1,136 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
Lie Group Analysis of Natural Convection Heat and Mass Transfer in an Inclined Surface
Natural convection heat transfer fluid flow past an inclined semiinfinite surface in the presence of solute concentration is investigated by Lie group analysis. The governing partial differential equations are reduced to a system of ordinary differential equations by the translation and scaling symmetries. An exact solution is obtained for translation symmetry and numerical solutions for scaling symmetry. It is found that the velocity increases and temperature and concentration of the fluid decrease with an increase in the thermal and solutal Grashof numbers. The velocity and concentration of the fluid decrease and temperature increases with increase in the Schmidt number
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
A Unified treatment of small and large- scale dynamos in helical turbulence
Helical turbulence is thought to provide the key to the generation of
large-scale magnetic fields. Turbulence also generically leads to rapidly
growing small-scale magnetic fields correlated on the turbulence scales. These
two processes are usually studied separately. We give here a unified treatment
of both processes, in the case of random fields, incorporating also a simple
model non-linear drift. In the process we uncover an interesting plausible
saturated state of the small-scale dynamo and a novel analogy between quantum
mechanical (QM) tunneling and the generation of large scale fields. The steady
state problem of the combined small/large scale dynamo, is mapped to a
zero-energy, QM potential problem; but a potential which, for non-zero mean
helicity, allows tunneling of bound states. A field generated by the
small-scale dynamo, can 'tunnel' to produce large-scale correlations, which in
steady state, correspond to a force-free 'mean' field.Comment: 4 pages, 1 figure, Physical Review Letters, in pres
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
Hydrodynamic flow between rotating eccentric cylinders with suction at the porous walls
The flow of a viscous, incompressible fluid
between two eccentric rotating porous cylinders with
suction/injection at both the cylinders, for very small clearance
ratio is studied. The expressions for various flow characteristics
are obtained using perturbation analysis. Streamlines and pressure
plots are shown graphically for various values of flow parameters
and discussed
Role of sulphated polysaccharides from Sargassum Wightii in Cyclosporine A-induced oxidative liver injury in rats
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
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.
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