In the Moody diagram, which two dimensionless parameters are plotted to determine friction factor?

• A. Friction factor and Reynolds number
• B. Pipe diameter and roughness
• C. Reynolds number and relative roughness
• D. Flow rate and head loss

Answer: (C)

The Moody diagram uses two dimensionless inputs to determine friction factor: Reynolds number and relative roughness. On the chart, curves correspond to fixed relative roughness ε/D, while the horizontal axis is the Reynolds number Re. To find the friction factor for a pipe, you pick the roughness of the pipe, look at the Re for your flow, and read off the friction factor from the appropriate curve. This shows how flow regime (via Re) and surface roughness (via ε/D) together govern the friction factor.