In pipe flow, when the fluid moves in orderly parallel layers with faster inner layers, this describes what type of flow?

• A. Cavitation
• B. Laminar flow
• C. Flow separation
• D. Turbulent flow

Answer: (B)

Laminar flow. When fluid moves in smooth, orderly layers with each layer sliding past the one next to it, and with the fastest motion at the center of the pipe, the flow is laminar. Viscous forces dominate and momentum is transferred between layers without the chaotic mixing seen in turbulence, producing a smooth, parabolic velocity profile for a Newtonian fluid under fully developed conditions. The no-slip condition at the wall forces the velocity to go to zero at the boundary, while the center runs fastest because it experiences the least shear. The other phenomena don’t describe this orderly, layered motion: cavitation involves vapor bubble formation from low pressure, flow separation refers to boundary-layer detachment and eddies, and turbulent flow shows chaotic fluctuations and significant mixing. In practice, laminar flow occurs at low enough Reynolds numbers (typically below about 2000 for pipe flow).