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

Abstract

We report on near-body measurements of nitric oxide (NO) rotational and vibrational temperatures and concentration in the hypersonic flow around a cylindrical test model in the Caltech T5 reflected shock tunnel. An array of quantum cascade lasers (QCLs) and distributed feedback lasers (DFBs) emitted infrared light in the vicinity of 5 microns resonant with several quantum transitions of NO. Tunable diode laser absorption spectroscopy (TDLAS) was employed to infer path-averaged flow parameters of temperature and NO concentration, both in front of and behind the bow shock around the cylinder, at a measurement rate of 50 kHz. The fractions of laser beam paths behind the shock at different spatial locations were also discerned, thus providing a measurement of the shock location. Two shots at the same nominal flow condition with stagnation enthalpy ~ 8 MJ/kg, freestream temperature ~ 1050 K, and freestream velocity ~ 3600 m/s were explored. Laser beams were pitched at three locations at radial distances of 9 mm, 19.5 mm, and 30 mm from the cylinder surface along a plane 120 degrees relative to the upstream direction. The cylinder was 1.75 inches in diameter with an aspect ratio of 3. Beam locations were specified relative to the cylinder using a custom-made alignment grid that could be rigidly attached to the cylinder end-span during the alignment step. To isolate measurements of the freestream and near-cylinder flow structure, optical arms with flow-cutting wedges were designed to divert the boundary-layer and shear-layer of the nozzle exit flow away from the field of view and deliver the laser beams directly into the core of the flow. Freestream measurements from this dataset show general agreement with those in previous works. Nevertheless, shot-to-shot variations are sufficient to confound the interpretation of the post-shock gas characteristics, which rely on the freestream conditions. A novel technique to extract freestream information from the TDLAS path-average is presented, permitting the independent measurement of the freestream each shot without reliance on previous characterization.