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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 1,711 records · Page 95
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NASA NTRS1988

Seasonal variation of the semiannual oscillation

Observations of zonally average temperature from the Nimbus-7 Stratospheric and Mesospheric Sounder in 1979-82 show the existence of a significant season variation in the equatorial semiannual oscillation. The first semiannual cycle, beginning in the Northern Hemisphere winter (December-May), is much stronger than the second (June-November) cycle. The balanced winds are calculated from satellite data, showing corresponding seasonality in the semiannual oscillation wind regimes. Strong coupling is observed during the first cycle between equatorial and North Polar temperature. A model of this coupling suggests that planetary Rossby wave momentum deposition in the northern winter is the cause of the seasonal variation in the easterly phase of the semiannual oscillation.

Delisi, Donald P.↗
NASA NTRS1988

Diffusional decay of striations

This paper investigates the amount of decay of growth striations in bulk crystals grown from melts and solutions, due to solid-state diffusion during the cooling of the crystal to room temperature. A model is developed based on the assumption of a sinusoidal variation in the solid composition at the interface of a semiinfinite ingot growing at constant velocity, and the damping behavior of this variation is followed as it moves away from the solid-liquid interface. Numerical results are presented showing that an increase in the diffusivity causes the striations to penetrate deeper into the solid and, consequently causes the distance for 99.9 percent decay to increase. Analytical results are also presented for the decay of striations in an isothermal ingot induced by a sinusoidal fluctuation in the rate of crystal growth.

Gray, R. T.↗
NASA NTRS1989

The development of an explicit thermochemical nonequilibrium algorithm and its application to compute three dimensional AFE flowfields

This study presents a three-dimensional explicit, finite-difference, shock-capturing numerical algorithm applied to viscous hypersonic flows in thermochemical nonequilibrium. The algorithm employs a two-temperature physical model. Equations governing the finite-rate chemical reactions are fully-coupled to the gas dynamic equations using a novel coupling technique. The new coupling method maintains stability in the explicit, finite-rate formulation while allowing relatively large global time steps. The code uses flux-vector accuracy. Comparisons with experimental data and other numerical computations verify the accuracy of the present method. The code is used to compute the three-dimensional flowfield over the Aeroassist Flight Experiment (AFE) vehicle at one of its trajectory points.

Palmer, Grant↗
NASA NTRS1989

SSME LOX post flow analysis/fluid structure interaction. Volume 2: Fluid structure interaction

The progress made in resolving the computational issues associated with modeling high temperature and pressure viscous flows in advanced propulsion systems is documented. An overview of adaptive computational methods for various classes of fluid-structure interaction problems is presented. The key features associated with several approaches are outlined and the advantages/disadvantages of each are summarized. A mathematical formulation is presented for general fluid-structure interaction problems with a moving domain. The two adaptive approaches are discussed used in solving the benchmark problems. The first scheme is a user interactive scheme which is quite versatile and easy to implement but requires an excessive amount of monitoring by the operator and is computationally inefficient. The second method is a new, local remeshing method for handling a general class of fluid-structure interaction problems. This method couples many of the attractive features of other approaches into a computationally efficient and versatile method. The results obtained for two test cases are presented. Results from both the user interactive and local remeshing procedure are described. The current status of the project and future goals to be completed are summarized.

Source record↗
NASA NTRS1991

Algorithm Computes Hypersonic Flow Of Air

Algorithm simulates three-dimensional hypersonic flow of air or another mixture of gases around blunt body. Accounts for nonequilibrium thermochemical effects via two-temperature mathematical model of molecular excitations and equations for finite-rate chemical reactions. These equations fully coupled to fluid dynamical equations.

Palmer, Grant↗
NASA NTRS1991

Effects Of Heat Sinks On VPPA Welds

Report describes theoretical and experimental study of absorption of heat by metal blocks in contact with metal plate while plate subjected to variable-polarity plasma-arc (VPPA) welding. Purpose of study to contribute to development of comprehensive mathematical model of temperature in weld region. Also relevant to welding of thin sheets of metal to thick blocks of metal, heat treatment of metals, and hotspots in engines.

Nunes, Arthur C.↗
NASA NTRS1990

Contribution of cool dust to the FIR brightness distribution of NGC 6946 Sc(s)2

A few galaxies have been observed at submillimeter wavelengths (Stark et al., 1988; Eales, Wynn-Williams, and Duncan, 1988). These observations do not support the hypothesis that there exist substantial amounts of cool dust generating submillimeter radiation in excess of predictions from modified blackbody curves of the form (1/lambda)exp 1.5 B sub v (v, T) fit to total 100 and 160 micron fluxes. However, researchers find relatively cold dust (approx. 17 to 23 degrees K) at several interarm locations in the central 5 minute by 5 minute region of NGC 6946 Sc(s)II. Since molecular gas is the predominate constituent of the interstellar medium (ISM) in this region, such low dust temperatures are indicative of quiescent molecular clouds and low star formation rates. Based on the contribution of cool grains to the 160 micron surface brightness distribution, regions of low star formation activity can be identified. A 1/lambda exp 1.5 emissivity law is assumed for this analysis. Researchers assumed a two temperature grain model in an effort to decompose the far infrared surface brightness distribution of NGC 6946 into contributions from cool, quiescent and active, star forming regions. This model has some validity; although an oversimplification of the dust energetics, researchers find from it that, where giant HII regions are sparse, a significant fraction of the 160 micron emission is emitted by a cool grain component which the Infrared Astronomy Satellite (IRAS) could barely detect.

