problem of elbow pressure drop

Tony

I'm simulating a special elbow's pressure drop by Fluent. The results I got are about 60% of tested data. Your help is appreciated.

Problem description: Galvanized elbow for air system, elbow inlet and oulet openings are 25.5 inches, upstream duct and downstream duct openings are 40 inches, duct is 24 inches thick. Inlet flow velocity is 1750 fpm.

I built a 2D model with 8 equivalent diameters upstream and 10 equivalent diameters downstream duct. Inlet velocity and outlet pressure boundary conditions. I have tried standand k-e,realizable k-e, RNG k-e, RSM modles with standard wall functions,and standard k-e with near wall function, but all pressure drops just seem too small compare to tested data.

Addition simulation with a turning vane in the elbow has same problem, pressure drop looks too small in Fluent.

I am wondersing if the air flows is too bad to simulate in this elbow?

Thank you for your comments.

Tony