Reynolds number
Compare inertial and viscous scales of a flow.
Key knowledge
The Reynolds number Re = ρUL/μ is dimensionless. Its chosen length and speed specify the flow scale; it informs similarity and the relative importance of inertia and viscosity.
Values and equations
Re = ρUL/μ
- Re
- Reynolds number
- ρ
- Mass density kg m⁻³
- U
- Characteristic speed m s⁻¹
- L
- Characteristic length m
- μ
- Dynamic viscosity Pa s
Conditions and scope
- For tube flow use diameter as L and the stated mean speed.
- Transition thresholds depend on geometry, disturbances and flow conditions; no universal turbulence cutoff is supplied.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Section 14.7, equation 14.20 and the tube-flow convention.
2026-10-10 · Dimensionless scale ratio only; tabulated transition numbers are not treated as universal.
Entry details
Permanent link to this entry ↗- Record identity
- physics-reynolds-number
- Revision
- university-reference.1
- Structured reference record
- University reference · University core expansion