Hi guys,

For a cylindrical geometry, I want to get the azimuth (i.e. radial or polar) angle \phi evaluated at all mesh coordinates.

I tried four approaches with interpolate/project and cpp/ufl expressions. However, if I use the `ufl.atan_2`

and try to project onto a simple Lagrange finite element function space, the angle is not correct at the \phi = \pi discontinuity (see screenshot). However, if a construct the angle with an cpp expression, it works using both interpolate and project.

So, the main question is, why the ufl expression behaves different than the cpp expression when using the project method (case 1 vs 3 in the MWE)?

MWE:

```
from dolfin import *
import matplotlib.pyplot as plt
import ufl
mesh = RectangleMesh(Point(-1.0, -1.0), Point(1.0, 1.0), 10, 10)
el = FiniteElement("Lagrange", mesh.ufl_cell(), degree=1)
V = FunctionSpace(mesh, el)
x = SpatialCoordinate(mesh)
# Approach 1: Fails with project
phi = project(ufl.atan_2(x[1], x[0]), V)
# phi = interpolate(ufl.atan_2(x[1], x[0]), V) # fails, no eval() method..
plt.subplot(221)
p1 = plot(phi, title="phi1")
plt.colorbar(p1)
file = File("phi1.pvd")
file.write(phi)
# Approach 2: Works with interpolate and UserExpression (has eval method)
class RadialAngle(UserExpression):
def eval(self, values, x):
values[0] = ufl.atan_2(x[1], x[0])
phi2 = interpolate(RadialAngle(), V)
plt.subplot(222)
p2 = plot(phi2, title="phi2")
plt.colorbar(p2)
file2 = File("phi2.pvd")
file2.write(phi2)
# Approach 3: Works with project
phi3 = project(Expression("std::atan2(x[1],x[0])", element=V.ufl_element()), V)
plt.subplot(223)
p3 = plot(phi3, title="phi3")
plt.colorbar(p3)
file3 = File("phi3.pvd")
file3.write(phi3)
# Approach 4: Works with interpolate
phi4 = interpolate(Expression("std::atan2(x[1],x[0])", element=V.ufl_element()), V)
plt.subplot(224)
p4 = plot(phi4, title="phi4")
plt.colorbar(p4)
file4 = File("phi4.pvd")
file4.write(phi4)
plt.rcParams['figure.figsize'] = [15, 10]
plt.show()
```