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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Long-Term Variability of Airborne Asian Dust Observed from TOMS

Recent studies suggest that airborne Asian dust may not only play an important role in the regional radiation budget, but also influence the air quality over North America through long-range transport. In this paper, we use satellite data to investigate the long-term variability of airborne Asian dust as well as the daily variation of the dust aerosol distribution. By combining the Total Ozone Mapping Spectrometer (TOMS) aerosol index with National Centers for Environmental Prediction (NCEP) wind data, our analysis shows a strong correlation between the generation of dust storms in the region and the passage of springtime weather fronts. This is consistent with earlier studies performed by other researchers. According to both the Nimbus-7 and Earth-Probe TOMS data the Takla Makan desert, the Gobi desert, and the and region of Inner Mongolia are major sources of the eastward-flowing airborne Asian dust. Heavily populated areas in eastern China (e.g., Beijing) are often on the primary path of the dust storms originating in these desert regions. The increasing desertification north of the Beijing region has served to exacerbate problems stemming from these storms. The time series derived from 20 years of TOMS aerosol index data shows the first significant satellite evidence of the atmospheric effect of increasing desertification, indicating that the amount of dust blown eastward has increased strongly during the past few years including the year 2000.

Herman, J. R.↗
NASA NTRS1977

A parametric study of the copper chloride laser

A parametric study of the double-pulsed copper chloride laser is reported. The effects of a wide range of variables on the laser energy density and on three characteristic time intervals (the minimum, maximum, and optimum delay time) between the two electrical-discharge pulses were studied. The geometric variables investigated included a tube diameter of 2.3 to 40 mm and a tube length of 3 to 60 cm. Three buffer gases, helium, neon, and argon, were studied over the pressure range 0.5-50 torr, and the tube temperature was varied from 270 to 500 C. The energy density and voltage of both the dissociation and pumping pulse were varied independently from less than 1 mJ/cu cm at 8.5 kV to over 500 mJ/cu cm at 20 kV. The optimum conditions for maximum laser energy density were found to be with 20 torr neon in a 10-mm by 30-cm tube at 400 C. The maximum energy density obtained was 22 microjoules/cu cm.

Nerheim, N. M.↗
NASA NTRS1986

The frequency spectrum of C sub n sup 2 from MST radar data

In a recent study (Nastrom et al., 1986), the variability of the refractivity turbulence structure constant, C sub n(2), was examined using observations from the stratosphere troposphere/mesosphere stratosphere troposphere (ST/MST) radar at Poker Flat, Alaska, and Platteville, Colorado. Variations of C2 with height, season, and weather conditions were examined. Also, the autocorrelation function and the frequency distribution of C sub n(2) were studied, and it was shown that C sub n(2), follows a log-normal frequency distribution. One of the more tentative results given in that paper is a first look at the spectrum of log C sub n(2), as a function of frequency at Poker Flat. This spectrum appears to obey a power law relation with frequency, P(F) approx. F(k), with k near -5/3 at periods between about 4 hours and 6 days, and with k near -1 at shorter periods. Power law behavior of a spectrum often helps us to infer the underlying dynamics which give rise to this spectrum, and it is thus of some concern to establish further confidence in the spectral shape. The purpose here is to address these questions.

Nastrom, G. D.↗
NASA NTRS2005

Connection Between X-Ray Emission and Relativistic Jets in the Radio Galaxies 3C 111 and 3C 120

