499 research outputs found
The impact of the stellar evolution of single and binary stars on the global, dynamical evolution of dense star clusters across cosmic time
Sternhaufen im Universum stellen dichte, selbstgravitierende und typischerweise dynamisch kollidierende
Umgebungen dar, die aus Tausenden bis Millionen von Sternen bestehen. Sie bevölkern galaktische Scheiben,
Halos und sogar galaktische Zentren im gesamten Kosmos und bilden eine grundlegende Einheit in einer
Hierarchie der kosmischen Strukturbildung. AuĂerdem sind sie in der Regel viel dichter als ihre Wirtsgalaxie, was sie zu unglaublich faszinierenden astronomischen Objekten macht. Anders als ihre Umgebung
erleben Sterne und kompakte Objekte in Sternhaufen hÀufige dynamische Streuungen, bilden dynamische
Doppelsterne, verschmelzen unter Aussendung von Gravitationswellen, werden durch Dreikörperdynamik
herausgeschleudert und stoĂen in seltenen FĂ€llen sogar direkt zusammen. Infolgedessen sind Sternhaufen
Fabriken aller exotischen Doppelsterne, von z.B. Thorne-Zytkow-Objekten und kataklysmischen Variablen
bis hin zu kompakten Doppelsternen, beispielsweise Doppelsterne, die aus schwarzen Löchern und Neutronensternen bestehen. DarĂŒber hinaus fangen mit zunehmender Teilchenzahl einzigartige Gravitationseffekte
von kollidierenden Vielteilchensystemen an die frĂŒhe Entwicklung des Haufens zu dominieren, die zu
zusammenziehenden und zunehmend schneller rotierenden Kernen der Sternhaufen fĂŒhren, die bevorzugt
massereiche Sterne und kompakte Objeckte sowie Doppelsterne enthalten, und einem sich ausdehnenden
Halo aus Sternen und kompakten Objekten geringerer Masse. Sternhaufen sind daher nicht nur ein Labor fĂŒr
die Gravitationsvielteilchenphysik, sondern auch fĂŒr die Sternentwicklung von Einzel- und Doppelsternen
sowie hierarchischen Sternensystemen höherer Ordnung. Alle diese physikalischen Prozesse können nicht
isoliert betrachtet werden - sie verstÀrken sich in Sternhaufen gegenseitig und viele passieren auf Àhnlichen
Zeitskalen. In dieser Arbeit möchte ich den Einfluss der Sternentwicklung auf die globale Dynamik von
Sternhaufen mit Hilfe von direkten gravitativen N-Körper und Hénon-Typ Monte-Carlo Simulationen von
Sternhaufen genauer studieren. Ich konzentriere mich auf die Entwicklung von metallarmen Sternpopulationen (Population II), die in Kugelsternhaufen und extrem metallarme Sternpopulationen (Population III), die
die Àltesten Sternpopulationen im Universum bilden
Effects of temporal variability of disturbance on the succession in marine fouling communities in northern-central Chile
We investigated the effects of temporal variability in a disturbance regime on fouling communities at two study sites in a northern-central Chilean bay. Fouling assemblages grown on artificial settlement substrata were disturbed by mechanical removal of biomass at different time intervals. Using one single disturbance frequency (10 disturbance events over 5 months) we applied 7 different temporal disturbance treatments: a constant disturbance regime (identical intervals between disturbance events), and 6 variable treatments where both variableness and sequences of intervals between disturbance events were manipulated. Two levels of temporal variableness (low and high, i.e. disturbance events were either dispersed or highly clumped in time) in the disturbance regime were applied by modifying the time intervals between subsequent disturbance events. To investigate the temporal coupling between disturbance events and other ecological processes (e.g. larval supply and recruitment intensity), three different sequences of disturbance intervals were nested in each of the two levels of temporal variableness. Species richness, evenness, total abundance, and structure of communities that experienced the various disturbance regimes were compared at the end of the experiment (15 days after the last disturbance event). Disturbance strongly influenced the community structure and led to a decrease in evenness and total abundance but not species richness. In undisturbed reference communities, the dominant competitor Pyura chilensis (Tunicata) occupied most available space while this species was suppressed in all disturbed treatments. Surprisingly, neither temporal variableness in the disturbance regime nor the sequence of intervals between disturbance events had an effect on community structure. Temporal variability in high disturbance regimes may be of minor importance for fouling communities, because they are dominated by opportunistic species that are adapted to rapidly exploit available space
