Concept reference

Reynolds Number and Flow Regime

C.1Plain-language definition

Reynolds number compares the tendency of moving fluid to keep moving with viscosity’s tendency to smooth disturbances. Low values support orderly laminar layers; high values support turbulent mixing. Transition is a band affected by disturbances and geometry, not a switch guaranteed at one exact number.

C.2Governing relation or decision logic

Re = ρVD/μ = VD/ν, dimensionless, where V is m/s, hydraulic diameter D is m, dynamic viscosity μ is Pa·s and kinematic viscosity ν is m²/s. For circular pipes, laminar Darcy friction is f = 64/Re. Turbulent friction also depends on relative roughness.

C.3The failure to name

Using water viscosity at the wrong temperature can shift Reynolds number, though ordinary municipal force mains usually remain turbulent. More damaging is feeding a laminar or transitional case into a turbulent-only friction correlation without flagging that its physics no longer applies.

C.4Field review checklist

• Use mean velocity and the correct hydraulic diameter.
• Record fluid temperature and the viscosity property used.
• Classify laminar, transitional or turbulent without claiming a universal sharp boundary.
• Choose a friction relation valid for that regime and geometry.

C.5Where it fits in the course

§3.1 develops regime intuition; §3.2 connects it to the Moody chart.