问题描述
When creating a newMagnetic Fieldsmodel in COMSOL Multiphysics 6.2 or newer and following a modeling workflow I got used to in COMSOL Multiphysics 6.1 or earlier, I get different results. It seems the material properties assigned to some of the domains have not been considered. Why does this happen, and what can I do about it?
解决方法
A new workflow and its logic
A new default feature,Free Space, was added to theMagnetic Fieldsinterface in COMSOL Multiphysics 6.2. It represents the physical conditions in the free space - typically air or vacuum - surrounding the modeled device. The electromagnetic material properties of the free space are defined by the feature, and therefore override any of the properties supplied by theMaterialsnode.
TheFree Spacefeature provides a canvas on top of which other features, such asAmpère’s Law in Fluids,Ampère’s Law in Solids,Magnet,Passive Conductor,Laminated Core, andCoilcan be added. One such feature must be applied to any domain where you want to use material properties different from those of free space. Additionally, some of these features allow you to specify an excitation source, such as a current flowing through aCoil.
Most of the example models featuring theMagnetic Fieldsinterface demonstrate this workflow. See for example this one. The next two sections discuss the extra benefit of using theFree Spacefeature, and the distinction between theAmpère’s Law in SolidsandAmpère’s Law in Fluidsfeatures.
The Free Space and the Stabilization conductivity
TheFree Spacefeature implements a conductivity governed by theStabilization Conductivitydropdown menu. In 3D, it defaults toAutomatic, which provides a stabilization conductivity forFrequency DomainandTime Dependentsimulations. The conductivity is defined such that the skin depth in free space becomes 100 times greater than the diagonal of the bounding box of the geometry. Such a small conductivity is expected to have a negligible impact on the fidelity of the results, but its presence improves the solver's reliability and speed.
User-defined options for manual tuning of the stabilization are available as well. In 2D, theStabilization Conductivityis off per default, and is rarely needed. More information and detailed specifications can be found inthe documentation.
The difference between Ampère’s Law in Solids and Ampère’s Law in Fluids
The new workflow has introduced two types of Ampère’s Law;Ampère’s Law in SolidsandAmpère’s Law in Fluids. These determine how materials behave and how material properties are interpreted when when mechanical deformation is involved. This matters if your model has aMoving Meshand/orSolid Mechanicsinterface coupled with the magnetic problem.
TheAmpère’s Law in Solidsfeature applies to solid materials whose material properties are affected during deformation and, for instance, depend on the material strain or material orientation. Features such asMagnet,Passive Conductor, andLaminated Corealso behave as solids. Meanwhile, theAmpère’s Law in Fluidsfeature applies to nonsolid materials, whose properties can be considered a independent of the deformations.
More information can be found inthe documentation.
NB: In COMSOL Multiphysics 6.1 and earlier, the distinction between solid and fluid materials was controlled within theAmpère’s Lawnode by specifyingSolid/Nonsolidin theMaterialtype setting. In newer versions, the same logic is still kept for theCoilfeature.
TheFree Spacedefault feature with theStabilizationoption, as demonstrated for a model of a three-phase power transformer. Please notice theAmpère’s Law in Solidsand severalCoilnodes added further down in the Model Tree.
The Free Space feature in some other AC/DC interfaces
A similar workflow has been introduced for theElectrostatics,Magnetic and Electric Fields, andMagnetic Fields, Currents Onlyinterfaces, starting from COMSOL Multiphysics 6.3. The motivation and the logic are the same.
NB: TheFree Spacefeature for theMagnetic and Electric Fieldsinterface comes with a built-inStabilizationconductivity option which is different from the one used for theMagnetic Fieldsinterface. By default, it adds a conductivity of 1 S/m to preventStationaryandTime Dependentstudies from becoming singular.Frequency Domainstudies with this interface often do not require any stabilization conductivity.
Backward compatibility
A model saved in an older version and created with the old workflow will be opened in the current COMSOL version's GUI with legacyAmpère's Lawnodes retained. You will have the possibility to add theFree Spacefeature manually if necessary.
Beware that the new workflow may impact backward compatibility with respect to API support. Any model method, Java® code, and MATLAB® code that has been saved using previous COMSOL versions may need to be revised in order to function properly. Please follow our recommendationshere.
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