Showing papers by "Karlsruhe Institute of Technology published in 1976"

Density profiles in argon and nitrogen shock waves measured by the absorption of an electron beam

Abstract: Accurate measurements of the density distribution in Ar and N2 shock waves have been made in a shock tube for the Mach number range from 1.55 to 9 and 10 respectively by the absorption of an electron beam. A modified absorption law has been used for data reduction. The density profiles were corrected for the influence of shock curvature and density rise behind the shock wave. The measurements in Ar agree to within 1% with those of Schmidt (1969) in the mean range of Ms but give a slightly smaller density gradient for Ms = 9. Comparison with various theories shows very good agreement with Bird's Monte Carlo simulation in the whole Mach number range for a simple repulsive intermolecular force law. Further, the agreement with the Mott-Smith density profile for the same interaction law is also good, and surprisingly is found to be better for lower than for higher Mach numbers. Qualitative agreement is obtained with the solutions of Hicks & Yen for hard-sphere and Maxwell molecules. The Navier-Stokes and BGK solutions are found to differ significantly from the present experiments even for the lowest measured Mach number (1·55), whereas the Burnett equation gives better agreement, especially with respect to the asymmetry of the profiles.The measured N2 profiles agree on the whole with the shock-tube measurements of other investigators but show substantial deviations from the low density wind-tunnel experiments of Robben & Talbot (1966b) for higher Mach numbers. Bird's ‘energy sink model’ (1971) is in agreement with the measured density profiles for a realistic interaction law and a suitable rotational collision number. Rotational relaxation in nitrogen is found to be very fast for all Mach numbers. Consequently the coupling between rotational and translational relaxation is very strong.

Population analysis based on occupation numbers of modified atomic orbitals (MAOs)

Abstract: A new interpretational scheme is proposed for the analysis of molecular wavefunctions. Starting from the molecular density operator we first construct a minimal set of MAOs from the requirement that the MOs can be represented as closely as possible by the MAOs. We then use the MAOs to compute atomic occupation numbers N and shared electron numbers σ. The molecular density is then discussed in terms of N and σ. This approach has the following advantages: 1) it is generally applicable, 2) the quantities N and σ are virtually basis set independent, 3) the quantities N and σ fulfil the intuitively expected boundary conditions, 4) the simultaneous consideration of N and σ allows for a more reliable description of chemical bonding than consideration of atomic charges only.

Experiments on a viscous fluid flow between concentric rotating spheres

Abstract: Some experimental results on incompressible viscous fluid flow in the gap between two concentric rotating spheres are discussed. The flow field in the spherical gap has been studied qualitatively by flow visualization (photographs) and quantitatively by measurements by the hot-wire technique. For a wide range of Reynolds numbers, the friction torque was measured for several gap widths and a relatively simple method of determining the torque theoretically is given. At higher Reynolds numbers instabilities appear. Their different behaviour for relatively small and large gap widths is demonstrated. For the larger gap widths, the different appearance of the Taylor–Gortler vortices, the reason for their generation, their regimes of existence as well as their influence on the friction torque are thoroughly treated. Detailed information is given on the new effect of the dependence of the wavelength of the vortices on the Reynolds number.

Fluctuating forces on a rigid circular cylinder in confined flow

Abstract: The fluctuating lift and drag acting on a long, rigidly supported circular cylinder placed symmetrically in a narrow rectangular duct were investigated for various blockage percentages over a wide range of Reynolds numbers around the critical value. The data obtained permit a full assessment of the effect of confinement on the mean-drag coefficient, the root-mean-square values of both the drag and the lift fluctuations, the Strouhal number of the dominant vortex shedding, and the Reynolds number marking transition from laminar to turbulent flow separation. Besides experimental information on a subject on which little is known so far, the paper provides a basis for the deduction of better correction procedures concerning the effects of blockage.

A Lithospheric Seismic Profile in Britain—I Preliminary Results*

Abstract: Summary The planning, execution and preliminary results of a major Anglo-German explosion seismic project are presented in this, paper I of a series. This Lithospheric Seismic Profile in Britain (LISPB) was planned as a reversed 1000 km line between two major sea-shot points off Cape Wrath in Scotland and one in the English Channel; additional sea-shots and intermediate land-shots were fired to give reversed and overlapping crustal coverage (to 180–400 km distance) along the line. In all, 29 shots were fired and 60 mobile magnetic tape stations recorded three-components of ground‘motion. The resulting 14 crustal and three long-range profiles have observations at intervals of typically 2–4 km. Recordings have been digitized and four examples of filtered, computer-plotted record sections are presented to illustrate data quality. In a preliminary analysis, phase correlations are discussed and some models presented; the latter especially are more relevant to future interpretations than to geological or tectonic problems. However, significant variations in crustal thickness and in the nature of the crust-mantle transition do seem to occur beneath the British Isles.

