problem with using fan BC to simulate turbine.
 

Hi forumers. I'm using fan BC (with and without rotation )to simulate a turbine.
I've got a and a' by BEMT and applyed it to fan BC.
And fan was set as △P=2ρa(1-a)U^2(for both conditions) and Vθ=2Ωra'(equals 0 if rotation is neglected) and radial velocity is 0(for both).
steady k-e was used

Results shows: (dot is experiment results and thin blue line is cfd result)
1.without rotation
http://s11.sinaimg.cn/orignal/002VbloFgy6SKf8EKTw0a&690
2.with rotation
http://s12.sinaimg.cn/orignal/002VbloFzy6SKeabyd5bb&690

you can see that results of model without rotation is acceptable.
however model with rotation shows an acceleration of wind speed at bottom area.
could anyone tell me why does it happpen?
is it wrong with the equation of △P=2ρa(1-a)U^2 and Vθ=2Ωra' ?
or if i add source term in N-S, instead of use fan BC, shall solve this problem?

pls feel free to ask any question and contact me!
thank you guys!