The large-scale flow structure in turbulent rotating Rayleigh–Bénard convection

We report on the influence of rotation about a vertical axis on the large-scale circulation (LSC) of turbulent Rayleigh–Bénard convection in a cylindrical vessel with aspect ratio Γ ≡ D/L = 0.50 (where D is the diameter and L the height of the sample). The working fluid is water at an average temperature Tav = 40 ◦C with a Prandtl number Pr = 4.38. For rotation rates Ω . 1 rad s−1, corresponding to inverse Rossby numbers 1/Ro between 0 and 20, we investigated the temperature distribution at the sidewall and from it deduced properties of the LSC. The work covered the Rayleigh-number range 2.3 × 109 . Ra . 7.2 × 1010. We measured the vertical sidewall temperature gradient, the dynamics of the LSC and flow-mode transitions from single-roll states (SRSs) to double-roll states (DRSs). We found that modest rotation stabilizes the SRSs. For modest 1/Ro . 1 we found the unexpected result that the vertical LSC plane rotated in the prograde direction (i.e. faster than the sample chamber), with the rotation at the horizontal midplane faster than near the top and bottom. This differential rotation led to disruptive events called half-turns, where the plane of the top or bottom section of the LSC underwent a rotation through an angle of 2π relative to the main portion of the LSC. The signature of the LSC persisted even for large 1/Ro where Ekman vortices are expected. We consider the possibility that this signature actually is generated by a two-vortex state rather than by a LSC. Whenever possible, we compare our results with those for a Γ = 1 sample by Zhong & Ahlers (J. Fluid Mech., vol. 665, 2010, pp. 300–333).