Speaker
Description
Background
The Electric Arc Shock Tube (EAST) at NASA Ames Research Center produces hypersonic shock waves for validation and improvement of non-equilibrium chemical kinetics and radiation models relevant to atmospheric entry flows. The main experimental technique employs four spectrometers, capable of achieving simultaneous spatially resolved emission spectra of the radiating gas from the vacuum ultraviolet to the mid-infrared wavelength range [1, 2]. Calibrated frames map the radiance emitted along the probing volume across the tube diameter, resolved in wavelength and spatial dimensions along the view window. The similarity to the stagnation line flow over a blunt body at equivalent free-stream conditions is exploited to numerically simulate the hypersonic shock wave and evaluate the agreement with radiative simulations using the Nonequilibrium Radiative Transport and Spectra Program (NEQAIR v15.3) [3]. Experimental uncertainties on the measured radiance are important to quantify the discrepancy with numerical models, and directly propagate to inferred quantities, e.g., when performing calibration of rate parameters.
Methodology
Leveraging recent experimental data collected in Test Series 66 [3], which included incident air shocks at nominal velocities of 6 km/s and 7 km/s, and fill pressures of 2 Torr and 1.4 Torr, respectively, this work analyzes the uncertainty on the experimental radiance, and its impact on the measured temperature and number densities. Comparison to concurrent laser absorption measurements, as well as to data collected at similar test conditions in the Oxford T6 facility, will be also presented.
A previous work from Brandis et al. [4] characterized the uncertainty on emission spectroscopy measurement in EAST based on the scatter among data points with respect to a global fit of different conditions. This work focuses on the individual contributions to the OES calibration uncertainty and propagates their impact to the measured spectral radiance for each single shot. These include the calibration source intensity, its spatial non-uniformity over the sensor field, as well as the camera non-reciprocity and intensity non-linearity. Measurement and background noise are characterized by means of statistical sampling. Uncertainties are propagated with a random sampling of the distributions through the calibration steps.
Additionally, the effect of different analytical representations of the Instrument Line Shape (ILS) and Spatial Resolution Function (SRF) is studied when comparing with simulated post-shock equilibrium spectra and spatial radiance profiles. We consider the effect of reducing the slit width on the background continuum level, as well as hardware binning the CCD sensors to reduce the gate time and improve the spatial resolution.
Finally, the work investigates the impact of the measurement uncertainties on temperature and species number density obtained through fitting of the measured spectra, and evaluates their agreement with concurrent laser absorption measurements, in view of providing a consistent description of the post-shock conditions.
Preliminary results
Preliminary analysis provides standard deviations close to 10% of the measured local radiance below 900 nm. The relative contribution of the different terms ultimately depends on the measured signal level, with calibration and measurement noise being the largest terms for intensified CCD cameras at the observed conditions. In this regard, the noise equivalent radiance is a useful quantity to define the lower detectability threshold, and relative uncertainties increase significantly when the measured signal reaches this level.
Broadening of traditional analytical shapes, such as the square root of the Voigt profile, or the mixed Gaussian-Lorentzian profiles, with an additional square function improves the fitting of the measured ILS in case of wide slit widths. Optical raytracing across the tube diameter defines the optical resolution function, providing consistent results with previous analytical derivations and showing negligible spherical aberration effects. The camera gating function dominates the SRF overall.
Spectral fitting of $\textrm{NO}$, $\textrm{N}_2$ and $\textrm{N}_2^+$ excited states is in progress. Due to the fact the measurements are affected by the spatial resolution of the optical system, a deconvolution procedure is attempted to retrieve local values.
Conclusions
This work quantifies the experimental uncertainty on spatially resolved radiance maps obtained by optical emission spectroscopy measurements of incident shock radiation in the EAST facility. The aim is to determine bounds to the local spectral radiance, which can better inform comparison to model predictions and calibration of rate parameters. The sources of uncertainty at each calibration step are identified and propagated to the measured radiance. Analysis of fitted quantities is in progress.
References
[1] Cruden, Brett, Ramon Martinez, Jay Grinstead, and Joeseph Olejniczak. “Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube.” In 41st AIAA Thermophysics Conference. San Antonio, TX, USA, 2009.
[2] Cruden, Brett A. “Absolute Radiation Measurements in Earth and Mars Entry Conditions.” Radiation and Gas-Surface Interaction Phenomena in High-Speed Reentry, VKI Lecture Series, 2014.
[3] Cruden, Brett A, and Aaron M Brandis. “Updates to the NEQAIR Radiation Solver.” In 6th International Workshop on Radiation of High Temperature Gas, St. Andrews, UK, 2014.
[4] Cruden, Brett A, and Augustin Tibere-Inglesse. “Radiative Emission in Incident Air Shocks from 3-7 Km/s.” to be presented at AIAA Aviation Forum, Las Vegas, NV, USA, 2024.
[5] Brandis, A. M., C. O. Johnston, B. A. Cruden, D. Prabhu, and D. Bose. “Uncertainty Analysis and Validation of Radiation Measurements for Earth Reentry.” Journal of Thermophysics and Heat Transfer 29, no. 2 (April 2015): 209–21.
Summary
This work quantifies the experimental uncertainty on spatially resolved radiance maps obtained by optical emission spectroscopy measurements of incident shock radiation in the EAST facility. The sources of uncertainty at each calibration step are identified and propagated to the measured radiance, as well as to fitted quantities, such as temperatures and state densities. The aim is to determine bounds to the experimental quantities in order to better inform comparison to model predictions and calibration of rate parameters.
