[ChemEng_1992]

Hello, dear engineers

In the Reynolds number formula, which in the general state of U represents the velocity characteristic, under what conditions can the velocity inlet, mean velocity, characteristic velocity, and free stream velocity in a pipe in turbulent flow for incompressible fluid be assumed to be the same?
In spiral, helical, and serpentine channels (without or with tube insert), in turbulent flow and velocity inlet equal to 2 m/s, is the hypothesis of equality of all types of velocities mentioned earlier acceptable?
Some Y+ online calculator sites have stated mean velocity or free stream velocity for the velocity parameter.
I don't know for sure which velocity is actually considered in the Reynolds number formula and Y+ calculation.

Thank you in advance for your guidance.