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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

An analysis of artificial viscosity effects on reacting flows using a spectral multi-domain technique

Analytical and experimental techniques for modeling the aerothermodynamics of hypersonic flight are assessed, together with the problems which will be encountered in developing reusable hypersonic vehicles. Emphasis is placed on a numerical coupling between a nonequilibrium chemistry model and hypersonic flow kinematics. Finite difference and finite element descriptions of flow fields in which molecules encounter a shock wave and undergo various motion (and thereby energy) transformations are discussed. The effects of artificial smearing of the shock wave are considered in terms of the resulting effects on the distribution of the energies and chemical composition of the transition region. Results are provided from schlieren photographs of shock-tube experiments, Navier-Stokes calculations of axisymmetric flow over a conical body, and calculations using a spectral multidomain approach for chemically reacting flows.

Macaraeg, Michele G.↗
NASA NTRS1987

The challenge of aerobraking

The system efficiencies obtainable through the synergistic interaction of a LEO spaceport, earth-to-spaceport transportation, and a space-based, reusable aerobraking orbit transfer vehicle, are discussed with a view to NASA's next-century requirements. Attention is given to low Reynolds number hypersonic aerothermodynamic phenomena that will be encountered in aerobraking maneuvers, together with tasks of characterizing nonequilibrium chemistry and thermodynamics through ground experiments, computational chemistry, and CFD. Also noted are the requirements of an aeroassist flight experiment furnishing verification of current understanding of aerobraking vehicle design criteria.

Ried, Robert C.↗
NASA NTRS1989

Analysis of artificial viscosity effects on reacting flows using a spectral multidomain technique

Standard techniques used to model chemically-reacting flows require an artificial viscosity for stability in the presence of strong shocks. The resulting shock is smeared over at least three computational cells, so that the thickness of the shock is dictated by the structure of the overall mesh and not the shock physics. A gas passing through a strong shock is thrown into a nonequilibrium state and subsequently relaxes down over some finite distance to an equilibrium end state. The artificial smearing of the shock envelops this relaxation zone which causes the chemical kinetics of the flow to be altered. A method is presented which can investigate these issues by following the chemical kinetics and flow kinetics of a gas passing through a fully resolved shock wave at hypersonic Mach numbers. A nonequilibrium chemistry model for air is incorporated into a spectral multidomain Navier-Stokes solution method. Since no artificial viscosity is needed for stability of the multidomain technique, the precise effect of this artifice on the chemical kinetics and relevant flow features can be determined.

Macaraeg, Michele G.↗
NASA NTRS1991

Monte Carlo analysis of dissociation and recombination behind strong shock waves in nitrogen

Computations are presented for the relaxation zone behind strong, 1D shock waves in nitrogen. The analysis is performed with the direct simulation Monte Carlo method (DSMC). The DSMC code is vectorized for efficient use on a supercomputer. The code simulates translational, rotational and vibrational energy exchange and dissociative and recombinative chemical reactions. A model is proposed for the treatment of three body-recombination collisions in the DSMC technique which usually simulates binary collision events. The model improves previous models because it can be employed with a large range of chemical-rate data, does not introduce into the flow field troublesome pairs of atoms which may recombine upon further collision (pseudoparticles) and is compatible with the vectorized code. The computational results are compared with existing experimental data. It is shown that the derivation of chemical-rate coefficients must account for the degree of vibrational nonequilibrium in the flow. A nonequilibrium-chemistry model is employed together with equilibrium-rate data to compute the flow in several different nitrogen shock waves.

Boyd, I. D.↗
NASA NTRS1991

Spectral simulations of reacting turbulent flows

Spectral methods for simulating flows are reviewed, emphasizing their recent applications to reacting flow problems. Various classifications of spectral methods and their convergence properties are described and the 'spectral element' method is presented, highlighting its flexibility in dealing with complex flow geometries. The applications considered include chemical reactions in homogeneous turbulence, temporally evolving mixing layers, variable-density simulations, nonequilibrium chemistry, and spatially evolving mixing layers.

Mcmurtry, Patrick A.↗
NASA NTRS1992

Computation of losses in a scramjet combustor

The losses in a conceptual scramjet combustor at flight Mach numbers of 8, 10, 12, 16 and 20 are computed. These losses are extracted from three-dimensional parabolized Navier-Stokes solutions of the turbulent, reacting combustor flow field. A combustor performance index was defined based on the rationale that an efficient scramjet combustor should add heat to the fluid in such a manner as to maximize the stream thrust at the combustor exit while minimizing the losses. This index showed a decrease of more than 40 percent as the flight Mach number increased from 8 to 20, indicative of a drop in the thrust-producing potential of the scramjet at the upper end of the speed regime studied. A breakdown of the losses showed that dissipation, nonequilibrium chemistry and heat diffusion contributed roughly 15 percent, 35 percent, and 50 percent to the irreversible increase in entropy at Mach 8 and 22 percent, 13 and 65 percent at Mach 20.

