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## does a periodic boundary condition for u implement inflow u (boundary1) = outflow u (boundary2) in newer comsol versions as well? (periodic boundary condition for the flux)

Dear everyone,

I have a question about the implementation of periodic boundary conditions for fluxes, so that the inflow through one boundary equals the outflow through the other. In comsol 3.5, this condition was 'automatically' implemented when chosing periodic boundary conditions for a dependent variable with Neumann boundary conditions (no flux). Is this the same in the newer versions? (And if not, how can i implement this instead? I am working with the matlab live link, but any hint is highly appreciated) I cannot find any information about this neither in the manuals nor in internet ressources, so if you know the answer, PLEASE let me know, it won't take you ong to write a line, but help me a lot :)

All best,
Sabine

8 Replies Last Post Jan 19, 2013, 6:38 PM EST
Posted: 5 years ago
Hi

check the "equation" tab of your periodic boundary condition node, for me it's written out fully thereunder (the tab is closed by default, except if you have enabled the default showin of the equations).
I find it useful to regularly check the equations it helps understanding whree you are and what you need to do, possibly help you identify errors in solvers, particularly for multi-physics since all physics do not accept all solver combinations

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Good luck
Ivar

Posted: 5 years ago
Hey Ivar and everybody,
thx for the reply. I had checked the equation view, but there it says only that the continuity or antiperiodicity condition, respectively, corresponds to var_source=+/- var_destination -- thats clear so far, but does not help me in knowing about any consequences for the flux of var or how to implement inflow=outflow if it is not already included :(
Sabine

Posted: 5 years ago
Here is what I see

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Good luck
Ivar

Posted: 5 years ago
Ah, ok, but youre using the fluid flow physics! We have it as well, but so far I have my problem implemented in the PDE mode and are generally quite happy with it. And there only usource=udestination appears as equation, so nothing said about the derivatives, right? So in tha PDE mode, I could just define myself a second periodic boundary condition, where I type as expression the flux by hand, no?

Posted: 5 years ago
oh no, I dont know how to do that, since there is no field where to type in the expression, I can only aplly the condition to the dependent variables by klicking on them ->?

Posted: 5 years ago
Hi

in PDE view you have the option for continuity or symmetric, and antisymmetric periodic conditions (4.3a), which means you flip the sign of the flow over the boundaries. This does NOT show up in the equation that remains static, but if you dig into the under-laying COMSOL equations you will see that you have a "-u" instead of "+u"

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Good luck
Ivar

Posted: 5 years ago
hey ivar, last comment to make sure i got you right - so the continuity/antisymmetry setting is for the FLOW, not for the dep. variable, i.e. the u in the equation ("-u" instead of "+u) does NOT stand for the dep. varialbe itself? (but usually this is the meaning of u, no? so it is not very intuitive then :( ) so whether i choose continutity or antisymmetry, for my variable say c it is always c_source=c_destination, but the flow is then flow_c_source=+flow_c_destination (continuity) or flow_c_source=-flow_c_destination (antisymmetry), respectively?
(and do you know if there has been any change in this from v.4.1 to the v.4.3a?)
thanks a lot for your time :) sabine

Posted: 5 years ago
Hi

by default COMSOL proposes "u" as dependent variable for a new PDE physics, but depending on how you rename it it might have anoher name.

Turn on the preferences equatin views andlook at the COMSOL equations

I know many persons prefer to click to link in phenomena but stll , often it's worth to take a look at the equations generated

in SPF I agree, you do not have the symmetry/antisymmetry condition as an option, then use rather inlet and outlet and a coupling variable

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Good luck
Ivar

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