3,377 research outputs found
Generalized Signal Models and Direct FID-Based Dielectric Parameter Retrieval in MRI
In this paper we present full-wave signal models for magnetic and electric
field measurements in magnetic resonance imaging (MRI). Our analysis is based
on a scattering formalism in which the presence of an object or body is taken
into account via an electric scattering source. We show that these signal
models can be evaluated, provided the Green's tensors of the background field
are known along with the dielectric parameters of the object and the
magnetization within the excited part of the object. Furthermore, explicit
signal expressions are derived in case of a small homogeneous ball that is
embedded in free space and for which the quasi-static Born approximation can be
applied. The conductivity and permittivity of the ball appear as explicit
parameters in the resulting signal models and allow us to study the sensitivity
of the measured signals with respect to these dielectric parameters. Moreover,
for free induction decay signals we show that under certain conditions it is
possible to retrieve the dielectric parameters of the ball from
noise-contaminated induction decay signals that are based on electric or
magnetic field measurements
Integrating Dynamics and Wear Modelling to Predict Railway Wheel Profile Evolution
The aim of the work described was to predict wheel
profile evolution by integrating multi-body dynamics
simulations of a wheelset with a wear model.
The wear modelling approach is based on a wear
index commonly used in rail wear predictions. This
assumes wear is proportional to Tγ, where T is tractive
force and γ is slip at the wheel/rail interface. Twin disc
testing of rail and wheel materials was carried out to
generate wear coefficients for use in the model.
The modelling code is interfaced with
ADAMS/Rail, which produces multi-body dynamics
simulations of a railway wheelset and contact conditions
at the wheel/rail interface. Simplified theory of rolling
contact is used to discretise the contact patches
produced by ADAMS/Rail and calculate traction and
slip within each.
The wear model combines the simplified theory of
rolling contact, ADAMS/Rail output and the wear
coefficients to predict the wear and hence the change of
wheel profile for given track layouts
Ecophysiological traits of grasses: resolving the effects of photosynthetic pathway and phylogeny
C4 photosynthesis is an important example of convergent evolution in plants, having arisen in eudicots, monocots and diatoms. Comparisons between such diverse groups are confounded by phylogenetic and ecological differences, so that only broad generalisations can be made about the role of C4 photosynthesis in
determining ecophysiological traits. However, 60% of C4 species occur in the grasses (Poaceae) and molecular phylogenetic techniques confirm that there are between 8 and 17 independent origins of C4 photosynthesis in the Poaceae. In a screening experiment, we compared leaf physiology and growth traits across several major
independent C3 & C4 groups within the Poaceae, asking 1) which traits differ consistently between photosynthetic
types and 2) which traits differ consistently between clades within each photosynthetic type
Performance of polarimetric beamformers for phased array radio telescopes
The results of four recently introduced beamforming schemes for phased array systems are discussed, each of which is capable to provide high sensitivity and accurate polarimetric performance of array-based radio telescopes. Ideally, a radio polarimeter should recover the actual polarization state of the celestial source, and thus compensate for unwanted polarization degradation effects which are intrinsic to the instrument. In this paper, we compare the proposed beamforming schemes through an example of a practical phased array system (APERTIF prototype) and demonstrate that the optimal beamformer, the max-SLNR beamformer, the eigenvector beamformer, and the bi-scalar beamformer are sensitivity equivalent but lead to different polarization state solutions, some of which are sub-optimal
Efficient Prediction of Array Element Patterns Using Physics-Based Expansions and a Single Far-Field Measurement
A method is proposed to predict the antenna array beam through employing a relatively small set of physics-based basis functions-called characteristic basis function patterns (CBFPs)-for modeling the embedded element patterns. The primary CBFP can be measured or extracted from numerical simulations, while additional (secondary) CBFPs are derived from the primary one. Furthermore, each numerically generated CBFP, which is typically simulated/measured for discrete directions only, can in turn be approximated by analytical basis functions with fixed expansion coefficients to evaluate the resulting array pattern at any angle through interpolation. This hierarchical basis reduces the number of unknown expansion coefficients significantly. Accordingly, the CBFP expansion coefficients can be determined through a single far-field measurement of only a few reference sources in the field of view. This is particularly important for multibeam array applications where only a limited number of reference sources are available for predicting the beam shape. Furthermore, this instantaneous beam calibration is fast, i.e., potentially capable to speed up the array calibration by one or two orders of magnitude, which is particularly important if the antenna radiation characteristics are subject to drifts
