1,260 research outputs found
Natural cement and monumental restoration
Natural cement, called "Romanâ cement, was invented at the end of the 19th century and played an important role in the development of civil engineering works until the 1860s. More surprisingly, it was also used to restore historic buildings, such as gothic cathedrals. This paper deals with the mineralogy and the durability of natural cement in the particular case of the Bourges Cathedral in France. This study illustrates the interest of this material particularly adapted in stone repair or substitution. Contrary to traditional mortars, the present samples are made of neat cement paste, revealed by the absence of mineral additions as quartz or carbonate sand. Several combined techniques (SEM-EDS, TGA, XRD) were carried out to determine the composition of the hydraulic binder rich in calcium aluminate hydrates. The raw marl at the origin of the cement production contains oxidized pyrites which consist in a potential source of sulphate pollution of the surrounding limestone. The exposition of the cement in urban environment leads to some weathering features as atmospheric sulfation. Finally a petrophysical approach, based on water porosity, capillary sorption and compressive strength, has been performed to demonstrate the durability and the compatibility of natural cement applied as an historical building restoration morta
Random perfect lattices and the sphere packing problem
Motivated by the search for best lattice sphere packings in Euclidean spaces
of large dimensions we study randomly generated perfect lattices in moderately
large dimensions (up to d=19 included). Perfect lattices are relevant in the
solution of the problem of lattice sphere packing, because the best lattice
packing is a perfect lattice and because they can be generated easily by an
algorithm. Their number however grows super-exponentially with the dimension so
to get an idea of their properties we propose to study a randomized version of
the algorithm and to define a random ensemble with an effective temperature in
a way reminiscent of a Monte-Carlo simulation. We therefore study the
distribution of packing fractions and kissing numbers of these ensembles and
show how as the temperature is decreased the best know packers are easily
recovered. We find that, even at infinite temperature, the typical perfect
lattices are considerably denser than known families (like A_d and D_d) and we
propose two hypotheses between which we cannot distinguish in this paper: one
in which they improve Minkowsky's bound phi\sim 2^{-(0.84+-0.06) d}, and a
competitor, in which their packing fraction decreases super-exponentially,
namely phi\sim d^{-a d} but with a very small coefficient a=0.06+-0.04. We also
find properties of the random walk which are suggestive of a glassy system
already for moderately small dimensions. We also analyze local structure of
network of perfect lattices conjecturing that this is a scale-free network in
all dimensions with constant scaling exponent 2.6+-0.1.Comment: 19 pages, 22 figure
The structure of invariant tori in a 3D galactic potential
We study in detail the structure of phase space in the neighborhood of stable
periodic orbits in a rotating 3D potential of galactic type. We have used the
color and rotation method to investigate the properties of the invariant tori
in the 4D spaces of section. We compare our results with those of previous
works and we describe the morphology of the rotational, as well as of the tube
tori in the 4D space. We find sticky chaotic orbits in the immediate
neighborhood of sets of invariant tori surrounding 3D stable periodic orbits.
Particularly useful for galactic dynamics is the behavior of chaotic orbits
trapped for long time between 4D invariant tori. We find that they support
during this time the same structure as the quasi-periodic orbits around the
stable periodic orbits, contributing however to a local increase of the
dispersion of velocities. Finally we find that the tube tori do not appear in
the 3D projections of the spaces of section in the axisymmetric Hamiltonian we
examined.Comment: 26 pages, 34 figures, accepted for publication in the International
Journal of Bifurcation and Chao
Definition of an indicator assessing the impact of a dam on the downstream river landscape
The increasing number of water withdrawals in Alpine regions represents a significant threat to aquatic ecosystems and river landscape (riverscape). To assess their sustainability, the impacts on river ecological status and landscape features need to be quantified with appropriate indicators. However, assessment of landscape attributes is a complex challenge, due to the lack of standardized methods. Moreover, few metrics quantifying the impacts of water withdrawal on downstream riverscape perception are available in the scientific literature. In this paper, a new indicator, named Landscape Protection Level (LPL), aimed at assessing the effects of water withdrawals on the river landscape, is presented. The indicator has been developed in Aosta Valley (NW Italian Alps), where the river network is heavily exploited by hundreds of withdrawals for hydropower production and irrigation, and it has been included in a multi-criteria analysis (MCA) procedure to assess the sustainability of water withdrawal licenses in relation to different flow release scenarios. The LPL indicator is based on three parameters, Constraint Factor, Release Factor, and Visual Elements Factor, quantifying the presence of landscape protection constraints, the ratio of flow released downstream of the dam to the available river discharge, and the impact on the visual perception of the bypassed stretch, respectively. Its application in four real case studies of existing hydropower plants is presented and discussed in the paper, demonstrating the indicator applicability to assess both specific release values and flow release scenarios varying over the year. Results are analyzed by highlighting the main strengths and weaknesses of the indicator and proposing some suggestions for future improvements. In particular, the reactiveness of the indicator, the representativeness of the stakeholdersâ interests, the transparency of the indicator calculation procedure, and the time required for data collection and processing are discussed. Finally, future activities aimed at further improving the indicator applicability and transferability to different river contexts are proposed
