Hi Eike,

Can you give some context. What you have described is a very unfiniteelement concept. The usual FE concept of the flux is the integral over a facet:

phi*dot(u, n) * ds

Where n is the FacetNormal. In that case everything gets evaluated at quadratic equation points so the issue you raised does not occur.

Cheers,

David

On Fri, 2 Oct 2015 at 20:21 Eike Mueller <E.Mueller@bath.ac.uk> wrote:
Dear firedrakers

if I have a scalar field phi and a vector field u, how can I calculate the flux f=phi*u in firedrake? I need to be able to loop over the dofs i of u, and for each one calculate f_i = sum_k phi_k*e_k(x_i)*u_i where e_k(x_i) is the k-th basis function of phi evaluated at the node x_i of u. But I somehow have to tell it that I want phi evaluated at u's nodes, not the other way round.

Thanks,

Eike

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