Convert ufl.algebra object to function for errornorm

1

Hello!
I have a function defined as

x = SpatialCoordinate(mesh)
u_D = -((pow(x[0],2)-pow(x[1],2))*x[2])/12

and I want to plug it into errornorm like this

errornorm(u_D, u, 'H1')

where u is the solution to some PDE.
Unfortunately, as it stands, u_D is an object of type ufl.algebra.Division, and errornorm complains that

AttributeError: 'Division' object has no attribute 'ufl_element'

so I am trying to convert this u_D into anything that can be used into errornorm.
I tried assembling the error manually but I get numerical instabilities; I know I can use Expression but I was advised to avoid it.

Is there any way I can convert my u_D in something I can use in errornorm?

2

I have no idea why anyone would advise you to avoid Expression…

Interpolate or project u_D onto an appropriate function space.

3

Thanks a lot for the reply!

Let’s leave aside the question of whether to Expression or not to Expression.

I tried projecting with

u_D = project(u_D, FunctionSpace(mesh,FiniteElement("CG", mesh.ufl_cell(), 1)))

but I always get the AttributeError above, then I tried interpolating with

u_e = Function(FunctionSpace(mesh,FiniteElement("CG", mesh.ufl_cell(), 1)))
u_e.interpolate(u_D)

but I got

AttributeError: 'Division' object has no attribute '_cpp_object'

Is this what you meant? What’s the correct way of doing it?

4

Hi, what FEniCS version do you run and what is the code that raises the attribute error? The following works on 2017.2.0, 2018.1.0.

from dolfin import *

mesh = UnitCubeMesh(4, 4, 4)

x = SpatialCoordinate(mesh)
u_D = -((pow(x[0],2)-pow(x[1],2))*x[2])/12

u_D = project(u_D, FunctionSpace(mesh,FiniteElement("CG", mesh.ufl_cell(), 1)))
5

I’m using 2019.1.0.
Now, as it’s customary in computer science, I pasted your code and it worked. Then I tried again the code I posted above, just the way it was, and now it worked. This totally eludes my comprehension XD

Now that I looked silly enough, I can go back to it.
I saw UMFPACK running out of memory at some point so it’s gonna be a fun day.

Thank you very much for your help, both! I would mark both your messages as “Solution” but I can only mark one, apparently.