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Direct numerical simulations of turbulent convection at high Rayleigh numbers in large aspect ratio cells are challenging because of the prohibitive computational resources required. One can achieve high Rayleigh numbers at affordable costs for low aspect ratios, but the effect of small aspect ratio remains to be understood fully. In this work, we explore integral quantities in convection in a cell with the small aspect ratio of 0.1 by varying both the Rayleigh and Prandtl numbers systematically. We find that the heat transport in this flow is within 10\% of that in cells with large aspect ratios for high enough Rayleigh numbers and for Prandtl numbers larger than unity. For low Prandtl numbers, the increase of the heat transport is steeper for low aspect ratios, approaching that in large aspect ratios as the Prandtl number increases. Further, the global momentum transport, quantified by the Reynolds number, is reduced for all Prandtl numbers, presumably because of the larger volume of flow affected by the friction from sidewalls, compared to that in cells of larger aspect ratio., Comment: 7 pages, 6 figures, to appear in Europhysics Letters

When the leaders of the world's main industrialized nations met at the G8 summit in Scotland in July, the issues of Africa and climate change were at the top of the agenda. Certainly it was good to see that aid to developing nations will be boosted by $50bn and that the debts of the 18 poorest nations will be cancelled. However, it was disappointing that science did not figure as prominently in the leaders' final communiquA© as one might have hoped.

Using experimental data on thermal convection, obtained at a Rayleigh number of 1.5 $\times 10^{11}$, it is shown that the temperature structure functions $$, where $\Delta T_r$ is the absolute value of the temperature increment over a distance $r$, can be well represented in an intermediate range of scales by $r^{\zeta_p} \phi (r)^{p}$, where the $\zeta_p$ are the scaling exponents appropriate to the passive scalar problem in hydrodynamic turbulence and the function $\phi (r) = 1-a(\ln r/r_h)^2$. Measurements are made in the midplane of the apparatus near the sidewall, but outside the boundary layer.

At least up to Rayleigh numbers of the order 1013, a feature of turbulent convection in confined containers is a self-organized and coherent large-scale motion ("mean wind"). For aspect ratio unity, the mean wind is comparable in scale to the container size. Its magnitude is measured here using short-time temperature correlations in a cylindrical container of aspect ratio unity; the working fluid is cryogenic helium and the Rayleigh numbers span from 106 to 1015. The self-organizing advection of "plumes" by the mean wind leads to periodic temperature oscillations near the sidewall. Comparisons of the observed oscillation frequency to the rotational rate of the mean wind, however, have differed by a factor of 2 in the recent literature. It is argued here that this apparent discrepancy is the result of the evolution of the shape of the mean wind, from a tilted and nearly elliptical shape at low Rayleigh numbers to a squarish shape at high Rayleigh numbers, thereby altering the effective path length from which the rotational rate of the mean wind is deduced.

We examine turbulent thermal convection at very high Rayleigh numbers using helium gas in a cylindrical container of diameter-to-height aspect ratios of 1 and 4, and confirm that the Nusselt number, Nu, follows an approximately 1/3 power of the Rayleigh number, Ra, up to some value of Ra that depends on various experimental conditions, such as the nearness to the critical point of helium as well as the aspect ratio. An enhancement of heat transport occurs for higher Ra, coinciding with a substantial increase in Prandtl number as well as in various measures of Boussinesq conditions, and marking the transition from the region in which Nu≈0.064Ra1/3 to another in which Nu≈0.078Ra1/3. By necessity, the Nusselt number in the transition region increases more steeply than 1/3. The transitional slope, which happens to be close to 1/2, does not occur at unique values of Ra for given Pr and so should not be mistaken for the 'ultimate regime' of Kraichnan. By comparing various experiments performed under different conditions (but in the apparatus with the same horizontal surfaces), we empirically find that substantially negative values of a non-dimensional parameter related to fluid conductivity and viscosity correlate well with the observed enhancement of heat transport.

