8,972 research outputs found
Flow field predictions for a slab delta wing at incidence
Theoretical results are presented for the structure of the hypersonic flow field of a blunt slab delta wing at moderately high angle of attack. Special attention is devoted to the interaction between the boundary layer and the inviscid entropy layer. The results are compared with experimental data. The three-dimensional inviscid flow is computed numerically by a marching finite difference method. Attention is concentrated on the windward side of the delta wing, where detailed comparisons are made with the data for shock shape and surface pressure distributions. Surface streamlines are generated, and used in the boundary layer analysis. The three-dimensional laminar boundary layer is computed numerically using a specially-developed technique based on small cross-flow in streamline coordinates. In the rear sections of the wing the boundary layer decreases drastically in the spanwise direction, so that it is still submerged in the entropy layer at the centerline, but surpasses it near the leading edge. Predicted heat transfer distributions are compared with experimental data
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
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Associated reading skills in children with a history of Specific Language Impairment (SLI)
A large cohort of 200 eleven-year-old children with Specific Language Impairment (SLI) were assessed on basic reading accuracy and on reading comprehension as well as language tasks. Reading skills were examined descriptively and in relation to early language and literacy factors. Using stepwise regression analyses in which age and nonverbal IQ were controlled for, it was found that a single word reading measure taken at 7 years was unsurprisingly a strong predictor of the two different types of reading ability. However, even with this measure included, a receptive syntax task (TROG) entered when reading accuracy score was the DV. Furthermore, a test of expressive syntax/narrative and a receptive syntax task completed at 7 years entered into the model for word reading accuracy. When early reading accuracy was excluded from the analyses, early phonological skills also entered as a predictor of both reading accuracy and comprehension at 11 years. The group of children with a history of SLI were then divided into those with no literacy difficulties at 11 and those with some persisting literacy impairment. Using stepwise logistic regression, and again controlling for IQ and age, 7 years receptive syntax score (but not tests of phonology, expressive vocabulary or expressive syntax/narrative) entered as a positive predictor of membership of the âno literacy problemsâ group regardless of whether early reading accuracy was controlled for in step one. The findings are discussed in relation to the overlap of SLI and dyslexia and the long term sequelae of language impairment
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Narrative skills in adolescents with a history of SLI in relation to non-verbal IQ scores
There is a debate about whether the language of children with primary language disorders and normal cognitive levels is qualitatively different from those with language impairments who have low or borderline non-verbal IQ (NVIQ). As children reach adolescence, this distinction may be even harder to ascertain, especially in naturalistic settings. Narrative may provide a useful, ecologically valid way in which to assess the language ability of adolescents with specific language impairment (SLI) who have intact or lowered NVIQ and to determine whether there is any discernable difference in every day language. Nineteen adolescents with a history of SLI completed two narrative tasks: a story telling condition and a conversational condition. Just under half the group (n = 8) had non-verbal IQs of 85. The remaining 11 had NVIQs in the normal range or above. Four areas of narrative (productivity, syntax, cohesion and performance) were assessed. There were no differences between the groups on standardized tests of language. However, the group with low NVIQ were poorer on most aspects of narrative, suggesting that cognitive level is important, even when language is the primary disorder. The groups showed similar patterns of differences between story telling and conversational narrative. It was concluded that adolescents with a history of SLI and poor cognitive levels have poorer narrative skills than those with normal range NVIQ even though these may not be detected by standardized assessment. Their difficulties present as qualitatively similar to those with normal range NVIQ and narratives appear impoverished rather than inaccurate
Observation of Fermi-Pasta-Ulam-Tsingou Recurrence and Its Exact Dynamics
One of the most controversial phenomena in nonlinear dynamics is the reappearance of initial
conditions. Celebrated as the Fermi-Pasta-Ulam-Tsingou problem, the attempt to understand how these
recurrences form during the complex evolution that leads to equilibrium has deeply influenced the entire
development of nonlinear science. The enigma is rendered even more intriguing by the fact that integrable
models predict recurrence as exact solutions, but the difficulties involved in upholding integrability for a
sufficiently long dynamic has not allowed a quantitative experimental validation. In natural processes,
coupling with the environment rapidly leads to thermalization, and finding nonlinear multimodal systems
presenting multiple returns is a long-standing open challenge. Here, we report the observation of more than
three Fermi-Pasta-Ulam-Tsingou recurrences for nonlinear optical spatial waves and demonstrate the
control of the recurrent behavior through the phase and amplitude of the initial field. The recurrence period
and phase shift are found to be in remarkable agreement with the exact recurrent solution of the nonlinear
Schrödinger equation, while the recurrent behavior disappears as integrability is lost. These results identify
the origin of the recurrence in the integrability of the underlying dynamics and allow us to achieve one of
the basic aspirations of nonlinear dynamics: the reconstruction, after several return cycles, of the exact
initial condition of the system, ultimately proving that the complex evolution can be accurately predicted in
