375 research outputs found
Vector potential-vorticity relationship for the Stokes flows: application to the Stokes eigenmodes in 2D/3D closed domain
The unsteady dynamics of the Stokes flows, where , is shown to verify the vector potential-vorticity ( ) correlation , where the field is the pressure-gradient vector potential defined by . This correlation is analyzed for the Stokes eigenmodes, , subjected to no-slip boundary conditions on any two-dimensional (2D) closed contour or three-dimensional (3D) surface. It is established that an asymptotic linear relationship appears, verified in the core part of the domain, between the vector potential and vorticity, , where is a constant offset field, possibly zer
Fundamental Stokes eigenmodes in the square: which expansion is more accurate, Chebyshev or Reid-Harris?
The well-known Reid-Harris expansions, applied to the stream function formulation, and the projection-diffusion Chebyshev Stokes solver, in primitive variables, are used to compute the fundamental Stokes eigenmodes of each of the symmetry families characterizing the Stokes solutions in the square. The numerical accuracy of both methods, applied with several discretizations, are compared, for both the eigenvalues and the main features of the corresponding eigenmodes. The Chebyshev approach is by far the most efficient, even though the associated solver does not provide a divergence free velocity but asymptoticall
Sensitivity of the liquid bridge hydrodynamics to local capillary contributions
International audienceIn the usual models of thermocapillary flows, a vorticity singularity occurs at the contact free surface-solid boundaries. The steady axisymmetric hydrodynamics of the side-heated liquid bridge of molten metal is addressed here for its sensitivity to the size delta of a length scale explicitly introduced to regularize the problem. By linear stability analysis of the flows, various stable steady states are predicted: The already known steady states which are reflection-symmetric about the mid-plane, but also others which do not possess this property. The thresholds in Ma of the associated bifurcations are strongly dependent on delta, and converge with delta-->0 towards low values. Published data give these results some physical relevance. (C) 2002 American Institute of Physics
Numerical Evaluation of the Accuracy and Stability Properties of High-order Direct Stokes Solvers with or without Temporal Splitting
The temporal stability and effective order of two different direct high-order Stokes solvers are examined. Both solvers start from the primitive variables formulation of the Stokes problem, but are distinct by the numerical uncoupling they apply on the Stokes operator. One of these solvers introduces an intermediate divergence free velocity for performing a temporal splitting (J. Comput. Phys. [1991] 97, 414-443) while the other treats the whole Stokes problem through the evaluation of a divergence free acceleration field (C.R. Acad. Sci. Paris [1994] 319 Serie I, 1455-1461; SIAM J. Scient. Comput. [2000] 22(4), 1386-1410). The second uncoupling is known to be consistent with the harmonicity of the pressure field (SIAM J. Scient. Comput. [2000] 22(4), 1386-1410). Both solvers proceed by two steps, a pressure evaluation based on an extrapolated in time (of theoretical order Je) Neumann condition, and a time implicit (of theoretical order Ji) diffusion step for the final velocity. These solvers are implemented with a Chebyshev mono-domain and a Legendre spectral element collocation schemes. The numerical stability of these four options is investigated for the sixteen combinations of (Je,Ji), 1 ≤ Je, Ji ≤
Gross thermodynamics of two-component core convection
We model the inner core by an alloy of iron and 8 per cent sulphur or silicon and the outer core by the same mix with an additional 8 per cent oxygen. This composition matches the densities of seismic model, Preliminary Reference Earth Model (PR-EM). When the liquid core freezes S and Si remain with the Fe to form the solid and excess 0 is ejected into the liquid. Properties of Fe, diffusion constants for S, Si, 0 and chemical potentials are calculated by first-principles methods under the assumption that S, 0, and Si react with the Fe and themselves, however, not with each other. This gives the parameters required to calculate the power supply to the geodynamo as the Earth's core cools. Compositional convection, driven by light O released at the inner-core boundary on freezing, accounts for half the entropy balance and 15 per cent of the heat balance. This means the same magnetic field can be generated with approximately half the heat throughput needed if the geodynamo were driven by heat alone. Chemical effects are significant: heat absorbed by disassociation of Fe and 0 almost nullify the effect of latent heat of freezing in driving the dynamo. Cooling rates below 69 K Gyr(-1) are too low to maintain thermal convection everywhere; when the cooling rate lies between 35 and 69 K Gyr(-1) convection at the top of the core is maintained compositionally against a stabilizing temperature gradient; below 35 K Gyr(-1) the dynamo fails completely. All cooling rates freeze the inner core in less than 1.2 Gyr, in agreement with other recent calculations. The presence of radioactive heating will extend the life of the inner core, however, it requires a high heat flux across the core-mantle boundary. Heating is dominated by radioactivity when the inner core age is 3.5 Gyr. We, also, give calculations for larger concentrations of O in the outer core suggested by a recent estimation of the density jump at the inner-core boundary, which is larger than that of PREM. Compositional convection is enhanced for the higher density jumps and overall heat flux is reduced for the same dynamo dissipation, however, not by enough to alter the qualitative conclusions based on PREM. Our preferred model has the core convecting near the limit of thermal stability, an inner-core age of 3.5 Gyr and a core heat flux of 9 TW or 20 per cent of the Earth's surface heat flux, 80 per cent of which originates from radioactive heating
Lack of phenotypic and evolutionary cross-resistance against parasitoids and pathogens in Drosophila melanogaster
