issue with 2D couette flow of a Newtonian fluid.

Hi,

I'm trying to solve a very simple problem which is a 2D couette flow of a Newtonian fluid.
I made the geometry and have specified the boundary conditions.

v_outercylinder = 0
vx_innercylinder = -(sqrt(x^2+y^2))*sin(atan(y/x))
vy_innercylinder = (sqrt(x^2+y^2))*cos(atan(y/x))

The problem is I don't get a converged result! Some one please help me to resolve this issue.
Thanks