compute normal derivative on boundary

Question

Is there a straight forward way to compute (and plot) $\vec n \cdot \nabla u$ on a boundary?

This was asked 3 years ago here, but there didn't seem to be a simple solution. Maybe this has changed by now?

Comments

My inclination is to first build n:

n = FacetNormal(mesh)

Then, assuming you're simply trying to include this in a weak formulation:

u = TrialFunction(U)
v = TestFunction(U)

problem = inner(nabla_grad(u)*n,v)*dx

OR You could simply compute grad(u) directly with the Dx() function (assuming u is a scalar function, and your domain is 2D, for simplicity):

problem = Dx(u,0)*n[0]*v*dx + Dx(u,1)*n[1]*v*dx