1,775 research outputs found
Gauge drivers for the generalized harmonic Einstein equations
The generalized harmonic representation of Einstein's equations is manifestly hyperbolic for a large class of gauge conditions. Unfortunately most of the useful gauges developed over the past several decades by the numerical relativity community are incompatible with the hyperbolicity of the equations in this form. This paper presents a new method of imposing gauge conditions that preserves hyperbolicity for a much wider class of conditions, including as special cases many of the standard ones used in numerical relativity: e.g., K freezing, Gamma freezing, Bona-Massó slicing, conformal Gamma drivers, etc. Analytical and numerical results are presented which test the stability and the effectiveness of this new gauge-driver evolution system
Mitigation of artifacts due to isolated acoustic heterogeneities in photoacoustic computed tomography using a variable data truncation-based reconstruction method
Photoacoustic computed tomography (PACT) is an emerging computed imaging
modality that exploits optical contrast and ultrasonic detection principles to
form images of the absorbed optical energy density within tissue. If the object
possesses spatially variant acoustic properties that are unaccounted for by the
reconstruction method, the estimated image can contain distortions. While
reconstruction methods have recently been developed to compensate for this
effect, they generally require the object's acoustic properties to be known a
priori. To circumvent the need for detailed information regarding an object's
acoustic properties, we previously proposed a half-time reconstruction method
for PACT. A half-time reconstruction method estimates the PACT image from a
data set that has been temporally truncated to exclude the data components that
have been strongly aberrated. However, this method can be improved upon when
the approximate sizes and locations of isolated heterogeneous structures, such
as bones or gas pockets, are known. To address this, we investigate PACT
reconstruction methods that are based on a variable data truncation (VDT)
approach. The VDT approach represents a generalization of the half-time
approach, in which the degree of temporal truncation for each measurement is
determined by the distance between the corresponding ultrasonic transducer
location and the nearest known bone or gas void location. Computer-simulated
and experimental data are employed to demonstrate the effectiveness of the
approach in mitigating artifacts due to acoustic heterogeneities
A holistic multi-scale approach to using 3D scanning technology in accident reconstruction
Three-dimensional scanning and documentation methods are becoming increasingly employed by law enforcement personnel for crime scene and accident scene recording. Three-dimensional documentation of the victim’s body in such cases is also increasingly used as the field of forensic radiology and imaging is expanding rapidly. These scanning technologies enable a more complete and detailed documentation than standard autopsy. This was used to examine a fatal pedestrian-vehicle collision where the pedestrian was killed by a van whilst crossing the road. Two competing scenarios were considered for the vehicle speed calculation: the pedestrian being projected forward by the impact or the pedestrian being carried on the vehicle’s bonnet. In order to assist with this, the impact area of the accident vehicle was scanned using laser surface scanning, the victim was scanned using postmortem CT and micro-CT and the data sets were combined to virtually match features of the vehicle to injuries on the victim. Micro-CT revealed additional injuries not previously detected, lending support to the pedestrian-carry theory
High-accuracy waveforms for binary black hole inspiral, merger, and ringdown
The first spectral numerical simulations of 16 orbits, merger, and ringdown
of an equal-mass non-spinning binary black hole system are presented.
Gravitational waveforms from these simulations have accumulated numerical phase
errors through ringdown of ~0.1 radian when measured from the beginning of the
simulation, and ~0.02 radian when waveforms are time and phase shifted to agree
at the peak amplitude. The waveform seen by an observer at infinity is
determined from waveforms computed at finite radii by an extrapolation process
accurate to ~0.01 radian in phase. The phase difference between this waveform
at infinity and the waveform measured at a finite radius of r=100M is about
half a radian. The ratio of final mass to initial mass is M_f/M = 0.95162 +-
0.00002, and the final black hole spin is S_f/M_f^2=0.68646 +- 0.00004.Comment: 15 pages, 11 figures; New figure added, text edited to improve
clarity, waveform made availabl
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