Hi!

In the code below u is a scalar function of x and y and U = (0, 0, u(x,y)). The code

`dot(grad(U) + grad(U).T, n)`

gives the error

`ufl.log.UFLException: Can't add expressions with different shapes.`

I am not quite able to figure out why. Any suggestions would be helpful. Here’s the MWE:

```
from __future__ import print_function
from fenics import *
import matplotlib.pyplot as plt
from dolfin import *
import meshio
from dolfin import Mesh, XDMFFile, File, MeshValueCollection, cpp
# Optimization options for the form compiler
parameters["form_compiler"]["cpp_optimize"] = True
ffc_options = {"optimize": True, \
"eliminate_zeros": True, \
"precompute_basis_const": True, \
"precompute_ip_const": True}
import numpy
import meshio
from mshr import Circle, generate_mesh
from dolfin import Mesh, File, MeshFunction, Point, BoundaryMesh, SubDomain, plot, File
import matplotlib.pyplot as plt
from dolfin import *
from mshr import *
class boundary(SubDomain):
def inside(self, x, on_boundary):
return on_boundary
class disk(SubDomain):
def inside(self, x, on_boundary):
return True
mesh = generate_mesh(Circle(Point(0, 0), 3), 50)
boundary_markers = MeshFunction("size_t", mesh, mesh.topology().dim()-1, 0)
surface_markers = MeshFunction("size_t", mesh, mesh.topology().dim(), 0)
boundary().mark(boundary_markers, 2)
disk().mark(surface_markers, 1)
ds = Measure('ds', subdomain_data=boundary_markers)
dx = Measure('dx', subdomain_data=surface_markers)
n = FacetNormal(mesh)
W1 = FunctionSpace(mesh, "Lagrange", 1)
n = FacetNormal(mesh)
G , mu = 1, 0.1
u_D=Constant(0.0)
bc = DirichletBC(W1, u_D, boundary_markers, 2)
# Define variational problem
u = TrialFunction(W1)
v = TestFunction(W1)
f = Constant(-G/mu) # f=-G/mu
a = dot(grad(u), grad(v))*dx
L = -f*v*dx
# Compute solution
u = Function(W1)
solve(a == L, u, bc)
# Calculating shear stress S
Ux=0.0
Uy=0.0
Uz=u
U = as_vector((Ux, Uy, u))
Sn = mu/2 * dot(grad(U) + grad(U).T, n)
```