4,125 research outputs found
Accurate estimators of power spectra in N-body simulations
abridged] A method to rapidly estimate the Fourier power spectrum of a point
distribution is presented. This method relies on a Taylor expansion of the
trigonometric functions. It yields the Fourier modes from a number of FFTs,
which is controlled by the order N of the expansion and by the dimension D of
the system. In three dimensions, for the practical value N=3, the number of
FFTs required is 20. We apply the method to the measurement of the power
spectrum of a periodic point distribution that is a local Poisson realization
of an underlying stationary field. We derive explicit analytic expression for
the spectrum, which allows us to quantify--and correct for--the biases induced
by discreteness and by the truncation of the Taylor expansion, and to bound the
unknown effects of aliasing of the power spectrum. We show that these aliasing
effects decrease rapidly with the order N. The only remaining significant
source of errors is reduced to the unavoidable cosmic/sample variance due to
the finite size of the sample. The analytical calculations are successfully
checked against a cosmological N-body experiment. We also consider the initial
conditions of this simulation, which correspond to a perturbed grid. This
allows us to test a case where the local Poisson assumption is incorrect. Even
in that extreme situation, the third-order Fourier-Taylor estimator behaves
well. We also show how to reach arbitrarily large dynamic range in Fourier
space (i.e., high wavenumber), while keeping statistical errors in control, by
appropriately "folding" the particle distribution.Comment: 18 Pages, 9 Figures. Accepted for publication in MNRAS. The
Fourier-Taylor module as well as the associated power spectrum estimator tool
we propose is available as an F90 package, POWMES, at
http://www.projet-horizon.fr or on request from the author
Time shifted aliasing error upper bounds for truncated sampling cardinal series
AbstractTime shifted aliasing error upper bound extremals for the sampling reconstruction procedure are fully characterized. Sharp upper bounds are found on the aliasing error of truncated cardinal series and the corresponding extremals are described for entire functions from certain specific Lp, p>1, classes. Analogous results are obtained in multidimensional regular sampling. Truncation error analysis is provided in all cases considered. Moreover, sharpness of bounding inequalities, convergence rates and various sufficient conditions are discussed
Numerical approximation of poroelasticity with random coefficients using Polynomial Chaos and Hybrid High-Order methods
In this work, we consider the Biot problem with uncertain poroelastic
coefficients. The uncertainty is modelled using a finite set of parameters with
prescribed probability distribution. We present the variational formulation of
the stochastic partial differential system and establish its well-posedness. We
then discuss the approximation of the parameter-dependent problem by
non-intrusive techniques based on Polynomial Chaos decompositions. We
specifically focus on sparse spectral projection methods, which essentially
amount to performing an ensemble of deterministic model simulations to estimate
the expansion coefficients. The deterministic solver is based on a Hybrid
High-Order discretization supporting general polyhedral meshes and arbitrary
approximation orders. We numerically investigate the convergence of the
probability error of the Polynomial Chaos approximation with respect to the
level of the sparse grid. Finally, we assess the propagation of the input
uncertainty onto the solution considering an injection-extraction problem.Comment: 30 pages, 15 Figure
A Neural Model of How the Brain Computes Heading from Optic Flow in Realistic Scenes
Animals avoid obstacles and approach goals in novel cluttered environments using visual information, notably optic flow, to compute heading, or direction of travel, with respect to objects in the environment. We present a neural model of how heading is computed that describes interactions among neurons in several visual areas of the primate magnocellular pathway, from retina through V1, MT+, and MSTd. The model produces outputs which are qualitatively and quantitatively similar to human heading estimation data in response to complex natural scenes. The model estimates heading to within 1.5° in random dot or photo-realistically rendered scenes and within 3° in video streams from driving in real-world environments. Simulated rotations of less than 1 degree per second do not affect model performance, but faster simulated rotation rates deteriorate performance, as in humans. The model is part of a larger navigational system that identifies and tracks objects while navigating in cluttered environments.National Science Foundation (SBE-0354378, BCS-0235398); Office of Naval Research (N00014-01-1-0624); National-Geospatial Intelligence Agency (NMA201-01-1-2016
Optical Identification of Cepheids in 19 Host Galaxies of Type Ia Supernovae and NGC 4258 with the Hubble Space Telescope
We present results of an optical search for Cepheid variable stars using the
Hubble Space Telescope (HST) in 19 hosts of Type Ia supernovae (SNe Ia) and the
maser-host galaxy NGC 4258, conducted as part of the SH0ES project (Supernovae
and H0 for the Equation of State of dark energy). The targets include 9 newly
imaged SN Ia hosts using a novel strategy based on a long-pass filter that
minimizes the number of HST orbits required to detect and accurately determine
Cepheid properties. We carried out a homogeneous reduction and analysis of all
observations, including new universal variability searches in all SN Ia hosts,
that yielded a total of 2200 variables with well-defined selection criteria --
the largest such sample identified outside the Local Group. These objects are
used in a companion paper to determine the local value of H0 with a total
uncertainty of 2.4%.Comment: ApJ, in press. v2 adds missing co-author to arXiv metadata and text
in acknowledgment
Higher-order derivative correlations and the alignment of small-scale structures in isotropic numerical turbulence
In a three dimensional simulation higher order derivative correlations, including skewness and flatness factors, are calculated for velocity and passive scalar fields and are compared with structures in the flow. The equations are forced to maintain steady state turbulence and collect statistics. It is found that the scalar derivative flatness increases much faster with Reynolds number than the velocity derivative flatness, and the velocity and mixed derivative skewness do not increase with Reynolds number. Separate exponents are found for the various fourth order velocity derivative correlations, with the vorticity flatness exponent the largest. Three dimensional graphics show strong alignment between the vorticity, rate of strain, and scalar-gradient fields. The vorticity is concentrated in tubes with the scalar gradient and the largest principal rate of strain aligned perpendicular to the tubes. Velocity spectra, in Kolmogorov variables, collapse to a single curve and a short minus 5/3 spectral regime is observed
Generation of Vorticity and Velocity Dispersion by Orbit Crossing
We study the generation of vorticity and velocity dispersion by orbit
crossing using cosmological numerical simulations, and calculate the
backreaction of these effects on the evolution of large-scale density and
velocity divergence power spectra. We use Delaunay tessellations to define the
velocity field, showing that the power spectra of velocity divergence and
vorticity measured in this way are unbiased and have better noise properties
than for standard interpolation methods that deal with mass weighted
velocities. We show that high resolution simulations are required to recover
the correct large-scale vorticity power spectrum, while poor resolution can
spuriously amplify its amplitude by more than one order of magnitude. We
measure the scalar and vector modes of the stress tensor induced by orbit
crossing using an adaptive technique, showing that its vector modes lead, when
input into the vorticity evolution equation, to the same vorticity power
spectrum obtained from the Delaunay method. We incorporate orbit crossing
corrections to the evolution of large scale density and velocity fields in
perturbation theory by using the measured stress tensor modes. We find that at
large scales (k~0.1 h/Mpc) vector modes have very little effect in the density
power spectrum, while scalar modes (velocity dispersion) can induce percent
level corrections at z=0, particularly in the velocity divergence power
spectrum. In addition, we show that the velocity power spectrum is smaller than
predicted by linear theory until well into the nonlinear regime, with little
contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new
appendix. Accepted by PR
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