5,448 research outputs found
The control of carbon dioxide cryodeposits
An experimental study has been conducted to investigate the parameters affecting the cryodeposition of carbon dioxide frost. In the investigation carbon dioxide frost was cryodeposited from a helium-carbon dioxide mixture into a layer of fibrous insulation surrounding a cylindrical cryogenic tank. Results of the study indicated that not only did deposition occur on the frost surface but also within the frost layer. Over the range of variables investigated both the frost density and the mass of frost deposited were most sensitive to the time of deposition, the percent of carbon dioxide in the purge-gas mixture, and the thickness of the insulation. Frost density and mass of frost deposition were found to increase with time and percent carbon dioxide, and to decrease with increasing insulation thickness
Performance of a hypersonic hot fuselage structure with a carbon dioxide frost projected, nonintegral cryogenic tank
A model which consisted of a hot structure and a nonintegral tank protected by a carbon dioxide frost thermal protection system was tested under the following conditions: (1) room temperature loading and (2) heating and loading corresponding to the Mach 8 flight of an air-breathing launch vehicle. In the simulated flight tests, liquid nitrogen inside the tank was withdrawn at the rate fuel would be consumed. Prior to each simulated flight test, carbon dioxide was cryodeposited in the insulation surrounding the tank; during the tests, subliming CO2 frost absorbed heat and provided a purge gas for the space between the tank and the structure. A method of flame spraying the joints between panels with a nickel-aluminum material was developed to prevent excessive leakage of the purge gas through the outer structure. The tests indicated that the hot structure (with a joint repaired by riveting), the nonintegral tank and suspension system, and the carbon dioxide frost thermal protection system provide a workable concept with predictable performance
Ambient temperature fatigue tests of elements of an actively cooled honeycomb sandwich structural panel
Elements of an actively cooled structural panel for a hypersonic aircraft have been investigated for fatigue characteristics. The study involved a bonded honeycomb sandwich panel with d-shaped coolant tubes. The curved portion of these tubes was embedded in the honeycomb, and the flat portion was bonded or soldered to the inner surface of the outer skin. The elements examined were two plain skin specimens (aluminum alloy); two specimens with skins attached to manifolds and tubes (one specimen was bonded, the other soldered); and a specimen representative of a corner section of the complete cooled sandwich. Sinusoidal loads were applied to all specimens. The honeycomb sandwich specimen was loaded in both tension and compression; the other specimens were loaded in tension only. The cooling tubes were pressurized with oil throughout the fatigue tests. The most significant results of these tests follow: All specimens exceeded their design life of 20,000 cycles without damage. Crack growth rates obtained in the plain skin specimens were used to determine the crack growth characteristics of aluminum alloy. Cracks in skins either bonded or soldered to cooling tubes propagated past the tubes without penetration. The coolant tubes served as crack arresters and temporarily stopped crack growth when a crack reached a tube-skin interface. The honeycomb core demonstrated that it could contain leakage from a tube
Radiative, actively cooled panel tests results
The radiative, actively cooled panel designed to withstand a uniform incident heat flux of 136 kW/sq m to a 444 K surface temperature was evaluated. The test program consisted of preliminary static thermal mechanical loading and aerothermal flow tests. Test results are briefly discussed
Charge-ice dynamics in the negative thermal expansion material Cd(CN)
We use variable-temperature (150--300\,K) single-crystal X-ray diffraction to
re-examine the interplay between structure and dynamics in the ambient phase of
the isotropic negative thermal expansion (NTE) material Cd(CN). We find
strong experimental evidence for the existence of low-energy vibrational modes
that involve off-centering of Cd ions. These modes have the effect of
increasing network packing density---suggesting a mechanism for NTE that is
different to the generally-accepted picture of correlated Cd(C/N) rotation
modes. Strong local correlations in the displacement directions of neighbouring
cadmium centres are evident in the existence of highly-structured diffuse
scattering in the experimental X-ray diffraction patterns. Monte Carlo
simulations suggest these patterns might be interpreted in terms of a basic set
of `ice-rules' that establish a mapping between the dynamics of Cd(CN) and
proton ordering in cubic ice VII.Comment: 5 pages, 5 figures, submitted to PR
Heavy-Meson Observables at One-Loop in Partially Quenched Chiral Perturbation Theory
I present one-loop level calculations of the Isgur-Wise functions for B ->
D^{(*)} + e + nu, of the matrix elements of isovector twist-2 operators in B
and D mesons, and the matrix elements for the radiative decays D^* -> D + gamma
in partially quenched heavy quark chiral perturbation theory. Such expressions
are required in order to extrapolate from the light quark masses used in
lattice simulations of the foreseeable future to those of nature.Comment: 13 pages, 3 fig
Decomposition, Condensation Defects, and Fusion
In this paper we outline the application of decomposition to condensation defects and their fusion rules. Briefly, a condensation defect is obtained by gauging a higher-form symmetry along a submanifold, and so there is a natural interplay with notions of decomposition, the statement that d-dimensional quantum field theories with global (Formula presented.) -form symmetries are equivalent to disjoint unions of other quantum field theories. We will also construct new (sometimes non-invertible) defects, and compute their fusion products, again utilizing decomposition. An important role will be played in all these analyses by theta angles for gauged higher-form symmetries, which can be used to select individual universes in a decomposition
Quark mass dependence of masses and decay constants of the pseudo-Goldstone bosons in QCD
The dependence of the pseudoscalar meson masses and decay constants on sea
and valence quark masses is compared to next-to-leading order (NLO) Chiral
Perturbation Theory (ChPT). The numerical simulations with two light dynamical
quark flavors are performed with the Wilson-quark lattice action at gauge
coupling beta=5.1 and hopping parameters kappa=0.176, 0.1765, 0.177 on a 16^4
lattice. O(a) lattice artifacts are taken into account by applying chiral
perturbation theory for the Wilson lattice action. The values of the relevant
combinations of Gasser-Leutwyler constants L_4, L_5, L_6 and L_8 are estimated.Comment: 21 pages, 4 figures; discussion of the sea quark mass dependence
changed, references adde
Chiral Perturbation Theory for the Quenched Approximation of QCD
[This version is a minor revision of a previously submitted preprint. Only
references have been changed.] We describe a technique for constructing the
effective chiral theory for quenched QCD. The effective theory which results is
a lagrangian one, with a graded symmetry group which mixes Goldstone bosons and
fermions, and with a definite (though slightly peculiar) set of Feynman rules.
The straightforward application of these rules gives automatic cancellation of
diagrams which would arise from virtual quark loops. The techniques are used to
calculate chiral logarithms in , , , and the ratio of
to . The leading
finite-volume corrections to these quantities are also computed. Problems for
future study are described.Comment: 14 page
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