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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Bragg scattering measurement of atmospheric plasma decay

The decay processes of the plasma layers generated by two intersecting microwave pulses in 1 torr dry air are investigated by Bragg scattering method. The results of measurement show that the electrons decay initially at the three-body attachment rate. However, when enough negative molecule ions are produced through the electron attachment process, the regeneration of electrons via detachment process is increased and eventually balances out the electron attachment loss. The net electron loss is then dominated by the recombination process. The temporal evolution of electron density has also been reproduced by the numerical result of a system of three modal equations for a best fit detachment rate.

Zhang, Y. S.↗
NASA NTRS1990

Pre-mixed flame simulations for non-unity Lewis numbers

A principal effect of turbulence on premixed flames in the flamelet region is to wrinkle the flame fronts. For non-unity Lewis numbers (Le), the local flame structure is altered in curved regions. This effect is examined using direct numerical simulations of the three dimensional, constant density, decaying isotropic turbulence with a single step, finite rate chemical reaction. Simulations of Lewis numbers 0.8, 1.0, and 1.2 are compared. The turbulent flame speed, S(sub T), increases as Le decreases. The correlation between S(sub T) and u prime found in previous Le = 1 simulations has a strong Lewis number dependency. The variance of the pdf of the flame curvature increases as Le decreases, indicating that the flames become more wrinkled. A strong correlation between local flame speed and curvature was found. For Le greater than 1, the flame speed increases in regions concave towards the products and decreases in convex regions. The opposite correlation was found for Le less than 1. The mean temperature of the products was also found to vary with Lewis number. For Le = 0.8, it is less than the adiabatic flame temperature and for Le = 1.2 it is greater.

Rutland, C. J.↗
NASA NTRS1992

Mode-medium instability and its correction with a Gaussian-reflectivity mirror

A high-power CO2 laser beam is known to deteriorate after a few microseconds due to a mode-medium instability (MMI) which results from an intensity-dependent heating rate related to the vibrational-to-translational decay of the upper and lower CO2 lasing levels. An iterative numerical technique is developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO2 resonator with a hard-edge output mirror for different parameters like the Fresnel number and the gas density. The results show that the mode of the hard edge unstable resonator deteriorates because of the diffraction ripples in the mode. A Gaussian-reflectivity mirror was used to correct the MMI. This mirror produces a smoother intensity profile which significantly reduces the effects of the MMI. Quantitative results on peak density variation and beam quality are presented.

Webster, K. L.↗
NASA NTRS1998

Time-Resolved O3 Chemical Chain Reaction Kinetics Via High-Resolution IR Laser Absorption Methods

Excimer laser photolysis in combination with time-resolved IR laser absorption detection of OH radicals has been used to study O3/OH(v = 0)/HO2 chain reaction kinetics at 298 K, (i.e.,(k(sub 1) is OH + 03 yields H02 + 02 and (k(sub 2) is H02 + 03 yields OH + 202). From time-resolved detection of OH radicals with high-resolution near IR laser absorption methods, the chain induction kinetics have been measured at up to an order of magnitude higher ozone concentrations ([03] less than or equal to 10(exp 17) molecules/cu cm) than accessible in previous studies. This greater dynamic range permits the full evolution of the chain induction, propagation, and termination process to be temporally isolated and measured in real time. An exact solution for time-dependent OH evolution under pseudo- first-order chain reaction conditions is presented, which correctly predicts new kinetic signatures not included in previous OH + 03 kinetic analyses. Specifically, the solutions predict an initial exponential loss (chain "induction") of the OH radical to a steady-state level ([OH](sub ss)), with this fast initial decay determined by the sum of both chain rate constants, k(sub ind) = k(sub 1) + k(sub 2). By monitoring the chain induction feature, this sum of the rate constants is determined to be k(sub ind) = 8.4(8) x 10(exp -14) cu cm/molecule/s for room temperature reagents. This is significantly higher than the values currently recommended for use in atmospheric models, but in excellent agreement with previous results from Ravishankara et al.

