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leroyv February 13, 2014 10:54

Understanding the cyclic boundary condition
 
2 Attachment(s)
Dear foamers,

I need to solve problems with cyclic BCs 'all around the place'. To be clearer, this is the geometry I am considering:
https://www.dropbox.com/s/amuzikceam...nder_array.png

On this screenshot, the colouring is the velocity (U) magnitude, U being obtained using the simpleFoam solver and a uniform inlet BC on the x = 0 face. What I want to do is simply simulate the flow in the (6,4) cell using periodic boundary conditions:
https://www.dropbox.com/s/v5nz4mvx6user06/6-4_cell.png

Firstly, let's have a look at what we want to reproduce. Here are the velocity magnitude and orientation in the (6,4) cell (let's call this the reference problem):
https://www.dropbox.com/s/8eivt2e1ai...eference_U.png
This velocity field seems periodic. If we subtract a periodic field from this one, we should get another periodic field, maybe better as an evidence. So we do that and plot the so-called deviation field, UTilde, obtained by subtracting the volume average of U (which is constant over the cell) from U:

UTilde = U - UAverage

Here is what we get:
https://www.dropbox.com/s/8gnccd1pzy...nce_UTilde.png
This deviation field is periodic 'x-wise' and 'y-wise'.

I tried to simulate the flow in the (6,4) cell using the simpleFoam solver (though if might not be the best, AFAIK; anyway, this is not the worst problem here). I therefore simply applied cyclic patches at the boundaries of the cell. Although this insufficiently constraints the problem, the SIMPLE algorithm converges to a solution which only depends on the initial condition, yielding the following velocity field:
https://www.dropbox.com/s/rufn5k53mf...periodic_U.png

Given that image, we can doubt that this field is periodic. The deviation field gives a clearer proof:
https://www.dropbox.com/s/nf81rnnj4h...dic_UTilde.png
UTilde is not periodic, while it should be.

So here is my question: Is there any reason why the periodic BC wouldn't be satisfied in the cyclic problem? Did I do something wrong?

The associated OpenFOAM cases come attached. Screenshot gallery:
https://www.dropbox.com/sh/hmq2qx6jcvjc6lx/LFKCB1X32b

leroyv February 13, 2014 13:08

Update
 
I ran an additional simulation with a correctly constrained problem, replacing the inlet and outlet cyclic patches with regular inlet and outlet patches.

Inlet: zeroGradient pressure, fixedValue uniform velocity
Outlet: fixedValue uniform pressure, zeroGradient velocity
https://www.dropbox.com/s/38s9ap0ev5...periodic_U.png

The 'y-wise' periodicity is not satisfied either.

snak February 14, 2014 06:30

1 Attachment(s)
hi,

something in cyclic patch in your original case is wrong.
checkMesh shows some error.

Using createPatch fixed this problem.
a modified case with createPatchDict and modified Allrun is here,
Attachment 28671

Periodic fields is obtained with this case. tested with OF222.


Quote:

Originally Posted by leroyv (Post 474832)
I ran an additional simulation with a correctly constrained problem, replacing the inlet and outlet cyclic patches with regular inlet and outlet patches.

Inlet: zeroGradient pressure, fixedValue uniform velocity
Outlet: fixedValue uniform pressure, zeroGradient velocity
https://www.dropbox.com/s/38s9ap0ev5...periodic_U.png

The 'y-wise' periodicity is not satisfied either.


leroyv February 14, 2014 13:38

Problem solved
 
Thank you very much! It seems like patch orientation is much more of a big deal than I expected.

Quote:

Originally Posted by snak (Post 474932)
hi,

something in cyclic patch in your original case is wrong.
checkMesh shows some error.

Using createPatch fixed this problem.
a modified case with createPatchDict and modified Allrun is here,
Attachment 28671

Periodic fields is obtained with this case. tested with OF222.


leroyv March 20, 2014 10:01

Dear foamers,

I have an additional question about the cyclic boundary condition: what does it actually do? Does it ensure equality of fields and derivatives at the associated boundaries? Or does it only ensure equality of fields?


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