Issue with currents computed by bSolver and MagnetoDynamics2DHarmonic

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mwdawson
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Issue with currents computed by bSolver and MagnetoDynamics2DHarmonic

Post by mwdawson » 11 Aug 2019, 02:05

I'm performing a 2D cylindrical coordinates simulation of a metal detector using MagnetoDynamics2DHarmonic solver.

The simulation is basically working but there is a strange anomaly. bSolver is giving me non-zero currents in regions where the
conductivity is zero. According to the models document the conduction current should be given by the equation

Jc = -j*sigma*omega*Az.

According to this equation it would be impossible for the current to be nonzero in any region with a conductivity (sigma) of zero.


Simulation files and some paraview screenshots scan be found at the link below.
https://1drv.ms/u/s!AkpB2-DyzyN6hro3esI ... A?e=QfQe1o

The metal detector coils are surrounded by a metal case where there are very high induced current densities. The case currents are
2000 times bigger than the spurious currents in air regions of zero conductivity.
Attachments
case.sif
(2.88 KiB) Downloaded 2 times

raback
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Re: Issue with currents computed by bSolver and MagnetoDynamics2DHarmonic

Post by raback » 11 Aug 2019, 18:41

Hi

The problem is that whereas Az is continuous sigma*Az is not. So what you are seeing is the effort to map discontinuous field to continuous basis functions. The remedy is to use discontinuous Galerkin (DG) basis as may be done with MagnetoDynamicsCalcFields (instead of BSolver).

-Peter

mwdawson
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Re: Issue with currents computed by bSolver and MagnetoDynamics2DHarmonic

Post by mwdawson » 12 Aug 2019, 07:15

I assumed that Elmer would model the conductivity as continuous. To deal with this I had added a very thin zero conductivity layer next to the high conductivity case region. (See attached figure). I'm seeing non-zero currents at nodes far away from the high conductivity regions (nodes where all adjacent nodes have zero conductivity). Shouldn't this result in the conductivity being zero in all triangles surrounding the node. How can BSolver still compute a non-zero current for such a node?
Attachments
spurios_current_mid_range_zoom.png
Spurious currents many cells away from non-zero conductivity region
spurios_current_mid_range_zoom.png (295.76 KiB) Viewed 55 times

raback
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Re: Issue with currents computed by bSolver and MagnetoDynamics2DHarmonic

Post by raback » 12 Aug 2019, 11:19

Hi, Its difficult to comment without the whole case. However, it is clear that if you you have a jump in currents over elements this cannot be presented by continuous function. This is difficult to ovecome without a discontinuous visualization space. -Peter

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