193 research outputs found
Identification of Nonlinear Normal Modes of Engineering Structures under Broadband Forcing
The objective of the present paper is to develop a two-step methodology
integrating system identification and numerical continuation for the
experimental extraction of nonlinear normal modes (NNMs) under broadband
forcing. The first step processes acquired input and output data to derive an
experimental state-space model of the structure. The second step converts this
state-space model into a model in modal space from which NNMs are computed
using shooting and pseudo-arclength continuation. The method is demonstrated
using noisy synthetic data simulated on a cantilever beam with a
hardening-softening nonlinearity at its free end.Comment: Journal pape
Comparison between control-based continuation and phase-locked loop methods for the identification of backbone curves and nonlinear frequency responses
Control-based continuation (CBC) and phase-locked loops (PLL) are two experimental testing methods that have demonstrated great potential for the non-parametric identification of key nonlinear dynamic features such as nonlinear frequency responses and backbone curves. Both CBC and PLL exploit stabilizing feedback control to steer the dynamics of the tested system towards the responses of interest and overcome important difficulties experienced when applying conventional testing methods such as sine sweeps to nonlinear systems. For instance, if properly designed, the feedback controller can prevent the system from exhibiting untimely transitions between coexisting responses or even losing stability due to bifurcations. This contribution aims to highlight the similarities that exist between CBC and PLL and present the first thorough comparison of their capabilities. Comparisons are supported by numerical simulations as well as experimental data collected on a conceptually simple nonlinear structure primarily composed of a thin curved beam. The beam is doubly clamped and exhibits nonlinear geometric effects for moderate excitation amplitudes
Comparison between control-based continuation and phase-locked loop methods for the identification of backbone curves and nonlinear frequency responses
Control-based continuation (CBC) and phase-locked loops (PLL) are two experimental testing methods that have demonstrated great potential for the non-parametric identification of key nonlinear dynamic features such as nonlinear frequency responses and backbone curves. Both CBC and PLL exploit stabilizing feedback control to steer the dynamics of the tested system towards the responses of interest and overcome important difficulties experienced when applying conventional testing methods such as sine sweeps to nonlinear systems. For instance, if properly designed, the feedback controller can prevent the system from exhibiting untimely transitions between coexisting responses or even losing stability due to bifurcations. This contribution aims to highlight the similarities that exist between CBC and PLL and present the first thorough comparison of their capabilities. Comparisons are supported by numerical simulations as well as experimental data collected on a conceptually simple nonlinear structure primarily composed of a thin curved beam. The beam is doubly clamped and exhibits nonlinear geometric effects for moderate excitation amplitudes
Magnetic chemically peculiar stars
Chemically peculiar (CP) stars are main-sequence A and B stars with
abnormally strong or weak lines for certain elements. They generally have
magnetic fields and all observables tend to vary with the same period.
Chemically peculiar stars provide a wealth of information; they are natural
atomic and magnetic laboratories. After a brief historical overview, we discuss
the general properties of the magnetic fields in CP stars, describe the oblique
rotator model, explain the dependence of the magnetic field strength on the
rotation, and concentrate at the end on HgMn stars.Comment: 9 pages, 4 figures, 2 tables, chapter in "Determination of
Atmospheric Parameters of B-, A-, F- and G-Type Stars", Springer (2014), eds.
E. Niemczura, B. Smalley, W. Pyc
Deep Impact : High Resolution Optical Spectroscopy with the ESO VLT and the Keck 1 telescope
We report on observations of comet 9P/Tempel 1 carried out before, during,
and after the NASA DEEP IMPACT event (UT July 4), with the optical
spectrometers UVES and HIRES mounted on the telescopes Kueyen of the ESO VLT
(Chile) and Keck 1 on Mauna Kea (Hawaii), respectively. A total observing time
of about 60 hours, distributed over 15 nights around the impact date, allowed
us (i) to find a periodic variation of 1.709 +/- 0.009 day in the CN and NH
flux, explained by the presence of two major active regions; (ii) to derive a
lifetime > ~ 5 x 10^4 s for the parent of the CN radical from a simple modeling
of the CN light curve after the impact; (iii) to follow the gas and dust
spatial profiles evolution during the 4 hours following the impact and derive
the projected velocities (400 m/s and 150 m/s respectively); (iv) to show that
the material released by the impact has the same carbon and nitrogen isotopic
composition as the surface material (12C/13C = 95 +/- 15 and 14N/15N = 145 +/-
20).Comment: Accepted for publication in ApJ Letter
The radius and effective temperature of the binary Ap star beta CrB from CHARA/FLUOR and VLT/NACO observations
The prospects for using asteroseismology of rapidly oscillating Ap (roAp)
stars are hampered by the large uncertainty in fundamental stellar parameters.
