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Magnetic field
Posted Nov 19, 2012, 3:11 a.m. EST Low-Frequency Electromagnetics Version 4.3a 10 Replies
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Does anybody know how to write this expression on comsol ( ∇/B/^2 )? I meant expression that replace "the gradient of the magnetic field norm square".
Thank you
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you need to write out the full expression the derivatives are defined inside COMSOl see the doc
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Good luck
Ivar
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But I have gotten only this expression mf.Br*d(mf.Br, r), which is not the right expression I want.
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if you are solving with MF your dependent variable is A the magnetic vector potential, then B is already the derivative of A, so if you want to further derive B, you should use a higher discretization (3rd or 4th order)
Thjen it's easier to express everything directly from A (check the COMSOl equations by turning on the equation view
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Good luck
Ivar
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Here is what I 'm using the expression for: I coupled magnetic field and structural mechanics, the expression is proportional to magnetic force and I'm using this expression as a body load. I used this product mf.Br*d(mf.br, r) as a body load and it works but now I want to replace it with the right expression, which is ∇/B/^2. Actually I 've tried to write it but couldn't manage to do so and every time I tried to run it, it comes up with expression error.
The equation view is already turned on.
Thank you
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So is it possible to express ∇/B/^2, in a way that comsol understands...?
Thank you
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you can develop the
grad(dot(B,B)) into "2*(dot(B,grad(B))+cross(B,curl(B))
(see i.e Wiki : Vector calculus identities) , knowing the variable names from the equation view, in your particular COMSOL space you have chosen, you can write this out, no ?
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Good luck
Ivar
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I guess it should be possible to write.
I was just wondering, if these two are the same. ∇/B/^2 = ∇/B^2/ ????
If so, ∇/B/^2 = 2[B×(∇×B) + (B . ∇)B] but then I couldn't be sure of B .∇ = ∇. B? I know that the divergence of the magnetic field is zero( ∇. B = 0)
Thank you very much
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the two are not really the same, there is a sign issue:
If I read you corretly (but perhpas I misinterpreated it) the square of the norm of a vector is for me B*B=B^2 = a scalar representing the dot product of B with itself
but if you take the norm of this scalar, for a complex valued case you will have an sign difference, no?
from the moment you take a suqare root you have to check your signs. So the true question what is the true vector product you are looking for ?
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Good luck
Ivar
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I'm not sure but may be you right, it could be different for complex valued case.
Actually I'm looking for a force which is proportional to the negative of the gradient of the magnetic field norm square.
i.e F = -∇/B/^2, B= B(z,t)= Bz(z)*sin(wt) in the Z direction or Z-hat
And I want to use this force as a body load.
Thank you for being concerned, I really appreciate all the help.
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one of the reasons I have some issues to give a cear response here is due to the complex nature of E and I do not fully understand in which environment / model you are looking at these equations.
See my comment to:
www.comsol.eu/community/forums/general/message/reply/91752/
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Good luck
Ivar