1,496 research outputs found
Lagrangian Based Methods for Coherent Structure Detection
There has been a proliferation in the development of Lagrangian analytical methods for detecting coherent structures in fluid flow transport, yielding a variety of qualitatively different approaches. We present a review of four approaches and demonstrate the utility of these methods via their application to the same sample analytic model, the canonical double-gyre flow, highlighting the pros and cons of each approach. Two of the methods, the geometric and probabilistic approaches, are well established and require velocity field data over the time interval of interest to identify particularly important material lines and surfaces, and influential regions, respectively. The other two approaches, implementing tools from cluster and braid theory, seek coherent structures based on limited trajectory data, attempting to partition the flow transport into distinct regions. All four of these approaches share the common trait that they are objective methods, meaning that their results do not depend on the frame of reference used. For each method, we also present a number of example applications ranging from blood flow and chemical reactions to ocean and atmospheric flows. (C) 2015 AIP Publishing LLC.ONR N000141210665Center for Nonlinear Dynamic
Effect of Photometric Redshift Uncertainties on Weak Lensing Tomography
We perform a systematic analysis of the effects of photometric redshift
uncertainties on weak lensing tomography. We describe the photo-z distribution
with a bias and Gaussian scatter that are allowed to vary arbitrarily between
intervals of dz = 0.1 in redshift.While the mere presence of bias and scatter
does not substantially degrade dark energy information, uncertainties in both
parameters do. For a fiducial next-generation survey each would need to be
known to better than about 0.003-0.01 in redshift for each interval in order to
lead to less than a factor of 1.5 increase in the dark energy parameter errors.
The more stringent requirement corresponds to a larger dark energy parameter
space, when redshift variation in the equation of state of dark energy is
allowed.Of order 10^4-10^5 galaxies with spectroscopic redshifts fairly sampled
from the source galaxy distribution will be needed to achieve this level of
calibration. If the sample is composed of multiple galaxy types, a fair sample
would be required for each. These requirements increase in stringency for more
ambitious surveys; we quantify such scalings with a convenient fitting formula.
No single aspect of a photometrically binned selection of galaxies such as
their mean or median suffices, indicating that dark energy parameter
determinations are sensitive to the shape and nature of outliers in the photo-z
redshift distribution.Comment: 10 pages, 12 figures, accepted by Ap
Recombination Induced Softening and Reheating of the Cosmic Plasma
The atomic recombination process leads to a softening of the matter equation
of state as reflected by a reduced generalized adiabatic index, with
accompanying heat release. We study the effects of this recombination softening
and reheating of the cosmic plasma on the ionization history, visibility
function, Cold Dark Matter (CDM) transfer function, and the Cosmic Microwave
Background (CMB) spectra. The resulting modifications of the CMB spectrm is
1/10 of WMAP's current error and is comparable to PLANCK's error. Therefore,
this effect should be considered when data with higher accuracy are analysed.Comment: 11 pages, 6 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society; as advised by referee, omit high-baryon mode
A Modified TreePM Code
We discuss the performance characteristics of using the modification of the
tree code suggested by Barnes \citep{1990JCoPh..87..161B} in the context of the
TreePM code. The optimisation involves identifying groups of particles and
using only one tree walk to compute force for all the particles in the group.
This modification has been in use in our implementation of the TreePM code for
some time, and has also been used by others in codes that make use of tree
structures. In this paper, we present the first detailed study of the
performance characteristics of this optimisation. We show that the
modification, if tuned properly can speed up the TreePM code by a significant
amount. We also combine this modification with the use of individual time steps
and indicate how to combine these two schemes in an optimal fashion. We find
that the combination is at least a factor of two faster than the modified
TreePM without individual time steps. Overall performance is often faster by a
larger factor, as the scheme of groups optimises use of cache for large
simulations.Comment: 16 pages, 5 figures; Accepted for publication in Research In
Astronomy and Astrophysics (RAA
Halo bias in the excursion set approach with correlated steps
In the Excursion Set approach, halo abundances and clustering are closely
related. This relation is exploited in many modern methods which seek to
constrain cosmological parameters on the basis of the observed spatial
distribution of clusters. However, to obtain analytic expressions for these
quantities, most Excursion Set based predictions ignore the fact that, although
different k-modes in the initial Gaussian field are uncorrelated, this is not
true in real space: the values of the density field at a given spatial
position, when smoothed on different real-space scales, are correlated in a
nontrivial way. We show that when the excursion set approach is extended to
include such correlations, then one must be careful to account for the fact
that the associated prediction for halo bias is explicitly a real-space
quantity. Therefore, care must be taken when comparing the predictions of this
approach with measurements in simulations, which are typically made in
Fourier-space. We show how to correct for this effect, and demonstrate that
ignorance of this effect in recent analyses of halo bias has led to incorrect
conclusions and biased constraints.Comment: 7 pages, 3 figures; v2 -- minor clarifications, accepted in MNRA
The Coyote Universe I: Precision Determination of the Nonlinear Matter Power Spectrum
Near-future cosmological observations targeted at investigations of dark
energy pose stringent requirements on the accuracy of theoretical predictions
for the clustering of matter. Currently, N-body simulations comprise the only