Engargiola, Greg↗
NASA NTRS1989

High resolution observations of compact H II regions at 230 GHz

Based on the idea that star formation goes on progressively in molecular clouds, a search was conducted for protostars by mapping compact H II regions at a frequency of 250 GHz. The IRAM 30 m radio telescope was used with a (3)He cooled bolometer. Twenty compact H II regions usually obtaining twice the expected free-free flux density, positionally coincident with the H II region, were observed. Even fine structure within the H II regions can be traced in the maps as in the case of G75.84+0.40 near ON-2. The high degree of coincidence between the 250 and 5 GHz map of Harris shows that the excess flux density observed must come from dust mixed with the ionized gas. Part of the dust must however be accumulated in the outer part of the H II region, since in some cases the contours are shifted outwards relative to the radio maps. This is consistent with the fact that in those cases where enough information is available to make a model fit, temperatures were derived of 80 + or - 30 K.

Wink, JOERN-E.↗
NASA NTRS1991

Progress toward a 30 percent-efficient, monolithic, three-junction, two-terminal concentrator solar cell for space applications

Component efficiencies of 0.2/sq cm cells at approximately 100x AMO light concentration and 80 C temperatures are not at 15.3 percent for a 1.9 eV AlGaAs top cell, 9.9 percent for a 1.4 eV GaAs middle cell under a 1.9 eV AlGaAs filter, and 2.4 percent for a bottom 1.0 eV InGaAs cell under a GaAs substrate. The goal is to continue improvement in these performance levels and to sequentially grow these devices on a single substrate to give 30 percent efficient, monolithic, two-terminal, three-junction space concentrator cells. The broad objective is a 30 percent efficient monolithic two-terminal cell that can operate under 25 to 100x AMO light concentrations and at 75 to 100 C cell temperatures. Detailed modeling predicts that this requires three junctions. Two options are being pursued, and both use a 1.9 eV AlGaAs top junction and a 1.4 eV GaAs middle junction grown by a 1 atm OMVPE on a lattice matched substrate. Option 1 uses a low-doped GaAs substrate with a lattice mismatched 1.0 eV InGaAs cell formed on the back of the substrate. Option 2 uses a Ge substrate to which the AlGaAs and GaAs top junctions are lattice matched, with a bottom 0.7 eV Ge junction formed near the substrate interface with the GaAs growth. The projected efficiency contributions are near 16, 11, and 3 percent, respectively, from the top, middle, and bottom junctions.

Partain, L. D.↗
NASA NTRS1991

The use of aircraft-based Thermal Infrared Multispectral Scanner (TIMS) data to measure surface energy budgets on a landscape scale

A series of Thermal Infrared Multispectral Scanner Data (TIMS) was collected over the H. J. Andrews experimental forest in western Oregon and at the Coweeta Hydrologic Laboratory in North Carolina. Flight lines were overlapped with an 8 to 28 minute time difference between flight lines. Concurrent radiosonde measurements of atmospheric profiles of air and dew point temperatures provided inputs to LOWTRAN6 for atmospheric radiance corrections of the TIMS data. Surface temperature differences over time between flight lines allowed the development of thermal response numbers (TRN) which characterized the thermal response of the different surface types. The polygons containing mostly soil and bare rock had the lowest TRN whereas the forested polygons were the highest. Results indicate that forest canopy temperatures measured by the TIMS are comparable to needle thermocouples temperatures. ET models developed from the TIMS data obtained similar ET rates as those using energy balance techniques.

Luvall, Jeffrey C.↗
NASA NTRS1990

HEAO 1 A-2 low-energy detector X-ray spectra of the Cygnus Loop

The Cygnus Loop supernova remnant was observed by the A-2 low-energy detector (LED) proportional counters on the HEAO 1 satellite. Recent improvements to the non-X-ray background rejection and detector response simulation have allowed production of the most accurate spectra of the Cygnus Loop to date. Three separate regions of the Cygnus Loop were observed. Single-temperature, Raymond-Smith models are inadequate to describe the spectra, but two component model fits are good. Temperature, column density, and emission measure variations across the Cygnus Loop are found. These results are interpreted and compared with previous work.

