693 research outputs found
High efficiency thermionic converter studies
Research in thermionic energy conversion technology is reported. The objectives were to produce converters suitable for use in out of core space reactors, radioisotope generators, and solar satellites. The development of emitter electrodes that operate at low cesium pressure, stable low work function collector electrodes, and more efficient means of space charge neutralization were investigated to improve thermionic converter performance. Potential improvements in collector properties were noted with evaporated thin film barium oxide coatings. Experiments with cesium carbonate suggest this substance may provide optimum combinations of cesium and oxygen for thermionic conversion
Introduction to dynamical horizons in numerical relativity
This paper presents a quasi-local method of studying the physics of dynamical
black holes in numerical simulations. This is done within the dynamical horizon
framework, which extends the earlier work on isolated horizons to
time-dependent situations. In particular: (i) We locate various kinds of
marginal surfaces and study their time evolution. An important ingredient is
the calculation of the signature of the horizon, which can be either spacelike,
timelike, or null. (ii) We generalize the calculation of the black hole mass
and angular momentum, which were previously defined for axisymmetric isolated
horizons to dynamical situations. (iii) We calculate the source multipole
moments of the black hole which can be used to verify that the black hole
settles down to a Kerr solution. (iv) We also study the fluxes of energy
crossing the horizon, which describes how a black hole grows as it accretes
matter and/or radiation.
We describe our numerical implementation of these concepts and apply them to
three specific test cases, namely, the axisymmetric head-on collision of two
black holes, the axisymmetric collapse of a neutron star, and a
non-axisymmetric black hole collision with non-zero initial orbital angular
momentum.Comment: 20 pages, 16 figures, revtex4. Several smaller changes, some didactic
content shortene
Numerical relativity with characteristic evolution, using six angular patches
The characteristic approach to numerical relativity is a useful tool in
evolving gravitational systems. In the past this has been implemented using two
patches of stereographic angular coordinates. In other applications, a
six-patch angular coordinate system has proved effective. Here we investigate
the use of a six-patch system in characteristic numerical relativity, by
comparing an existing two-patch implementation (using second-order finite
differencing throughout) with a new six-patch implementation (using either
second- or fourth-order finite differencing for the angular derivatives). We
compare these different codes by monitoring the Einstein constraint equations,
numerically evaluated independently from the evolution. We find that, compared
to the (second-order) two-patch code at equivalent resolutions, the errors of
the second-order six-patch code are smaller by a factor of about 2, and the
errors of the fourth-order six-patch code are smaller by a factor of nearly 50.Comment: 12 pages, 5 figures, submitted to CQG (special NFNR issue
Relative palatability and efficacy of brodifacoum-25D conservation rodenticide pellets for mouse eradication on Midway Atoll
Invasive mice (Mus spp.) can negatively impact island species and ecosystems. Because fewer island rodent eradications have been attempted for mice compared to rats (Rattus spp.), less is known about efficacy and palatability of rodenticide baits for mouse eradications. We performed a series of bait acceptance and efficacy cage trials using a standard formulation of brodifacoum-based rodenticide on wild-caught mice from Sand Island, Midway Atoll, to help inform a proposed eradication there. Mice were offered ad libitum brodifacoum pellets along with various alternative food sources, and a “no choice” treatment group received only bait pellets. Mortality in the no choice trial was 100%; however, when offered alternative foods, mice preferred the alternative diets to the bait, leading to low mortality (40%). Because there was concern that the bittering agent Bitrex® in the formulation may have reduced palatability, we conducted a subsequent trial comparing brodifacoum bait with and without Bitrex. Mortality in the with-Bitrex treatment group was slightly higher, indicating that the bittering agent was not likely responsible for low efficacy. Laboratory trials cannot account for the numerous environmental and behavioral factors that influence bait acceptance nor replicate the true availability of alternative food sources in the environment, so low efficacy results from these trials should be interpreted cautiously and not necessarily as a measure of the likelihood of success or failure of a proposed eradication
Hyperboloidal slices for the wave equation of Kerr-Schild metrics and numerical applications
We present new results from two open source codes, using finite differencing
and pseudo-spectral methods for the wave equations in (3+1) dimensions. We use
a hyperboloidal transformation which allows direct access to null infinity and
simplifies the control over characteristic speeds on Kerr-Schild backgrounds.
