19,352 research outputs found
A localised subgrid scale model for fluid dynamical simulations in astrophysics II: Application to type Ia supernovae
The dynamics of the explosive burning process is highly sensitive to the
flame speed model in numerical simulations of type Ia supernovae. Based upon
the hypothesis that the effective flame speed is determined by the unresolved
turbulent velocity fluctuations, we employ a new subgrid scale model which
includes a localised treatment of the energy transfer through the turbulence
cascade in combination with semi-statistical closures for the dissipation and
non-local transport of turbulence energy. In addition, subgrid scale buoyancy
effects are included. In the limit of negligible energy transfer and transport,
the dynamical model reduces to the Sharp-Wheeler relation. According to our
findings, the Sharp-Wheeler relation is insuffcient to account for the
complicated turbulent dynamics of flames in thermonuclear supernovae. The
application of a co-moving grid technique enables us to achieve very high
spatial resolution in the burning region. Turbulence is produced mostly at the
flame surface and in the interior ash regions. Consequently, there is a
pronounced anisotropy in the vicinity of the flame fronts. The localised
subgrid scale model predicts significantly enhanced energy generation and less
unburnt carbon and oxygen at low velocities compared to earlier simulations.Comment: 13 pages, 10 figures, accepted for publication in Astron. Astrophys.;
3D visualisations not included; complete PDF version can be downloaded from
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/SGSModel_II_AA.pd
Electronic states and transport properties in the Kronig-Penney model with correlated compositional and structural disorder
We study the structure of the electronic states and the transport properties
of a Kronig-Penney model with weak compositional and structural disorder. Using
a perturbative approach we obtain an analytical expression for the localisation
length which is valid for disorder with arbitrary correlations. We show how to
generate disorder with self- and cross-correlations and we analyse both the
known delocalisation effects of the long-range self-correlations and new
effects produced by cross-correlations. We finally discuss how both kinds of
correlations alter the transport properties in Kronig-Penney models of finite
size.Comment: 23 pages, 5 figure
Effective models for charge transport in DNA nanowires
The rapid progress in the field of molecular electronics has led to an
increasing interest on DNA oligomers as possible components of electronic
circuits at the nanoscale. For this, however, an understanding of charge
transfer and transport mechanisms in this molecule is required. Experiments
show that a large number of factors may influence the electronic properties of
DNA. Though full first principle approaches are the ideal tool for a
theoretical characterization of the structural and electronic properties of
DNA, the structural complexity of this molecule make these methods of limited
use. Consequently, model Hamiltonian approaches, which filter out single
factors influencing charge propagation in the double helix are highly valuable.
In this chapter, we give a review of different DNA models which are thought to
capture the influence of some of these factors. We will specifically focus on
static and dynamic disorder.Comment: to appear in "NanoBioTechnology: BioInspired device and materials of
the future". Edited by O. Shoseyov and I. Levy. Humana Press (2006
Local helioseismology and the active Sun
The goal of local helioseismology is to elicit three-dimensional information
about the sub-surface (or far-side) structure and dynamics of the Sun from
observations of the helioseismic wave field at the surface. The physical
quantities of interest include flows, sound-speed deviations and magnetic
fields. However, strong surface magnetic fields induce large perturbations to
the waves making inversions difficult to interpret. The purpose of this paper
is to outline the methods of analysis used in local helioseismology, review
discoveries associated with the magnetic Sun made using local helioseismology
from the past three years, and highlight the efforts towards imaging the
interior in the presence of strong magnetic fields.Comment: 6 pages, 4th HELAS International Conference, Lanzarote, Spain, 1-5
February 201
Time-distance analysis of the emerging active region NOAA 10790
We investigate the emergence of Active Region NOAA 10790 by means of time – distance helioseismology. Shallow regions of increased sound speed at the location of increased magnetic activity are observed, with regions becoming deeper at the locations of sunspot pores. We also see a long-lasting region of decreased sound speed located underneath the region of the flux emergence, possibly relating to a temperature perturbation due to magnetic quenching of eddy diffusivity, or to a dense flux tube. We detect and track an object in the subsurface layers of the Sun characterised by increased sound speed which could be related to emerging magnetic-flux and thus obtain a provisional estimate of the speed of emergence of around 1 km s−1
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