Kulcke, Axel↗
NASA NTRS2012

Estimation of Eddy Dissipation Rates from Mesoscale Model Simulations

The Eddy Dissipation Rate is an important metric for representing the intensity of atmospheric turbulence and is used as an input parameter for predicting the decay of aircraft wake vortices. In this study, the forecasts of eddy dissipation rates obtained from the current state-of-the-art mesoscale model are evaluated for terminal area applications. The Weather Research and Forecast mesoscale model is used to simulate the planetary boundary layer at high horizontal and vertical mesh resolutions. The Bougeault-Lacarrer and the Mellor-Yamada-Janji schemes implemented in the Weather Research and Forecast model are evaluated against data collected during the National Aeronautics and Space Administration s Memphis Wake Vortex Field Experiment. Comparisons with other observations are included as well.

Ahmad, Nashat N.↗
NASA NTRS1985

Some characteristics of the solar flare event of February 16, 1984

In the morning of February 16, 1984 a solar cosmic ray event (GLE) was recorded by the world wide network of neutron monitors (NM). The counting rate vs. time profile of the Goose Bay NM (geog. lat. = 53.3 deg. N, deog. long. = 299.6 deg E) where the increase is expressed as percent of the counting rate of an equatorial sea level NM is presented. The Goose Bay NM was observed to have the maximum response to the solar particles. Its counting rate vs. time profile exhibits a rapid increase to maximum, has a large amplitude (approx. 170%) and decays rapidly to background in approx. 90 min. In Fig. 1 we also show the counting rate vs. time profile for the Tixie Bay NM (71.6 deg, 128.9 deg) which recorded an increase of only a few percent. Since the NMs at Goose Bay and Tixie Bay have asymptotic viewing directions approx. 180 deg apart in longitude, the anisotropy of the solar particle flux at Earth from these stations.

Debrunner, H.↗
NASA NTRS1996

Testing of hypotheses about altitude decompression sickness by statistical analyses

This communication extends a statistical analysis of forced-descent decompression sickness at altitude in exercising subjects (J Appl Physiol 1994; 76:2726-2734) with a data subset having an additional explanatory variable, rate of ascent. The original explanatory variables for risk-function analysis were environmental pressure of the altitude, duration of exposure, and duration of pure-O2 breathing before exposure; the best fit was consistent with the idea that instantaneous risk increases linearly as altitude exposure continues. Use of the new explanatory variable improved the fit of the smaller data subset, as indicated by log likelihood. Also, with ascent rate accounted for, replacement of the term for linear accrual of instantaneous risk by a term for rise and then decay made a highly significant improvement upon the original model (log likelihood increased by 37 log units). The authors conclude that a more representative data set and removal of the variability attributable to ascent rate allowed the rise-and-decay mechanism, which is expected from theory and observations, to become manifest.

NASA Center JSC↗
NASA NTRS1978

High rate of reversal in Nicd cells

Pressure response to reverse current, pressure decay after reversal, and discharge performance of nickel cadmium cells are presented.

Killian, H.↗
NASA NTRS2003

Resistance of virus to extinction on bottleneck passages: study of a decaying and fluctuating pattern of fitness loss

RNA viruses display high mutation rates and their populations replicate as dynamic and complex mutant distributions, termed viral quasispecies. Repeated genetic bottlenecks, which experimentally are carried out through serial plaque-to-plaque transfers of the virus, lead to fitness decrease (measured here as diminished capacity to produce infectious progeny). Here we report an analysis of fitness evolution of several low fitness foot-and-mouth disease virus clones subjected to 50 plaque-to-plaque transfers. Unexpectedly, fitness decrease, rather than being continuous and monotonic, displayed a fluctuating pattern, which was influenced by both the virus and the state of the host cell as shown by effects of recent cell passage history. The amplitude of the fluctuations increased as fitness decreased, resulting in a remarkable resistance of virus to extinction. Whereas the frequency distribution of fitness in control (independent) experiments follows a log-normal distribution, the probability of fitness values in the evolving bottlenecked populations fitted a Weibull distribution. We suggest that multiple functions of viral genomic RNA and its encoded proteins, subjected to high mutational pressure, interact with cellular components to produce this nontrivial, fluctuating pattern.

