# Discussion Forum

## Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

 Topics: RF Module
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Nan Zhang

February 10, 2013 8:10pm UTC

Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Hi all,

Recently I confront one confusion about periodic conditions in the tutorial model - Periodic Boundary Condition. In Periodic Condition, it selects only boundaries 1 and 6 which are parallel with the yz-plane. As you know in the model, the cube should be a repeated cell and certainly repeated on the xy-plane both in the x direction and in the y direction. However, boundaries 1 and 6 only correspond to x direction. So what about repetition in the y direction?

Then I try to select both boundary 1, 6 parallel with yz-plane and boundary 2, 5 parallel with xz-plane in the Periodic Condition. But some problem occurs just as follows:

Errors during multiphysics.
Failed to find destination boundaries or destination selection empty
- Feature: Periodic Condition 1 (pc1)

I don't know why this happens and how to set Periodic Condition that means a repeated cell in the xy-plane both in the x direction and in the y direction.

Thank you so much.

Alexander Kuznetsov

February 11, 2013 7:17pm UTC in response to Nan Zhang

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

As far as I remember, this example considers particular set of angles of incidence and polarization, at which having periodic BC in y-direction is not required. For more general case, you will need to have periodic BC in both x and y directions. They should be defined as two different sets of BC, one for each of the directions. Then if you define your k-vectors properly, everything should work just fine.

Also make sure your mesh is identical on symmetric BC.

Nan Zhang

February 11, 2013 7:51pm UTC in response to Alexander Kuznetsov

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Thank you so much for your reply. But I'm still confused about two possible situations when I set periodic BC in both x and y directions.

OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOＯOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO

O
O
O
OOOOＯOOOOO
O
O
O

I hope you can understand what I mean for the above pictures. Could you please help me to explain that?

Thank you.

Nan Zhang

February 11, 2013 9:41pm UTC in response to Alexander Kuznetsov

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

There are some mistakes for the display of the second picture. The "O" in the first three lines and last three lines should be exactly above and under the "Ｏ". That's what I really mean.

Alexander Kuznetsov

February 12, 2013 5:48pm UTC in response to Nan Zhang

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Sorry, I don't get what those O-s mean.

Nan Zhang

February 12, 2013 6:31pm UTC in response to Alexander Kuznetsov

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Sorry to confuse you. I mean "Ｏ" is the original single cell.

When I set the x direction periodic BC, it will become like this

OOOOOＯOOOOO

When I set the y direction periodic BC, it will become like this

O
O
O
O

O
O
O
O

But when I set both x and y direction periodic BC, what will it be? Probably it can be periodic on the x-y plane.

OOOOOOOOOO
OOOOOOOOOO
OOOOOＯOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO

However, there exists another probability that it may become a "cross" periodic model just like this

O
O
O
O
ＯOOOOOO
O
O
O
O

So I'm confused about these two probabilities and can't decide which it should be.

Thank you.

Alexander Kuznetsov

February 12, 2013 10:34pm UTC in response to Nan Zhang

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

It's the first one with square of O's

Nan Zhang

February 13, 2013 4:26am UTC in response to Alexander Kuznetsov

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Thank you so much.

Duy Duc Nguyen

April 10, 2015 12:29pm UTC in response to Nan Zhang

Re: Problems about Periodic Conditions in the tutorial model “Periodic Boundary Condition”

Hi Nan Zhang

I would like to study Periodic condition in Comsol too. However, I can not locate where are the tutorial files of field in Comsol's library. Can you help me ? (Or models in www.comsol.com/models)

Thank you very much
Best regards
Duc

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