### Rayleigh number

**(Ra)**
(measure reflecting a fluid system's propensity to turbulence)

A **Rayleigh number** (**Ra**) is a measure of a condition within a fluid
that might cause convection, a ratio between a tendency toward
conduction and a tendency toward convection, each tendency
represented by a timescale.
Sufficiently low, there is no convection, sufficiently high, there
is turbulent convection, and between, there is smoother
convection (**laminar flow**).

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

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

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

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