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Non-Oberbeck-Boussinesq (NOB) effects on the Nusselt number Nu and Reynolds number Re in strongly turbulent Rayleigh-Benard convection in liquids were investigated both experimentally and theoretically. In the experiment, the heat current, the temperature difference, and the temperature at the horizontal mid-plane were measured. Three cells of different heights L, all filled with water and all with aspect ratio T close to 1 were used. For each L, about 1.5 decades in Ra were covered, together spanning the ränge 108 < Ra < 1011. For the largest temperature difference between the bottom and top plates of ? = 40K the kinematic viscosity and the thermal expansion coefficient, due to their temperature dependence, varied by more than a factor of two. The Oberbeck-Boussinesq (OB) approximation of temperature independent material parameters thus was no longer valid. The ratio Ï? of the temperature drops across the bottom and top thermal boundary layers became as small as Ï? = 0.83, as compared to the ratio Ï? = 1 in the OB case. Nevertheless, the Nusselt number Nu was found to be only slightly smaller (at most 1.4%) than in the next larger cell with the same Rayleigh number, where the material parameters were still nearly height-independent. The Reynolds numbers in the OB and NOB case agreed with each other within the experimental resolution of about 2%, showing that NOB effects for this parameter were small as well. Thus Nu and Re are rather insensitive against even significant deviations from OB conditions. Theoretically, we first account for the robustness of Nu with respect to NOB corrections: the NOB effects in the top boundary layer cancel those which arise in the bottom boundary layer as long as they are linear in the temperature difference ?. The net effects on Nu are proportional to ?2 and thus increase only slowly and still remain minor despite drastic material parameter changes. We then extend the Prandtl-Blasius boundary-layer theory to NOB Rayleigh-Benard flow with temperature dependent viscosity and thermal diffusivity. This allows the calculation of the shift of the bulk temperature, the temperature drops across the boundary layers, and the ratio Ï? without introducing any fitting parameter. The calculated quantities are in very good agreement with experiment. When in addition we use the experimental finding that for water the sum of the top and bottom thermal boundary-layer widths (based on the slopes of the temperature profiles at the plates) remains unchanged under NOB effects within experimental resolution, the theory also gives the measured small Nusseltnumber reduction for the NOB case. In addition, it predicts an increase by about 0.5% of the Reynolds number, which is also consistent with the experimental data. By theoretically studying hypothetical liquids with only one of the material parameters being temperature dependent, we shed further light on the origin of NOB corrections in water: While the NOB deviation of x from its OB value Ï? = 1 mainly originates from the temperature dependence of the viscosity, the NOB correction of the Nusselt number primarily originates from the temperature dependence of the thermal diffusivity. Finally, we give the predictions from our theory for the NOB corrections if glycerol is used as operating liquid.
Die Erscheinungsform einer bestimmten Einzelsprache resultiert aus dem Nebeneinander und Zusammenwirken vertikaler und horizontaler Transmission von sprachlichen Merkmalen und umfasst demzufolge normalerweise neben Phänomenen, die über eine Kette früherer Sprechergenerationen ererbt wurden, auch solche, die aus anderen Sprachen entlehnt wurden. Deutliche Spuren einer komplexen Entstehungsgeschichte und der Interaktion verschiedener Sprachgemeinschaften lassen sich nicht erst im Altägyptischen oder gar erst in der Sprache des Neuen Reiches ausmachen, sondern bereits die frühesten schriftlichen Quellen aus der Zeit um 3000 v. Chr. erlauben – trotz ihrer Knappheit – überraschende Einblicke in die zeitgenössischen sprachlichen Verhältnisse. Bislang nur durch Sprachvergleich rekonstruierbare Entwicklungen sind nunmehr historisch nachweisbar, und das phonologische System einer dem Altägyptischen vorausgehenden und erheblich von ihm abweichenden Sprachstufe wird greifbar. Da das Voraltägyptische ein für eine afroasiatische Sprache eher untypisches Konsonanteninventar besitzt und sich zudem lexikalische Übereinstimmungen (nicht-genetischer Natur) mit indoeuropäischen Sprachen abzeichnen, erscheinen fundierte Hypothesen über die Herausbildung des Ägyptischen nicht länger unmöglich. Damit wäre das Ägyptische nicht nur die am längsten bezeugte Einzelsprache der Menschheitsgeschichte, sondern wohl auch die einzige, die von ihrer Entstehung bis zu ihrem Aussterben schriftlich dokumentiert ist.
