[mrshb4]

Dear Foamers
I'm modeling a centrifugal fan. It has an almost large radius half-sphere at inlet because of modeling air in the room. And also has an almost long duct at outlet.
I've used these boundary conditions for inlet and outlet:

Inlet
P : total pressure 0
U : pressureInletVelocity;
k : fixed value 1e-6;
epsilon : fixed value 1e-9;

outlet
P : fixedvalue 0 (or fixedMeanValue 0)
U : zeroGradient
k : zeroGradient
epsilon : zeroGradient

but in my first iterations (more than 500 iterations) flow is from outlet to inlet and after that the flow will turn around and will be correct. I don't know the reason. Anyone know why this happens?!!