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Transitions in heat transport by turbulent convection at Rayleigh numbers up to 10(15)
No full textWe describe a pressure vessel for conducting experiments in helium (He), air, nitrogen (N-2) or sulfur hexafluoride (SF6) under pressures of up to 19 bars, and facilities for the study of Rayleigh-Benard convection inside this pressure vessel. The convection cells, known as the high pressure convection facilities (HPCFs), can have interior heights up to L = 2.3 m and diameters up to D = 1.2 m. Measurements of the Nusselt number Nu for Rayleigh numbers Ra up to Ra = 4 x 10(13) and a Prandtl number Pr similar or equal to 0.8 gave Nu proportional to Ra-gamma eff with gamma(eff) similar or equal to 0.308. At Ra there was a sharp transition to a new regime. The Nusselt number was continuous at Ra , but the exponent characterizing its dependence on Ra changed suddenly to gamma(eff) = 0.25. Near Ra = Ra similar or equal to 3 x 10(14), there was a further change in the Ra-dependence of Nu. A new state with gamma(eff) similar or equal to 0.17 evolved and there was bistability of the gamma(eff) = 0.25 and the gamma(eff) = 0.17 branches
Heat transport by turbulent Rayleigh–Bénard convection for Pr ≃ 0.8 and 3 × 1012 ≲ Ra ≲ 1015: aspect ratio Γ = 0.50
No full textWe report on the experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh-Benard convection (RBC) in a cylindrical sample of aspect ratio Gamma equivalent to D/L = 0.50 (D = 1.12m is the diameter and L = 2.24m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bar as the fluid. They are for the Rayleigh-number range 3 x 10(12) less than or similar to Ra less than or similar to 10(15) and for Prandtl numbers Pr between 0.79 and 0.86. For Ra < Ra-1 similar or equal to 1.4 x 10(13) we find Nu = N-0 Ra-gamma eff with gamma(eff) = 0.312 +/- 0.002, which is consistent with classical turbulent RBC in a system with laminar boundary layers below the top and above the bottom plate. For Ra-1 < Ra < Ra-2 (with Ra-2 similar or equal to 5 x 10(14)) gamma(eff) gradually increases up to 0.37 +/- 0.01. We argue that above Ra-2 the system is in the ultimate state of convection where the boundary layers, both thermal and kinetic, are also turbulent. Several previous measurements for Gamma = 0.50 are re-examined and compared with our results. Some of them show a transition to a state with gamma(eff) in the range from 0.37 to 0.40, albeit at values of Ra in the range from 9 x 10(10) to 7 x 10(11) which is much lower than the present Ra-1 or Ra-2 . The nature of the transition found by them is relatively sharp and does not reveal the wide transition range observed in this work. In addition to the results for the genuine Rayleigh-Benard system, we present measurements for a sample which was not completely sealed; the small openings permitted external currents, imposed by density differences and gravity, to pass through the sample. That system should no longer be regarded as genuine RBC because the externally imposed currents modified the heat transport in a major way. It showed a sudden decrease of gamma(eff) from 0.308 for Ra < Ra-t similar or equal to 4 x 10(13) to 0.25 for larger Ra. A number of possible experimental effects are examined in a sequence of appendices; none of these effects is found to have a significant influence on the measurements
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No full textMax-Planck-Gesellschaft http://dx.doi.org/10.13039/501100004189National Natural Science Foundation of China http://dx.doi.org/10.13039/50110000180
Turbulent Rayleigh–Bénard convection for a Prandtl number of 0.67
No full textFor the Rayleigh-number range 10 7 ≲ Ra ≲ 10 11 we report measurements of the Nusselt number Nu and of properties of the large-scale circulation (LSC) for cylindrical samples of helium gas (Prandtl number Pr = 0.674) that have aspect ratio Γ ≡ D / L = 0.50 ( D and L are the diameter and the height respectively) and are heated from below. The results for Nu are consistent with recent direct numerical simulations. We measured the amplitude δ of the azimuthal temperature variation induced by the LSC at the sidewall, and the LSC circulation-plane orientation θ 0 , at three vertical positions. For the entire Ra range the LSC involves a convection roll that is coherent over the height of the system. However, this structure frequently collapses completely at irregular time intervals and then reorganizes from the incoherent flow. At small δ the probability distribution p (δ) increases linearly from zero; for Γ = 1 and Pr = 4.38 this increase is exponential. No evidence of a two-roll structure, with one above the other, was observed. This differs from recent direct numerical simulations for Γ = 0.5 and Pr = 0.7, where a one-roll LSC was found to exist only for Ra ≲ 10 9 to 10 10 , and from measurements for Γ = 0.5 and Pr ≃ 5, where one- and two-roll structures were observed with transitions between them at random time intervals
