667 research outputs found
Phase field modeling of Widmanstatten plate formation in Zr–2.5Nb material
AbstractGrowth of Widmanstatten side-plates from pre-nucleated α in a matrix of β is a diffusion controlled process. The phase-field approach is formulated for Widmanstatten side plate formation through β-Zr (B.C.C)→α-Zr (HCP) transformation in binary substitutional Zr–2.5Nb alloy. Using Gibbs energy functional and diffusional mobility, microstructural evolution of parallel side-plates from pre-nucleated α phase has been simulated. The highly anisotropic nature of Widmanstatten plate and the presence of thin interface at the parent-product interface make the task challenging for FEM analysis. Based on this model, an estimate on the range of temperature over which Widmanstatten parallel side-plates can form is estimated and the extent of anisotropy of surface free energy on the morphology of the plates is analyzed
Microbial mat-related structures shared by both siliciclastic and carbonate formations
AbstractMicrobiota has always been the dominant life form, records of which are preserved in delicate forms within siliciclastic rocks. More pronounced record in the form of stromatolites possibly obscured the fact that many of the same delicate structures may be recognizable within carbonate rocks too. The Neoproterozoic Bhander Limestone in central India bears many such structures that are quintessentially similar to microbial mat-related structures reported from the Paleoproterozoic Chorhat Sandstone preserved within the same, Vindhyan Basin. Extensive microscopic, ultramicroscopic, and geochemical studies address the apprehension that such bedding plane structures in carbonate rocks could be merely weathering products. Trapping, binding and stabilitization of sediment by microbial mats are all evident. Preferred pyritization along the inferred, predefined microbial mats confirmed on the basis of EPMA (Electron Probe Microanalysis) results, and the enhanced carbon content along these mats layers and within suspected mat chips associated with them, are revealing. Raman spectroscopy, indeed, evinces enhanced kerogen content within both mats and mat chips.Interestingly, these microbial mat layers are recognized selectively within the lower of the two tiers of the Bhander Limestone. The lagoonal carbonate of the lower tier of the Bhander Limestone is muddy and contains a substantial proportion of silt-sized quartz grains that possibly impeded stromatolite growth. Stromatolites abound in the wave agitated upper tier of the Bhander Limestone which is dominated by oosparite. This paper provides evidence that the delicate microbial mat-related structures reported so far only from siliciclastic rocks can also be recognized within carbonate formations, and hopes to stimulate the search for additional such features, more preferably within carbonates originated in shallow and quiet water
Quantum Communication in 6G Satellite Networks: Entanglement Distribution Across Changing Topologies
As LEO/VLEO satellites offer many attractive features, such as low
transmission delay, they are expected to be an integral part of 6G. Global
entanglement distribution over LEO and VLEO satellites network must reckon with
satellite movement over time. Current studies do not fully capture the dynamic
nature of satellite constellations. We model a dynamic LEO/VLEO satellite
network as a time-varying graph and construct a sequence of static graphs to
represent a dynamic network. We study the entanglement distribution problem
between a set of source-destination node pairs in this dynamic network
utilizing Multi-commodity Flow (MCF). Solving MCF over a sequence of graphs
independently for each graph may produce a completely different set of paths.
Changing the set of paths every time the graph topology changes may involve a
significant amount of overhead, as an established set of paths must be taken
down and a new set of paths established. We propose a technique that will avoid
this overhead by computing only one set of paths P to be used over all the
graphs in the sequence. The degraded performance offered by P may be viewed as
the cost of using P. The benefit of using P is the overhead cost of path
switching that can be avoided. We provide a cost-benefit analysis in a LEO/VLEO
constellation for entanglement distribution between multiple source-destination
pairs. Our extensive experimentation shows that a significant amount of savings
in overhead can be achieved if one is willing to accept a slightly degraded
performance
On the compatibility of a flux transport dynamo with a fast tachocline scenario
The compatibility of the fast tachocline scenario with a flux transport
dynamo model is explored. We employ a flux transport dynamo model coupled with
simple feedback formulae relating the thickness of the tachocline to the
amplitude of the magnetic field or to the Maxwell stress. The dynamo model is
found to be robust against the nonlinearity introduced by this simplified fast
tachocline mechanism. Solar-like butterfly diagrams are found to persist and,
even without any parameter fitting, the overall thickness of the tachocline is
well within the range admitted by helioseismic constraints. In the most
realistic case of a time and latitude dependent tachocline thickness linked to
the value of the Maxwell stress, both the thickness and its latitude dependence
are in excellent agreement with seismic results. In the nonparametric models,
cycle related temporal variations in tachocline thickness are somewhat larger
than admitted by helioseismic constraints; we find, however, that introducing a
further parameter into our feedback formula readily allows further fine tuning
of the thickness variations.Comment: Accepted in Solar Physic
Differential rotation and meridional flow in the solar supergranulation layer: Measuring the eddy viscosity
We measure the eddy viscosity in the outermost layers of the solar convection
zone by comparing the rotation law computed with the Reynolds stress resulting
from f-plane simulations of the angular momentum transport in rotating
convection with the observed differential rotation pattern. The simulations
lead to a negative vertical and a positive horizontal angular momentum
transport. The consequence is a subrotation of the outermost layers, as it is
indeed indicated both by helioseismology and the observed rotation rates of
sunspots. In order to reproduce the observed gradient of the rotation rate a
value of about 1.5 x 10^{13} cm/s for the eddy viscosity is necessary.
