Thermal Instability and Multiphase Gas in the Simulated Interstellar
  Medium with Conduction, Viscosity and Magnetic Fields
  Thermal instability (TI) plays a crucial role in the formation of multiphase
structures and their dynamics in the Interstellar Medium (ISM) and is a leading
theory for cold cloud creation in various astrophysical environments. In this
paper we use two-dimensional (2D) simulations to investigate thermal
instability under the influence of various initial conditions and physical
processes. We experiment with Gaussian random field (GRF) density perturbations
of different initial power spectra. We also enroll thermal conduction and
physical viscosity in isotropic hydrodynamic and anisotropic
magnetohydrodynamic (MHD) simulations. We find that the initial GRF spectral
index $\alpha$ has a dramatic impact on the pure hydrodynamic development of
thermal instability, influencing the size, number and motions of clouds. Cloud
fragmentation happens due to two mechanisms: tearing and contraction rebound.
In the runs with isotropic conduction and viscosity, the structures and
dynamics of the clouds are dominated by evaporation and condensation flows in
the non-linear regime, and the flow speed is regulated by viscosity. Cloud
disruptions happen as a result of the Darrieus--Landau instability (DLI).
Although at very late times, all individual clouds merge into one cold
structure in all hydrodynamic runs. In the MHD case, the cloud structure is
determined by both the initial perturbations and the initial magnetic field
strength. In high $\beta$ runs, anisotropic conduction causes dense filaments
to align with the local magnetic fields and the field direction can become
reoriented. Strong magnetic fields suppress cross-field contraction and cold
filaments can form along or perpendicular to the initial fields.
