ufl team mailing list archive

[Kristian Oelgaard <k.b.oelgaard@xxxxxxxxxx>]

Quoting Martin Sandve Alnæs <martinal@xxxxxxxxx>:

> Hi all,
> thanks for all the nice comments and feedback at the workshop.
> There was in particular some interest in how to extend UFL
> with experimental features without having to interfere with the
> main development branch. I'd like to compile a list of the kind
> of features that are most interesting to people, so we can figure
> out what to make extensible. Then I'll see what I can do.

Extending UFL with lifting operators.

Recall that for the Poisson equation we have:

a = \int (grad(v) + R(v))(grad(u) + R(u)) d\Omega
    + \sum_e \int (\eta r^e(v) r^e(u) d\Omega

with

R(u) = \sum_e r^e(u)
and
\int_{E = E^{+} + E^{-}} w r^e(u) d\Omega = - \int_e {w}[u] d\Gamma

where the last equation is the lifting operation.

I think we need to introduce two new functions in order to distinguish terms
like grad(v)R(u) and the stabilisation term r^e(v)r^e(u).
As mentioned at the workshop, we probably also need a new integral type.

So (some of) the terms in the Poisson equation could be represented as:

element  = FiniteElement("Discontinuous Lagrange", triangle, 1)
l_space = VectorElement("Discontinuous Lagrange", triangle, 0)

v = TestFunction(element)
u = TrialFunction(element)
R = LiftingFunction(l_space)
r = LiftingOperator(l_space)

a = inner(grad(v), R(u))*dE + inner(r(v), r(u))*dE

For this to work, we probably have to introduce a new type of integral in UFC
and modify the assembly in DOLFIN.

Kristian

> Martin
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