Creating a Multiphysics Model
There are two ways to create and use the available predefined multiphysics couplings: using Predefined Multiphysics Interfaces or Adding Predefined Multiphysics Couplings to Physics Interfaces using The Add Multiphysics Window or in The Multiphysics Branch. You can also create multiphysics couplings, in the physics interface settings, using a model input or by directly typing an expression using a dependent variable from another physics interface, for example (see Specifying Equation Coefficients and Material Properties for information about what you can include in such expressions).
Predefined Multiphysics Interfaces
The Joule Heating Interface is an example of a predefined multiphysics interface. Many other multiphysics interfaces are available depending on the products included in your COMSOL license. After Joule Heating is selected from The Model Wizard, the Heat Transfer in Solids interface, the Electric Currents interface, and a Multiphysics node, including the default feature applicable to the multiphysics coupling (Electromagnetic Heating), are displayed under the Added physics interfaces list as in Figure 3-14. Figure 3-15 shows you what is included in the Model Builder when a predefined multiphysics interface is added. Compare to Figure 3-16 where individual physics interfaces are added, and these features are initially accessible only from the context menu. There can also be moving mesh nodes added as part of a multiphysics interface; they then appear under Definitions in the Added physics interfaces list.
You can add physics interfaces when you start creating the model with The Model Wizard or at any time with The Add Physics Window.
Figure 3-14: When Joule Heating is selected in the Model Wizard, the default physics interfaces and coupling feature are displayed under Added physics interfaces.
Figure 3-15: An example of what is added to the Model Builder when Joule Heating is selected in the Model Wizard. The Electromagnetic Heating feature is automatically included under the Multiphysics node.
Adding Predefined Multiphysics Couplings to Physics Interfaces
An empty Multiphysics node is added automatically when two (or more) physics interfaces are set up in a model and when there is the possibility to couple the physics interfaces. In other words, if you add physics interfaces one at a time, and the software identifies these physics interfaces as being of the multiphysics category, the Multiphysics node is automatically added to the Model Builder. The relevant predefined multiphysics coupling features are then available from the context menu (right-click the Multiphysics node) as well as from the Physics toolbar, in the Multiphysics menu. See Figure 3-16. You can also add predefined multiphysics couplings from the Add Multiphysics window (see The Add Multiphysics Window), which then adds all necessary multiphysics coupling nodes under the Multiphysics node. Using a workflow where you add physics interfaces and multiphysics couplings manually makes it possible to analyze and validate one physics at the time before solving the full multiphysics model.
Figure 3-16: An example of when a Multiphysics node is automatically added to the model. The specific multiphysics features are made available from the context menu (left) or (partially shown here) Physics toolbar, Multiphysics menu (right) based on the physics interfaces in the model. The difference when the predefined Joule Heating interface is added is that these features are included under Multiphysics and there are some modified settings automatically applied. In either case, the available features depend on the COMSOL products that the license includes.