Convergence properties of the intermolecular force series (1/R-expansion)

Abstract: It is proven that the total energy of interacting molecular systems A and B at large intermolecular distancesR can be expanded in a semi-convergent series in powers of 1/R. It is further proven that “exchange forces” vanish faster than any power of 1/R.

Anomalous photovoltage in BaTiO3

Abstract: In ferroelectric melt-grown BaTiO3 single crystals steady-state short-circuit photocurrents have been observed parallel to the c-axis. At room temperature light polarized parallel and perpendicular to the c-axis produces short-circuit photocurrents with opposite sign. Along with the temperature dependence a physical model of this effect is outlined.

The friction and deformation behaviour of rock joints

Abstract: The Friction and Deformation Behaviour of Rock Joints The present investigation deals with the influence of joint roughness and material strength on the friction and deformation behaviour of rough indented joints. Besides the analytical determination of the joint roughness by the dilatation behaviour of the joint, friction tests on models with natural joint morphology and rock samples have been conducted. A material law is developed, describing the friction resistance and the dilatation behaviour at large deformations in their dependence on normal stress and material strength.

Mechanismen der Haftkraftverstärkung bei der Partikelhaftung durch plastisches Verformen, Sintern und viskoelastisches Fließen

Abstract: Haftkraftverstarkungen treten augenblicklich und im Laufe der Zeit ein Augenblickliche Haftkraftverstarkungen beruhen auf Umlagerungen und plastischer Verformung Die Haftkrafte selbst sind im allgemeinen hierfur nicht ausreichend, dagegen konnen asere Krafte — wie Versuche von Schutz und Schubert mit Kalksteinpartikeln zeigten — erhebliche Verstarkungen bis zu einer Grosenordnung bewirken Zeitbedingte Haftkraftverstarkungen beruhen auf viskoelastischer Abplattung und Sinterung Am System Kugel/Kugel wurden beide Mechanismen theoretisch und experimentell untersucht Unterhalb einer kritischen Zeit uberwiegt die viskose Abplattung, oberhalb das Sinterhalswachstum, das von der Grenzflachenspannung und dem auseren Anpresdruck angetrieben werden kann Die von der Theorie gelieferten quantitativen Zusammenhange stehen unter Berucksichtigung des hypothetischen Modellcharakters in gutem Einklang mit den experimentellen Befunden

Compact high-power N 2 laser: Circuit theory and design

Abstract: Based on measurements of the transient voltage across the cavity of a transversely excited N 2 laser an equivalent circuit is developed which provides better understanding of the contribution of all individual components to maximum peak power. Consideration of the design criteria established guarantees about one million light pulses with more than 1 MW from simple high-efficiency construction.

Oxygen diffusion in BaTiO3 ceramic

Abstract: High resistivity diffusion layers are formed by reoxidation between 380 and 750 °C in reduced BaTiO3 ceramic. The potential distribution is measured with an applied voltage at these samples. In addition, observations with the polarizing microscope, resistance and gravimetric measurements are made. The results show that the diffusion of oxygen vacancies is hindered by grain boundaries. The quantities controlling the diffusion mechanism, such as the grain boundary diffusion velocity and the effective diffusion coefficient at longer diffusion distances, are determined experimentally. In reduzierter BaTiO3-Keramik werden durch Reoxidation im Temperaturbereich von 380 bis 750 °C hochohmige Diffusionsrandschichten erzeugt. An solchen von Strom durch-flossenen Proben wird die Potentialverteilung abgetastet. Zudem werden Beobachtungen im Polarisationsmikroskop, Widerstands- und gravimetrische Messungen durchgefuhrt. Die Ergebnisse zeigen, das die Sauerstoffleerstellendiffusion durch die Korngrenzen behindert wird. Die den Diffusionsvorgang bestimmenden Grosen, wie Korngrenzendiffusionsgeschwindigkeit und effektiver Diffusionkoeffizient bei groseren Diffusionswegen, werden experimentell ermittelt.