viable approach to this problem. In this paper we demonstrate that N-body
simulations can indeed be sufficiently controlled to fulfill these requirements
for the needs of ongoing and near-future weak lensing surveys. By performing a
large suite of cosmological simulation comparison and convergence tests we show
that results for the nonlinear matter power spectrum can be obtained at 1%
accuracy out to k~1 h/Mpc. The key components of these high accuracy
simulations are: precise initial conditions, very large simulation volumes,
sufficient mass resolution, and accurate time stepping. This paper is the first
in a series of three, with the final aim to provide a high-accuracy prediction
scheme for the nonlinear matter power spectrum.Comment: 18 pages, 22 figures, minor changes to address referee repor
Upholding heightened expectations of Indigenous children? Parents do, teachers do not
This paper argues that a component of increasing the rate of Aboriginal and Torres Strait Islander children and youths completing their secondary education is having parents and teachers maintain heightened expectations of these children in achieving this goal. To understand this phenomenon, we investigate the importance of, and discrepancies between, primary caregiver and teacher outlooks regarding Indigenous youths completing year 12. For the purpose of this paper, we adopt the term âprimary caregiverâ in place of parent. This is because the majority (87.7%) of P1s analysed are the biological mothers with the remainder being close female relatives. P2s analysed are all male, 93.3% are biological fathers; remainder are step-fathers or adoptive fathers. This paper uses quantitative data from the Longitudinal Study of Indigenous Children to measure expectations from parents and teachers of Indigenous children. Results suggest that parents maintain exceptionally high expectations of their children, while teacher's expectations significantly decline over the course of Indigenous children's primary and secondary schooling years. We suggest that relationships and communication between parents and teachers, regarding expectations of students, are important to establishing an equilibrium in expectations of children, and that teachers may benefit from further training to address any underlying biases towards Indigenous children
Redshift Evolution of the Nonlinear Two-Point Correlation Function
This paper presents a detailed theoretical study of the two-point correlation
function for both dark matter halos and the matter density field in five
cosmological models with varying matter density and neutrino
fraction . The objectives of this systematic study are to evaluate
the nonlinear gravitational effects on , to contrast the behavior of
for halos vs. matter, and to quantify the redshift evolution of and its
dependence on cosmological parameters. Overall, for halos exhibits
markedly slower evolution than for matter, and its redshift dependence is
much more intricate than the single power-law parameterization used in the
literature. Of particular interest is that the redshift evolution of the
halo-halo correlation length depends strongly on and
, being slower in models with lower or higher
. Measurements of to higher redshifts can therefore be a
potential discriminator of cosmological parameters. The evolution rate of
for halos within a given model increases with time, passing the phase of fixed
comoving clustering at to 3 toward the regime of stable clustering at
. The shape of the halo-halo , on the other hand, is well
approximated by a power law with slope -1.8 in all models and is not a
sensitive model discriminator.Comment: 22 pages, 8 postscript figures, AAS LaTeX v4.0. Accepted for
publication in The Astrophysical Journal, Vol. 510 (January 1 1999
Power spectrum for the small-scale Universe
The first objects to arise in a cold dark matter universe present a daunting
challenge for models of structure formation. In the ultra small-scale limit,
CDM structures form nearly simultaneously across a wide range of scales.
Hierarchical clustering no longer provides a guiding principle for theoretical
analyses and the computation time required to carry out credible simulations
becomes prohibitively high. To gain insight into this problem, we perform
high-resolution (N=720^3 - 1584^3) simulations of an Einstein-de Sitter
cosmology where the initial power spectrum is P(k) propto k^n, with -2.5 < n <
-1. Self-similar scaling is established for n=-1 and n=-2 more convincingly
than in previous, lower-resolution simulations and for the first time,
self-similar scaling is established for an n=-2.25 simulation. However, finite
box-size effects induce departures from self-similar scaling in our n=-2.5
simulation. We compare our results with the predictions for the power spectrum
from (one-loop) perturbation theory and demonstrate that the renormalization
group approach suggested by McDonald improves perturbation theory's ability to
predict the power spectrum in the quasilinear regime. In the nonlinear regime,
our power spectra differ significantly from the widely used fitting formulae of
Peacock & Dodds and Smith et al. and a new fitting formula is presented.
Implications of our results for the stable clustering hypothesis vs. halo model
debate are discussed. Our power spectra are inconsistent with predictions of
the stable clustering hypothesis in the high-k limit and lend credence to the
halo model. Nevertheless, the fitting formula advocated in this paper is purely
empirical and not derived from a specific formulation of the halo model.Comment: 30 pages including 10 figures; accepted for publication in MNRA
Explicit computation of shear three-point correlation functions: the one-halo model case
We present a method for calculating explicit expressions of the shear
three-point function for various cosmological models. The method is applied
here to the one-halo model in case of power law density profiles for which
results are detailed. The three-point functions are found to reproduce to a
large extent patterns in the shear correlations obtained in numerical
simulations and may serve as a guideline to implement optimized methods for
detecting the shear three-point function. In principle, the general method
presented here can also be applied for other models of matter correlation.Comment: 8 pages, 6 figures, submitted to A
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