Subsonic-supersonic condition for shocks

The necessary conditions for stability of a shock, that the shock travel with supersonic velocity with respect to the medium ahead and with subsonic velocity with respect to the medium behind, are shown to be implied by the second law of thermodynamics for very general classes of viscous, heat-conducting fluids; the Weyl conditions are not invoked. The results are shown to be also compatible with the Le Chatelier–Braun principle. They further imply that under certain conditions it is not thermodynamically permissible to assume the existence of a shock transition layer in which entropy production is due to heat conduction alone.