Showing papers by "Technische Universität Darmstadt published in 1989"

Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese

Abstract: Total-energy band calculations, including an antiferromagnetic extension of the fixed-spin-moment procedure, are used to study magnetovolume effects in bulk fcc iron and maganese. By constraining these systems to have a fixed total magnetic moment in a single-atom fcc unit cell, we find magnetovolume instabilities in the form of first-order transitions from nonmagnetic to ferromagnetic behavior. Constraining the moments to have fixed values in a CuAu unit cell of two atoms to allow for antiferromagnetic (and field-induced ferrimagnetic) order alters these instabilities and yields second-order transitions from nonmagnetic to antiferromagnetic behavior at volumes coincident with the equilibrium volumes for both metals.

A Self-organizing Associative Memory System for Control Applications

Abstract: The CMAC storage scheme has been used as a basis for a software implementation of an associative memory system AMS, which itself is a major part of the learning control loop LERNAS. A major disadvantage of this CMAC-concept is that the degree of local generalization (area of interpolation) is fixed. This paper deals with an algorithm for self-organizing variable generalization for the AKS, based on ideas of T. Kohonen.

The Two Roles of Nested Relations in the DASDBS Project

Abstract: The paper gives an overview of the nested relational model and its two roles in the Darmstadt Database System (DASDBS) project, which was started in 1982 to develop an extensible database architecture supporting a variety of application specific front-ends with a common kernel system. In its first role the nested relational model serves as a model for the kernel interface describing hierarchical storage clusters. In its second role the nested relational algebra appears as a basic language for a KL-ONE-oriented semantic data model at an object-oriented layer upon the kernel.

Numerical studies on cold fronts Part II: Orographic effects on gravity flows

Abstract: A two-dimensional nonhydrostatic numerical model was used to investigate the behaviour of a cold air gravity current, moving along complex terrain. It is found, that the model with a high horizontal and vertical resolution and with a closure scheme, using the turbulent kinetic energy, is suitable to simulate currents, which have the main features of those found in laboratory experiments.

Numerical studies on cold fronts Part I: Gravity flows in a neutral and stratified atmosphere

Abstract: We have used a two dimensional version of a nonhydrostatic mesoscale model to simulate atmospheric gravity currents for different thermal stratification. The horizontal and vertical grid increments are chosen such, that the major features of a current like head and elevated nose are resolvable.

Turbulence Manipulation to Increase Effective Reynolds Numbers in Vehicle Aerodynamics

Abstract: In the testing of small-scale models of road vehicles Reynolds number requirements usually are violated. Instead of increasing the freestream velocity (Mach number problems), the approach in the present paper makes use of Taylor's theory on the interdependence between the intensity and structure of freestream turbulence and the Reynolds number corresponding to a defined location of the point of transition. Two differently scaled principle models of a passenger car were tested in the wind tunnel. Reynolds numbers (based on overall length) varied from 1.7 x 10 to 5.2 X 10. The resulting drag coefficients (CD) decreased from 0.40 to 0.28, and the overall flow structure changed significantly, which can be seen from the pressure distributions. When a screen of adequate mesh and bar size in the nozzle outlet is used, pressure distributions corresponding to different Reynolds numbers nearly fall onto a single curve that is equivalent to an increase of the effective Reynolds number. Analogous to the critical Reynolds number of the sphere, a Reynolds number corresponding to a fixed value of CD can be defined.

Molecular Fluids and Mixtures

Abstract: On the basis of a WCA-like perturbation theory various potentials for polyatomic molecules are dis- cussed. These are multicenter potentials of the Lennard-Jones and the exp-6-8 type, and also other distributions than site- site potentials. For mixtures, the sensitivity of excess properties to the form of the potential and to shape effects is demonstrated. Various combining rules are discussed for the calculation of the potential between unlike partners from pure component properties.

