Gravitating discs around black holes

Fluid discs and tori around black holes are discussed within different approaches and with the emphasis on the role of disc gravity. First reviewed are the prospects of investigating the gravitational field of a black hole--disc system by analytical solutions of stationary, axially symmetric Einstein's equations. Then, more detailed considerations are focused to middle and outer parts of extended disc-like configurations where relativistic effects are small and the Newtonian description is adequate. Within general relativity, only a static case has been analysed in detail. Results are often very inspiring, however, simplifying assumptions must be imposed: ad hoc profiles of the disc density are commonly assumed and the effects of frame-dragging and completely lacking. Astrophysical discs (e.g. accretion discs in active galactic nuclei) typically extend far beyond the relativistic domain and are fairly diluted. However, self-gravity is still essential for their structure and evolution, as well as for their radiation emission and the impact on the environment around. For example, a nuclear star cluster in a galactic centre may bear various imprints of mutual star--disc interactions, which can be recognised in observational properties, such as the relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be available from http://www.iop.org/EJ/journal/CQ

Modulare Echtzeitsimulation in der elektrischen Energietechnik

The theoretical fundamentals of the following modules are discussed: Synchronous generator, asynchronous motor, six-pulse bridge, and circuit of concentrated elements. The algorithmic coupling of the modules by means of an iterative process and the numerical integration of the systems of differential equatins encountered, some of which are nonlinear, are described. Computer systems with digital signal processors are employed, which are described in some detail. Also described are the complete models with processor units, binary and analog input and output. The practical applicability of the concept and the constructed real-time models are illustrated by two applications each of the field of energy engineering (generator model for testing voltage control elements, simulation of the system 'pulse inverter and asynchronous motor'). The problems connected with the integration of the model in the closed control circuit are explained as well as the solution strategies applied. (orig.)In diesem Bericht werden die theoretischen Grundlagen der Teilmodelle Synchrongenerator, Asynchronmotor, Sechspulsbruecke und Schaltung aus konzentrierten Elementen erlaeutert. Die algorithmische Kopplung der Teileinheiten mittels iterativer Verfahren und die numerische Integration der auftretenden zum Teil nichtlinearen Differentialgleichungssysteme werden beschrieben. Fuer die Bereitstellung der Rechenleistung werden Rechnersysteme mit digitalen Signalprozessoren eingesetzt. Auf diese Klasse von Prozessoren und auf die verschiedenen verwendeten Rechnersysteme wird naeher eingegangen. Auch die kompletten Modelle mit Prozessorbaugruppen, binaeren und analogen Ein- und Ausgaben werden beschrieben. Die Praxistauglichkeit des vorgestellten Konzepts und der ausgefuehrten Echtzeitmodelle wird durch je zwei Anwendungen aus dem Bereich Energietechnik (Generatormodell zum Test von Spannungsreglern und als Teilkomponente eines analogen Netzmodells) und Antriebstechnik (Simulation eine Stromrichtermotors, Simulation des Systems Pulswechselrichter und Asynchronmaschine) illustriert. Dabei werden auch die jeweiligen Schwierigkeiten der Integration des Modells in den geschlossenen Regelkreis und die eingesetzten Loesungsstrategien erklaert. (orig.)SIGLEAvailable from TIB Hannover: H93B5245 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman