Need help in modeling flow through orifice
 

Dear all,
I am a beginner to cfd and have been trying to model flow through an orifice to validate the Discharge coefficient values, but the values i get are around 0.7 instead of 0.6. I have taken the geometry from ISO 5167 - Measurement instruments. In my model the duct dia is 50mm and the orifice dia is 12mm. Hence the ratio beta is 0.24. I us prostar for the analysis and the meshing is done using harpoon. Mostly hexahedral cells with 2mm size. Near the orifice, the cells have been refined to 0.2mm. Inlet condition is velocity and outlet is pressure. I am using a log function to model flows near the wall and the Y plus values are in acceptable range. Relaxation factors are also standard of prostar. Problem arises as the analysis converges before the jet fully develops. I have reduced the residual values to 1e-5 and this gives better jet formation but the Cd value is still very high. I take the value obtained from the simulation as real mass flow and for ideal mass flow im using a standard formula obtained from bernoulli's equation. If anyone has any experience with air jets and such kindly share... desperately need to figure out where im going wrong....

Thanks,
Kunal