Comparison with the magnetic eddy diffusivity derived from the sunspot decay
yields a surprisingly large magnetic Prandtl number of 150 for the
supergranulation layer. The negative gradient of the rotation rate also drives
a surface meridional flow towards the poles, in agreement with the results from
Doppler measurements. The successful reproduction of the abnormally positive
horizontal cross correlation (on the northern hemisphere) observed for bipolar
groups then provides an independent test for the resulting eddy viscosity.Comment: 6 pages, 8 figures, Astronomy and Astrophysics (subm.
The asymmetry of sunspot cycles and Waldmeier relations as due to nonlinear surface-shear shaped dynamo
The paper presents a study of a solar dynamo model operating in the bulk of
the convection zone with the toroidal magnetic field flux concentrated in the
subsurface rotational shear layer. We explore how this type of dynamo may
depend on spatial variations of turbulent parameters and on the differential
rotation near the surface. The mean-field dynamo model takes into account the
evolution of magnetic helicity and describes its nonlinear feedback on the
generation of large-scale magnetic field by the -effect. We compare the
magnetic cycle characteristics predicted by the model, including the cycle
asymmetry (associated with the growth and decay times) and the duration -
amplitude relation (Waldmeier's effects), with the observed sunspot cycle
properties. We show that the model qualitatively reproduces the basic
properties of the solar cycles.Comment: 28 pages, 7 figures(Second revision, figures updates
Morphology and evolution of umbral dots and their substructures
Substructures - dark lanes and tails - of umbral dots (UDs) were predicted by
numerical simulations of magnetoconvection. We analyse a 6 h 23 min time series
of broadband images of a large umbra in the active region NOAA 10634, acquired
with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A
43 min part of this series was reconstructed with the MFBD method, reaching a
spatial resolution of 0.14". We measure brightness, size, lifetime, and
horizontal velocities of various umbral structures. Most (90 %) of UDs and
bright point-like features in faint LBs split and merge, and their median
lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is
considered as the end of their life. Both UDs and features in faint LBs that do
not split or merge are clearly smaller (0.15") than the average size (0.17") of
all features. Horizontal motions of umbral bright small-scale features are
directed either into the umbra or along faint LBs with mean horizontal
velocities of 0.34 km/s. Features faster than 0.4 km/s appear mostly at the
periphery of the umbra. The intensity of dark lanes, measured in four bright
central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs.
The width of dark lanes is probably less than the resolution limit 0.14". The
characteristic time of substructure changes of UDs is ~4 min. We observe narrow
(0.14") bright and dark filaments connected with PUDs. Usually one dark and two
bright filaments form a 0.4" wide tail attached to one PUD, resembling a short
dark-cored penumbral filament. Our results indicate the similarity between PUDs
and PGs located at the tips of bright penumbral filaments. The features seen in
numerical MHD simulations are consistent with our observations of dark lanes in
CUDs and tails attached to PUDs.Comment: 8 pages, 8 figures, Astronomy & Astrophysics, in pres
The Sun's Preferred Longitudes and the Coupling of Magnetic Dynamo Modes
Observations show that solar activity is distributed non-axisymmetrically,
concentrating at "preferred longitudes". This indicates the important role of
non-axisymmetric magnetic fields in the origin of solar activity. We
investigate the generation of the non-axisymmetric fields and their coupling
with axisymmetric solar magnetic field. Our kinematic generation (dynamo) model
operating in a sphere includes solar differential rotation, which approximates
the differential rotation obtained by inversion of helioseismic data, modelled
distributions of the turbulent resistivity, non-axisymmetric mean helicity, and
meridional circulation in the convection zone. We find that (1) the
non-axisymmetric modes are localised near the base of the convection zone,
where the formation of active regions starts, and at latitudes around
; (2) the coupling of non-axisymmetric and axisymmetric modes
causes the non-axisymmetric mode to follow the solar cycle; the phase relations
between the modes are found. (3) The rate of rotation of the first
non-axisymmetric mode is close to that determined in the interplanetary space.Comment: 22 pages, 18 figures. Accepted for publication in the Astrophysical
Journa
Magnetic field confinement by meridional flow and the solar tachocline
We show that the MHD theory that explains the solar tachocline by an effect
of the magnetic field can work with the decay modes of a fossil field in the
solar interior if the meridional flow of the convection zone penetrates
slightly the radiative zone beneath. An equatorward flow of about 10 m/s
penetrating to a maximum depth of 1000 km below the convection zone is able to
generate almost horizontal field lines in the tachocline region so that the
internal field is almost totally confined to the radiative zone. The theory of
differential solar rotation indeed provides meridional flows of about 10 m/s
and a penetration depth of < 1000 km for viscosity values that are
characteristic of a stable tachocline.Comment: 5 pages, 6 figures, submitted to A&
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