Jay H. Grinstead

Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube

We report on new characterization capabilities recently implemented in the NASA Ames Electric Arc Shock Tube (EAST) facility. A new optical configuration, completely enclosed within a high vacuum chamber and attached to the shock tube, has enabled observation of spatially (and hence, temporally) resolved radiation through the shock layer. These imaging optics are coupled with four spectrometers covering the complete wavelength range of 1201700 nm, allowing for simultaneous measurement, at the same axial location, of spectral features over a broad range at various spectral resolutions. Measurements in the new system have addressed several of the discrepancies between model and experiment in prior EAST testing. The presence of CN impurity emission has been nearly eliminated, while atomic C and H emissions have been reduced. The presence of background continuum radiation has been confirmed as a real effect in the shock tube. Stark broadening measurements have been performed on Hydrogen Balmer-α line and show the electron number densities in the shock to be somewhat higher than model predictions. Experimental artifacts in the old configuration have been discovered and explain disagreements in the measured absolute magnitude of radiance.

Shock Radiation Measurements for Mars Aerocapture Radiative Heating Analysis

NASAs In-Space Propulsion Technology program is supporting the development of shock radiation transport models for aerocapture missions to Mars and Venus. Phenomenological models of nonequilibrium shock radiation will be incorporated into high-fidelity flowfield computations used to predict the aerothermal environments for a Mars or Venus aerocapture entry vehicle. These models are validated with shock radiance measurements obtained at flight-relevant conditions. A comprehensive test series in the NASA Ames Electric Arc Shock Tube facility at a representative freestream condition was recently completed. The facilitys optical instrumentation enabled spectral measurements of shocked gas radiation from the vacuum ultraviolet to the near infrared. The instrumentation captured the nonequilibrium postshock excitation and relaxation dynamics of dispersed spectral features. A description of the shock tube facility, optical instrumentation, and examples of the test data are presented.

Analysis and Model Validation of Shock Layer Radiation in Air

This paper analyzes the shock layer radiative heating environment for a large entry vehicle on a lunar return trajectory. Modeling results show that much of the shock layer plasma is in local thermodynamic equilibrium (LTE) and is not optically thin. The ionization level is generally high (15%) and the air is almost fully dissociated. A significant amount of vacuum ultraviolet (VUV) radiation is produced due to bound-bound and bound-free transitions of N and O atoms. The sensitivity of total radiation to Stark broadening, which dominates over other line broadening mechanisms, is quantified. The latter part of this paper reports the status of ongoing validation of the current radiation models with measurements in the Electric-Arc Shock Tube (EAST) facility at NASA Ames Research Center. Model predictions are compared with the calibrated radiation spectra measured in the equilibrium portion of the shock layer at 0.3 Torr. The reasons for discrepancy between model and measurements are also discussed with possible hypotheses presented for further investigation.

Absolute Radiation Measurement in High Mass Venus and Mars Entry Conditions

Comparisons of experimental characterization and model predictions of entry radiation relevant to Mars and Venus exploration are presented. Characterization is performed in the recently upgraded Ames Electric Arc Shock Tube (EAST) facility. Tests are performed in Mars (96% CO2, 4% N2) and Venus (96.5% CO2, 3.5% N2) simulant gases at downstream pressures and incident velocities spanning from 0.1-2.0 Torr and 3-12 km/s. Velocity and pressure conditions were chosen based on expected flight conditions (direct entry or aerocapture) in Mars and Venus atmospheres. The absolute radiance data are spatially and spectrally resolved and span the vacuum ultraviolet (VUV) through mid-IR (120-1650 nm, 35 μm). Resolved spectra of the CO 4 positive band in the VUV are reported for the first time. Measurements of CO2 molecular vibrational radiation is also attempted at low velocity conditions. Radiation modeled under equilibrium assumption with NEQAIR code compares favorably to measured radiation under some, but not all, conditions.

Measurement of Ultraviolet Radiative Heating Augmentation in HIEST Reflected Shock Tunnel

Radiance measurements in air at enthalpies from 8-20 MJ/kg have been made over a 250 mm diameter flat-faced test article in Japan Aerospace Exploration Agencys HIghEnthalpy Shock Tunnel (HIEST). Measurements were made in the ultraviolet region (200400 nm wavelength) in an attempt to resolve the long-standing discrepancy between theory and measurements of heat flux over a blunt body; this discrepancy is often attributed to radiation. The spectra obtained indicate the presence of atomic iron vapor in the flowfield. At the highest enthalpies, the radiance is at the blackbody limit. An attempt to model the radiance is made by taking a nominal CFD flowfield without any contamination products and processing it through a line-by-line radiation simulation tool. Iron vapor is introduced into the shocked gas ahead of the model and radiation computations are repeated; the mole fraction of iron vapor is adjusted to match the data. For the higher enthalpy conditions, the radiance was strongly absorbed and it was necessary to adjust the temperature and NO density in the freestream to match the signal below 300 nm. Once the observed spectra were satisfactorily matched, the radiance to the stagnation point was then computed. It is shown that the impurity radiation is sufficiently large to explain the discrepancy.

Consolidated Laser-Induced Fluorescence Diagnostic Systems for the NASA Ames Arc Jet Facilities

The spectroscopic diagnostic technique of two photon absorption laser-induced fluorescence (TALIF) of atomic species for non-intrusive arc jet flow property measurement was first implemented at NASA Ames in the mid-1990s. Use of TALIF expanded at NASA Ames and to NASA Johnsons arc jet facility in the late 2000s. In 2013-2014, NASA combined the agencys large-scale arc jet test capabilities at NASA Ames. Concurrent with that effort, the agency also sponsored a project to establish two comprehensive LIF diagnostic systems for the Aerodynamic Heating Facility (AHF) and Interaction Heating Facility (IHF) arc jets. The scope of the project enabled further engineering development of the existing IHF LIF system as well as the complete reconstruction of the original AHF LIF system. The updated LIF systems are identical in design and capability. They represent the culmination of over 20 years of development experience in transitioning a specialized laboratory research tool into a measurement system for large-scale, high-demand test facilities. This paper documents the overall system design from measurement requirements to implementation. Representative data from the redeveloped AHF and IHF LIF systems are also presented.