Experimental determination of test gas caloric quantities in high-enthalpy, hypersonic ground testing is impeded by excessive pressure and temperature levels as well as minimum test time scales of short-duration facilities. However, accurate knowledge of test gas conditions and stagnation enthalpy prior to nozzle expansion is vital to achieve a valid comparison of experimental data with numerical results. In order to facilitate a more accurate quantification of caloric nozzle inlet conditions in hypersonic shock tunnels, a deliberate combination of in situ non-intrusive measurements within the nozzle reservoir with ex situ intrusive free-stream measurements and numerical rebuilding is devised. For this purpose, in situ measurements of post-reflected shock stagnation temperature in the nozzle reservoir of the HELM facility (High-enthalpy laboratory Munich) are carried out by resonant, homodyne laser-induced grating spectroscopy (LIGS). For incident shock waves up to Mach 3.6, test conditions in air are advanced up to moderate stagnation enthalpies of 2.1 MJ/kg and stagnation pressures of 220 bar. Technical measures which were required to reach this goal are discussed in detail. In order to lay the foundation for this effort, detailed and comprehensive operation conditions for the HELM facility across the entire test envelope and up to the nominal range of 1000 bar burst pressure and 25 MJ/kg stagnation enthalpy are developed, according to different theories for tuned free-piston driver (FPD) and tailored contact surface operation for the first time. In order to complement and validate numerical predictions, continuous measurements of instantaneous piston acceleration by an on-board accelerometer facilitate to reconstruct and validate the time-resolved compression piston trajectory for a full-stroke and up to 30,000 m/s2 peak deceleration. Measurements are corrected by and validated against localized waypoint markers. This work represents the first effort to systematically characterize the free-stream regime at the HELM nozzle exit by a deliberate combination of intrusive experimental measurements and numerical simulations - the use of experimental-numerical rebuilding routines, which are implemented for this purpose, will facilitate a deduction of caloric stagnation conditions from ex situ measurements and comparison to in situ measurements in the future, eventually contributing to a higher accuracy of boundary conditions in high-enthalpy, short-duration ground testing.    «

Experimental determination of test gas caloric quantities in high-enthalpy, hypersonic ground testing is impeded by excessive pressure and temperature levels as well as minimum test time scales of short-duration facilities. However, accurate knowledge of test gas conditions and stagnation enthalpy prior to nozzle expansion is vital to achieve a valid comparison of experimental data with numerical results. In order to facilitate a more accurate quantification of caloric nozzle inlet conditions in...    »