[HLo]

Hello experts,
I was looking for some means for time integration of a quantity to establish some kind of control (PI-control). There are proposed approaches:
1) User fortran
2) some calc with a prevoius value, which is not availabe by default (keyword: Update Loop = TRANS_LOOP)
3) sth. I worked upon using internal equations: http://www.cfd-online.com/Forums/cfx...tml#post465756
x) more ?
As I want to control to a certain steady state, I may tend to do a steady state analysis but in this case, the approaches have disadvantages
1) fortran: I didn't find any hint how to get values of the "false timestep" (or related values) used in steady state analyses to be able to calculate the integral (in fact a accumulated sum of quantity*timestep)
2) trans_loop: the setup for previous value needs a two step setup, and for the aquisition of the timestep value it is the same problem
3) internal equations: solution is more complex, but should yield (more) correct(?) values, as the internal equations' integration underlies the same timestep as the steady state solution itself
What is your opinion?
What are your proposals?
Thanks in advance for upcoming feedback
HLo

[ghorrocks]

It will be very difficult to implement a PI type controller inside a steady state simulation. You are going to have convergence difficulties. You are almost certainly going to have to run this transient.

What are you trying to control anyway? Implementing a controller will be tricky and difficult to converge so there might be another way.

[HLo]

Hello Glenn,
I investigate some switch setup with different housing and thermal insulating material. What I wanted to achieve is a constant steady state temperature (or at end of trans. run) in the electric current carrying part (some kind of heating) and derive and compare the heating powers needed from PI input. Of course works with a P-only control if one accepts a certain deviation from target temperature (but deviation may evolve different in different setups).

Thanks
HLo

[ghorrocks]

Why not use a boundary condition to fix the temperature at that region then? Or a source term to fix the temperature in that region? These options are MUCH simpler.

[HLo]

Hello Glenn,
thought about that, but then -I assumed- I get no information about the electrical power needed to achieve the same target temperature for different setups.
BTW: the electrical power is input by profile data from an ANSYS electrical calc with a weight factor, the latter PI-controlled
But your remark now takes me to an idea.
I'll gonna check for equivalence: surface integral (areaInt@...) of all fluxes around the heating should be equal to input power in final steady state.
You agree?
... and: what -apart from the actual example- is your opinion to the integration proposals?
... and: is there access to the stead state analysis' "false timesteps" in user fortran?
HLo

[ghorrocks]

If the electrical side of things generates a known temperature and that temperature generates (or interacts with) some fluid flow and/or heat transfer then the electrical stuff and the fluid/thermal stuff is decoupled. You can do the electrical stuff in one simulation in the knowledge that it gives a known temperature, and the fluid/thermal stuff can have the known temperature as a boundary condition. This is far simpler than the everything model you propose.

Or is there some reason why this is inappropriate? Maybe the temperature distribution is important?

To answer your specific questions:
1) All 3 methods should work. All are a bit yukky, but should work given perservence. I would give each a go and use whatever looks the most promising after you have tried them.
2) I do not know this, but hopefully it is documented. If not contact ANSYS support. Or you might be lucky and somebody else on the forum knows it.

[HLo]

Hello Glenn,
thanks for your answers.
Temperature distribution is a point of interest, but maybe in a pre-analysis one may find that temp is spatially almost the same (within 5 .. 10 K tolerance at 500 K). Then I can applay a constant temp and the total heat flow balance at surface "heating" should give the power input.
I'll keep you and the forum on track when I got further.
HLo

[ghorrocks]

That sounds much more sensible.

You could consider doing a preliminary analysis with constant temperature, and that gives you the heat flow. Then you can repeat the analysis with the heat flow you just got and that will give you an approximation of the temperature distribution. This would give you a better representation of the temperature distribution, but keep the electrical and fluid/thermal stuff uncoupled.

[HLo]

some more on time integration pros and cons see Post #21 in: http://www.cfd-online.com/Forums/cfx...tml#post471348

HLo