The Distribution of Bar and Spiral Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamental property
of the galaxy population that has only begun to be explored. We have applied
the bar/spiral separation method of Buta, Block, and Knapen to derive the
distribution of maximum relative gravitational bar torques, Q_b, for 147 spiral
galaxies in the statistically well-defined Ohio State University Bright Galaxy
Survey (OSUBGS) sample. Our goal is to examine the properties of bars as
independently as possible of their associated spirals. We find that the
distribution of bar strength declines smoothly with increasing Q_b, with more
than 40% of the sample having Q_b <= 0.1. In the context of recurrent bar
formation, this suggests that strongly-barred states are relatively short-lived
compared to weakly-barred or non-barred states. We do not find compelling
evidence for a bimodal distribution of bar strengths. Instead, the distribution
is fairly smooth in the range 0.0 <= Q_b < 0.8. Our analysis also provides a
first look at spiral strengths Q_s in the OSU sample, based on the same torque
indicator. We are able to verify a possible weak correlation between Q_s and
Q_b, in the sense that galaxies with the strongest bars tend also to have
strong spirals.Comment: Accepted for publication in the Astronomical Journal, August 2005
issue (LaTex, 23 pages + 11 figures, uses aastex.cls
Boundary Limitation of Wavenumbers in Taylor-Vortex Flow
We report experimental results for a boundary-mediated wavenumber-adjustment
mechanism and for a boundary-limited wavenumber-band of Taylor-vortex flow
(TVF). The system consists of fluid contained between two concentric cylinders
with the inner one rotating at an angular frequency . As observed
previously, the Eckhaus instability (a bulk instability) is observed and limits
the stable wavenumber band when the system is terminated axially by two rigid,
non-rotating plates. The band width is then of order at small
() and agrees well with
calculations based on the equations of motion over a wide -range.
When the cylinder axis is vertical and the upper liquid surface is free (i.e.
an air-liquid interface), vortices can be generated or expelled at the free
surface because there the phase of the structure is only weakly pinned. The
band of wavenumbers over which Taylor-vortex flow exists is then more narrow
than the stable band limited by the Eckhaus instability. At small
the boundary-mediated band-width is linear in . These results are
qualitatively consistent with theoretical predictions, but to our knowledge a
quantitative calculation for TVF with a free surface does not exist.Comment: 8 pages incl. 9 eps figures bitmap version of Fig
Regularization of Linear Ill-posed Problems by the Augmented Lagrangian Method and Variational Inequalities
We study the application of the Augmented Lagrangian Method to the solution
of linear ill-posed problems. Previously, linear convergence rates with respect
to the Bregman distance have been derived under the classical assumption of a
standard source condition. Using the method of variational inequalities, we
extend these results in this paper to convergence rates of lower order, both
for the case of an a priori parameter choice and an a posteriori choice based
on Morozov's discrepancy principle. In addition, our approach allows the
derivation of convergence rates with respect to distance measures different
from the Bregman distance. As a particular application, we consider sparsity
promoting regularization, where we derive a range of convergence rates with
respect to the norm under the assumption of restricted injectivity in
conjunction with generalized source conditions of H\"older type
Structure of Disk Dominated Galaxies I. Bulge/Disk Parameters, Simulations, and Secular Evolution
(Abridged) A robust analysis of galaxy structural parameters, based on the
modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented
for 121 face-on and moderately inclined late-type spirals. Each surface
brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge
and an exponential disk. The reliability and limitations of our bulge-to-disk
(B/D) decompositions are tested with extensive simulations of galaxy brightness
profiles (1D) and images (2D). Galaxy types are divided into 3 classes
according to their SB profile shapes; Freeman Type-I and Type-II, and a third
``Transition'' class for galaxies whose profiles change from Type-II in the
optical to Type-I in the infrared. We discuss possible interpretations of
Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I
late-type spirals shows a range between n=0.1-2 but, on average, the underlying
surface density profile for the bulge and disk of these galaxies is adequately
described by a double-exponential distribution. We confirm a coupling between
the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or
h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent
N-body simulations of disk formation and models of secular evolution. This
ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types.
The similar scaling relations for early and late-type spirals suggest
comparable formation and/or evolution scenarios for disk galaxies of all Hubble
types.Comment: 78 pages with 23 embedded color figures + tables of galaxy structural
parameters. Accepted for publication in the Astrophysical Journal. The
interested reader is strongly encouraged to ignore some of the low res
figures within; instead, download the high resolution version from
http://www.astro.ubc.ca/people/courteau/public/macarthur02_disks.ps.g
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