force coefficient computation
 

I an simulating flow around hemisphere cylinder configuration. In supersonic range the force and moment coefficients match very well with experimental data. But in low speed subsonic range theses coefficients are not in good agreement with experimental data.What is the reason? I am using TASCflow and CFX5.5. I have tried meshes of different resolutions different discretization schemes and different turbulence models but I am unable to get accurate results. In some cases percetage difference between experimental and CFD results is about 200%.