Stretching the boundaries in turbulent convection

Laboratory experiments investigating turbulent Rayleigh-Benard convection (RBC) have nearly always been confined to cylindrical cells of small diameter-to-height aspect ratio Γ (typically 1/2). The motivation for this has been that the governing control parameter, the Rayleigh Number Ra, depends on the cube of the height, and so laboratory experiments have been made as tall as possible, which for technical reasons has precluded them from having lateral dimensions substantially larger than this. Obtaining large Γ simply through a reduction in cell height, at least for for ordinary fluids and/or cryogenic apparatus which are small, has the unfortunate consequence of also moving the experiment substantially out of the turbulent regime. The work reviewed here achieves concurrently large aspect ratio and high Ra, taking advantage of an apparatus of relatively large diameter and the use of an optimum test fluid, namely low temperature helium gas. This allows the experiment to approach more closely geophysically relevant conditions as well as theoretical analysis which typically assumes an infinite horizontal extent.