Magnetohydrodynamics

) multiphysics coupling feature is used to model the interaction of magnetic fields and conducting fluids. It couples one of the magnetic field interfaces and the interface. The supported magnetic field interfaces are the , , , and interfaces in the AC/DC Module. By default, the coupling feature passes the Lorentz force F = J × B from the magnetic field interface to the interface and the electromotive force (the induced electric field) E = v × B from the interface to the magnetic fieldinterface.

The feature is available in 2D, 2D axisymmetric, and 3D geometries and is allowed for both Stationary and Time Dependent studies (including Frequency-Stationary and Frequency-Transient studies). When the deformation of the conducting fluids cannot be neglected (that is, when topology changes are important), the feature (added from the ) is usually used together with the feature.


	The Magnetohydrodynamics feature cannot be used to model magnetizable fluids (ferrohydrodynamics). In magnetizable fluids, the magnetization forces can be much larger than the Lorentz force. In such cases, add a Force Calculation feature from the magnetic field interface and a Volume Force feature from the Laminar Flow 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 field interface and Laminar Flow interface is implemented with the Force Calculation feature and the Volume Force feature, the Magnetohydrodynamics feature should not be used in the same domain.

Settings

The is the multiphysics coupling feature name. The default (for the first multiphysics coupling feature in the model) is Magnetohydrodynamics 1.

The 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 field. The first character must be a letter.

The default (for the first multiphysics coupling feature in the model) is mhd1.

Coupled Interfaces

This section defines the physics involved in the multiphysics coupling. The and lists include all applicable physics interfaces.

When the Magnetohydrodynamics node is added from the ribbon (Windows users), contextual toolbar (macOS and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.

You can also select from either list to uncouple the Magnetohydrodynamics node from a physics interface. If the physics interface is removed from the , for exampleis deleted, then the list defaults to 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 interface. See Multiphysics Modeling Workflow in the COMSOL Multiphysics Reference Manual.

Coupled Settings

The coupling behavior can be easily controlled by selecting or clearing the and check boxes. For example, if is selected and is cleared, only the Lorentz force is considered and the electromotive force is neglected. This is usually valid when the magnetic Reynolds number is very low.

There are several library examples that demonstrate its usage:


	Hartmann Boundary Layer: Application Library path ACDC_Module/Electromagnetics_and_Fluids/hartmann_boundary_layer Magnetohydrodynamics Pump: Application Library path ACDC_Module/Electromagnetics_and_Fluids/magnetohydrodynamics_pump