epl draft Breakdown of large - scale circulation in turbulent rotating convection

Turbulent rotating convection in a cylinder is investigated both numerically and experimentally at Rayleigh number Ra = 10 and Prandtl number σ = 6.4. In this Letter we discuss two topics: the breakdown under rotation of the domain-filling large-scale circulation (LSC) typical for confined convection, and the convective heat transfer through the fluid layer, expressed by the Nusselt number. The presence of the LSC is addressed for several rotation rates. For Rossby numbers Ro . 1.2 no LSC is found (the Rossby number indicates relative importance of buoyancy over rotation, hence small Ro indicates strong rotation). For larger Rossby numbers a precession of the LSC in anticyclonic direction (counter to the background rotation) is observed. It is shown that the heat transfer has a maximal value close to Ro = 0.18 being about 15% larger than in the non-rotating case Ro = ∞. Since the LSC is no longer present at this Rossby value we conclude that the peak heat transfer is independent of the LSC. Introduction. – The flow in a fluid layer confined between two horizontal plates and driven by a destabilising temperature gradient is commonly known as Rayleigh– Bénard convection [1]. The dimensionless parameters that govern this flow are the Rayleigh and Prandtl numbers, defined respectively as Ra ≡ gα∆TH 3 νκ , σ ≡ ν κ . (1) Here g is the gravitational acceleration, ∆T the temperature difference between the plates, H their vertical separation, and ν, κ and α the kinematic viscosity, thermal diffusivity and thermal expansion coefficient of the fluid, respectively. In most experimental investigations of Rayleigh–Bénard convection a cylindrical geometry is used. The cylinder axis is aligned vertically with gravity. This introduces an additional parameter, the diameterto-height aspect ratio Γ ≡ D/H . In this work we will use Γ = 1 geometries. An interesting addition to this flow problem is the presence of a background rotation. In this case the rotation vector points antiparallel to gravity. The rotation rate Ω is incorporated in the dimensionless Rossby number Ro, which states the relative importance of buoyancy and rotation: