Braginskii viscosity on an unstructured, moving mesh accelerated with
  super-time-stepping
  We present a method for efficiently modelling Braginskii viscosity on an
unstructured, moving mesh. Braginskii viscosity, i.e., anisotropic transport of
momentum with respect to the direction of the magnetic field, is thought to be
of prime importance for studies of the weakly collisional plasma that comprises
the intracluster medium (ICM) of galaxy clusters. Here anisotropic transport of
heat and momentum has been shown to have profound consequences for the
stability properties of the ICM. Our new method for modelling Braginskii
viscosity has been implemented in the moving mesh code Arepo. We present a
number of examples that serve to test the implementation and illustrate the
modified dynamics found when including Braginskii viscosity in simulations.
These include (but are not limited to) damping of fast magneto-sonic waves,
interruption of linearly polarized Alfv\'en waves by the firehose instability
and the inhibition of the Kelvin-Helmholtz instability by Braginskii viscosity.
An explicit update of Braginskii viscosity is associated with a severe time
step constraint that scales with $(\Delta x)^2$ where $\Delta x$ is the grid
size. In our implementation, this restrictive time step constraint is
alleviated by employing 2nd order accurate Runge-Kutta-Legendre
super-time-stepping. We envision including Braginskii viscosity in future
large-scale simulations of Kelvin-Helmholtz unstable cold fronts in cluster
mergers and AGN-generated bubbles in central cluster regions.
