Problem with 3D Pipe Flow
 

I want to simulate a fully developed flow in a pipe. I can’t use a 2D mesh with axisymmetric because then I have to add an axisymmetric heat flux (q”=f(angle)). This problem has still a symmetry plane so I have tried to use a mesh (hexa mesh, block structured generated with ICEM CFD) representing half of the pipe.
I have used a symmetry boundary condition for the symmetry plane and the periodic conditions, imposing a pressure gradient (pa/m), to simulate a fully developed flow.
However, the final solutions has a wall shear stress not uniform on the wall. It grows approaching the symmetry plane. I expected the wall shear stress to be uniform. This problem disappears if I simulate the entire pipe without using the symmetry condition.
Has anybody found the same problem? Does anybody know why I cannot obtain a good solution of the velocity field if I simulate half pipe?
Thank you for any help