Weird error when differentiating TrialFunction

The relevant code-snippets I use is

def normal_grad(f,n):
return f.Diff(x)*n[0] + f.Diff(y)*n[1]

Vh = H1(mesh, order=order, dirichlet=[1,2,3,4],flags={"dgjumps": True})
v = Vh.TestFunction()

nF = specialcf.normal(2)

vn = normal_grad(v,n)
vno = normal_grad(v.Other(),nF)

vnno = normal_grad(vno,nF)
vnn = normal_grad(vn,nF)



and I get error


NgException Traceback (most recent call last)
<ipython-input-197-5729456d113a> in <module>
179 jump_u_2 = normal_grad(un,nF) - normal_grad(uno,nF)
180 vnno = normal_grad(vno,nF)
--> 181 vnn = normal_grad(vn,nF)
182 jump_v_2 = vnn-vnno
183

<ipython-input-191-e5403275507c> in normal_grad(f, n)
1 def normal_grad(f,n):
----> 2 return f.Diff(x)*n[0] + f.Diff(y)*n[1]
3

NgException: Deriv not implemented for CF N5ngfem23NormCoefficientFunctionE

Has anyone stumbled upon this error before? I find it weird since I can do the same thing for a TestFunction, and it works. I am trying to implement higher order directional derivatives.

Best regards,

Henrik

Hi Henrik,

I needed to change "vn = normal_grad(v,n)" to "vn = normal_grad(v,nF)" to get your code-snippet running, but I did not get any further error. Do you have the latest NGSolve version?

To compute the normal derivative of a H1 test/trial function u you have to write as Diff(x) is used for differentiating CoefficientFunctions depending explicitly on x. Using u.Diff(x) returns 0.

Best
Michael

Thank you so much! Did not spot that error. That solved it.

Small addition:
With the above correction you will only get first (or with "hesse") second order normal derivatives.
In ngsxfem we have a DifferentialOperator "dn" for higher order normal derivatives implemented with limited scope of application. However, perhaps you want to have a look at the "dnjump" use case in the example
github.com/ngsxfem/ngsxfem/blob/release/.../fictdom_ghostpen.py

Best,
Christoph

Thank you very much Christoph!

Henrik