Quasi-steady Stokes approximation ($\mathrm{Re}\ll 1,L^2/(\nu T)\ll 1$)

Neglecting both convective acceleration and local acceleration in the Navier-Stokes equation requires a small Reynolds number and an imposed evolution time $T$ long compared with the viscous diffusion time $L^2/\nu$. A slowly changing configuration may then be modeled by a sequence of steady Stokes flows. Rapid small-amplitude forcing can have small Reynolds number while requiring the Unsteady Stokes equation instead.