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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Retooling CFD for hypersonic aircraft

The CFD facility requirements of hypersonic aircraft configuration design development are different from those thus far employed for reentry vehicle design, because (1) the airframe and the propulsion system must be fully integrated to achieve the desired performance; (2) the vehicle must be reusable, with minimum refurbishment requirements between flights; and (3) vehicle performance must be optimized for a wide range of Mach numbers. An evaluation is presently made of flow resolution within shock waves, transition and turbulence phenomenon tractability, chemical reaction modeling, and hypersonic boundary layer transition, with state-of-the-art CFD.

Dwoyer, Douglas L.↗
NASA NTRS1989

Spectral diagnostics from X-ray to radio wavelengths

A spectral diagnostic is defined as an observable spectral quantity that can be used to measure a physical parameter characterizing the observed plasma. The first step in establishing a spectral line is to determine whether the upper state of the transition is excited primarily by collisions (typically by electrons), by a radiative process, or by recombination (either direct or cascade). The ultraviolet portion of the spectrum is emphasized as it is well studied, the background photospheric continuum is weak for cool stars, and the spectrum consists of an interesting mix of lines formed by all three mechanisms.

Linsky, Jeffrey L.↗
NASA NTRS1990

Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF By

Data from two conjugate HF radars currently operating at Goose Bay (Labrador) and the Halley Station (Antarctica), obtained for a single 45-min period about local noon on April 22, 1988, were used to study the near-instantaneous conjugate two-dimensional patterns of plasma convection in the vicinity of the cusp. In particular, the response of these plasma convection patterns to changes in the By component of the IMF was examined. Results indicate that, under quasi-stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard (1987) and that the patterns respond rapidly to changes in the IMF By component. Results also show that transitions between convection states begin to occur within minutes of the time that an IMF state change is incident on the magnetospheric boundary, and that the convection reconfigurations expand poleward, completely filling the field of view of an HF radar within 6 min of the time of onset.

Greenwald, R. A.↗
NASA NTRS1989

Red fluorescence and 3-12 micron emission in NGC 2023, HD 44179, M 82, and Lynds 1780

A red excess observed in the Red Rectangle (HD 44179), was attributed to a possible molecular fluorescence mechanism was discovered in NGC 2023 and analyzed in subsequent work in this and other nebulae. An unexpected red light excess was also noticed in a high latitude dark cloud L 1780. The fluorescence was attributed to hydrogenated amorphous carbon by Duley (1985), on the basis of laboratory work. Alternatively, transitions between electronic states of free polycyclic aromatic hydrocarbon molecules, by-passing the cascade along the vibrational states was considered. In L 1780, the red excess was related to the 12 micron emission detected by IRAS. A quantitative comparison of the intensity of the red fluorescence and that of the 3 to 12 micron features is thus warranted in helping assess the physical properties of large interstellar molecules. The red fluorescence radiation, F(R), appears as a bump on the spectra between 0.6 and 0.9 micron. Values were deduced from the spectra for HD 44179, and for the high latitude cloud L 1780. Corrections for the extinction, both interstellar and internal to the nebulae, were included. The 3 to 12 micron brightness, F(IR), was obtained through integration of the spectra for NGC 2023, and for HD 44179 after removal of a smooth continuum due to hot large grains. The values of the ratio of fluorescence flux to the infrared flux, F(R)/F(IR), are summarized. Red fluorescence and infrared radiation are two separate ways to access to the size of the molecules through observation, and it is rewarding that both approaches give similar results. These findings bring a striking coherence into the physical description of the particles, and add further support to the initial attribution of the infrared features to polycyclic aromatic hydrocarbons (PAHs).

