Speaker
Description
Background
Hypersonic flows are characterized by complex thermochemical processes that are difficult to simulate. Ultimately, there is a need for experimental data to validate the multiple modeling approaches present in the literature. Optical emission spectroscopy (OES) can be used as a non-intrusive measurement to probe the excited thermo-chemical state of important radiating species, such as nitric oxide (NO). NO readily forms in hypersonic flows and is the dominant radiator in the ultraviolet (UV). In the current study, spatially resolved OES measurements of NO in the UV are taken within the shock layer over a cylinder and wedge geometry in the Sandia Hypersonic Shock Tunnel (HST). These spatially resolved emission spectra are fit using two spectral modeling codes, NASA NEQAIR and the Sandia Spectral Physics Environment for Advanced Remote Sensing (SPEARS). The spectral model fits are used to extract the NO rotational and vibrational temperatures and are compared to direct simulation Monte Carlo (DSMC) simulations of the hypersonic flow-field that utilize the latest thermo-chemical models. The modeling approach is described in more detail in an accompanying abstract submitted by the authors. The OES measurements and comparisons to simulation are made in regions of rapid flow expansion, where thermal non-equilibrium between the NO rotational and vibrational temperature may be induced.
Methodology
OES measurements are made in Sandia’s HST, a free-piston shock tunnel that produces high-enthalpy Mach 8-10 flow. Spatially resolved OES measurements are performed in the shock layer formed around two models placed in the core flow of the HST: a 50.8mm cylinder and 50.8mm square wedge. For the cylinder case, OES measurements are made through the shock layer along the stagnation streamline and at angles 30 and 60 degrees to the horizontal. In the wedge case, measurements are made along a horizontal line 1.59 and 6.59 mm below the tip of the expansion corner. Measurements were performed for two freestream conditions generated in the HST.
Shock layer emission was collected using a 50.8mm diameter lens mounted into the shock tunnel side wall. A 2f imaging setup directly imaged the shock layer onto the slit of a spectrometer mounted onto an angular rotation stage. This allows the vertical extent of the slit to align with the various angles measured in the cylinder geometry. The imaging spectrometer captured a single shot image for each shock tunnel run, where the vertical extent of the image represented a spatial dimension through the shock layer and the horizontal captured a wavelength dimension ranging from 205-265nm. Vertical bins of 30 pixels were taken to determine NO spectra within a given spatial region of the shock layer.
The spectra measured at each location are corrected for relative intensity and fit using two spectra modeling codes: NASA's NEQAIR v15.1 and the Sandia Spectral Physics Environment for Advanced Remote Sensing (SPEARS). Four NO emission bands will radiate in the UV range: the γ, β,δ, and ε bands. Spectral fits performed using NEQAIR utilize all four band systems while SPEARS only includes the first three taken from the ExoMol NO linelist. Spectral fits using both codes assume a multi-Boltzmann distribution for independent rotational and vibrational states. The NEQAIR spectral fits use a non-Boltzmann quasi-steady-state (QSS) solver to solve for the electronic state distribution function while the SPEARS spectral fits assume a Boltzmann distribution.
Results
For each freestream condition and measurement location, OES measurements were repeated a minimum of three times. The vibrational and rotational temperatures determined through spectral modeling fits of the repeated cases were averaged and the variance across each repetition was small, giving confidence in the repeatability of the measurement.
Initial temperature fits of the OES measurements made along the stagnation streamline for the cylinder geometry show the vibrational and rotational NO temperature are in thermal equilibrium and agree with DSMC simulations. At the 30 and 60 degree locations on the cylinder geometry the spectral radiance decreases rapidly as the flow expands and cools. Initial results show a decrease in the fitted rotational and vibrational temperature at larger angles to the horizontal. The fitted rotational temperatures found using both spectral modeling codes agree with DSMC predictions. However, only the vibrational temperature fit found using SPEARS agrees with simulation. Fits performed using NEQAIR predict vibrational temperatures ~2000K higher than the rotational temperature.
The sources of the discrepancy between the fitted vibrational temperatures produced using SPEARS and NEQAIR will be explored and discussed in the full presentation. Further, the data collected on the wedge geometry will be fully processed and the results will be presented for the full presentation.
Conclusion
OES measurements were made of NO produced within the shock layer for a cylinder and wedge geometry in Sandia’s HST facility. The collected emission was fit using Sandia’s SPEARS code and NEQAIR to determine line profiles of the vibrational and rotational temperature of NO. Measurements were collected over multiple areas on each geometry and targeted areas of flow expansion where thermodynamic non-equilibrium may occur. Fitted temperatures are compared to DSMC simulations utilizing the latest thermo-chemical models.
Initial results show the fitted rotational temperature agrees with DSMC predictions while the NEQAIR vibrational temperature fits yield temperatures ~2000K higher than the DSMC predictions and the SPEARS vibrational temperature fits. Potential sources of these disagreements and the measurements on the wedge geometry will be discussed in the full presentation.
Note
The authors are submitting a companion abstract which discusses in detail the simulation and modeling results whereas this abstract describes the experimental effort.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
Summary
Optical Emission Spectroscopy (OES) measurements were made of nitric oxide (NO) produced within the shock layer for a cylinder and wedge geometry in Sandia’s Hypersonic Shock Tunnel (HST) facility. The collected emission was fit using Sandia’s SPEARS code and NEQAIR to determine line profiles of the vibrational and rotational temperature of NO. Measurements were collected over multiple areas on each geometry and targeted areas of flow expansion where thermodynamic non-equilibrium may occur. Fitted temperatures are compared to DSMC simulations utilizing the latest thermo-chemical models.
Initial results show the fitted rotational temperature agrees with DSMC predictions while the NEQAIR vibrational temperature fits yield temperatures ~2000K higher than the DSMC predictions and the SPEARS vibrational temperature fits. Potential sources of these disagreements and the measurements on the wedge geometry will be discussed in the full presentation.
