7,885 research outputs found
Development of a computer code for calculating the steady super/hypersonic inviscid flow around real configurations. Volume 1: Computational technique
A numerical procedure has been developed to compute the inviscid super/hypersonic flow field about complex vehicle geometries accurately and efficiently. A second order accurate finite difference scheme is used to integrate the three dimensional Euler equations in regions of continuous flow, while all shock waves are computed as discontinuities via the Rankine Hugoniot jump conditions. Conformal mappings are used to develop a computational grid. The effects of blunt nose entropy layers are computed in detail. Real gas effects for equilibrium air are included using curve fits of Mollier charts. Typical calculated results for shuttle orbiter, hypersonic transport, and supersonic aircraft configurations are included to demonstrate the usefulness of this tool
A continuous population of variable stars up to about 1.5 mag above the horizontal branch?
Increasing samples of pulsating variable stars populating the classical
instability strip from the horizontal branch to a few magnitudes brighter are
being found in several Local Group galaxies, irrespective of the galaxy
morphological type. We will review the observational scenario focusing in
particular on the Anomalous Cepheids and related objects.Comment: 5 pages, 2 figures, invited review in JENAM 2003, "Minisymposium:
Asteroseismology and Stellar Evolution", Communications in Asteroseismology,
in pres
Kepler observations of A-F pre-main sequence stars in Upper Scorpius: Discovery of six new ~Scuti and one ~Doradus stars
We present light curves and periodograms for 27 stars in the young Upper
Scorpius association (age=\,Myr) obtained with the Kepler spacecraft.
This association is only the second stellar grouping to host several pulsating
pre-main sequence (PMS) stars which have been observed from space. From an
analysis of the periodograms, we identify six ~Scuti variables and one
~Doradus star. These are most likely PMS stars or else very close to
the zero-age main sequence. Four of the ~Scuti variables were observed
in short-cadence mode, which allows us to resolve the entire frequency
spectrum. For these four stars, we are able to infer some qualitative
information concerning their ages. For the remaining two ~Scuti stars,
only long-cadence data are available, which means that some of the frequencies
are likely to be aliases. One of the stars appears to be a rotational variable
in a hierarchical triple system. This is a particularly important object, as it
allows the possibility of an accurate mass determination when radial velocity
observations become available. We also report on new high-resolution echelle
spectra obtained for some of the stars of our sample.Comment: 19 pages, 9 figures. Accepted for publication on MNRA
Model Identification and Adaptive State Observation for a Class of Nonlinear Systems
In this article, we consider the joint problems of state estimation and model identification for a class of continuous-time nonlinear systems in the output-feedback canonical form. An adaptive observer is proposed that combines an extended high-gain observer and a discrete-time identifier. The extended observer provides the identifier with a dataset permitting the identification of the system model and the identifier adapts the extended observer according to the new estimated model. The design of the identifier is approached as a system identification problem and sufficient conditions are presented that, if satisfied, allow different identification algorithms to be used for the adaptation phase. The cases of recursive least-squares and multiresolution black-box identification via wavelet-based identifiers are specifically addressed. Stability results are provided relating the asymptotic estimation error to the prediction capabilities of the identifier. Robustness with respect to additive disturbances affecting the system equations and measurements is also established in terms of an input-to-state stability property relative to the noiseless estimates
"Class-Type" identification-based internal models in multivariable nonlinear output regulation
The paper deals with the problem of output regulation in a “non-equilibrium” context for a special class of multivariable nonlinear systems stabilizable by high-gain feedback. A post-processing internal model design suitable for the multivariable nature of the system, which might have more inputs than regulation errors, is proposed. Uncertainties in the system and exosystem are dealt with by assuming that the ideal steady state input belongs to a certain “class of signals" by which an appropriate model set for the internal model can be derived. The adaptation mechanism for the internal model is then cast as an identification problem and a least square solution is specifically developed. In line with recent developments in the field, the vision that emerges from the paper is that approximate, possibly asymptotic, regulation is the appropriate way of approaching the problem in a multivariable and uncertain context. New insights about the use of identification tools in the design of adaptive internal models are also presented
Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
We present time series observations of intermediate mass PMS stars belonging
to the young star cluster IC 348. The new data reveal that a young member of
the cluster, H254, undergoes periodic light variations with delta Scuti-like
characteristics. This occurrence provides an unambiguous evidence confirming
the prediction that intermediate-mass pre-main sequence (PMS) stars should
experience this transient instability during their approach to the
main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able
to constrain the intrinsic stellar parameters of H254 by means of linear, non
adiabatic, radial pulsation models. The range of the resulting luminosity and
effective temperature permitted by the models is narrower than the
observational values. In particular, the pulsation analysis allows to derive an
independent estimate of the distance to IC 348 of about 320 pc. Further
observations could either confirm the monoperiodic nature of H254 or reveal the
presence of other frequencies.Comment: 7 pages, including 7 postscript figures, accepted for publication on
A&
High resolution spectroscopic analysis of seven giants in the bulge globular cluster NGC 6723
Globular clusters associated with the Galactic bulge are important tracers of
stellar populations in the inner Galaxy. High resolution analysis of stars in
these clusters allows us to characterize them in terms of kinematics,
metallicity, and individual abundances, and to compare these fingerprints with
those characterizing field populations. We present iron and element ratios for
seven red giant stars in the globular cluster NGC~6723, based on high
resolution spectroscopy. High resolution spectra () of seven K
giants belonging to NGC 6723 were obtained with the FEROS spectrograph at the
MPG/ESO 2.2m telescope. Photospheric parameters were derived from FeI
and FeII transitions. Abundance ratios were obtained from line-to-line spectrum
synthesis calculations on clean selected features. An intermediate metallicity
of [Fe/H] dex and a heliocentric radial velocity of
were found for NGC 6723. Alpha-element
abundances present enhancements of dex,
dex, dex, and
dex. Similar overabundance is found for the iron-peak Ti with
dex. Odd-Z elements Na and Al present abundances of
dex and dex, respectively. Finally,
the s-element Ba is also enhanced by dex. The enhancement
levels of NGC 6723 are comparable to those of other metal-intermediate bulge
globular clusters. In turn, these enhancement levels are compatible with the
abundance profiles displayed by bulge field stars at that metallicity. This
hints at a possible similar chemical evolution with globular clusters and the
metal-poor of the bulge going through an early prompt chemical enrichment
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