Near-Body Measurements in Hypersonic Wedge Flows in the T5 Reflected Shock Tunnel by Tunable Diode Laser Absorption Spectroscopy

Abstract

We report on spectroscopic measurements of nitric oxide (NO) rotational and vibrational temperatures and concentration in the hypersonic flow fields around two symmetric wedge test models in the Caltech T5 reflected shock tunnel. Three quantum cascade lasers (QCLs), targeting 13 distinct quantum rovibrational transitions of NO, probed the T5 flows at a measurement rate of 50 kHz. Via tunable diode laser absorption spectroscopy (TDLAS), absorption from these rovibrational transitions permitted the path-averaged inferences of temperature and NO concentration both in front of and behind the shocks around the test articles. The fractional proportion of laser absorption in front and behind the shocks was also determined, enabling the measurement of shock locations at several lines of sight in the flow. Two different symmetric wedge test articles were utilized, one characterized by a wedge angle of 48o, and the other 38o. Three different T5 flow conditions were interrogated, a M = 5.5, ∼8 MJ/kg stagnation enthalpy case, a M = 5.3, ∼11 MJ/kg stagnation enthalpy case, and a M = 4.9, ∼18 MJ/kg stagnation enthalpy case. Beam locations were specified relative to the wedge using alignment fins, Cartesian grids of holes which attached rigidly to the model endspans during alignment. The optical configuration allowed for independent characterization of the freestream and post-shock regions. Freestream measurements demonstrate general temperature and NO density decline over the duration of test time, typical of what has been previously measured in T5. Post-shock measurements demonstrate thermal equilibrium at elevated temperatures and high NO densities on the upstream sides of the wedges. Past the vertices of the wedges, post-shock measurements are colder, with thermal non-equilibrium sometimes detected.

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