Ryter, Charles↗
NASA NTRS1991

Industrial modification of the lunar atmosphere

The sources and sinks of the present trace lunar atmosphere can be accounted for. In particular, it is clear that the solar wind is the primary agent for rapidly removing gases, other than H2 which can escape thermally. But the solar wind has only a finite capability for stripping. If gases are added at rates approaching 100 g/sec, an atmosphere will begin to accumulate. Transition to a state where thermal escape dominates, solar wind loss occurs when the atmosphere has accumulated a total mass of about 108 kg. Assuming that disturbing the lunar regolith liberates about 10-4 by mass of trapped gases, this implies that some kinds of mining operation could handle up to 109 tons of material before creating a long lived, albeit still exceedingly tenuous, atmosphere which would have no appreciable influence on most lunar activities. Should the lunar atmosphere ever exceed an abundance matching that of the Earth at 120 km altitude, ever larger UV wavelengths ranges would be inhibited or lost, as would probably be the ability to use the Moon as a base for very low frequency radio astronomy.

Smith, Harlan J.↗
NASA NTRS1989

Nonequilibrium hypersonic aerothermodynamics

Nonequilibrium phenomena in hypersonic flows are examined on the basis of theoretical models and selected experimental data, in an introduction intended for second-year graduate students of aerospace engineering. Chapters are devoted to the physical nature of gas atoms and molecules, transitions of internal states, the formulation of the master equation of aerothermodynamics, the conservation equations, chemical reactions in CFD, the behavior of air flows in nonequilibrium, experimental aspects of nonequilibrium flow, a review of experimental results, and gas-solid interaction. Diagrams, graphs, and tables of numerical data are provided.

Park, Chul↗
NASA NTRS1991

A planar oblique wave model for the Euler equations

A numerical method in which any transition between neighboring states in a multidimensional flowfield is reconstructed by a pattern of oblique waves. The wave pattern is obtained from a superposition of model waves which arise in a local linearization based on Rankine-Hugoniot averaging. The strengths and orientations of these waves are derived from a minimum path length rule applied in state space. Two test cases with comparisons to other methods in planar flow problems are presented to demonstrate the strengths of the method, and to point out its weaknesses.

Parpia, Ijaz H.↗
NASA NTRS1991

Optical absorption by free holes in heavily doped GaAs

Optical absorption in p-type GaAs with hole concentrations between 10 exp 19 and 10 exp 20/cu cm has been measured for wavelengths between 2 and 20 microns and compared with results of theoretical calculations. In contrast to previous measurements at lower doping levels, the occupied hole states are far from the zone center, where the heavy- and light-hole bands become parallel. This gives rise to a large joint density of states for optical transitions. It is found that the overall magnitude of the observed absorption is explained correctly by the theory, with both the free-carrier (indirect) and the inter-valence-band (direct) transitions contributing significantly to the total absorption. The strength of the absorption (a about 20,000/cm for N(A) = 5 x 10 exp 19/cu cm) is attractive for long-wavelength infrared-detector applications.

Huberman, M. L.↗
NASA NTRS1992

Electron-impact vibrational relaxation in high-temperature nitrogen

Vibrational relaxation process of N2 molecules by electron-impact is examined for the future planetary entry environments. Multiple-quantum transitions from excited states to higher/lower states are considered for the electronic ground state of the nitrogen molecule N2 (X 1Sigma-g(+)). Vibrational excitation and deexcitation rate coefficients obtained by computational quantum chemistry are incorporated into the 'diffusion model' to evaluate the time variations of vibrational number densities of each energy state and total vibrational energy. Results show a non-Boltzmann distribution of number densities at the earlier stage of relaxation, which in turn suppresses the equilibrium process but affects little the time variation of total vibrational energy. An approximate rate equation and a corresponding relaxation time from the excited states, compatible with the system of flow conservation equations, are derived. The relaxation time from the excited states indicates the weak dependency of the initial vibrational temperature. The empirical curve-fit formula for the improved e-V relaxation time is obtained.

Lee, Jong-Hun↗
NASA NTRS1992

Rate coefficients for the excitation of infrared and ultraviolet lines in C II, N III, and O IV

New and improved collision strengths and Maxwellian-averaged rate coefficients for temperatures between 1000 and 40,000 K are presented for all the prominent transitions in the spectra of C II, N III, and O IV. The collision strengths show extensive autoionization structures that are delineated in detail and which enhance the rate coefficients for several transitions by a considerable amount. Maxwellian-averaged collision strengths are calculated for all possible fine-structure transitions along the states included in the eigenfunction expansion of the target ion. All calculations are carried out in the close-coupling approximation using the R-matrix method as adapted for the Opacity Project.

