### Stokes number

**(Stk, St)**
(measure of a particle's tendency to flow the flow of a fluid)

A **Stokes number** (**Stk** or **St**) of a particle in a fluid is the
ratio of two characteristic times:

Stk = t_{0} u_{0} / l_{0}

- t
_{0} - **relaxation time** of the particle.
- u
_{0} - fluid velocity.
- l
_{0} - 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 due to inertia, i.e., a more massive particle.

(*measure,fluid dynamics,ratio*)
**Further reading:**

http://en.wikipedia.org/wiki/Stokes_number

Index