The Magnetic Fields, No Currents, Boundary Elements Interface
The Magnetic Fields, No Currents, Boundary Elements (mfncbe) interface (), found under the AC/DC>Magnetic Fields, No Currents branch when adding a physics interface, is used to compute magnetostatic fields from permanent magnets and other current free magnetic sources. It can also be coupled to The Magnetic Fields, Currents Only Interface or The Magnetic Fields Interface to account for outer, current free regions as an alternative to using Infinite Elements. The formulation is stationary. The physics interface solves Gauss’ Law for the magnetic field using the scalar magnetic potential as the dependent variable.
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For a discussion about the boundary element method, see Theory for the Boundary Elements PDE in the COMSOL Multiphysics Reference Manual.
When this physics interface is added, these default nodes are also added to the Model Builder — Magnetic Flux Conservation, Magnetic Insulation (on exterior boundaries), and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, magnetic scalar potential and magnetic flux density conditions. You can also right-click Magnetic Fields, No Currents, Boundary Elements to select physics features from the context menu.
If both The Magnetic Fields, Currents Only Interface and The Magnetic Fields, No Currents, Boundary Elements Interface are available, the Magnetic Scalar-Scalar Potential Coupling node is available from the Multiphysics menu in the Physics toolbar or by right-clicking the Multiphysics Couplings node in Model Builder.
If both The Magnetic Fields Interface and The Magnetic Fields, No Currents, Boundary Elements Interface are available, the Magnetic Vector-Scalar Potential Coupling node is available from the Multiphysics menu in the Physics toolbar or by right-clicking the Multiphysics Couplings node in Model Builder.
Physics-Controlled Mesh
The physics-controlled mesh is controlled from the Mesh node’s Settings window (if the Sequence type is Physics-controlled mesh). There, in the table in the Physics-Controlled Mesh section, find the physics interface in the Contributor column and select or clear the check box in the Use column on the same table row for enabling (the default) or disabling contributions from the physics interface to the physics-controlled mesh.
Information from the physics will be used to automatically set up an appropriate meshing sequence.
In the COMSOL Multiphysics Reference Manual see the Physics-Controlled Mesh section for more information about how to define the physics-controlled mesh.
Domain Selection
From the Selection list, select any of the options — Manual, All domains, All voids, or All domains and voids (the default). The geometric entity list displays the selected domain entity numbers. Edit the list of selected domain entity numbers using the selection toolbar buttons to the right of the list or by selecting the geometric entities in the Graphics window. Entity numbers for voids can be entered by clicking the Paste () button in the selection toolbar and supplying the entity numbers in the dialog box. The entity number for the infinite void is 0, and finite voids have negative entity numbers.
Selections can also be entered using the Selection List window, available from the Windows menu in the Home toolbar.
For more information about making selections, see Working with Geometric Entities in the COMSOL Multiphysics Reference Manual.
Physics Symbols
Select the Enable physics symbols check box to display symmetry planes (in 3D) and lines (in 2D) in the Graphics window, as specified in the Symmetry settings.
Symmetry
For 3D components, from the Condition for the x = x0 plane, Condition for the y = y0 plane, and Condition for the z = z0 plane lists, choose Off (the default), Symmetric (infinite magnetic insulation plane), or Antisymmetric (infinite ground plane). Then enter the value for the plane location x0, y0, or z0 (SI unit: m) as required.
For 2D components, from the Condition for the x = x0 plane and Condition for the y = y0 plane lists, choose Off (the default), Symmetric (infinite magnetic insulation line), or Antisymmetric (infinite ground line). Then enter the value for the plane location x0 or z0 (SI unit: m) as required.
Far Field Approximation
To display this section, click the Show More Options button () and select Advanced Physics Options.
For more information about the Far Field Approximation settings, see Far-Field Approximation Settings in the COMSOL Multiphysics Reference Manual.
Quadrature
To display this section, click the Show More Options button () and select Advanced Physics Options.
For more information about the Quadrature settings, see Quadrature in the COMSOL Multiphysics Reference Manual.
Thickness (2D components)
For 2D components, enter a default value for the Out-of-plane thickness d (SI unit: m). The default value of 1 unit length is typically not representative for a thin domain. Instead it describes a unit thickness that makes the 2D equation identical to the equation used for 3D components.
Infinity Condition
For 3D components, select the Infinity condition — Magnetic insulation (the default) or Zero magnetic scalar potential at infinity. If there is an antisymmetric symmetry in the potential field, it acts as an added infinite ground plane with a fixed value of the magnetic scalar potential. The value at infinity is fixed to 0 by the presence of the infinite ground plane, so for this case there is only a fixed Zero magnetic scalar potential at infinity condition.
For 2D components, select the Infinity condition — Magnetic insulation (the default) or Zero magnetic scalar potential at reference distance. If there is an antisymmetric symmetry in the potential field, it acts as an added ground line with a fixed value of the magnetic scalar potential. The value at a reference distance is fixed to 0 by the presence of the ground line, so for this case there is only a fixed Zero magnetic scalar potential at reference distance condition.
Discretization
From the Magnetic scalar potential/Magnetic flux density list, choose from predefined options for the boundary element discretization order for the magnetic scalar potential variable and the magnetic flux density variable, respectively. The predefined options represent the suitable combinations of element orders such as Quadratic/Linear (the default).
The settings under Value types when using splitting of complex variables are important for sensitivity and optimization computations. See the description of the built-in operators fsens and fsensimag.
Dependent Variables
The dependent variable (field variable) is for the Magnetic scalar potential Vm. The name can be changed but the names of fields and dependent variables must be unique within a model.