Numerical experiments on strongly turbulent thermal convection in a slender cylindrical cell

Numerical experiments are conducted to study high-Rayleigh-number convective turbulence (Ra ranging from 2 x 10(6) up to 2 x 10(11)) in a Gamma = 1/2 aspect-ratio cylindrical cell heated from below and cooled from above and filled with gaseous helium (Pr = 0.7). The numerical approach allows three-dimensional velocity, vorticity and temperature fields to be analysed. Furthermore, several numerical probes are placed within the fluid volume, permitting point-wise velocity and temperature time series to be extracted. Taking advantage of the data accessibility provided by the direct numerical simulation the flow dynamics has been explored and separated into its mean large-scale and fluctuating components, both in the bulk and in the boundary layer regions. The existence of large-scale structures creating a mean flow sweeping the horizontal walls has been confirmed. However, the presence of a single recirculation cell filling the whole volume was observed only for Ra < 10(9)-10(10) and with reduced intensity compared to axisymmetric toroidal vortices attached to the horizontal plates. At larger Ra the single cell is no longer observed, and the bulk recirculation breaks up into two counter-rotating asymmetric unity-aspect-ratio rolls. This transition has an appreciable impact on the boundary layer structure and on the global heat transfer properties. The large-scale structure signature is reflected in the statistics of the bulk turbulence as well, which, taking advantage of the large number of numerical probes available, is examined both in terms of frequency spectra and of temperature structure functions. The present results are also discussed within the framework of recent theoretical developments showing that the effect of the aspect ratio on the global heat transfer properties at large Ra still remains an open question.