Fluid U-flow in a cylindrical tube

I am trying to model a U type fluid flow through a cylindrical tube. But every time while computing, it shows this error,
" Failed to find a solution.
Maximum number of Newton iterations reached.
Returned solution is not converged."
Please help me with this.( the result which i am getting is like this )

And when I tried to change the iteration number, the flow profile is changing very differently.
What is the role of "iteration number" in flow model results?