Kamath, Pradeep S.↗
NASA NTRS1992

Solution strategies and heat transfer calculations for three-dimensional configurations at hypersonic speeds

A procedure which reduces the memory requirements for computing the viscous flow over a modified Orbiter geometry at a hypersonic flight condition is presented. The Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) code which incorporates a thermochemical nonequilibrium chemistry model, a finite rate catalytic wall boundary condition and wall temperature distribution based on radiation equilibrium is used in this study. In addition, the effect of choice of 'min mod' function, eigenvalue limiter and grid density on surface heating is investigated. The surface heating from a flowfield calculation at Mach number 22, altitude of 230,000 ft and 40 deg angle of attack is compared with flight data from three Orbiter flights.

Weilmuenster, K. J.↗
NASA NTRS1992

A hybrid multigrid technique for computing steady-state solutions to supersonic flows

Recently, Li and Sanders have introduced a class of finite difference schemes to approximate generally discontinuous solutions to hyperbolic systems of conservation laws. These equations have the form together with relevant boundary conditions. When modelling hypersonic spacecraft reentry, the differential equations above are frequently given by the compressible Euler equations coupled with a nonequilibrium chemistry model. For these applications, steady state solutions are often sought. Many tens (to hundreds) of super computer hours can be devoted to a single three space dimensional simulation. The primary difficulty is the inability to rapidly and reliably capture the steady state. In these notes, we demonstrate that a particular variant from the schemes presented can be combined with a particular multigrid approach to capture steady state solutions to the compressible Euler equations in one space dimension. We show that the rate of convergence to steady state coming from this multigrid implementation is vastly superior to the traditional approach of artificial time relaxation. Moreover, we demonstrate virtual grid independence. That is, the rate of convergence does not depend on the degree of spatial grid refinement.

Sanders, Richard↗
NASA NTRS1992

Stochastic stellar wind models - Evolved stars

Observational, semiempirical, and theoretical results are presented for individual cool giants and supergiants which show that stochastic stellar wind flows are an extremely important feature in these stars. It is shown that the status of the existing theoretical models is very uncertain, due to the fact that appropriate models must simultaneously take into account a broad variety of strongly coupled physical processes. The potentially important physical processes include global oscillation modes; convection; magnetic fields; the generation, propagation, and interaction of stochastic waves; three-dimensional radiative transfer effects; time-dependent hydrogen ionization; thermal bifurcation and instabilities; and nonequilibrium chemistry, particularly the formation and destruction of molecules and dust.

Cuntz, Manfred↗
NASA NTRS1995

Experimental and Computational Analysis of Shuttle Orbiter Hypersonic Trim Anomaly

During the high-Mach-number, high-altitude portion of the first entry of the Shuttle Orbiter, the vehicle exhibited a nose-up pitching moment relative to preflight prediction of approximately Delta Cm = 0.03. This trim anomaly has been postulated to be due to compressibility, viscous, and/or real-gas (lowered specific heat ratio gamma) effects on basic body pitching moment, body-flap effectiveness, or both. In order to assess the relative contribution of each of these effects, an experimental study was undertaken to examine the effects of Mach number, Reynolds number, and ratio of specific heats. Complementary computational solutions were obtained for wind-tunnel and flight conditions. The primary cause of the anomaly was determined to be lower pressures on the aft windward surface of the Orbiter than deduced from hypersonic wind-tunnel tests with ideal- or near-ideal-gas test flow. The lower pressure levels are a result of the lowering of the flowfield gamma due to high-temperature effects. This phenomenon was accurately simulated in a hypersonic wind tunnel using a heavy gas, which provided a lower, gamma, and was correctly predicted by Navier-Stokes computations using nonequilibrium chemistry.

Brauckmann, Gregory J.↗
NASA NTRS1997

Application of Chimera Grid Scheme to Combustor Flowfields at all Speeds

A CFD method for solving combustor flowfields at all speeds on complex configurations is presented. The approach is based on the ALLSPD-3D code which uses the compressible formulation of the flow equations including real gas effects, nonequilibrium chemistry and spray combustion. To facilitate the analysis of complex geometries, the chimera grid method is utilized. To the best of our knowledge, this is the first application of the chimera scheme to reacting flows. In order to evaluate the effectiveness of this numerical approach, several benchmark calculations of subsonic flows are presented. These include steady and unsteady flows, and bluff-body stabilized spray and premixed combustion flames.

Yungster, Shaye↗
NASA NTRS1991

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields, volume 1

The following subject areas are covered: the development of detailed nonequilibrium radiation models for molecules along with appropriate models for atoms; the inclusion of nongray radiation gasdynamic coupling in the VSL (Viscous Shock Layer) code; the development and evaluation of various electron-electronic energy models; and an examination of the effects of shock slip.

