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 .