pump prevalence at low mass flow rates
 

Hi everybody,
my question deals with simulation of water pumps.

I simulated a water pump at different mass flow rates as inlet condition and compared my results with experimental data. I worked with MRF model with a commercial code.
At maximum mass flow rates (water valve completely open) prevalence results agree with experimental data very well. At contrary, at lower mass flow rates prevalence is over-estimated, and error grows at diminishing of mass flow rate and at growing of rotational velocity.

Has anybody a suggestion to explain this phenomenon?

Thanks,

Caterina