278 research outputs found
Renormalization Group theory outperforms other approaches in statistical comparison between upscaling techniques for porous media
Determining the pressure differential required to achieve a desired flow rate
in a porous medium requires solving Darcy's law, a Laplace-like equation, with
a spatially varying tensor permeability. In various scenarios, the permeability
coefficient is sampled at high spatial resolution, which makes solving Darcy's
equation numerically prohibitively expensive. As a consequence, much effort has
gone into creating upscaled or low-resolution effective models of the
coefficient while ensuring that the estimated flow rate is well reproduced,
bringing to fore the classic tradeoff between computational cost and numerical
accuracy. Here we perform a statistical study to characterize the relative
success of upscaling methods on a large sample of permeability coefficients
that are above the percolation threshold. We introduce a new technique based on
Mode-Elimination Renormalization-Group theory (MG) to build coarse-scale
permeability coefficients. Comparing the results with coefficients upscaled
using other methods, we find that MG is consistently more accurate,
particularly so due to its ability to address the tensorial nature of the
coefficients. MG places a low computational demand, in the manner that we have
implemented it, and accurate flow-rate estimates are obtained when using
MG-upscaled permeabilities that approach or are beyond the percolation
threshold.Comment: 15 pages, 7 figures, Physical Review
The impedance boundary condition for acoustics in swirling ducted flow
The acoustics of a straight annular lined duct containing a swirling mean flow is considered. The classical Ingard–Myers impedance boundary condition is shown not to be correct for swirling flow. By considering behaviour within the thin boundary layers at the duct walls, the correct impedance boundary condition for an infinitely thin boundary layer with swirl is derived, which reduces to the Ingard–Myers condition when the swirl is set to zero. The correct boundary condition contains a spring-like term due to centrifugal acceleration at the walls, and consequently has a different sign at the inner (hub) and outer (tip) walls. Examples are given for mean flows relevant to the interstage region of aeroengines. Surface waves in swirling flows are also considered, and are shown to obey a more complicated dispersion relation than for non-swirling flows. The stability of the surface waves is also investigated, and as in the non-swirling case, one unstable surface wave per wall is found
The dust properties and physical conditions of the interstellar medium in the LMC massive star forming complex N11
We combine Spitzer and Herschel data of the star-forming region N11 in the
Large Magellanic Cloud to produce detailed maps of the dust properties in the
complex and study their variations with the ISM conditions. We also compare
APEX/LABOCA 870um observations with our model predictions in order to decompose
the 870um emission into dust and non-dust (free-free emission and CO(3-2) line)
contributions. We find that in N11, the 870um can be fully accounted for by
these 3 components. The dust surface density map of N11 is combined with HI and
CO observations to study local variations in the gas-to-dust mass ratios. Our
analysis leads to values lower than those expected from the LMC low-metallicity
as well as to a decrease of the gas-to-dust mass ratio with the dust surface
density. We explore potential hypotheses that could explain the low observed
gas-to-dust mass ratios (variations in the XCO factor, presence of CO-dark gas
or of optically thick HI or variations in the dust abundance in the dense
regions). We finally decompose the local SEDs using a Principal Component
Analysis (i.e. with no a priori assumption on the dust composition in the
complex). Our results lead to a promising decomposition of the local SEDs in
various dust components (hot, warm, cold) coherent with that expected for the
region. Further analysis on a larger sample of galaxies will follow in order to
understand how unique this decomposition is or how it evolves from one
environment to another.Comment: 24 pages, 16 figures, accepted for publication in MNRA
The applicability of FIR fine-structure lines as Star Formation Rate tracers over wide ranges of metallicities and galaxy types
We analyze the applicability of far-infrared fine-structure lines [CII] 158
micron, [OI] 63 micron and [OIII] 88 micron to reliably trace the star
formation rate (SFR) in a sample of low-metallicity dwarf galaxies from the
Herschel Dwarf Galaxy Survey and compare with a broad sample of galaxies of
various types and metallicities in the literature. We study the trends and
scatter in the relation between the SFR (as traced by GALEX FUV and MIPS 24
micron) and far-infrared line emission, on spatially resolved and global galaxy