Blum, Robert D.↗
NASA NTRS1992

Impact craters and Venus resurfacing history

The history of resurfacing by tectonism and volcanism on Venus is reconstructed by means of an analysis of Venusian impact crater size-frequency distributions, locations, and preservation states. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size-frequency distribution is close to the atmosphere-free case. An age of cessation of rapid resurfacing of about 500 Ma is obtained. It is inferred that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end-member model is developed to quantify resurfacing scenarios. It is argued that Venus has been resurfacing at an average rate of about 1 sq km/yr. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the completely spatially random location restraint for Venusian craters: a is less than 0.0003 (4 deg diameter circle) and a is greater than 0.1 (74 deg diameter circle).

Phillips, Roger J.↗
NASA NTRS1993

Optical probing of cold trapped atoms

Transitions between excited states of laser-cooled and laser-trapped rubidium and cesium atoms are probed by use of fiber and diode lasers. High-resolution Doppler-free spectra are detected by observation of the absorption and fluorescence of light from the intermediate level of two-step cascade systems. The optical double-resonance spectra show Autler-Townes splitting in the weak probe limit and more complicated spectra for a strongly coupled three-level system.

Fox, R. W.↗
NASA NTRS1995

Reduction Of Sizes Of Semi-Markov Reliability Models

Trimming technique reduces computational effort by order of magnitude while introducing negligible error. Error bound depends on only three parameters from semi-Markov model: maximum sum of rates for failure transitions leaving any state, maximum average holding time for recovery-mode state, and operating time for system. Error bound computed before any model generated, enabling modeler to decide immediately whether or not model can be trimmed. Trimming procedure specified by precise and easy description, making it easy to include trimming procedure in program generating mathematical models for use in assessing reliability. Typical application of technique in design of digital control systems required to be extremely reliable. In addition to aerospace applications, fault-tolerant design has growing importance in wide range of industrial applications.

White, Allan L.↗
NASA NTRS1995

Solar convection zone dynamics and rotation

Helioseismology reveals that there appears to be a discrepancy between the differential rotation profile with the radius and latitude deduced from the inversion of the observed frequency splitting of p-modes and the rotation profile anticipated from various global simulations of convection in rotating shells. The problem may be caused by deductions in numerical simulations of convection which was considered near laminar flows. A high performance computing offers paths for studying the properties of such astrophysical turbulence. A range of approaches to study the basic dynamics of convection is reviewed, along with its ability to redistribute angular momentum in rotating systems. The results of recent three-dimensional simulations of turbulent compressible convection constrained by the effects of rotation are described for the cases in spherical shells and local area f-planes. The turbulence possesses intense vortex tubes with intricate interactions and instabilities. The theoretical studies reveal that the transition to turbulent states has contributed to significant changes in flow symmetries and transports, and that such a turbulent convection can drive substantial mean flows, which are distinctly different from those in which the convection is dominantly laminar.

Toomre, Juri↗
NASA NTRS1996

Line Coupling in Atmospheric Spectra

The theoretical modeling of atmospheric spectra is important for a number of different applications: for instance, in the determination of minor atmospheric constituents such as ozone, carbon dioxide, CFC's etc.; in monitoring the temperature profile for climate studies; and in measuring the incoming and outgoing radiation to input into global climate models. In order to accomplish the above mentioned goal, one needs to know the spectral parameters characterizing the individual spectral lines (frequency, width, strength, and shape) as well as the physical parameters of the atmosphere (temperature, abundances, and pressure). When all these parameters are known, it is usually assumed that the resultant spectra and concomitant absorption coefficient can then be calculated by a superposition of individual profiles of appropriate frequency, strength and shape. However, this is not true if the lines are 'coupled'. Line coupling is a subtle effect that takes place when lines of a particular molecule overlap in frequency. In this case when the initial states and the final states of two transitions are connected by collisions, there is a quantum interference resulting in perturbed shapes. In general, this results in the narrowing of Q-branches (those in which the rotational quantum number does not change), and vibration-rotational R- and P branches (those in which the rotational quantum number changes by +/- 1), and in the spectral region beyond band heads (regions where the spectral lines pile up due to centrifugal distortion). Because these features and spectral regions are often those of interest in the determination of the abundances and pressure-temperature profiles, one must take this effect into account in atmospheric models.

