Rayleigh number

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A dimensionless value used to estimate when convection commences in a fluid. The Ra depends on the density and depth of the fluid, the coefficient of thermal expansion, the gravitational field, the temperature gradient (in excess of the adiabatic gradient), the thermal diffusivity, and the kinematic viscosity. Convection usually starts when Ra is 1000 or more, while heat transfer is entirely by conduction when Ra is less than 10.

Subjects: Earth Sciences and Geography — Astronomy and Astrophysics.

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