3,657 research outputs found
Forming simulation of a thermoplastic commingled woven textile on a double dome
This paper presents thermoforming experiments and FE simulations of a commingled glass-PP woven composite on a double dome geometry, with the aim of assessing the correspondence of predicted and experimental shear angles. Large local deformations - especially in-plane shear, i.e. relative rotation between the two yarn families â occur when draping a textile on a three dimensional part and eventually unwanted phenomena like wrinkling or tearing may occur. The macroscopic drape behaviour of a weave is generally subdivided into: 1) The high tensile resistance along the yarn directions, expressed as non-linear stress-strain curves, and 2) The shear resistance, expressed as non-linear shear force versus shear angle curves. The constitutive model is constituted of a dedicated non-orthogonal hypo-elastic shear resistance model, previously described in [1, 2], combined with truss elements that represent the high tensile resistance along the yarn directions. This model is implemented in a user subroutine of the ABAQUS explicit FE solver. The material parameters have been identified via textile biaxial tensile tests at room temperature and bias extension tests at 200°. Thermoforming experiments are performed on a rectangular blank with the warp direction along the second symmetry plane of the tool, with a preheating temperature of 200°C, a constant mold temperature of about 70°C, and a blankholder ring. It was concluded that the shear angles were fairly well predicted for this particular case study, which could be expected in view of the fact that no wrinkles had formed during the thermoforming experiment
Current Accuracy of Augmented Reality Neuronavigation Systems: Systematic Review and Meta-Analysis
BACKGROUND
Augmented reality neuronavigation (ARN) systems can overlay three-dimensional anatomy and pathology without the need for a two-dimensional external monitor. Accuracy is crucial for their clinical applicability. We performed a systematic review regarding the reported accuracy of ARN systems and compared them with the accuracy of conventional infrared neuronavigation (CIN).
OBJECTIVE
Explore the current navigation accuracy of ARN systems and compare them with CIN.
METHODS
Pubmed and Embase were searched for ARN and CIN systems. For ARN: type of system, method of patient-to-image registration, accuracy method and accuracy of the system was noted. For CIN: navigation accuracy, expressed as target registration error (TRE), was noted. A meta-analysis was performed comparing the TRE of ARN and CIN systems.
RESULTS
35 studies were included, 12 for ARN and 23 for CIN. ARN systems were divided into head-mounted display and heads-up display. In ARN, four methods were encountered for patient-to-image registration, of which point-pair matching was the one most frequently used. Five methods for assessing accuracy were described. 94 TRE measurements of ARN systems were compared with 9058 TRE measurements of CIN systems. Mean TRE was 2.5 mm (CI 95% 0.7 - 4.4) for ARN systems and 2.6 mm (CI 95% 2.1 - 3.1) for CIN systems.
CONCLUSIONS
In ARN, there seems to be lack of agreement regarding the best method to assess accuracy. Nevertheless, ARN systems seem able to achieve an accuracy comparable with CIN systems. Future studies should be prospective and compare TREs which should be measured in a standardized fashion
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field
We observe mixing between two-electron singlet and triplet states in a double
quantum dot, caused by interactions with nuclear spins in the host
semiconductor. This mixing is suppressed by applying a small magnetic field, or
by increasing the interdot tunnel coupling and thereby the singlet-triplet
splitting. Electron transport involving transitions between triplets and
singlets in turn polarizes the nuclei, resulting in striking bistabilities. We
extract from the fluctuating nuclear field a limitation on the time-averaged
spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction
will therefore be crucial for the coherent manipulation of individual electron
spins.Comment: 4 pages main text, 4 figure
The first accurate parallax distance to a black hole
Using astrometric VLBI observations, we have determined the parallax of the
black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds,
corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the
previously accepted value. This model-independent estimate is the most accurate
distance to a Galactic stellar-mass black hole measured to date. With this new
distance, we confirm that the source was not super-Eddington during its 1989
outburst. The fitted distance and proper motion imply that the black hole in
this system likely formed in a supernova, with the peculiar velocity being
consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred
to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations
of a larger sample of such systems would provide useful insights into the
formation and properties of accreting stellar-mass black holes.Comment: Accepted for publication in ApJ Letters. 6 pages, 2 figure
Holonomic quantum computing in symmetry-protected ground states of spin chains
While solid-state devices offer naturally reliable hardware for modern
classical computers, thus far quantum information processors resemble vacuum
tube computers in being neither reliable nor scalable. Strongly correlated many
body states stabilized in topologically ordered matter offer the possibility of
naturally fault tolerant computing, but are both challenging to engineer and
coherently control and cannot be easily adapted to different physical
platforms. We propose an architecture which achieves some of the robustness
properties of topological models but with a drastically simpler construction.
