As described in Constraints, a feature that puts some sort of constraint on a dependent variable (a Dirichlet boundary condition, for example) needs a Constraint node ().

To add a Constraint, first add a Component node under Building Blocks>Components, then:

In the Expression field, you enter the constraint expression that the solver forces to zero when solving (for example, the expression T-293.15[K] makes T equal to 293.15 K). The options in the Constraint force list let you change how the constraint affects the governing equations. The Use type from constraint settings option means that the user can choose between Bidirectional, symmetric and Unidirectional constraints in the Constraint type list of an instance of this feature in the Model Builder. If you choose Customized, a Force expression field becomes visible. Here you can write an expression for controlling the constraint force.

The options in the Selection list and Output entities list define the selection where this variable definition is valid. See Specifying Selections for more information.

Under the Shape Declaration section you choose what shape function to use for the constraint. Similar to the Dependent Variable Definition, you can choose to specify a reference to a Dependent Variable Declaration, and the constraint then uses the same shape function as that dependent variable. To use this option, choose Same as dependent variable from the Constraint shape function list. With the option Use specific shape in this list, you can specify an arbitrary shape function for the constraint from the Shape function list. If you choose User-defined product, you get additional choices for individual shape functions and discretization for the base geometry and extra dimensions, when working with extra dimensions.

The Allow weak constraints check box lets you deactivate support for weak constraints, which is selected by default. When selected, the constraint can potentially generate a weak form equation. Therefore, it is necessary to support a Base vector system list selection similar to the Weak Form Equation. You can also select Customized from the Weak expression list instead of Automatic, which is the default, and then enter a custom weak-form expression for the constraint in the Weak expression field. In addition, if you have selected Use specific shape from the Constraint shape function list in the Shape Declaration section above, then you can select Customized from the Lagrange multiplier name list instead of Automatic, which is the default, and then enter a Lagrange multiplier base name in the Base name field (default: name_lm). Otherwise, the Lagrange multiplier’s name is determined automatically. Under When selected, the Lagrange multiplier may differ from the reaction forces, you can select the Use volume factor in axial symmetry or for non-orthonormal systems check box, which adds a 2πr volume factor in axisymmetric models. It is also possible to select the Assume constant volume factor check box in this section.

Clearing the Allow weak constraints check box means that it is not possible to activate weak constraints for an instance of the feature owning this constraint in the Model Builder. Then all the settings above are disabled.

Select the Add excluding selections automatically check box to add such excluding selections for the constraint settings automatically. Any Excluding Selection subnodes are then disabled and not used.

Select the Exclude from constraint grouping check box to make sure that the constraint is not affected by constraint grouping. This setting is only relevant when constraint grouping is activated for the physics feature. To activate constraint grouping add an Auxiliary Settings node to the parent feature and select the setting Enable constraint groups.