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Using a Probe Value for a Stop Condition
Posted Jan 15, 2013, 6:25 p.m. EST Studies & Solvers Version 4.3a 4 Replies
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Can someone refer me to a model or tell me how to solve my problem.
In a system undergoing resistive heating I want to use the calculated resistive energy as the stop condition for a resistive heating step. The User Manual talks about using Probes to monitor the development of a scalar quantity and says that a Probe variable can be used as a stop condition.
In order to test the concept I developed the attached simple 2D case. It is easy enough to keep track of the resistive power by volume integrating ec.Qrh and calling it POWER. Next, I tried to keep track of resistive energy by setting up a Global Variable Probe and defining it as the time integral of power (timeint(0,t,POWER)).
When I ran the Study I received the warning that the time integral operator cannot be used in this context. Nevertheless, the resulting table looked OK. It is clear that I need help!
Also, I don’t have to be precise with regard to the stopping point. I can always control my time steps to have them close to each other within my time zone of interest.
So here is my two part question: How should I keep track of resistive energy and how do I use it as a stop condition?
Any help will be greatly appreciated.
Joe Williams
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first of all timeint() can only be used in Results evaluation, not during solving. It's explicitly written at the end of the help/doc section on timeint(). This applies to several functions, be aware !
Another way is to define a Global variable under the EC (turn on the advanced physics options view) as MyPQ = MyPt - intop1(ec.Qrh) and then access MyP
the MyPt menas that you set the "time derivative of MyP to the intop1() value and that should work. See also the KB at www.comsol.eu/support/knowledgebase/1101/
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Good luck
Ivar
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Thank you for getting back to me with your help. However, I still need a little more.
I generated a Global variable under the ec as you suggested (I think). (I also did it separately under the mathematics interface with the same results).
I defined it as MyE = MyEt – intop1(ec.Qrh) and was able to get nice plot of MyE vs. time with no error messages.
Next I put in 8000 – MyE as a Stop Condition and received the error message that ‘Expression could not be evaluated.’
I still must be doing something wrong. Would you point out where I am making my mistake.
My file is attached.
Thanks in advance,
Joe
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it seems that MyE is not defined before the solver has gone through one iteration, but the stop condition is tested before MyE exits => hence the error message.
The workaround is simple: add a Global Definitions - Parameter
MyE = 0
for the first iteration and off you go ;)
Oups I was a bit too quick there :) correct your stop condition (to really react) to be
8000-mod1.MyE
without the full name "mod1.MyE" it will not stop correctly for me ;)
Even if you dump the global variable myE and mod1.MyE and they give the same results, during solving they are differentiated, probably a casting issue
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Good luck
Ivar
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Thank you very much!! Your solution to my problem was complete, elegant, and very instructive.
Joe