Correctly setting initial conditions using Expression() in DG problem

Legacy DOLFIN
1

I have discovered an issue when trying to use to avg and jump operators with expressions for the initial conditions in a DG problem. dolfin version ==2019.2.0.dev0

I have been unable to correctly define the expression on both sides of the facet - one side is always ~0. For example:

from fenics import *

mesh = UnitIntervalMesh(10)

V = FunctionSpace(mesh, 'DG', 1)

f = Expression('1', element = V.ufl_element())

# f = interpolate(f, V) #problem persists with or without interpolation step, which I have seen in other codes

print(project(avg(f), mesh=mesh).vector().get_local())

# prints list of 0.5s, but should print list of 1s as f=1 on both sides of each facet

Any insight would be greatly apprecited!

2

Maybe I’m missing something crucial, but I’m actually surprised this didn’t throw an error.

See the source. Note that it expects a formulation posed only within cell volumes. And consider Where to find 'project'-function in dolfinx? - #2 by nate.

The projection operation you’re proposing doesn’t make sense for an operator (avg(.)) defined on facets only. I’d suggest you construct your own projection operation as you require.

1 Like
3

Hi Nate, thanks for your answer that makes sense!

I am trying to debug a numerical flux made up of a combination of avg() and jump() operators. Erroneously, I’ve been using project() as a way to evaluate the result and observe the values at the facets. Is there a more correct way of doing this? (avg(f) is a Product object)

4

Make a manual projection operator, as nate suggests.
See for instance:

2 Likes
5

Ok thank you both, I will try this