Measurements of Time-Resolved Air Freestream Nitric Oxide Rotational, Vibrational Temperature and Concentration in the T5 Reflected Shock Tunnel

We report on measurements of freestream temperature and nitric oxide (NO) collected in the Caltech T5 reflected shock tunnel facility. Quantum cascade lasers (QCL), emitting mid-infrared light resonant with fundamental rovibrational NO transitions, were directed through the supersonic (Mach ~5) flow immediately downstream of the nozzle exit plane. Tunable diode laser absorption spectroscopy (TDLAS) was used to measure the path-averaged rotational and vibrational temperature of NO in the flow, in addition to the NO partial pressure. The temperature measurements demonstrate strong evidence of NO rotational and vibrational equilibrium during the test-period (from 1 to 2 ms after shock reflection). Agreement between vibrational and rotational temperature was observed in all experiments, including one lower-enthalpy (h₀,∞ ≈ 8 MJ/kg) and four higher-enthalpy (h₀,∞ ≈ 18 MJ/kg) shots, during and after the nominal test time. Slight disagreement between the rotational and vibrational temperatures was observed during the start-up period preceding the test time. Unexpected absorption from CO and H₂O was also observed in the TDLAS measurements, yielding estimates of their partial pressures in the flow. The goal of these and future experiments is to help to inform improvements of existing models and solvers used for prediction of freestream conditions. Future efforts will include a more quantitative evaluation of freestream conditions, including core flow velocity and uniformity, and TDLAS measurements near relevant flow models.