plot value of dot(n, grad(u)) over boundary

Question

Given a function u over some bounded domain, is there a way to plot dot(n, grad(u)) over the boundary? (Maybe as a FacetFunction.)

Comments

It is easy to plot this function when $\mathbf{n}$ is known and you can form it without using the built-in FacetNormal(). It would be handy to have a way to form $\mathbf{n}$ for use with cell integrals. The interior could just be set to zero.

---

Is there a simple solution by now?

Answer 1

This does not quite answer your question, but hopefully is useful.

A solution on computing dot(t, grad(u)) has been posted here: How to compute tangential gradient, where t is the tangent along the boundary.