What is the Moody diagram used for in pipe hydraulics?

• A. Relates the friction factor f to Reynolds number and relative roughness to estimate head losses.
• B. Provides a direct method to compute pipe diameter from flow rate.
• C. Predicts the onset of turbulent flow based on Reynolds number only.
• D. Gives the relationship between flow velocity and pressure drop in laminar flow only.

Answer: (A)

The Moody diagram is a graphical tool that relates the friction factor to Reynolds number and relative roughness to estimate head losses. The friction factor f is the dimensionless coefficient that appears in the Darcy–Weisbach equation for head loss: h_f = f (L/D) (v^2 / 2g). By knowing the flow velocity (to get Re = ρvD/μ) and the pipe’s relative roughness ε/D, you locate the corresponding curve on the diagram and read off f. In the laminar region, f = 64/Re, so f depends only on Re. In smooth turbulent flow, f decreases with Re, while in rough turbulent flow, f becomes largely set by ε/D and changes little with Re at high Re. This makes it a convenient way to estimate head losses without solving the implicit Colebrook–White equation.