What Reynolds number tells you
Reynolds number is a dimensionless quantity that compares inertial effects with viscous effects in fluid flow. It helps engineers identify the flow regime and compare flows under different conditions.
Re = ρ V D / μ = V D / ν
Variables
- ρ = fluid density.
- V = characteristic flow velocity.
- D = characteristic length; for a full circular pipe this is normally the pipe diameter.
- μ = dynamic viscosity.
- ν = kinematic viscosity, μ/ρ.
Pipe-flow interpretation
For internal flow in a circular pipe, common textbook classifications use approximate boundaries around Re = 2300 and Re = 4000. Below the lower boundary flow is generally treated as laminar, above the upper boundary as turbulent, and the interval between them is commonly described as transitional. These boundaries are useful for coursework, but the exact behavior depends on the flow and experimental conditions.
Calculation checklist
- Use consistent SI units.
- Identify the correct characteristic length.
- Use dynamic or kinematic viscosity consistently with the chosen form of the equation.
- Check the order of magnitude of the result.
Do not classify flow from Reynolds number alone when the problem has special geometry or a different characteristic length. First identify the physical flow configuration.