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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 487 records · Page 27
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NASA NTRS1987

Removal of spurious modes encountered in solving stability problems by spectral methods

A technique based on the Galerkin approximation is developed to remove spurious roots arising when Chebyshev spectral methods are used to solve eigenvalue problems in hydrodynamic stability. The derivation of Galerkin-Chebyshev approximations is explained, and numerical results for the Orr-Sommerfeld equations of plane Poiseuille flow and a Blasius profile are presented in tables and compared with those obtained by the method of Zebib (1984). It is pointed out that the present method does not increase the size of the algebraic system to be solved.

Zebib, Abdelfattah↗
NASA NTRS1988

Overview of heat transfer and fluid flow problem areas encountered in Stirling engine modeling

NASA Lewis Research Center has been managing Stirling engine development programs for over a decade. In addition to contractual programs, this work has included in-house engine testing and development of engine computer models. Attempts to validate Stirling engine computer models with test data have demonstrated that engine thermodynamic losses need better characterization. Various Stirling engine thermodynamic losses and efforts that are underway to characterize these losses are discussed.

Tew, Roy C., Jr.↗
NASA NTRS1987

Modeling Halley before and after the encounters

Numerical models developed prior to the 1986 spacecraft flybys at comet P/Halley described anticipated dust and gas environments. Predicted values for gas production, for dynamic ranges of gas and dust experiments, and for likely spacecraft effects were matched during the flybys well within expected margins; in particular the dust-impact-generated attitude disturbance experienced by Giotto several seconds before closest approach was within the envelope of expectations from the models. Three major areas are identified in which 1986 results for Halley might be used to improve the models, namely (1) gas production values from earth-based and in situ observations, (2) dust flux and fluence values in the coma from three spacecraft, and (3) dust size distributions from the in situ data which require the presence of numerous particles at masses less than 10 to the -17th kg.

Divine, N.↗
NASA NTRS1988

Performance limits for optimal microburst encounter

An effort has been made to ascertain the envelope-edges for uneventful aircraft penetrations of microburst windshears on the basis of optimal aircraft control strategies. Over 1100 such trajectories have been computed for contemporary airliners and general aviation aircraft, in the case of idealized microbursts, using a successive quadratic program trajectory optimization algorithm able to directly handle inequality constraints. Variations of optimal performance with microburst type, intensity, length scale, and location, define performance limits; these limits fall into short, intermediate, and long microburst length scale regimes. The ability to safely transit a microburst also varies strongly with microburst location.

Psiaki, Mark L.↗
NASA NTRS1988

Solution of an integral equation encountered in studies on radiative transfer in completely absorbing leaf canopies

The leaf normal distribution function is related to the extinction coefficient of direct solar radiation by a Fredholm integral equation of the first kind. The integral equation is solved by the method of constrained least squares. The solutions are stable and correct only in extreme cases and, in general, it is not possible to recover an arbitrary leaf normal distribution function.

Myneni, Ranga B.↗
NASA NTRS1989

Investigation and suppression of high dynamic response encountered on an elastic supercritical wing

The DAST Aeroelastic Research Wing had been previously in the NASA Langley TDT and an unusual instability boundary was predicted based upon supercritical response data. Contrary to the predictions, no instability was found during the present test. Instead a region of high dynamic wing response was observed which reached a maximum value between Mach numbers 0.92 and 0.93. The amplitude of the dynamic response increased directly with dynamic pressure. The reponse appears to be related to chordwise shock movement in conjunction with flow separation and reattachment on the upper and lower wing surfaces. The onset of flow separation coincided with the occurrence of strong shocks on a surface. A controller was designed to suppress the wing response. The control law attenuated the response as compared with the uncontrolled case and added a small but significant amount of damping for the lower density condition.

Seidel, David A.↗
NASA NTRS1988

Multiple-encounter shock-drift acceleration in nearly perpendicular shocks

A model in which low-energy ions are accelerated by a nearly perpendicular MHD shock without upstream turbulence is presented. In the present scheme, vigorous particle cross-field scattering by downstream turbulence competes with the convection downstream, sending particles back to the shock front for acceleration. Some fraction of the low-energy ions are accelerated up to maximal energy prior to escaping upstream to possibly undergo diffusion shock acceleration. The model can explain particle injection of the diffusive shock acceleration along with ion acceleration during solar flares.

Chiueh, Tzihong↗
NASA NTRS1988

Asteroid/comet encounter opportunities for the Galileo VEEGA mission

The opportunity for the Galileo spacecraft to perform a close flyby of an asteroid or distant observation of a comet while on the Venus-Earth-Earth-Gravity-Assist (VEEGA) mission to Jupiter is discussed. More than 120 nominal trajectories were used in a scan program to identify asteroids passing within 30 million km of the spacecraft. A total of 47 asteroids were examined to determine the propellant cost of a close flyby. The possible flybys include a double asteroid flyby with No. 951 in October, 1991, with a flyby of No. 243 in August 1993. The factors considered in the selection of an asteroid include the propellant margin cost of modifying a nominal trajectory to include a close flyby, the size and type of asteroid, and the Jupiter arrival date.

