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Rayleigh number

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The Rayleigh number is the ratio of the buoyancy force to the viscous force in a medium.

Ra = \frac{g \alpha \Gamma d^4}{\nu \kappa}

where g = acceleration due to gravity, \alpha = coefficient of thermal expansion, \Gamma = background temperature gradient, d = length scale of flow, \nu = kinematic viscosity and \kappa = thermal diffusivity

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