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All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Planetary protection issues for the MESUR mission: Probability of growth (Pg)

The purpose of the workshop was to re-evaluate the existing guidelines for the probability of growth of terrestrial organisms on Mars that were established by the National Academy of Sciences following the Viking Mission. A panel of specialists in microbiology and allied fields reviewed this issue in the light of current information about the physical and chemical environments expected on Mars. Their deliberations resulted in the virtually unanimous conclusion that the existing Pg guidelines were either appropriate or that the values for Pg should be further reduced. Individual assessments of this problem by each of the participants, together with those of additional invited experts are included.

Klein, Harold P.↗
NASA NTRS1992

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n- and p-doped QWIPs, as well as intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasi-continuum, and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path; net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

Levine, B. F.↗
NASA NTRS1993

On the probability of magnification by cosmologically distributed gravitational lenses

An analytical method for calculating the statistical properties of source magnification caused by gravitational lenses randomly distributed throughout the universe is presented. Two lenses are considered at different redshifts to show that such an assumption is a statistically adequate approximation. The derived general formulas are applied to point-mass lenses with both point and extended sources. Analytical results of the magnification probability for point sources are accurate to within 10 percent in comparison with the available numerical simulations to moderate redshifts of less than about 2. In terms of the flux conservation, the results are accurate to within 18 percent at a redshift of 6 with respect to the Dyer-Roeder model of a clumpy universe. It is concluded that the present formulas are adequate for statistical studies of magnification by random gravitational lenses on cosmological scales.

Pei, Yichuan C.↗
NASA NTRS1992

Space Station Freedom structure floating potential and the probability of arcing

The interaction between a space system and the space environment has been one of the driving questions for the design of spacecraft since the dawn of the space age. The Space Station Freedom will represent a significant increase in spacecraft size, power, and activity relative to spacecraft that are currently in orbit. The structure floating potential on Space Station Freedom is studied with simple analytical models of the current collection. The probability of arcing due to dielectric breakdown is assessed.

Hastings, Daniel E.↗
NASA NTRS1993

Measurement of the transition probability of the C III 190.9 nanometer intersystem line

A radio-frequency ion trap has been used to store C(2+) ions created by electron bombardment of CO. The transition probability for the 2s2p 3Po1-2s2 1S0 intersystem line of C m has been measured by recording the radiative decay at 190.9 nm. The measured A-value is 121 +/- 7/s and agrees, within mutual uncertainty limits, with that of Laughlin et al. (1978), but is 20 percent larger than that of Nussbaumer and Storey (1978). The effective collision mixing rate coefficient among the fine structure levels of 3Po and the combined quenching and charge transfer rate coefficients out of the 3Po1 level with the CO source gas have also been measured.

Kwong, Victor H. S.↗
NASA NTRS1993

Probability-matched reflectivity-rainfall relations for a hurricane from aircraft observations

The relation between the distribution of equivalent reflectivity, Z(sub e), measured by an airborne C-band radar and that for rain rate, R, measured concurrently by a disdrometer on the same aircraft in the eyewall and outer bands of Hurricane Anita (1977) was investigated using the probability-matching method (PMM). It was found that the pdf of R is much narrower in the outer rainbands of Hurricane Anita than in the eyewall, and that the mean rain rates are also much smaller. It is shown that the use of PMM provides more realistic relations than the conventional power-law Z-R regression relations based upon scatter-plots of disdrometer measurements, showing higher eyewall reflectivities for the same rain rates at rates in excess of 6.3 mm/h.

Marks, Frank D., Jr.↗
NASA NTRS1991

Investigations of turbulent scalar fields using probability density function approach

Scalar fields undergoing random advection have attracted much attention from researchers in both the theoretical and practical sectors. Research interest spans from the study of the small scale structures of turbulent scalar fields to the modeling and simulations of turbulent reacting flows. The probability density function (PDF) method is an effective tool in the study of turbulent scalar fields, especially for those which involve chemical reactions. It has been argued that a one-point, joint PDF approach is the one to choose from among many simulation and closure methods for turbulent combustion and chemically reacting flows based on its practical feasibility in the foreseeable future for multiple reactants. Instead of the multi-point PDF, the joint PDF of a scalar and its gradient which represents the roles of both scalar and scalar diffusion is introduced. A proper closure model for the molecular diffusion term in the PDF equation is investigated. Another direction in this research is to study the mapping closure method that has been recently proposed to deal with the PDF's in turbulent fields. This method seems to have captured the physics correctly when applied to diffusion problems. However, if the turbulent stretching is included, the amplitude mapping has to be supplemented by either adjusting the parameters representing turbulent stretching at each time step or by introducing the coordinate mapping. This technique is still under development and seems to be quite promising. The final objective of this project is to understand some fundamental properties of the turbulent scalar fields and to develop practical numerical schemes that are capable of handling turbulent reacting flows.

