310 research outputs found
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Numerical optimisation of mechanical ring reinforcement for bulk high-Temperature superconductors
Abstract
The finite element method has been used extensively in recent years to solve various problems related to applied superconductivity and provides a useful tool for analysing and predicting experimental results. Based on a recently-developed modelling framework, implemented in the finite element software package COMSOL Multiphysics, investigations on the minimum ring reinforcement required to prevent mechanical failure in bulk high-temperature superconducting magnets have been carried out. Assuming homogeneous Jc
(B,T) across the bulk sample irrespective of its dimensions, the maximum magnetic stresses experienced, and the minimum ring thickness required to prevent the hoop and radial stresses from exceeding the tensile strength of the bulk superconductor have been determined for varying values of the Young’s modulus, radius, height and temperature of a representative single-grain Ag-containing Gd-Ba-Cu-O bulk sample. This comprehensive analysis details the influence each of these key parameters has on the magnetic stress and hence their impact on the necessary ring thickness to prevent mechanical failure in any given system, i.e., for any combination of material properties and sample dimensions.</jats:p
Measuring cluster peculiar velocities with the Sunyaev-Zeldovich effects: scaling relations and systematics
The fluctuations in the Cosmic Microwave Background (CMB) intensity due to
the Sunyaev-Zeldovich (SZ) effect are the sum of a thermal and a kinetic
contribution. Separating the two components to measure the peculiar velocity of
galaxy clusters requires radio and microwave observations at three or more
frequencies, and knowledge of the temperature T_e of the intracluster medium
weighted by the electron number density. To quantify the systematics of this
procedure, we extract a sample of 117 massive clusters at redshift z=0 from an
N-body hydrodynamical simulation, with 2x480^3 particles, of a cosmological
volume 192 Mpc/h on a side of a flat Cold Dark Matter model with Omega_0=0.3
and Lambda=0.7. Our simulation includes radiative cooling, star formation and
the effect of feedback and galactic winds from supernovae. We find that (1) our
simulated clusters reproduce the observed scaling relations between X-ray and
SZ properties; (2) bulk flows internal to the intracluster medium affect the
velocity estimate by less than 200 km/s in 93 per cent of the cases; (3) using
the X-ray emission weighted temperature, as an estimate of T_e, can
overestimate the peculiar velocity by 20-50 per cent, if the microwave
observations do not spatially resolve the cluster. For spatially resolved
clusters, the assumptions on the spatial distribution of the ICM, required to
separate the two SZ components, still produce a velocity overestimate of 10-20
per cent, even with an unbiased measure of T_e. Thanks to the large size of our
cluster samples, these results set a robust lower limit of 200 km/s to the
systematic errors that will affect upcoming measures of cluster peculiar
velocities with the SZ effect.Comment: 14 pages, 12 figures, MNRAS, in press. Figures 3 and 4 now contain
more recent observational data. Other minor revisions according to referee's
comment
The Gaussian cell two-point "energy-like" equation: Application to large scale galaxy redshift and peculiar motion surveys
We introduce a simple linear equation relating the line-of-sight peculiar
velocity and density contrast correlation functions. The relation, which we
call the "Gaussian cell two-point energy-like equation", is valid in the
distant-observer-limit and requires Gaussian smoothed fields. In the variance
case, i.e., at zero lag, the equation is similar in its mathematical form to
the Layzer-Irvine cosmic energy equation. \beta estimation with this equation
from the PSC redshift galaxy survey and the SEcat catalogue of peculiar
velocities is carried out, returning a value of \beta = 0.44+-0.08. The
applicability of the method for the 6dF galaxy redshift and peculiar motions
survey is demonstrated with mock data where it is shown that beta could be
determined with ~5% accuracy. The prospects for constraining the dark energy
equation of state with this method from the kinematic and thermal
Sunyaev-Zel'dovich cluster surveys are discussed. The equation is also used to
construct a nonparametric mass density power spectrum estimator from peculiar
velocity data.Comment: Accepted for publication in MNRA
Modeling intracranial aneurysm stability and growth: An integrative mechanobiological framework for clinical cases
We present a novel patient-specific fluid-solid-growth framework to model the mechanobiological state of clinically detected intracranial aneurysms (IAs) and their evolution. The artery and IA sac are modeled as thick-walled, non-linear elastic fiber-reinforced composites. We represent the undulation distribution of collagen fibers: the adventitia of the healthy artery is modeled as a protective sheath whereas the aneurysm sac is modeled to bear load within physiological range of pressures. Initially, we assume the detected IA is stable and then consider two flow-related mechanisms to drive enlargement: (1) low wall shear stress; (2) dysfunctional endothelium which is associated with regions of high oscillatory flow. Localized collagen degradation and remodelling gives rise to formation of secondary blebs on the aneurysm dome. Restabilization of blebs is achieved by remodelling of the homeostatic collagen fiber stretch distribution. This integrative mechanobiological modelling workflow provides a step towards a personalized risk-assessment and treatment of clinically detected IAs
Non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus
The objective of this work was to determine the linear and non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus and to study the changes in mechanical properties throughout the thickness of the thrombus. Samples are gathered from thrombi of seven patients. Linear viscoelastic data from oscillatory shear experiments show that the change of properties throughout the thrombus is different for each thrombus. Furthermore the variations found within one thrombus are of the same order of magnitude as the variation between patients. To study the non-linear regime, stress relaxation experiments are performed. To describe the phenomena observed experimentally, a non-linear multimode model is presented. The parameters for this model are obtained by fitting this model successfully to the experiments. The model cannot only describe the average stress response for all thrombus samples but also the highest and lowest stress responses. To determine the influence on the wall stress of the behavior observed the model proposed needs to implemented in the finite element wall stress analysis
Trouble for cluster parameter estimation from blind SZ surveys?
