Knudsen number

The Knudsen number is a dimensionless quantity in fluid dynamics that compares the molecular mean free path to a characteristic physical length scale.

When the Knudsen number is very small, fluids behave as continuous media; when large, individual molecular behavior dominates and conventional fluid equations break down.

Named for Danish physicist Martin Knudsen, it determines whether gas flow requires kinetic theory or can use standard continuum mechanics.