  Truncated accretion disks are commonly invoked to explain the
spectro-temporal variability from accreting black holes in both small systems,
i.e. state transitions in galactic black hole binaries (GBHBs), and large
systems, i.e. low-luminosity active galactic nuclei (LLAGNs). In the canonical
truncated disk model of moderately low accretion rate systems, gas in the inner
region of the accretion disk occupies a hot, radiatively inefficient phase,
which leads to a geometrically thick disk, while the gas in the outer region
occupies a cooler, radiatively efficient phase that resides in the standard
geometrically thin disk. Observationally, there is strong empirical evidence to
support this phenomenological model, but a detailed understanding of the
dynamics of truncated disks is lacking. We present a well-resolved viscous,
hydrodynamic simulation that uses an ad hoc cooling prescription to drive a
thermal instability and, hence, produce the first sustained truncated accretion
disk. With this simulation, we perform a study of the dynamics, angular
momentum transport, and energetics of a truncated disk. We find that time
variability introduced by the quasi-periodic transition of gas from efficient
cooling to inefficient cooling impacts the evolution of the simulated disk. A
consequence of the thermal instability is that an outflow is launched from the
hot/cold gas interface which drives large, sub-Keplerian convective cells in
the disk atmosphere. The convective cells introduce a viscous $\theta-\phi$
stress that is less than the generic $r-\phi$ viscous stress component, but
greatly influences the evolution of the disk. In the truncated disk, we find
that the bulk of the accreted gas is in the hot phase.
