Pressure drop due to contraction and a subsequent expansion
 

I need to know pressure drop acoss an obsrtuction which has contraction and a subsequent expansion (imagine an extended orifice in a tube)
I solved the problem in 2D and 3D both but the 3D results are closer to the theoretical calculations.
The 2D results almost deviate by 100-110%
The solver is FLUENT.
Can anyone please help???
Is it because of the vorticity?? I am unable to get it.