45 research outputs found
Baroclinically-driven flows and dynamo action in rotating spherical fluid shells
The dynamics of stably stratified stellar radiative zones is of considerable interest due to the availability of increasingly detailed observations of Solar and stellar interiors. This article reports the first non-axisymmetric and time-dependent simulations of flows of anelastic fluids driven by baroclinic torques in stably stratified rotating spherical shells – a system serving as an elemental model of a stellar radiative zone. With increasing baroclinicity a sequence of bifurcations from simpler to more complex flows is found in which some of the available symmetries of the problem are broken subsequently. The poloidal component of the flow grows relative to the dominant toroidal component with increasing baroclinicity. The possibility of magnetic field generation thus arises and this paper proceeds to provide some indications for self-sustained dynamo action in baroclinically-driven flows. We speculate that magnetic fields in stably stratified stellar interiors are thus not necessarily of fossil origin as it is often assumed
Quasi-geostrophic approximation of anelastic convection
The onset of convection in a rotating cylindrical annulus with parallel ends filled with a compressible fluid is studied in the anelastic approximation. Thermal Rossby waves propagating in the azimuthal direction are found as solutions. The analogy to the case of Boussinesq convection in the presence of conical end surfaces of the annular region is emphasised. As in the latter case, the results can be applied as an approximation for the description of the onset of anelastic convection in rotating spherical fluid shells. Reasonable agreement with three-dimensional numerical results published by Jones, Kuzanyan & Mitchell (J. Fluid Mech., vol. 634, 2009, pp. 291–319) for the latter problem is found. As in those results, the location of the onset of convection shifts outwards from the tangent cylinder with increasing number Nρof density scale heights until it reaches the equatorial boundary. A new result is that at a much higher number Nρ the onset location returns to the interior of the fluid shell
Dynamo Effects Near The Transition from Solar to Anti-Solar Differential Rotation
Numerical MHD simulations play increasingly important role for understanding
mechanisms of stellar magnetism. We present simulations of convection and
dynamos in density-stratified rotating spherical fluid shells. We employ a new
3D simulation code for the solution of a physically consistent anelastic model
of the process with a minimum number of parameters. The reported dynamo
simulations extend into a "buoyancy-dominated" regime where the buoyancy
forcing is dominant while the Coriolis force is no longer balanced by pressure
gradients and strong anti-solar differential rotation develops as a result. We
find that the self-generated magnetic fields, despite being relatively weak,
are able to reverse the direction of differential rotation from anti-solar to
solar-like. We also find that convection flows in this regime are significantly
stronger in the polar regions than in the equatorial region, leading to
non-oscillatory dipole-dominated dynamo solutions, and to concentration of
magnetic field in the polar regions. We observe that convection has different
morphology in the inner and at the outer part of the convection zone
simultaneously such that organized geostrophic convection columns are hidden
below a near-surface layer of well-mixed highly-chaotic convection. While we
focus the attention on the buoyancy-dominated regime, we also demonstrate that
conical differential rotation profiles and persistent regular dynamo
oscillations can be obtained in the parameter space of the rotation-dominated
regime even within this minimal model.Comment: Published in the Astrophysical Journa
Flows and dynamos in a model of stellar radiative zones
Stellar radiative zones are typically assumed to be motionless in standard
models of stellar structure but there is sound theoretical and
observational evidence that this cannot be the case. We investigate
by direct numerical simulations a three-dimensional and time-dependent
model of stellar radiation zones consisting of an electrically-conductive
and stably-stratified anelastic fluid confined to a rotating spherical
shell and driven by a baroclinic torque. As the baroclinic driving is
gradually increased a sequence of transitions from an axisymmetric and
equatorially-symmetric time-independent flow to flows with a strong
poloidal component and lesser symmetry are found. It is shown that
all flow regimes characterised with significant non-axisymmetric
components are capable of generating self-sustained magnetic field.
