Effective viscosity from cloud-cloud collisions in three-dimensional
  global SPH simulations
  Analytic estimates of the viscous time-scale due to cloud-cloud collisions
have been as high as thousands of Gyr. Consequently, cloud collisions are
widely ignored as a source of viscosity in galactic disks. However, capturing
the hydrodynamics of discs in simple analytic models is a challenge, both
because of the wide dynamic range and importance of 2D and 3D effects. To test
the validity of analytic models we present estimates for the viscous time-scale
that are measured from three dimensional SPH simulations of disc formation and
evolution. We have deliberately removed uncertainties associated with
star-formation and feedback thereby enabling us to place lower bounds on the
time-scale for this process. We also contrast collapse simulations with results
from simulations of initially stable discs and examine the impact of numerical
parameters and assumptions on our work, to constrain possible systematics in
our estimates. We find that cloud-collision viscous time-scales are in the
range of 0.6-16 Gyr, considerably shorter than previously estimated. This large
discrepency can be understood in terms of how the efficiency of collisions is
included in the analytical estimates. We find that the viscous time-scale only
depends weakly on the number of clouds formed, and so while the viscous
time-scale will increase with increasing resolution, this effect is too weak to
alter our conclusions.