We show that this method is ideal for attaching hyperboloidal slices or for
adapting the numerical resolution in certain spacetime regions. As an example
application, we study late-time Kerr tails of sub-dominant modes and obtain new
insight into the splitting of decay rates. The involved conformal wave equation
is freed of formally singular terms whose numerical evaluation might be
problematically close to future null infinity.Comment: 15 pages, 12 figure
Stability of general-relativistic accretion disks
Self-gravitating relativistic disks around black holes can form as transient
structures in a number of astrophysical scenarios such as binary neutron star
and black hole-neutron star coalescences, as well as the core-collapse of
massive stars. We explore the stability of such disks against runaway and
non-axisymmetric instabilities using three-dimensional hydrodynamics
simulations in full general relativity using the THOR code. We model the disk
matter using the ideal fluid approximation with a -law equation of
state with . We explore three disk models around non-rotating black
holes with disk-to-black hole mass ratios of 0.24, 0.17 and 0.11. Due to metric
blending in our initial data, all of our initial models contain an initial
axisymmetric perturbation which induces radial disk oscillations. Despite these
oscillations, our models do not develop the runaway instability during the
first several orbital periods. Instead, all of the models develop unstable
non-axisymmetric modes on a dynamical timescale. We observe two distinct types
of instabilities: the Papaloizou-Pringle and the so-called intermediate type
instabilities. The development of the non-axisymmetric mode with azimuthal
number m = 1 is accompanied by an outspiraling motion of the black hole, which
significantly amplifies the growth rate of the m = 1 mode in some cases.
Overall, our simulations show that the properties of the unstable
non-axisymmetric modes in our disk models are qualitatively similar to those in
Newtonian theory.Comment: 30 pages, 21 figure
Visual Fixations Duration as an Indicator of Skill Level in eSports
Using highly interactive systems like computer games requires a lot of visual
activity and eye movements. Eye movements are best characterized by visual
fixation - periods of time when the eyes stay relatively still over an object.
We analyzed the distributions of fixation duration of professional athletes,
amateur and newbie players. We show that the analysis of fixation durations can
be used to deduce the skill level in computer game players. Highly skilled
gaming performance is characterized by more variability in fixation durations
and by bimodal fixation duration distributions suggesting the presence of two
fixation types in high skill gamers. These fixation types were identified as
ambient (automatic spatial processing) and focal (conscious visual processing).
The analysis of computer gamers' skill level via the analysis of fixation
durations may be used in developing adaptive interfaces and in interface
design.Comment: 10 pages, 3 figure
Properties of the Volume Operator in Loop Quantum Gravity II: Detailed Presentation
The properties of the Volume operator in Loop Quantum Gravity, as constructed
by Ashtekar and Lewandowski, are analyzed for the first time at generic
vertices of valence greater than four. The present analysis benefits from the
general simplified formula for matrix elements of the Volume operator derived
in gr-qc/0405060, making it feasible to implement it on a computer as a matrix
which is then diagonalized numerically. The resulting eigenvalues serve as a
database to investigate the spectral properties of the volume operator.
Analytical results on the spectrum at 4-valent vertices are included. This is a
companion paper to arXiv:0706.0469, providing details of the analysis presented
there.Comment: Companion to arXiv:0706.0469. Version as published in CQG in 2008.
More compact presentation. Sign factor combinatorics now much better
understood in context of oriented matroids, see arXiv:1003.2348, where also
important remarks given regarding sigma configurations. Subsequent
computations revealed some minor errors, which do not change qualitative
results but modify some numbers presented her
The neuroscience of vision-based grasping: a functional review for computational modeling and bio-inspired robotics
The topic of vision-based grasping is being widely studied using various techniques and
with different goals in humans and in other primates. The fundamental related findings are
reviewed in this paper, with the aim of providing researchers from different fields, including
intelligent robotics and neural computation, a comprehensive but accessible view on the
subject. A detailed description of the principal sensorimotor processes and the brain areas
involved in them is provided following a functional perspective, in order to make this survey
especially useful for computational modeling and bio-inspired robotic application
Properties of the Volume Operator in Loop Quantum Gravity I: Results
We analyze the spectral properties of the volume operator of Ashtekar and
Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the
classical volume expression for regions in three dimensional Riemannian space.
Our analysis considers for the first time generic graph vertices of valence
greater than four. Here we find that the geometry of the underlying vertex
characterizes the spectral properties of the volume operator, in particular the
presence of a `volume gap' (a smallest non-zero eigenvalue in the spectrum) is
found to depend on the vertex embedding. We compute the set of all
non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of
valence 5--7, and argue that these sets can be used to label spatial
diffeomorphism invariant states. We observe how gauge invariance connects
vertex geometry and representation properties of the underlying gauge group in
a natural way. Analytical results on the spectrum on 4-valent vertices are
included, for which the presence of a volume gap is proved. This paper presents
our main results; details are provided by a companion paper arXiv:0706.0382v1.Comment: 36 pages, 7 figures, LaTeX. See also companion paper
arXiv:0706.0382v1. Version as published in CQG in 2008. See arXiv:1003.2348
for important remarks regarding the sigma configurations. Subsequent
computations have revealed some minor errors, which do not change the
qualitative results but modify some of the numbers presented her
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