48 research outputs found
Nonlinear Properties of the Shear Dynamo Model
No abstract available
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Learning from the learners' experience: e-Learning@greenwich post-conference reflections
This publication comprises papers from presenters who, having made a conference presentation, were invited to author an academic paper about their work
Large-to small-scale dynamo in domains of large aspect ratio: kinematic regime
The Sun’s magnetic field exhibits coherence in space and time on much larger scales than
the turbulent convection that ultimately powers the dynamo. In this work, we look for numerical
evidence of a large-scale magnetic field as the magnetic Reynolds number, Rm, is
increased. The investigation is based on the simulations of the induction equation in elongated
periodic boxes. The imposed flows considered are the standard ABC flow (named after
Arnold, Beltrami & Childress) with wavenumber ku = 1 (small-scale) and a modulated ABC
flow with wavenumbers ku = m, 1, 1 ± m, where m is the wavenumber corresponding to
the long-wavelength perturbation on the scale of the box. The critical magnetic Reynolds
number Rcrit
m decreases as the permitted scale separation in the system increases, such that
Rcrit
m ∝ [Lx /Lz]
−1/2. The results show that the α-effect derived from the mean-field theory
ansatz is valid for a small range of Rm after which small scale dynamo instability occurs and the
mean-field approximation is no longer valid. The transition from large- to small-scale dynamo
is smooth and takes place in two stages: a fast transition into a predominantly small-scale
magnetic energy state and a slower transition into even smaller scales. In the range of Rm
considered, the most energetic Fourier component corresponding to the structure in the long
x-direction has twice the length-scale of the forcing scale. The long-wavelength perturbation
imposed on the ABC flow in the modulated case is not preserved in the eigenmodes of the
magnetic field
On the necessary conditions for bursts of convection within the rapidly rotating cylindrical annulus
Zonal flows are often found in rotating convective systems. Not only are
these jet-flows driven by the convection, they can also have a profound effect
on the nature of the convection. In this work the cylindrical annulus geometry
is exploited in order to perform nonlinear simulations seeking to produce
strong zonal flows and multiple jets. The parameter regime is extended to
Prandtl numbers that are not unity. Multiple jets are found to be spaced
according to a Rhines scaling based on the zonal flow speed, not the convective
velocity speed. Under certain conditions the nonlinear convection appears in
quasi-periodic bursts. A mean field stability analysis is performed around a
basic state containing both the zonal flow and the mean temperature gradient
found from the nonlinear simulations. The convective growth rates are found to
fluctuate with both of these mean quantities suggesting that both are necessary
in order for the bursting phenomenon to occur
The dynamics and excitation of torsional waves in geodynamo simulations
The predominant force balance in rapidly rotating planetary cores is between Coriolis, pressure, buoyancy and Lorentz forces. This magnetostrophic balance leads to a Taylor state where the spatially averaged azimuthal Lorentz force is compelled to vanish on cylinders aligned with the rotation axis. Any deviation from this state leads to a torsional oscillation, signatures of which have been observed in the Earth's secular variation and are thought to influence length of day variations via angular momentum conservation. In order to investigate the dynamics of torsional oscillations (TOs), we perform several 3-D dynamo simulations in a spherical shell. We find TOs, identified by their propagation at the correct Alfvén speed, in many of our simulations. We find that the frequency, location and direction of propagation of the waves are influenced by the choice of parameters. Torsional waves are observed within the tangent cylinder and also have the ability to pass through it. Several of our simulations display waves with core traveltimes of 4–6 yr. We calculate the driving terms for these waves and find that both the Reynolds force and ageostrophic convection acting through the Lorentz force are important in driving TOs
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Designing for learning: e-Learning@greenwich post-conference reflections and abstracts
The e-learning@greenwich/conference 2007, Designing for Learning, was the fifth conference in the series and has developed from its original focus on local institutional issues to a conference that focuses on global themes in e-learning attracting international participation. Our conferences are characterised by being practitioner focused and sector inclusive. Conference evaluations suggest that its intimate and friendly atmosphere, within the grounds of the world heritage site at Greenwich, allows practitioner-delegates to freely mix with delegates of international research repute and have an influence on practice, whether in the classroom or in educational software development.
We felt it was now time to produce a series of publications to share the important work being captured and disseminated at the conferences to a wider public in the form of post-conference reflective proceedings
Solenoidal force balances in numerical dynamos
Numerical simulations of the geodynamo (and other planetary dynamos) have
made significant progress in recent years. As computing power has advanced,
some new models claim to be ever more appropriate for understanding Earth's
core dynamics. One measure of the success of such models is the ability to
replicate the expected balance between forces operating within the core;
Coriolis and Lorentz forces are predicted to be most important. The picture is
complicated for an incompressible flow by the existence of the pressure
gradient force which renders the gradient parts of all other forces dynamically
unimportant. This can confuse the situation, especially when the scale
dependence of forces are considered. In this work we investigate force balances
through the alternative approach of eliminating gradient parts of each force to
form `solenoidal force balances'. We perform a lengthscale dependent analysis
for several spherical simulations and find that removal of gradient parts
offers an alternative picture of the force balance compared to looking at
traditional forces alone. Solenoidal force balances provide some agreement with
the results of previous studies but also significant differences. They offer a
cleaner overall picture of the dynamics and introduce differences at smaller
scales. This has implications for geodynamo models purporting to have reached
Earth-like regimes: in order to achieve a meaningful comparison of forces, only
the solenoidal part of forces should be considered.Comment: 12 pages, 4 figures, 1 tabl