193,839 research outputs found
Hybrid SGP4 orbit propagator
Two-Line Elements (TLEs) continue to be the sole public source of orbiter
observations. The accuracy of TLE propagations through the Simplified General
Perturbations-4 (SGP4) software decreases dramatically as the propagation
horizon increases, and thus the period of validity of TLEs is very limited. As
a result, TLEs are gradually becoming insufficient for the growing demands of
Space Situational Awareness (SSA). We propose a technique, based on the hybrid
propagation methodology, aimed at extending TLE validity with minimal changes
to the current TLE-SGP4 system in a non-intrusive way. It requires that the
institution in possession of the osculating elements distributes hybrid TLEs,
HTLEs, which encapsulate the standard TLE and the model of its propagation
error. The validity extension can be accomplished when the end user processes
HTLEs through the hybrid SGP4 propagator, HSGP4, which comprises the standard
SGP4 and an error corrector.Comment: 18 pages, 4 figure
Anisotropic Inflation from Charged Scalar Fields
We consider models of inflation with U(1) gauge fields and charged scalar
fields including symmetry breaking potential, chaotic inflation and hybrid
inflation. We show that there exist attractor solutions where the anisotropies
produced during inflation becomes comparable to the slow-roll parameters. In
the models where the inflaton field is a charged scalar field the gauge field
becomes highly oscillatory at the end of inflation ending inflation quickly.
Furthermore, in charged hybrid inflation the onset of waterfall phase
transition at the end of inflation is affected significantly by the evolution
of the background gauge field. Rapid oscillations of the gauge field and its
coupling to inflaton can have interesting effects on preheating and
non-Gaussianities.Comment: minor changes, references added, figures are modified, conforms JCAP
published versio
Langevin PDF simulation of particle deposition in a turbulent pipe flow
The paper deals with the description of particle deposition on walls from a
turbulent flow over a large range of particle diameter, using a Langevin PDF
model. The first aim of the work is to test how the present Langevin model is
able to describe this phenomenon and to outline the physical as- pects which
play a major role in particle deposition. The general features and
characteristics of the present stochastic model are first recalled. Then,
results obtained with the standard form of the model are presented along with
an analysis which has been carried out to check the sensitivity of the
predictions on different mean fluid quantities. These results show that the
physical repre- sentation of the near-wall physics has to be improved and that,
in particular, one possible route is to introduce specific features related to
the near-wall coherent structures. In the following, we propose a simple
phenomenological model that introduces some of the effects due to the presence
of turbulent coherent structures on particles in a thin layer close to the
wall. The results obtained with this phenomenological model are in good
agreement with experimental evidence and this suggests to pursue in that
direction, towards the development of more general and rigorous stochastic
models that provide a link between a geometrical description of turbulent flow
and a statistical one.Comment: 40 pages, 8 figure
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