The influence of the large scale circulation on the transition to the "Ultimate state of thermal convection"

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Thermal convection is one of the most important heat transport mechanism and plays a major role in many geo- and astrophysical systems, as well as for industrial applications. Such natural convection systems are highly turbulent due to their strong thermal driving. In laboratory experiments, a fluid is usually confined by rigid plates from the top and the bottom, at which thermal and viscous boundary layers (BL) develop. Since the vertical heat flux through the cell is usually limited by the heat flux through these BL, scaling laws have been derived by considering thermal and kinetic energy dissipation inside the BL. The flow in the bulk, although turbulent, develops large scale circulations structures (LSC), that extent from the bottom to the top. It is predicted, that at sufficiently large thermal driving (expressed by the dimensionless Rayleigh number Ra), the shear caused by the LSC, renders the BL turbulent. This in fact would cause a significant increase in the heat transport, and the system would enter a regime, where scaling laws are expected to hold for arbitrary large Ra, aka the "ultimate state of convection". The objective of this work is to investigate the influence of the LSC on the stability of the viscous BL and thus on the heat transport in a Rayleigh-Benard setup. We are especially interested in the Ra-range close to the transition to the ultimate regime. We want to understand how the large scale circulation (the turbulent superstructure of thermal convection) affects the stability of the BL. In particular, we plan in a first experiment to increase the strength of the LSC with a fan and by this try to reach the transition to the ultimate regime already at lower Ra. In a second experiment, we want to directly investigate the influence of a shear on the boundary layer above a heated plate, with optical vlocimetry methods such as particle image velocimetry or laser Doppler velocimetry.