High temperature and thermal non-equilibrium effects on the determination of free-stream flow properties in hypersonic wind tunnels

Various methodologies used to derive free-stream conditions in hypersonic wind tunnels using different sets of experimental inputs are reviewed. The accuracy of a relevant but seldom used approach, involving free-stream static pressure measurements, is improved in the present work by solving numerically shock conservation equations and by accounting for the high-temperature effects typically expected in hypersonic flows (vibrational excitation, dissociation, and ionization). The numerical method implemented is also extended to handle free-stream thermal non-equilibrium provided that free-stream vibrational temperature measurements are available. The performances of the present approach are evaluated against conventional methods by determining free-stream flow properties in the von Karman Institute Longshot hypersonic wind tunnel. Uncertainties are quantified for each of the derived free-stream flow conditions. It is demonstrated that methodologies limited to Pitot pressure diagnostics in the test section fail to detect departures from ideal nozzle flow expansions, due to their inherent isentropic flow assumptions. Involving free-stream static pressure diagnostics does, however, improve the results, as validated by independent investigations. It is shown that the non-ideal nozzle flow expansion is not induced by free-stream thermal non-equilibrium since this phenomenon leads to opposite trends on free-stream flow properties with respect to those observed. Overall, free-stream static pressure probes represent a useful complement and their benefits for free-stream rebuilding methodologies are emphasized. The present free-stream rebuilding methodology is recommended to enhance the relevance of every hypersonic ground experiment.