844 research outputs found
The three dimensional skeleton: tracing the filamentary structure of the universe
The skeleton formalism aims at extracting and quantifying the filamentary
structure of the universe is generalized to 3D density fields; a numerical
method for computating a local approximation of the skeleton is presented and
validated here on Gaussian random fields. This method manages to trace well the
filamentary structure in 3D fields such as given by numerical simulations of
the dark matter distribution on large scales and is insensitive to monotonic
biasing. Two of its characteristics, namely its length and differential length,
are analyzed for Gaussian random fields. Its differential length per unit
normalized density contrast scales like the PDF of the underlying density
contrast times the total length times a quadratic Edgeworth correction
involving the square of the spectral parameter. The total length scales like
the inverse square smoothing length, with a scaling factor given by 0.21 (5.28+
n) where n is the power index of the underlying field. This dependency implies
that the total length can be used to constrain the shape of the underlying
power spectrum, hence the cosmology. Possible applications of the skeleton to
galaxy formation and cosmology are discussed. As an illustration, the
orientation of the spin of dark halos and the orientation of the flow near the
skeleton is computed for dark matter simulations. The flow is laminar along the
filaments, while spins of dark halos within 500 kpc of the skeleton are
preferentially orthogonal to the direction of the flow at a level of 25%.Comment: 17 pages, 11 figures, submitted to MNRA
Vlasov versus N-body: the H\'enon sphere
We perform a detailed comparison of the phase-space density traced by the
particle distribution in Gadget simulations to the result obtained with a
spherical Vlasov solver using the splitting algorithm. The systems considered
are apodized H\'enon spheres with two values of the virial ratio, R ~ 0.1 and
0.5. After checking that spherical symmetry is well preserved by the N-body
simulations, visual and quantitative comparisons are performed. In particular
we introduce new statistics, correlators and entropic estimators, based on the
likelihood of whether N-body simulations actually trace randomly the Vlasov
phase-space density. When taking into account the limits of both the N-body and
the Vlasov codes, namely collective effects due to the particle shot noise in
the first case and diffusion and possible nonlinear instabilities due to finite
resolution of the phase-space grid in the second case, we find a spectacular
agreement between both methods, even in regions of phase-space where nontrivial
physical instabilities develop. However, in the colder case, R=0.1, it was not
possible to prove actual numerical convergence of the N-body results after a
number of dynamical times, even with N=10 particles.Comment: 19 pages, 11 figures, MNRAS, in pres
Two-point correlation functions on the light cone: testing theoretical predictions against N-body simulations
We examine the light-cone effect on the two-point correlation functions using numerical simulations for the first time. Specifically, we generate several sets of dark matter particle distributions on the light-cone up to z=0.4 and z=2 over the field-of-view of \pi degree^2 from cosmological N-body simulations. Then we apply the selection function to the dark matter distribution according to the galaxy and QSO luminosity functions. Finally we compute the two-point correlation functions on the light-cone both in real and in redshift spaces using the pair-count estimator and compare with the theoretical predictions. We find that the previous theoretical modeling for nonlinear gravitational evolution, linear and nonlinear redshift-distortion, and the light-cone effect including the selection function is in good agreement with our numerical results, and thus is an accurate and reliable description of the clustering in the universe on the light-cone
The effect of baryons on the variance and the skewness of the mass distribution in the Universe at small scales
We study the dissipative effects of baryon physics on cosmic statistics at small scales using a cosmological simulation of a (50 Mpc hâ1)3 volume of universe. The MareNostrum simulation was performed using the adaptive mesh refinement (AMR) code ramses, and includes most of the physical ingredients which are part of the current theory of galaxy formation, such as metal-dependent cooling and UV heating, subgrid modelling of the interstellar medium, star formation and supernova feedback. We reran the same initial conditions for a dark matter only universe, as a reference point for baryon-free cosmic statistics. In this paper, we present the measured small-scale amplification of Ï2 and S3 due to baryonic physics and their interpretation in the framework of the halo model. As shown in recent studies, the effect of baryons on the matter power spectrum can be accounted for at scales kâČ 10 h Mpcâ1 by modifying the halo concentration parameter. We propose to extend this result by using a composite halo profile, which is a linear combination of a Navarro, Frenk and White profile for the dark matter component and an exponential disc profile mimicking the baryonic component at the heart of the halo. This halo profile form is physically motivated and depends on two parameters, the mass fraction f d of baryons in the disc and the ratio λd of the disc's characteristic scale to the halo's virial radius. We find this composite profile to reproduce both the small-scale variance and skewness boosts measured in the simulation up to k⌠102 h Mpcâ1 for physically meaningful values of the parameters f d and λd. Although simulations like the one presented here usually suffer from various problems when compared to observations, our modified halo model could be used as a fitting model to improve the determination of cosmological parameters from weak lensing convergence spectra and skewness measurement
Analytical solution of the full-range behavior of adhesively bonded FRP-steel joints made with toughened adhesives
