Head loss due to friction: which mechanisms contribute to the energy loss? In internal and open-channel flows, the frictional loss of head includes the energy dissipation arising from which physical effects?

Introduction / Context:
Head loss quantifies mechanical energy degraded to heat through internal friction and chaotic motion. Correctly identifying the sources of frictional loss is essential for sizing pipes, pumps, and channels.

Given Data / Assumptions:

• Single-phase Newtonian fluids.
• Fully developed sections for representative behavior.

Concept / Approach:
In laminar flow, energy is dissipated by viscous shear alone. In turbulent flow, additional dissipation arises from eddying motion—turbulent shear—which ultimately converts kinetic energy into heat at the smallest scales through viscosity. Thus, both viscosity and turbulence are responsible for frictional head losses across practical flow regimes.

Step-by-Step Solution:

Verification / Alternative check:
Compare Darcy–Weisbach friction factor: f_laminar = 64/Re (purely viscous), while turbulent correlations (e.g., Colebrook–White) capture roughness and turbulence effects.

Why Other Options Are Wrong:
Neither viscosity alone nor turbulence alone explains losses across all regimes; “none” is clearly incorrect.

Common Pitfalls:
Assuming turbulence “replaces” viscosity; actually viscous dissipation remains the final sink for kinetic energy.

Final Answer:
both (a) and (b)