1,185 research outputs found
Multiscale model of global inner-core anisotropy induced by hcp-alloy plasticity
Multiscale model of inner-core anisotropy produced by hcp alloy
deformation 5 to 20% single-crystal elastic anisotropy and plastic
deformation by pyramidal slip Low-degree inner-core formation model
with faster crystallization at the equatorThe Earth's solid inner-core exhibits
a global seismic anisotropy of several percents. It results from a coherent
alignment of anisotropic Fe-alloy crystals through the inner-core history that
can be sampled by present-day seismic observations. By combining
self-consistent polycrystal plasticity, inner-core formation models,
Monte-Carlo search for elastic moduli, and simulations of seismic measurements,
we introduce a multiscale model that can reproduce a global seismic anisotropy
of several percents aligned with the Earth's rotation axis. Conditions for a
successful model are an hexagonal-close-packed structure for the inner-core
Fe-alloy, plastic deformation by pyramidal \textless{}c+a\textgreater{} slip,
and large-scale flow induced by a low-degree inner-core formation model. For
global anisotropies ranging between 1 and 3%, the elastic anisotropy in the
single crystal ranges from 5 to 20% with larger velocities along the c-axis
Earth's Inner Core dynamics induced by the Lorentz force
Seismic studies indicate that the Earth's inner core has a complex structure
and exhibits a strong elastic anisotropy with a cylindrical symmetry. Among the
various models which have been proposed to explain this anisotropy, one class
of models considers the effect of the Lorentz force associated with the
magnetic field diffused within the inner core. In this paper we extend previous
studies and use analytical calculations and numerical simulations to predict
the geometry and strength of the flow induced by the poloidal component of the
Lorentz force in a neutrally or stably stratified growing inner core, exploring
also the effect of different types of boundary conditions at the inner core
boundary (ICB). Unlike previous studies, we show that the boundary condition
that is most likely to produce a significant deformation and seismic anisotropy
is impermeable, with negligible radial flow through the boundary. Exact
analytical solutions are found in the case of a negligible effect of buoyancy
forces in the inner core (neutral stratification), while numerical simulations
are used to investigate the case of stable stratification. In this situation,
the flow induced by the Lorentz force is found to be localized in a shear layer
below the ICB, which thickness depends on the strength of the stratification,
but not on the magnetic field strength. We obtain scaling laws for the
thickness of this layer, as well as for the flow velocity and strain rate in
this shear layer as a function of the control parameters, which include the
magnitude of the magnetic field, the strength of the density stratification,
the viscosity of the inner core, and the growth rate of the inner core. We find
that the resulting strain rate is probably too small to produce significant
texturing unless the inner core viscosity is smaller than about Pa.s.Comment: submitted to Geophysical Journal Internationa
Experimental study of super-rotation in a magnetostrophic spherical Couette flow
We report measurements of electric potentials at the surface of a spherical
container of liquid sodium in which a magnetized inner core is differentially
rotating. The azimuthal angular velocities inferred from these potentials
reveal a strong super-rotation of the liquid sodium in the equatorial region,
for small differential rotation. Super-rotation was observed in numerical
simulations by Dormy et al. [1]. We find that the latitudinal variation of the
electric potentials in our experiments differs markedly from the predictions of
a similar numerical model, suggesting that some of the assumptions used in the
model - steadiness, equatorial symmetry, and linear treatment for the evolution
of both the magnetic and velocity fields - are violated in the experiments. In
addition, radial velocity measurements, using ultrasonic Doppler velocimetry,
provide evidence of oscillatory motion near the outer sphere at low latitude:
it is viewed as the signature of an instability of the super-rotating region
Recommended from our members
Minimizing the Cost of Innovative Nuclear Technology Through Flexibility: The Case of a Demonstration Accelerator-Driven Subcritical Reactor Park
Presented is a methodology to analyze the expected Levelised Cost Of Electricity (LCOE) in the face of technology uncertainty for Accelerator-Driven Subcritical Reactors (ADSRs). It shows that flexibility in the design and deployment strategy of an ADSR park demonstrator significantly reduces its expected LCOE. The methodology recognizes in the conceptual design a range of possible technological outcomes for the ADSR accelerator system. It identifies flexibility âonâ and âinâ the design to modify the future development path in light of such uncertain scenarios. Uncertainty and flexibility are incorporated in the ADSR valuation. The resulting economic assessment is more realistic than typical discounted cash flow analysis that does not consider a range of development outcomes, or the flexibility to change development path
Numerical Simulations of Dynamos Generated in Spherical Couette Flows
We numerically investigate the efficiency of a spherical Couette flow at
generating a self-sustained magnetic field. No dynamo action occurs for
axisymmetric flow while we always found a dynamo when non-axisymmetric
hydrodynamical instabilities are excited. Without rotation of the outer sphere,
typical critical magnetic Reynolds numbers are of the order of a few
thousands. They increase as the mechanical forcing imposed by the inner core on
the flow increases (Reynolds number ). Namely, no dynamo is found if the
magnetic Prandtl number is less than a critical value .
