138 research outputs found
Phase-field modeling of equilibrium precipitate shapes under the influence of coherency stresses
Coherency misfit stresses and their related anisotropies are known to
influence the equilibrium shapes of precipitates. Additionally, mechanical
properties of the alloys are also dependent on the shapes of the precipitates.
Therefore, in order to investigate the mechanical response of a material which
undergoes precipitation during heat treatment, it is important to derive the
range of precipitate shapes that evolve. In this regard, several studies have
been conducted in the past using sharp interface approaches where the influence
of elastic energy anisotropy on the precipitate shapes has been investigated.
In this paper, we propose a diffuse interface approach which allows us to
minimize grid-anisotropy related issues applicable in sharp-interface methods.
In this context, we introduce a novel phase-field method where we minimize the
functional consisting of the elastic and surface energy contributions while
preserving the precipitate volume. Using this method we reproduce the
shape-bifurcation diagrams for the cases of pure dilatational misfit that have
been studied previously using sharp interface methods and then extend them to
include interfacial energy anisotropy with different anisotropy strengths which
has not been a part of previous sharp-interface models. While we restrict
ourselves to cubic anisotropies in both elastic and interfacial energies in
this study, the model is generic enough to handle any combination of
anisotropies in both the bulk and interfacial terms. Further, we have examined
the influence of asymmetry in dilatational misfit strains along with
interfacial energy anisotropy on precipitate morphologies
Microstructural Evolution in Elastically-stressed Solids: A Phase-field Simulation
Simulation of microstructures under different processing conditions is important for fine- tuning the processing window as well as to understand the mechanisms. Phase field simulation has gained importance for problems with diffuse interfaces. Since in this simulation, thermodynamic driving forces (chemical as well as non-chemical) and kinetic constraints have been naturally incorporated, it has the potential to simulate microstructures under different processing and service conditions. In this paper, DMRL's initiatives on using phase field simulations to understand microstructural evolution in both the phase separating and precipitating model systems have been presented. The influence of misfit stresses on the morphology of microstructures has been described. Output from actual thermodynamic calculations can be combined with these simulations to study systems of technological importance.Defence Science Journal, 2011, 61(4), pp.383-393, DOI:http://dx.doi.org/10.14429/dsj.61.65
Seeing-Induced Errors in Solar Doppler Velocity Measurements
Imaging systems based on a narrow-band tunable filter are used to obtain
Doppler velocity maps of solar features. These velocity maps are created by
taking the difference between the blue- and red-wing intensity images of a
chosen spectral line. This method has the inherent assumption that these two
images are obtained under identical conditions. With the dynamical nature of
the solar features as well as the Earth's atmosphere, systematic errors can be
introduced in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data for
identifying their reliability. It is shown, under such conditions, that there
is a strong cross-talk from the total intensity to the velocity estimates.
These spurious velocities are larger in magnitude for the umbral regions
compared to the penumbra or quiet-sun regions surrounding the sunspots. The
variable seeing can induce spurious velocities up to about 1 km/s It is also
shown that adaptive optics, in general, helps in minimising this effect.Comment: 14 page
On the Weakening of Chromospheric Magnetic Field in Active Regions
Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields
at the photosphere and chromosphere are presented. Fe I line at
and at are used respectively for deriving the
physical parameters at photospheric and chromospheric heights. The LOS magnetic
field obtained through the center-of-gravity method show a linear relation
between photospheric and chromospheric field for field strengths less than 700
G. But in strong field regions, the LOS magnetic field values derived from
are much weaker than what one gets from the linear relationship
and also from those expected from the extrapolation of the photospheric
magnetic field. We discuss in detail the properties of magnetic field observed
in from the point of view of observed velocity gradients. The
bisector analysis of Stokes profiles show larger velocity
gradients in those places where strong photospheric magnetic fields are
observed. These observations may support the view that the stronger fields
diverge faster with height compared to weaker fields.Comment: accepted for publication in Ap
Performance of transverse tripoles vs. longitudinal tripoles with anode intensification (AI) in spinal cord stimulation:Computational modeling study
Radiative transfer effects on Doppler measurements as sources of surface effects in sunspot seismology
We show that the use of Doppler shifts of Zeeman sensitive spectral lines to
observe wavesn in sunspots is subject to measurement specific phase shifts
arising from, (i) altered height range of spectral line formation and the
propagating character of p mode waves in penumbrae, and (ii) Zeeman broadening
and splitting. We also show that these phase shifts depend on wave frequencies,
strengths and line of sight inclination of magnetic field, and the polarization
state used for Doppler measurements. We discuss how these phase shifts could
contribute to local helioseismic measurements of 'surface effects' in sunspot
seismology.Comment: 12 pages, 4 figures, Accepted for publication in the Astrophysical
Journal Letter
High Resolution Observations using Adaptive Optics: Achievements and Future Needs
Over the last few years, several interesting observations were obtained with
the help of solar Adaptive Optics (AO). In this paper, few observations made
using the solar AO are enlightened and briefly discussed. A list of
disadvantages with the current AO system are presented. With telescopes larger
than 1.5m are expected during the next decade, there is a need to develop the
existing AO technologies for large aperture telescopes. Some aspects of this
development are highlighted. Finally, the recent AO developments in India are
also presented
Dissecting star-formation in the "Atoms-for-Peace" galaxy: Ultra-Violet Imaging Telescope observations of the post-merger galaxy NGC7252
The tidal tails of post-merger galaxies exhibit ongoing star formation far
from their disks. The study of such systems can be useful for our understanding
of gas condensation in diverse environments. The ongoing star formation in the
tidal tails of post-merger galaxies can be directly studied from ultraviolet
(UV) imaging observations. The post merger galaxy NGC7252 ("Atoms-for-Peace"
galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband
NUV and FUV filters to isolate the star forming regions in the tidal tails and
study the spatial variation in star formation rates. Based on ultraviolet
imaging observations, we discuss star formation regions of ages 200 Myrs in
the tidal tails. We measure star formation rates in these regions and in the
main body of the galaxy. The integrated star formation rate of NGC7252 (i.e.,
that in the galaxy and tidal tails combined) without correcting for extinction
is found to be 0.81 0.01 M/yr. We show that the integrated star
formation rate can change by an order of magnitude if the extinction correction
used in star formation rates derived from other proxies are taken into
consideration. The star formation rates in the associated tidal dwarf galaxies
(NGC7252E, SFR=0.02 M/yr and NGC7252NW, SFR=0.03 M/yr) are
typical of dwarf galaxies in the local Universe. The spatial resolution of the
UV images reveals a gradient in star formation within the tidal dwarf galaxy.
The star formation rates show a dependence on the distance from the centre of
the galaxy. This can be due to the different initial conditions responsible for
the triggering of star formation in the gas reservoir that was expelled during
the recent merger in NGC7252.Comment: Accepted for publication in Astronomy & Astrophysic
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