102,470 research outputs found
Numerical simulations of Rayleigh-Benard convection for Prandtl numbers between 10(-1) and 10(4) and Rayleigh numbers between 10(5) and 10(9)
Get PDFWe summarize the results of an extensive campaign of direct numerical simulations of Rayleigh–Bénard convection at moderate and high Prandtl numbers (10−1 ≤ Pr ≤ 104) and moderate Rayleigh numbers (105 ≤ Ra ≤ 109). The computational domain is a cylindrical cell of aspect ratio Γ = 1/2, with the no-slip condition imposed on all boundaries. By scaling the numerical results, we find that the free-fall velocity should be multiplied by in order to obtain a more appropriate representation of the large-scale velocity at high Pr. We investigate the Nusselt and the Reynolds number dependences on Ra and Pr, comparing the outcome with previous numerical and experimental results. Depending on Pr, we obtain different power laws of the Nusselt number with respect to Ra, ranging from Ra2/7 for Pr = 1 up to Ra0.31 for Pr = 103. The Nusselt number is independent of Pr. The Reynolds number scales as , neglecting logarithmic corrections. We analyse the global and local features of viscous and thermal boundary layers and their scaling behaviours with respect to Ra and Pr, and with respect to the Reynolds and Péclet numbers. We find that the flow approaches a saturation state when Reynolds number decreases below the critical value, Res ≃ 40. The thermal-boundary-layer thickness increases slightly (instead of decreasing) when the Péclet number increases, because of the moderating influence of the viscous boundary layer. The simulated ranges of Ra and Pr contain steady, periodic and turbulent solutions. A rough estimate of the transition from the steady to the unsteady state is obtained by monitoring the time evolution of the system until it reaches stationary solutions. We find multiple solutions as long-term phenomena at Ra = 108 and Pr = 103, which, however, do not result in significantly different Nusselt numbers. One of these multiple solutions, even if stable over a long time interval, shows a break in the mid-plane symmetry of the temperature profile. We analyse the flow structures through the transitional phases by direct visualizations of the temperature and velocity fields. A wide variety of large-scale circulation and plume structures has been found. The single-roll circulation is characteristic only of the steady and periodic solutions. For other regimes at lower Pr, the mean flow generally consists of two opposite toroidal structures; at higher Pr, the flow is organized in the form of multi-jet structures, extending mostly in the vertical direction. At high Pr, plumes mainly detach from sheet-like structures. The signatures of different large-scale structures are generally well reflected in the data trends with respect to Ra, less in those with respect to Pr
Turbulent thermal convection at high Rayleigh numbers for a Boussinesq fluid of constant Prandtl number
No full textThe results from direct numerical simulations of turbulent Boussinesq convection are briefly presented. The flow is computed for a cylindrical cell of aspect ratio 1/2 in order to compare with the results from recent experiments. The results span eight decades of Ra from 2×10^6 to 2×10^14 and form the baseline data for a strictly Boussinesq fluid of constant Prandtl number (Pr = 0.7). A conclusion is that the Nusselt number varies nearly as the 1/3 power of Ra for about four decades towards the upper end of the Ra range covered
Étude de l’Integral view de G. Sreenivasan : l’émotion comme état exécutif et le contre‐exemple du courage
Get PDFQuelle est la nature de la vertu ? Quels sont les ingrédients psychologiques
nécessaires pour qualifier un agent de vertueux ? La réponse de Sreenivasan dans
Emotion and Virtue (à paraître) suggère que l’émotion constitue la vertu et est
essentielle à la psychologie des individus considérés comme vertueux. Si un sujet
manque de sympathie, il ne peut être qualifié de véritablement compassionné. La
raison ? Il n’est pas disposé à voir et à juger qu’une autre personne est dans le
besoin et fait appel à son aide. L’émotion oriente ainsi l’attention de l’agent sur les
données externes pertinentes et sur les réponses comportementales appropriées.
Mais si l’émotion exerce bien un contrôle cognitif sur la motivation, par la
sélection d’informations, cela suffit-il à entrainer l’action ? Selon l’auteur, oui,
mais son explication du cas de la vertu de courage, faisant appel au désir, suggère
l’inverse
Bibliographie Hilarion G. Petzold 1958 – 2009 mit Anhang als Einführung
No full textDieses Archiv enthält die Gesamtbibliographie der Werke des Autors nebst einiger Texte „Über H. G. Petzold“ im Schlussteil der Bibliographie sowie einen Anhang mit einer Einführung in die Architektur des Werkes in seinem wissenslogischen Aufbau als Ausarbeitung seines „Tree of Science Modells“ (2007).This archive contains the complete bibliography of the author and some texts about H. G. Petzold, moreover an epilogue with an introduction to the architecture of the works in its epistemological structure and composition and as an elaborations of Petzold’s „Tree of Science Modell (2007).https://www.fpi-publikation.de/polyloge/01-2009-petzold-h-g-gesamtbibliographie-h-g-petzold-1958-2009-updating-november2009/peerReviewedpublishedVersio
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Non Locality of Collective Effects Related to Dynamical Friction in Elliptical Galaxies
No full textTo a large extent, the basic mechanism of dynamical friction remains an open problem. In fact, the classical idealized local description, given by Chandrasekhar, is known to be subject to a number of limitations, possibly related to the global nature of the collective processes that are involved. Here we present a study of the properties of dynamical friction in galaxy models, derived from a realistic distribution function, that are characterized by significant density concentration and by pressure anisotropy biased in the radial direction. The problem is addressed by means of dedicated N-body simulations. By following the fall of heavy objects inside the host galaxy, we find that the classical theory is better suited to describe dynamical friction only outside cores of models with low concentration, but fails in more concentrated models, more appropriate to describe elliptical galaxies. In turn, pressure anisotropy in the host galaxy does not appear to have significant effects
Étude de l’Integral view de G. Sreenivasan : l’émotion comme état exécutif et le contre‐exemple du courage
Get PDFQuelle est la nature de la vertu ? Quels sont les ingrédients psychologiques
nécessaires pour qualifier un agent de vertueux ? La réponse de Sreenivasan dans
Emotion and Virtue (à paraître) suggère que l’émotion constitue la vertu et est
essentielle à la psychologie des individus considérés comme vertueux. Si un sujet
manque de sympathie, il ne peut être qualifié de véritablement compassionné. La
raison ? Il n’est pas disposé à voir et à juger qu’une autre personne est dans le
besoin et fait appel à son aide. L’émotion oriente ainsi l’attention de l’agent sur les
données externes pertinentes et sur les réponses comportementales appropriées.
Mais si l’émotion exerce bien un contrôle cognitif sur la motivation, par la
sélection d’informations, cela suffit-il à entrainer l’action ? Selon l’auteur, oui,
mais son explication du cas de la vertu de courage, faisant appel au désir, suggère
l’inverse
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