Thank you for reply.
My Reynolds number is 20000. The theoretical formula I used, f=0.079 * Re ^ (-0.25), seems to be applicable only in fully developed turbulence, which is why I only use data from the area near the outlet for calculation. The pressure drop inferred from this formula should be approximately 0.23Pa/m, and the shear stress should be approximately 0.0115pa. The shear stress basically conforms to the theory. The figure shows the static pressure curve of the channel centerline calculated using a longer channel. Even for the entire flow area, the pressure drop still does not meet the requirements.
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