This work represents a part of a longterm study of the X-ray flux variability in radio galaxies and its relation to flux and structural changes in the associated radio jet. The work described here included: 1) continued study of the emission properties of the FR I radio galaxy 3C 120 known to exhibit a jet/disk connection from our past work; and 2) the commencement of monitoring of a second radio galaxy, the FR I1 object 3C 111 which was selected because of similar radio and X-ray properties to 3C 120, including the presence of Fe K a emission. The association between X-ray dips and new superluminal components, suggesting a picture in which the radio jet is fed by accretion events near the black hole, was identified in 3C 120 using combined RXTE and radio flux monitoring data and bi-monthly to monthly imaging data from the VLBA at 43 GHz. Such data were also obtained for both targets during the period described here. Specific goals were to more broadly investigate the X-ray dip/superluminal connection in 3C 120, thereby determining the epochs of X-ray minima and superluminal ejections more accurately (and hence more precisely determining the distance between the accretion disk and the core of the radio jet), and to determine whether a similar pattern is present in the data for a second radio galaxy. In 3C 111 a different time scale (longer time delays between X-ray dips and superluminal ejections) was expected due to the higher black hole mass implied by its higher radio luminosity: no black hole mass is published for this object but one can be determined from a PDS analysis of the RXTE data. The addition of the second source to the study would identify whether a similar connection was present in other sources and, if found, would provide important information on how time scale (and hence size scale) of accretion disk/jet systems depends on black hole mass. The grant included funding for the reduction and analysis of data obtained during the time period of Rossi XTE cycle 9. During this time window, the project was awarded 2 RXTE pointings weekly for 3C 111 and 4 pointing weekly for 3C 120, excluding time when the target was too close to the sun for observation. T

Aller, Margo F.↗
NASA NTRS1997

CGRO Guest Investigator Program

The following are highlights from the research supported by this grant: (1) Theory of gamma-ray blazars: We studied the theory of gamma-ray blazars, being among the first investigators to propose that the GeV emission arises from Comptonization of diffuse radiation surrounding the jet, rather than from the synchrotron-self-Compton mechanism. In related work, we uncovered possible connections between the mechanisms of gamma-ray blazars and those of intraday radio variability, and have conducted a general study of the role of Compton radiation drag on the dynamics of relativistic jets. (2) A Nonlinear Monte Carlo code for gamma-ray spectrum formation: We developed, tested, and applied the first Nonlinear Monte Carlo (NLMC) code for simulating gamma-ray production and transfer under much more general (and realistic) conditions than are accessible with other techniques. The present version of the code is designed to simulate conditions thought to be present in active galactic nuclei and certain types of X-ray binaries, and includes the physics needed to model thermal and nonthermal electron-positron pair cascades. Unlike traditional Monte-Carlo techniques, our method can accurately handle highly non-linear systems in which the radiation and particle backgrounds must be determined self-consistently and in which the particle energies span many orders of magnitude. Unlike models based on kinetic equations, our code can handle arbitrary source geometries and relativistic kinematic effects In its first important application following testing, we showed that popular semi-analytic accretion disk corona models for Seyfert spectra are seriously in error, and demonstrated how the spectra can be simulated if the disk is sparsely covered by localized 'flares'.

Begelman, Mitchell C.↗
NASA NTRS1975

Propulsion-induced interference effects on jet-lift VTOL aircraft

Recent investigations of propulsion-induced aerodynamics are reviewed. Experimental and analytical studies of the fundamental properties of lifting jet wakes and wind-tunnel investigations of VTOL configuration variables are discussed. The jet-wake flow studies will provide the data base for evaluating theoretical performance prediction methods; while the configuration studies will provide guidance for optimizing airframe/propulsion system integration. The influence of results to date on high-performance VTOL designs emphasizing minimum jet-induced losses and improved thrust vectoring in forward flight (VIFF) capability is illustrated.

Winston, M. M.↗
NASA NTRS2012

Probabilistic Sensitivity Analysis for Launch Vehicles with Varying Payloads and Adapters for Structural Dynamics and Loads

This paper examines Probabilistic Sensitivity Analysis (PSA) methods and tools in an effort to understand their utility in vehicle loads and dynamic analysis. Specifically, this study addresses how these methods may be used to establish limits on payload mass and cg location and requirements on adaptor stiffnesses while maintaining vehicle loads and frequencies within established bounds. To this end, PSA methods and tools are applied to a realistic, but manageable, integrated launch vehicle analysis where payload and payload adaptor parameters are modeled as random variables. This analysis is used to study both Regional Response PSA (RRPSA) and Global Response PSA (GRPSA) methods, with a primary focus on sampling based techniques. For contrast, some MPP based approaches are also examined.

