3,121 research outputs found
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
Transitions in heat transport by turbulent convection at Rayleigh numbers up to 10<sup>15</sup>
No full textHe et al. Reply:
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
Contrasting activity profile of two distributed cortical networks as a function of attentional demands
No full textThe original publication is available at http://www.jneurosci.orgThis work was supported by R01 grant MH-073610 from the National Institutes of Health to Denis Paré
Comment 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
Is Tolerance Political? An Interview with Denis Lacorne
No full textcontribution à un site webDenis Lacorne is the author of "The Limits of Tolerance. Enlightenment Values and Religious Fanaticism" (Columbia University Press, 2019), the English translation of "Les limites de la tolérance" (Gallimard, awarded the Prix Montyon by the Académie Française). In his book, which is intellectually very inspiring because of the many questions it addresses and raises, Denis Lacorne traces the emergence of the notion of tolerance from its early thinkers to the Age of Enlightenment and finally questions the notion and its various understandings through more recent events in France and the United States. What is tolerance? Is tolerance political? Interview by Miriam Périer, CER
Timing of impulses from the central amygdala and bed nucleus of the stria terminalis to the brainstem
No full textThe amygdala and bed nucleus of the stria terminalis (BNST) are thought to subserve distinct functions with the former mediating rapid fear responses to discrete sensory cues and the latter longer “anxiety-like” states in response to diffuse environmental contingencies. Yet, these structures are reciprocally connected and their projection sites overlap extensively. To shed light on the significance of BNST-amygdala connections, we compared the antidromic response latencies of BNST and central amygdala (CE) neurons to brainstem stimulation. Whereas the frequency distribution of latencies was unimodal in BNST neurons (~10 ms mode), that of CE neurons was bimodal (~10 and ~30 ms modes). However, after stria terminalis (ST) lesions, only short-latency antidromic responses were observed, suggesting that CE axons with long conduction times course through the ST. Compared to the direct route, the ST greatly lengthens the path of CE axons to the brainstem, an apparently disadvantageous arrangement. Since BNST and CE share major excitatory basolateral amygdala (BL) inputs, lengthening the path of CE axons might allow synchronization of BNST and CE impulses to brainstem when activated by BL. To test this, we applied electrical BL stimuli and compared orthodromic response latencies in CE and BNST neurons. The latency difference between CE and BNST neurons to BL stimuli approximated that seen between the antidromic responses of BNST cells and CE neurons with long-conduction times. These results point to a hitherto unsuspected level of temporal coordination between the inputs and outputs of CE and BNST neurons, supporting the idea of shared functions.The original publication is available at: http://jn.physiology.org/cgi/content/full/100/6/342
Rehab Depot de la Plaine Saint-Denis
No full textRedesign for workshop Atelier Revision Intermediaire at the Depot de la Plaine Saint-Denis with a rehabilitation center as new functionRMITArchitecture and The Built Environmen
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