Gao, Feng↗
NASA NTRS1993

Probability density distribution of velocity differences at high Reynolds numbers

Recent understanding of fine-scale turbulence structure in high Reynolds number flows is mostly based on Kolmogorov's original and revised models. The main finding of these models is that intrinsic characteristics of fine-scale fluctuations are universal ones at high Reynolds numbers, i.e., the functional behavior of any small-scale parameter is the same in all flows if the Reynolds number is high enough. The only large-scale quantity that directly affects small-scale fluctuations is the energy flux through a cascade. In dynamical equilibrium between large- and small-scale motions, this flux is equal to the mean rate of energy dissipation epsilon. The pdd of velocity difference is a very important characteristic for both the basic understanding of fully developed turbulence and engineering problems. Hence, it is important to test the findings: (1) the functional behavior of the tails of the probability density distribution (pdd) represented by P(delta(u)) is proportional to exp(-b(r) absolute value of delta(u)/sigma(sub delta(u))) and (2) the logarithmic decrement b(r) scales as b(r) is proportional to r(sup 0.15) when separation r lies in the inertial subrange in high Reynolds number laboratory shear flows.

Praskovsky, Alexander A.↗
NASA NTRS1993

Heterogeneous reaction probabilities, solubilities, and the physical state of cold volcanic aerosols

On 19 January 1992, heterogeneous loss of HNO3, ClNO3, and HCl was observed in part of the Mount Pinatubo volcanic cloud that had cooled as a result of forced ascent. Portions of the volcanic cloud froze near 191 kelvin. The reaction probability of ClNO3 and the solubility of HNO3 were close to laboratory measurements on liquid sulfuric acid. The magnitude of the observed loss of HCl suggests that it underwent a heterogeneous reaction. Such reactions could lead to substantial loss of HCl on background sulfuric acid particles and so be important for polar ozone loss.

Toon, O.↗
NASA NTRS1993

Asteroid orbit determination using Bayesian probabilities

Bayesian a priori and a posteriori probability densities are employed to determine asteroid orbital elements from optical astrometric observations. For near-earth asteroids and Jupiter Trojans, the two-body approximation can yield grossly erroneous uncertainty estimates because of the scattering effect of close planetary encounters or prolonged weak planetary perturbations. For main-belt asteroids, the two-body approximation works efficiently, from which it is inferred that, although the ephemeris must be computed from an integrated orbit, only approximate partial derivatives are needed to attain sufficient accuracy in predicting ephemeris uncertainties. The semimajor and semiminor axes of the 2D 1-sigma sky-plane uncertainty ellipse during the years 1993 to 2005 in the many-body and two-body linear approximations are shown.

Muinonen, Karri↗
NASA NTRS1995

Probability of detection of defects in coatings with electronic shearography

The goal of this research was to utilize statistical methods to evaluate the probability of detection (POD) of defects in coatings using electronic shearography. The coating system utilized in the POD studies was to be the paint system currently utilized on the external casings of the NASA space transportation system reusable solid rocket motor boosters. The population of samples was to be large enough to determine the minimum defect size for 90-percent POD of 95-percent confidence POD on these coatings. Also, the best methods to excite coatings on aerospace components to induce deformations for measurement by electronic shearography were to be determined.

Russell, S. S.↗
NASA NTRS1994

Probability density function approach for compressible turbulent reacting flows

The objective of the present work is to extend the probability density function (PDF) tubulence model to compressible reacting flows. The proability density function of the species mass fractions and enthalpy are obtained by solving a PDF evolution equation using a Monte Carlo scheme. The PDF solution procedure is coupled with a compression finite-volume flow solver which provides the velocity and pressure fields. A modeled PDF equation for compressible flows, capable of treating flows with shock waves and suitable to the present coupling scheme, is proposed and tested. Convergence of the combined finite-volume Monte Carlo solution procedure is discussed. Two super sonic diffusion flames are studied using the proposed PDF model and the results are compared with experimental data; marked improvements over solutions without PDF are observed.

Hsu, A. T.↗
NASA NTRS1995

Transition probabilities for the UV0.01 multiplet in N III

We report on large-scale ab initio multiconfiguration Hartree-Fock calculations for the UV0.01 multiplet, 2s(sup 2)2p(sup 2)P(sub J) - 2s2p(sup 2 4)P(sub J prime), in N III. The resulting transition probabilities agree very well with recent semiempirical calculations, and the lifetimes for two of the three upper levels agree with experiments. The deviation for the third level is discussed. Comparisons made with the highest quality IUE echelle spectra available -- those of RR Tel and V1016 Cyg (both photoionized sources with electron densities below 10(exp 8)/cu cm) -- show that computed branching ratios of lines sharing a common upper level are in agreement with observations to within uncertainties of +/- 10%. High-quality solar limb data or stellar data from the Hubble Space Telescope (HST) could, in principle, be used to determine whether the theoretical or measured lifetimes for the discrepant level are in error. Unfortunately, stellar data for high-density plasmas (N(sub e) greater than 10(exp 11)/cu cm are needed) do not yet exist, and existing solar data lack the photometric precision to address this problem.

