  In this paper, the nonlinear evolution of a bistable interstellar medium is
investigated using two-dimensional simulations with a realistic cooling rate,
thermal conduction, and physical viscosity. The calculations are performed
using periodic boundary conditions without any external dynamical forcing. As
the initial condition, a spatially uniform unstable gas under thermal
equilibrium is considered. At the initial stage, the unstable gas quickly
segregates into two phases, or cold neutral medium (CNM) and warm neutral
medium (WNM). Then, self-sustained turbulence with velocity dispersion of
$0.1-0.2\;\mathrm{km\;s^{-1}}$ is observed in which the CNM moves around in the
WNM. We find that the interfacial medium (IFM) between the CNM and WNM plays an
important role in sustaining the turbulence. The self-sustaining mechanism can
be divided into two steps. First, thermal conduction drives fast flows
streaming into concave CNM surfaces towards the WNM. The kinetic energy of the
fast flows in the IFM is incorporated into that of the CNM through the phase
transition. Second, turbulence inside the CNM deforms interfaces and forms
other concave CNM surfaces, leading to fast flows in the IFM. This drives the
first step again and a cycle is established by which turbulent motions are
self-sustained.
