High-Temperature Real-Gas Effects
Regarding the influence of high-temperature real-gas effects on instability and transition, two basic scenarios appear to play a role. The first is characterized by flow in thermo-chemical equilibrium, and by frozen flow, the second by flow in thermo-chemical non-equilibrium.
In view of the first scenario we note that high-temperature real-gas effects affect properties of the attached viscous flow, for instance the temperature and the density distribution in the direction normal to the surface. Hence they will have an indirect influence on instability and transition phenomena in the same way as pressure gradients, the thermal state of the wall, and so on have.
For the investigation and prediction of instability and transition of a boundary layer they must be taken into account therefore in order to determine the mean flow properties with the needed high accuracy. We remember that the point-of-inflection properties of a boundary layer are governed by the first and the second derivative of u(y), the thermal state of the surface, i. e., the first derivative of T(y) and—via the viscosity—the wall temperature T
Thermal and chemical non-equilibrium effects on the other hand affect the stability properties of boundary layers directly. Relaxation of chemical non-equilibrium has been shown experimentally and theoretically to stabilize boundary-layer flow, see, e. g., [61, 62]. Relaxation of rotational energy stabilizes, whereas relaxation of vibrational energy, contrary to what was believed until recently, can destabilize the flow strongly . This holds in particular for boundary layers downstream of a blunt nose or leading edge, which is the standard situation on the hypersonic flight vehicles considered in this book.
Important again is that the wind-tunnel situation must be distinguished from the free-flight situation. In the wind-tunnel frozen vibrational nonequilibrium might exist in the free-stream flow of the test section ahead of the flight vehicle model, Fig. 5.9 in Sub-Section 5.5.2, whereas in flight the atmosphere ahead of the flight vehicle is in equilibrium. In  it is shown that only for the thin flat plate in the free-flight situation with weak nonequilibrium the influence of vibrational relaxation is slightly stabilizing for second-mode instabilities.