5,884 research outputs found
Shuttle orbiter boundary layer transition at flight and wind tunnel conditions
Hypersonic boundary layer transition data obtained on the windward centerline of the Shuttle orbiter during entry for the first five flights are presented and analyzed. Because the orbiter surface is composed of a large number of thermal protection tiles, the transition data include the effects of distributed roughness arising from tile misalignment and gaps. These data are used as a benchmark for assessing and improving the accuracy of boundary layer transition predictions based on correlations of wind tunnel data taken on both aerodynamically rough and smooth orbiter surfaces. By comparing these two data bases, the relative importance of tunnel free stream noise and surface roughness on orbiter boundary layer transition correlation parameters can be assessed. This assessment indicates that accurate predications of transition times can be made for the orbiter at hypersonic flight conditions by using roughness dominated wind tunnel data. Specifically, times of transition onset and completion is accurately predicted using a correlation based on critical and effective values of a roughness Reynolds number previously derived from wind tunnel data
The Kovacs effect in model glasses
We discuss the `memory effect' discovered in the 60's by Kovacs in
temperature shift experiments on glassy polymers, where the volume (or energy)
displays a non monotonous time behaviour. This effect is generic and is
observed on a variety of different glassy systems (including granular
materials). The aim of this paper is to discuss whether some microscopic
information can be extracted from a quantitative analysis of the `Kovacs hump'.
We study analytically two families of theoretical models: domain growth and
traps, for which detailed predictions of the shape of the hump can be obtained.
Qualitatively, the Kovacs effect reflects the heterogeneity of the system: its
description requires to deal not only with averages but with a full probability
distribution (of domain sizes or of relaxation times). We end by some
suggestions for a quantitative analysis of experimental results.Comment: 17 pages, 6 figures; revised versio
Relaxation of spherical systems with long-range interactions: a numerical investigation
The process of relaxation of a system of particles interacting with
long-range forces is relevant to many areas of Physics. For obvious reasons, in
Stellar Dynamics much attention has been paid to the case of 1/r^2 force law.
However, recently the interest in alternative gravities emerged, and
significant differences with respect to Newtonian gravity have been found in
relaxation phenomena. Here we begin to explore this matter further, by using a
numerical model of spherical shells interacting with an 1/r^alpha force law
obeying the superposition principle. We find that the virialization and
phase-mixing times depend on the exponent alpha, with small values of alpha
corresponding to longer relaxation times, similarly to what happens when
comparing for N-body simulations in classical gravity and in Modified Newtonian
Dynamics.Comment: 6 pages, 3 figures, accepted in the International Journal of
Bifurcation and Chao
On a conjecture by Boyd
The aim of this note is to prove the Mahler measure identity
which was conjectured by
Boyd. The proof is achieved by proving relationships between regulators of both
curves
Orion revisited. II. The foreground population to Orion A
Following the recent discovery of a large population of young stars in front
of the Orion Nebula, we carried out an observational campaign with the DECam
wide-field camera covering ~10~deg^2 centered on NGC 1980 to confirm, probe the
extent of, and characterize this foreground population of pre-main-sequence
stars. We confirm the presence of a large foreground population towards the
Orion A cloud. This population contains several distinct subgroups, including
NGC1980 and NGC1981, and stretches across several degrees in front of the Orion
A cloud. By comparing the location of their sequence in various color-magnitude
diagrams with other clusters, we found a distance and an age of 380pc and
5~10Myr, in good agreement with previous estimates. Our final sample includes
2123 candidate members and is complete from below the hydrogen-burning limit to
about 0.3Msun, where the data start to be limited by saturation. Extrapolating
the mass function to the high masses, we estimate a total number of ~2600
members in the surveyed region. We confirm the presence of a rich, contiguous,
and essentially coeval population of about 2600 foreground stars in front of
the Orion A cloud, loosely clustered around NGC1980, NGC1981, and a new group
in the foreground of the OMC-2/3. For the area of the cloud surveyed, this
result implies that there are more young stars in the foreground population
than young stars inside the cloud. Assuming a normal initial mass function, we
estimate that between one to a few supernovae must have exploded in the
foreground population in the past few million years, close to the surface of
Orion A, which might be responsible, together with stellar winds, for the
structure and star formation activity in these clouds. This long-overlooked
foreground stellar population is of great significance, calling for a revision
of the star formation history in this region of the Galaxy.Comment: Accepted for publication in A&
On the Adam-Gibbs-Wolynes scenario for the viscosity increase in glasses
We reformulate the interpretation of the mean-field glass transition scenario
for finite dimensional systems, proposed by Wolynes and collaborators.
This allows us to establish clearly a temperature dependent length xi* above
which the mean-field glass transition picture has to be modified. We argue in
favor of the mosaic state introduced by Wolynes and collaborators, which leads
to the Adam-Gibbs relation between the viscosity and configurational entropy of
glass forming liquids.
Our argument is a mixture of thermodynamics and kinetics, partly inspired by
the Random Energy
Model: small clusters of particles are thermodynamically frozen in low energy
states, whereas large clusters are kinetically frozen by large activation
energies. The relevant relaxation time is that of the smallest `liquid'
clusters. Some physical consequences are discussed.Comment: 8 page
Linear and non linear response in the aging regime of the 1D trap model
We investigate the behaviour of the response function in the one dimensional
trap model using scaling arguments that we confirm by numerical simulations. We
study the average position of the random walk at time tw+t given that a small
bias h is applied at time tw. Several scaling regimes are found, depending on
the relative values of t, tw and h. Comparison with the diffusive motion in the
absence of bias allows us to show that the fluctuation dissipation relation is,
in this case, valid even in the aging regime.Comment: 5 pages, 3 figures, 3 references adde
The Seven Sisters DANCe III: Projected spatial distribution
Methods. We compute Bayesian evidences and Bayes Factors for a set of
variations of the classical radial models by King (1962), Elson et al. (1987)
and Lauer et al. (1995). The variations incorporate different degrees of model
freedom and complexity, amongst which we include biaxial (elliptical) symmetry,
and luminosity segregation. As a by-product of the model comparison, we obtain
posterior distributions and maximum a posteriori estimates for each set of
model parameters. Results. We find that the model comparison results depend on
the spatial extent of the region used for the analysis. For a circle of 11.5
parsecs around the cluster centre (the most homogeneous and complete region),
we find no compelling reason to abandon Kings model, although the Generalised
King model, introduced in this work, has slightly better fitting properties.
Furthermore, we find strong evidence against radially symmetric models when
compared to the elliptic extensions. Finally, we find that including mass
segregation in the form of luminosity segregation in the J band, is strongly
supported in all our models. Conclusions. We have put the question of the
projected spatial distribution of the Pleiades cluster on a solid probabilistic
framework, and inferred its properties using the most exhaustive and least
contaminated list of Pleiades candidate members available to date. Our results
suggest however that this sample may still lack about 20% of the expected
number of cluster members. Therefore, this study should be revised when the
completeness and homogeneity of the data can be extended beyond the 11.5
parsecs limit. Such study will allow a more precise determination of the
Pleiades spatial distribution, its tidal radius, ellipticity, number of objects
and total mass.Comment: 39 pages, 31 figure
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