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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 The Oberbeck-Boussinesq (OB) approximation of temperature independent material parameters thus was no longer valid.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.
Der Vortrag über den im Titel „Naturwissenschaft und Menschenbild“ umschriebenen Problemkreis, der natur- und kulturwissenschaftliche Aspekte betrifft, bildete den Abschluss des Symposiums über das Thema „Wie entstehen neue Qualitäten in komplexen Systemen“ am 18. Dezember 1998 in Berlin zum 50-jährigen Gründungsjubiläum der Max-Planck-Gesellschaft. Schwerpunkte sind Reichweite und Grenzen naturwissenschaftlicher Erklärung von Bewusstsein, evolutionsbiologische Grundlagen von Kooperativität und Empathie, sowie die kulturellen Verallgemeinerungs- und Gestaltungsmöglichkeiten biologisch angelegter Fähigkeiten, insbesondere was die Aktivierung der fragilen und begrenzten, aber durchaus realen und wichtigen Ressource „Gemeinsinn“ angeht.
Abstract (ger): Die Flüssigkeitsschichten, die um ein sich bewegendes Molekül herumfließen, haben infolge der endlichen Abmessungen der Flüssigkeitsmolekeln eine endliche Dicke. Die Berücksichtigung dieses Umstandes führt zu einer Modifikation der Stokesschen Gesetze der Kontinuumstheorie für den Zusammenhang zwischen Reibung und Viskosität. Es ergibt sich die richtige Größenordnung und ungefähr die richtige Radienabhängigkeit der beobachteten Mikroreibung, und zwar sowohl für die Rotation als auch für die Translation.
Die Entstehung der modernen Naturwissenschaften beruhte auf sehr spezifischen Merkmalen der daran beteiligten Kulturen, und doch sind ihre Erkenntnisse und Ergebnisse transkulturell und weltweit akzeptiert. So waren die Elektrizitätslehre und die Elektrotechnik spezifische Produkte der europäischen Kultur der Neuzeit, die ihrerseits auf einer bestimmten Sequenz und Kombination kultureller und interkultureller Entwicklungen bis zurück zur altgriechischen Philosophie aufbauten. Sie entstanden nicht in China, wo die Kraft des theoretischen Denkens nicht in gleicher Weise eingeschätzt wurde. Warum wurden dann aber moderne Wissenschaft und Technik transkulturell wirksam? Ein Hauptgrund dafür dürfte darin bestehen, daß die zugrunde liegenden kognitiven Fähigkeiten - Fähigkeiten der Abstraktion, des symbolischen und strategischen Denkens - auf einer biologischen Basis beruhen, die der gesamten heutigen Spezies Mensch gemeinsam ist. Gegenstand wissenschaftlicher Erkenntnis sind aber auch die prinzipiellen Grenzen der Erkenntnis; sie bedingen, daß naturwissenschaftliches Denken, das seine eigenen Grenzen kritisch reflektiert, auf der metatheoretischen Ebene mit verschiedenen philosophischen und kulturellen Interpretationen des Menschen und der Welt vereinbar ist. Dazu gehören auch religiöse Interpretationen, die die Ordnung der Natur mit dem menschlichen Geist verbinden und es dem Menschen aufgeben, diese Ordnung mit Hilfe seines Denkens zu erleben und zu erfahren.
The short paper introduces the concept of possible branches of double-stranded DNA (later sometimes called palindromes): Certain sequences of nucleotides may be followed, after a short unpaired stretch, by a complementary sequence in reversed order, such that each DNA strand can fold back on itself, and the DNA assumes a cruciform or tree-like structure. This is postulated to interact with regulatory proteins.
Upon separation of the protein from the nucleic acid component of tobacco mosaic virus by phenol, using a fast and gentle procedure, the nucleic acid is infective in assays on tobacco leaves. A series of qualitative and quantitative control experiments demonstrates that the biological activity cannot depend on residual proteins in the preparation, but is a property of isolated nucleic acid which is thus the genetic material of the virus.
In dem Buch über „Wissenschaft und Menschenbild“ werden verschiedene Wege zu einem Verständnis unserer Spezies „Mensch“ begangen: Zum einen zeigt uns die Geschichte und die geistige Struktur der modernen Naturwissenschaft sowohl die umfassende Reichweite als auch die prinzipiellen Grenzen menschlicher Erkenntnis auf, und zwar wohl besser und genauer als jede andere Kulturleistung. Zum anderen ergibt die Evolutions- und die Gehirnbiologie Einsichten in menschliche Grundfähigkeiten wie Sprache, Selbstrepräsentation und strategisches Denken. Sie sind Ergebnisse genetischer Evolution, bildeten aber dann die Voraussetzung dynamischer kultureller Entwicklung, die nicht mehr auf genetischen Änderungen beruht. Die moderne Wissenschaft ist, indem sie ihre eigenen Grenzen begründet, offen für verschiedene, natürlich nicht für alle kulturellen und philosophischen Interpretationen des Menschen und der Welt. Menschliches Bewusstsein ist ein Ergebnis der Evolution des Gehirns, und doch ist die Gehirn-Geist-Beziehung aus entscheidungstheoretischen Gründen vermutlich nicht vollständig dekodierbar. Eine wesentliche Fähigkeit unserer Spezies Mensch ist kognitionsgestützte Empathie. Sie entstand vielleicht im Kontext der Evolution strategischen Denkens, indem sie es erleichtert, das Verhalten anderer vorherzusehen, ist aber auch Motivation für altruistisches Verhalten. Stereotype Kontroversen zwischen Sozialwissenschaftlern und Soziobiologen erscheinen heute eher überflüssig; es gibt eine, wenn auch begrenzte, biologische Basis auch für freundliche menschliche Eigenschaften wie Kooperativität und Vertrauen und nicht nur für Egoismus. Moralische Vorstellungen sollten die biologischen Grundlagen menschlichen Verhaltens respektieren: Gemeinwohl ist eine durchaus reale, aber doch begrenzte Ressource unserer Spezies „Mensch“. Sie ist eher behutsam zu aktivieren, und moralische Überforderungen sind kontraproduktiv.
Understanding cooperative human behaviour depends on insights into the biological basis of human altruism, as well as into socio-cultural development. In terms of evolutionary theory, kinship and reciprocity are well established as underlying cooperativeness. Reasons will be given suggesting an additional source, the capability of a cognition-based empathy that may have evolved as a by-product of strategic thought. An assessment of the range, the intrinsic limitations, and the conditions for activation of human cooperativeness would profit from a systems approach combining biological and socio-cultural aspects. However, this is not yet the prevailing attitude among contemporary social and biological scientists who often hold prejudiced views of each other's notions. It is therefore worth noticing that the desirable integration of aspects has already been attempted, in remarkable and encouraging ways, in the history of thought on human nature. I will exemplify this with the ideas of the fourteenth century Arab-Muslim historian Ibn Khaldun. He set out to explicate human cooperativeness - "asabiyah" - as having a biological basis in common descent, but being extendable far beyond within social systems, though in a relatively unstable and attenuated fashion. He combined psychological and material factors in a dynamical theory of the rise and decline of political rulership, and related general social phenomena to basic features of human behaviour influenced by kinship, expectation of reciprocity, and empathic emotions.
This is the invited evening lecture of the biannual workshop on hydroid development of 1999. Its topic is the role of hydra as a rather puristic model for the de-novo generation of spatial patterns in development, and our work in this field. Emphasis is placed not only on experimental studies, but also on theoretical analysis, because the understanding of spatial order requires a systems approach involving the combination of knowledge on molecules, cells and tissues with mathematical analysis, laws and facts.
Biological evolution and technological innovation, while differing in many respects, also share common features. In particular, the implementation of a new technology in the market is analogous to the spreading of a new genetic trait in a population. Technological innovation may occur either through the accumulation of quantitative changes, as in the development of the ocean clipper, or it may be initiated by a new combination of features or subsystems, as in the case of steamships. Other examples of the latter type are electric networks that combine the generation, distribution, and use of electricity, and containerized transportation that combines standardized containers, logistics, and ships. Biological evolution proceeds, phenotypically, in many small steps, but at the genetic level novel features may arise not only through the accumulation of many small, common mutational changes, but also when distinct, relatively rare genetic changes are followed by many further mutations. New evolutionary directions may be initiated by, in particular, some rare combinations of regulatory sections within the genome. The combinatorial type of mechanism may not be a logical prerequisite for biological innovation, but it can be efficient, especially when novel features arise out of already highly developed systems. Such is the case with the evolution of general, widely applicable capabilities of the human brain. Hypothetical examples include the evolution of strategic thought, which encompasses multiple self-representations, cognition-based empathy, meta-levels of abstraction, and symbolic language. These capabilities of biologically modern man may have been initiated, perhaps some 150 000 years ago, by one or few accidental but distinct combinations of modules and subroutines of gene regulation which are involved in the generation of the neural network in the cerebral cortex. This hypothesis concurs with current insights into the molecular biology of the combinatorial and hierarchical facets of gene regulation that underlie brain development. A theory of innovation encompassing technological as well as biological development cannot per se dictate alternative explanations of biological evolution, but it may help in adding weight and directing attention to notions outside the mainstream, such as the hypothesis that few distinct genetic changes were crucial for the evolution of modern man.
