What is the Reynolds number range that marks the transition from laminar to turbulent flow in smooth pipes?

• A. Approximately Re ≈ 2000–2300
• B. Approximately Re ≈ 100–200
• C. Approximately Re ≈ 5000–7000
• D. Approximately Re ≈ 10000

Answer: (A)

The question tests how Reynolds number indicates the switch from smooth, orderly pipe flow to chaotic, turbulent flow in a smooth pipe. Reynolds number compares inertial effects (which tend to destabilize the flow and promote mixing) to viscous effects (which damp disturbances). In a smooth pipe, viscous forces can keep the flow laminar up to a fairly high but finite level of inertia. Empirically, the onset of instability and transition to turbulence occurs in the range around 2000 to 2300. So the transition appears in that band, rather than at very low values or well into turbulence. That makes the approximate range of 2000–2300 the best answer.

Why the other ranges don’t fit: 100–200 is far below the threshold, so flow remains laminar. 5000–7000 and 10000 are well into turbulent flow, not the point where turbulence just begins.