Editing Knudsen number (section)

{{Short description|Dimensionless number relating to mean free path of a particle}}
{{more citations needed|date=March 2011}}
{{distinguish|text=[[KN number]], used by [[United States]] for describing [[North Korea]]n missiles}}

The '''Knudsen number''' ('''Kn''') is a [[dimensionless number]] defined as the [[ratio]] of the molecular [[mean free path]] length to a [[Characteristic dimension|representative physical length scale]]. This length scale could be, for example, the [[radius]] of a body in a fluid. The number is named after [[Denmark|Danish]] physicist [[Martin Knudsen]] (1871–1949).

The Knudsen number helps determine whether [[statistical mechanics]] or the [[continuum mechanics]] formulation of [[fluid dynamics]] should be used to model a situation. If the Knudsen number is near or greater than one, the mean free path of a molecule is comparable to a length scale of the problem, and the continuum assumption of [[fluid mechanics]] is no longer a good approximation.  In such cases, statistical methods should be used.