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time dependent study, external current density in mf model.

Posted May 25, 2011, 7:31 a.m. EDT Low-Frequency Electromagnetics Version 4.1, Version 4.2 9 Replies

Isabel Gil
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Hello,

I am using COMSOL version 4.2, module AC/DC, Magnetic Fields (mf) model, 2D axisymmetric wizard and time dependent study.

I have created in "Global Definitions" a function of type "Sawtooth" named by "F1saw".

As " External Current Density" I want to use "Jc*F1saw(t)" but the program says that it is "unexpected unit of input", where "Jc" is given as a parameter and is units are "A/m^2".

I really don't know what I am doing wrongly.

Could you help me?

Thanks in advance.

Best regards,
Isa
9 Replies Last Post May 27, 2011, 4:56 a.m. EDT
Edgar Kaiser
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Posted: 1 decade ago May 25, 2011, 8:15 a.m. EDT
Isa,

did you assign a unit to your Sawtooth function? It should be unitless for your application.

Cheers
Edgar
Isa, did you assign a unit to your Sawtooth function? It should be unitless for your application. Cheers Edgar
Isabel Gil
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Posted: 1 decade ago May 25, 2011, 8:41 a.m. EDT
Hello,
thank-you Edgar.

No, I define it simply. But when I plot it, as title appears "F1Saw(t)", so it seems that by default it takes the variable time, doesn't it?

How should I do it?

Thanks again in advance.
Best regards,
Isa
Hello, thank-you Edgar. No, I define it simply. But when I plot it, as title appears "F1Saw(t)", so it seems that by default it takes the variable time, doesn't it? How should I do it? Thanks again in advance. Best regards, Isa
L. Queval
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Posted: 1 decade ago May 26, 2011, 10:43 a.m. EDT
Sorry I still didn't received Comsol 4.2 but with 4.1 I set up a model with a triangle function multiplied by Jc.
Sorry I still didn't received Comsol 4.2 but with 4.1 I set up a model with a triangle function multiplied by Jc.
Isabel Gil
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Posted: 1 decade ago May 26, 2011, 11:55 a.m. EDT
Dear Loic,

Thank-you very much for your help!

I have opened your file in both, comsol 4.1 and 4.2, and yes, you are right, it allows me to introduce the external current density as "Jc*tria1(t)" but you can see that it put itself in orange colour (in both versions), which points out that it is "unexpectd unit of input"... It is like a warning, not an error because later the program runs.

I think it is because tria1(t) has units of "seconds", do you know if we can do it unitless?

Thanks in advance!

Best regards,
Isa
Dear Loic, Thank-you very much for your help! I have opened your file in both, comsol 4.1 and 4.2, and yes, you are right, it allows me to introduce the external current density as "Jc*tria1(t)" but you can see that it put itself in orange colour (in both versions), which points out that it is "unexpectd unit of input"... It is like a warning, not an error because later the program runs. I think it is because tria1(t) has units of "seconds", do you know if we can do it unitless? Thanks in advance! Best regards, Isa
L. Queval
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Posted: 1 decade ago May 27, 2011, 12:07 a.m. EDT
I don't know if it is possible to do it unitless but as you said, it is a warning not an error. Comsol is just saying that he cannot determine it automatically.
I don't know if it is possible to do it unitless but as you said, it is a warning not an error. Comsol is just saying that he cannot determine it automatically.
Isabel Gil
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Posted: 1 decade ago May 27, 2011, 2:50 a.m. EDT
Ok. Understood.
Again, thank-you!
Best regards,
Isa
Ok. Understood. Again, thank-you! Best regards, Isa
Edgar Kaiser
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Posted: 1 decade ago May 27, 2011, 2:59 a.m. EDT
Isa,

you might consider to define a constant with an appropriate unit, assign the value 1 and multiply with your function to remove the unit issue.

Cheers Edgar
Isa, you might consider to define a constant with an appropriate unit, assign the value 1 and multiply with your function to remove the unit issue. Cheers Edgar
Sven Friedel COMSOL Employee

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Posted: 1 decade ago May 27, 2011, 4:17 a.m. EDT
Hi,

the correct way calling a time function *without orange* marks is:

sawtooth(t[1/s])[A/m^2]

The [1/s] removes the unit [s] and creates a unitless input for the function call.
The [A/m^2] adds the correct unit required for the entry field.

Sven
Hi, the correct way calling a time function *without orange* marks is: sawtooth(t[1/s])[A/m^2] The [1/s] removes the unit [s] and creates a unitless input for the function call. The [A/m^2] adds the correct unit required for the entry field. Sven
Isabel Gil
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Posted: 1 decade ago May 27, 2011, 4:56 a.m. EDT
Dear Sven,

I have tried your suggestion and, obviously (it makes sense), it has worked!

Thank-you very much!
Best regards,
Isa
Dear Sven, I have tried your suggestion and, obviously (it makes sense), it has worked! Thank-you very much! Best regards, Isa

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