Experimental investigation on turbulent rotating thermal convection at large Rayleigh numbers

Thermal convection is of major importance in various astro- and geophysical systems, exemplary are buoyancy driven flows in the atmosphere or in the stellar interior. It has been studied for decades in an idealized model system - the Rayleigh-Bénard convection (RBC) - which consists of a horizontal fluid layer heated at the bottom and cooled at the top. Within the Oberbeck-Boussinesq approximation this system is controlled by two parameters only. These are the Rayleigh number (Ra), which represents the thermal driving and the Prandtl number (Pr) that relates the momentum and thermal diffusivities of the fluid. Convection flows in geo- and astrophysics are often influenced by Coriolis forces due to the rotation of the planet or the star. In RBC-system, Coriolis forces are introduced by rotating the convection cell around its vertical axis. The rotation is expressed by an additional dimensionless control parameter, i.e., the inverse Rossby number 1/Ro. We study experimentally the influence of rotation on the heat transport and the temperature field at very large Ra in the High Pressure Convection Facility (HPCF) in Göttingen. The facility consists of a cylindrical cell of 1.10m diameter and 2.20m height that is filled with pressurized sulfur hexafluoride (SF₆) at up to 19bar. The height of the cell and the large density of SF₆ enable us to reach very large Ra (up to 8×10¹⁴) at 0.74<Pr<0.96. The cell is mounted on a rotating table and connected to the non-rotating world via water feed-throughs and slip rings. With these, the signals of more than 100 thermistors close to the sidewalls are collected.
We find a monotonic decrease of the heat transport with increasing rotation rate. Furthermore, we measure quantities of the flow close to the lateral side walls of the convection cylinder. For large rotation rates we analyze this as part of the recently proposed “Boundary Zonal Flow” (BZF), where the vertical heat transport is enhanced and warm (cold) up (down) flow self-organizes in a periodic manner. In the experiment we observe the BZF most notably in the probability density function of the temperature, which develops a bimodal Gaussian distribution. We also find that the periodic warm-cold structure drifts in anti-cyclonic direction and thus form traveling waves of the temperature field.