Making the Partial Transitive Closure an Elementary Database Operation

Abstract: A framework for processing recursive queries is briefly described. In this framework, computation of partial transitive closures (PTC’s) is considered as an elementary and essentially new database operation which deals with the recursion. To realize this operation, a breath-first searching algorithm is proposed which works on a successor-list representation of the directed graph representating a base relation. The usage of these searching strategy and data structure leads to a very efficient database PTC algorithm. Since the algorithm processes each relevant successor-list only once, we show that it is not only computationally very efficient (O(e′) where e′; is the number of the relevant tuples to the query), but also most suitable in the context of databases (I/O’s). The locality of the computation allows a fairly simple implementation on top of the DASDBS kernel.

In-situ measurement of permeability of concrete cover by overpressure

Abstract: Various methods have been applied to concrete for assessing the permeability of concrete cover. A new (in–situ) method is presented, which is less affected by moisture and which is quick in use.

On-line identification of inertia, friction and gravitational forces applied to an industrial robot

Abstract: In this paper on-line identification results are presented which were obtained with an industrial robot. It is shown that physical parameters - inertia, friction, damping coefficients - and the gravitational forces for the robot joints can be estimated on-line. For the on-line identification the armature currents (process input signals) and the actuator velocities (process output signals) are measured. Least-squares algorithms are used for the parameter estimation.

Beta decay of22O

Abstract: The beta-gamma spectroscopic study of22O is presented. This nucleus, produced as a projectile-like fragment from the interaction of a 60 MeV/n40Ar beam with a Be target, has been separated by the LISE spectrometer. Several gamma rays from22O decay have been observed, from which a half-life of (2.25±0.15)s has been determined. Accurate excitation energies have been deduced for several states in22F. A partial beta decay scheme of22O has been established. Experimental results have been compared with shell model calculations.

Fast volatilization of some lanthanide and actinide elements from titanium surfaces

Abstract: The ad- and desorption of trace quantities of the lanthanide elements lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, and ytterbium and of the actinide elements americium and curium were investigated by thermochromatographic and isothermal measurements with hot titanium columns. The adsorption enthalpies deduced from the experimental data are in reasonable agreement with calculated values which indicate considerable variations in volatility under such conditions. More volatile elements can quickly be separated from less volatile elements by heating the mixture on titanium in high vacuum.

Fast determination of structure, homogeneity, and composition of targets for heavy-ion experiments from backscattering and pixe with 1.6-MeV protons

Abstract: An experimental setup has been installed to perform backscattering spectroscopy and proton induced X-ray emission for fast determinations of structures, homogeneities, and compositions including the identification of impurities in targets for heavy-ion experiments. With a well collimated beam of 1.6-MeV protons of 50 nA the measurement of a single target point takes only 5–10 min of irradiation time. Additionally, an automatic scanning of the whole target can be performed for a precise determination of the lateral distribution of these quantities. The elastic-scattering resonances for proton scattering on light elements are particularly useful for the efficient investigation of multilayer targets where the target material is sandwiched between carbon foils. The results enter into the interpretation of data from atomic and nuclear interaction studies at the UNILAC of GSI; they lead as well to developments in the preparation of heavy-ion targets for future experiments.

Using 100 GeV/nucleon accelerators for fusion and plasma experiments

Abstract: With larger E / M it becomes easier to design accelerators to produce high specific deposition power, although overall efficiency becomes poor. To re

Status report on the SIS/ESR project

Abstract: The heavy-ion synchrotron of GSI is nearing completion. Perspectives of beam properties in an early stage of operation are outlined together with a con

WKB theory of wave tunneling for Hermitian and nearly Hermitian vector systems of integral equations

Abstract: A general theory of wave tunneling in one dimension for Hermitian and nearly Hermitian vector systems of integral equations is presented. It describes mode conversion in terms of the general dielectric tensor of the medium and properly accounts for the forward and backward nature of the waves without regard to specific models. Energy conservation in the WKB approximation can be obtained for general Hermitian systems by the use of modified Furry rules that are similar to those used by Heading for second‐order differential equations. Wave energy absorption can then be calculated perturbatively using the conservation properties of the dominant Hermitian operator. Operational graphical rules are developed to construct global wave solutions and to determine the direction of energy flow for spatially disconnected roots. In principle, these rules could be applied to systems with arbitrary mode complexity. Coupling coefficients for wave tunneling problems with up to four interacting modes are calculated explicitly.

