The friction factor (f) for laminar flow in a pipe is given by

Concept:

For laminar flow:

Friction factor (f) \(= \frac{{64}}{{{{\rm{R}}_{\rm{e}}}}}\)

For turbulent flow:

Case I: Smooth Pipe

Friction factor (f) \( = \frac{{0.316}}{{{{\left( {{{\rm{R}}_{\rm{e}}}} \right)}^{1/4}}}}\)

Case II: Rough Pipe

\(\frac{1}{{\sqrt {\rm{f}} }} = 2{\log _{10}}\left( {\frac{{\rm{r}}}{{\rm{k}}}} \right) + 1.74\)

Case III: In between smooth & rough (Transition)

Friction (f) = \(\rm{\phi (R_e, \frac{k}{D})}\)

∴ For laminar flow friction factor depends only upon the Reynolds number of the flow and is independent of the contact surface.

As can observe, the friction factor is inversely proportional to Reynold’s number. Also, the velocity distribution of laminar flow through the pipe is parabolic.

∴ Statement 3 and 4 are correct.

Mistake Points

• For the Laminar flow, the friction factor depends only on the Reynolds number and is independent of the internal condition of the pipe. 
• For turbulent flow, when the Reynolds number Re > 4000, the friction factor f depends not only on Re but also on the internal roughness of the pipe.