Sorry for bothering you, and I sincerely hope for your response and need your help.

Currently I am running a numerical simulation with ANSYS FLUENT on a air-cooled LOUVERED FIN heat exchanger, and I encountered a serious problem that, when I conduct the data reduction on my simulation results, I found the Fanning friction factor f does not change reglualr with the increase of velcoity,  this abnormal trend is different from other researchers's results. The results of mine and other researchers' are presented in the following pics. It's clear that my simulation is wrong, but I do not know what causes this.

The configuration of my simulation is: single precision, Y+=1, RNG ke & Ehanced wall treatment, velocity inlet, pressure outlet, top & down periodic, left & right symmetry, for velocity and pressure coupling is SIMPLEC, Gradient is Least Squares Based, other scalars are all discretized with the second-order unpwind scheme.

Meanwhile, to solve my problem, I tried a lot to modify the simulation configurations, but sadly it does not work.

So, can you please give me some advice?

BEST REGARDS TO YOU!