McGhee, David S.↗
NASA NTRS2015

North Atlantic Simulations in Coordinated Ocean-Ice Reference Experiments Phase II (CORE-II) : Inter-Annual to Decadal Variability - Part II

Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958−2007 period from twenty global ocean - sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The study is Part II of our companion paper (Danabasoglu et al., 2014) which documented the mean states in the North Atlantic from the same models. A major focus of the present study is the representation of Atlantic meridional overturning circulation (AMOC) variability in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until the mid- to late-1970s, AMOC is relatively steady, remaining lower than its long-term (1958−2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in the mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about the mid-1970s and the mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric datasets which include the influence of ocean dynamics from nature superimposed onto anthropogenic effects. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres.

Ocean model comparisons↗
NASA NTRS2012

Variable-Speed Power-Turbine for the Large Civil Tilt Rotor

Turbine design concepts were studied for application to a large civil tiltrotor transport aircraft. The concepts addressed the need for high turbine efficiency across the broad 2:1 turbine operating speed range representative of the notional mission for the aircraft. The study focused on tailoring basic turbine aerodynamic design design parameters to avoid the need for complex, heavy, and expensive variable geometry features. The results of the study showed that good turbine performance can be achieved across the design speed range if the design focuses on tailoring the aerodynamics for good tolerance to large swings in incidence, as opposed to optimizing for best performance at the long range cruise design point. A rig design configuration and program plan are suggested for a dedicated experiment to validate the proposed approach.

Suchezky, Mark↗
NASA NTRS1993

Elements Of Theory Of Multidimensional Complex Variables

Two reports describe elements of theory of multidimensional complex variables, with emphasis on three dimensions. First report introduces general theory. Second, presents further developments in theory of analytic functions of single three-dimensional variable and applies theory to representation of ideal flows. Results of preliminary studies suggest analytic functions of new three-dimensional complex variables useful in numerous applications, including representing of three-dimensional flows and potentials.

Martin, E. Dale↗
NASA NTRS1994

The use of stellite scatterometer winds to drive a primitive equation model of the Indian Ocean: The impact of bandlike sampling

The aim of this study is to evaluate the impact of the bandlike sampling of spaceborne scatterometers on the ability of scatterometer winds to successfully force the mean flow and seasonal cycle of an ocean model in the context of equatorial and tropical dynamics. The equatorial ocean is simulated with a four-layer, primitive equation, reduced gravity model of the Indian Ocean. The variable wind stress used in this study is derived from one year (1988) of 6-hour analyses of the 10-m wind vector over the Indian Ocean performed at the European Centre for Medium-Range Weather Forecasts (ECMWF). It is applied as a forcing at every grid point of the model to drive a reference circulation. Scatterometer winds are simulated from ECMWF winds, using the nominal configurations and orbital parameters of the European Remote Sensing 1 (ERS-1) and NASA Scatterometer (NSCAT) missions. The model is forced in real time under swaths with the raw scatterometer winds of ERS-1 and NSCAT, with a persistence condition (i.e., the wind is kept constsnt until the next passage of the satellite provides a new value). The circulation obtained for each of the scatterometer experiments is compared with the reference circulation. The seasonal circulation of the Indian Ocean with NSCAT winds is very similar to the reference. The perturbations introduced by the bandlike sampling and the persistance condition have an impact similar to that of a small uncorrelated noise added to the reference forcing. The persistence condition for ERS-1 does not give results which are as good as those obtained for NSCAT.

