Self-Sustained Large-Scale Flow in Turbulent Cryogenic Convection

Turbulent convection is studied in a cell filled with cryogenic helium gas. At high Rayleigh numbers a large-scale circulation persists and has a size comparable to the container. Over a wide range of time scales greater than its characteristic turnover time, this mean flow exhibits occasional and irregular reversals of direction without a change in magnitude. We study this feature in an apparatus of aspect ratio unity, in which the highest attainable Rayleigh number is about 10 16.