  We explore the scaling behavior of an unsteady flow that is generated by an
oscillating body of finite size in a gas. If the gas is gradually rarefied, the
Navier-Stokes equations begin to fail and a kinetic description of the flow
becomes more appropriate. The failure of the Navier-Stokes equations can be
thought to take place via two different physical mechanisms: either the
continuum hypothesis breaks down as a result of a finite size effect; or local
equilibrium is violated due to the high rate of strain. By independently tuning
the relevant linear dimension and the frequency of the oscillating body, we can
experimentally observe these two different physical mechanisms. All the
experimental data, however, can be collapsed using a single dimensionless
scaling parameter that combines the relevant linear dimension and the frequency
of the body. This proposed Knudsen number for an unsteady flow is rooted in a
fundamental symmetry principle, namely Galilean invariance.