scales, in dwarf galaxies. We assemble far-infrared line measurements from the
literature and infer whether the far-infrared lines can probe the SFR (as
traced by the total-infrared luminosity) in a variety of galaxy populations. In
metal-poor dwarfs, the [OI] and [OIII] lines show the strongest correlation
with the SFR with an uncertainty on the SFR estimates better than a factor of
2, while the link between [CII] emission and the SFR is more dispersed
(uncertainty factor of 2.6). The increased scatter in the SFR-L([CII]) relation
towards low metal abundances, warm dust temperatures, large filling factors of
diffuse, highly ionized gas suggests that other cooling lines start to dominate
depending on the density and ionization state of the gas. For the literature
sample, we evaluate the correlations for a number of different galaxy
populations. The [CII] and [OI] lines are considered to be reliable SFR tracers
in starburst galaxies, recovering the star formation activity within an
uncertainty of factor 2. [Abridged]Comment: 35 pages, 13 figures, accepted for publication in A&A on May 7th 201
Dust in the Extremely Metal-Poor Blue Compact Dwarf Galaxy IZw18: The Spitzer Mid-Infrared View
IZw18, a blue compact dwarf (BCD) galaxy with the 2nd lowest metallicity
measured in a star-forming object, has been observed with all three instruments
on board the Spitzer Space Telescope. We present the deepest 5-36 micron
mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6
to 70 micron imaging results. As with SBS0335-052E, another BCD with similar
metallicity, IZw18 shows no detectable emission from polycyclic aromatic
hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 micron, of
IZw18 has a much steeper slope, more characteristic of a typical starburst
galaxy of solar abundance. The neon abundance as measured from the infrared
fine-structure lines is ~1/23 Z_sun, and the sulfur abundance is ~ 1/35 Z_sun,
generally consistent with the nebular oxygen abundance of 1/30 Z_sun derived
from optical lines. This suggests that the extinction to the infrared emitting
regions of this galaxy is low, also in agreement with the optical Balmer line
ratios.Comment: Accepted by ApJ, 7 pages, 5 figure
Dissecting the Mid-Infrared Heart of M83 with JWST
We present a first look at the MRS observations of the nucleus of the spiral
galaxy M83, taken with MIRI onboard JWST. The observations show a rich set of
emission features from the ionized and warm molecular gas, as well as traces of
the dust properties in this highly star forming environment. To begin
dissecting the complex processes taking place in this part of the galaxy, we
divide the nucleus observations into four different regions. We find that the
strength of the emission features appears to strongly vary in all four regions,
with the south-east region displaying the weakest features tracing both the
dust continuum and ISM properties. Comparison between the cold molecular gas
traced by the CO (1-0) transition with ALMA and the H 0-0 S(1)
transition showed a similar spatial distribution throughout the nucleus. This
is in contrast to the distribution of the much warmer H emission from the
S(7) transition found to be concentrated mainly around the optical nucleus. We
modeled the H excitation using the rotational emission lines and estimate a
total molecular gas mass accounting for the warm H component of M(50
K) = 59.33 () 10 M. We
compared this value to the total molecular gas mass inferred by probing the
cold H gas through the CO (1-0) emission, M(CO) =
14.99 10 M. Our findings indicate that 75\% of
the total molecular gas mass in the core of M83 is contained in the warm H
component. We also identify [OIV]25.89 m and [FeII]25.99 m emission
(indicative of shocks) in all four nuclear regions with the strongest emission
originating from the north-west section. We propose that the diffuse
[FeII]25.99 m emission is an indication of the combined effects of both
the collective supernova explosions and the starbursts themselves.Comment: 13 pages, 3 Tables, 8 Figures, to be submitted to Ap
Novel Multi-Linear Quantitative Brain Volume Formula For Manual Radiological Evaluation Of Brain Atrophy
This research article published by Research Square, 2020The brain is a dynamic organ that develops and involutes in volume. The process of volume loss known as brain atrophy commonly occurs in elderly. However, some conditions have been implicated to provoke this paradoxical process in childhood and making it important to have methods and techniques of quantifying brain volume. Automated quantitative methods are very important in brain atrophy assessment but these tools have limited availability in developing countries. The simplified linear radiological methods are poorly reproducible and hence there is a need to develop an alternative formula that is reproducible and applicable at all healthcare levels
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