New question #133595 on DOLFIN:
https://answers.launchpad.net/dolfin/+question/133595
I have a time-dependent Cahn-Hilliard-Type problem and I need to
evaluate the angle from the x-axis of a gradient in order to define
my variational problem. The angle of the gradient can be calculated
by using phi = arctan( dc/dy / dc/dx ). This is needed to model an
anisotropic N-fold surface tension with the help of cos(N*phi).
A very native approach was to use the following lines of code. It
doesn't work of course. But I think it helps to understand my
problem.
-------------------- import numpy as np from dolfin import *
...
V = FunctionSpace(mesh, "CG", 2) w = TestFunction(V)
c = Function (V) # current solution c0 = Function(V) # old
solution
...
# Anisotropic N-fold surface tension: def anisotropic_tension(c0):
return np.cos(N*np.arctan2(c0.dx(1),c0.dx(0)))
L = L - dt*anisotropic_tension(c0)*(inner(div(grad(w)),
div(grad(c)))*dx)
...
--------------------
I think one basic problem is the interaction with numpy and the
encapsuled evaluation of the derivatives. I'm glad for any hint how
to proper evaluate the angle and how to use it in order to define L.
