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7 Replies
Last post: January 22, 2012 2:07pm UTC
July 2, 2010 9:19am UTC
calculation of derivative in 3D
Hi all,
I want to run a calculation in comsol, and for that, I need to calculate the derivative of a function
I already ran this calculation in 2D and it works, but while I get to 3D, the derivative is calculated as zero although it is not:
To check that I went to:
Postprocessing -> cross section Plot Parameters
In line/extrusion tab, I choose magnetic flux density, x component, I have a graph that its derivative cannot be zero (attached)
Then I type d(Bx_emqa,x) in expression case, the graph of the result is zero(attached)!
What am I doing wrong? the syntax of derivative in 3D is different than the one in 2D?
thanks
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July 16, 2011 4:59pm UTC in response to Rana Afshar
Re: calculation of derivative in 3D
Hey
how are you?
did you solve described problem?
becuse i have exactly the same problem
Thanks a lot and
Best regards,
Arye Nudelman
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August 2, 2011 6:53pm UTC in response to Arye Nudelman
Re: calculation of derivative in 3D
I also have exactly the same problem.
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August 14, 2011 3:33pm UTC in response to Matt Eichenfield
Re: calculation of derivative in 3D
Hi
I do not have access to my COMSOL running PC, but there are a few things to know abot derivatives: COMSOl can only extract them if the shape functions are of high enough order (2 or more). Then COMSOl has already defined as variables most of the first and second order derivatives. Do not forget that for ACDC you solve for A, the B is then derived from the derivative of A and the gradient of B from the second derivative of A. So if you are looking for the first derivative of the gradient, your are using the third derivative of the dependet variable A, hence you need at least 3rd order shape functions.
I suspect this is the reason why you get only "0". the > 2nd derivative of a second ordr equation is "0" so mathemataically it is correct ;)
Even if COMSOl is looking after most of the physics and math behind the scene, you must be sure you understand too, the FEM limitations underneath
--
Good luck
Ivar
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August 25, 2011 8:10am UTC in response to Ivar Kjelberg
Re: calculation of derivative in 3D
I also encountered the same problem
I have tried different cross section line and ended up with zero result when I differentiated it
anyone knows the solution?
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September 22, 2011 10:23pm UTC in response to Hendrik Santoso Sugiarto
Re: calculation of derivative in 3D
I'm afraid I have the same question. I'm not able to derive dB/dz from A. It says it cannot be evaluation as shown in the picture attached. I think it should be Axyz but it does not work.
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January 21, 2012 5:49pm UTC in response to Ivar Kjelberg
Re: calculation of derivative in 3D
Hi, i try to get the derivative of H in x,y,z direction in MF module(3D) by using the command d(Hx,x). while the result is zero. i already changed the discretization option to cubic. can you help me out. thanks
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January 22, 2012 2:07pm UTC in response to Chen Chen
Re: calculation of derivative in 3D
Hi
indeed in my up to date 4.2a in 2D I can plot the derivative d(mf.Hx,x) even with default second order discretisation, but in 3D it's all = 0 even if I select higher order discretisation.
Not sure why, it seems that B is calculated slightly differently internally between 2D and 3D, could it be from the way the Bx = curl of Ax is extracted ?
It's worth to try to ask support if nobody else has a good reply that we have missed ;)
--
Good luck
Ivar
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