Fiber-reinforced polymer (FRP) composites represent an effective solution to strengthen and retrofit existing steel members. Namely, bonded or unbonded carbon FRP (CFRP) plates have been employed to improve the strength, fatigue behavior, and durability of steel bridges. In bonded solutions, the effectiveness of the CFRP reinforcement strongly depends on the adhesive employed to bond the plate, as failure usually occurs due to debonding. Within this framework, the use of toughened adhesives is particularly attractive since they may improve the load carrying capacity of the CFRP-steel interface, also providing a certain ductility. Debonding in CFRP-steel joints was previously studied using a cohesive approach. However, solutions able to describe the full-range behavior of joints with toughened adhesives and finite bonded length are not available in the literature. In this paper, a trapezoidal (trilinear) cohesive material law (CML) is employed to model the bond behavior of pultruded carbon FRP-steel joints made with a rubber-toughened epoxy adhesive, which showed cohesive debonding within the adhesive layer. The analytical solution provided is employed to study the experimental response of nine CFRP-steel joints tested using a single-lap direct shear set-up. Comparisons of analytical and experimental results of joints with three different bonded lengths confirm the effectiveness of the solution proposed
Source-lens clustering effects on the skewness of the lensing convergence
The correlation between source galaxies and lensing potentials causes a
systematic effect on measurements of cosmic shear statistics, known as the
source-lens clustering (SLC) effect. The SLC effect on the skewness of lensing
convergence, , is examined using a nonlinear semi-analytic approach and is
checked against numerical simulations. The semi-analytic calculations have been
performed in a wide variety of generic models for the redshift distribution of
source galaxies and power-law models for the bias parameter between the galaxy
and dark matter distributions. The semi-analytic predictions are tested
successfully against numerical simulations. We find the relative amplitude of
the SLC effect on to be of the order of five to forty per cent. It
depends significantly on the redshift distribution of sources and on the way
the bias parameter evolves. We discuss possible measurement strategies to
minimize the SLC effects.Comment: 14 pages, 14 figures, accepted for publication in MNRA
A comparison of estimators for the two-point correlation function
Nine of the most important estimators known for the two-point correlation
function are compared using a predetermined, rigorous criterion. The indicators
were extracted from over 500 subsamples of the Virgo Hubble Volume simulation
cluster catalog. The ``real'' correlation function was determined from the full
survey in a 3000Mpc/h periodic cube. The estimators were ranked by the
cumulative probability of returning a value within a certain tolerance of the
real correlation function. This criterion takes into account bias and variance,
and it is independent of the possibly non-Gaussian nature of the error
statistics. As a result for astrophysical applications a clear recommendation
has emerged: the Landy & Szalay (1993) estimator, in its original or grid
version Szapudi & Szalay (1998), are preferred in comparison to the other
indicators examined, with a performance almost indistinguishable from the
Hamilton (1993) estimator.Comment: aastex, 10 pages, 1 table, 1 figure, revised version, accepted in
ApJ
Long-Term Behavior of PBO FRCM and Comparison with Other Inorganic-Matrix Composites
Fabric-reinforced cementitious matrix (FRCM) composites, comprising high-strength fiber textiles embedded within inorganic matrices, represent an effective, cost-efficient, and low-invasive solution for strengthening and retrofitting existing masonry and reinforced concrete structures. Among different textiles employed in FRCM composites, polyparaphenylene benzo-bisoxazole (PBO) textiles are adopted due to their high tensile strength and good adhesion with the matrix. Although several experimental, numerical, and analytical works were performed to investigate the mechanical properties of PBO FRCM composites, limited information is available on their long-term behavior, as well as in the case of exposure to aggressive environments. This paper presents and discusses the results of a wide experimental campaign aimed at investigating the effect of different environmental conditions on the long-term tensile behavior of a PBO FRCM composite. Tests are performed using a clamping-grip tensile test set-up. The effect of various aggressive environments on the composite matrix cracking stress, composite tensile strength, ultimate strain, and fully cracked stage slope is investigated by comparing the results of nominally equal conditioned and unconditioned (control) specimens. These results are also compared with those of other FRCM composites comprising glass and carbon textiles subjected to the same conditionings, collected from the literature. The results show only limited reductions in the tensile properties, even after long exposure to aggressive environments
Effect of cyclic load on the tensile behavior of a PBO FRCM composite
The use of externally bonded fiber-reinforced cementitious matrix (FRCM) composites represents a valid alternative to traditional techniques for the strengthening and retrofitting of existing reinforced concrete and masonry structures. FRCM composites are comprised of high strength textiles embedded within inorganic matrices and can be directly applied to the external surface of the existing structural element to increase its displacement and load capacity (i.e., axial, flexural, and shear strength). Thus, FRCM have a low invasiveness and a high strength-to-weight ratio. Recently, investigations on the bond behavior of FRCM composites showed that the presence of friction between the textile and matrix can induce damage to the fiber, which in turn determines possible reductions in the strengthened element capacity. This effect appears particularly critical in the case of cyclic and dynamic loads. In this paper, the cyclic behavior of a PBO FRCM composite is experimentally investigated using low-cycle tensile tests on composite specimens. Namely, FRCM rectangular coupons are subjected to clamping- and clevis-grip tensile tests. These tests provide important information on the effect of low-frequency dynamic loading on the composite tensile properties under different test configurations
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