A Few Remarks on Simulation Issues

Thermo-chemical freezing phenomena in the nozzles of hot hypersonic ground – simulation facilities seem not to be a major problem for the simulation of blunt-body (RV-type configuration) flows [24, 25]. The pressure is not much affected, but the bow shock stand-off distance may be wrong, which can be a problem in view of interaction phenomena on downstream configuration elements, Sub-Section 6.4.1. Also the determination of the test section Mach number can be problematic, Sub-Section 5.5.2. The possible large difference between the test section Mach number and the flight Mach number should be no problem, if the former is large enough that Mach-number independence is ensured [6]. However, the Space Shuttle Orbiter experience regarding the pitching-moment anomaly, [26], must be taken very seriously.[52]

Another problem are the thermal surface effects. For RV’s they concern as thermo-chemical effects at least the catalytic surface recombination. In ground-simulation facilities we have in general cold model surfaces. In reality the surface is hot, Tw ^ 2,000 K, and thermal loads determined with a cold model will have deficiencies.

The situation is different for CAV’s. Thermo-chemical equilibrium or non­equilibrium will occur in the nose region, but downstream of it, where the body slope is small, freezing may set in. At a forebody with pre-compression and at the inlet ramps then again the situation will change. It is not clear what effect thermo-chemical freezing phenomena in the facility nozzle flow will have in this case. In [27] it was found that a species separation due to pressure-gradient induced mass diffusion can happen when the frozen free – stream flow passes the body (flat plate) induced oblique shock wave. Whether this can be of importance is not known.

Not much more is known about the influence of frozen nozzle flow on ther­mal surface effects, in this case regarding predominantly viscous flow phe­nomena (viscous effects). Boundary-layer instability and laminar-turbulent transition will be affected, Section 8.2, but for CAV’s the cold model surfaces will lead in any case to major adverse effects.

Regarding computational simulation it depends on the flight speed, the altitude and the flight-vehicle type, and on the critical phenomenon/pheno- mena, whether an equilibrium or a non-equilibrium high-temperature real-gas model must be employed. This concerns not only the pressure field and near- wall/wall viscous and thermo-chemical phenomena, but via the shock stand­off distance also strong interaction effects, Section 9.2.2. Non-equilibrium thermo-chemical, but also radiation phenomena, however, must be regarded in any case at the very high speeds of, for instance, AOTV-type vehicles, see,

e. g., [28].