Oscillating quadrupolar dynamos are present in the vicinity of the dynamo
onset. Saturated magnetic fields obtained in supercritical regimes (either
or ) correspond to the equipartition between magnetic and
kinetic energies. A global rotation of the system (Ekman numbers ) yields to a slight decrease (factor 2) of the critical magnetic
Prandtl number, but we find a peculiar regime where dynamo action may be
obtained for relatively low magnetic Reynolds numbers (). In this
dynamical regime (Rossby number , spheres in opposite direction) at
a moderate Ekman number (), a enhanced shear layer around the inner
core might explain the decrease of the dynamo threshold. For lower
() this internal shear layer becomes unstable, leading to small
scales fluctuations, and the favorable dynamo regime is lost. We also model the
effect of ferromagnetic boundary conditions. Their presence have only a small
impact on the dynamo onset but clearly enhance the saturated magnetic field in
the ferromagnetic parts. Implications for experimental studies are discussed
Seismic response and anisotropy of a model hcp iron inner core
International audienceWe present a framework for simulating the measurement of seismic anisotropy in a model inner core by computing travel time residuals of synthetic seismic rays propagated through the model. The method is first tested on simple inner core structural models consisting of layers with distinct anisotropy, as often proposed in the literature. Those models are not consistent with geodynamics. Hence, we extend the method to a numerically grown inner core composed of Δ-Fe with flow generated from an excess of crystallization in the equatorial belt, inducing polycrystalline textures. The global inner core anisotropy is 7 times smaller than that of the single crystal. Compositional stratification amplifies the global anisotropy by 15% while the addition of solidification textures reduces it by a factor of 2. As such, and within the tested geodynamical models, no published elastic model of Δ-Fe at inner core conditions allows for reproducing the 3% cylindrical anisotropy reported in seismology publications. In addition, our models demonstrate that additional information such as the depth dependence and the spread of the observed anisotropy is a key for revealing the dynamics and history of the inner core
Recommended from our members
Simultaneous mesoscopic and two-photon imaging of neuronal activity in cortical circuits.
Spontaneous and sensory-evoked activity propagates across varying spatial scales in the mammalian cortex, but technical challenges have limited conceptual links between the function of local neuronal circuits and brain-wide network dynamics. We present a method for simultaneous cellular-resolution two-photon calcium imaging of a local microcircuit and mesoscopic widefield calcium imaging of the entire cortical mantle in awake mice. Our multi-scale approach involves a microscope with an orthogonal axis design where the mesoscopic objective is oriented above the brain and the two-photon objective is oriented horizontally, with imaging performed through a microprism. We also introduce a viral transduction method for robust and widespread gene delivery in the mouse brain. These approaches allow us to identify the behavioral state-dependent functional connectivity of pyramidal neurons and vasoactive intestinal peptide-expressing interneurons with long-range cortical networks. Our imaging system provides a powerful strategy for investigating cortical architecture across a wide range of spatial scales
New Si-based multilayers for solar cell applications
In this article, we have fabricated and studied a new multilayer structure Si-SiO2/SiNx by reactive magnetron sputtering. The comparison between SiO2 and SiNx host matrices in the optical properties of the multilayers is detailed. Structural analysis was made on the multilayer structures using Fourier transform infrared spectroscopy. The effect of specific annealing treatments on the optical properties is studied and we report a higher visible luminescence with a control over the thermal budget when SiO2 is replaced by the SiNx matrix. The latter seems to be a potential candidate to replace the most sought SiO2 host matrix
Geometric singular perturbartion theory for non-smooth dynamical systems
In this article we deal with singularly perturbed Filippov systems ZΔ: (1) Ëx = ( F(x, y, Δ) if h(x, y, Δ) †0, G(x, y, Δ) if h(x, y, Δ) â„ 0, ΔyË = H(x, y, Δ), where Δ â R is a small parameter, x â Rn, n â„ 2, and y â R denote the slow and fast variables, respectively, and F, G, h, and H are smooth maps. We study the effect of singular perturbations at typical singularities of Z0. Special attention will be dedicated to those points satisfying q â {h(x, y, 0) = 0} â© {H(x, y, 0) = 0} where F or G is tangent to {h(x, y, 0) = 0}. The persistence and the stability properties of those objects are investigated.Fundação de Amparo Ă Pesquisa do Estado de SĂŁo Paulo (FAPESP
Zonal shear and super-rotation in a magnetized spherical Couette flow experiment
We present measurements performed in a spherical shell filled with liquid
sodium, where a 74 mm-radius inner sphere is rotated while a 210 mm-radius
outer sphere is at rest. The inner sphere holds a dipolar magnetic field and
acts as a magnetic propeller when rotated. In this experimental set-up called
DTS, direct measurements of the velocity are performed by ultrasonic Doppler
velocimetry. Differences in electric potential and the induced magnetic field
are also measured to characterize the magnetohydrodynamic flow. Rotation
frequencies of the inner sphere are varied between -30 Hz and +30 Hz, the
magnetic Reynolds number based on measured sodium velocities and on the shell
radius reaching to about 33. We have investigated the mean axisymmetric part of
the flow, which consists of differential rotation. Strong super-rotation of the
fluid with respect to the rotating inner sphere is directly measured. It is
found that the organization of the mean flow does not change much throughout
the entire range of parameters covered by our experiment. The direct
measurements of zonal velocity give a nice illustration of Ferraro's law of
isorotation in the vicinity of the inner sphere where magnetic forces dominate
inertial ones. The transition from a Ferraro regime in the interior to a
geostrophic regime, where inertial forces predominate, in the outer regions has
been well documented. It takes place where the local Elsasser number is about
1. A quantitative agreement with non-linear numerical simulations is obtained
when keeping the same Elsasser number. The experiments also reveal a region
that violates Ferraro's law just above the inner sphere.Comment: Phys Rev E, in pres
- âŠ