Tipping, R. H.↗
NASA NTRS2001

Responses of the Jovian Atmosphere to Cometary Particles and Photon Impacts

Detailed calculations were performed of the deposition of energetic oxygen ions into the atmosphere of Jupiter. A Monte Carlo simulation was used. Similar processes occur in other astrophysical environments to which our methods can be applied. In particular Cravens has suggested that the X-ray emissions seen from comets are due to transitions from excited states following capture of electrons by solar wind ions colliding with the atmosphere of the comet. Alternative proposals have been advanced for the source of the cometary X-rays. We have carried out a study of the spectra and have shown that with the spectral resolution of about 20 eV the different excitation mechanisms can be distinguished. The response of the two components of the solar wind predict a spectrum that is consistent with the cometary observations. X-rays from the comets arise from the slow solar wind.

Dalgarno, Alex↗
NASA NTRS1991

Asymptotic expansions of the hydrogenic radial dipole integral

A method is presented for an asymptotic expansion of the hydrogenic radial dipole integral for discrete-discrete transitions from initial state n'l' to final state nl. The advantage of this simple method is that it allows all the necessary algebraic manipulations to be done by the computer programming package REDUCE-2. Expansion coefficients up to the coefficient of the l/n17 term are obtained for all initial states with n' < or = 10. However, only the coefficients which are not calculated by Klarsfeld are tabulated in this paper.

NASA Discipline Radiation Health↗
NASA NTRS2000

Homologous recombination as a potential target for caffeine radiosensitization in mammalian cells: reduced caffeine radiosensitization in XRCC2 and XRCC3 mutants

The radiosensitizing effect of caffeine has been associated with the disruption of multiple DNA damage-responsive cell cycle checkpoints, but several lines of evidence also implicate inhibition of DNA repair. The role of DNA repair inhibition in caffeine radiosensitization remains uncharacterized, and it is unknown which repair process, or lesion, is affected. We show that a radiosensitive cell line, mutant for the RAD51 homolog XRCC2 and defective in homologous recombination repair (HRR), displays significantly diminished caffeine radiosensitization that can be restored by expression of XRCC2. Despite the reduced radiosensitization, caffeine effectively abrogates checkpoints in S and G2 phases in XRCC2 mutant cells indicating that checkpoint abrogation is not sufficient for radiosensitization. Another radiosensitive line, mutant for XRCC3 and defective in HRR, similarly shows reduced caffeine radiosensitization. On the other hand, a radiosensitive mutant (irs-20) of DNA-PKcs with a defect in non-homologous end-joining (NHEJ) is radiosensitized by caffeine to an extent comparable to wild-type cells. In addition, rejoining of radiation-induced DNA DSBs, that mainly reflects NHEJ, remains unaffected by caffeine in XRCC2 and XRCC3 mutants, or their wild-type counterparts. These observations suggest that caffeine targets steps in HRR but not in NHEJ and that abrogation of checkpoint response is not sufficient to explain radiosensitization. Indeed, immortalized fibroblasts from AT patients show caffeine radiosensitization despite the checkpoint defects associated with ATM mutation. We propose that caffeine radiosensitization is mediated by inhibition of stages in DNA DSB repair requiring HRR and that checkpoint disruption contributes by allowing these DSBs to transit into irreparable states. Thus, checkpoints may contribute to genomic stability by promoting error-free HRR.

NASA Discipline Radiation Health↗