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Knudsen number

Knudsen number

It is a dimensionless number that shows whether flow can be treated as a continuum.

It can be expressed with the following equation:

Kn : Knudsen number
λ : Mean free path [m]
: Characteristic length [m]
k: Boltzmann constant (1.3807 × 10-23 [J/K])
: Temperature [K]
σ : Diameter of molecule [m]
: Absolute pressure [Pa]

Fluid can be treated as a continuum only when Knudsen number is smaller than 1. In other cases, fluid must be treated as a dilute fluid with the consideration of motions of atoms and molecules.

Knudsen number is named after a Danish physicist Martin Knudsen.