David R. Olynick

Computational Aerothermodynamic Design Issues for Hypersonic Vehicles

A brief review of the evolutionary progress in computational aerothermodynamics is presented. The current status of computational aerothermodynamics is then discussed, with emphasis on its capabilities and limitations for contributions to the design process of hypersonic vehicles. Some topics to be highlighted include: (1) aerodynamic coefficient predictions with emphasis on high temperature gas effects; (2) surface heating and temperature predictions for thermal protection system (TPS) design in a high temperature, thermochemical nonequilibrium environment; (3) methods for extracting and extending computational fluid dynamic (CFD) solutions for efficient utilization by all members of a multidisciplinary design team; (4) physical models; (5) validation process and error estimation; and (6) gridding and solution generation strategies. Recent experiences in the design of X-33 will be featured. Computational aerothermodynamic contributions to Mars Path finder, METEOR, and Stardust (Comet Sample return) will also provide context for this discussion. Some of the barriers that currently limit computational aerothermodynamics to a predominantly reactive mode in the design process will also be discussed, with the goal of providing focus for future research.

Comparison of coupled radiative flow solutions with Project Fire II flight data

A nonequilibrium, axisymmetric, Navier-Stokes flow solver with coupled radiation has been developed for use in the design or thermal protection systems for vehicles where radiation effects are important. The present method has been compared with an existing now and radiation solver and with the Project Fire 2 experimental data. Good agreement has been obtained over the entire Fire 2 trajectory with the experimentally determined values of the stagnation radiation intensity in the 0.2-6.2 eV range and with the total stagnation heating. The effects of a number of flow models are examined to determine which combination of physical models produces the best agreement with the experimental data. These models include radiation coupling, multitemperature thermal models, and finite rate chemistry. Finally, the computational efficiency of the present model is evaluated. The radiation properties model developed for this study is shown to offer significant computational savings compared to existing codes.

Comparison of Coupled Radiative Navier-Stokes Flow Solutions with the Project Fire II Flight Data

A nonequilibrium, axisymmetric, Navier-Stokes flow solver with coupled radiation has been developed to use in the design of thermal protection systems for vehicles where radiation effects are important. The present method has been compared with an existing flow and radiation solver and with the Project Fire II experimental data. Very good agreement has been obtained over the entire Fire II trajectory with the experimentally determined values of the stagnation radiation intensity in the .2 to 6.2 eV range and with the total stagnation heating. The agreement was significantly better than previous numerical predictions. The effects of a number of flow models are examined to determine which combination of physical models produces the best agreement with the experimental data. These models include radiation coupling, multi-temperature thermal models, finite-rate chemistry, and a quasi-steady-state or Boltzmann assumption for the calculation of the excited electronic states. Finally, the computational efficiency of the present model is evaluated. The radiation properties model developed for this study is shown to offer significant computational savings compared to existing codes.

Thermo-chemical nonequilibrium effects on the aerothermodynamics of aerobraking vehicles

A three-dimensional computational fluid dynamics algorithm is developed to study the effect of chemical and thermal nonequilibrium on blunt body aerodynamics. Both perfect gas and five species air reacting gas models are used to compute the flow over the Apollo command module. The reacting gas air mixture is assumed to be governed by a translational-rotational temperature and a vibrational temperature. The Navier-Stokes computations are compared to wind-tunnel and flight-aerodynamic data from the Apollo missions. The effects of chemical reaction and vibrational excitation on lift-to-drag ratio and trim angle are investigated. It is shown that including real gas effects results in a lower trim angle and L/D than predicted by nonreacting gas wind-tunnel simulations. The reacting gas numerical results are consistent with flight data from the unmanned Apollo AS-202 mission, whereas the perfect gas computations agree with the extrapolated preflight wind-tunnel results.

