convergence problem in the turbomachinery problem.
 

what are the issue for the convergence of a simulation. In my case I am solving a centrifugal fan problem. In which I am simulating the impeller as well as the volute. I imposed the boundary conditions at
1) inlet : total pressure (0 pa relative pressure)
2) Outlet : mass flow rate:
3) impeller : Rotating wall;
4) casing :stationary wall;
I am using the k- omega turbulence model for solving the turbulent quantities.
I set the convergence criteria as 10e-4 for the velocities and the turbulence quantities.
I am solving the momentum equations with High resolution scheme.
I got struck when the simulation is not conversing up to the 10e-4. but I want it must touch the criteria of the 10e-5.
Can anyone help me where may be the problem? Please suggest me. Thank you.