The Constraints Section
The CONSTRAINTS section, which is optional, is used to apply integral constraints to the system. These constraints can be used to eliminate ambiguities that would otherwise occur in steady state systems, such as mechanical and chemical reaction systems, or when only derivative boundary conditions are specified.
The CONSTRAINTS section, when used, normally contains one or more statements of the form
INTEGRAL ( argument ) = expression
CONSTRAINTS should not be used with steady state systems which are unambiguously defined by their boundary conditions, or in time-dependent systems.
A CONSTRAINT creates a new auxiliary functional which is minimized during the solution process. If there is a conflict between the requirements of the CONSTRAINT and those of the PDE system or boundary conditions, then the final solution will be a compromise between these requirements, and may not strictly satisfy either one.
CONSTRAINTS can be applied to any of the INTEGRAL operators.
CONSTRAINTS cannot be used to enforce local requirements, such as positivity, to nodal variables.