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complex current density

Posted 16 gen 2013, 10:51 GMT-5 5 Replies

J Bohnker
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Hi all,
I'm working on getting the frequency dependent conductance and susceptance (real and imaginary admittance) of a simple resonating PZT block using the Piezoelectric Devices physics. I seem to be getting a reasonable admittance by integrating the normal current density over the top electrode and dividing by the applied voltage.

My problem comes in when I try to separate the admittance into conductance and susceptance. It seems like this should be done by integrating the real and imaginary parts of the normal current density, but the current density doesn't seem to have an imaginary part - giving a susceptance of zero. Is there a setting I need to change to find the imaginary part of the current density? I've checked the boxes "Allow complex-valued output from functions with real input" and "split complex variables in real and imaginary parts" in the solver, but left everything else as the default option.

Please help! The model is attached.

Thanks,
Jillian
5 Replies Last Post 2 nov 2016, 13:50 GMT-4
Robert Koslover Certified Consultant
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Posted: 1 decade ago 16 gen 2013, 14:35 GMT-5
Hi Jillian. Although I can't open your file because it requires licenses I don't have, I'm going to take a wild guess that you are failing to find the imaginary part of your expression because of your syntax. When a complex quantity (let's call it "MyVariable") is simply pasted into an expression, it can sometimes be evaluated (By Comsol Multiphysics) as only the real part of it! To extract the real and imaginary parts explicitly, use the real() and imag() functions, i.e.,

To get the real part.... real(MyVariable)
To get the imaginary part.... imag(MyVariable)

Hope that helps.
Hi Jillian. Although I can't open your file because it requires licenses I don't have, I'm going to take a wild guess that you are failing to find the imaginary part of your expression because of your syntax. When a complex quantity (let's call it "MyVariable") is simply pasted into an expression, it can sometimes be evaluated (By Comsol Multiphysics) as only the real part of it! To extract the real and imaginary parts explicitly, use the real() and imag() functions, i.e., To get the real part.... real(MyVariable) To get the imaginary part.... imag(MyVariable) Hope that helps.
Edgar Kaiser
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Posted: 1 decade ago 17 gen 2013, 04:12 GMT-5
Jillian,

you used pzd.normJ for your integration. The norm of a quantity is real by definition.

You might consider to add a circuit physics to your model. This will provide complex impedances and admittances of your device as global results.

Cheers
Edgar
Jillian, you used pzd.normJ for your integration. The norm of a quantity is real by definition. You might consider to add a circuit physics to your model. This will provide complex impedances and admittances of your device as global results. Cheers Edgar
J Bohnker
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Posted: 1 decade ago 17 gen 2013, 12:23 GMT-5
Thanks for pointing that out - I was assuming that "norm" referred to the component of the current density normal to the surface. As a side question, where would I find the definitions of each of the model outputs? I've had trouble locating the correct documentation to avoid misunderstandings like that one.

However, even if I integrate the imaginary part of the z-component of the current density, I get zero. Is that not the correct quantity to integrate to find the imaginary component of current? I'll try out your suggestion of adding circuit physics.

Thanks,
Jillian
Thanks for pointing that out - I was assuming that "norm" referred to the component of the current density normal to the surface. As a side question, where would I find the definitions of each of the model outputs? I've had trouble locating the correct documentation to avoid misunderstandings like that one. However, even if I integrate the imaginary part of the z-component of the current density, I get zero. Is that not the correct quantity to integrate to find the imaginary component of current? I'll try out your suggestion of adding circuit physics. Thanks, Jillian
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17 gen 2013, 15:00 GMT-5
Hi

to see the COMSOL default equations, turn pn the "equation view" in the preferences, then you will get an addition node level with the list of equations, you might even change them at will (at your own risk for the results ;)

--
Good luck
Ivar
Hi to see the COMSOL default equations, turn pn the "equation view" in the preferences, then you will get an addition node level with the list of equations, you might even change them at will (at your own risk for the results ;) -- Good luck Ivar
Aj
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Posted: 8 years ago 2 nov 2016, 13:50 GMT-4
how can I see ec.normJ implementation in comsol
how can I see ec.normJ implementation in comsol

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