### 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 sitting in a fluid
is a characterization of its tendency to flow with the fluid.
It is the ratio of two characteristic times:

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

- t
_{0} - **relaxation time** of the particle (characterizing the rate at which the particle catches up to the flow).
- 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

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