Numerical heat conduction in hydrodynamical models of colliding hypersonic flows

E. R. Parkin (1,2) & J. M. Pittard (2)

(1) Institut d'Astrophysique et de Geophysique, Universite de Liege, Belgium
(2) School of Physics & Astronomy, The University of Leeds, UK

Hydrodynamical models of colliding hypersonic flows are presented which explore the de-
pendence 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 post-shock gas of one flow
behaves quasi-adiabatically while the post-shock 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 in the amount of numerical conduction, leads to a
commensurate reduction in spurious X-ray emission, though the dynamics of the collision are
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.

Reference: Mon. Not. R. Astron. Soc. 406, 2373–2385 (2010)
Status: Manuscript has been accepted


Comments: 13 pages, 10 figures