Truncation method


The truncation method consists of closing the study domain with an outer boundary sufficiently far away from the device so as not to interfere with the results.

At what distance should be placed the border ?

To evaluate at what distance one should place the boundary, it is necessary to take into account the studied phenomena.

Generally we can say that:

  • when the field is strongly confined within the structure (flux directed by flux concentrators, Faraday cages, capacitor, etc.), a small quantity of air is sufficient. The boundary can be placed directly on the device outline or near it.
  • when the field spreads strongly outside the structure (EMC, etc.), a large quantity of air is necessary. The difficulty consists in the estimation of this quantity.


Some rules for the positioning of the boundary for an open boundary magnetic problem (device surrounded by air) are as follows:

  • for a 2D plane study or a 3D study:

    the boundary should be placed at a distance ranging between 5 to 10 times the longest dimension of the device.

  • for a 2D axi-symmetric study:

    in the direction normal to the revolution axis, the boundary should be placed at a distance ranging between 10 to 20 times the longest dimension of the device, the variable r*A Z decreasing slowly in this direction.

Boundary conditions

The user must impose boundary conditions on the external boundaries of the study domain.