Brage, Tomas↗
NASA NTRS1995

Energization in regions of CIRs unconnected to shocks are probably not the result of cross-field transport

Corotating energetic ion populations (CEIPs) associated with the forward and reverse shocks of corotating interaction regions (CIRs) are observed in CIRs at places where models say are magnetically unconnected to either shock. Such disconnections between CEIPs and shocks are common and have been documented with data from Pioneers 10 and 11 and confirmed with data from Ulysses. They pose a problem for models that account for these CEIPs in terms of ion energization at the shocks followed by ion propagation along field lines. Two possible resolutions to this problem have been suggested: diffusion of the ions across field lines and extension of the ion energization process to regions beyond the shock waves. Here we quantitatively examine the first of these possibilities. We give the Green's function solution to the convection-diffusion equation applied to idealized CIR geometry, with a source at the reverse shock -- the main producer of CEIPs. Two kinds of diffusion are considered: resonant diffusion and stochastic field line diffusion. We find that for resonant diffusion the computed ratio is many orders of magnitudes below the observed ratio. For stochastic field line diffusion, the computed ratio approximately equals the observed ratio if a diffusion coefficient appropriate to the free solar wind is used. It is several orders of magnitude below the observed ratio, however, if a diffusion coefficient appropriate to CIRs is used. We conclude that cross-field diffusion probably does not account for the presence of energetic ions in regions of CIRs that are magnetically unconnected to its shock waves. We suggest that the alternative possibility -- the energetic ions in regions magnetically unconnected to shocks result from an acceleration process that is independent of shocks -- be pursued to the point where quantitative tests can be performed.

Intriligator, Devrie S.↗
NASA NTRS1995

Space Debris Surfaces (Computer Code): Probability of No Penetration Versus Impact Velocity and Obliquity

A unique collection of computer codes, Space Debris Surfaces (SD_SURF), have been developed to assist in the design and analysis of space debris protection systems. SD_SURF calculates and summarizes a vehicle's vulnerability to space debris as a function of impact velocity and obliquity. An SD_SURF analysis will show which velocities and obliquities are the most probable to cause a penetration. This determination can help the analyst select a shield design that is best suited to the predominant penetration mechanism. The analysis also suggests the most suitable parameters for development or verification testing. The SD_SURF programs offer the option of either FORTRAN programs or Microsoft-EXCEL spreadsheets and macros. The FORTRAN programs work with BUMPERII. The EXCEL spreadsheets and macros can be used independently or with selected output from the SD_SURF FORTRAN programs. Examples will be presented of the interaction between space vehicle geometry, the space debris environment, and the penetration and critical damage ballistic limit surfaces of the shield under consideration.

Elfer, N.↗
NASA NTRS1996

Bounds on Block Error Probability for Multilevel Concatenated Codes

Maximum likelihood decoding of long block codes is not feasable due to large complexity. Some classes of codes are shown to be decomposable into multilevel concatenated codes (MLCC). For these codes, multistage decoding provides good trade-off between performance and complexity. In this paper, we derive an upper bound on the probability of block error for MLCC. We use this bound to evaluate difference in performance for different decompositions of some codes. Examples given show that a significant reduction in complexity can be achieved when increasing number of stages of decoding. Resulting performance degradation varies for different decompositions. A guideline is given for finding good m-level decompositions.

Lin, Shu↗
NASA NTRS1995

Probability Density Functions of Observed Rainfall in Montana

The question of whether a rain rate probability density function (PDF) can vary uniformly between precipitation events is examined. Image analysis on large samples of radar echoes is possible because of advances in technology. The data provided by such an analysis easily allow development of radar reflectivity factors (and by extension rain rate) distribution. Finding a PDF becomes a matter of finding a function that describes the curve approximating the resulting distributions. Ideally, one PDF would exist for all cases; or many PDF's that have the same functional form with only systematic variations in parameters (such as size or shape) exist. Satisfying either of theses cases will, validate the theoretical basis of the Area Time Integral (ATI). Using the method of moments and Elderton's curve selection criteria, the Pearson Type 1 equation was identified as a potential fit for 89 percent of the observed distributions. Further analysis indicates that the Type 1 curve does approximate the shape of the distributions but quantitatively does not produce a great fit. Using the method of moments and Elderton's curve selection criteria, the Pearson Type 1 equation was identified as a potential fit for 89% of the observed distributions. Further analysis indicates that the Type 1 curve does approximate the shape of the distributions but quantitatively does not produce a great fit.

Larsen, Scott D.↗