10,411 research outputs found
Digital adaptive controllers for VTOL vehicles. Volume 1: Concept evaluation
A digital self-adaptive flight control system was developed for flight test in the VTOL approach and landing technology (VALT) research aircraft (a modified CH-47 helicopter). The control laws accept commands from an automatic on-board guidance system. The primary objective of the control laws is to provide good command-following with a minimum cross-axis response. Three attitudes and vertical velocity are separately commanded. Adaptation of the control laws is based on information from rate and attitude gyros and a vertical velocity measurement. The final design resulted from a comparison of two different adaptive concepts--one based on explicit parameter estimates from a real-time maximum-likelihood estimation algorithm, the other based on an implicit model reference adaptive system. The two designs were compared on the basis of performance and complexity
Digital adaptive controllers for VTOL vehicles. Volume 2: Software documentation
The VTOL approach and landing test (VALT) adaptive software is documented. Two self-adaptive algorithms, one based on an implicit model reference design and the other on an explicit parameter estimation technique were evaluated. The organization of the software, user options, and a nominal set of input data are presented along with a flow chart and program listing of each algorithm
Final state interactions in two-particle interferometry
We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".Comment: 12 pages RevTeX, 2 ps-figures included, submitted to Phys. Rev.
Causality Violations in Cascade Models of Nuclear Collisions
Transport models have successfully described many aspects of intermediate
energy heavy-ion collision dynamics. As the energies increase in these models
to the ultrarelativistic regime, Lorentz covariance and causality are not
strictly respected. The standard argument is that such effects are not
important to final results; but they have not been seriously considered at high
energies. We point out how and why these happen, how serious of a problem they
may be and suggest ways of reducing or eliminating the undesirable effects.Comment: RevTeX, 23 pages, 9 (uuencoded) figures; to appear in Phys. Rev
Molecular Realism in Default Models for Information Theories of Hydrophobic Effects
This letter considers several physical arguments about contributions to
hydrophobic hydration of inert gases, constructs default models to test them
within information theories, and gives information theory predictions using
those default models with moment information drawn from simulation of liquid
water. Tested physical features include: packing or steric effects, the role of
attractive forces that lower the solvent pressure, and the roughly tetrahedral
coordination of water molecules in liquid water. Packing effects (hard sphere
default model) and packing effects plus attractive forces (Lennard-Jones
default model) are ineffective in improving the prediction of hydrophobic
hydration free energies of inert gases over the previously used Gibbs and flat
default models. However, a conceptually simple cluster Poisson model that
incorporates tetrahedral coordination structure in the default model is one of
the better performers for these predictions. These results provide a partial
rationalization of the remarkable performance of the flat default model with
two moments in previous applications. The cluster Poisson default model thus
will be the subject of further refinement.Comment: 5 pages including 3 figure
Revisiting scaling relations for giant radio halos in galaxy clusters
Many galaxy clusters host Megaparsec-scale radio halos, generated by
ultrarelativistic electrons in the magnetized intracluster medium. Correlations
between the power of radio halos and the thermal properties of the hosting
clusters were established in the last decade, including the connection between
the presence of a halo and cluster mergers. The X-ray luminosity and redshift
limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for
statistical studies of the halos. We uniformly analyze the radio and X-ray data
for the GMRT cluster sample, and use the new Planck SZ catalog, to revisit the
correlations between the power of halos and the thermal properties of galaxy
clusters. We find that the radio power of halos at 1.4 GHz scales with the
cluster X-ray (0.1--2.4 keV) luminosity computed within R_500 as P_1.4
L_500^2.0. Our bigger and more homogenous sample confirms that the X-ray
luminous (L_500 > 5x10^44 erg/s) clusters branch into two populations --- radio
halos lie on the correlation, while clusters with upper limits to radio-halo
emission are well below that correlation. This bimodality remains if we excise
cool cores from the X-ray luminosities. Correlating with Planck data, we find
that P_1.4 scales with the cluster integrated SZ signal within R_500 as P_1.4
Y_500^2.1, in line with previous findings. However, contrary to previous
studies that were limited by incompleteness and small sample size, we find that
the "SZ-luminous" Y_500 > 6x10^-5 Mpc^2 clusters show a bimodal behavior
similar to that in the radio-X-ray diagram. Bimodality of both correlations can
be traced to clusters dynamics, with radio halos found exclusively in merging
clusters. These results confirm the key role of mergers for the origin of giant
radio halos, suggesting that they trigger the relativistic particle
acceleration.Comment: 16 pages, 7 figures, accepted for publication in ApJ on September 12,
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