How does roughness affect the friction factor in turbulent pipe flow, and how is it reflected on the Moody chart?

• A. Increased roughness raises friction factor in the fully rough region; the Moody chart shows f vs Re for several ε/D values.
• B. Increased roughness lowers friction factor in the fully rough region.
• C. Roughness has no effect on the friction factor in turbulent flow.
• D. The Moody chart plots friction factor against velocity rather than Reynolds number.

Answer: (A)

In turbulent pipe flow, wall roughness directly increases the shear stress at the wall. When the flow is in the fully rough regime, the roughness elements dominate the near-wall turbulence, so the friction factor becomes controlled mainly by the roughness size relative to the pipe diameter (ε/D) and is largely independent of Reynolds number.

That’s why increasing roughness raises the friction factor in this regime. On the Moody chart, you see friction factor plotted against Reynolds number for several fixed ε/D values. As ε/D increases, the corresponding curves sit higher, showing a larger f. In the high-Reynolds-number (fully rough) portion, the curves become nearly horizontal, indicating that f no longer depends on Re and is determined by roughness. This visualization matches the idea that rougher pipes have higher friction factors in turbulent flow, especially when Re is large.