Discussion Forum

Hey Guys?
The first question is In COMSOL 3.5a, the periodic boundary condition can be express as u in the source and U*exp(i*k*a?in the destination, but it doesn't works in COMSOL4.

Hi,
in an effort to make the interface more clean and easy to use,
the periodic boundary conditions in 4.0 is used for pure periodic or antiperiodic conditions. What you outline can be done with a Model Coupling, found under Definitions.

The other quesiton is the PML can be defined in Structural Moudule in COMSOL 3.5a, But it also can not works in COMSOL 4.

PMLs in solid mechanics will be added in the 4.0a release, available soon.
Niklas Rom, COMSOL

Thanks for your reply!

about the periodic boundary condition,how to use modle coupling to achieve this work, u in the source and U*exp(i*G*a) in the destination. Can you send me a simple example, I'll really appreciate it.

Hi,
I am new to migrant to COMSOL 4. I am also interested with this skill.
I have tried to realize it by model coupling, but I failed. I could not find somewhere to input my condition equation, for example I want to write u1=u0+4*L1.
Is there some examples to this problem.
It is not easy to understand it in new version of comsol.
Thanks a lot for your attention

Hi,

I am also interested in this topic. Can anybody elaborate how I can implement arbitrary boundary condition equations?

Hi Guys,

did anyone of you have solved this already? I am also stuck with the same problem.

thanks,
kodanda

Anybody ever answer this as I have the same issue, which modal coupling do you use?

This is the response I got from COMSOL support:

We are currently working on implementing Floquet-type periodic boundary
conditions also for solid mechanics, but it is not yet certain when this
will be available. However, it is indeed possible to set up Floquet
conditions with a few extra steps. Here is how:

First you must create a linear extrusion coupling from one of the periodic
boundaries to the other. Linear extrusions are available in under the model
definitions in the Model Builder.

Then, create a pointwise constraint for each dependent variable you want to
have periodicity for. In
order to add a pointwise constraint, you must first have "advanced Physics
Interface Options" active in the "Show" menu.

A normal periodic boundary condition would have the following
expressions:

Constrain expression:
linext1(u)-u

Constraint force expression:
test(linext1(u)-u)

However, with Floquet boundary conditions, you must include a phase shift
in the expressions:

Constraint expression:
linext1(u)-u*exp(i*phase_shift)

Constraint force expression:
test(linext1(u)*exp(i*phase_shift)-u)

Yes, it works! Thanks a lot!

Hello,

Could somebody explain me why in the constraint expr 'exp(i*phase_shift)' is multiplied with 'u' while in the constraint force expr 'linext1(u)' should be multiplied by this phase shift? Should the phase shift not be multiplied with the same variable?

see:
Constraint expression:
linext1(u)-u*exp(i*phase_shift)

Constraint force expression:
test(linext1(u)*exp(i*phase_shift)-u)

The answer to my question can possibly lie in the definition of this mysterious test-function, which I do not find and do therefore not understand.

Bruno

I'm trying to use this in wave equation module, but the result is not changing when I apply this. can you please share a model?

i just deleted my post

May I ask that what the operator linext1 in "linext1(u)" stand for ?