The Reynolds number (Re) is used to predict similar flow patterns in different fluid flow situations.
The Reynolds Number formula
Re = (ρ u2) / (μ u / L) = ρ u L / μ = u L / ν
where
Re = Reynolds Number (non-dimensional)
ρ = density (kg/m3, lbm/ft3)
u = velocity based on the actual cross section area of the duct or pipe (m/s, ft/s)
μ = dynamic viscosity (Ns/m2, lbm/s ft)
L = characteristic length (m, ft)
ν = kinematic viscosity (m2/s, ft2/s)