(Abriged version) The Sunyaev-Zel'dovich (SZ) effect of galaxy clusters is a
tool to measure three quantities: Compton parameter, electron temperature, and
cluster peculiar velocity. However, a major problem is non-removed
contamination by astrophysical sources that emit in the SZ frequencies. This
includes interstellar dust emission, infra-red (IR) galaxies, and radio sources
in addition to primary Cosmic Microwave Background (CMB) anisotropies. The
three former contaminations induce systematic shifts in the three SZ
parameters. In this study, we carefully estimated, both for a large beam
experiment (namely Planck Surveyor) and a small beam experiment (ACT-like), the
systematic errors that result if a fraction of the expected levels of emission
from dust, IR galaxies, and radio sources remains non-removed. ...Comment: Accepted version (09/03/2005) in press in A&A. Modified content and
title, SPT section remove
Genetic Evidence for a Tight Cooperation of TatB and TatC during Productive Recognition of Twin-Arginine (Tat) Signal Peptides in Escherichia coli
The twin arginine translocation (Tat) pathway transports folded proteins across the cytoplasmic membrane of bacteria. Tat signal peptides contain a consensus motif (S/T-R-R-X-F-L-K) that is thought to play a crucial role in substrate recognition by the Tat translocase. Replacement of the phenylalanine at the +2 consensus position in the signal peptide of a Tat-specific reporter protein (TorA-MalE) by aspartate blocked export of the corresponding TorA(D+2)-MalE precursor, indicating that this mutation prevents a productive binding of the TorA(D+2) signal peptide to the Tat translocase. Mutations were identified in the extreme amino-terminal regions of TatB and TatC that synergistically suppressed the export defect of TorA(D+2)-MalE when present in pairwise or triple combinations. The observed synergistic suppression activities were even more pronounced in the restoration of membrane translocation of another export-defective precursor, TorA(KQ)-MalE, in which the conserved twin arginine residues had been replaced by lysine-glutamine. Collectively, these findings indicate that the extreme amino-terminal regions of TatB and TatC cooperate tightly during recognition and productive binding of Tat-dependent precursor proteins and, furthermore, that TatB and TatC are both involved in the formation of a specific signal peptide binding site that reaches out as far as the end of the TatB transmembrane segment
Whole-body diffusion-weighted imaging for staging malignant lymphoma in children
CT is currently the mainstay in staging malignant lymphoma in children, but the risk of second neoplasms due to ionizing radiation associated with CT is not negligible. Whole-body MRI techniques and whole-body diffusion-weighted imaging (DWI) in particular, may be a good radiation-free alternative to CT. DWI is characterized by high sensitivity for the detection of lesions and allows quantitative assessment of diffusion that may aid in the evaluation of malignant lymphomas. This article will review whole-body MRI techniques for staging malignant lymphoma with emphasis on whole-body DWI. Furthermore, future considerations and challenges in whole-body DWI will be discussed
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Dark Energy Survey year 1 results: Joint analysis of galaxy clustering, galaxy lensing, and CMB lensing two-point functions
We perform a joint analysis of the auto and cross-correlations between three
cosmic fields: the galaxy density field, the galaxy weak lensing shear field,
and the cosmic microwave background (CMB) weak lensing convergence field. These
three fields are measured using roughly 1300 sq. deg. of overlapping optical
imaging data from first year observations of the Dark Energy Survey and
millimeter-wave observations of the CMB from both the South Pole Telescope
Sunyaev-Zel'dovich survey and Planck. We present cosmological constraints from
the joint analysis of the two-point correlation functions between galaxy
density and galaxy shear with CMB lensing. We test for consistency between
these measurements and the DES-only two-point function measurements, finding no
evidence for inconsistency in the context of flat CDM cosmological
models. Performing a joint analysis of five of the possible correlation
functions between these fields (excluding only the CMB lensing autospectrum)
yields and . We test
for consistency between these five correlation function measurements and the
Planck-only measurement of the CMB lensing autospectrum, again finding no
evidence for inconsistency in the context of flat CDM models.
Combining constraints from all six two-point functions yields
and .
These results provide a powerful test and confirmation of the results from the
first year DES joint-probes analysis
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