11,965 research outputs found
A model for Rayleigh-B\'enard magnetoconvection
A model for three-dimensional Rayleigh-B\'{e}nard convection in
low-Prandtl-number fluids near onset with rigid horizontal boundaries in the
presence of a uniform vertical magnetic field is constructed and analyzed in
detail. The kinetic energy , the convective entropy and the
convective heat flux () show scaling behaviour with near
onset of convection, where is the reduced Rayleigh number. The model is
also used to investigate various magneto-convective structures close to the
onset. Straight rolls, which appear at the primary instability, become unstable
with increase in and bifurcate to three-dimensional structures. The
straight rolls become periodically varying wavy rolls or quasiperiodically
varying structures in time with increase in depending on the values of
Prandtl number . They become irregular in time, with increase in . These
standing wave solutions bifurcate first to periodic and then quasiperiodic
traveling wave solutions, as is raised further. The variations of the
critical Rayleigh number and the frequency at the onset
of the secondary instability with are also studied for different values of
Chandrasekhar's number .Comment: 11 pages (To appear in EPJB
Role of uniform horizontal magnetic field on convective flow
The effect of uniform magnetic field applied along a fixed horizontal
direction in Rayleigh-B\'enard convection in low-Prandtl-number fluids has been
studied using a low dimensional model. The model shows the onset of convection
(primary instability) in the form of two dimensional stationary rolls in the
absence of magnetic field, when the Rayleigh number is raised above a
critical value . The flow becomes three dimensional at slightly higher
values of Rayleigh number via wavy instability. These wavy rolls become chaotic
for slightly higher values of in low-Prandtl-number () fluids. A
uniform magnetic field along horizontal plane strongly affects all kinds of
convective flows observed at higher values of in its absence. As the
magnetic field is raised above certain value, it orients the convective rolls
in its own direction. Although the horizontal magnetic field does not change
the threshold for the primary instability, it affects the threshold for
secondary (wavy) instability. It inhibits the onset of wavy instability. The
critical Rayleigh number at the onset of wavy instability, which
depends on Chandrasekhar's number and , increases monotonically with
for a fixed value of . The dimensionless number scales with as . A stronger magnetic field suppresses chaos
and makes the flow two dimensional with roll pattern aligned along its
direction.Comment: 6 pages, 8 figure
Augmented Superfield Approach to Gauge-invariant Massive 2-Form Theory
We discuss the complete sets of the off-shell nilpotent (i.e. s^2_{(a)b} = 0)
and absolutely anticommuting (i.e. s_b s_{ab} + s_{ab} s_b = 0)
Becchi-Rouet-Stora-Tyutin (BRST) (s_b) and anti-BRST (s_{ab}) symmetries for
the (3+1)-dimensional (4D) gauge-invariant massive 2-form theory within the
framework of augmented superfield approach to BRST formalism. In this
formalism, we obtain the coupled (but equivalent) Lagrangian densities which
respect both BRST and anti-BRST symmetries on the constrained hypersurface
defined by the Curci-Ferrari type conditions. The absolute anticommutativity
property of the (anti-)BRST transformations (and corresponding generators) is
ensured by the existence of the Curci-Ferrari type conditions which emerge very
naturally in this formalism. Furthermore, the gauge-invariant restriction plays
a decisive role in deriving the proper (anti-)BRST transformations for the
St{\"u}ckelberg-like vector field.Comment: LaTeX file, 22 pages, no figures, version to appear in Eur. Phys. J.
C (2017
Oscillatory instability and fluid patterns in low-Prandtl-number Rayleigh-B\'{e}nard convection with uniform rotation
We present the results of direct numerical simulations of flow patterns in a
low-Prandtl-number () fluid above the onset of oscillatory convection
in a Rayleigh-B\'{e}nard system rotating uniformly about a vertical axis.
Simulations were carried out in a periodic box with thermally conducting and
stress-free top and bottom surfaces. We considered a rectangular box () and a wide range of Taylor numbers () for the purpose. The horizontal aspect ratio of the box
was varied from to . The primary instability appeared in the form of
two-dimensional standing waves for shorter boxes ( and ). The flow patterns observed in boxes with and
were different from those with and . We observed a
competition between two sets of mutually perpendicular rolls at the primary
instability in a square cell () for , but observed a set
of parallel rolls in the form of standing waves for . The
three-dimensional convection was quasiperiodic or chaotic for , and then bifurcated into a two-dimensional periodic flow for . The convective structures consisted of the appearance and disappearance
of straight rolls, rhombic patterns, and wavy rolls inclined at an angle with the straight rolls.Comment: 32 pages, 14 figures, 1 tabl
Secost: Sequential co-supervision for large scale weakly labeled audio event detection
Weakly supervised learning algorithms are critical for scaling audio event
detection to several hundreds of sound categories. Such learning models should
not only disambiguate sound events efficiently with minimal class-specific
annotation but also be robust to label noise, which is more apparent with weak
labels instead of strong annotations. In this work, we propose a new framework
for designing learning models with weak supervision by bridging ideas from
sequential learning and knowledge distillation. We refer to the proposed
methodology as SeCoST (pronounced Sequest) -- Sequential Co-supervision for
training generations of Students. SeCoST incrementally builds a cascade of
student-teacher pairs via a novel knowledge transfer method. Our evaluations on
Audioset (the largest weakly labeled dataset available) show that SeCoST
achieves a mean average precision of 0.383 while outperforming prior state of
the art by a considerable margin.Comment: Accepted IEEE ICASSP 202
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