Computational modelling of gas-liquid-solid multiphase free surface flow with and without evaporation
Gas-liquid-solid multiphase systems are ubiquitous in engineering
applications, e.g. inkjet printing, spray drying and coating. Developing a
numerical framework for modelling these multiphase systems is of great
significance. An improved, resolved CFD-DEM framework is developed to model the
multiphase free surface flow with and without evaporation. An improved
capillary force model is developed to compute the capillary interactions for
partially floating particles at a free surface. Two well-known benchmark cases,
namely drag coefficient calculation and the single sphere settling, are
conducted to validate the resolved CFD-DEM model. It turns out that the
resolved CFD-DEM model developed in this paper can accurately calculate the
fluid-solid interactions and predict the trajectory of solid particles
interacting with the liquid phase. Numerical demonstrations, namely two
particles moving along a free surface when the liquid phase evaporates, and
particle transport and accumulations inside an evaporating sessile droplet show
the performance of the resolved model.Comment: 54 pages, 19 figures, 9 table
Variational free energy based macroscopical modeling of ferroelectroelasticity
In this paper, a thermodynamically consistent minimum-type variational model for ferroelectric materials in a macroscopical continuum approach is presented. The motivation for this results from the lack of models in the literature that have on the one hand a Helmholtz free energy based variational structure and on the other hand are able to represent all important characteristic phenomena of ferroelectrics under quasi-static conditions. First of all, a unified variational theory for the material response of dissipative electro-mechanical solids in line with the framework of the generalized standard materials (GSM) is outlined. A macroscopic ferroelectric model with microscopically motivated internal state variables representing the switching processes taking place at the material microscale is adapted to the above mentioned variational structure. Additionally, a mixed variational principle for the global electro-mechanical boundary value problem is introduced in order to embed the Helmholtz free energy based local theory in a suitable finite element formulation. The solution processes for the resulting local and global variational problems is described in detail to enable easy implementation. The capability of the presented methods to reproduce the real behavior of ferroelectric systems is demonstrated by numerical examples. Here, a comparison to experimental results from the literature is a particular focus
A Thermal Discrete Element Analysis of EU Solid Breeder Blanket subjected to Neutron Irradiation
Due to neutron irradiation, solid breeder blankets are subjected to complex
thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed
is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of
granular material, it exhibits strong coupling between temperature and stress
fields. In this paper, we study these thermo-mechanical problems by developing
a thermal discrete element method (Thermal-DEM). This proposed simulation tool
models each individual ceramic pebble as one element and considers grain-scale
thermo-mechanical interactions between elements. A small section of solid
breeder pebble bed in HCPB is modelled using thousands of individual pebbles
and subjected to volumetric heating profiles calculated from neutronics under
ITER-relevant conditions. We consider heat transfer at the grain-scale between
pebbles through both solid-to-solid contacts and the interstitial gas phase,
and we calculate stresses arising from thermal expansion of pebbles. The
overall effective conductivity of the bed depends on the resulting compressive
stress state during the neutronic heating. The thermal-DEM method proposed in
this study provides the access to the grain-scale information, which is
beneficial for HCPB design and breeder material optimization, and a better
understanding of overall thermo-mechanical responses of the breeder units under
fusion-relevant conditions.Comment: 6 Pages, 3 Tables, 4 Figures, Fusion Science and Technology, 201
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