Darstellung und “disproportionierende Hydrolyse” von Cyclopropanonthioacetalen Eine neue Homologisierungsmethode1, 2)

Abstract: Nach einer allgemeinen Diskussion der Moglichkeiten zur Darstellung von Cyclopropanonthioacetalen 1 (Schema 1) werden zwei praparativ ergiebige Zugange zu dieser Substanzklasse beschrieben. Der eine geht von Epoxiden aus (Tab.1) und stellt eine verbesserte “Eintopfvariante” einer bekannten Reaktion dar, der andere, neue und vielseitigere verwendet die allgemein zuganglichen Dibromcyclopropane 3 als Ausgangsmaterialien, in welchen die Br-Atome in hohen Ausbeuten durch RS-Gruppen ersetzt werden (34561). In wasr. Trifluoressigsaure gehen die Titelverbindungen 1, RCH3, nach Gl. (2) glatt in Ketone 11 und Dimethyldisulfid uber (Tab. 2). Ein mit allen experimentellen Befunden vereinbarer Mechanismus wird fur diese Reaktion in Schema 3 vorgeschlagen. Die Reaktionsfolge Olefin Cyclopropanonthioacetal Keton erlaubt die in Gl. (4) dargestellte Einschiebung einer Carbonylgruppe zwischen die C-Atome der Doppelbindung nicht aktivierter Olefine. Vorteile und Grenzen dieser Methode werden in Abschnitt D diskutiert. Preparation and “Disproportionative Hydrolysis” of Cyclopropanone Thioacetals A New Method of Homologation1, 2) After a general discussion of the conceivable ways of preparing cyclopropanone thioacetals 1 (scheme 1) two practical accesses to this class of compounds are described. One starts from epoxides (table 1) and is an improved one-pot-modification of a previously described method. the other one is new and of broader scope: it uses the generally available dibromocarbene adducts 3 to olefins as precursors in which the bromines are successively exchanged by RS-groups (3451) in high yields. Aqueous trifluoroacetic acid converts the title compounds 1. RCH3, into ketones 11 and dimethyl disulfide (equation (2), table 2). A mechanism which is compatible with all experimental results is proposed in scheme 3 for this reaction. The sequence olefin cyclopropanone thioacetal ketone allows the insertion of a carbonyl group in between sp2-carbons of an olefin (equation (4)). Advantages, scope and limitations of the method are evaluated in section D.

On computing the range of a rational function of n variables over a bounded region

Abstract: We seek an efficient method for computing the range of values of a function ofn variables over a bounded domain.

Velocity profiles and fence drag for a turbulent boundary layer along smooth and rough flat plates

Abstract: The properties of a turbulent boundary layer were investigated as they relate to the form drag on a two-dimensional fence. Detailed measurements were performed at zero pressure gradient of velocity profiles along smooth, rough and transitional flat plates. Upon comparison with other published data, these measurements resulted in simple formulae for the displacement thickness and the local shear coefficient and in a modification to the universal velocity defect law for equilibrium boundary layers.With these boundary layers, experiments were performed to determine the drag on a two-dimensional fence. These data were analysed along with data from previous investigations. It was found that after suitable blockage corrections all form-drag coefficients for two-dimensional fences collapsed on a single curve if they were calculated with the shear velocity as the reference velocity and plotted against the ratio of the fence height to the characteristic roughness parameter of the approaching flow.

A neutron-diffraction study of liquid acetonitrile: II. CD3C15N

Abstract: Neutron-diffraction studies on liquid acetonitrile CD3C15N at 20°C were carried out at neutron wavelengths of 0·5 A and 0·7 A. The data were corrected for background, absorption, multiple scatterin...

Coercive field in fine-grained plzt ceramics

Abstract: Principally the polarization reversal in ferroelectric ceramics with tetragonal symmetry can be derived from 90° or 180° domain reorientations, in ceramics with rhombohedral symmetry it involves 71° and 109° or 180° domain reorientations. Especially the PLZT system contains certain ceramics characterized by predominating 71° and 109° processes. For these materials the coercive field is calculated by means of the domain wall pressure involved from the internal stress state of the ceramics. Measurements of the coercive field of fine-grained (mean grain size 1-2 μm), lanthanum-doped lead zirconate-titanate ceramics (PLZT) of the composition 6/65/35 are consistent with the requirements of the reported theory.

Domain wall motion concept to describe ferroelectric rhombohedral PLZT ceramics

Abstract: Recent studies of hot-pressed PLZT 6/65/35 ceramics have shown that the electrically induced polarization reversal in these materials is characterized by non-180° processes. External and internal produced 71° and 109° domain wall pressures have proved to be responsible for domain wall motions. In this paper the data for electrically and mechanically caused depolarization of initially poled ferroelectric ceramic samples are given and shown to be in quantitative accordance with theoretical requirements of a domain wall motion concept. Furthermore, a mathematical description is given for the hysteretic polarization and strain behaviour of these materials based upon the most significant results of the experiments concerning their domain wall motion properties.