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Application
The Knudsen number can be used to determine the rarefaction of a flow:


 * $$\mathrm{Kn} < 0.01 $$: Continuum flow
 * Navier-Stokes no-slip boundary condition; molecules interact unaffected by relative velocities from other particles due to relative size.


 * $$0.01 < \mathrm{Kn} < 0.1 $$: Slip flow
 * Navier-Stokes slip boundary condition; layers of molecules interact depending on relative velocity between neighboring molecules. Considers sheer force, allowed by the increase in space from larger length scale.
 * $$ 0.1 < \mathrm{Kn} < 10 $$: Transitional flow
 * $$\mathrm{Kn} > 10 $$: Free-molecular flow, Appendix N, page 434

This regime classification is empirical and problem dependent but has proven useful to adequately model flows.