35,022 research outputs found
Preferential concentration of inertial sub-kolmogorov particles. The roles of mass loading of particles, Stokes and Reynolds numbers
Turbulent flows laden with inertial particles present multiple open questions
and are a subject of great interest in current research. Due to their higher
density compared to the carrier fluid, inertial particles tend to form high
concentration regions, i.e. clusters, and low concentration regions, i.e.
voids, due to the interaction with the turbulence. In this work, we present an
experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov
particles in homogeneous isotropic turbulent flows. Three control parameters
have been varied over significant ranges: ,
and volume fraction . The scaling of clustering characteristics, such as the distribution
of Vorono\"i areas and the dimensions of cluster and void regions, with the
three parameters are discussed. In particular, for the polydispersed size
distributions considered here, clustering is found to be enhanced strongly
(quasi-linearly) by and noticeably (with a square-root
dependency) with , while the cluster and void sizes, scaled with the
Kolmogorov lengthscale , are driven primarily by . Cluster
length scales up to , measured
at the highest , while void length
scaled also with is typically two times larger ().
The lack of sensitivity of the above characteristics to the Stokes number lends
support to the "sweep-stick" particle accumulation scenario. The non-negligible
influence of the volume fraction, however, is not considered by that model and
can be connected with collective effects
From patterned response dependency to structured covariate dependency: categorical-pattern-matching
Data generated from a system of interest typically consists of measurements
from an ensemble of subjects across multiple response and covariate features,
and is naturally represented by one response-matrix against one
covariate-matrix. Likely each of these two matrices simultaneously embraces
heterogeneous data types: continuous, discrete and categorical. Here a matrix
is used as a practical platform to ideally keep hidden dependency among/between
subjects and features intact on its lattice. Response and covariate dependency
is individually computed and expressed through mutliscale blocks via a newly
developed computing paradigm named Data Mechanics. We propose a categorical
pattern matching approach to establish causal linkages in a form of information
flows from patterned response dependency to structured covariate dependency.
The strength of an information flow is evaluated by applying the combinatorial
information theory. This unified platform for system knowledge discovery is
illustrated through five data sets. In each illustrative case, an information
flow is demonstrated as an organization of discovered knowledge loci via
emergent visible and readable heterogeneity. This unified approach
fundamentally resolves many long standing issues, including statistical
modeling, multiple response, renormalization and feature selections, in data
analysis, but without involving man-made structures and distribution
assumptions. The results reported here enhance the idea that linking patterns
of response dependency to structures of covariate dependency is the true
philosophical foundation underlying data-driven computing and learning in
sciences.Comment: 32 pages, 10 figures, 3 box picture
The Panchromatic Hubble Andromeda Treasury. Progression of Large-Scale Star Formation across Space and Time in M31
We investigate the clustering of early-type stars younger than 300 Myr on
galactic scales in M31. Based on the stellar photometric catalogs of the
Panchromatic Hubble Andromeda Treasury program that also provides stellar
parameters derived from the individual energy distributions, our analysis is
focused on the young stars in three star-forming regions, located at
galactocentric distances of about 5, 10, and 15 kpc, corresponding to the inner
spiral arms, the ring structure, and the outer arm, respectively. We apply the
two-point correlation function to our selected sample to investigate the
clustering behavior of these stars across different time- and length-scales. We
find that young stellar structure survives across the whole extent of M31
longer than 300 Myr. Stellar distribution in all regions appears to be
self-similar, with younger stars being systematically more strongly clustered
than the older, which are more dispersed. The observed clustering is
interpreted as being induced by turbulence, the driving source for which is
probably gravitational instabilities driven by the spiral arms, which are
stronger closer to the galactic centre.Comment: 10 pages, 5 figures. To appear in "LESSONS FROM THE LOCAL GROUP - A
Conference in Honour of David Block and Bruce Elmegreen" eds. Freeman, K.C.,
Elmegreen, B.G., Block, D.L. & Woolway, M. (Springer: New York
Probing the Radio Loud/Quiet AGN dichotomy with quasar clustering
We investigate the clustering properties of 45441 radio-quiet quasars (RQQs)
and 3493 radio-loud quasars (RLQs) drawn from a joint use of the Sloan Digital
Sky Survey (SDSS) and Faint Images of the Radio Sky at 20 cm (FIRST) surveys in
the range . This large spectroscopic quasar sample allow us to
investigate the clustering signal dependence on radio-loudness and black hole
(BH) virial mass. We find that RLQs are clustered more strongly than RQQs in
all the redshift bins considered. We find a real-space correlation length of
and
{\normalsize{}for} RQQs and
RLQs, respectively, for the full redshift range. This implies that RLQs are
found in more massive host haloes than RQQs in our samples, with mean host halo
masses of and
, respectively. Comparison with
clustering studies of different radio source samples indicates that this mass
scale of is characteristic for the
bright radio-population, which corresponds to the typical mass of galaxy groups
and galaxy clusters. The similarity we find in correlation lengths and host
halo masses for RLQs, radio galaxies and flat-spectrum radio quasars agrees
with orientation-driven unification models. Additionally, the clustering signal
shows a dependence on black hole (BH) mass, with the quasars powered by the
most massive BHs clustering more strongly than quasars having less massive BHs.
We suggest that the current virial BH mass estimates may be a valid BH proxies
for studying quasar clustering. We compare our results to a previous
theoretical model that assumes that quasar activityComment: 15 pages, 13 figures, A&A in pres
Measuring the growth of matter fluctuations with third-order galaxy correlations
Measurements of the linear growth factor at different redshifts are
key to distinguish among cosmological models. One can estimate the derivative
from redshift space measurements of the 3D anisotropic galaxy
two-point correlation , but the degeneracy of its transverse (or
projected) component with galaxy bias , i.e. , introduces large errors in the growth measurement. Here we present a
comparison between two methods which break this degeneracy by combining second-
and third-order statistics. One uses the shape of the reduced three-point
correlation and the other a combination of third-order one- and two-point
cumulants. These methods use the fact that, for Gaussian initial conditions and
scales larger than Mpc, the reduced third-order matter
correlations are independent of redshift (and therefore of the growth factor)
while the third-order galaxy correlations depend on . We use matter and halo
catalogs from the MICE-GC simulation to test how well we can recover and
therefore with these methods in 3D real space. We also present a new
approach, which enables us to measure directly from the redshift evolution
of second- and third-order galaxy correlations without the need of modelling
matter correlations. For haloes with masses lower than
M, we find deviations between the different estimates of
, which are comparable to current observational errors. At higher masses we
find larger differences that can probably be attributed to the breakdown of the
bias model and non-Poissonian shot noise.Comment: 24 pages, 20 figures, 2 tables, accepted for publication in MNRA
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