Surface coarseness of the workpiece surface is determined on the basis of the relative roughness relationship described above.

To find λ, it’s necessary to preliminary determine the relative roughness (e1) of the pipe and the Reynolds criterion (Re1):

Now, let’s find the relative roughness for a 450mm-diameter pipe section and choose the formula for calculating the coefficient of friction:

The resistance coefficient depends upon the Reynolds number of flow and the relative roughness e/D, where e is the average size of the roughness element.

Solution: Since the flow velocity v and the pipe diameter d are unknown, it’s impossible to calculate either the Reynolds number Re, or the relative roughness ɛ/d.