19,632 research outputs found
Propagation of a Three-dimensional Weak Shock Front Using Kinematical Conservation Laws
In this paper we present a mathematical theory and a numerical method to
study the propagation of a three-dimensional (3-D) weak shock front into a
polytropic gas in a uniform state and at rest, though the method can be
extended to shocks moving into nonuniform flows. The theory is based on the use
of 3-D kinematical conservation laws (KCL), which govern the evolution of a
surface in general and a shock front in particular. The 3-D KCL, derived purely
on geometrical considerations, form an under-determined system of conservation
laws. In the present paper the 3-D KCL system is closed by using two
appropriately truncated transport equations from an infinite hierarchy of
compatibility conditions along shock rays. The resulting governing equations of
this KCL based 3-D shock ray theory, leads to a weakly hyperbolic system of
eight conservation laws with three divergence-free constraints. The
conservation laws are solved using a Godunov-type central finite volume scheme,
with a constrained transport technique to enforce the constraints. The results
of extensive numerical simulations reveal several physically realistic
geometrical features of shock fronts and the complex structures of kink lines
formed on them. A comparison of the results with those of a weakly nonlinear
wavefront shows that a weak shock front and a weakly nonlinear wavefront are
topologically same. The major important differences between the two are
highlighted in the contexts of corrugational stability and converging shock
fronts
Minimum Dominating Set for a Point Set in \IR^2
In this article, we consider the problem of computing minimum dominating set
for a given set of points in \IR^2. Here the objective is to find a
minimum cardinality subset of such that the union of the unit radius
disks centered at the points in covers all the points in . We first
propose a simple 4-factor and 3-factor approximation algorithms in and time respectively improving time complexities by a
factor of and respectively over the best known result
available in the literature [M. De, G.K. Das, P. Carmi and S.C. Nandy, {\it
Approximation algorithms for a variant of discrete piercing set problem for
unit disk}, Int. J. of Comp. Geom. and Appl., to appear]. Finally, we propose a
very important shifting lemma, which is of independent interest and using this
lemma we propose a -factor approximation algorithm and a PTAS for
the minimum dominating set problem.Comment: 14 pages, 8 figure
Einstein energy-momentum complex for a phantom black hole metric
In this paper we calculate the energy distribution E(r) associated with a
static spherically symmetric non-singular phantom black hole metric in
Einstein's prescription in general relativity. As required for Einstein
energy-momentum complex, we perform calculations in quasi-Cartesian
coordinates. We also calculate momentum components and get zero values as
expected from the geometry of the metric.Comment: 5 pages, 2 figure
Thermo-optic coefficient measurement of liquids using silicon photonic microring resonators
On-chip measurement of thermo-optic coefficient (TOC) of samples along with
on-chip temperature measurements can be used to compensate for thermal
fluctuation induced noise in refractometry using integrated photonic sensors.
In this article we demonstrate a device design and describe the method to
extract TOCs of liquid samples using resonant wavelength shifts of a silicon
microring resonator. The TOCs of three standard fluids; De-ionized water,
Ethanol and Isopropanol, were determined using our sensor and show a good
agreement with values reported in literature. A mechanism for tracking of
on-chip temperature variations is also included for ensuring accuracy of TOC
measurements. Potential applications of the demonstrated on-chip TOC sensor
include improvements in accuracy of refractive index measurements and
multiparametric analysis of biochemical analytes
Energy Efficiency in Massive MIMO-Based 5G Networks: Opportunities and Challenges
As we make progress towards the era of fifth generation (5G) communication
networks, energy efficiency (EE) becomes an important design criterion because
it guarantees sustainable evolution. In this regard, the massive multiple-input
multiple-output (MIMO) technology, where the base stations (BSs) are equipped
with a large number of antennas so as to achieve multiple orders of spectral
and energy efficiency gains, will be a key technology enabler for 5G. In this
article, we present a comprehensive discussion on state-of-the-art techniques
which further enhance the EE gains offered by massive MIMO (MM). We begin with
an overview of MM systems and discuss how realistic power consumption models
can be developed for these systems. Thereby, we discuss and identify few
shortcomings of some of the most prominent EE-maximization techniques present
in the current literature. Then, we discuss "hybrid MM systems" operating in a
5G architecture, where MM operates in conjunction with other potential
technology enablers, such as millimetre wave, heterogenous networks, and energy
harvesting networks. Multiple opportunities and challenges arise in such a 5G
architecture because these technologies benefit mutually from each other and
their coexistence introduces several new constraints on the design of
energy-efficient systems. Despite clear evidence that hybrid MM systems can
achieve significantly higher EE gains than conventional MM systems, several
open research problems continue to roadblock system designers from fully
harnessing the EE gains offered by hybrid MM systems. Our discussions lead to
