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1/r in 2D-axisymmetric volume force: How to deal with it on the axis?

Posted Jul 10, 2019, 1:10 p.m. EDT Low-Frequency Electromagnetics, Computational Fluid Dynamics (CFD) Version 5.4 3 Replies

Milad Saadat
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Hi all, I'm simulating ferrohydrodynamics and I'm trying to impose a Volume Force in the Laminar Flow physics. This volume force has a part like this:

((1/r)(d(r(d(A,r))^2,r))+d(d(A,r)d(A,z),z))mu0_const

where "A" is the magnetic scalar potential. When I plot this term, the closest layer to the axis has not been taken into account actually (take a look at the attached image please, the black vertical line is the axis.) How can I prevent this problem? I have instabilities on the axis because of this issue. Thanks

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3 Replies Last Post Jul 10, 2019, 5:03 p.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 5 years ago Jul 10, 2019, 3:41 p.m. EDT
Updated: 5 years ago Jul 10, 2019, 3:40 p.m. EDT

Milad,

the 1/r term goes to infinity at the axis (r=0). You need to prevent that some way, maybe by adding a constant>0 in the denominator: 1/(r + const). And you need to be aware that the symmetry does not allow any nonzero radial component at the axis.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Milad, the 1/r term goes to infinity at the axis (r=0). You need to prevent that some way, maybe by adding a constant>0 in the denominator: 1/(r + const). And you need to be aware that the symmetry does not allow any nonzero radial component at the axis. Cheers Edgar
Milad Saadat
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Posted: 5 years ago Jul 10, 2019, 4:25 p.m. EDT
Updated: 5 years ago Jul 10, 2019, 4:24 p.m. EDT

Edgar Thanks for your response. This term is a divergence from a field, that's why it makes 1/r. Consider the continuity equation for instance: how this equation is written in 2D-axisymmetric? Does the COMSOL itself prevent 1/0 by adding a constant to its denominator too? Thanks, Milad

Edgar Thanks for your response. This term is a divergence from a field, that's why it makes 1/r. Consider the continuity equation for instance: how this equation is written in 2D-axisymmetric? Does the COMSOL itself prevent 1/0 by adding a constant to its denominator too? Thanks, Milad
Edgar J. Kaiser Certified Consultant
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Posted: 5 years ago Jul 10, 2019, 5:03 p.m. EDT
Updated: 5 years ago Jul 10, 2019, 5:03 p.m. EDT

Comsol isn't modifiying your expression. You need to take care of cases like this by yourself.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Comsol isn't modifiying your expression. You need to take care of cases like this by yourself.

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