Towards the ultimate regime in Rayleigh-Bénard turbulence

Rayleigh-Bénard convection – a fluid flow in a container heated from below and cooled from above – is one of the paradigmatic systems in fluid dynamics. This also holds for the turbulent case. Here the key response of the system is the heat transport (Nusselt number Nu) and the key question is: how does Nu depend on the thermal driving strength (Rayleigh number Ra)? We start this talk with a brief digression into the history of the theory of heat transport scaling relations for large Ra, and in particular for the ultimate regime, where the scaling laws do not change anymore with the further growing Ra. We discuss not only the outcome but also the difference in the assumptions of the various scaling models, which helps to understand the applicability limits of the various models. We then focus on the factors that influence the transition to the ultimate regime, including the container shape, wall roughness, specific thermal boundary conditions, and possible non-Oberbeck-Boussinesq effects, as well as the multiple-state nature of turbulent thermal convection.<br type="_moz" />