13,523 research outputs found
High temperature measuring device
Ultrasonic pulse technique for measuring average gas temperature in nuclear rocket engine - sound propagation and environmental studie
Information requirements for guidance and control systems
Control or guidance system performance dependency on information handling by subsystem
Paper Session II-B - Smarter Software for Enhanced Vehicle Health Monitoring and Inter-Planetary Exploration
The existing philosophy for space mission control was born in the early days of the space program when technology did not exist to put significant control responsibility onboard the spacecraft. NASA relied on a team of ground control experts to troubleshoot systems when problems occurred. As computing capability improved, more responsibility was handed over to the systems software. However, there is still a large contingent of both launch and flight controllers supporting each mission. New technology can update this philosophy to increase mission assurance and reduce the cost of inter-planetary exploration.
The advent of model-based diagnosis and intelligent planning software enables spacecraft to handle most routine problems automatically and allocate resources in a flexible way to realize mission objectives. The manifests for recent missions include multiple subsystems and complex experiments. Spacecraft must operate at longer distances from earth where communications delays make earthbound command and control impractical.
NASA’s Ames Research Center (ARC) has demonstrated the utility of onboard diagnosis and planning with the Remote Agent experiment in 1999. KSC has pioneered model-based diagnosis and demonstrated its utility for ground support operations. KSC and ARC are cooperating in research to improve the state of the art of this technology. This paper highlights model-based reasoning applications for Moon and Mars missions including in-situ resource utilization and enhanced vehicle health monitoring
Relativistic nucleon optical potentials with isospin dependence in Dirac Brueckner Hartree-Fock approach
The relativistic optical model potential (OMP) for nucleon-nucleus scattering
is investigated in the framework of Dirac-Brueckner-Hartree-Fock (DBHF)
approach using the Bonn-B One-Boson- Exchange potential for the bare
nucleon-nucleon interaction. Both real and imaginary parts of isospin-dependent
nucleon self-energies in nuclear medium are derived from the DBHF approach
based on the projection techniques within the subtracted T -matrix
representation. The Dirac potentials as well as the corresponding Schrodinger
equivalent potentials are evaluated. An improved local density approximation is
employed in this analysis, where a range parameter is included to account for a
finite-range correction of the nucleon-nucleon interaction. As an example the
total cross sections, differential elastic scattering cross sections, analyzing
powers for n, p + 27Al at incident energy 100 keV < E < 250 MeV are calculated.
The results derived from this microscopic approach of the OMP are compared to
the experimental data, as well as the results obtained with a phenomenological
OMP. A good agreement between the theoretical results and the measurements can
be achieved for all incident energies using a constant value for the range
parameter.Comment: 10 pages, 16 figure
A method for the estimation of p-mode parameters from averaged solar oscillation power spectra
A new fitting methodology is presented which is equally well suited for the
estimation of low-, medium-, and high-degree mode parameters from -averaged
solar oscillation power spectra of widely differing spectral resolution. This
method, which we call the "Windowed, MuLTiple-Peak, averaged spectrum", or
WMLTP Method, constructs a theoretical profile by convolving the weighted sum
of the profiles of the modes appearing in the fitting box with the power
spectrum of the window function of the observing run using weights from a
leakage matrix that takes into account both observational and physical effects,
such as the distortion of modes by solar latitudinal differential rotation. We
demonstrate that the WMLTP Method makes substantial improvements in the
inferences of the properties of the solar oscillations in comparison with a
previous method that employed a single profile to represent each spectral peak.
