally periodic faces line up. This is an absolute requirement of any kind of periodicity, since this is physically what you are trying to model.

-CycLone

 CycLone December 8, 2008 10:51

Re: roto-translational Periodic BC in CFX

I see what you mean. Do as I suggested in the other post (translational and rotational interfaces), but rather than specifying your translational interface with the "Translational Periodicity" option, use the "General Connection" option and specify the "Frame Change/Mixing Model" as "Frozen Rotor". Note that you may need to specify a "Rotational Offset" to make sure the walls line up appropriately. The rotational offset simply clocks the interface alignment in the theta direction.

The Frozen Rotor interface will connect these parts, despite the fact that they are not lined up in space. It maps each side into a non-dimensional space in R and Theta, then computes the connections.

You can still apply a Mass Flow Rate or Pressure Drop at the Frozen Rotor interface.

Note that you will require a Full Capability Solver or a Multiple Frames of Reference module license to use the Frozen Rotor interface.

-CycLone

 CycLone December 8, 2008 10:51

See my other post for the solution