  The evolution of thin axisymmetric viscous accretion disks is a classic
problem in astrophysics. While models based on this simplified geometry provide
only approximations to the true processes of instability-driven mass and
angular momentum transport, their simplicity makes them invaluable tools for
both semi-analytic modeling and simulations of long-term evolution where two-
or three-dimensional calculations are too computationally costly. Despite the
utility of these models, the only publicly-available frameworks for simulating
them are rather specialized and non-general. Here we describe a highly
flexible, general numerical method for simulating viscous thin disks with
arbitrary rotation curves, viscosities, boundary conditions, grid spacings,
equations of state, and rates of gain or loss of mass (e.g., through winds) and
energy (e.g., through radiation). Our method is based on a conservative,
finite-volume, second-order accurate discretization of the equations, which we
solve using an unconditionally-stable implicit scheme. We implement Anderson
acceleration to speed convergence of the scheme, and show that this leads to
factor of $\sim 5$ speed gains over non-accelerated methods in realistic
problems, though the amount of speedup is highly problem-dependent. We have
implemented our method in the new code Viscous Accretion Disk Evolution
Resource (VADER), which is freely available for download from
https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public
License.
