15,197 research outputs found
Method of growing composites of the type exhibiting the Soret effect
A predetermine amount of segregation is introduced into a molten sample of a composite that exhibits the Soret effect, such amount approximating the amount of segregation resulting from directional solidification of the sample. The molten sample is then directionally solidified starting at the end opposite the end richer in the constituent that would migrate toward the cooler part of a liquid solution of the composite maintained in a temperature gradient. Since solidification commences at the end deficient in such constituent, its migration toward the interface between the solid and liquid during the solidification will compensate for the deficiency, yielding a more homogeneous product
Searches for radio transients
Exploration of the transient Universe is an exciting and fast-emerging area
within radio astronomy. Known transient phenomena range in time scales from
sub-nanoseconds to years or longer, thus spanning a huge range in time domain
and hinting a rich diversity in their underlying physical processes. Transient
phenomena are likely locations of explosive or dynamic events and they offer
tremendous potential to uncover new physics and astrophysics. A number of
upcoming next-generation radio facilities and recent advances in computing and
instrumentation have provided a much needed impetus for this field which has
remained a relatively uncharted territory for the past several decades. In this
paper we focus mainly on the class of phenomena that occur on very short time
scales (i.e. from milliseconds to nanoseconds), known as {\it
fast transients}, the detections of which involve considerable signal
processing and data management challenges, given the high time and frequency
resolutions required in their explorations, the role of propagation effects to
be considered and a multitude of deleterious effects due to radio frequency
interference. We will describe the techniques, strategies and challenges
involved in their detections and review the world-wide efforts currently under
way, both through scientific discoveries enabled by the ongoing large-scale
surveys at Parkes and Arecibo, as well as technical developments involving the
exploratory use of multi-element array instruments such as VLBA and GMRT. Such
developments will undoubtedly provide valuable inputs as next-generation arrays
such as LOFAR and ASKAP are designed and commissioned. With their wider fields
of view and higher sensitivities, these instruments, and eventually the SKA,
hold great potential to revolutionise this relatively nascent field, thereby
opening up exciting new science avenues in astrophysics.Comment: To appear in the special issue of the Bulletin of the Astronomical
Society of India on Transients at different wavelengths, eds D.J. Saikia and
D.A. Green. 21 pages, 5 figures. http://www.ncra.tifr.res.in/~bas
Rolling contact fatigue life of chromium ion plated 440C bearing steel
Rolling contact fatigue (RCF) test specimens of heat treated 440C bearing steel were chromium ion plated in thicknesses from 0.1 to 8.0 micron and tested in RCF tester using 700 ksi maximum Hertzian stress. Heavy coatings, greater than about 5 micron in thickness, peeled off or spalled readily, whereas thin coatings, less than 3 micron thick, were tenacious and did not come off. Furthermore, significant improvement in RCF life was obtained with thin chromium ion plated test specimens. The average increase in B10 life was 75% compared with unplated 440C. These preliminary results indicate that ion plating is a promising way to improve bearing life
Pulsar Scintillation and the Local Bubble
We present here the results from an extensive scintillation study of twenty
pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 caried out using
the Ooty Radio Telescope (ORT) at 327 MHz, to investigate the distribution of
ionized material in the local interstellar medium. Observations were made
during the period January 1993 to August 1995, in which the dynamic
scintillation spectra of these pulsars were regularly monitored over 10 - 90
epochs spanning 100 days. Reliable and accurate estimates of strengths of
scattering have been deduced from the scintillation parameters averaged out for
their long-term fluctuations arising from refractive scintillation (RISS)
effects. Our analysis reveals several anomalies in the scattering strength,
which suggest tht the distribution of scattering material in the Solar
neighborhood is not uniform.
We have modelled these anomalous scattering effects in terms of
inhomogeneities in the distribution of electron dnsity fluctuations in the
local interstellar medium (LISM). Our model suggests the presence of a low
density bubble surrounded by a shell of much higher density fluctuations. We
are able to put constraints on geometrical and scattering properties of such a
structure, and find it to be morphologically similar to the Local Bubble known
from other studies.Comment: 35 pages, 12 figure
Long-Term Scintillation Studies of Pulsars: III. Testing Theoretical Models of Refractive Scintillation
Refractive interstellar scintillation (RISS) is thought to be the cause
behind a variety of phenomena seen at radio wavelengths in pulsars and compact
radio sources. Though there is substantial observational data to support
several consequences of it, the quantitative predictions from theories have not
been thoroughly tested. In this paper, data from our long-term scintillation
study of 18 pulsars are used to test the predictions. The fluctuations of
decorrelation bandwidth (), scintillation time scale () and flux
density (F) are examined for their cross-correlations and compared with the
predictions. The theory predicts a strong correlation between and
, and strong anti-correlations between and F, and and
F. For 5 pulsars, we see a reasonable agreement. There is considerable
difficulty in reconciling the results for the rest of the pulsars. Our analysis
shows the underlying noise sources can sometimes reduce the correlation, but
cannot cause an absence of correlation. It is also unlikely that the poor flux
correlations arise from a hitherto unrecognized intrinsic flux variations. For
PSR B0834+06, which shows anomalous behaviour of persistent drift slopes,
positive correlation is found between and the drift-corrected .
Many pulsars show an anti-correlation between and the drift slope, and
this is in accordance with the simple models of RISS. The detections of
correlated variations of observables and a reasonable agreement between the
predicted and measured correlations for some pulsars confirm RISS as the
primary cause of the observed fluctuations. However, the complexity seen with
the detailed results suggests the necessity of more comprehensive theoretical
treatments for describing refractive fluctuations and their correlations.Comment: 27 pages, 6 Figures, 6 Tables. Accepted for publication in The
Astrophysical Journa
An analytical model of prominence dynamics
Solar prominences are magnetic structures incarcerating cool and dense gas in
an otherwise hot solar corona. Prominences can be categorized as quiescent and
active. Their origin and the presence of cool gas (~K) within the hot
(~K) solar corona remains poorly understood. The structure and dynamics
of solar prominences was investigated in a large number of observational and
theoretical (both analytical and numerical) studies. In this paper, an analytic
model of quiescent solar prominence is developed and used to demonstrate that
the prominence velocity increases exponentially, which means that some gas
falls downward towards the solar surface, and that Alfven waves are naturally
present in the solar prominences. These theoretical predictions are consistent
with the current observational data of solar quiescent prominences.Comment: Update Final Journal Print Version along with other Metadat
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