Showing papers by "Technische Universität Darmstadt published in 1983"

On the asymptotic convergence of collocation methods

Abstract: We prove quasioptimal and optimal order estimates in various Sobolev norms for the approximation of linear strongly elliptic pseudodifferential equations in one independent variable by the method of nodal collocation by odd degree polynomial splines. The analysis pertains in particular to many of the boundary element methods used for numerical computation in engineering applications. Equations to which the analysis is applied include Fredholm integral equations of the second kind, certain first kind Fredholm equations, singular integral equations involving Cauchy kernels, a variety of integro-differential equations, and two-point boundary value problems for ordinary differential equations. The error analysis is based on an equivalence which we establish between the collocation methods and certain nonstandard Galerkin methods. We compare the collocation method with a standard Galerkin method using splines of the same degree, showing that the Galerkin method is quasioptimal in a Sobolev space of lower index and furnishes optimal order approximation for a range of Sobolev indices containing and extending below that for the collocation method, and so the standard Galerkin method achieves higher rates of convergence.

Investigation of the Laminar-Turbulent Transition of Boundary Layers Disturbed by Wakes

Abstract: The boundary layer transition under instationary afflux conditions as present in the stages of turbomachines is investigated. A model for the transition process is introduced by means of time-space distributions of the turbulent spots during transition and schematic drawings of the instantaneous boundary layer thicknesses. To confirm this model, measurements of the transition with zero and favorable pressure gradient are performed.

High-j proton and neutron alignment in the odd actinides235U and237Np

Abstract: The investigation of the odd-neutron nucleus235U and odd-proton nucleus237Np provides the possibility to study separately the alignment effects in thei13/2 proton andj15/2 neutron intruder states in the actinides. Using Coulomb excitation with208Pb projectiles we were able to extend the yrast bands up to the 57/2 levels in both cases. The experimental results consistently show — at least in the region of the rotational frequencies studied — the dominant role of thei13/2 proton alignment, as well as the weakness of the interaction strength of thej15/2 neutrons-configuration forN=144.

Applications of an orbiting gravity gradiometer

Abstract: Considering present attempts to develop a gradiometer with an accuracy between 10−3 E and 10−4 E, two applications for such a device have been studied: (a) mapping the gravitational field of the Earth, and (b) estimating the geocentric distance of a satellite carrying the instrument. Given a certain power spectrum for the signal and 10−4 E (rms) of white measurement noise, the results of an error analysis indicate that a six-month mission in polar orbit at a height of 200 km, with samples taken every three seconds, should provide data for estimating the spherical harmonic potential coefficients up to degree and order 300 with less than 50% error, and improve the coefficients through degree 30 by up to four orders of magnitude compared to existing models. A simulation study based on numerical orbit integrations suggests that a simple adjustment of the initial conditions based on gradiometer data could produce orbits where the geocentric distance is accurate to 10 cm or better, provided the orbits are 2000 km high and some improvement in the gravity field up to degree 30 is first achieved. In this sense, the gravity-mapping capability of the gradiometer complements its use in orbit refinement. This idea can be of use in determining orbits for satellite altimetry. Furthermore, by tracking the gradiometer-carrying spacecraft when it passes nearly above a terrestrial station, the geocentric distance of this station can also be estimated to about one decimeter accuracy. This principle could be used in combination with VLBI and other modern methods to set up a world-wide 3-D network of high accuracy.

Beam energy dependence of spontaneous positron creation in delayed nuclear collisions

Abstract: We compute positron emission from U+Cm collisions within a quantum mechanical model for delayed nuclear collisions. We demonstrate a striking beam energy dependence of the strength of the spontaneous positron peak.

First Results on γ-Ray Spectroscopy in the Second Minimum of 238U and 240Pu

Abstract: γ-transitions preceding the fission decay of the shape isomers in 238U and 240Pu have been measured with the crystal ball. Lines at 616 keV and 808 keV are observed in 238U and 240Pu, respectively, which are tentatively interpreted as deexcitations of the β-vibrational band in the second minimum.

