45 research outputs found
Evaluation of candidate metals in a simulated space shuttle main engine environment for application as turbine blade dampers
The high-pressure pumps for the space shuttle main engine are driven by combustion products which are 50 percent H2 and 50 percent H2O (by weight). The Haynes alloy 188, used in the dampers, experienced erosion. The erosion producing characteristics of wet H2 were evaluated using a controlled environment produced by a ballistic compressor. Four candidate materials were evaluated: H-188, A-286, Pt/Rh and Rh. Rh was clearly the most erosion resistant. Comparisons with AISI 4340 and Fe specimens of known uniformity indicate that metallurgical defects in the candidate materials are primary contributors to the higher than usual erosion
Measuring emission coordinates in a pulsar-based relativistic positioning system
A relativistic deep space positioning system has been proposed using four or
more pulsars with stable repetition rates. (Each pulsar emits pulses at a fixed
repetition period in its rest frame.) The positioning system uses the fact that
an event in spacetime can be fully described by emission coordinates: the
proper emission time of each pulse measured at the event. The proper emission
time of each pulse from four different pulsars---interpolated as
necessary---provides the four spacetime coordinates of the reception event in
the emission coordinate system. If more than four pulsars are available, the
redundancy can improve the accuracy of the determination and/or resolve
degeneracies resulting from special geometrical arrangements of the sources and
the event.
We introduce a robust numerical approach to measure the emission coordinates
of an event in any arbitrary spacetime geometry. Our approach uses a continuous
solution of the eikonal equation describing the backward null cone from the
event. The pulsar proper time at the instant the null cone intersects the
pulsar world line is one of the four required coordinates. The process is
complete (modulo degeneracies) when four pulsar world lines have been crossed
by the light cone.
The numerical method is applied in two different examples: measuring emission
coordinates of an event in Minkowski spacetime using pulses from four pulsars
stationary in the spacetime; and measuring emission coordinates of an event in
Schwarzschild spacetime using pulses from four pulsars freely falling toward a
static black hole.
These numerical simulations are merely exploratory, but with improved
resolution and computational resources the method can be applied to more
pertinent problems. For instance one could measure the emission coordinates,
and therefore the trajectory, of the Earth.Comment: 9 pages, 2 figures, v3: replaced with version accepted by Phys. Rev.
Tracking Black Holes in Numerical Relativity
This work addresses and solves the problem of generically tracking black hole
event horizons in computational simulation of black hole interactions.
Solutions of the hyperbolic eikonal equation, solved on a curved spacetime
manifold containing black hole sources, are employed in development of a robust
tracking method capable of continuously monitoring arbitrary changes of
topology in the event horizon, as well as arbitrary numbers of gravitational
sources. The method makes use of continuous families of level set viscosity
solutions of the eikonal equation with identification of the black hole event
horizon obtained by the signature feature of discontinuity formation in the
eikonal's solution. The method is employed in the analysis of the event horizon
for the asymmetric merger in a binary black hole system. In this first such
three dimensional analysis, we establish both qualitative and quantitative
physics for the asymmetric collision; including: 1. Bounds on the topology of
the throat connecting the holes following merger, 2. Time of merger, and 3.
Continuous accounting for the surface of section areas of the black hole
sources.Comment: 14 pages, 16 figure
Revisiting Event Horizon Finders
Event horizons are the defining physical features of black hole spacetimes,
and are of considerable interest in studying black hole dynamics. Here, we
reconsider three techniques to localise event horizons in numerical spacetimes:
integrating geodesics, integrating a surface, and integrating a level-set of
surfaces over a volume. We implement the first two techniques and find that
straightforward integration of geodesics backward in time to be most robust. We
find that the exponential rate of approach of a null surface towards the event
horizon of a spinning black hole equals the surface gravity of the black hole.
In head-on mergers we are able to track quasi-normal ringing of the merged
black hole through seven oscillations, covering a dynamic range of about 10^5.
Both at late times (when the final black hole has settled down) and at early
times (before the merger), the apparent horizon is found to be an excellent
approximation of the event horizon. In the head-on binary black hole merger,
only {\em some} of the future null generators of the horizon are found to start
from past null infinity; the others approach the event horizons of the
individual black holes at times far before merger.Comment: 30 pages, 15 figures, revision
Adaptive Event Horizon Tracking and Critical Phenomena in Binary Black Hole Coalescence
This work establishes critical phenomena in the topological transition of
black hole coalescence. We describe and validate a computational front tracking
event horizon solver, developed for generic studies of the black hole
coalescence problem. We then apply this to the Kastor - Traschen axisymmetric
analytic solution of the extremal Maxwell - Einstein black hole merger with
cosmological constant. The surprising result of this computational analysis is
a power law scaling of the minimal throat proportional to time. The minimal
throat connecting the two holes obeys this power law during a short time
immediately at the beginning of merger. We also confirm the behavior
analytically. Thus, at least in one axisymmetric situation a critical
phenomenon exists. We give arguments for a broader universality class than the
restricted requirements of the Kastor - Traschen solution.Comment: 13 pages, 20 figures Corrected labels on figures 17 through 20.
Corrected typos in references. Added some comment