Comparisons of direct simulation Monte Carlo modeling of nitric oxide emissions with electric arc shock tube measurements

Emission measurements taken at the NASA Ames Electric Arc Shock Tube facility as part of Test Series 66 [Cruden and Tibère-Inglesse, “Radiative emission in incident air shocks from 3–7 km/s,” AIAA Paper No. 2024-3653, 2024] included air shocks in the range of 3–5 km/s. In this work, we use the direct simulation Monte Carlo (DSMC) method to model unsteady one-dimensional shocks and replicate flow conditions observed for Shots 26 and 27 from this test series. The DSMC solutions are used along with a radiation solver, Nonequilibrium and Equilibrium Radiative Transport and Spectra Program, to compute nitric oxide (NO) emission in the infrared (IR) and ultraviolet (UV) spectral regions in the shock layer. The numerically generated estimates of NO emission provide good agreement with emission measurements in the IR spectral region in terms of absolute spectral radiance as well as relative spectral intensity providing validation of the thermochemical models used to model the NO ground electronic state. This was also observed from comparisons of NO ground electronic state populations and temperatures predicted by DSMC calculations and values derived from IR emission and tunable diode laser absorption measurements. However, numerical predictions of NO emission in the UV spectral region over-predict the spectral intensity in comparison with emission measurements while providing partial agreement for the relative spectral intensity highlighting a need for improvement in the modeling of mechanisms critical to the excitation and de-excitation of NO electronic state populations. This was further supported by comparisons of numerical predictions of NO excited electronic state populations and electronic temperatures with those inferred from UV emission measurements. The possible effect of the boundary layer, which is colder than the core flow, on NO IR and UV emission signals in the shock layer was also explored.