Reynolds Number
Aerodynamics / Flow Properties
Table of Contents
About this calculator
Reynolds Number estimates the flow regime by combining density, velocity, characteristic length, and dynamic viscosity.
Choose characteristic length to match the problem geometry, such as airfoil chord, pipe diameter, or missile/UAV body length.
Advanced mode can estimate density from ISA altitude and dynamic viscosity from Sutherland's law for quick atmospheric checks.
Inputs
Enter the required values used in the formula for Reynolds Number.
Calculation mode
Advanced mode computes density from ISA and dynamic viscosity from Sutherland's law using altitude.
Required inputs for calculation
- Density (rho) (kg/m^3) - Fluid density
- Velocity (V) (m/s) - Flow speed
- Characteristic Length (L) (m) - Relevant physical length, such as chord, diameter, or body length
- Dynamic Viscosity (mu) (Pa.s) - Fluid dynamic viscosity
Results
Results are hidden until you click Calculate.
Formula used for calculations
Re = (rho * V * L) / mu
Choose characteristic length L for the actual geometry
Airfoil -> chord length
Pipe -> diameter
Missile/UAV -> body length
Optional advanced altitude mode
Input altitude h instead of density and viscosity.
Density rho is computed from ISA.
Dynamic viscosity mu is computed from Sutherland's law using ISA temperature.
Reynolds number compares inertial and viscous effects using the standard aerodynamics relation. Keep SI units consistent: rho in kg/m^3, V in m/s, L in m, and mu in Pa.s.
Assumptions
- Use consistent SI units throughout the calculation.
- Advanced mode assumes standard ISA conditions up to 86 km and air viscosity from Sutherland's law.
- Characteristic length should represent the dominant dimension of the flow problem.
Recommendations
- For airfoils, use chord length; for pipes, use diameter; for missiles or UAVs, use body length.
- If measured density or viscosity is available, prefer manual mode over atmospheric estimates.
- Interpret the result alongside the geometry-specific transition criteria for your application.
FAQs
What characteristic length should I use?
Use the dimension that governs the flow problem: chord for an airfoil, diameter for a pipe, and body length for a missile or UAV.
What does advanced altitude mode do?
It replaces manual density and viscosity inputs by computing density from ISA altitude and dynamic viscosity from Sutherland's law.
Is this suitable for engineering work?
Yes for preliminary engineering analysis, provided you use the correct characteristic length and validate against measured properties when available.