Christopher L. Strand

Spatially resolved measurements of temperature and molecular oxygen concentration are needed to understand hypersonic phenomena such as vibrational relaxation, dissociation, and recombination. To address this need, the present work develops and demonstrates a two-color, fluorescence-based diagnostic capable of measuring both of these key quantities in repeated experiments. The diagnostic leverages a narrow-linewidth, continuous-wave laser source to probe the Schumann–Runge bands of O ₂ near 225 nm behind reflected shock waves. Measured absorption cross-sections and normalized fluorescence signals show good agreement with computational models, exhibiting an average discrepancy of 10.1% and 6.9% for absorption and fluorescence, respectively. Demonstration measurements show good agreement with expected post-shock conditions and exhibit average 2 σ temperature and O ₂ number density uncertainties as low as 8.5% and 5.4%, respectively.

Quantitative, high-bandwidth measurements of temperature, pressure, and velocity are needed to characterize high-speed flows and validate numerical simulations. To that end, this work demonstrates rapid, multi-parameter spectrally resolved planar laser-induced fluorescence (SR-PLIF) for spatially resolved, calibration-free measurements of these three key quantities using the gamma-bands of nitric oxide near 225.33 nm. The SR-PLIF technique was validated from 295–420 K and 3.48–20.4 kPa using a static gas cell and exhibits an average temperature and pressure error of 2% and 4%, respectively. The diagnostic was then used to study an underexpanded jet at rates up to 1 kHz and a spatial resolution of 220µm, with results showing favorable agreement with numerical simulations.

A ring-amplified shock tube (RAST) is presented as a new facility design for high-sensitivity laser absorption spectroscopy (LAS) in shock-heated experiments. By integrating a confocal, segmented circular multi-pass cell (SC-MPC) into the test section, the RAST decouples optical pathlength from tube diameter, enabling multi-pass absorption spectroscopy (MPAS) with selectable pathlengths ranging from 0.15 m to 11 m using single-entry alignment. Pathlengths well in excess of 11 m are possible by combining multiple cavity modes through re-entrant beam alignment. Design details of the constructed facility are provided and complemented with a generalized analytical framework for calculating the modes and beam pattern characteristics of SC-MPC optical cavities. A high-temperature air chemistry experiment targeting O₂(X³Σ) at 221.6 nm, O(3s ⁵S₂ ^o ) at 777 nm, N₂(A³Σ) at 868 nm, and N(3s ⁴P_5/2) at 868 nm demonstrates multi-wavelength, variable-gain measurements of absorption transitions previously undetectable at the target conditions. Results confirm robust beam alignment, low cavity noise, and minimal emission interference. The RAST provides substantial improvements in sensitivity and diagnostic flexibility for kinetics and spectroscopy applications.

Vibrational relaxation of the ground to twelfth excited vibrational levels (v″ = 0–12) of carbon monoxide (CO) has been probed with multi-pass absorption spectroscopy (MPAS) on a ring amplified shock tube (RAST) targeting absorption transitions near 5055 nm (1978 cm−1). Streicher et al., “High-temperature measurements of nitrogen vibrational relaxation through pathlengthamplified probing of carbon monoxide vibrational states in shock-tube experiments,” AIAA Paper No. 2025–1991, 2025. Experiments probed mixtures of 0.02%–5% CO diluted in nitrogen (N2) from 2000 to 7500 K and 0.07 to 0.84 atm, with conditions selected for sensitive inference of N2–N2 vibration-translation (VT) relaxation times (τVTN2−N2) using CO as an optically accessible tracer of N2. Two additional types of experiments leveraged argon (Ar) dilution of either 50% or 91%; 50% Ar was used to extend the test time, while 91% Ar was used to access a condition at 10 250 K where chemical reactions occur. High-temperature experiments probed 24 absorption features covering all vibrational states v″ = 0–12 and many rotational levels from J″ = 2–116, as well as weak features from the C13O16 isotopologue. Low-lying vibrational states follow an expected Boltzmann distribution, although states above v″ = 5 are observed to populate faster than expected from their Boltzmann populations—both for full N2 dilution and for 50% Ar dilution. Modal temperatures, inferred from v″ = 0–5 states, remain in good agreement with simulations of the vibrational relaxation processes based on literature values of vibrational relaxation times. Non-Boltzmann behavior for v″ > 5 was investigated with a state-to-state model, with model predictions improving when multiquantum transitions were included in the model. Overall, the inferred values of τVTN2−N2 remain consistent with literature values, although the low scatter and uncertainty of the RAST measurements suggest a rate approximately 10% faster than the Millikan and White correlation.

A ring-amplified shock tube (RAST) is presented as a new facility design for high-sensitivity laser absorption spectroscopy (LAS) in shock-heated experiments. By integrating a confocal, segmented circular multi-pass cell (SC-MPC) into the test section, the RAST decouples optical pathlength from tube diameter, enabling multi-pass absorption spectroscopy (MPAS) with selectable pathlengths ranging from 0.15 m to 11 m using single-entry alignment. Pathlengths well in excess of 11 m are possible by combining multiple modes through re-entrant beam alignment. A high-temperature air chemistry experiment targeting O2(X3Σ) at 221.6 nm, O(3s5So2) at 777 nm, N2(A3Σ) at 868 nm, and N(3s4P5/2) at 868 nm demonstrates multi-wavelength, variable-gain measurements of absorption transitions previously undetectable at the target conditions. Results confirm robust beam alignment, low cavity noise, and minimal emission interference. The RAST provides substantial improvements in sensitivity and diagnostic flexibility for kinetics and spectroscopy applications.