sebastien.cadalen 
July 10, 2013 08:28 
Add a constraint equation
Dear all,
This post is about the way to impose an integral relation on the problem variables. As an example, let us consider a steadystate thermal transport problem with the following constraint:
where is the fluid domain, is the volume of the fluid domain, and a given temperature.
From a mathematical point of view this is equivalent to introduce a constant source term in the domain to satisfy the constraint. If the unconstrain problem is writen in the discrete space hence the constrained problem is:
where is a row vector composed of (the cell volume divided by the total volume) and is the Lagrange multiplier associated to the constraint.
This way the constraint is implicitly solved. We can also imagine the same kind of contraint on a part of the domain, on a surface of the domain, or involving more than one unknown, etc.
Is there any way to solve such a problem with implicit or explicit method?
If yes, how behaves the linear solvers with such a matrix to invert?
Best regards,
Sébastien
