19,599 research outputs found
Thermal and structural assessments of a ceramic wafer seal in hypersonic engines
The thermal and structural performances of a ceramic wafer seal in a simulated hypersonic engine environment are numerically assessed. The effects of aerodynamic heating, surface contact conductance between the seal and its adjacent surfaces, flow of purge coolant gases, and leakage of hot engine flow path gases on the seal temperature were investigated from the engine inlet back to the entrance region of the combustion chamber. Finite element structural analyses, coupled with Weibull failure analyses, were performed to determine the structural reliability of the wafer seal
Life assessment of combustor liner using unified constitutive models
Hot section components of gas turbine engines are subject to severe thermomechanical loads during each mission cycle. Inelastic deformation can be induced in localized regions leading to eventual fatigue cracking. Assessment of durability requires reasonably accurate calculation of the structural response at the critical location for crack initiation. In recent years nonlinear finite element computer codes have become available for calculating inelastic structural response under cyclic loading. NASA-Lewis sponsored the development of unified constitutive material models and their implementation in nonlinear finite element computer codes for the structural analysis of hot section components. These unified models were evaluated with regard to their effect on the life prediction of a hot section component. The component considered was a gas turbine engine combustor liner. A typical engine mission cycle was used for the thermal and structural analyses. The analyses were performed on a CRAY computer using the MARC finite element code. The results were compared with laboratory test results, in terms of crack initiation lives
Localization for MCMC: sampling high-dimensional posterior distributions with local structure
We investigate how ideas from covariance localization in numerical weather
prediction can be used in Markov chain Monte Carlo (MCMC) sampling of
high-dimensional posterior distributions arising in Bayesian inverse problems.
To localize an inverse problem is to enforce an anticipated "local" structure
by (i) neglecting small off-diagonal elements of the prior precision and
covariance matrices; and (ii) restricting the influence of observations to
their neighborhood. For linear problems we can specify the conditions under
which posterior moments of the localized problem are close to those of the
original problem. We explain physical interpretations of our assumptions about
local structure and discuss the notion of high dimensionality in local
problems, which is different from the usual notion of high dimensionality in
function space MCMC. The Gibbs sampler is a natural choice of MCMC algorithm
for localized inverse problems and we demonstrate that its convergence rate is
independent of dimension for localized linear problems. Nonlinear problems can
also be tackled efficiently by localization and, as a simple illustration of
these ideas, we present a localized Metropolis-within-Gibbs sampler. Several
linear and nonlinear numerical examples illustrate localization in the context
of MCMC samplers for inverse problems.Comment: 33 pages, 5 figure
Broadband double-layered coplanar patch antennas with adjustable CPW feeding structure
In this paper, we have presented the double-layered coplanar patch antennas of enhanced impedance bandwidth and adjustable conductor-backed coplanar waveguide feed lines. The proposed structure retains the advantage of laying the coplanar patch and coplanar waveguide (CPW) feed line on the same surface, which makes direct integration with other devices easier. In addition, the substrate thickness of the radiating patch can be adjusted to achieve a wider impedance bandwidth while the dimensions of the CPW feed line are kept unchanged. Simulation has been done by using commercial electromagnetic (EM) simulation software. Four testing antennas, which have centre frequency at about 10 GHz, were designed. The four testing antennas had the same total thickness, but different thickness combinations. From the measured return loss, gain, and radiation patterns of the antennas, it was demonstrated that different thickness combinations do not affect the characteristics of the antennas seriously. Therefore, the dimensions of the CPW feed structure of the antennas can be adjusted individually and can be selected for different applications
Structural assessment of a space station solar dynamic heat receiver thermal energy storage canister
The structural performance of a space station thermal energy storage (TES) canister subject to orbital solar flux variation and engine cold start up operating conditions was assessed. The impact of working fluid temperature and salt-void distribution on the canister structure are assessed. Both analytical and experimental studies were conducted to determine the temperature distribution of the canister. Subsequent finite element structural analyses of the canister were performed using both analytically and experimentally obtained temperatures. The Arrhenius creep law was incorporated into the procedure, using secondary creep data for the canister material, Haynes 188 alloy. The predicted cyclic creep strain accumulations at the hot spot were used to assess the structural performance of the canister. In addition, the structural performance of the canister based on the analytically determined temperature was compared with that based on the experimentally measured temperature data
Ultrafast response of surface electromagnetic waves in an aluminum film perforated with subwavelength hole arrays
The ultrafast dynamics of surface electromagnetic waves photogenerated on
aluminum film perforated with subwavelength holes array was studied in the
visible spectral range by the technique of transient photomodulation with 100
fs time resolution. We observed a pronounced blueshift of the resonant
transmission band that reveals the important role of plasma attenuation in the
optical response of nanohole arrays. The blueshift is inconsistent with
plasmonic mechanism of extraordinary transmission and points to the crucial
role of interference in the formation of transmission bands. The transient
photomodulation spectra were successfully modeled within the Boltzmann equation
approach for the electron-phonon relaxation dynamics, involving non-equilibrium
hot electrons and quasi-equilibrium phonons.Comment: 4 pages, 3 figure
A second-order class-D audio amplifier
Class-D audio amplifiers are particularly efficient, and this efficiency has led to their ubiquity in a wide range of modern electronic appliances. Their output takes the form of a high-frequency square wave whose duty cycle (ratio of on-time to off-time) is modulated at low frequency according to the audio signal. A mathematical model is developed here for a second-order class-D amplifier design (i.e., containing one second-order integrator) with negative feedback. We derive exact expressions for the dominant distortion terms, corresponding to a general audio input signal, and confirm these predictions with simulations. We also show how the observed phenomenon of “pulse skipping” arises from an instability of the analytical solution upon which the distortion calculations are based, and we provide predictions of the circumstances under which pulse skipping will take place, based on a stability analysis. These predictions are confirmed by simulations
Superconformal Vortex Strings
We study the low-energy dynamics of semi-classical vortex strings living
above Argyres-Douglas superconformal field theories. The worldsheet theory of
the string is shown to be a deformation of the CP^N model which flows in the
infra-red to a superconformal minimal model. The scaling dimensions of chiral
primary operators are determined and the dimensions of the associated relevant
perturbations on the worldsheet and in the four dimensional bulk are found to
agree. The vortex string thereby provides a map between the A-series of N=2
superconformal theories in two and four dimensions.Comment: 22 pages. v2: change to introductio
Symmetry, dark matter and LHC phenomenology of the minimal SM
A sterile neutrino with a mass of a few keV can play the role of a warm dark
matter(DM). This can be realized in seesaw models with 3 left- and 3
right-handed neutrinos. It is possible to identify the keV neutrino to be one
of the right-handed neutrinos leaving the other two to be much more heavier,
the SM model. We show that with this realization of keV neutrino DM, the
model has an approximate Friedberg-Lee symmetry providing a natural explanation
for the lightness of the right-handed neutrino. We also find that in this model
the mixing parameters couple light and heavy neutrinos are strongly correlated,
and can be large enough to have testable effects at the LHC for the two heavy
right-handed neutrinos to be in the hundred-GeV range.Comment: 20 pages, 5 figures, discussions expanded, references added, to
appear in PR
- …