1,159 research outputs found
Natural Convection in a Square Cavity in the Presence of Heated Plate
Natural convection heat transfer in a square cavity induced by heated plate is studied numerically. Top and bottom of the cavity are adiabatic, the two vertical walls of the cavity have constant temperature lower than the plate’s temperature. The flow is assumed to be two-dimensional. The discretized equations were solved by finite difference method using Alternating Direction Implicit technique and Successive OverRelaxation method. The study was performed for different values of Grashof number ranging from 103 to 105 for different aspect ratios and position of heated plate. Air was chosen as a working fluid (Pr = 0.71). The effect of the position and aspect ratio of heated plate on heat transfer and flow were addressed. With increase of Gr heat transfer rate increased in both vertical and horizontal position of the plate. When aspect ratio of heated thin plate is decreased the heat transfer also decreases. For the vertical situation of thin plate heat transfer becomes more enhanced than for horizontal situation
Weighing neutrinos using high redshift galaxy luminosity functions
Laboratory experiments measuring neutrino oscillations, indicate small mass
differences between different mass eigenstates of neutrinos. The absolute mass
scale is however not determined, with at present the strongest upper limits
coming from astronomical observations rather than terrestrial experiments. The
presence of massive neutrinos suppresses the growth of perturbations below a
characteristic mass scale, thereby leading to a decreased abundance of
collapsed dark matter halos. Here we show that this effect can significantly
alter the predicted luminosity function (LF) of high redshift galaxies. In
particular we demonstrate that a stringent constraint on the neutrino mass can
be obtained using the well measured galaxy LF and our semi-analytic structure
formation models. Combining the constraints from the Wilkinson Microwave
Anisotropy Probe 7 year (WMAP7) data with the LF data at z = 4, we get a limit
on the sum of the masses of 3 degenerate neutrinos \Sigma m_\nu < 0.52 eV at
the 95 % CL. The additional constraints using the prior on Hubble constant
strengthens this limit to \Sigma m_\nu < 0.29 eV at the 95 % CL. This neutrino
mass limit is a factor of order 4 improvement compared to the constraint based
on the WMAP7 data alone, and as stringent as known limits based on other
astronomical observations. As different astronomical measurements may suffer
from different set of biases, the method presented here provides a
complementary probe of \Sigma m_\nu . We suggest that repeating this exercise
on well measured luminosity functions over different redshift ranges can
provide independent and tighter constraints on \Sigma m_\nu .Comment: 14 pages, 7 figures, submitted to PR
Constrained semi-analytical models of Galactic outflows
We present semi-analytic models of galactic outflows, constrained by
available observations on high redshift star formation and reionization.
Galactic outflows are modeled in a manner akin to models of stellar wind blown
bubbles. Large scale outflows can generically escape from low mass halos
(M<10^9 M_sun) for a wide range of model parameters but not from high mass
halos (M> 10^{11} M_sun). The gas phase metallicity of the outflow and within
the galaxy are computed. Ionization states of different metal species are
calculated and used to examine the detectability of metal lines from the
outflows. The global influence of galactic outflows is also investigated.