Serial Passage↗
NASA NTRS2004

The North Alabama Lightning Mapping Array: Recent Severe Storm Observations and Future Prospects

The North Alabama Lightning Mapping Array became operational in November 2001 as a principal component of a severe weather test bed to infuse new science and technology into the short-term forecasting of severe and hazardous weather, principally within nearby National Weather Service forecast offices. Since the installation of the LMA, it has measured the total lightning activity of a large number of severe weather events, including three supercell tornado outbreaks, two supercell hailstorm events, and numerous microburst-producing storms and ordinary non-severe thunderstorms. The key components of evolving storm morphology examined are the time rate-of-change (temporal trending) of storm convective and precipitation characteristics that can be diagnosed in real-time using NEXRAD WSR-88D Doppler radar (echo growth and decay, precipitation structures and velocity features, outflow boundaries), LMA (total lightning flash rate and its trend) and National Lightning Detection Network (cloud-to- ground lightning, its polarity and trends). For example, in a transitional season supercell tornado outbreak, peak total flash rates for typical supercells in Tennessee reached 70-100/min, and increases in the total flash rate occurred during storm intensification as much as 20-25 min prior to at least some of the tornadoes. The most intense total flash rate measured during this outbreak (over 800 flashes/min) occurred in a storm in Alabama. In the case of a severe summertime pulse thunderstorm in North Alabama, the peak total flash rate reached 300/min, with a strong increase in total lightning evident some 9 min before damaging winds were observed at the surface. In this paper we provide a sampling of LMA observations and products during severe weather events to illustrate the capability of the system, and discuss the prospects for improving the short-term forecasting of convective weather using total lightning data.

Goodman, S. J.↗
NASA NTRS1992

Theoretical and computational study of rotating isotropic turbulence

The various flow regimes that occur in rotating isotropic turbulence and their associated turbulence structure are studied. Direct numerical simulations are conducted for moderate and rapid rotation rates and comparisons are made with the predictions of generalized Eddy Damped Quasi-Normal Markovian (EDQNM) approximations. It is shown that at high rotation rates the nonlinear transfer terms remain small and that the development of the spectrum is through pure viscous decay. This shows that the effect of rapid rotation on the turbulence kinetic energy is through the shutting off of the production term in the dissipation rate equation. At moderate and relatively rapid rates, the rotation causes a discernible anisotropy to develop in the integral length scale with a mild trend toward a two-dimensionalization of the flow. As the flow decays, the Rossby number decreases, leading again to the shutting off of the nonlinear transfer terms. For extremely rapid rotation rates, the anisotropies in the integral length scales are small and direct numerical simulation results are in good agreement with rapid distortion theory.

Mansour, N. N.↗
NASA NTRS1973

Ratemeter

An instantaneous reading tachometer in which reoccurring events to be measured in rate, trigger a threestate timing generator in which the first two states are of fixed duration and the third state is of variable duration is described. An electrical decay circuit is set to a reference level by the second state and the third state causes this reference level to decay until the reoccurrence of an event. This triggers a new first state which in turn triggers a sample and hold circuit to hold the decayed level. The decayed level is amplified and provided as an output indicative of the instantaneous rate of occurence of the last two successive events.

Currie, J. R.↗
NASA NTRS1994

Searching for hard X-ray directivity during the rise, peak, and decay phases of solar flares

We have identified 72 large solar flares (peak counting rates more than 1000 counts/s) observed by Hard X-ray Burst Spectroscopy (HXRBS) on-board the Solar Maximum Mission (SMM). Using a database of these flares, we have studied hard X-ray (50-850 keV) spectral center-to-limb variation and its evolution with time. The major results are the following: (1) During the rise phase, the center-to-limb spectral variation is small, with a hardness of delta delta = 0.02 +/- 0.25, and a statistical significance of 0.1 sigma. (2) During the peak phase, the center-to-limb variation is delta delta = 0.13 +/- 0.13, with a statistical significance of 1 sigma. (3) During the decay phase, the center-to-limb variation changes to softening. The softness is relatively large with delta delta = -0.25 +/- 0.21, and a statistical significance of 1.2 sigma. (4) The linear least-squares fits to the spectral center-to-limb variations do not have slopes significantly different from zero during all those three phases. (5) The center events and limb events spectral distributions are shown to be not different by using Kolmogorov-Smirnov two-samples test. (6) The fraction of events detected near the limb is marginally consistent with that expected from isotropically emitting flares. (7) On average, flares evolve as soft-hard-soft. These results suggest that there is no statistically significant evidence for hard X-ray directivity during the rise, peak, and decay phases of solar flares. The hard X-ray radiation pattern at those energies is almost isotropic during all those phases. This lack of directivity (or anisotropy) found in this study is not in agreement with the results discovered by Vestrand et al. (1987) in which they found energetic photon source is anisotropic, using SMM Gamma-Ray Spectrometer (GRS) data at a much higher energy band of 0.3-1 MeV. If we want to interpret the results of Vestrand et al. (1987) and our present results in a self-consistent way, we must conclude that at energies of greater than 50 keV, the Compton backscattering suppresses the hard X-ray anisotropy sufficiently so that no significant directivity can be measured during the rise, peak, and decay phases of solar flares. during the rise, peak, and decay phases of solar flares.

