Definition
The Knudsen number is a dimensionless number defined as:
where
- = mean free path
- = representative physical length scale .
For an ideal gas, the mean free path may be readily calculated so that:
where
- is the Boltzmann constant (1.3806504(24) × 10−23 J/K in SI units),
- is the thermodynamic temperature,
- is the particle hard shell diameter,
- is the total pressure, .
For particle dynamics in the atmosphere, and assuming standard temperature and pressure, i.e. 25 °C and 1 atm, we have ≈ 8 × 10−8 m.
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