Christopher R. Alba

Boundary-Layer Stability Calculations for the HIFiRE-1 Transition Experiment

Boundary-layer stability analysis is performed by computational fluid dynamic simulation of experiments conducted in the National Aeronautics and Space Administration Langley Research Center 20-in. Mach 6 Air Tunnelinsupportofthe first flightoftheHypersonicInternationalFlightResearchExperimentationprogram.From the laminar computational flow solutions, disturbances are calculated using the linear parabolized stability equations to obtain integrated disturbance growth rates. Comparisons are made between the experimentally observed transition locations and the results of the stability analysis. The stability results from the NASA Langley Research Center Air Tunnel are combined with previous work done for the Calspan University at Buffalo Research Center Large-Energy National Shock Tunnel to show excellent correlation between predicted and observed boundary-layer transition locations. Roughness calculations are also performed and a Reynolds number based on trip height is tabulated with experimental results.

Aerothermodynamic Testing and Boundary-Layer Trip Sizing of the HIFiRE Flight 1 Vehicle

An experimental wind tunnel test was conducted in the NASA Langley Research Center’s 20-Inch Mach 6 Air Tunnel in support of the Hypersonic International Flight Research Experimentation Program. The information in this report is focused on the Flight 1 configuration, the first in a series of flight experiments. This report documents experimental measurements made over a range of Reynolds numbers and angles of attack on several scaled ceramic heat transfer models of the Flight 1 payload. Global heat transfer was measured using phosphor thermography and the resulting images and heat transfer distributions were used to infer the state of the boundary layer on the vehicle windside and leeside surfaces. Boundary layer trips were used to force the boundary layer turbulent, and a brief study was conducted to determine the effectiveness of the trips with various heights. The experimental data highlighted in this test report were used to size and place the boundary layer trip for the flight test vehicle.

Three-dimensional hypersonic boundary layer stability analysis with STABL-3D

A suite of computer codes, developed at the University of Minnesota, for the analysis of reacting hypersonic boundary layer stability has been used successfully by many different researchers in a wide range of applications. However, these applications have been limited by the software to the analysis of axisymmetric or two-dimensional flows or to analysis in the symmetry planes of three-dimensional flows. This paper introduces the latest version of the software suite which has been extended for the analysis of three-dimensional flows. Comparisons are made with the already well-tested two-dimensional version of the codes, and results are presented to illustrate the new capabilities for analysis of three-dimensional flows.

Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Thermal protection system design for atmospheric reentry vehicles remains a challenging and complex problem. Recent advances in computational modeling of air–carbon interactions consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops several modifications to the Zhluktov and Abe air–carbon model that account for all three reaction mechanisms with the goal of improving cyanogen shock-layer radiation predictions to recent experimental results. First, the study examines two possible paths for carbon nitridation and then assesses the augmented surface reaction model in a representative blunt-body reentry flow. Second, a sensitivity analysis is performed to determine which surface reactions have the most impact on altering cyanogen radiance predictions. It was...

Influence of Carbon Nitridation in a Nonequilibrium Finite-Rate Ablation Model

Thermal protection system design for atmospheric re-entry vehicles remains to be a challenging and complex problem. There have been several attempts to model surface chemical kinetics but there is no universally supported model among investigators. Recent advances in computational modeling of air-carbon interactions now consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops a modification to the Zhluktov and Abe air-carbon interaction model that accounts for all three reaction mechanisms. The study examines two possible paths that represents carbon nitridation as a direct or a surface participating reaction. The augmented surface reaction model is assessed in a representative blunt body re-entry flow and compared against the well-known Park model. It is shown that the implemented direct nitridation mechanism has the most significant impact on predicted surface mass fluxes and species mass fractions. There are notable differences between the Park and direct nitridation mechanisms, particularly at carbon sublimation surface temperatures. More detailed measurements of the carbon nitridation reaction at higher surface temperatures are required to further validate and improve the rate parameters derived in this study.

Comparison of carbon ablative shock-layer radiation with high surface temperatures

Despite the prominence of carbon-based materials for use in thermal protection systems,much uncertainty remains in predicting thermochemical ablation rates at high surface temperatures. To address this issue, a series of experiments using pre-heated graphite models with surface temperatures up to 3280 K was conducted in the X-2 expansion tunnel at The University of Queensland. Calibrated shock-layer emission measurements in the wavelength region from 353 to 391 nm were taken to observe the effect of surface temperature on radiation from the CN Violet ∆v = 0 and ∆v = +1 bands. Numerical simulations were conducted using US3D with modified Park and Zhluktov and Abe surface thermochemistry models. The simulation results were applied in NEQAIR to reproduce the experimental radiance profiles. The experiments showed reduced CN emissions at the highest surface temperatures, which runs contrary to numerical simulations. This result may be due to blockage by spalled particles which were observed in high speed footage of the tests.

Characterization of Carbon Ablation Models Including Effects of Gas-Phase Chemical Kinetics

The modeling of the oxidation and sublimation of carbon-based ablative thermal protection system materials remains an area of active research. In this paper, two gas–surface interaction models for carbon are studied at representative reentry conditions. One model is based on the widely used B′ approach with an equilibrium saturated state assumption for the surface composition, and the second uses a detailed finite-rate chemical kinetics model for the gas–surface reactions. Key modeling parameters are varied in the finite-rate model to assess its sensitivity to modeling uncertainties. The gas-phase chemical kinetics models are also varied to characterize their effects on the ablation process. The models were evaluated using a generic sphere–cone geometry at four representative reentry conditions. It is found that there are notable differences in the predicted overall surface mass flux, and particularly in the details of the individual species mass fluxes to and from the surface. In addition, the gas-phase ...