82 research outputs found
Global simulations of Tayler instability in stellar interiors: a long-time multi-stage evolution of the magnetic field
Magnetic fields have been observed in massive Ap/Bp stars and presumably are
also present in the radiative zone of solar-like stars. Yet, to date there is
no clear understanding of the dynamics of the magnetic field in stably
stratified layers. A purely toroidal magnetic field configuration is known to
be unstable, developing mainly non-axisymmetric modes. Rotation and a small
poloidal field component may lead to a stable configuration. Here we perform
global MHD simulations with the EULAG-MHD code to explore the evolution of a
toroidal magnetic field located in a layer whose stratification resembles the
solar tachocline. Our numerical experiments allow us to explore the initial
unstable phase as well as the long-term evolution of the magnetic field. During
the first Alfven cycles, we observe the development of the Tayler instability
with the prominent longitudinal wavenumber, . Rotation decreases the
growth rate of the instability, and eventually suppresses it. However, after a
stable phase, sudden energy surges lead to the development of higher order
modes even for fast rotation. These modes extract energy from the initial
toroidal field. Nevertheless, our results show that sufficiently fast rotation
leads to a lower saturation energy of the unstable modes, resulting in a
magnetic topology with only a small fraction of poloidal field which remains
steady for several hundreds of Alfven travel times. At this stage, the system
becomes turbulent and the field is prone to turbulent diffusion. The final
toroidal-poloidal configuration of the magnetic field may represent an
important aspect of the field generation and evolution in stably-stratified
layers.Comment: 15 pages, 16 figures, submitted to MNRA
A finite-volume module for simulating global all-scale atmospheric flows
This paper was accepted for publication in the Journal of Computational Physics and the definitive published version is available at http://dx.doi.org/10.1016/j.jcp.2016.03.015.The paper documents the development of a global nonhydrostatic finite-volume module designed to enhance an established spectral-transform based numerical weather prediction (NWP) model. The module adheres to NWP standards, with formulation of the governing equations based on the classical meteorological latitude-longitude spherical framework. In the horizontal, a bespoke unstructured mesh with finite-volumes built about the reduced Gaussian grid of the existing NWP model circumvents the notorious stiffness in the polar regions of the spherical framework. All dependent variables are co-located, accommodating both spectral-transform and grid-point solutions at the same physical locations. In the vertical, a uniform finite-difference discretisation facilitates the solution of intricate elliptic problems in thin spherical shells, while the pliancy of the physical vertical coordinate is delegated to generalised continuous transformations between computational and physical space. The newly developed module assumes the compressible Euler equations as default, but includes reduced soundproof PDEs as an option. Furthermore, it employs semi-implicit forward-in-time integrators of the governing PDE systems, akin to but more general than those used in the NWP model. The module shares the equal regions parallelisation scheme with the NWP model, with multiple layers of parallelism hybridising MPI tasks and OpenMP threads. The efficacy of the developed nonhydrostatic module is illustrated with benchmarks of idealised global weather
A finite-volume module for simulating global all-scale atmospheric flows
The paper documents the development of a global nonhydrostatic finite-volume module designed to enhance an established spectral-transform based numerical weather prediction (NWP) model. The module adheres to NWP standards, with formulation of the governing equations based on the classical meteorological latitude-longitude spherical framework. In the horizontal, a bespoke unstructured mesh with finite-volumes built about the reduced Gaussian grid of the existing NWP model circumvents the notorious stiffness in the polar regions of the spherical framework. All dependent variables are co-located, accommodating both spectral-transform and grid-point solutions at the same physical locations. In the vertical, a uniform finite-difference discretisation facilitates the solution of intricate elliptic problems in thin spherical shells, while the pliancy of the physical vertical coordinate is delegated to generalised continuous transformations between computational and physical space. The newly developed module assumes the compressible Euler equations as default, but includes reduced soundproof PDEs as an option. Furthermore, it employs semi-implicit forward-in-time integrators of the governing PDE systems, akin to but more general than those used in the NWP model. The module shares the equal regions parallelisation scheme with the NWP model, with multiple layers of parallelism hybridising MPI tasks and OpenMP threads. The efficacy of the developed nonhydrostatic module is illustrated with benchmarks of idealised global weather
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