mf and mef huge difference

Topics: AC/DC Module, 4.3a

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Julien Givernaud

Julien Givernaud

January 18, 2013 11:08am UTC

mf and mef huge difference

Hi,

I want to compare mf and mef physics in order to know which one I have to use for my study.
I work with a coil and a conductive core. I'm interesting in the electromagnetic power dissipated in conductive elements mf.Qh and mef.Qh respectively.
Under same excitation set up (current surface like in cold crucible tutorial) I obtain a huge difference between values of dissipated power in the core: mf.Qh=12.14 kW and mef.Qh=3.7 kW
Mesh is the same for the 2 physics and increasing mesh density has no significant impact on results.

Magnetic fields seems to be very different in the 2 cases, i join a picture of streamlines

I attach as well my simple model.

Can someone can explain this huge difference?

Thank you

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Ivar Kjelberg

Ivar Kjelberg

January 18, 2013 1:37pm UTC in response to Julien Givernaud

Re: mf and mef huge difference

Hi

check the dependent variables: in MF it's the magnetic filed via the magnetic vector potential A, MEF adds in also the Electric Potential = Voltage (hence more degrees of freedom and heavier to solve)

If you study the induced currents, you would probably need the voltage for solving correctly, but in MEF you can add an "Amper law's" alone, skipping the current conservation or Electric potential part for some domains, check the equations

--
Good luck
Ivar

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Julien Givernaud

Julien Givernaud

January 18, 2013 2:30pm UTC in response to Ivar Kjelberg

Re: mf and mef huge difference

Thank you Ivar for your answer,

In fact I don't understand why I have to add a new Ampères law because in mef there is already by default the Ampères law and current conservation in the first node.

Can you explain to me why I have to add the same equation in a new node?

I try what you say in adding a new Ampères law which overriddes the first on the core domain, it give some crazy results extremely sensitive to the mesh density (I use boundary layers), I think no realistic results.

Thank you

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Julien Givernaud

Julien Givernaud

January 18, 2013 3:01pm UTC in response to Julien Givernaud

Re: mf and mef huge difference

I have tried a new excitation type;

-10 V terminal for mef
-10 V single turn coil domain for mf

It gives me the same results in power dissipation in the core for the 2 physics.
I don't understand what's happens really but it works,

thank you Ivar

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Ivar Kjelberg

Ivar Kjelberg

January 18, 2013 3:11pm UTC in response to Julien Givernaud

Re: mf and mef huge difference

Hi

you do not need to, but if you have i.e. some air without any conductivity, you cannot get a "current conservation" variable running correctly with 0 conductivity.
Then its better to add a simple "Ampere law" node and select only the air domain (and leave the rest in Ampere law with current conservation), and solve in MEF with V there where the current can flow and as MF where there is no current flowing

Check carefully the equation tab (not COMSOL equation node) for ampere law and Ampere law with current conservation

--
Good luck
Ivar

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Julien Givernaud

Julien Givernaud

January 18, 2013 3:57pm UTC in response to Ivar Kjelberg

Re: mf and mef huge difference

When I add an Ampère law on the gas domain results change
it gives more power dissipated in core with mef, around 50 % more, and about the same power dissipated in the coil
Thank you again

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