Magnetohydrodynamics
The 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 Laminar Flow interface. The supported magnetic field interfaces are the Magnetic Fields, Magnetic and Electric Fields, Magnetic Field Formulation, and Rotating Machinery, Magnetic 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 Laminar Flow interface and the electromotive force (the induced electric field) E = v × B from the Laminar Flow interface to the magnetic field interface.
The Magnetohydrodynamics 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 Moving Mesh feature (added from the Definitions) is usually used together with the Magnetohydrodynamics feature.
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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.
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The difference between the Magnetic and Electric Fields interface and the Magnetic Fields interface is that the former solves the current conservation equation explicitly, while the latter assumes the conservation of current implicitly. In 3D magnetohydrodynamics modeling, the most general approach is to use the Magnetic and Electric Fields interface to solve the magnetic field. The Magnetic Fields, Magnetic Field Formulation, and Rotating Machinery, Magnetic interfaces can be useful in specific cases.
Settings
The Label is the multiphysics coupling feature name. The default Label (for the first multiphysics coupling feature in the model) is Magnetohydrodynamics 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 mhd1.
Coupled Interfaces
This section defines the physics involved in the multiphysics coupling. The Electromagnetic and Fluid flow lists include all applicable physics interfaces.
When the Magnetohydrodynamics node is added from the Physics ribbon (Windows users), Physics 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 None from either list to uncouple the Magnetohydrodynamics node from a physics interface. If the physics interface is removed from the Model Builder, for example Solid Mechanics is deleted, then the 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 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 Include Lorentz force and Include electromotive force check boxes. For example, if Include Lorentz force is selected and Include electromotive force 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