# Knudsen Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Knudsen number, abbreviated as Kn, a dimensionless number, is used in fluid dynamics to determine what type of analysis should be used to model an object moving through a fluid.  It is defined as the ratio of the molecular mean free path length to a representative physical length scale.  This calculation determines the Knudsen number, the length or the lambda value based on your inputs.

## Knudsen Number formula

 $$\large{ Kn = \frac{ \lambda }{ l } }$$

### Where:

 Units English Metric $$\large{ Kn }$$ = Knudsen number $$\large{dimensionless}$$ $$\large{ l }$$ = length $$\large{in}$$ $$\large{mm}$$ $$\large{ \lambda }$$ (Greek symbol lambda) = mean free path $$\large{in}$$ $$\large{mm}$$

### Solve For:

 $$\large{ l = \frac{ \lambda }{ Kn } }$$ $$\large{ \lambda = Kn \; l }$$