On Sun, 14 Jan 2018, Jonas Ballani wrote:
I have a question regarding constrained dofs that are themselves constrained by
other dofs within the same element. Apparently, this leads to a non-symmetric
constrained element matrix which I illustrate here by a small example:
I consider an element with dof indices = (0,1,2,3). I impose the following
contraint rows
{
constrained_dof_index = 0;
constraint_row[1] = 1.0;
constraint_row[2] = -1.0;
}
{
constrained_dof_index = 1;
constraint_row[0] = 1.0;
constraint_row[3] = -1.0;
}
So x0 = x1 - x2 is the first constraint, and
x1 = x0 - x3 is the second constraint?
You're constraining DoF 0 in terms of DoF 1, *and* you're constraining
DoF 1 in terms of DoF 0?
I'm afraid we don't support constraint equations with cycles. You can
have a sequence in which you constrain a DoF with respect to a second
DoF which is itself constrained with respect to a third DoF and so on
indefinitely, *except* that you can never end up at a constraint which
was already referenced earlier in that sequence.
I don't think that limitation would be easy to change, either. It
would turn DofMap::enforce_constraints_exactly() from a simple sweep
into an outright linear solve.
---
Roy
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