9 research outputs found
Simulation of the Magnetothermal Instability
In many magnetized, dilute astrophysical plasmas, thermal conduction occurs
almost exclusively parallel to magnetic field lines. In this case, the usual
stability criterion for convective stability, the Schwarzschild criterion,
which depends on entropy gradients, is modified. In the magnetized long mean
free path regime, instability occurs for small wavenumbers when (dP/dz)(dln
T/dz) > 0, which we refer to as the Balbus criterion. We refer to the
convective-type instability that results as the magnetothermal instability
(MTI). We use the equations of MHD with anisotropic electron heat conduction to
numerically simulate the linear growth and nonlinear saturation of the MTI in
plane-parallel atmospheres that are unstable according to the Balbus criterion.
The linear growth rates measured from the simulations are in excellent
agreement with the weak field dispersion relation. The addition of isotropic
conduction, e.g. radiation, or strong magnetic fields can damp the growth of
the MTI and affect the nonlinear regime. The instability saturates when the
atmosphere becomes isothermal as the source of free energy is exhausted. By
maintaining a fixed temperature difference between the top and bottom
boundaries of the simulation domain, sustained convective turbulence can be
driven. MTI-stable layers introduced by isotropic conduction are used to
prevent the formation of unresolved, thermal boundary layers. We find that the
largest component of the time-averaged heat flux is due to advective motions as
opposed to the actual thermal conduction itself. Finally, we explore the
implications of this instability for a variety of astrophysical systems, such
as neutron stars, the hot intracluster medium of galaxy clusters, and the
structure of radiatively inefficient accretion flows.Comment: Accepted for publication in Astrophysics and Space Science as
proceedings of the 6th High Energy Density Laboratory Astrophysics (HEDLA)
Conferenc
Turbulent Compressible Convection with Rotation - Penetration above a Convection Zone
We perform Large eddy simulations of turbulent compressible convection in
stellar-type convection zones by solving the Navi\'{e}r-Stokes equations in
three dimensions. We estimate the extent of penetration into the stable layer
above a stellar-type convection zone by varying the rotation rate
({\boldmath}), the inclination of the rotation vector () and
the relative stability () of the upper stable layer. The computational
domain is a rectangular box in an f-plane configuration and is divided into two
regions of unstable and stable stratification with the stable layer placed
above the convectively unstable layer. Several models have been computed and
the penetration distance into the stable layer above the convection zone is
estimated by determining the position where time averaged kinetic energy flux
has the first zero in the upper stable layer. The vertical grid spacing in all
the model is non-uniform, and is less in the upper region so that the flows are
better resolved in the region of interest. We find that the penetration
distance increases as the rotation rate increases for the case when the
rotation vector is aligned with the vertical axis. However, with the increase
in the stability of the upper stable layer, the upward penetration distance
decreases. Since we are not able to afford computations with finer resolution
for all the models, we compute a number of models to see the effect of
increased resolution on the upward penetration. In addition, we estimate the
upper limit on the upward convective penetration from stellar convective cores.Comment: Accepted for Publication in Asttrophysics & Space Scienc
The Relaxation Oscillation of Turbulent Convection in Rotating Cylindrical Annulus
Engineering, EnvironmentalMechanicsCPCI-S(ISTP)