Numerical experiments are conducted to study high-Rayleigh-number convective turbulence ($Ra$ ranging from $2\times 10^6$ up to $2\times 10^{11}$) 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.

In the present study the mechanisms of evolution of propeller tip and hub\ud vortices in the transitional region and the far field are investigated experimentally.\ud The experiments involved detailed time-resolved visualizations and velocimetry\ud measurements and were aimed at examining the effect of the spiral-to-spiral distance\ud on the mechanisms of wake evolution and instability transition. In this regard, three\ud propellers having the same blade geometry but different number of blades were\ud considered. The study outlined dependence of the wake instability on the spiralto-\ud spiral distance and, in particular, a streamwise displacement of the transition\ud region at the increasing inter-spiral distance. Furthermore, a multi-step grouping\ud mechanism among tip vortices was highlighted and discussed. It is shown that such\ud a phenomenon is driven by the mutual inductance between adjacent spirals whose\ud characteristics change by changing the number of blades

It has recently been recognized that the convective velocities achieved in the current solar convection simulations might be overestimated. The newly-revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this paper, we propose and numerically confirm a possible suppression mechanism of convective velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global convective amplitude in the high-Prandtl thermal convection is thus reduced especially in the lower part of the convection zone via the change in the mean entropy profile which becomes more subadiabatic near the base and less superadiabatic in the bulk.

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Experimental data obtained in various turbulent flows are analysed by means of orthogonal wavelet transforms. Several configurations are analysed: homogeneous grid turbulence at low and very low Reλ, and fully developed jet turbulence at moderate and high Reλ. It is shown by means of the wavelet decomposition in combination with the form of scaling named extended self-similarity that some statistical properties of fully developed turbulence may be extended to low-Reλ flows. Indeed, universal properties related to intermittency are observed down to Reλ≃10. Furthermore, the use of a new conditional averaging technique of velocity signals, based on the wavelet transform, permits the identification of the time signatures of coherent structures which may or may not be responsible for intermittency depending on the scale of the structure itself. It is shown that in grid turbulence, intermittency at the smallest scales is related to structures with small characteristic size and with a shape that may be related to the passage of vortex tubes. In jet turbulence, the longitudinal velocity component reveals that intermittency may be induced by structures with a size of the order of the integral length. This effect is interpreted as the signature of the characteristic jet mixing layer structures. The structures identified on the transverse velocity component of the jet case turn out on the other hand not to be affected by the mixing layer and the corresponding shape is again correlated with the signature of vortex tubes.