Global Constraint
Use a Global Constraint node (), for example, to make the sum of some global variables (space-independent) equal to a fixed number. To add a Global Constraint, either right-click The Global ODEs and DAEs Interface and select it from the context menu or click Global Constraint on the toolbar.
In any other physics interface, click the Show More Options button () and select Equation-Based Contributions in the Show More Options dialog box. Then right-click the physics interface and select Global>Global Constraint.
Global Constraint
Choose an option from the Apply reaction terms on list: All physics (symmetric) (the default) or User defined to define reaction terms. For either choice, enter a Constraint expression. The default is 0. For example, entering 2-(u+v) constrains u+v to the value 2. For User defined, enter a Constraint force expression.
The Constraint force expression must use the test() or var() operator. For example, write test(-u) to enforce the constraint by modifying only the u equation with reaction terms. See Constraint Reaction Terms and Pointwise Constraint for more information.