As the value of the Prandtl number is decreased and the value of the
Ekman number is decreased flows become strongly time-dependent with
progressively complex spatial structure and dynamos can be generated
at lower values of the magnetic Prandtl number
Regimes of thermo-compositional convection and related dynamos in rotating spherical shells
Convection and magnetic field generation in the Earth and planetary interiors are driven by both thermal and compositional gradients. In this work numerical simulations of finite-amplitude double-diffusive convection and dynamo action in rapidly rotating spherical shells full of incompressible two-component electrically-conducting fluid are reported. Four distinct regimes of rotating double-diffusive convection identified in a recent linear analysis (Silva, Mather and Simitev, Geophys. Astrophys. Fluid Dyn. 2019, 113, 377) are found to persist significantly beyond the onset of instability while their regime transitions remain abrupt. In the semi-convecting and the fingering regimes characteristic flow velocities are small compared to those in the thermally- and compositionally-dominated overturning regimes, while zonal flows remain weak in all regimes apart from the thermally-dominated one. Compositionally-dominated overturning convection exhibits significantly narrower azimuthal structures compared to all other regimes while differential rotation becomes the dominant flow component in the thermally-dominated case as driving is increased. Dynamo action occurs in all regimes apart from the regime of fingering convection. While dynamos persist in the semi-convective regime they are very much impaired by small flow intensities and very weak differential rotation in this regime which makes poloidal to toroidal field conversion problematic. The dynamos in the thermally-dominated regime include oscillating dipolar, quadrupolar and multipolar cases similar to the ones known from earlier parameter studies. Dynamos in the compositionally-dominated regime exhibit subdued temporal variation and remain predominantly dipolar due to weak zonal flow in this regime. These results significantly enhance our understanding of the primary drivers of planetary core flows and magnetic fields
Kelvin-Helmholtz instability and collapse of a twisted magnetic nullpoint with anisotropic viscosity
Context. Magnetic null points are associated with high-energy coronal phenomena such as solar flares and are often sites of reconnection and particle acceleration. Dynamic twisting of a magnetic null point can generate a Kelvin-Helmholtz instability (KHI) within its fan plane and can instigate spine-fan reconnection and an associated collapse of the null point under continued twisting.
Aims. This article aims to compare the effects of isotropic and anisotropic viscosity in simulations of the KHI and collapse in a dynamically twisted magnetic null point.
Methods. We performed simulations using the 3D magnetohydrodynamics code Lare3d with a custom anisotropic viscosity module. A pair of high-resolution simulations were performed, one using isotropic viscosity and another using anisotropic viscosity, keeping all other factors identical. We analysed the results in detail. A further parameter study was performed over a range of values for viscosity and resistivity.
Results. Both viscosity models permit the growth of the KHI and the eventual collapse of the null point. Over all studied parameters, anisotropic viscosity allows a faster growing instability, while isotropic viscosity damps the instability to the extent of stabilisation in some cases. Although the viscous heating associated with anisotropic viscosity is generally smaller, the ohmic heating dominates and is enhanced by the current sheets generated by the instability. This leads to a greater overall heating rate when using anisotropic viscosity. The collapse of the null point occurs significantly sooner when anisotropic viscosity is employed
Estimation of parameters for an archetypal model of cardiomyocyte membrane potentials
Contemporary realistic mathematical models of single-cell cardiac electrical
excitation are immensely detailed. Model complexity leads to parameter uncertainty, high
computational cost and barriers to mechanistic understanding. There is a need for reduced
models that are conceptually and mathematically simple but physiologically accurate.
To this end, we consider an archetypal model of single-cell cardiac excitation that replicates
the phase-space geometry of detailed cardiac models, but at the same time has a simple
piecewise-linear form and a relatively low-dimensional configuration space. In order to make
this archetypal model practically applicable, we develop and report a robust method for
estimation of its parameter values from the morphology of single-stimulus action potentials
derived from detailed ionic current models and from experimental myocyte measurements.
The procedure is applied to five significant test cases and an excellent agreement with target
biomarkers is achieved. Action potential duration restitution curves are also computed
and compared to those of the target test models and data, demonstrating conservation of
dynamical pacing behaviour by the fine-tuned archetypal model. An archetypal model that
accurately reproduces a variety of wet-lab and synthetic electrophysiology data offers a
number of specific advantages such as computational efficiency, as also demonstrated in the
study. Open-source numerical code of the models and methods used is provided