Li, Peng↗
NASA NTRS1973

Decay time of type III solar bursts observed at kilometric wavelengths.

Type III bursts were observed between 3.5 MHz and 50 kHz by the University of Michigan radio astronomy experiment aboard the OGO-5 satellite. Decay times were measured and then combined with published data ranging up to about 200 MHz. The observed decay times increase with decreasing frequency, but at a rate considerably slower than that expected from electron-proton Coulomb collisions. At 50 kHz, values differ by about a factor of 100. Using Hartle and Sturrock's solar wind model, Coulomb collisional frequencies were computed and compared with the apparent collisional frequencies deduced from the observations. It was found that the ratio of observed to computed values varies with heliocentric distance according to an inverse 0.71 power.

Alvarez, H.↗
NASA NTRS1981

I/asterisk/ /6s 4P/ collisional quenching - Application to the IF 491-nm laser

Measurements of the rate constants for quenching of the excited I (6s 4P) states by Ar, Xe, CF39I, UF6, and NF3 are described. Each rate constant is determined by recording the exponential time decay of the excited IF or excited I2 (342 nm) fluorescence in the afterglow of the e-beam-excited plasmas containing Ar, CF3I, NF3, and the desired quenching gas. In addition, further experimental evidence in support of neutral channel formation of excited IF in e-beam-pumped Ar/CF3I/NF3 mixtures is presented. Details of the experimental apparatus and gas handling procedures are given, and the kinetics model developed to interpret the experimental data is described.

Hutchison, S. B.↗
NASA NTRS1985

Experimental studies on the reaction kinetics of 1,1-dimethylhydrazine and oxygen

Measurements behind shock waves in highly diluted 1,1 dimethylhydrazine-O2-Ar mixtures clearly showed a two-stage reaction. In the first stage, UDMH decomposes via a unimolecular step; in the second stage, clearly separated from the first one, the decay products react further with O2. Values for the rate constant of unimolecular decomposition were obtained.

Just, T.↗
NASA NTRS1985

A simplified model of the turbulent microburst

A simple model was developed to describe the low altitude shear associated with a microburst (or small downburst) in the atmosphere. Such microbursts are thought to be the cause of convective radial flows near the ground which represent potentially hazardous conditions to aircraft during approach and landing. Closed form solutions are presented for the equations of mass, momentum, and thermal energy to give the spreading rate of the microburst and the velocity and temperature decay due to turbulence. An analysis of the impact of the microburst and its radial spreading over the ground is given and illustrated examples are provided. The solution for the downward and outward wind velocities are given in terms of simple dimensionless algebraic parameters which can be computed readily in realtime during a piloted simulation of an aircraft flying through a microburst.

Roberts, L.↗
NASA NTRS1987

A new interpretation of plasma-line overshoot phenomena

The temporal evolution of Langmuir waves excited by high-power, high-frequency (HF) radio waves in the ionosphere is studied theoretically. This study is motivated by past observations made with the 450 MHz radar at Arecibo Observatory in Puerto Rico. Two kinds of nonlinear damping to the parametric decay instability are considered in the derivation of the rate equation for the spectral intensity of enhanced Langmuir waves. They are Langmuir wave cascading caused by nonlinear Landau damping and cross-field electron diffusion. The first damping process leads to the saturation of individual unstable Langmuir wave. The second process, which results from the incoherent scattering of electron orbits by the total excited Langmuir waves, yields anomalous damping that applies to each Langmuir wave. Consequently, Langmuir waves with smaller growth rates will be suppressed by those with larger growth rates. Such a mode competition process may cause the overshoot of the HF-enhanced plasma line observed with the Arecibo 430 MHz radar. Favorable agreement is obtained between theory and the Arecibo observations.

Kuo, S. P.↗