Johannesen, Jennie R.↗
NASA NTRS1988

Information content of radiometric and optical data during planetary encounter

An assessment is made of the feasibility of determining the flyby position and velocity parameters from optical data. Closed form solutions for the various parameter estimates are derived to show the functional dependence of the estimates on the nominal flyby parameters as well as the data arc characteristics. A strong correlation between the downtrack position and crosstrack velocity estimates is observed for the case of a short optical data arc, and a similarly strong correlation between the flyby planet mass and the flyby distance estimates is demonstrated for the case of radiometric data.

Kechichian, Jean A.↗
NASA NTRS1989

Interacting binary galaxies. VI - The fast encounter of NGC 2672 and NGC 2673 (Karachentsev 175, Arp 167)

The isolated pair of interacting ellipticals NGC 2672/2673 (Arp 167; Karachentsev 175), whose line-of-sight relative velocity is about 520 km/s, is studied. This very unequal mass system features two low surface brightness plumes emerging from the secondary galaxy NGC 2673. Photometric data on the pair are described, and the distortions in the galaxies and their photometric properties are discussed. Spectroscopic observations of the pair and the velocity structures derived from these are addressed. A physical solution for the pair is presented and explained.

Balcells, Marc↗
NASA NTRS1989

The far ultraviolet reflection spectrum of Uranus - Results from the Voyager encounter

The present analysis of the 1250-1700 A region Uranus spectrum obtained by Voyager's US spectrometer characterizes these observation results as due primarily to solar light reflected from an H2 Rayleigh and Raman scattering atmosphere with small but measurable hydrocarbon absorption. The hydrocarbon abundances obtained are substantially lower than those at comparable levels of the Saturn or Jupiter atmospheres; it is suggested, in one-dimensional terms, that this is due to diffusive separation, in conjunction with photochemical depletion caused by a very low eddy-diffusion coefficient. Strong latitudinal variations in the hydrocarbon abundances are suggested in the subsolar, polar stratosphere.

Yelle, Roger V.↗
NASA NTRS1989

Numerical studies of multiple-encounter shock drift acceleration

Test particle simulation is performed to examine the mechanism previously proposed by Chiueh for particle acceleration by a nearly perpendicular shock where only downstream MHD turbulence is present. A nonnegligible fraction of low-energy particles is observed gaining energy by more than three orders of magnitude in a few hundred cyclotron periods. Criteria for effective acceleration given previously are examined and confirmed. The numerical results suggest that in the nonrelativistic energy range, a power-law particle energy spectrum, with the power index determined by the degree to which the criteria are satisfied, may form.

Chiueh, Tzihong↗
NASA NTRS1987

Performance of laminar-flow leading-edge test articles in cloud encounters

An extensive data bank of concurrent measurements of laminar flow (LF), particle concentration, and aircraft charging state was gathered for the first time. From this data bank, 13 flights in the simulated airline service (SAS) portion were analyzed to date. A total of 6.86 hours of data at one-second resolution were analyzed. An extensive statistical analysis, for both leading-edge test articles, shows that there is a significant effect of cloud and haze particles on the extent of laminar flow obtained. Approximately 93 percent of data points simulating LFC flight were obtained in clear air conditions; approximately 7 percent were obtained in cloud and haze. These percentages are consistent with earlier USAF and NASA estimates and results. The Hall laminar flow loss criteria was verified qualitatively. Larger particles and higher particle concentrations have a more marked effect on LF than do small particles. A particle spectrometer of a charging patch are both acceptable as diagnostic indicators of the presence of particles detrimental to laminar flow.

Davis, Richard E.↗
NASA NTRS1990

Voyager 2's encounter with Neptune

The results of Voyager 2 observations of Neptune are reviewed. Observations of Neptune's Great Dark Spot, rotation atmosphere, magnetic field, rings, and satellites are discussed. Also, observations of Triton are considered, noting the presence of geyser activity on the satellite. Several photographs of features on both Neptune and Triton are presented.

Mclaughlin, William I.↗
NASA NTRS1990

Small-scale waves encountered during AASE

Variability on scales of 200 km or less in the lower stratosphere is examined using potential temperature data collected during the AASE campaign. Enhanced variability is found over regions containing topography presumably due to gravity waves. The effect of gravity wave critical levels on topographically induced variability is examined. A significant reduction in variability is found when a critical level is present.

Bacmeister, Julio T.↗
NASA NTRS1990

Voyager 2's encounter with Neptune answered many questions about the 'blue' planet

Voyager 2 observations of Neptune from August 1989 are examined. Voyager 2 discovered 6 new moons around Neptune and collected information on the shape and composition of Neptune's rings. The spots and clouds detected in the planet's atmosphere are described. Consideration is given to Neptune's magnetic field and auroras.

Tsurutani, Bruce T.↗