Magnetomechanics
Use the Magnetomechanics () coupling to model interaction between deformable solids and magnetic fields. It can be used to model cases with or without induction currents, depending on which magnetic field physics interface is used. The supported magnetic field interfaces are Magnetic Fields and Magnetic Fields, No Currents, which is coupled with the Solid Mechanics interface. The coupling can also handle the case when the deformation of the structure significantly affect the magnetic field distribution.
The presence of such a multiphysics node in the model will enforce the Include geometric nonlinearity option on all applicable study nodes, unless the Only use Lorentz force check box is selected.
Settings
The Label is the multiphysics coupling feature name. The default Label (for the first multiphysics coupling feature in the model) is Magnetomechanics 1.
The Name is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first multiphysics coupling feature in the model) is mmcpl1.
Domain Selection
The domain selection includes by default all applicable domains. Such domains represent an intersection of the applicable domains under the corresponding Magnetic Fields and Solid Mechanics interfaces selected in the coupling feature.
If the Only use Lorentz force check box is not selected, the following domain features are applicable in the Magnetic Fields and Magnetic Fields, No Currents interfaces:
•
 Ampère’s Law in Solids. Use this domain feature for solid magnetic materials.
•
 Ampère’s Law, Nonlinear Magnetostrictive
•
 Magnetic Flux Conservation, if its material type input is set to Solid. Use this domain feature for solid magnetic materials.
•
 Coil, if its material type input is set to Solid. You can use this feature to represent current carrying solid domains such as coils and wires.
•
 Magnet
•
 Passive Conductor
In the Solid Mechanics interface, the following domain material features are applicable:
•
 Linear Elastic Material
•
 Nonlinear Magnetostrictive Material
•
 Nonlinear Elastic Material (with the Nonlinear Structural Materials Module)
•
 Hyperelastic Material (with the Nonlinear Structural Materials Module)
•
 Rigid Material
•
For nonsolid magnetic domains, remove them from the Solid Mechanics interface selection but keep them selected in the Magnetic Fields or Magnetic Fields, No Currents interface selection.
•
Ampère’s Law, Nonlinear Magnetostrictive is a part of The Nonlinear Magnetostriction Interface. The corresponding multiphysics coupling feature, Nonlinear Magnetostriction, can be combined with Magnetomechanics.
See also
•
Magnetomechanics
Coupled interfaces
This section defines the physics interfaces involved in this Magnetomechanics coupling. The Solid mechanics and Magnetic Fields lists include all applicable physics interfaces.
The default values depend on how the node is created.
•
If it is added from the Physics ribbon (Windows users), Physics contextual toolbar (Mac and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.
•
If it is added automatically when the physics interface is chosen in the Model Wizard or Add Physics window, then the two participating physics interfaces are selected.
You can also select None from either list to uncouple the coupling from a physics interface. If the physics interface is removed from the Model Builder, for example Solid Mechanics is deleted, then the Solid mechanics list defaults to None as there is nothing to couple to.
If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the physics interface again from the lists. This is applicable to all multiphysics coupling nodes that would normally default to the once present physics interface. See Multiphysics Modeling Workflow in the COMSOL Multiphysics Reference Manual.
Lorentz Coupling
Select the Only use Lorentz force check box in order to model the interaction by only applying a Lorentz force to the mechanics side and a Lorentz velocity term to the electromagnetics side. The Lorentz force F = J × B from the Magnetic Fields interface is passed to the Solid Mechanics interface and, passes the induced electric field E = v × B from the Solid Mechanics interface to the Magnetic Fields interface. The Lorentz force effect on a moving structure is similar to that of the Rayleigh alpha-damping, where the damping coefficient is proportional to the background magnetic field and the material electric conductivity.
The Lorentz force and induced electric field can also be added separately from the interfaces, for instance, by using the Body Load feature from the Solid Mechanics interface and the Velocity (Lorentz term) feature (in 2D and 2D axisymmetric) from the Magnetic Fields interface. Once the Magnetomechanics feature is applied with this setting, those interface features with the same purpose should not be used to avoid double counting. When modeling electroacoustic transducers, like loudspeakers, the coupling is typically added in the voice coil domain.
The coupling feature handles frames as well as the correct formulation for transient and frequency domain problems. When the displacement of the vibrating or moving part of the model cannot be neglected (when topology changes are important), the Moving Mesh feature (added from the Definitions) is usually used together with the Magnetomechanics feature. In this case, the Material Type of the moving part in the Magnetic Fields interface has to set to Solid. In the Moving Mesh setting, the Prescribed Mesh Displacement is usually set as the dependent variables of the Solid Mechanics interface.
 
When the Lorentz Coupling is used in a domain with magnetic materials such as a magnetizable armature, the Lorentz force is no longer the only force contribution. In this case, the force contribution from the magnetic polarization might need to be considered. This can be done by adding a Force Calculation feature from the Magnetic Fields interface and adding a Body Load feature from the Solid Mechanics interface. Note that the Force Calculation feature includes the force contribution not only from magnetic polarization but also from the Lorentz force, that is, the total electromagnetic force. Therefore, when the coupling between the Magnetic Fields interface and Solid Mechanics interface is implemented with the Force Calculation feature and Body Load feature, the Only use Lorentz force setting should not be used in the same domain.
There are several library examples that demonstrate its usage:
Loudspeaker Driver — Frequency-Domain Analysis: Application Library path Acoustics_Module/Electroacoustic_Transducers/loudspeaker_driver
Loudspeaker Driver — Transient Analysis: Application Library path Acoustics_Module/Electroacoustic_Transducers/loudspeaker_driver_transient
Magnetic Damping of Vibrating Conducting Solids: Application Library path ACDC_Module/Electromagnetics_and_Mechanics/magnetic_damping
AC Contactor with Shading Coil: Application Library path ACDC_Module/Electromagnetics_and_Mechanics/contactor_shading_coil
Deformation of an Iron Plate by Magnetic Force: Application Library path ACDC_Module/Electromagnetics_and_Mechanics/plate_deflected_by_magnet