Constraint
The Constraint boundary condition specifies an expression R which is constrained to be equal to zero on the selection, R = 0. By default, this is a bidirectional constraint, meaning that all variables in R are affected by reaction terms. You can also add Constraint nodes for edges in 3D components and for points in 2D and 3D components by right-clicking the main PDE interface node and choosing Constraint from the Edges and Points submenus.
Constraint
Enter a value or expression for the value of R in the constraint R = 0. For example, to constrain u to 2, enter 2-u in the field for R. The default value, 0, does not add any constraint.
The sign in front of u in the constraint controls the sign of the implicit Lagrange multiplier μ, as well as the sign of reaction forces computed using the reacf() operator. For consistency with the way predefined physics interfaces implement constraints, write 2-u rather than u-2 in R.
Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. Normally these settings do not need to be changed. This section contains settings for specifying the type of constraint and whether to use a pointwise or weak constraint. See Constraint Settings for more information.
You can add subnodes to exclude the constraint from any surrounding boundary, edge, or point. See Excluded Points, Excluded Edges, Excluded Surfaces.
See Coefficient Form PDE for all the settings and Compact and Standard Notations for Classical PDEs for the equations that the Classical PDE interfaces solve.