Leahy, D. A.↗
NASA NTRS1991

Air-sea fluxes and surface layer turbulence around a sea surface temperature front

The observed effects of sharp changes in sea surface temperature (SST) on the air-sea fluxes, surface roughness, and the turbulence structure in the surface layer and the marine atmospheric boundary layer are discussed. In situ flux and turbulence observations were carried out from three aircraft and two ships within the FASINEX framework. Three other aircraft used remote sensors to measure waves, microwave backscatter, and lidar signatures of cloud tops. Descriptions of the techniques, intercomparison of aircraft and ship flux data, and use of different methods for analyzing the fluxes from the aircraft data are described. Changing synoptic weather on three successive days yielded cases of wind direction both approximately parallel and perpendicular to a surface temperature front. For the wind perpendicular to the front, wind over both cold-to-warm and warm-to-cold surface temperatures occurred. Model results consistent with the observations suggest that an internal boundary layer forms at the SST.

Friehe, C. A.↗
NASA NTRS1991

A boundary element method for steady incompressible thermoviscous flow

A boundary element formulation is presented for moderate Reynolds number, steady, incompressible, thermoviscous flows. The governing integral equations are written exclusively in terms of velocities and temperatures, thus eliminating the need for the computation of any gradients. Furthermore, with the introduction of reference velocities and temperatures, volume modeling can often be confined to only a small portion of the problem domain, typically near obstacles or walls. The numerical implementation includes higher order elements, adaptive integration and multiregion capability. Both the integral formulation and implementation are discussed in detail. Several examples illustrate the high level of accuracy that is obtainable with the current method.

Dargush, G. F.↗
NASA NTRS1991

Navier-Stokes analysis of a liquid rocket engine disk cavity

This paper presents a Navier-Stokes analysis of hydrodynamic phenomena occurring in the aft disk cavity of a liquid rocket engine turbine. The cavity analyzed in the Space Shuttle Main Engine Alternate Turbopump currently being developed by NASA and Pratt and Whitney. Comparison of results obtained from the Navier-Stokes code for two rotating disk datasets available in the literature are presented as benchmark validations. The benchmark results obtained using the code show good agreement relative to experimental data, and the turbine disk cavity was analyzed with comparable grid resolution, dissipation levels, and turbulence models. Predicted temperatures in the cavity show that little mixing of hot and cold fluid occurs in the cavity and the flow is dominated by swirl and pumping up the rotating disk.

Benjamin, Theodore G.↗
NASA NTRS1990

Thermal design, analysis, and testing of the CETA Space Shuttle Flight Experiment

Attention is given to the Crew and Equipment Translation Aid (CETA) Space Shuttle flight experiment designed to demonstrate techniques and equipment for propelling and restraining crew during EVA. Emphasis is placed on the thermal analysis of the CETA hardware, including thermal design trade-offs, modeling assumptions, temperature predictions, and testing activities.

Witsil, Amy K.↗
NASA NTRS1992

Airframe life prediction

The required research to develop improved life prediction methods for metallic and composite structures under severe thermomechanical loading must include the development of a verified thermoinelastic fracture criterion. There has been much work in this area with many fracture criteria being proposed. Due to the lack of adequate experimental verification none of them are widely accepted. Research must also be performed to develop and implement improved thermoinelasticity theories that properly model large temperature excursions and high temperature gradient. This research is required to provide confidence in the simpler theories currently used for thermoinelastic analysis. Finally, experimental data is needed to define the behavior of and damage accumulation process in thermoinelastic materials. Special emphasis must be placed on understanding failure mode transitions under thermomechanical loading conditions.

Sendeckyj, G. P.↗
NASA NTRS1993

Electron cyclotron thruster new modeling results preparation for initial experiments

The following topics are discussed: a whistler-based electron cyclotron resonance heating (ECRH) thruster; cross-field coupling in the helicon approximation; wave propagation; wave structure; plasma density; wave absorption; the electron distribution function; isothermal and adiabatic plasma flow; ECRH thruster modeling; a PIC code model; electron temperature; electron energy; and initial experimental tests. The discussion is presented in vugraph form.

Hooper, E. Bickford↗
NASA NTRS1991

Hypersonic cone flow predictions using an implicit upwind space-marching code

Solutions have been computed and results are presented for Problem 1, the case of Mach 9 transitional flow past a 7 deg half-angle cone at zero incidence. The solutions were computed using a code developed for the integration of the parabolized Navier-Stokes equations. The algorithm employed in the code is based on a Roe-type flux-difference-splitting scheme applied following a finite-volume approach. The basic algorithm has been modified to make it implicit and second-order accurate in the crossflow directions. Results are presented in terms of surface pressure and heat transfer as well as boundary layer profiles of pitot pressure, Mach number, and tangential velocity. The case was recalculated several times in an effort to determine sensitivities to such parameters as grid density, wall temperature, turbulence model parameters, as well as freestream expansion. Comparisons with the experimental data are presented and discussed.

Lawrence, Scott L.↗