Polarimetry With Phased Array Antennas: Theoretical Framework and Definitions
For phased array receivers, the accuracy with which the polarization state of a received signal can be measured depends on the antenna configuration, array calibration process, and beamforming algorithms. A signal and noise model for a dual-polarized array is developed and related to standard polarimetric antenna figures of merit, and the ideal polarimetrically calibrated, maximum-sensitivity beamforming solution for a dual-polarized phased array feed is derived. A practical polarimetric beamformer solution that does not require exact knowledge of the array polarimetric response is shown to be equivalent to the optimal solution in the sense that when the practical beamformers are calibrated, the optimal solution is obtained. To provide a rough initial polarimetric calibration for the practical beamformer solution, an approximate single-source polarimetric calibration method is developed. The modeled instrumental polarization error for a dipole phased array feed with the practical beamformer solution and single-source polarimetric calibration was -10 dB or lower over the array field of view for elements with alignments perturbed by random rotations with 5 degree standard deviation
Generic Bell correlation between arbitrary local algebras in quantum field theory
We prove that for any two commuting von Neumann algebras of infinite type,
the open set of Bell correlated states for the two algebras is norm dense. We
then apply this result to algebraic quantum field theory -- where all local
algebras are of infinite type -- in order to show that for any two spacelike
separated regions, there is an open dense set of field states that dictate Bell
correlations between the regions. We also show that any vector state cyclic for
one of a pair of commuting nonabelian von Neumann algebras is entangled (i.e.,
nonseparable) across the algebras -- from which it follows that every field
state with bounded energy is entangled across any two spacelike separated
regions.Comment: Third version; correction in the proof of Proposition
Dyadic Characteristics and Intimate Partner Violence among Men Who Have Sex with Men
Objective: Although the research community has begun to recognize intimate partner violence (IPV) as an important issue in same-sex relationships, there has been a lack of attention to characteristics of these relationships that may be associated with IPV. In particular, there has been a lack of attention paid to the associations between dyadic characteristics and IPV in same-sex relationships. This paper examined associations between dyadic characteristics, including relationship satisfaction, communal coping and efficacy, and perpetrating and experiencing IPV among a sample of United States men who have sex with men (MSM). Methods: We collected data via an online survey with 528 MSM, who were greater than 18 years of age and reported at least one male sex partner in the last 12 months. The analysis examined dyadic factors associated with reporting of experiencing and perpetrating emotional violence, physical violence, and sexual violence. Results: The prevalence of violence in the sample ranged from nine percent reporting perpetrating sexual violence to 33% of men reporting experiencing emotional violence. MSM who reported greater satisfaction with their relationship or who reported a higher degree of concordance with their partner on lifestyle choices were less likely to report experiencing or perpetrating emotional violence. MSM who perceived a stigma to being in a male same-sex couple were less likely to report experiencing or perpetrating sexual violence. Conclusion: The results presented here demonstrate high levels of IPV among MSM and that dyadic characteristics are associated with the occurrence of IPV. Understanding relationship characteristics associated with increased IPV among same-sex male couples can contribute to the development of more accurate IPV screening tools, and more sensitively and appropriately designed intervention messages. [West J Emerg Med. 2011;12(3):324-332.
Comparative study of Clâ‚‚, Clâ‚‚/Oâ‚‚, and Clâ‚‚/Nâ‚‚ inductively coupled plasma processes for etching of high-aspect-ratio photonic-crystal holes in InP
An extensive investigation has been performed on inductively coupled plasmaetching of InP. An important motivation for this work is the fabrication of high-aspect-ratio holes for photonic crystals. The essential chemistry is based on Clâ‚‚ with the addition of Nâ‚‚ or Oâ‚‚ for sidewall passivation. The influence of different process parameters such as gas flows, temperature,pressure, ion energy, and inductively coupled plasma power on the hole geometry is presented. It is concluded that photonic crystals can be etched with Clâ‚‚ only; however, temperature and pressure control is critical. Adding passivation gases largely broadens the window in the parameter space for hole etching. Most importantly, etching of narrow holes can be carried out at higher temperatures where the etching is mass limited and spontaneous etching of InP by Clâ‚‚ occurs.Part of this research is supported by NanoNed, a technology program of the Dutch
Ministry of Economic Affairs
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