  We compare the results of the numerical simulation of the viscous-like
interaction of the solar wind with the plasma tail of a comet, with velocities
of H2O+ ions in the tail of comet Swift-Tuttle determined by means of
spectroscopic, ground based observations. Our aim is to constrain the value of
the basic parameters in the viscous-like interaction model: the effective
Reynolds number of the flow and the interspecies coupling timescale. We find
that in our simulations the flow rapidly evolves from an arbitrary initial
condition to a quasi-steady state for which there is a good agreement between
the simulated tailward velocity of H2O+ ions and the kinematics derived from
the observations. The fiducial case of our model, characterized by a low
effective Reynolds number (Re \approx 20 and selected on the basis of a
comparison to in situ measurements of the plasma flow at comet Halley, yields
an excellent fit to the observed kinematics. Given the agreement between model
and observations, with no ad hoc assumptions, we believe that this result
suggests that viscous-like momentum transport may play an important role in the
interaction of the solar wind and the cometary plasma environment.
