(measure of a particle's tendency to follow the flow of a fluid)
A Stokes number (Stk or St) of a particle sitting in a fluid
is a characterization of its tendency to flow with the fluid.
It is the ratio of two characteristic times:
Stk = t0 u0 / l0
Stk - Stokes number.
t0 - relaxation time of the particle (characterizing the rate at which the particle catches up to the flow).
u0 - fluid velocity.
l0 - diameter of the particle
(The latter two divide out to be the reciprocal of a time.)
Particles with a low Stokes number tend to "stay with" the fluid flow,
whereas with a higher stokes number take time to respond to the
fluid motion, presumably more massive particles with more inertia.