The development of modern science has depended strongly on specific features of the cultures involved; however, its results are widely and transculturally accepted and applied. The science and technology of electricity, for example, emerged as a specific product of post-Renaissance Europe, rooted in the Greek philosophical tradition that encourages explanations of nature in theoretical terms. It did not evolve in China presumably because such encouragement was missing. The transcultural acceptance of modern science and technology is postulated to be due, in part, to the common biological dispositions underlying human cognition, with generalizable capabilities of abstract, symbolic and strategic thought. These faculties of the human mind are main prerequisites for dynamic cultural development and differentiation. They appear to have evolved up to a stage of hunters and gatherers perhaps some 100 000 years ago. However, the extent of the correspondence between some constructions of the human mind and the order of nature, as revealed by science, is a late insight of the last two centuries. Unless we subscribe to extreme forms of constructivism or historical relativism, we may take the success and the formal structure of science as indications of a close, intrinsic relation between the physical and the mental, between the order of nature and the structure of human cognition. At the metatheoretical level, however, modern science is consistent with philosophical and cultural diversity.
When thinking about the historic landscape gardens of the Berlin-Brandenburg region we tend to focus automatically on the royal gardens that Peter Joseph Lenné originally included in his beautification plan for the Potsdam area that are now a UNESCO World Heritage Site. There is good reason for this focus. The complex of Sanssouci, Charlottenhof, Neuer Garten, Babelsberg, Glienicke, and Pfaueninsel constitutes an extraordinarily beautiful panorama. But from a broader perspective the unique character of these gardens can be seen to have informed the garden landscape of all of Berlin in a series of social transformations that occurred during the course of industrialisation in the nineteenth century. For this purpose it is useful to focus on the role of steam power. By no means, however, did the significance of steam power (or the lack of it) end with its utilitarian function, for it informed the aesthetic character of landscape gardens, too. And this character changed with the historical transformations that associated different sorts of gardens with people in different social strata. I will schematise three such »classes« of people and gardens.
The example of Spain illustrates how the production of socio-ecological scales is centred on the social transformation of nature and the construction of socio-ecological and political-ecological scalar gestalts. Concrete geographies, with choreographies of uneven and shifting social power relations, are etched into these ecological, social, political or institutional scalar configurations. These processes are infused with contested and contestable strategies of individuals and social groups, who mobilise spatial scales as part of struggles for control and empowerment, and contest the power geometries of extant scalar gestalts. Needless to say, the mobilisation of scale, the occupation of geographical scale, and the production of scale are central moments in such processes of socio-spatial change. Struggling for the command of scale, or strategizing around excluding particular groups from the performative capabilities of certain scales, shapes social processes, defines relative empowerment and disempowerment and gives rise to very specific socio-spatial relations.
The generation of viral mutants in vitro was demonstrated by treatment of the isolated RNA of Tobacco Mosaic Virus by nitrous acid. This agent causes deaminations converting cytosine into uracil, and adenine into hypoxanthine. Our assay for mutagenesis was the production of local lesions on a tobacco variety on which the untreated strain produces systemic infections only. A variety of different mutants are generated in this way. Quantitative analysis of the kinetics of mutagenesis leads to the conclusion that alteration of a single out of the 6000 nucleotides of the viral RNA is sufficient for causing a mutation.