188 research outputs found
Imprints of Dark Energy on Cosmic Structure Formation: III. Sparsity of Dark Matter Halo Profiles
We study the imprint of Dark Energy on the density profile of Dark Matter
halos using a set of high-resolution large volume cosmological N-body
simulations from the Dark Energy Universe Simulation Series (DEUSS). We first
focus on the analysis of the goodness-of-fit of the Navarro-Frenk-White (NFW)
profile which we find to vary with halo mass and redshift. We also find that
the fraction of halos ill-fitted by NFW varies with cosmology, thus indicating
that the mass assembly of halos with perturbed density profiles carries a
characteristic signature of Dark Energy. To access this information
independently of any parametric profile, we introduce a new observable
quantity: the halo sparsity . This is defined as the mass ratio
, i.e. the ratio of mass inside a sphere of radius
to that contained within a radius , enclosing 200 and times
the mean matter density respectively. We find the average sparsity to be nearly
independent of the total halo mass, while its value can be inferred to better
than a few percent from the ratio of the integrated halo mass functions at
overdensities and 200 respectively. This provides a consistency
relation that can validate observational measurements of the halo sparsity.
Most importantly, the sparsity significantly varies with the underlying Dark
Energy model, thus providing an alternative cosmological probe.Comment: 12 pages, 16 figures. accepted by MNRA
Definition of a linear equivalent model for a non-linear system with impacts
International audienceModal characteristics of non-linear system are typically studied through response to harmonic excitation and using various definitions of non-linear modes. However, few results are available for systems under broadband excitation. The end objective sought here is to generate a linear system, in some sense equivalent to the non-linear system, whose modal characteristics evolve with a level of non-linearity. The considered application is the contact non-linearity found between the tubes of heat exchangers and their support plates. Such tubes, present in nuclear plants, participate to the nuclear safety and can be significantly excited by the fluid flow, so that their dynamic behavior is critical. The turbulent nature of the flow implies broadband excitation and the small gaps between the tubes and the support plate generate very significant non-linear behavior. The proposed equivalent linear system is based on a bilateral contact law whose stiffness and damping characteristics evolve with the amplitude of excitation. A non-linear model is first validated by correlation with experiments. It is then shown that three different indicators (bandwidth of main resonance, operational modal analysis of non-linear power spectral density and correlation of operational deflection shapes) lead to similar values of contact stiffness and damping in the equivalent linear model. This model is hus shown to be a very efficient tool to analyze the impact of the amplitude dependence of the non-linear behavior in the considered system
Fatigue analysis of catenary contact wires for high speed trains
The fatigue fracture is one of the most critical failures which may occur on the high speed network because it is undetectable and it has a huge impact on traffic disruption. The contact wire lifespan of a high speed line is estimated at more than 50 years and thus it is necessary to consider the risk of fatigue. The Railway Technical Research Institute in Japan studied this phenomenon for a long time and performed experimental tests. Using these results and by comparing with failures occurred in France, a preliminary analysis is carried out to identify parameters which significantly influence the fatigue phenomenon. This analysis consists in using the numerical software OSCAR© to evaluate the loads, perform a fatigue assessment of the contact wire. The procedure, using a one-dimensional and a three-dimensional model, is described in this article
OpenFEM: an open source finite element toolbox
International audienceOpenFEM is a finite element toolbox designed to be used within a matrix computing environment MATLAB and SCILAB. This toolbox provides general specifications for models, multiphysics elements, and self extending pre- and post-processing tools. This paper describes the software architecture and the existing tools.OpenFEM est une boîte à outils éléments finis développée dans les environnements de calculs matriciels MATLAB et SCILAB. Cette toolbox fournit des outils de générations de modèles, d'éléments multi-physiques, de pré- et post-traitement. Cet article décrit son architecture logicielle et ses fonctionnalités
Merging sensor data from multiple temperature scenarios for vibration monitoring of civil structures
The ambient temperature effect may result in limitations of vibration-based structural health monitoring (SHM) approaches for civil engineering structures. This paper addresses the issue of discriminating changes in modal parameters due to damages and changes in modal parameters due to temperature effects. A non parametric damage detection algorithm is proposed, which only assumes that several datasets are recorded on the safe structure at different and unknown temperatures, and smoothes out the temperature effect using an averaging operation
Bayesian approach to gravitational lens model selection: constraining H_0 with a selected sample of strong lenses
Bayesian model selection methods provide a self-consistent probabilistic
framework to test the validity of competing scenarios given a set of data. We
present a case study application to strong gravitational lens parametric
models. Our goal is to select a homogeneous lens subsample suitable for
cosmological parameter inference. To this end we apply a Bayes factor analysis
to a synthetic catalog of 500 lenses with power-law potential and external
shear. For simplicity we focus on double-image lenses (the largest fraction of
lens in the simulated sample) and select a subsample for which astrometry and
time-delays provide strong evidence for a simple power-law model description.
Through a likelihood analysis we recover the input value of the Hubble constant
to within 3\sigma statistical uncertainty. We apply this methodology to a
sample of double image lensed quasars. In the case of B1600+434, SBS 1520+530
and SDSS J1650+4251 the Bayes' factor analysis favors a simple power-law model
description with high statistical significance. Assuming a flat \LambdaCDM
cosmology, the combined likelihood data analysis of such systems gives the
Hubble constant H_0=76+15-5 km/s/Mpc having marginalized over the lens model
parameters, the cosmic matter density and consistently propagated the
observational errors on the angular position of the images. The next generation
of cosmic structure surveys will provide larger lens datasets and the method
described here can be particularly useful to select homogeneous lens subsamples
adapted to perform unbiased cosmological parameter inferenceComment: 13 pages; 13 figures; includes Bayesian analysis of a synthetic lens
catalog generated with GRAVLENS, several additional results; matches MNRAS
accepted versio
Subspace-based damage detection on steel frame structure under changing excitation
International audienceDamage detection can be performed by detecting changes in the modal parameters between a reference state and the current (possibly damaged) state of a structure from measured output-only vibration data. Alternatively, a subspace-based damage detection test has been proposed and applied successfully, where changes in the modal parameters are detected, but the estimation of the modal parameters themselves is avoided. Like this, the test can run in an automated way directly on the vibration measurements. However, it was assumed that the unmeasured ambient excitation properties during measurements of the structure in the reference and possibly damaged condition stay constant, which is hardly satisfied by any application. A new version of the test has been derived recently that is robust to such changes in the ambient excitation. In this paper, the robust test is recalled and its performance is evaluated both on numerical simulations and a real application, where a steel frame structure is artificially damaged in the lab
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