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## temperature in global equation

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RSS feed   |   Turn on email notifications   |   3 Replies   Last post: October 9, 2012 11:52am UTC

Bin Xu

October 8, 2012 7:38pm UTC

temperature in global equation

Hello,

I have a Joule heating model, this model includes two blocks, the upper block size is 2cm*2cm*0.5cm and the lower block size is the same.

It is assumed that the voltage difference of upper block is U1 and the voltage difference of lower block is U2.

The sum of U1 and U2 is a known value, the ratio of U1 and U2 is related to T(temperature)

I used global equation to try to find out the value of U1 and U2, the software shows the error information

'Failed to evaluate variable.
- Variable: mod1.T
- Global scope
Failed to evaluate expression.
- Expression: mod1.T'

I have given the initial value to U1, U2 and T, they are all 300.

Thanks

Ivar Kjelberg

October 8, 2012 8:47pm UTC in response to Bin Xu

Re: temperature in global equation

Hi

it's very difficult for us to tell, just with the explanations you give as it can come from many things.
In Joule heating the dependent variable T is mostly referred to as T and not mod1.T. Except if you have defined variables referring to Tin the Global Variable nodes, instead of having set them inthe Model Definitions Variable node

difficut to guess more ...

--
Good luck
Ivar

Bin Xu

October 9, 2012 8:44am UTC in response to Ivar Kjelberg

Re: temperature in global equation

Hello,Ivar:

I have done as you said, but it still des not work.

I atached my model so that you could see it clearly.

Thank you very much

Jesus Lucio

October 9, 2012 11:52am UTC in response to Bin Xu

Re: temperature in global equation

Hello, Bin,

I think you are confusing the field variable T with some temperature, you call also T as a variable (in Model/Definitions/Variables). Really, do you want that the T appearing in your Global Equations be the dependent variable T (this is the error given by COMSOL, and he is telling you that T is not a global variable), or some global temperature (for instance, the average one)?
I suppose your election is the second one, as U1 and U2 are already global variables. In that case, you can define that temperature as an average over both domains.

Bye,

Jesus.

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