All study step Settings windows contain a Physics and Variables Selection section, which you can use to control which physics interfaces (or even specific variables and physics nodes) to solve for. This can be useful for:

By default, you can select from the participating physics interfaces and moving frames. To select individual physics nodes and variables and other model settings, select the Modify model configuration for study step check box (see Modifying the Model Configuration below).

The Physics interface column contains the names of all true physics interfaces in the model. Some special auxiliary interfaces, notably Optimization and Sensitivity, are not shown when their dependent variables are controlled in other ways. The Moving Mesh interface itself is also excluded, but instead there is a special entry with the same name for controlling the mesh motion in each component. This entry appears in the list as soon as you add features under Definitions>Moving Mesh, or if your model contains any interface controlling the spatial frame, and the spatial frame uses dependent variables.

You can choose to not solve for one or more of the physics interfaces or the spatial frame by clicking the  button in the Solve for column (by default, a study solves for all physics interfaces). Those physics interfaces are then not solved for but can still provide values for the degrees of freedom (dependent variables) according to the settings for values of variables not solved for (see Values of Dependent Variables). Click the button ( when the physics is deactivated) again to solve for the physics interface.

In the Discretization list you can specify which discretization to use. The default (and often the only) choice is Physics settings, which means that the study uses the discretization from the main physics interface node’s settings. Changing it affects the discretization order used by this study. To add another discretization, use a separate Discretization node in the physics interface. The leftmost column is usually empty but contains a warning () if the physics’ degrees of freedom are not solved for regardless of the setting in the Solve for column. This can be the case if the physics interface is not compatible with the study step.

The Multiphysics couplings column contains the names of all multiphysics couplings in the model. You can choose to not solve for one or more of the multiphysics couplings by clicking the  button in the Solve for column (by default, a study solves for all physics interfaces). If you clear the Solve for button, any equations and dependent variables that the multiphysics coupling adds are not included, but the multiphysics coupling still affects the definition of variables.

If the model contains a Model Reduction study, there is also a Reduced model column, where the Reduced Model node that contains the reduced model appears. Select the Store output dependent variables check box to store the output from the reduced model as dependent variables for postprocessing. This option is available if you have selected the Add output dependent variables check box in the Reduced Model node’s settings.

If reconstruction is included in the model reduction, you can choose a compatible reduced model to use for reconstruction under Reduced model for the physics interface that is to be reconstructed under Reconstruction. This section is only available if the physics interface is not being solved for (the Solve for check box is cleared in the Physics interface section above). By default, this reduced model applies to all fields and states in the physics interface. If desired, it is possible to use different reconstruction for different fields and states by choosing another Reduced Model node in the Field and State nodes.

If the Modify model configuration for study step check box is selected, you can modify the model configuration by specifying which variables and physics features to include in the model that you solve. You can also control Perfectly Matched Layers, Infinite Element Domain, and other features under Component>Definitions as well as specifying which physics that manages which frame. The Physics and Variables Selection section then contains a tree that is a copy that include the following parts of the model tree in the Model Builder (see Figure 20-4):

It is possible to include or exclude all variables, physics interfaces, and physics nodes in a study step (that are not disabled in the model tree). Select one or more nodes in the tree and right-click or use the buttons at the bottom of the section (below the tree) to change their status. Click the Go to Source button () to move to the corresponding original node in the model tree. Click the Collapse All () and Expand All () buttons if desired to completely collapse or expand the physics tree. The following additional options are available:

Click Disable (or right-click to select from the context menu) to disable enabled nodes that are possible to disable. The contributions, conditions, or variables in a node that you disable are not included in the study when solving. You can also disable selected nodes by clicking the Disable button () underneath the tree. A disabled node is unavailable in the tree. You can also right-click any applicable node in the model tree to select Disable in all studies. That node is then disabled for all studies.

Click Enable (or right-click to select from the context menu) to enable disabled nodes. The contributions, conditions, or variables in a node that you enable are included in the study when solving. You can also enable selected nodes by clicking the Enable button () underneath the tree.

An asterisk displays in the upper-right corner of nodes for which the state has been changed in the study step’s selection tree compared to the state in the Model Builder. In this example, under the Physics and Variables Selection section, a Transport in Diluted Species interface () is disabled (unavailable), provides no degrees of freedom (red dot in the lower-right corner), and has a change of state indicated by the asterisk. The asterisk means the Laminar Flow interface in the Model Builder is not disabled. Also see Figure 20-4 for another example. In general, any variable or physics node in the Model Builder that is disabled in any study step gets an asterisk in the upper-right corner. For physics interfaces, this applies also when you have not selected the Modify model configuration for study step check box and the physics interface is disabled in the Solve for column in the Physics and Variables Selection section.

The dynamic visual icon indicators for overridden and contributing nodes also appear in the tree in the Settings window for the study steps when you have selected the Modify model configuration for study step check box in a study step’s Settings window. When you select a physics node in the tree, the override and contribution icon indicators appear in the same way as in the Model Builder when you select a physics node, but if you disable any physics node in the study step’s tree, the icon indicators then show how the physics node overrides and contributes to the model when one of more physics nodes are disabled in the study step.

The following options are available for the main physics nodes under the Physics and Variables Selection tree. Right-click a node and select one of the following from the context menu or click the button beneath the tree (see Figure 20-4). Selecting these options affects the entire physics interface. Select:

Multiple physics interfaces controlling the same frame are not allowed on the same selection. It is, however,. possible to use multiple frame-controlling physics with overlapping selection, but you then have to explicitly disable frame control on all but one of the physics. You can choose different physics to control the frame in different study steps. In the physics tree, frame-controlling interfaces have the label Controls spatial frame or Controls material frame. Toggle the frame control on those physics interfaces by selecting them in the tree and toggling the Control Frame Deformation button () in the toolbar or by right-clicking the physics interface node and selecting Control Frame Deformation from the context menu.

You can also right-click a physics node in the selection tree to select the discretization. The discretizations appear at the bottom of the context menu (underneath the horizontal divider). In most cases, the only option, and the default, is Physics Settings, which takes the discretization from the physics interface’s Settings window, but if you have added separate Discretization nodes, you can select from one of those instead of Physics Settings.