  Hydrodynamical models of colliding hypersonic flows are presented which
explore the dependence of the resulting dynamics and the characteristics of the
derived X-ray emission on numerical conduction and viscosity. For the purpose
of our investigation we present models of colliding flow with plane-parallel
and cylindrical divergence. Numerical conduction causes erroneous heating of
gas across the contact discontinuity which has implications for the rate at
which the gas cools. We find that the dynamics of the shocked gas and the
resulting X-ray emission are strongly dependent on the contrast in the density
and temperature either side of the contact discontinuity, these effects being
strongest where the postshock gas of one flow behaves quasi-adiabatically while
the postshock gas of the other flow is strongly radiative. Introducing
additional numerical viscosity into the simulations has the effect of damping
the growth of instabilities, which in some cases act to increase the volume of
shocked gas and can re-heat gas via sub-shocks as it flows downstream. The
resulting reduction in the surface area between adjacent flows, and therefore
of the amount of numerical conduction, leads to a commensurate reduction in
spurious X-ray emission, though the dynamics of the collision are compromised.
The simulation resolution also affects the degree of numerical conduction. A
finer resolution better resolves the interfaces of high density and temperature
contrast and although numerical conduction still exists the volume of affected
gas is considerably reduced. However, since it is not always practical to
increase the resolution, it is imperative that the degree of numerical
conduction is understood so that inaccurate interpretations can be avoided.
This work has implications for the dynamics and emission from astrophysical
phenomena which involve high Mach number shocks.
