Undergrad Shielding behavior of a steel cuboid in a DC magnetic field

[thinksolid]

TL;DR

Simulating a steel shielding box in a 100 mT DC magnetic field shows counterintuitive results depending on orientation. I'm trying to understand why less magnetic field penetrates the shorter side of the cuboid.

Hello everyone,

I'm currently working on simulating magnetic shielding.
The shielding enclosure is a rectangular steel box placed in a homogeneous DC magnetic field of 100 mT in air.
The steel box has a wall thickness of 3 mm on every side. The inside is filled with air.


Here’s the issue:

• When the box is oriented with its long side perpendicular to the magnetic field, the internal field drops to around 20 mT.
• When the short side is perpendicular to the field, the internal field rises to 40 mT.

This result is the opposite of what I expected. I assumed that having the long side perpendicular to the field would couple more magnetic flux into the material, pushing the shielding closer to saturation and thus allowing more field to penetrate into the interior.


Why is my assumption incorrect? Measurements of the box in realiy showed the same result.


Any insights would be appreciated!

Arrows indicate the direction of the external magnetic field.
(~20 mT max. inside)

(~40 mT max. inside)

Simulation environment, external field in green

 

[Baluncore]

Welcome to PF.

Why is my assumption incorrect?

I suspect that the longer flux path increases the reluctance, so reduces the shielding.
Try thickening the long-side walls proportionally, to compensate for the increased length.