a problem in rotor-stator interaction (help)!

feng

I am simulating the impeller-diffuser interactions including an impeller, an vaned diffuser and an vaned return chnnel of a radial hydraulic pump. For the transient process, I set some monitor points in different regions to judge the flow periodicity. I found that for the pressure field, it gets periodicity very fast. But the velocity field is far from being periodic. The calculation has been running for dozens of impeller revolutions. The boundary conditions are total pressure at the inlet and mass flow at the outlet. Who can give me some advices? thanks a lot!!