Which statement describes the typical laminar-turbulent criterion for pipe flow based on Reynolds number?

• A. Re < ~2000 is typically laminar
• B. Re > ~4000 is laminar
• C. 2000–4000 is always laminar
• D. Reynolds number does not indicate flow regime

Answer: (C)

Reynolds number compares inertial forces to viscous damping in pipe flow, so it tells you whether the flow stays smooth or becomes chaotic. In typical pipe conditions, low Re means viscosity wins and the flow remains laminar, while high Re means inertia wins and the flow turns turbulent. Practically, laminar flow is expected when Re is below about 2000–2100, and fully turbulent flow when Re is above about 4000. The range in between is a transition region where the actual state depends on disturbances, pipe roughness, and entry effects. Therefore, describing 2000–4000 as always laminar isn’t correct—this band is transitional and can be laminar or turbulent depending on conditions.