Modern science, based on the laws of physics, claims validity for all events in space and time. However, it also reveals its own limitations, such as the indeterminacy of quantum physics, the limits of decidability, and, presumably, limits of decodability of the mind-brain relationship. At the philosophical level, these intrinsic limitations allow for different interpretations of the relation between human cognition and the natural order. In particular, modern science may be logically consistent with religious as well as agnostic views of humans and the universe. These points are exemplified through the transcript of a discussion between Kurt Gödel and Rudolf Carnap that took place in 1940. Gödel, discoverer of mathematical undecidability, took a proreligious view; Carnap, one of the founders of analytical philosophy, an antireligious view. By the time of the discussion, Carnap had liberalized his ideas on theoretical concepts of science: he believed that observational terms do not suffice for an exhaustive definition of theoretical concepts. Then, responded Gödel, one should formulate a theory or metatheory that is consistent with scientific rationality, yet also encompasses theology. Carnap considered such theories unproductive. The controversy remained unresolved, but its emphasis shifted from rationality to wisdom, not only in the Gödel-Carnap discussion but also in our time.
Applying mild methods of preparation, part of the ribosomes of rabbit reticulocytes are found in aggregates (later called polyribosomes) of up to six ribosomal units. Upon treatment with RNA-ase, they desintegrate into single ribosomes. The fast-sedimenting aggregates are found to be more active in protein synthesis in terms of incorporation of radioactive amino acids, whereas the single ribosomes are more receptive to stimulation by the artificial messenger RNA poly-U. The findings indicate that the linkage of ribosomes into aggregates is due to the messenger RNA. They support a tape-reading mechanism of protein synthesis whereby growth of the peptide chain is accompanied by shifting the active site of the ribosome from one coding group of nucleotides of the messenger RNA to the next.
We expose analogies between turbulence in a fluid heated from below (Rayleigh-Bénard (RB) flow) and shear flows: The unifying theory for RB flow (S.Grossmann and D.Lohse, J.Fluid Mech. 407, 27-56 (2000) and subsequent refinements) can be extended to the flow between rotating cylinders (Taylor-Couette flow) and pipe flow. We identify wind dissipation rates and momentum fluxes that are analogous to the dissipation rate and heat flux in RB flow. The proposed unifying description for the three cases is consistent with the experimental data.
Naturwissenschaftliches Denken, wie es von altgriechischen Naturphilosophen begründet und in der Antike weiterentwickelt wurde, verfiel mit dem Aufstieg der monotheistischen Offenbarungsreligionen, die die Neugier auf die natürliche Wirklichkeit als eitle Bemühung ansahen, welche nichts zum Seelenheil beitrage. Die neuzeitliche Naturwissenschaft nahm ihren Ausgang in der Renaissance, die die kreativen Fähigkeiten des menschlichen Denkens wiederentdeckte. Sie verdankt aber auch sehr viel dem Aufbruch philosophisch-theologischen Denkens im Mittelalter, der dem „Buch der Natur“ eine Anerkennung als gleichberechtigten Zugang zur Wahrheit neben dem „Buch der Offenbarung“ verschaffte. Die ersten Ansätze hierzu zeigen sich besonders im Werk eines überragenden, wenn auch oft verkannten Denkers des 9. Jahrhunderts - Johannes ERIUGENA. Er postulierte den Vorrang der Vernunft vor der Autorität, erlaubte sich sehr weitgehende sinngemäße Interpretationen biblischer Überlieferungen und sah die Erkenntnis der natürlichen Wirklichkeit mit Begriffen menschlicher Vernunft als gottgewollt und gottgegeben an. Der Mensch selbst sei die Zusammenfassung der gesamten Schöpfung. Gleichzeitig und unabhängig von ERIUGENA forderte AL-KINDI in Bagdad - auf der Grundlage ähnlichen Wissens und ähnlicher philosophischer Ideen - die positive Bewertung wissenschaftlicher Bemühungen ein. In der frühen Renaissance war es besonders CUSANUS, der Ideen von ERIUGENA aufnahm. Mit der Betonung der menschlichen Kreativität, der Forderung nach quantitativen Experimenten, besonders aber mit seiner Philosophie des positiven Wissens um die Grenzen des Wissens, ging CUSANUS aber auch wesentlich über ERIUGENAs Gedanken hinaus. Als dann, über sieben Jahrhunderte nach ERIUGENA, GALILEI die moderne Naturwissenschaft begründete, rechtfertigte er sein freizügiges Denken im „Brief an CASTELLI“ mit Argumenten, die in erstaunlichem Maße mit Vorstellungen ERIUGENAS übereinstimmen.+++++++ An abbrived English version is also available online (17 pages): <a href="http://nbn-resolving.de/urn:nbn:de:kobv:b4360-1001048">Alfred Gierer (2000), Eriugena and al-Kindi, Nineth century protagonists pro-scientific cultural change</a>.
