Reynolds Number
A dimensionless quantity in fluid mechanics predicting flow patterns, equal to ratio of inertial forces to viscous forces.
Governing formula
Re = (ρ v L) / η
SI unit
Dimensionless
In depth
Low Re < 2000 indicates smooth laminar flow; high Re > 4000 indicates turbulent flow dominated by chaotic eddies.
Examples in the real world
Classifying smooth water flow in a pipe (Re < 2000) vs chaotic white-water river turbulence (Re >> 4000).