Barnier, Bernard↗
NASA NTRS1975

Advanced supersonic propulsion study, phase 2

A continuation of the NASA/P and WA study to evaluate various types of propulsion systems for advanced commercial supersonic transports has resulted in the identification of two very promising engine concepts. They are the Variable Stream Control Engine which provides independent temperature and velocity control for two coannular exhaust streams, and a derivative of this engine, a Variable Cycle Engine that employs a rear flow-inverter valve to vary the bypass ratio of the cycle. Both concepts are based on advanced engine technology and have the potential for significant improvements in jet noise, exhaust emissions and economic characteristics relative to current technology supersonic engines. Extensive research and technology programs are required in several critical areas that are unique to these supersonic Variable Cycle Engines to realize these potential improvements. Parametric cycle and integration studies of conventional and Variable Cycle Engines are reviewed, features of the two most promising engine concepts are described, and critical technology requirements and required programs are summarized.

Howlett, R. A.↗
NASA NTRS1980

Performance and noise of a low pressure ratio variable pitch fan designed for general aviation applications

A limited study has been conducted to establish the performance and noise characteristics of a low design tip speed (168 m/s, 550 ft/sec) low pressure ratio (1.04) variable pitch fan which was tested in the Langley 30 X 60 tunnel. This fan was designed for minimum noise when installed in the tail mount location of a twin engine aircraft which normally has both nose and tail mounted propulsors. Measurements showed the fan noise to be very close to predictions made during the design of the fan and extremely low in level (65 dBA at 1000 ft) with no acoustic treatment. This is about 8 dB lower than the unshrouded 2 blade propeller normally used in this installation. On the basis of tests conducted during this program, it appears that this level could be further reduced by 2 dBA if optimized acoustic treatments were installed in the fan duct. Even the best of the shrouded propellers tested previously were 7 dB higher in level than the Q-Fan without acoustic treatment. It was found that the cruise performance of this fan was within 5% of the predicted efficiency of 72%. Evaluation of the performance data indicated that disturbances in the inflow to the fan were the probable cause of the reduced performance.

Metzger, F. B.↗
NASA NTRS1988

Physical studies of small asteroids and cometary cores

The main goal of the research is to carry on an extensive study of physical properties (colors and variability) of asteroids in the 1 to 10 km diameter range and of cometary cores, with the use of a charge coupled device (CCD) camera, PMT photometer, or both. Particular attention is paid to asteroids being observed by radar because the greatest gain is found from the combination of radar results with the data obtained by optical techniques. To satisfy the goal, 100 nights/year have been scheduled on the 2.3 m and 1.5 m telescopes. During the past 2 years small asteroids were observed. Out of 22 asteroids for which periods of rotation could be precisely measured, 17 have periods of rotation less than 5 hours. This indicates that small objects rotate faster. By now researchers know 14 asteroids with rotation periods in the 2 hour range. At the same time they confirmed the existence of a number of exceptionally slow rotators e.g., 1367 Nongoma with P = 5.65 days. Taxonomic observations lead to a conclusion that Apollo, Amor, and Aten asteroids represent a variety of classes and are not predominantly of class S. Apollo asteroid 3361 Orpheus was found to belong to the rare class V. In collaboration with Dr A. Harris of JPL the opposition effect was studied for 30 Urania and 64 Angelina. Seven comets: P/Helin, P/Brooks 2, P/Klemola, P/Borrelly, Wilson (1986 1) and Shoemaker (1987 o) were monitored for variability. The results were negative with the exception of P/Brooks 2 for which 0.35 mag amplitude was detected.

Wisniewski, Wieslaw Z.↗
NASA NTRS1989

Scattering from nonspherical Chebyshev particles. III - Variability in angular scattering patterns

Shape-induced variability in the scattered intensity from randomly oriented nonspherical particles is studied. A comparison is made between bands of shape-induced variability for six size intervals within the size parameter range of 1 to 20 and corresponding spherical intensities. It is found that averaging spherical intensities over narrow size ranges produces effects that are qualitatively similar to a slightly distorted single sphere. The results also show that, for Chebyshev particles which are neither small nor large compared to the wavelength, shape-induced variability is often comparable to the mean.