Ancient Greek philosophers were the first to postulate the possibility of explaining nature in theoretical terms and to initiate attempts at this. With the rise of monotheistic religions of revelation claiming supremacy over human reason and envisaging a new world to come, studies of the natural order of the transient world were widely considered undesirable. Later, in the Middle Ages, the desire for human understanding of nature in terms of reason was revived. This article is concerned with the fundamental reversal of attitudes, from “undesirable” to “desirable”, that eventually led into the foundations of modern science. One of the earliest, most ingenious and most interesting personalities involved was Eriugena, a theologian at the Court of Charles the Bald in the 9th century. Though understanding what we call nature is only one of the several aspects of his theological work, his line of thought implies a turn into a pro-scientific direction: the natural order is to be understood in abstract terms of ‘primordial causes’; understanding nature is considered to be the will of God; man encompasses the whole of creation in a physical as well as a mental sense. Basically similar ideas on the reconciliation of scientific rationality and monotheistic religions of revelation were conceived, independently and nearly simultaneously, by the Arab philosopher al-Kindi in Bagdad. Eriugena was more outspoken in his claim that reason is superior to authority. This claim is implicit in the thought of Nicholas of Cusa with his emphasis on human mental creativity as the image of God’s creativity; and it is the keynote of Galileo’s ‘Letter to the Grand Duchess Christina’ some 800 years later, the manifesto expressing basic attitudes of modern science. This article in English is based on the monography (in German): A. Gierer “Eriugena, al-Kindi, Nikolaus von Kues - Protagonisten einer wissenschaftsfreundlichen Wende im philosophischen und theologischen Denken”, Acta Historica Leopoldina 29 (1999), Barth Verlag in MVH Verlage Heidelberg, ISBN: 3-335-00652-6
In dem Buch werden vielfältige und unterschiedliche Vorstellungen über den Nutzen und die Ausgestaltung wissenschaftlicher Politikberatung präsentiert. Sie geben zum einen die seit langem geführte akademische Diskussion zu diesem Thema wieder und zum anderen die konkreten Erfahrungen von Wissenschaftlerinnen und Wissenschaftlern, die in der Politikberatung tätig sind. Mit Beiträgen u. a. von Ortwin Renn, Naomi Oreskes, Roger A. Pielke Jr., Ottmar Edenhofer u. Martin Kowarsch, Peter Weingart u. a.
Early-career funding in German-African academic cooperation: achievements, challenges, perspectives
(2024)
This paper analyses experiences, challenges and potentials in German-African academic cooperation in the field of early-career funding considering the humanities and social sciences as well as natural sciences and medicine. It is based on a comprehensive overview of existing German funding formats and an exemplary survey of the experiences of African cooperation partners with these programmes. The authors propose the establishment of an interface between academic research, the practice of science funding, and African researchers. According to the authors, such a contact and information point would contribute to the improvement of German-African science cooperation and be an important element of Germany’s scientific diplomacy in the long term. This paper is the English translation of Denkanstoß 13 (2023): Early-Career-Förderung in der deutsch-afrikanischen Wissenschaftskooperation. Leistungen, Herausforderungen, Perspektiven. Berlin.