514 research outputs found
Comparative thermal fatigue resistances of twenty-six nickel and cobalt base alloys
Thermal fatigue resistances were determined from fluidized bed tests. Cycles to cracking differed by almost three orders of magnitude for these materials with directional solidification and surface protection of definite benefit. The alloy-coating combination with the highest thermal fatigue resistance was directionally solidified NASA TAZ-8A with an RT-XP coating. It oxidation resistance was excellent, showing almost no weight change after 15 000 fluidized bed cycles
Self-similarity and singularity formation in a coupled system of Yang-Mills-dilaton evolution equations
We study both analytically and numerically a coupled system of spherically
symmetric SU(2) Yang-Mills-dilaton equation in 3+1 Minkowski space-time. It has
been found that the system admits a hidden scale invariance which becomes
transparent if a special ansatz for the dilaton field is used. This choice
corresponds to transition to a frame rotated in the plane at a
definite angle. We find an infinite countable family of self-similar solutions
which can be parametrized by the - the number of zeros of the relevant
Yang-Mills function. According to the performed linear perturbation analysis,
the lowest solution with N=0 only occurred to be stable. The Cauchy problem has
been solved numerically for a wide range of smooth finite energy initial data.
It has been found that if the initial data exceed some threshold, the resulting
solutions in a compact region shrinking to the origin, attain the lowest N=0
stable self-similar profile, which can pretend to be a global stable attractor
in the Cauchy problem. The solutions live a finite time in a self-similar
regime and then the unbounded growth of the second derivative of the YM
function at the origin indicates a singularity formation, which is in agreement
with the general expectations for the supercritical systems.Comment: 10 pages, 5 figure
On the Definition of Averagely Trapped Surfaces
Previously suggested definitions of averagely trapped surfaces are not
well-defined properties of 2-surfaces, and can include surfaces in flat
space-time. A natural definition of averagely trapped surfaces is that the
product of the null expansions be positive on average. A surface is averagely
trapped in the latter sense if and only if its area and Hawking mass
satisfy the isoperimetric inequality , with similar inequalities
existing for other definitions of quasi-local energy.Comment: 4 page
On weakly turbulent instability of anti-de Sitter space (Preprint)
We study the nonlinear evolution of a weakly perturbed anti-de Sitter (AdS) spacetime by solving numerically the four-dimensional spherically symmetric Einstein-massless-scalar field equations with negative cosmological constant. Our results suggest that AdS spacetime is unstable under arbitrarily small generic perturbations. We conjecture that this instability is triggered by a resonant mode mixing which gives rise to diffusion of energy from low to high frequencies
Thermal fatigue testing of simulated turbine blades
Burner rig tests on simulated turbine blades, with and without internal cooling, to predict thermal fatigue performanc
Real-time demonstration hardware for enhanced DPCM video compression algorithm
The lack of available wideband digital links as well as the complexity of implementation of bandwidth efficient digital video CODECs (encoder/decoder) has worked to keep the cost of digital television transmission too high to compete with analog methods. Terrestrial and satellite video service providers, however, are now recognizing the potential gains that digital video compression offers and are proposing to incorporate compression systems to increase the number of available program channels. NASA is similarly recognizing the benefits of and trend toward digital video compression techniques for transmission of high quality video from space and therefore, has developed a digital television bandwidth compression algorithm to process standard National Television Systems Committee (NTSC) composite color television signals. The algorithm is based on differential pulse code modulation (DPCM), but additionally utilizes a non-adaptive predictor, non-uniform quantizer and multilevel Huffman coder to reduce the data rate substantially below that achievable with straight DPCM. The non-adaptive predictor and multilevel Huffman coder combine to set this technique apart from other DPCM encoding algorithms. All processing is done on a intra-field basis to prevent motion degradation and minimize hardware complexity. Computer simulations have shown the algorithm will produce broadcast quality reconstructed video at an average transmission rate of 1.8 bits/pixel. Hardware implementation of the DPCM circuit, non-adaptive predictor and non-uniform quantizer has been completed, providing realtime demonstration of the image quality at full video rates. Video sampling/reconstruction circuits have also been constructed to accomplish the analog video processing necessary for the real-time demonstration. Performance results for the completed hardware compare favorably with simulation results. Hardware implementation of the multilevel Huffman encoder/decoder is currently under development along with implementation of a buffer control algorithm to accommodate the variable data rate output of the multilevel Huffman encoder. A video CODEC of this type could be used to compress NTSC color television signals where high quality reconstruction is desirable (e.g., Space Station video transmission, transmission direct-to-the-home via direct broadcast satellite systems or cable television distribution to system headends and direct-to-the-home)
Real-time transmission of digital video using variable-length coding
Huffman coding is a variable-length lossless compression technique where data with a high probability of occurrence is represented with short codewords, while 'not-so-likely' data is assigned longer codewords. Compression is achieved when the high-probability levels occur so frequently that their benefit outweighs any penalty paid when a less likely input occurs. One instance where Huffman coding is extremely effective occurs when data is highly predictable and differential coding can be applied (as with a digital video signal). For that reason, it is desirable to apply this compression technique to digital video transmission; however, special care must be taken in order to implement a communication protocol utilizing Huffman coding. This paper addresses several of the issues relating to the real-time transmission of Huffman-coded digital video over a constant-rate serial channel. Topics discussed include data rate conversion (from variable to a fixed rate), efficient data buffering, channel coding, recovery from communication errors, decoder synchronization, and decoder architectures. A description of the hardware developed to execute Huffman coding and serial transmission is also included. Although this paper focuses on matters relating to Huffman-coded digital video, the techniques discussed can easily be generalized for a variety of applications which require transmission of variable-length data
Mass of Colored Black Holes
New results pertaining to colored static black hole solutions to the
Einstein-Yang-Mills equations are obtained. The isolated horizons framework is
used to define the concept of Hamiltonian Horizon Mass of the black hole. An
unexpected relation between the ADM and Horizon masses of the black hole
solution with the ADM mass of the corresponding Bartnik-McKinnon soliton is
found. These results can be generalized to other non-linear theories and they
suggest a general testing bed for the instability of the corresponding hairy
black holes.Comment: 8 pages, no figures, Revtex file. Minor changes made to clarify some
formulas. References updated. Final version to appear in PRD/15
Space Networking Demonstrated for Distributed Human-Robotic Planetary Exploration
Communications and networking experts from the NASA Glenn Research Center designed and implemented an innovative communications infrastructure for a simulated human-robotic planetary mission. The mission, which was executed in the Arizona desert during the first 2 weeks of September 2002, involved a diverse team of researchers from several NASA centers and academic institutions
Hairy Black Holes, Horizon Mass and Solitons
Properties of the horizon mass of hairy black holes are discussed with
emphasis on certain subtle and initially unexpected features. A key property
suggests that hairy black holes may be regarded as `bound states' of ordinary
black holes without hair and colored solitons. This model is then used to
predict the qualitative behavior of the horizon properties of hairy black
holes, to provide a physical `explanation' of their instability and to put
qualitative constraints on the end point configurations that result from this
instability. The available numerical calculations support these predictions.
Furthermore, the physical arguments are robust and should be applicable also in
more complicated situations where detailed numerical work is yet to be carried
out.Comment: 25 pages, 5 (new) figures. Revtex file. Final version to appear in
CQ
- …