Physical mechanisms for the electret behavior of PETP

Abstract: Thermally-stimulated discharge (TSD) currents of polyethyleneterephtha-late (PETP) electrets were measured by a number of authors [e.g. 1–10]. Both, open-[6,9,10] and short-circuit [1–8] TSD arrangements were utilized. For samples charged above the glass-transition temperature T g , the TSD response consists of a dipole-deorientation peak around ? (“heterocharge”) and a space-charge peak at a higher temperature T (“homocharge”) which are opposite in sign (e.g. Fig. 3) [1,4,5]. In contrast, a typical open-circuit TSD response of corona-charged PETP exhibits two peaks of the same sign (Fig. 1). Conflicting interpretations of these responses include a two-trap space-charge model [9], a four-trap space-charge model [2], and a dipole-deorientation model [3,4,7,8] in addition to the above-mentioned dipole + space-charge model [1,4,5]. Recently, a new interpretation for the TSD response (Fig.1) of samples charged at room temperature (RT) was suggested [10]: the first peak at T g is attributed to the orientation of dipoles in the field of the trapped charges, whereas the second peak is thought to originate from detrapping of the charges. In the present study, this qualitative model [10] and the previous dipole+space-charge model [1] are combined, further refined, and supported by several experimental results.

Minimum Weight Design for Structural Eigenvalue Problems by Optimal Control Theory

Abstract: The application of optimal control theory to minimum weight design of continuous one-dimensional structural elements subject to eigenvalue constraints is discussed. If not only the value of an eigenvalue is prescribed but also its position in the sequence of the ordered eigenvalues—for example, the critical buckling load of a column—the corresponding optimal control problem is shown to include necessarily all eigenvalues. Considering the unspecified eigenvalues as free parameters, necessary conditions for minimum weight design are derived. These conditions are compared with those obtained by use of variational methods. Attention is focused on the special case of multimodal solutions.

Radiation-induced charge transport in polymer electrets

Abstract: The mobilities of electrons and holes prove to be important parameters for the numerical results of our physical model. Experimental data for electron-beam-irradiated FEP, PETP, and PVDF foils could be clearly identified with three special cases of the general model: In FEP, the mobility of the electrons is small compared to the mobility of the holes, while in PETP the opposite is true. In PVDF, both charge carriers are mobile and their mobilities are of the same order. Quantitative agreement between theory and experiment has already been achieved for FEP [1]; parameter fitting for PETP and PVDF is presently under way,

Aufgaben und Lösungswege für die Datenverarbeitung in Einem Forschüngszentrum

Abstract: Als zentrale Dienstleistungseinrichtung fur wissenschaftlich-technische Aufgabenstellungen versorgt das Rechenzentrum der GSI die am Forschungs- und Entwicklungsprogramm der GSI beteiligten Arbeitsgruppen mit dem DV-spezifischen Leistungs- und Funktionsspektrum. Als Grosforschungseinrichtung der Bundesrepublik Deutschland und des Landes Hessen (Finanzierung 9:1) betreibt GSI mehrere Grosgerate, darunter den zur Zeit leistungsfahigsten Schwerionenbeschleuniger UNILAC (Universal Linear Accelerator).

Properties of Low-Density Polyethylene Produced with Organic Peroxides Together with Oxygen

Abstract: Both organic peroxides and molecular oxygen can be used for the high-pressure polymerization of ethylene. In order to answer the question if the product quality is influenced by the choice of the initiator, polyethylenes were produced in a stirred autoclave under 1700 bar at temperatures of 200–320°C with DTBP, oxygen, and with mixtures of both initiators. The products were compared for density, short- and long-chain branching, the average molar mass, and the molar mass distribution. Except for a slight increase in the number of long-chain branches, and as a consequence of this a slightly broader molar mass distribution, no significant changes of product properties could be noted. There are no objections against the combined use of peroxide and oxygen; for example, when the conversion from oxygen to peroxide takes place.

Moments of charge distributions in the reaction 92 Mo+ 92 Mo at 14.7 MeV/u

Abstract: First and second moments of primary charge distributions were determined as a function of the total kinetic energy loss in the reaction92Mo+92Mo at an incident energy of 14.7 MeV/u. Up to energy losses of 200 MeV no significant drift of the first moments is observed. The second moments are in agreement with predictions of a stochastic model derived at lower bombarding energies.