Transitions in heat transport by turbulent convection at Rayleigh numbers up to 10<sup>15</sup>
No full textComment on "Effect of Boundary Layers Asymmetry on Heat Transfer Efficiency in Turbulent Rayleigh-Bénard Convection at Very High Rayleigh Numbers"
No full textA Comment on the Letter by P. Urban, P. Hanzelka, T. Kralik, V. Musilova, A. Srnka, and L. Skrbek, Phys. Rev. Lett. 109, 154301 (2012). The authors of the Letter offer a Reply
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Experimental study of instabilities in turbulent Rayleigh Benard Convection and Path instability of rising bubbles
No full textLes présents travaux portent sur deux problèmes classiques de mécanique des fluides: la convection turbulente de Rayleigh-Bénard en rotation et l’instabilité de la trajectoire de bulles en ascension dans un liquide. Pour ce qui concerne la convection turbulente de Rayleigh-Bénard en rotation, les champs de vitesse et de température ont été mesurés respectivement par PIV (Particle Image Velocimetry) et par LIF (Laser Induced Fluorescence) dans le plan de symétrie vertical de notre cellule cylindrique de rapport d’aspect 1. Les résultats de PIV confirment l’affaiblissement de la circulation à grande échelle en fonction des nombres Rossby décroissants – jusqu’à sa disparition totale – ainsi que la formation de colonnes de vortex en régime dominé par la rotation. Par des corrélations croisées sur le champ de vitesse, il a été possible de montrer expérimentalement que la vorticité dans les colonnes changeait de signe au centre de la cellule. Les fluctuations de vitesses dans la cellule sont fortement anisotropiques et suivent une loi d’échelle en Ro0.2 en régime affecté par la rotation. La température des colonnes de vortex tout comme celle des panaches a été estimée par des mesures LIF. Pour ce qui concerne l’instabilité de la trajectoire de bulles en ascension, les bulles petites montent verticalement, alors qu’au-dessus d’une certaine taille, elles montent en zigzag ou en suivant une trajectoire hélicoïdale. De nouveaux points expérimentaux sur la courbe de stabilité marginale ont été obtenus en travaillant dans des huiles de silicone de 5 et 10 cst et dans l’eau. L’accord avec les résultats des simulations numériques les plus récentes n’est que partiel. La vitesse ascensionnelle, la fréquence et l’amplitude des oscillations ont été mesurées et suggèrent une bifurcation de Hopf supercritiqueThe present work focuses on two common fluid flow problems namely, Turbulent Rayleigh-Bénard Convection and Path instability of rising bubbles immersed in a liquid. Concerning Rotating Turbulent Rayleigh-Bénard Convection, the flow field and temperature field were measured respectively by Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) in a vertical plan of symmetry of our cylindrical cell of aspect ratio 1. The weakening of the Large Scale Circulation with decreasing Rossby number - leading to its complete disappearance - was confirmed as well as the formation of vortex columns in the rotation dominated regime. By doing velocity cross correlations, it has been possible to prove experimentally that the vorticity of the columns change direction in the cell’s center. The velocity fluctuations in the cell are highly anisotropic and follow a scaling of Ro0.2 in the rotation affected regime. The temperature of the vortex columns as well as of individual plumes has been estimated by LIF measurements. Concerning the Path instability of rising bubbles, small bubbles rise in straight path, whereas beyond a critical size, bubbles rise in zigzag or helical path. Some new experimental points on the marginal stability curve have been obtained by working in silicon oils of 5 and 10 cst and in water. The agreement with the most recent numerical simulations is only partial. The rise velocity, frequency and amplitude of oscillation have also been measured and suggest a supercritical Hopf bifurcation
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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