On Thu, 1 Mar 2018, David Knezevic wrote:
On Thu, Mar 1, 2018 at 10:50 AM, Roy Stogner <royst...@ices.utexas.edu> wrote:
If you have something vector or tensor valued, like e.g. a *velocity*
variable, and your formulation doesn't already use polar coordinates
for the components of that variable, then you're in trouble if you
want anything other than 0/90/180/270 degree rotations, because we
don't currently have any way to specify a periodic BC for one variable
as a weighted sum of other variables.
Ah, OK. I was hoping to do cases other than 0/90/180/270 degree rotations, and
I'm considering elasticity, hence (u,v,w) displacement variables.
As a result I think the current implementation won't work for me since I'd need
one variable to be a weighted sum of the others.
I can look into adding this,
That would be great!
any suggestions on where to start?
I think the right place to add this would be PeriodicBoundaryBase, but
I'm not sure what the right way to add it would be. Right now we have
a set of variables associated with each periodic boundary, but to do
this properly we'd also need the user to supply a matrix representing
the transformation of those variables from one boundary to the other,
and we'd need to cache the inverse of that matrix for use in a number
of (often literally!) corner cases where we have to apply constraint
equations to the "source" DoFs.
compute_periodic_constraints() in fe_base.C is (other than the header
and caching that matrix inverse) the only function you'd need to
change, but it's 700 lines of confusing and easily-broken (especially
in the DistributedMesh case IIRC) code.
We'd probably need to use DenseMatrix to take the matrix inverse, but
in the short run we'd want to have some semantics like "hold a
unique_ptr<DenseMatrix>, and if that pointer is null then treat it
like an identity matrix", to avoid suddenly adding an O(N_variables)
complexity factor to compute_periodic_constraints.
---
Roy
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