Although the equations governing turbulent flow of fluids are well known, understanding the overwhelming richness of flow phenomena, especially in high Reynolds number turbulent flows, remains one of the grand challenges in physics and engineering. High Reynolds number turbulence is ubiquitous in aerospace engineering, ground transportation systems, flow machinery, energy production (from gas turbines to wind and water turbines), as well as in nature, e.g. various processes occurring in the planetary boundary layer. High Reynolds number turbulence is not easily obtained in the laboratory, since in order to have good spatial resolution for measurements, the size of the facility itself has to be large. In this paper, we discuss limitations of various existing facilities and propose a new facility that will allow good spatial resolution even at high Reynolds number. The work is carried out in the framework of the Center for International Cooperation in Long Pipe Experiments (CICLoPE), an international collaboration that many in the turbulence community have shown an interest to participate in.

Laboratory experiments investigating turbulent Rayleigh-Benard convection (RBC) have nearly always been confined to cylindrical cells of small diameter-to-height aspect ratio Γ (typically 1/2). The motivation for this has been that the governing control parameter, the Rayleigh Number Ra, depends on the cube of the height, and so laboratory experiments have been made as tall as possible, which for technical reasons has precluded them from having lateral dimensions substantially larger than this. Obtaining large Γ simply through a reduction in cell height, at least for for ordinary fluids and/or cryogenic apparatus which are small, has the unfortunate consequence of also moving the experiment substantially out of the turbulent regime. The work reviewed here achieves concurrently large aspect ratio and high Ra, taking advantage of an apparatus of relatively large diameter and the use of an optimum test fluid, namely low temperature helium gas. This allows the experiment to approach more closely geophysically relevant conditions as well as theoretical analysis which typically assumes an infinite horizontal extent.

A new technique is used to explore the structure of turbulent convecting flows. A sinusoidal temperature perturbation is superimposed on the bottom plate of a cylindrical convection cell filled with cryogenic helium gas and its decay is measured at the cell mid-height. The measurements allow the first dynamic estimate of the core part of the flow which conducts fluid effectively without any thermal resistance. The estimate is quantitatively consistent with recent numerical simulations.

We study the effects of severe non-Boussinesq conditions on thermal convection at the moderate Rayleigh numbers of Ra = 2◊10 8 and 2◊10 9 by resorting to direct numerical computations of the full governing equations. We illustrate the effects by considering low temperature gaseous helium. The properties of helium are allowed to depend on the temperature around the mean of 5.4K. The Nusselt number is shown to decrease as the system departs from the Boussinesq approximation. For the Rayleigh numbers chosen here, the role of viscosity in thermal convection is limited to smudging the plume generation at the bottom surface, whereas the thermal expansion coefficient is demonstrated to have a larger impact on heat transport.

We investigate the stochasticity in temperature fluctuations in the cosmic microwave background (CMB) radiation data from {\it Wilkinson Microwave Anisotropy Probe}. We show that the angular fluctuations of the temperature is a Markov process with a {\it Markov angular scale}, $\Theta_{\rm Markov}=1.01^{+0.09}_{-0.07}$. We characterize the complexity of the CMB fluctuations by means of a Fokker-Planck or Langevin equation and measure the associated Kramers-Moyal coefficients for the fluctuating temperature field $T(\hat n)$ and its increment, $\Delta T =T(\hat n_1) - T(\hat n_2)$. Through this method we show that temperature fluctuations in the CMB has fat tails compared to a Gaussian distribution., Comment: 10 pages, 8 figures Accepted in J. Stat. Mech

Just as their powers begin to bloom, thousands of bright young scientists from less prosperous countries emigrate every year to more affluent parts of the world. This exodus of talent is not a total write-off. Some successful expatriates do contribute from abroad to causes in their home countries, where they are often a source of pride. However, on balance, the drain of youthful talent is damaging and hurts even populous countries such as China and India, which have no dearth of raw talent. This migration, which is largely an economic issue, will not cease unless the prevailing socioeconomic imbalances correct themselves.

Pulsation models for the enigmatic F-type supergiant variable, UU Her are investigated. It is shown that irregular pulsations with small amplitude is realized, if UU her is assumed to be an intermediate mass F type supergiant star which has evolved with mass-loss and rotation mixing and thus highly luminous, compared with what is obtained by conventional evolution.

A stability analysis of electrostatic perturbations in a positive column is reported. The positive column is considered as a three-component, low-pressure fluid, in which ionization and collisions play a significant role. The application of the generalized Hurwitz criterion for stability is found to predict an instability when the electron drift velocity exceeds 5/6 of the electron thermal velocity, in excellent agreement with experiments on current limitation in positive columns.