Access limitations to US databases for users. A critique of a report to the Commission of the European Communities

Abstract: In February 1988, a conference entitled ‘International Information Flows between Eastern and Western Europe: Towards Confidence, Mutual Understanding and Co‐operation’, was held at Arnoldshain, Schtmitten, in West Germany. Europe Speaks to Europe is a collection of papers given at the conference. This paper was presented by Professor Jorg Becker, a co‐editor of the hook, which is due to be published by Pergamon Press in July 1989.

Reduction of unnecessary dotted rules in the earley-algorithm

Abstract: With the high developed hardware from the PC’s today, there arise possibilities to implement programming environments on such kind of computers. To reduce the amount of calculation time and required memory space from implemented algorithms, new optimization approaches in the algorithm design are demanded. The purpose of this work is to explore and analyse possibilities to reduce the required memory space through elimination of superfluous grammar rules created during the process of recognition.

A survey of radiation transport effects in homogeneous targets heated by heavy-ion beams from the GSI darmstadt SIS

Abstract: A preliminary survey of possible simple experiments at the GSI, Darmstadt SIS, to bring out the role of thermal radiation transport, has been carried o

Narrow Positron Line Emission in Collisions of Very Heavy Ions

Abstract: Close collisions of very heavy ions at beam energies neighbouring the Coulomb barrier provide a particular situation in view of both nuclear and atomic aspects. The two extended nuclei of high charge impinge at a high initial velocity of β≃0.1 until they nearly come into touch. As a consequence, nuclear transfer and strong Coulomb excitation of collective nuclear modes simultanously takes place. Depending on their relative orientation the collision may even lead to partial nuclear overlap and a coherent nuclear volume of large size may be formed during a sticking period of 10-21 to 10-22 [ sec]; the actual existence of even longer living di-nuclear systems is not excluded. Independent of the nuclear formation the combined charge of Z U = Z1 + Z2 results in an extreme Coulomb potential forming a quasiatom for a period of ~10-21 sec. The inner electronic states become strongly localized ( 1/2≃100 [fm]) reaching binding energies which may exceed the critical value of 2m e c 2. The quasiatomic (QA) situation together with related nuclear aspects is most comprehensively described in [1].

Experiments on Low Energy Bhabha Scattering Search for Neutral Resonances in the MeV/c2 Mass Region

Abstract: In the last years, a series of experiments on low-energy Bhabha scattering was prompted by the evidence for correlated back-to-back e+e- emission, found in heavy-ion experiments [1–3]. As summarized in the previous contribution [4], several monoenergetic e+e- pairs, with sharp sum energies between 500 keV and 815 keV, were observed. Also reported was the detection of correlated back-to-back emission of two γ-quanta with a sum energy of 1062 keV [5]. One is led to conclude that one deals with the decay of various excited states of a composite extended object with internal structure into e+e- or γγ-pairs. Bearing this in mind, numerous experimental groups addressed themselves to the question of whether such a hypothetical e+e- object could be created in e+e- collisions and whether it would manifest its eventual decay as a resonance in the excitation function for Bhabha scattering (or, in the excitation function for anihilation-in-flight) [6–22]. For the sake of clarity, the list of references is restricted to experimental works only, already published or submitted for publication.

A real-time knowledge scheme for sensory-controlled robot assembly tasks

Abstract: In the field of assembly automation with industrial robots the research on the application of artificial intelligence techniques is getting increasing importance. This paper describes the hierarchical structure of a knowledge–based sensory-controlled robot assembly system under development which is capable to plan and execute assembly tasks under real–time requirements. The hybrid knowledge representation scheme combining the rule–based and object–oriented approach to represent the assembly domain–specific knowledge is discussed. Furtheron, the knowledge processing concept based upon the representation scheme is explained. A first prototype of the system has been implemented in a real robotic test–bed. Several peg/hole part mating sequences validated the capability of the system to execute assembly tasks in an uncertain environment using sensory information.