Interferometric measurement of temperature gradient reversal in a layer of convecting air

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Abstract
Experiments in which a horizontal layer of convecting air is probed by one beam of a Michelson interferometer are described. When the localized interference fringes are horizontal, they indicate that the beam is wide enough to provide a suitable horizontal average. When the fringes are oriented nearly vertically, quantitative temperature measurements may be made. Results are presented for ratios (λ) of Rayleigh number to critical Rayleigh number of 1·48, 3·81 and 16·0. The temperature profile becomes more distorted from linear as the Rayleigh number is increased. An isothermal central region and thermal boundary regions each occupy one-third of the layer at λ = 3·81. By λ = 16·0 each boundary region occupies only one-quarter of the layer thickness, and the central region shows a reversed gradient. No full calculation is presently available to compare to the measurements. However, the shape assumption, using the first eigenfunctions of the linear stability problem, predicts the profile rather well for λ = 1·48 and 3·81 if Nusselt number agreement is imposed.

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