Thank you Amine for your kind suggestion. As I mentioned, I monitored "Area weighted avg of radial velocity" over a plane by creating a plane monitor and used the final steady-state value when the solution is converged. I mean from this that

i) Monitored radial velocity reached the steady-state

ii) Residuals have reduced to 10-04

Then I picked the final value and plotted it vs no. of cells.

Now as you said it is better to use it as residual velocity. I don't understand your point completely. Would you please explain it more? 

How can I plot residual for radial velocity?