The Reynolds number is a dimensionless number linking viscosity The density , and reference length .

The reference length can be :

The pipe diameter (ducts)

To study the drag of non-geometric body shapes, this reference surface is the width of the front surface (perpendicular to flow)

To study the lift and drag of body sections, this reference surface is the maximum projected area, this length is taken along a flow .

To study the frictional drag of flat plates, this reference surface is the length of the wetted surface, this length is taken along a flow .

Re = V L / u

Re = (( average speed ) x ( reference length )) / ( kinematic viscosity of the fluid )

or

Re = (( density ) x ( average speed ) x ( reference length )) / ( dynamic viscosity of the fluid )

The Reynolds number determines the flow regime, Laminar , Transitional or Turbulent and equations to use.