Results in the literature for the effective temperature (Teff) often span a
range of 1000 K. Our goal is to reduce systematic errors and improve the Teff
calibration of Ap stars based on new interferometric measurements. We obtained
long-baseline interferometric observations of beta CrB using the CHARA/FLUOR
instrument. To disentangle the flux contributions of the two components of this
binary star, we obtained VLT/NACO adaptive optics images. We determined limb
darkened angular diameters of 0.699+-0.017 mas for beta CrB A (from
interferometry) and 0.415+-0.017 mas for beta CrB B (from surface brightness-
color relations), corresponding to radii of 2.63+-0.09 Rsun (3.4 percent
uncertainty) and 1.56+-0.07 Rsun (4.5 percent). The combined bolometric flux of
the A and B components was determined from satellite UV data, spectrophotometry
in the visible and broadband data in the infrared. The flux from the B
component constitutes 16+-4 percent of the total flux and was determined by
fitting an ATLAS9 model atmosphere to the broad-band NACO J and K magnitudes.
Combining the flux of the A component with its measured angular diameter, we
determine the effective temperature Teff(A) = 7980+-180 K (2.3 percent). Our
new interferometric and imaging data enable a nearly model-independent
determination of the effective temperature of beta CrB A. Including our recent
study of alpha Cir, we now have direct Teff measurements of two of the
brightest roAp stars, providing a strong benchmark for an improved calibration
of the Teff scale for Ap stars. This will support the use of potentially strong
constraints imposed by asteroseismic studies of roAp stars.Comment: 7 pages, accepted by A&
Lambda Boo stars with composite spectra
We examine the large sample of lambda Boo candidates collected in Table 1 of
Gerbaldi et al. (2003) to see how many of them show composite spectra. Of the
132 lambda Boo candidates we identify 22 which definitely show composite
spectra and 15 more for which there are good reasons to suspect a composite
spectrum. The percentage of lambda Boo candidates with composite spectra is
therefore > 17 and possibly considerably higher. For such stars the lambda Boo
classification should be reconsidered taking into account the fact that their
spectra are composite. We argue that some of the underabundances reported in
the literature may simply be the result of the failure to consider the
composite nature of the spectra. This leads to the legitimate suspicion that
some, if not all, the lambda Boo candidates are not chemically peculiar at all.
A thorough analysis of even a single one of the lambda Boo candidates with
composite spectra, in which the composite nature of the spectrum is duly
considered, which would demonstrate that the chemical peculiarities persist,
would clear the doubt we presently have that the stars with composite spectra
may not be lambda Boo at all.Comment: Accepted for publication by A&A on June 3rd 200
Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one?
The eclipses in binary stars give precise information of orbital period
changes. Goodricke discovered the 2.867 days period in the eclipses of Algol in
the year 1783. The irregular orbital period changes of this longest known
eclipsing binary continue to puzzle astronomers. The mass transfer between the
two members of this binary should cause a long-term increase of the orbital
period, but observations over two centuries have not confirmed this effect.
Here, we present evidence indicating that the period of Algol was 2.850 days
three millenia ago. For religious reasons, the ancient Egyptians have recorded
this period into the Cairo Calendar, which describes the repetitive changes of
the Raging one. Cairo Calendar may be the oldest preserved historical document
of the discovery of a variable star.Comment: 26 pages, 5 figures, 11 table
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