Re: [Libmesh-users] Constraint matrix

[Roy S. <roy...@ic...>]

On Fri, 5 Jan 2018, Renato Poli wrote:

> I need to tie three dofs together. (u1 + C u2 + D u3=0). I read about the
> constraint matrix in the list, but I did not find an example to follow. The
> penalty method seems also a nice alternative.
>
> Is there any example of either to suggest?

In systems_of_equations_ex2.C there's an example of using a penalty
term to pin pressure at one point; that's probably the closest analogy
we have to what you want to do.

Using DofMap::add_constraint_row() would be better (you'd be solving
the correct equations!) but I don't think we have any use of it in our
example codes, and it's slightly trickier: you need to ensure that the
DoF you constrain isn't already constrained by e.g. hanging node
constraints or boundary condition constraints.
---
Roy