Models with only atomic cooled halos significantly fill the IGM at z~3 with
metals (with -2.5>[Z/Z_sun]>-3.7), the actual extent depending on the
efficiency of winds, the IMF, the fractional mass that goes through star
formation and the reionization history of the universe. In these models, a
large fraction of outflows at z~3 are supersonic, hot (T> 10^5 K) and have low
density, making metal lines difficult to detect. They may also result in
significant perturbations in the IGM gas on scales probed by the Lyman-alpha
forest. On the contrary, models including molecular cooled halos with a normal
mode of star formation can potentially volume fill the universe at z> 8 without
drastic dynamic effects on the IGM, thereby setting up a possible metallicity
floor (-4.0<[Z/Z_sun]<-3.6). Interestingly, molecular cooled halos with a
``top-heavy'' mode of star formation are not very successful in establishing
the metallicity floor because of the additional radiative feedback, that they
induce. (Abridged)Comment: 27 pages, 31 figures, 2 tables, pdflatex. Accepted for publication in
MNRA
Magnetic helicity density and its flux in weakly inhomogeneous turbulence
A gauge invariant and hence physically meaningful definition of magnetic
helicity density for random fields is proposed, using the Gauss linking
formula, as the density of correlated field line linkages. This definition is
applied to the random small scale field in weakly inhomogeneous turbulence,
whose correlation length is small compared with the scale on which the
turbulence varies. For inhomogeneous systems, with or without boundaries, our
technique then allows one to study the local magnetic helicity density
evolution in a gauge independent fashion, which was not possible earlier. This
evolution equation is governed by local sources (owing to the mean field) and
by the divergence of a magnetic helicity flux density. The role of magnetic
helicity fluxes in alleviating catastrophic quenching of mean field dynamos is
discussed.Comment: 4 pages, accepted by Ap
Identification of rare nonsynonymous variants in SYNE1/CPG2 in bipolar affective disorder
Background: Bipolar affective disorder (BPD) is a severe
mood disorder with a prevalence of ∼ 1.5% in the
population. The pathogenesis of BPD is poorly understood;
however, a strong heritable component has been identified.
Previous genome-wide association studies have indicated a
region on 6q25, coding for the SYNE1 gene, which increases
disease susceptibility. SYNE1 encodes the synaptic nuclear
envelope protein-1, nesprin-1. A brain-specific splice variant
of SYNE1, CPG2 encoding candidate plasticity gene 2, has
been identified. The intronic single-nucleotide
polymorphism with the strongest genome-wide significant
association in BPD, rs9371601, is present in both SYNE1
and CPG2. / Methods: We screened 937 BPD samples for genetic
variation in SYNE1 exons 14–33, which covers the CPG2
region, using high-resolution melt analysis. In addition, we
screened two regions of increased transcriptional activity,
one of them proposed to be the CPG2 promoter region. / Results and Conclusion: We identified six nonsynonymous
and six synonymous variants. We genotyped three rare
nonsynonymous variants, rs374866393, rs148346599 and
rs200629713, in a total of 1099 BPD samples and 1056
controls. Burden analysis of these rare variants did not show
a significant association with BPD. However, nine patients
are compound heterozygotes for variants in SYNE1/CPG2,
suggesting that rare coding variants may contribute
significantly towards the complex genetic architecture
underlying BPD. Imputation analysis in our own wholegenome
sequencing sample of 99 BPD individuals
identified an additional eight risk variants in the CPG2
region of SYNE1
CMB Anisotropy Due to Tangled magnetic fields in re-ionized models
Primordial tangled cosmological Magnetic Fields source rotational velocity
perturbations of the baryon fluid, even in the post-recombination universe.
These vortical modes inturn leave a characteristic imprint on the temperature
anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be
re-scatterred after recombination. Observations from WMAP indicate that the
Universe underwent a relatively early re-ionization (), which
does indeed lead to a significant optical depth for re-scattering of CMB
photons after the re-ionization epoch. We compute the resulting additional
temperature anisotropies, induced by primordial magnetic fields in the
post-recombination universe. We show that in models with early re-ionization, a
nearly scale-invariant spectrum of tangled magnetic fields which redshift to a
present value of Gauss, produce vector mode
perturbations which in turn induce additional temperature anisotropy of about
0.3 to 0.4 K over very small angular scales, with upto
or so.Comment: 12 pages. Contains one figure. Submitted to Phys. Rev.
Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy
We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength
Microwave Background Signals from Tangled Magnetic Fields
An inhomogeneous cosmological magnetic field will create Alfven-wave modes
that induce a small rotational velocity perturbation on the last scattering
surface of the microwave background radiation. The Alfven-wave mode survives
Silk damping on much smaller scales than the compressional modes. This, in
combination with its rotational nature, ensures that there will be no sharp
cut-off in anisotropy on arc-minute scales. We estimate that a magnetic field
which redshifts to a present value of Gauss produces
temperature anisotropies at the 10 micro Kelvin level at and below 10 arc-min
scales. A tangled magnetic field, which is large enough to influence the
formation of large scale structure is therefore potentially detectable by
future observations.Comment: 5 pages, Revtex, no figure
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