Quantum information is stored in the symmetry-protected degenerate ground
states of spin-1 chains, while quantum gates are performed by adiabatic
non-Abelian holonomies using only single-site fields and nearest-neighbor
couplings. Gate operations respect the symmetry, and so inherit some protection
from noise and disorder from the symmetry-protected ground states.Comment: 19 pages, 4 figures. v2: published versio
Spin states of the first four holes in a silicon nanowire quantum dot
We report measurements on a silicon nanowire quantum dot with a clarity that
allows for a complete understanding of the spin states of the first four holes.
First, we show control of the hole number down to one. Detailed measurements at
perpendicular magnetic fields reveal the Zeeman splitting of a single hole in
silicon. We are able to determine the ground-state spin configuration for one
to four holes occupying the quantum dot and find a spin filling with
alternating spin-down and spin-up holes, which is confirmed by
magnetospectroscopy up to 9T. Additionally, a so far inexplicable feature in
single-charge quantum dots in many materials systems is analyzed in detail. We
observe excitations of the zero-hole ground-state energy of the quantum dot,
which cannot correspond to electronic or Zeeman states. We show that the most
likely explanation is acoustic phonon emission to a cavity between the two
contacts to the nanowire.Comment: 24 pages, 8 figures, both including supporting informatio
Pulsar Kicks and Spin Tilts in the Close Double Neutron Stars PSR J0737-3039, PSR B1534+12 and PSR B1913+16
In view of the recent measurement of the scintillation velocity for PSR
J0737-3039, we examine the complete set of constraints imposed on the pulsar B
natal kicks (magnitude and orientation) and predict the most favorable pulsar
kick velocity and spin tilt for both isotropic and polar kicks. Our analysis
takes into account both currently unknown parameters: the orientation of the
orbital plane on the sky (Omega) and the radial component of the systemic
velocity (V_r). Assuming that the system's peculiar velocity is entirely due to
the second supernova explosion, we find that the system may have crossed the
Galactic plane multiple times since the birth of the second neutron star and
that the post-supernova peculiar velocity could have been as high as 1200km/s.
We also confirm the absolute lower and upper limits on the physical parameters
derived in our earlier study. For specific combinations of the two unknown
parameters Omega and V_r, however, we find much tighter constraints on the
pre-supernova binary configuration and natal kicks imparted to pulsar B, as
well as on the age of system. Once Omega is measured in the coming year, it
will be straightforward to use the results presented here to further constrain
the natal kicks and the spin-tilt predictions. We complete our comprehensive
study and derive similar constraints and spin-tilt predictions for PSR
B1534+12, where the only free parameter is V_r. Lastly, for PSR B1913+16, we
update the progenitor and kick constraints using the measured pulsar spin tilt
and allowing for Roche-lobe overflow from the progenitor of the pulsar
companion.Comment: Replaced Fig. 16 with corrected version. See ApJ 616, p. 414 for
high-resolution figures and notes added in proo
Very high quality factor measured in annealed fused silica
We present the results of quality factor measurements for rod samples made of
fused silica. To decrease the dissipation we annealed our samples. The highest
quality factor that we observed was for a mode at
384 Hz. This is the highest published value of in fused silica measured to
date.Comment: 8 pages, 2 figure
Resonant tunnelling features in the transport spectroscopy of quantum dots
We present a review of features due to resonant tunnelling in transport
spectroscopy experiments on quantum dots and single donors. The review covers
features attributable to intrinsic properties of the dot as well as extrinsic
effects, with a focus on the most common operating conditions. We describe
several phenomena that can lead to apparently identical signatures in a bias
spectroscopy measurement, with the aim of providing experimental methods to
distinguish between their different physical origins. The correct
classification of the resonant tunnelling features is an essential requirement
to understand the details of the confining potential or predict the performance
of the dot for quantum information processing.Comment: 18 pages, 7 figures. Short review article submitted to
Nanotechnology, special issue on 'Quantum Science and Technology at the
Nanoscale
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