  In this article some of the hydrodynamical (HD) aspects of steady shocks as
described by the steady-state shock model are reviewed and discussed. It is
found that, at least in some of the contexts in which the steady-state model is
used, the steady-state assumption cannot be satisfied. Moreover, the main
result of the present work is that even if the assumptions on steadiness and on
the geometry are fully satisfied, serious limitations in the application of the
model are found: (i) in the absence of down-stream boundary conditions the
model is not related to the physical process(es) that originate the shock, (ii)
matter shocked during the presumed phase of steadiness of the shock is not
hydrodynamically interacting with previously shocked matter, and (iii) the
steady-state model assumes that the flow is stable against perturbations.
Furthermore, even if boundary conditions were assumed, the link between the
steady model and the astrophysical context would not be strictly speaking the
correct HD link. Time-dependent HD computations in different astrophysical
contexts (e.g. SNRs and molecular shocks) show that the steady-state
approximation is inadequate to describe these post-shock structures. Based on
the HD limitations of the steady-state model, it is advised that the model be
used to describe post-shock structures only in those astrophysical contexts
where full time-dependent HD models have already positively tested the
steadiness of the flow. Alternatively, it is suggested to replace the
steady-state model either with time-dependent HD models, or with less
problematic approximations.