Mugnai, Alberto↗
NASA NTRS2008

Validation of Cardiovascular Parameters During NASA's Functional Task Test

Microgravity-induced physiological changes, including cardiovascular deconditioning may impair crewmembers f capabilities during exploration missions on the Moon and Mars. The Functional Task Test (FTT), which will be used to assess task performance in short and long duration astronauts, consists of 7 functional tests to evaluate crewmembers f ability to perform activities to be conducted in a partial-gravity environment or following an emergency landing on Earth. The Recovery from Fall/Stand Test (RFST) tests both the subject fs ability to get up from a prone position and orthostatic intolerance. PURPOSE: Crewmembers have never become presyncopal in the first 3 min of quiet stand, yet it is unknown whether 3 min is long enough to cause similar heart rate fluctuations to a 5-min stand. The purpose of this study was to validate and test the reliability of heart rate variability (HRV) analysis of a 3-min quiet stand. METHODS: To determine the validity of using 3 vs. 5-min of standing to assess HRV, 7 healthy subjects remained in a prone position for 2 min, stood up quickly and stood quietly for 6 min. ECG and continuous blood pressure data were recorded. Mean R-R interval and spectral HRV were measured in minutes 0-3 and 0-5 following the heart rate transient due to standing. Significant differences between the segments were determined by a paired t-test. To determine the reliability of the 3-min stand test, 13 healthy subjects completed 3 trials of the complete FTT on separate days, including the RFST with a 3-min stand test. Analysis of variance (ANOVA) was performed on the HRV measures. RESULTS: Spectral HRV measures reflecting autonomic activity were not different (p>0.05) during the 0-3 and 0-5 min segment (mean R-R interval: 738+/-74 ms, 728+/-69 ms; low frequency to high frequency ratio: 6.5+/-2.2, 7.7+/-2.7; normalized high frequency: 0.19+/-0.03, 0.18+/-0.04). The average coefficient of variation for mean R-R interval, systolic and diastolic blood pressures in the prone position and stand test were less than 8% for the test sessions. ANOVA results yielded a greater inter-subject variability (p.0.006) than inter-session variability (p>0.05) for HRV in the stand test. CONCLUSION: These studies show that a 3 minute stand delivers repeatable cardiovascular heart rate and BP data in the context of this larger series of tests such as the FTT.

Arzeno, N. M.↗
NASA NTRS1987

Far-infrared variability in active galactic nuclei

Pointed IRAS observations of 20 active galaxies were examined in the first detailed search for far-infrared variability in a large sample of active galaxies. Less extensive survey data were also checked for evidence of strong 6-month variability in 45 active galaxies. The far-infrared fluxes of three highly polarized objects ('blazars') appeared to vary by up to a factor of 2 on time scales of a few months. No convincing cases of variability greater than about 15 percent (rms) were found in any of the normal quasars or Seyfert galaxies studied. It appears that blazars tend to be variable at all wavelengths, but most active galaxies may be systematically less variable in the far-infrared than at higher frequencies.

Edelson, R. A.↗
NASA NTRS1988

Accuracy analysis of the semi-analytical method for shape sensitivity calculation

The semianalytical method, widely used for calculating derivatives of static response with respect to design variables for structures modeled by finite elements, is studied in this paper. The paper shows that the method can have serious accuracy problems for shape design variables in structures modeled by beam, plate, truss, frame, and solid elements. The errors are shown to be associated with the structural model. An error index is developed to test the accuracy of the semianalytical method. It characterizes the difference in errors between a general finite-difference method and the semianalytical method. Moreover, a method for improving the accuracy of the semianalytical method (when possible) is provided. Examples are presented to demonstrate the use of the error index.

Barthelemy, Bruno↗