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Hi

it depends a bit in which context you use it,

But for example to fix a temperature on a boundary you might create an internal dependent variable that drives a heat flux Q such that the Twall-Tset = 0 value is achieved, this would be for me an internal DoF

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Good luck
Ivar

I cannot find the term “internal degree of freedom” in the manuals? What do you mean?

Haha! Neither could I!

I never saw the term before but now in v4.3 when I solve some models, under Messages when COMSOL reports the number of degrees of freedom solved for sometimes it will have in parentheses the text "(internal XXX)".

So that it looks something like this:

Number of degrees of freedom solved for: 2300 (internal 46000)

This came up for me when I added plasticity to my linear elastic material model. The internal DOF were about 20X the regular DOF.

Hi
but I believe COMSOl is adding now more internal variables to resolve the commplex equations of non linear material formulas, there were already some for Solid rigid body too, but I agree not really mentioned so far

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Good luck
Ivar

They are probably components of the plastic strain tensor or something similar. That would explain the large number since it is a tensor quantity and it is present at every integration point. However, in my experience, you should not see any significant increase in solution time or memory requirements due to these internal variables. I am curious if your example shows otherwise.

Nagi Elabbasi
Veryst Engineering

Nagi and Ivar -

Thanks for your replies.

Nagi, I ran the plasticity model in v4.2a and v4.3. Here are the solution time results:

v4.2a - Sol'n time = 2.05 hrs
v4.3 - Sol'n time = 2.61 hrs

That's about a 25% increase. I don't think this is due to the added internal variables that Ivar mentioned. I think the internal variables were there in v4.2a just not posted in the Messages tab as being internal. Here's the output in the Messages tab:

v4.2a - Number of degrees of freedom solved for: 46.25 MDoF
v4.3 - Number of degrees of freedom solved for: 2.35 MDoF (Internal 46.25 MDoF)

Now, I am wondering why the slow down. Has anyone seen similar slow-down when moving to v4.3?

-Josh

P.S. I ran the v4.2a solution overnight so I wasn't able to monitor the memory usage.

Hi Josh,

That is actually a small increase. If you refine the mesh using the elastic material to reach 46 million DOFs instead of 2.4 you will see several orders of magnitude increase in solution time! I really like that distinction COMSOL made in V4.3 between internal and regular DOFs for that reason.

That extra 25% time is probably going to evaluation of plasticity equations and extra solver iterations since the model is more nonlinear with plasticity. You can check the number of iterations from the log output.

Nagi Elabbasi
Veryst Engineering

Oh, I did not realize that the solution times you provided where for plasticity in V4.2A and V4.3. I thought they were between models with and without plasticity.

Yeah. That was what surprised me. I didn't expect such an increase in sol'n time for the same model file.

Hi,
The new Double Dogleg solver addresses a larger class of highly nonlinear simulations and complements an earlier Newton solver, and it is the default solver for plasticity in version 4.3. That might be the reason for the increase in computational time in your model.
The Double Dogleg solver might be slightly slower (for a given set of loads, mesh, material parameters, etc.), but it is much more robust. You can change the settings and use the nonlinear Newton solver if you prefer.

Also, Nagi is correct, the "internal" degree of freedom are due to the plasticity variables.

Thanks, Ed. I appreciate the response.