Navier-Stokes Heating Calculations for Benchmark Thermal Protection System Sizing

A study was carried out to identify, select, and benchmark simulation techniques needed for thermal protection material selection and sizing for reusable launch vehicles. Fully viscous, chemically reacting, Navier-Stokes solutions for the flow around a sphere are generated and compared using three different flow solvers. The effects of grid resolution, algorithm, transport modeling, and surface boundary conditions on the magnitude and convergence of the predicted heat transfer rate are examined. A third-order Van Leer inviscid upwind flux formulation was found to be a good method for surface heat transfer predictions. A number of three-dimensional, chemically reacting, Navier-Stokes flow solutions are generated for the nose of a single-stage-to-orbit rocket at angle of attack. A methodology for thermal protection system material selection is demonstrated. The strong influence of thermal protection system material selection on predicted heat transfer rates and surface temperatures is demonstrated. Further, it is shown that the changes in surface emissivity and catalycity at the interfaces between different thermal protection system concepts can produce large jumps in the predicted surface temperature. These gradients must be accounted for in the thermal protection system and vehicle design process.

A Web-based Analysis System for Planetary Entry Vehicle Design

An integrated analysis environment for designing planetary entry vehicles has been developed. The system uses a Web-based graphical user interface, so that members of a geographically dispersed design team, with heterogeneous hardware, can readily access analysis modules located at other sites. The analyses and their implementation are briefly described. A sample application illustrates the value of this system, and indicates where further development effort would provide significant return.

Comparisons between Monte Carlo methods and Navier-Stokes equations for re-entry flows

A detailed comparison is made between Navier-Stokes and direct stimulation Monte Carlo calculations for flows near the continuum limit to assess the accuracy of the continuum equations in this regime. Meaningful comparisons require the use of similar physical models. This necessitates the inclusion of a separate rotational energy equation and use of slip boundary conditions. Inclusion of slip boundary conditions resulted in improved agreement between surface properties. Moreover, good agreement was obtained for the various temperatures in the nonequilibrium portion of the flowfield that does not contain the shock region. Departures are noted in the shock region and in regions where thermal diffusion effects are important. 21 refs.

Comparisons between DSMC and the Navier-Stokes equations for reentry flows

A detailed comparison is made between Navier-Stokes and DSMC calculations for flows near the continuum limit to assess the accuracy of the continuum equations in this regime. Meaningful comparisons require the use of similar physical models. This necessitates the inclusion of a separate rotational energy equation and use of slip boundary conditions. Inclusion of slip boundary conditions resulted in improved agreement between surface properties. Moreover, good agreement was obtained for the various temperatures in the nonequilibrium portion of the flow field that does not contain the shock region. Departures are noted in the shock region and in regions where thermal diffusion effects are important.

Computational aerothermodynamic design issues for hypersonic vehicles

A brief review of the evolutionary progress in computational aerothermodynamics is presented. The current status of computational aerothermodynamics is then discussed, with emphasis on its capabilities and limitations for contributions to the design process of hypersonic vehicles. Some topics to be highlighted include: घ1ङ aerodynamic coeaecient predictions with emphasis on high temperature gas eaeects; घ2ङ surface heating and temperature predictions for thermal protection system घTPSङ design in a high temperature, thermochemical nonequilibrium environment; घ3ङ methods for extracting and extending computational aeuid dynamic घCFDङ solutions for eaecient utilization by all members of a multidisciplinary design team; घ4ङ physical models; घ5ङ validation process and error estimation; and घ6ङ gridding and solution generation strategies. Recent experiences in the design of X-33 will be featured. Computational aerothermodynamic contributions to Mars Pathaender, METEOR, and Stardust घComet Sample returnङ will also provide context for this discussion. Some of the barriers that currently limit computational aerothermodynamics to a predominantly reactive mode in the design process will also be discussed, with the goal of providing focus for future research.

Upwind methods for ionized flows

Numerical issues resulting from the electron pressure term in the electron energy equation are discussed. It is shown that when a separate rotational energy equation is employed, the heavy particle equations are essentially decoupled from the electron energy equation. This decoupling is exhibited by the fact that the total pressure has no explicit dependence on the electron energy. As a result, ambiguities in determining eigenvalues and eigenvectors are removed. The approach is implemented in solving the NavierStokes equations over the Project Fire I1 vehicle at two points early in its trajectory using an eleven species, four temperature model. Results show that for low density flows significant thermal nonequilibrium can exist between the electrons and other modes. Moreover, the numerical scheme employed removes numerical instabilities that appear in flows characterized by low degrees of ionization. -