BackgroundWhen organisms are attacked by multiple natural enemies, the evolution of a resistance mechanism to one natural enemy will be influenced by the degree of cross-resistance to another natural enemy. Cross-resistance can be positive, when a resistance mechanism against one natural enemy also offers resistance to another; or negative, in the form of a trade-off, when an increase in resistance against one natural enemy results in a decrease in resistance against another. Using Drosophila melanogaster, an important model system for the evolution of invertebrate immunity, we test for the existence of cross-resistance against parasites and pathogens, at both a phenotypic and evolutionary level.MethodsWe used a field strain of D. melanogaster to test whether surviving parasitism by the parasitoid Asobara tabida has an effect on the resistance against Beauveria bassiana, an entomopathogenic fungus; and whether infection with the microsporidian Tubulinosema kingi has an effect on the resistance against A. tabida. We used lines selected for increased resistance to A. tabida to test whether increased parasitoid resistance has an effect on resistance against B. bassiana and T. kingi. We used lines selected for increased tolerance against B. bassiana to test whether increased fungal resistance has an effect on resistance against A. tabida.Results/ConclusionsWe found no positive cross-resistance or trade-offs in the resistance to parasites and pathogens. This is an important finding, given the use of D. melanogaster as a model system for the evolution of invertebrate immunity. The lack of any cross-resistance to parasites and pathogens, at both the phenotypic and the evolutionary level, suggests that evolution of resistance against one class of natural enemies is largely independent of evolution of resistance against the other
Automatic detection of limb prominences in 304 A EUV images
A new algorithm for automatic detection of prominences on the solar limb in 304 A EUV images is presented, and results of its application to SOHO/EIT data discussed. The detection is based on the method of moments combined with a
classifier analysis aimed at discriminating between limb prominences, active regions, and the quiet corona. This classifier analysis is based on a Support Vector Machine (SVM). Using a set of 12 moments of the radial intensity profiles, the algorithm performs well in discriminating between the above three categories of limb structures, with a misclassification rate of 7%. Pixels detected as belonging to a prominence are then used as starting point to reconstruct the whole prominence by morphological image processing techniques. It is planned that a catalogue of limb prominences identified in SOHO and STEREO data using this method will be made publicly available to the scientific community
Thermal and electrical conductivity of iron at Earth's core conditions
The Earth acts as a gigantic heat engine driven by decay of radiogenic
isotopes and slow cooling, which gives rise to plate tectonics, volcanoes, and
mountain building. Another key product is the geomagnetic field, generated in
the liquid iron core by a dynamo running on heat released by cooling and
freezing to grow the solid inner core, and on chemical convection due to light
elements expelled from the liquid on freezing. The power supplied to the
geodynamo, measured by the heat-flux across the core-mantle boundary (CMB),
places constraints on Earth's evolution. Estimates of CMB heat-flux depend on
properties of iron mixtures under the extreme pressure and temperature
conditions in the core, most critically on the thermal and electrical
conductivities. These quantities remain poorly known because of inherent
difficulties in experimentation and theory. Here we use density functional
theory to compute these conductivities in liquid iron mixtures at core
conditions from first principles- the first directly computed values that do
not rely on estimates based on extrapolations. The mixtures of Fe, O, S, and Si
are taken from earlier work and fit the seismologically-determined core density
and inner-core boundary density jump. We find both conductivities to be 2-3
times higher than estimates in current use. The changes are so large that core
thermal histories and power requirements must be reassessed. New estimates of
adiabatic heat-flux give 15-16 TW at the CMB, higher than present estimates of
CMB heat-flux based on mantle convection; the top of the core must be thermally
stratified and any convection in the upper core driven by chemical convection
against the adverse thermal buoyancy or lateral variations in CMB heat flow.
Power for the geodynamo is greatly restricted and future models of mantle
evolution must incorporate a high CMB heat-flux and explain recent formation of
the inner core.Comment: 11 pages including supplementary information, two figures. Scheduled
to appear in Nature, April 201
On acceleration of Krylov-subspace-based Newton and Arnoldi iterations for incompressible CFD: replacing time steppers and generation of initial guess
We propose two techniques aimed at improving the convergence rate of steady
state and eigenvalue solvers preconditioned by the inverse Stokes operator and
realized via time-stepping. First, we suggest a generalization of the Stokes
operator so that the resulting preconditioner operator depends on several
parameters and whose action preserves zero divergence and boundary conditions.
The parameters can be tuned for each problem to speed up the convergence of a
Krylov-subspace-based linear algebra solver. This operator can be inverted by
the Uzawa-like algorithm, and does not need a time-stepping. Second, we propose
to generate an initial guess of steady flow, leading eigenvalue and eigenvector
using orthogonal projection on a divergence-free basis satisfying all boundary
conditions. The approach, including the two proposed techniques, is illustrated
on the solution of the linear stability problem for laterally heated square and
cubic cavities
Solar science with the Atacama Large Millimeter/submillimeter Array - A new view of our Sun
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful
tool for observing the Sun at high spatial, temporal, and spectral resolution.
These capabilities can address a broad range of fundamental scientific
questions in solar physics. The radiation observed by ALMA originates mostly
from the chromosphere - a complex and dynamic region between the photosphere
and corona, which plays a crucial role in the transport of energy and matter
and, ultimately, the heating of the outer layers of the solar atmosphere. Based
on first solar test observations, strategies for regular solar campaigns are
currently being developed. State-of-the-art numerical simulations of the solar
atmosphere and modeling of instrumental effects can help constrain and optimize
future observing modes for ALMA. Here we present a short technical description
of ALMA and an overview of past efforts and future possibilities for solar
observations at submillimeter and millimeter wavelengths. In addition, selected
numerical simulations and observations at other wavelengths demonstrate ALMA's
scientific potential for studying the Sun for a large range of science cases.Comment: 73 pages, 21 figures ; Space Science Reviews (accepted December 10th,
2015); accepted versio
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