We also present an inversion for the internal solar structure which is based
upon 6,366 modes that we have computed using the WMLTP method on the 66-day
long 2010 SOHO/MDI Dynamics Run. To improve both the numerical stability and
reliability of the inversion we developed a new procedure for the
identification and correction of outliers in a frequency data set. We present
evidence for a pronounced departure of the sound speed in the outer half of the
solar convection zone and in the subsurface shear layer from the radial sound
speed profile contained in Model~S of Christensen-Dalsgaard and his
collaborators that existed in the rising phase of Solar Cycle~24 during
mid-2010
Fingerprinting Soft Materials: A Framework for Characterizing Nonlinear Viscoelasticity
We introduce a comprehensive scheme to physically quantify both viscous and
elastic rheological nonlinearities simultaneously, using an imposed large
amplitude oscillatory shear (LAOS) strain. The new framework naturally lends a
physical interpretation to commonly reported Fourier coefficients of the
nonlinear stress response. Additionally, we address the ambiguities inherent in
the standard definitions of viscoelastic moduli when extended into the
nonlinear regime, and define new measures which reveal behavior that is
obscured by conventional techniques.Comment: 10 pages, 3 figures, full-page double-space preprint forma
Unconventional magnets in external magnetic fields
This short review surveys phenomena observed when a magnetic field is applied
to a system of localised spins on a lattice. Its focus is on frustrated magnets
in dimension . The interplay of field and entropy is illustrated in
the context of their unusual magnetocaloric properties, where field-tuned
degeneracies assert themselves. Magnetisation plateaux can reveal the physics
of fluctuations, with unusual excitations (such as local modes, extended string
defects or monopoles) involved in plateau termination. Field-tuning lattice
geometry is the final topic, where mechanisms for dimensional reduction and
conversion between different lattice types are discussed.Comment: Plenary Talk at HFM 2008 Conferenc
Analytical theory for the initial mass function: CO clumps and prestellar cores
We derive an analytical theory of the prestellar core initial mass function
based on an extension of the Press-Schechter statistical formalism. With the
same formalism, we also obtain the mass spectrum for the non self-gravitating
clumps produced in supersonic flows. The mass spectrum of the self-gravitating
cores reproduces very well the observed initial mass function and identifies
the different mechanisms responsible for its behaviour. The theory predicts
that the shape of the IMF results from two competing contributions, namely a
power-law at large scales and an exponential cut-off (lognormal form) centered
around the characteristic mass for gravitational collapse. The cut-off exists
already in the case of pure thermal collapse, provided that the underlying
density field has a lognormal distribution. Whereas pure thermal collapse
produces a power-law tail steeper than the Salpeter value, dN/dlog M\propto
M^{-x}, with x=1.35, this latter is recovered exactly for the (3D) value of the
spectral index of the velocity power spectrum, n\simeq 3.8, found in
observations and in numerical simulations of isothermal supersonic turbulence.
Indeed, the theory predicts that x=(n+1)/(2n-4) for self-gravitating structures
and x=2-n'/3 for non self-gravitating structures, where n' is the power
spectrum index of log(rho). We show that, whereas supersonic turbulence
promotes the formation of both massive stars and brown dwarfs, it has an
overall negative impact on star formation, decreasing the star formation
efficiency. This theory provides a novel theoretical foundation to understand
the origin of the IMF and to infer its behaviour in different environments. It
also provides a complementary approach and useful guidance to numerical
simulations exploring star formation, while making testable predictions.Comment: To appear in Ap
High-pressure study of substrate material ScAlMgO4
We report on the structural properties of ScAlMgO4 studied under
quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up
to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at
300 K and 100 K. We found that the low-pressure phase remains stable up to 24
GPa. At 28 GPa, we detected a reversible phase transformation. The
high-pressure phase is assigned to a monoclinic distortion of the low-pressure
phase. No additional phase transition is observed up to 40 GPa. In addition,
the equation of state, compressibility tensor, and thermal expansion
coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found
to be 143(8) GPa, with a strong compressibility anisotropy. For the trigonal
low-pressure phase, the compressibility along the c-axis is twice than
perpendicular one. A perfect lattice match with ZnO is retained under pressure
in the pressure range of stability of wurtzite ZnO.Comment: 22 pages, 5 figures, 4 tables, 24 reference
Nonlinear propagation of planet-generated tidal waves
The propagation and evolution of planet-generated density waves in
protoplanetary disks is considered. The evolution of waves, leading to the
shock formation and wake dissipation, is followed in the weakly nonlinear
regime. The local approach of Goodman & Rafikov (2001) is extended to include
the effects of surface density and temperature variations in the disk as well
as the disk cylindrical geometry and nonuniform shear. Wave damping due to
shocks is demonstrated to be a nonlocal process spanning a significant fraction
of the disk. Torques induced by the planet could be significant drivers of disk
evolution on timescales of the order 1-10 Myr even in the absence of strong
background viscosity. A global prescription for angular momentum deposition is
developed which could be incorporated into the study of gap formation in a
gaseous disk around the planet.Comment: AASTeX, 26 pages, 4 figures, 1 table, submitted to Ap
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