the conclusion that hybrid MM systems offer a sustainable evolution towards 5G
networks and are therefore an important research topic for future work.Comment: IEEE Wireless Communications, under revie
Low-Dimensionality of Noise-Free RSS and its Application in Distributed Massive MIMO
We examine the dimensionality of noise-free uplink received signal strength
(RSS) data in a distributed multiuser massive multiple-input multiple-output
system. Specifically, we apply principal component analysis to the noise-free
uplink RSS and observe that it has a low-dimensional principal subspace. We
make use of this unique property to propose RecGP - a reconstruction-based
Gaussian process regression (GP) method which predicts user locations from
uplink RSS data. Considering noise-free RSS for training and noisy test RSS for
location prediction, RecGP reconstructs the noisy test RSS from a low-
dimensional principal subspace of the noise-free training RSS. The
reconstructed RSS is input to a trained GP model for location prediction. Noise
reduction facilitated by the reconstruction step allows RecGP to achieve lower
prediction error than standard GP methods which directly use the test RSS for
location prediction.Comment: submitted to IEEE Wireless Communication Letters, July 201
A Deep Learning Approach for Similar Languages, Varieties and Dialects
Deep learning mechanisms are prevailing approaches in recent days for the
various tasks in natural language processing, speech recognition, image
processing and many others. To leverage this we use deep learning based
mechanism specifically Bidirectional- Long Short-Term Memory (B-LSTM) for the
task of dialectic identification in Arabic and German broadcast speech and Long
Short-Term Memory (LSTM) for discriminating between similar Languages. Two
unique B-LSTM models are created using the Large-vocabulary Continuous Speech
Recognition (LVCSR) based lexical features and a fixed length of 400 per
utterance bottleneck features generated by i-vector framework. These models
were evaluated on the VarDial 2017 datasets for the tasks Arabic, German
dialect identification with dialects of Egyptian, Gulf, Levantine, North
African, and MSA for Arabic and Basel, Bern, Lucerne, and Zurich for German.
Also for the task of Discriminating between Similar Languages like Bosnian,
Croatian and Serbian. The B-LSTM model showed accuracy of 0.246 on lexical
features and accuracy of 0.577 bottleneck features of i-Vector framework.Comment: 17 page
Dissipation scale lengths of density turbulence in the inner solar wind
Knowing the lengthscales at which turbulent fluctuations dissipate is key to
understanding the nature of weakly compressible magnetohydrodynamic turbulence.
We use radio wavelength interferometric imaging observations which measure the
extent to which distant cosmic sources observed against the inner solar wind
are scatter-broadened. We interpret these observations to determine that the
dissipation scales of solar wind density turbulence at heliocentric distances
of 2.5 -- 20.27 range from 13500 to 520 m. Our estimates
from 10--20 suggest that the dissipation scale
corresponds to the proton gyroradius. They are relevant to in-situ observations
to be made by the Parker Solar Probe, and are expected to enhance our
understanding of solar wind acceleration.Comment: Accepted for publication in The Astrophysical Journal (8 pages, 4
figures, 1 table
LASCO Measurements of the Energetics of Coronal Mass Ejections
We examine the energetics of Coronal Mass Ejections (CMEs) with data from the
LASCO coronagraphs on SOHO. The LASCO observations provide fairly direct
measurements of the mass, velocity and dimensions of CMEs. Using these basic
measurements, we determine the potential and kinetic energies and their
evolution for several CMEs that exhibit a flux-rope morphology. Assuming flux
conservation, we use observations of the magnetic flux in a variety of magnetic
clouds near the Earth to determine the magnetic flux and magnetic energy in
CMEs near the Sun. We find that the potential and kinetic energies increase at
the expense of the magnetic energy as the CME moves out, keeping the total
energy roughly constant. This demonstrates that flux rope CMEs are magnetically
driven. Furthermore, since their total energy is constant, the flux rope parts
of the CMEs can be considered to be a closed system above 2 R_{\sun}.Comment: Accepted for publication, Astrophysical Journal. Uses AASTeX 5.
The Receiver System for the Ooty Wide Field Array
The legacy Ooty Radio Telescope (ORT) is being reconfigured as a 264-element
synthesis telescope, called the Ooty Wide Field Array (OWFA). Its antenna
elements are the contiguous 1.92 m sections of the parabolic cylinder. It will
operate in a 38-MHz frequency band centred at 326.5 MHz and will be equipped
with a digital receiver including a 264-element spectral correlator with a
spectral resolution of 48 kHz. OWFA is designed to retain the benefits of
equatorial mount, continuous 9-hour tracking ability and large collecting area
of the legacy telescope and use modern digital techniques to enhance the
instantaneous field of view by more than an order of magnitude. OWFA has unique
advantages for contemporary investigations related to large scale structure,
transient events and space weather watch. In this paper, we describe the RF
subsystems, digitizers and fibre optic communication of OWFA and highlight some
specific aspects of the system relevant for the observations planned during the
initial operation.Comment: 10 pages, 5 figures, 1 table, (Accepted for publication in J.
Astrophysics and Astronomy
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