  Colliding hypersonic flows play a decisive role in many astrophysical
objects. In this paper, we look at the idealized model of a 2D plane parallel
isothermal slab (CDL) and at symmetric settings, where both flows have equal
parameters. We performed a set of high-resolution simulations with upwind Mach
numbers, 5 < M_u < 90.
  We find that the CDL is irregularly shaped and has a patchy and filamentary
interior. The size of these structures increases with l_cdl, the extension of
the CDL. On average, but not at each moment, the solution is about self-similar
and depends only on M_u. We find the root mean square Mach number to scale as
M_rms ~ 0.2 M_u. Independent of M_u is the mean density, rho_m ~ 30 rho_u. The
fraction f_eff of the upwind kinetic energy that survives shock passage scales
as f_eff= 1 - M_rms^(-0.6). This dependence persists if the upwind flow
parameters differ from one side to the other of the CDL, indicating that the
turbulence within the CDL and its driving are mutually coupled. In the same
direction points the finding that the auto-correlation length of the confining
shocks and the characteristic length scale of the turbulence within the CDL are
proportional.
  In summary, larger upstream Mach numbers lead to a faster expanding CDL with
more strongly inclined confining interfaces relative to the upstream flows,
more efficient driving, and finer interior structure relative to the extension
of the CDL.