Carlson, Leland A.↗
NASA NTRS1991

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields

The primary tasks performed are: (1) the development of a second order local thermodynamic nonequilibrium (LTNE) model for atoms; (2) the continued development of vibrational nonequilibrium models; and (3) the development of a new multicomponent diffusion model. In addition, studies comparing these new models with previous models and results were conducted and reported.

Carlson, Leland A.↗
NASA NTRS1991

Effect of non-equilibrium flow chemistry and surface catalysis on surface heating to AFE

The effect of nonequilibrium flow chemistry on the surface temperature distribution over the forebody heat shield on the Aeroassisted Flight Experiment (AFE) vehicle was investigated using a reacting boundary-layer code. Computations were performed by using boundary-layer-edge properties determined from global iterations between the boundary-layer code and flow field solutions from a viscous shock layer (VSL) and a full Navier-Stokes solution. Surface temperature distribution over the AFE heat shield was calculated for two flight conditions during a nominal AFE trajectory. This study indicates that the surface temperature distribution is sensitive to the nonequilibrium chemistry in the shock layer. Heating distributions over the AFE forebody calculated using nonequilibrium edge properties were similar to values calculated using the VSL program.

Stewart, David A.↗
NASA NTRS1994

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields

The primary accomplishments of the project were as follows: (1) From an overall standpoint, the primary accomplishment of this research was the development of a complete gasdynamic-radiatively coupled nonequilibrium viscous shock layer solution method for axisymmetric blunt bodies. This method can be used for rapid engineering modeling of nonequilibrium re-entry flowfields over a wide range of conditions. (2) Another significant accomplishment was the development of an air radiation model that included local thermodynamic nonequilibrium (LTNE) phenomena. (3) As part of this research, three electron-electronic energy models were developed. The first was a quasi-equilibrium electron (QEE) model which determined an effective free electron temperature and assumed that the electronic states were in equilibrium with the free electrons. The second was a quasi-equilibrium electron-electronic (QEEE) model which computed an effective electron-electronic temperature. The third model was a full electron-electronic (FEE) differential equation model which included convective, collisional, viscous, conductive, vibrational coupling, and chemical effects on electron-electronic energy. (4) Since vibration-dissociation coupling phenomena as well as vibrational thermal nonequilibrium phenomena are important in the nonequilibrium zone behind a shock front, a vibrational energy and vibration-dissociation coupling model was developed and included in the flowfield model. This model was a modified coupled vibrational dissociation vibrational (MCVDV) model and also included electron-vibrational coupling. (5) Another accomplishment of the project was the usage of the developed models to investigate radiative heating. (6) A multi-component diffusion model which properly models the multi-component nature of diffusion in complex gas mixtures such as air, was developed and incorporated into the blunt body model. (7) A model was developed to predict the magnitude and characteristics of the shock wave precursor ahead of vehicles entering the Earth's atmosphere. (8) Since considerable data exists for radiating nonequilibrium flow behind normal shock waves, a normal shock wave version of the blunt body code was developed. (9) By comparing predictions from the models and codes with available normal shock data and the flight data of Fire II, it is believed that the developed flowfield and nonequilibrium radiation models have been essentially validated for engineering applications.

Carlson, Leland A.↗
NASA NTRS1993

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields

The period from Jan. 1993 thru Aug. 1993 is covered. The primary tasks during this period were the development of a single and multi-vibrational temperature preferential vibration-dissociation coupling model, the development of a normal shock nonequilibrium radiation-gasdynamic coupling model based upon the blunt body model, and the comparison of results obtained with these models with experimental data. In addition, an extensive series of computations were conducted using the blunt body model to develop a set of reference results covering a wide range of vehicle sizes, altitudes, and entry velocities.

Carlson, Leland A.↗
NASA NTRS1991

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields, volume 3

The computer programs developed to calculate the shock wave precursor and the method of using them are described. This method calculated the precursor flow field in a nitrogen gas including the effects of emission and absorption of radiation on the energy and composition of gas. The radiative transfer is calculated including the effects of absorption and emission through the line as well as the continuum process in the shock layer and through the continuum processes only in the precursor. The effects of local thermodynamic nonequilibrium in the shock layer and precursor regions are also included in the radiative transfer calculations. Three computer programs utilized by this computational scheme to calculate the precursor flow field solution for a given shock layer flow field are discussed.

Carlson, Leland A.↗
NASA NTRS1991

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields, volume 2

A technique was developed for predicting the character and magnitude of the shock wave precursor ahead of an entry vehicle and the effect of this precursor on the vehicle flow field was ascertained. A computational method and program were developed to properly model this precursor. Expressions were developed for the mass production rates of each species due to photodissociation and photoionization reactions. Also, consideration was given to the absorption and emission of radiation and how it affects the energy in each of the energy modes of both the atomic and diatomic species. A series of parametric studies were conducted covering a range of entry conditions in order to predict the effects of the precursor on the shock layer and the radiative heat transfer to the body.

Carlson, Leland A.↗