experimental conditions
The Stellar Content of Obscured Galactic Giant H II Regions
Near infrared images of the Galactic giant HII region W43 reveal a dense
stellar cluster at its center. Broad band JHK photometry of the young cluster
and K-band spectra of three of its bright stars are presented. The 2 micron
spectrum of the brightest star in the cluster is very well matched to the
spectra of Wolf-Rayet stars of sub-type WN7. Two other stars are identified as
O type giants or supergiants by their NIII and CIV emission. The close spatial
clustering of O and the hydrogen WN type stars is analogous to the intense star
burst clusters R136 in the Large Magellanic Cloud and NGC3603 in the Galaxy.Comment: 22 pages (LaTex), including 7 figures (eps
Harmonic damped oscillators with feedback. A Langevin study
We consider a system in direct contact with a thermal reservoir and which, if
left unperturbed, is well described by a memory-less equilibrium Langevin
equation of the second order in the time coordinate. In such conditions, the
strength of the noise fluctuations is set by the damping factor, in accordance
with the Fluctuation and Dissipation theorem. We study the system when it is
subject to a feedback mechanism, by modifying the Langevin equation
accordingly. Memory terms now arise in the time evolution, which we study in a
non-equilibrium steady state. Two types of feedback schemes are considered, one
focusing on time shifts and one on phase shifts, and for both cases we evaluate
the power spectrum of the system's fluctuations. Our analysis finds application
in feedback cooled oscillators, such as the Gravitational Wave detector AURIGA.Comment: 17 page
Infrared Observations of the Candidate LBV 1806-20 & Nearby Cluster Stars
We report near-infrared photometry, spectroscopy, and speckle imaging of the
hot, luminous star we identify as candidate LBV 1806-20. We also present
photometry and spectroscopy of 3 nearby stars, which are members of the same
star cluster containing LBV 1806-20 and SGR 1806-20. The spectroscopy and
photometry show that LBV 1806-20 is similar in many respects to the luminous
``Pistol Star'', albeit with some important differences. They also provide
estimates of the effective temperature and reddening of LBV 1806-20, and
confirm distance estimates, leading to a best estimate for the luminosity of
this star of . The nearby cluster stars have
spectral types and inferred absolute magnitudes which confirm the distance (and
thus luminosity) estimate for LBV 1806-20. If we drop kinematic measurements of
the distance ( kpc), we have a lower limit on the distance
of kpc, and on the luminosity of , based on
the cluster stars. If we drop both the kinematic and cluster star indicators
for distance, an ammonia absorption feature sets yet another lower limit to the
distance of kpc, with a corresponding luminosity estimate of for the candidate LBV 1806-20. Furthermore, based on very high
angular-resolution speckle images, we determine that LBV 1806-20 is not a
cluster of stars, but is rather a single star or binary system. Simple
arguments based on the Eddington luminosity lead to an estimate of the total
mass of LBV 1806-20 (single or binary) exceeding . We discuss
the possible uncertainties in these results, and their implications for the
star formation history of this cluster.Comment: 36 pages, including 8 figures (Figures 1 and 7 in JPG format due to
space); Accepted for publication in Ap
The Stellar Content of Obscured Galactic Giant H II Regions III.: W31
We present near infrared (J, H, and K) photometry and moderate resolution
(lambda/Deltalambda = 3000) K-band spectroscopy of the embedded stellar cluster
in the giant H II region W31. Four of the brightest five cluster members are
early O--type stars based on their spectra. We derive a spectro--photometric
distance for W31 of 3.4 +/- 0.3 kpc using these new spectral types and infrared
photometry. The brightest cluster source at K is a red object which lies in the
region of the J - H vs. H - K color--color plot inhabited by stars with excess
emission in the K-band. This point source has an H plus K-band spectrum which
shows no photospheric features, which we interpret as being the result of
veiling by local dust emission. Strong Brackett series emission and permitted
FeII emission are detected in this source; the latter feature is suggestive of
a dense inflow or outflow. The near infrared position of this red source is
consistent with the position of a 5 GHz thermal radio source seen in previous
high angular resolution VLA images. We also identify several other K-band
sources containing excess emission with compact radio sources. These objects
may represent stars in the W31 cluster still embedded in their birth cocoons.Comment: LaTeX2e/aastex, 29 pages including 9 figures, 3 table
WR 7a: a V Sagittae or a qWR star?
The star WR 7a, also known as SPH 2, has a spectrum that resembles that of V
Sagittae stars although no O VI emission has been reported. The Temporal
Variance Spectrum - TVS - analysis of our data shows weak but strongly variable
emission of O VI lines which is below the noise level in the intensity
spectrum. Contrary to what is seen in V Sagittae stars, optical photometric
monitoring shows very little, if any, flickering. We found evidence of periodic
variability. The most likely photometric period is P(phot) = 0.227(14) d, while
radial velocities suggest a period of P(spec) = 0.204(13) d. One-day aliases of
these periods can not be ruled out. We call attention to similarities with HD
45166 and DI Cru (= WR 46), where multiple periods are present. They may be
associated to the binary motion or to non-radial oscillations. In contrast to a
previous conclusion by Pereira et al. (1998), we show that WR 7a contains
hydrogen. The spectrum of the primary star seems to be detectable as the N V
4604A absorption line is visible. If so, it means that the wind is optically
thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR - quasi
Wolf-Rayet - star. Its classification is WN4h/CE in the Smith et al. (1996)
three dimensional classification system.Comment: 6 pages, 6 figures, preprint of an article accepted for publication
in Monthly Notices of The Royal Astronomical Societ
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