Knudsen Number - Definition

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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