[AMu9]

I am trying to calculate average friction factor (f = ΔP Dh / 2 ρ L V^2) for 2D rectangular duct that has entry, test and exit sections with 225, 125 and 215 mm length, respectively, and 20mm height. It has roughness ribs.
I used RNG k-ε with enhanced wall treatment. I take the difference between total pressures at inlet and outlet of test section for calculating f.
for Re ≥ 10000 the pattern of f seems ok (see image bellow). However, I could not get the same pattern at Re < 10000 even with refining the mesh to 40 μm near the ribs and 100 μm for the test section with inflation starting from 10 μm.

Nusselt number values are acceptable.

How can I find the correct values for f at Re < 10000 ?

https://ibb.co/7XtGKRN