72,445 research outputs found
Research study on stabilization and control: Modern sampled-data control theory. Design of the large space telescope system
Conditions of self-sustained oscillations in a two-axis model of the nonlinear LST system are studied. The describing function of the CMG frictional nonlinearity of the LST system is used for the analysis, as well as continuous-data and discrete-data models of the simplified LST control system. A numerical-iterative method is described for the analysis of the two-axis system. Approximation methods and the direct plotting of the stability equation are implemented in the study. It is shown that although the dynamics of the two axes are identical, the amplitudes of self-sustained oscillations in the two axes may in principle be different. Analysis shows that the LST systems are of equal amplitudes but with 180-degree phase shift
Research study on stabilization and control modern sampled-data control theory
The methods of continuous and discrete describing function analysis were applied to predicting the existence of self-sustained oscillations in the single-axis model of the large space telescope system with nonlinear control moment gyroscope friction characteristics. It is shown that the stability equations may be solved by a numerical-iterative technique using the describing function analysis, instead of the usual graphical methods. The numerical method is found to be effective in leading to a convergent solution rapidly, with an appropriate guess of the initial condition
Research study on stabilization and control modern sampled-data control theory. Design of the Large Space Telescope system
The numerical technique is applied to the prediction of self-sustained oscillations in a two-axis model of the nonlinear system with sampled data. The sampled-data two-axis LST system model, and its stability equation are analyzed along with the exact solution of the stability equation by numerical-iterative techniques
A Cost-Benefit Study of Doing Astrophysics On The Cloud: Production of Image Mosaics
Utility grids such as the Amazon EC2 and Amazon S3 clouds offer computational and storage resources that can be used on-demand for a fee by compute- and data-intensive applications. The cost of running an application on such a cloud depends on the compute, storage and communication resources it will provision and consume. Different execution plans of the same application may result in significantly different costs. We studied via simulation the cost performance trade-offs of different execution and resource provisioning plans by creating, under the Amazon cloud fee structure, mosaics with the Montage image mosaic engine, a widely used data- and compute-intensive application. Specifically, we studied the cost of building mosaics of 2MASS data that have sizes of 1, 2 and 4 square degrees, and a 2MASS all-sky mosaic. These are examples of mosaics commonly generated by astronomers. We also study these trade-offs in the context of the storage and communication fees of Amazon S3 when used for long-term application data archiving. Our results show that by provisioning the right amount of storage and compute resources cost can be significantly reduced with no significant impact on application performance
Pair Correlation Functions and a Free-Energy Functional for the Nematic Phase
In this paper we have presented the calculation of pair correlation functions
in a nematic phase for a model of spherical particles with the long-range
anisotropic interaction from the mean spherical approximation(MSA) and the
Percus-Yevick (PY) integral equation theories. The results found from the MSA
theory have been compared with those found analytically by Holovko and
Sokolovska (J. Mol. Liq. , 161(1999)). A free energy functional which
involves both the symmetry conserving and symmetry broken parts of the direct
pair correlation function has been used to study the properties of the nematic
phase. We have also examined the possibility of constructing a free energy
functional with the direct pair correlation function which includes only the
principal order parameter of the ordered phase and found that the resulting
functional gives results that are in good agreement with the original
functional. The isotropic-nematic transition has been located using the grand
thermodynamic potential. The PY theory has been found to give nematic phase
with pair correlation function harmonic coefficients having all the desired
features. In a nematic phase the harmonic coefficient of the total pair
correlation function connected with the correlations
of the director transverse fluctuations should develop a long-range tail. This
feature has been found in both the MSA and PY theories.Comment: 27 pages, 11 figures, Accepted in J. Chem. Phy
A New Narrow-Line Seyfert 1 galaxy : RXJ1236.9+2656
We report identification of a narrow-line Seyfert 1 galaxy RXJ1236.9+2656.
X-ray emission from the NLS1 galaxy undergoes long-term variability with
0.1--2.0 keV flux changing by a factor of 2 within about 3 yr. The ROSAT PSPC
spectrum of RXJ1236.9+2656 is well represented by a power-law of Gamma = 3.7
absorbed by matter in our own Galaxy (N_H = 1.33X10^20 cm**-2). Intrinsic soft
X-ray luminosity of the NLS1 galaxy is estimated to be 1.5X10^43 erg/s in the
energy band of 0.1-2.0 keV. The optical spectrum of RXJ1236.9+2656 is typical
of NLS1 galaxies and shows narrow Balmer emission lines (1100 km/s < FWHM <
1700 km/s) of Hbeta, Halpha, and forbidden lines of [O III] and [N II]. Fe II
multiplets, usually present in optical spectra of NLS1 galaxies, are also
detected in RXJ1236.9+2656.Comment: 4 pages, A&A style Latex, To apear in A&A as a research not
Electronic Structure and Thermoelectric Prospects of Phosphide Skutterudites
The prospects for high thermoelectric performance in phosphide skutterudites
are investigated based on first principles calculations. We find that
stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in
that it is metallic. As such the band structure must be modified if high
thermopowers are to be achieved. In analogy to the antimonides it is expected
that this may be done by filling with La. Calculations for LaFe_4P_12 show that
a gap can in fact be opened by La filling, but that the valence band is too
light to yield reasonable p-type thermopowers at appropriate carrier densities;
n-type La filled material may be more favorable.Comment: 3 pages, 3 figures, 1 tabl
A 10-day ASCA Observation of the Narrow-line Seyfert~1 galaxy IRAS 13224-3809
(Abridged) We present an analysis of a 10-day continuous ASCA observation of
the narrow-line Seyfert 1 galaxy IRAS 13224-3809. The soft (0.7-1.3 keV) and
hard (1.3-10 keV) X-ray band light curves binned to 5000s reveal trough-to-peak
variations by a factor >25 and 20, respectively. The light curves in the soft
and hard bands are strongly correlated without any significant delay. However,
this correlation is not entirely due to changes in the power-law flux alone but
also due to changes in the soft X-ray hump emission above the power law. The
presence of a soft X-ray hump below 2 keV, previously detected in ROSAT and
ASCA data, is confirmed. Time resolved spectroscopy using daily sampling
reveals changes in the power-law slope, with Gamma in the range 1.74-2.47,
however, day-to-day variations in Gamma are not significant. The Soft hump
emission is found to dominate the observed variability on a timescale of a
week, but on shorter timescales (20000s) the power-law component appears to
dominate the observed variability. Flux resolved spectroscopy reveals that at
high flux levels the power law becomes steeper and the soft hump more
pronounced. The steepening of the photon index with the fluxes in the soft and
hard bands can be understood in the framework of disk/corona models in which
accretion disk is heated by viscous dissipation as well as by reprocessing of
hard X-rays following an X-ray flare resulting from coronal dissipation through
magnetic reconnection events.Comment: 29 pages, 16 figures, To apear in A&
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