2,117 research outputs found
Probing spatial homogeneity with LTB models: a detailed discussion
Do current observational data confirm the assumptions of the cosmological
principle, or is there statistical evidence for deviations from spatial
homogeneity on large scales? To address these questions, we developed a
flexible framework based on spherically symmetric, but radially inhomogeneous
Lemaitre-Tolman-Bondi (LTB) models with synchronous Big Bang. We expanded the
(local) matter density profile in terms of flexible interpolation schemes and
orthonormal polynomials. A Monte Carlo technique in combination with recent
observational data was used to systematically vary the shape of these profiles.
In the first part of this article, we reconsider giant LTB voids without dark
energy to investigate whether extremely fine-tuned mass profiles can reconcile
these models with current data. While the local Hubble rate and supernovae can
easily be fitted without dark energy, however, model-independent constraints
from the Planck 2013 data require an unrealistically low local Hubble rate,
which is strongly inconsistent with the observed value; this result agrees well
with previous studies. In the second part, we explain why it seems natural to
extend our framework by a non-zero cosmological constant, which then allows us
to perform general tests of the cosmological principle. Moreover, these
extended models facilitate explorating whether fluctuations in the local matter
density profile might potentially alleviate the tension between local and
global measurements of the Hubble rate, as derived from Cepheid-calibrated type
Ia supernovae and CMB experiments, respectively. We show that current data
provide no evidence for deviations from spatial homogeneity on large scales.
More accurate constraints are required to ultimately confirm the validity of
the cosmological principle, however.Comment: 18 pages, 12 figures, 2 tables; accepted for publication in A&
An exactly solvable quantum-lattice model with a tunable degree of nonlocality
An array of N subsequent Laguerre polynomials is interpreted as an
eigenvector of a non-Hermitian tridiagonal Hamiltonian with real spectrum
or, better said, of an exactly solvable N-site-lattice cryptohermitian
Hamiltonian whose spectrum is known as equal to the set of zeros of the N-th
Laguerre polynomial. The two key problems (viz., the one of the ambiguity and
the one of the closed-form construction of all of the eligible inner products
which make Hermitian in the respective {\em ad hoc} Hilbert spaces) are
discussed. Then, for illustration, the first four simplest, parametric
definitions of inner products with and are explicitly
displayed. In mathematical terms these alternative inner products may be
perceived as alternative Hermitian conjugations of the initial N-plet of
Laguerre polynomials. In physical terms the parameter may be interpreted as
a measure of the "smearing of the lattice coordinates" in the model.Comment: 35 p
A Wiener-Laguerre model of VIV forces given recent cylinder velocities
Slender structures immersed in a cross flow can experience vibrations induced
by vortex shedding (VIV), which cause fatigue damage and other problems. VIV
models in engineering use today tend to operate in the frequency domain. A time
domain model would allow to capture the chaotic nature of VIV and to model
interactions with other loads and non-linearities. Such a model was developed
in the present work: for each cross section, recent velocity history is
compressed using Laguerre polynomials. The compressed information is used to
enter an interpolation function to predict the instantaneous force, allowing to
step the dynamic analysis. An offshore riser was modeled in this way: Some
analyses provided an unusually fine level of realism, while in other analyses,
the riser fell into an unphysical pattern of vibration. It is concluded that
the concept is promissing, yet that more work is needed to understand orbit
stability and related issues, in order to further progress towards an
engineering tool
Stochastic model for the 3D microstructure of pristine and cyclically aged cathodes in Li-ion batteries
It is well-known that the microstructure of electrodes in lithium-ion
batteries strongly affects their performance. Vice versa, the microstructure
can exhibit strong changes during the usage of the battery due to aging
effects. For a better understanding of these effects, mathematical analysis and
modeling has turned out to be of great help. In particular, stochastic 3D
microstructure models have proven to be a powerful and very flexible tool to
generate various kinds of particle-based structures. Recently, such models have
been proposed for the microstructure of anodes in lithium-ion energy and power
cells. In the present paper, we describe a stochastic modeling approach for the
3D microstructure of cathodes in a lithium-ion energy cell, which differs
significantly from the one observed in anodes. The model for the cathode data
enhances the ideas of the anode models, which have been developed so far. It is
calibrated using 3D tomographic image data from pristine as well as two aged
cathodes. A validation based on morphological image characteristics shows that
the model is able to realistically describe both, the microstructure of
pristine and aged cathodes. Thus, we conclude that the model is suitable to
generate virtual, but realistic microstructures of lithium-ion cathodes
Thermal disequilibration of ions and electrons by collisionless plasma turbulence
Does overall thermal equilibrium exist between ions and electrons in a weakly
collisional, magnetised, turbulent plasma---and, if not, how is thermal energy
partitioned between ions and electrons? This is a fundamental question in
plasma physics, the answer to which is also crucial for predicting the
properties of far-distant astronomical objects such as accretion discs around
black holes. In the context of discs, this question was posed nearly two
decades ago and has since generated a sizeable literature. Here we provide the
answer for the case in which energy is injected into the plasma via Alfv\'enic
turbulence: collisionless turbulent heating typically acts to disequilibrate
the ion and electron temperatures. Numerical simulations using a hybrid
fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an
increasing function of the thermal-to-magnetic energy ratio,
: it ranges from at to at
least for . This energy partition is
approximately insensitive to the ion-to-electron temperature ratio
. Thus, in the absence of other equilibrating
mechanisms, a collisionless plasma system heated via Alfv\'enic turbulence will
tend towards a nonequilibrium state in which one of the species is
significantly hotter than the other, viz., hotter ions at high
, hotter electrons at low . Spectra of
electromagnetic fields and the ion distribution function in 5D phase space
exhibit an interesting new magnetically dominated regime at high and
a tendency for the ion heating to be mediated by nonlinear phase mixing
("entropy cascade") when and by linear phase mixing
(Landau damping) when $\beta_\mathrm{i}\gg1
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